TY - JOUR A1 - Kopf, A. A1 - Saalfrank, Peter T1 - Electron transport through molecules treated by LCAO-MO Green's functions with absorbing boundaries N2 - In this Letter, we present a method for calculating transport properties of molecular conductors using a time- independent scattering approach based on Green's functions with absorbing boundaries. The method, which has been used before for chemical reaction dynamics in a grid basis [Seideman, Miller, J. Chem. Phys. 96 (1992) 4412], is formulated here in an LCAO-MO form within simple Huckel theory and extended Huckel theory (EHT), respectively. Test calculations are for a quasi-one-dimensional atom chain. As a more realistic application, the organic molecules benzene- 1,4-dithiolate and biphenyl-4,4'-dithiolate between gold electrodes are studied. (C) 2004 Elsevier B.V. All rights reserved Y1 - 2004 SN - 0009-2614 ER - TY - JOUR A1 - Saalfrank, Peter T1 - Theory of photon- and STM-induced bond cleavage at surfaces N2 - In this contribution, recent advances in the theory of laser and, to a lesser extent, of scanning tunneling microscope (STM) induced cleavage of bonds between an adsorbate and a solid surface, will be reviewed. Special emphasis will be given to the quantum dynamics of electronically non-adiabatic reactions. (c) 2005 Elsevier Ltd. All rights reserved Y1 - 2004 SN - 1359-0286 ER - TY - JOUR A1 - Krause, Pascal A1 - Klamroth, Tillmann A1 - Saalfrank, Peter T1 - Time-dependent configuration-interaction calculations of laser-pulse-driven many-electron dynamics : Controlled dipole switching in lithium cyanide N2 - We report simulations of laser-driven many-electron dynamics by means of the time-dependent configuration interaction singles (doubles) approach. The method accounts for the correlation of ground and excited states, is capable of describing explicitly time-dependent, nonlinear phenomena, and is systematically improvable. Lithium cyanide serves as a molecular test system in which the charge distribution and hence the dipole moment are shown to be switchable, in a controlled fashion, by (a series of) laser pulses which induce selective, state-to-state electronic transitions. One focus of our time-dependent calculations is the question of how fast the transition from the ionic ground state to a specific excited state that is embedded in a multitude of other states can be made, without creating an electronic wave packet. (c) 2005 American Institute of Physics Y1 - 2005 SN - 0021-9606 ER - TY - JOUR A1 - Saalfrank, Peter A1 - Klamroth, Tillmann A1 - Huber, C. A1 - Krause, Pascal T1 - Laser-driven electron dynamics at interfaces N2 - In this paper we present time-dependent, quantum-dynamical simulations of photoinduced processes at solid surfaces involving nonadiabatic transitions of electrons to and from short-lived intermediate excited states. In particular, two-photon photoemission (2PPE) spectra of naked metal surfaces and free-standing metal films are considered. One major problem in both cases is the presence of electron-electron scattering, which is treated here in various ways. The first way is to adopt an open-system density matrix approach, in which a single electron is weakly coupled to a "bath" of other electrons. The second approach is based on a many-electron Schrodinger equation, which is solved with the help of a time-dependent configuration interactions singles (TD-CIS) method Y1 - 2005 SN - 0021-2148 ER - TY - JOUR A1 - Nest, Mathias A1 - Klamroth, Tillmann A1 - Saalfrank, Peter T1 - The multiconfiguration time-dependent Hartree-Fock method for quantum chemical calculations N2 - We apply the multiconfiguration time-dependent Hartree-Fock method to electronic structure calculations and show that quantum chemical information can be obtained with this explicitly time-dependent approach. Different equations of motion are discussed, as well as the numerical cost. The two-electron integrals are calculated using a natural potential expansion, of which we describe the convergence behavior in detail Y1 - 2005 SN - 0021-9606 ER - TY - JOUR A1 - Klamroth, Tillmann A1 - Kroner, Dominic A1 - Saalfrank, Peter T1 - Laser-driven coupled electron-nuclear dynamics : Quantum mechanical simulation of molecular photodesorption from metal films N2 - In this paper we report dynamical simulations of laser-driven, coupled nuclear-electron dynamics for a molecule- surface system. Specifically, the laser desorption of a small molecule (NO) from a metal slab (Pt) in the so-called DIET limit (Desorption Induced by Electronic Transitions), is studied. The excitation of the metal electrons by a laser pulse followed by the formation of a negative ion resonance, its subsequent decay, and the simultaneous desorption of the molecule are all treated within a single quantum mechanical model. This model is based on an earlier theory of Harris and others [S. M. Harris, S. Holloway, and G. R. Darling, J. Chem. Phys. 102, 8235 (1995)], according to which a nuclear degree of freedom is coupled to an electronic one, both propagated on a single non-Born-Oppenheimer potential energy surface. The goals of the present contribution are (i) to make a conceptual connection of this model to the frequently adopted nonadiabatic "multi-state" models of photodesorption, (ii) to understand details of the desorption mechanism, (iii) to explicitly account for the laser pulse, and (iv) to study the photodesorption as a function of the thickness of the metal film, and the laser parameters. As an important methodological aspect we also present a highly efficient numerical scheme to propagate the wave packet in a problem-adapted diabatic basis Y1 - 2005 SN - 1098-0121 ER - TY - JOUR A1 - Martinazzo, Rocco A1 - Nest, Mathias A1 - Saalfrank, Peter A1 - Tantardini, Gian Franco T1 - A local coherent-state approximation to system-bath quantum dynamics JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - A novel quantum method to deal with typical system-bath dynamical problems is introduced. Subsystem discrete variable representation and bath coherent-state sets are used to write down a multiconfigurational expansion of the wave function of the whole system. With the help of the Dirac-Frenkel variational principle, simple equations of motion-a kind of Schrodinger-Langevin equation for the subsystem coupled to (pseudo) classical equations for the bath-are derived. True dissipative dynamics at all times is obtained by coupling the bath to a secondary, classical Ohmic bath, which is modeled by adding a friction coefficient in the derived pseudoclassical bath equations. The resulting equations are then solved for a number of model problems, ranging from tunneling to vibrational relaxation dynamics. Comparison of the results with those of exact, multiconfiguration time-dependent Hartree calculations in systems with up to 80 bath oscillators shows that the proposed method can be very accurate and might be of help in studying realistic problems with very large baths. To this end, its linear scaling behavior with respect to the number of bath degrees of freedom is shown in practice with model calculations using tens of thousands of bath oscillators. Y1 - 2006 U6 - https://doi.org/10.1063/1.2362821 SN - 0021-9606 SN - 1089-7690 VL - 125 IS - 19 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Andrianov, Ivan A1 - Saalfrank, Peter T1 - Free vibrational relaxation of H adsorbed on a Si(100) surface investigated with the multi-configurational time-dependent Hartree method JF - Chemical physics letters N2 - The results of a quantum-mechanical study of vibrational relaxation of hydrogen adsorbed on a Si(100) surface with the multi-configurational time-dependent Hartree (MCTDH) method are presented. A two-dimensional subsystem is coupled non-linearly to a bath of harmonic oscillators (phonons of the Si bulk), and the relaxation of subsystem vibrations proceeds primarily via a two-phonon process. Characteristic times of the system evolution agree well with our previous perturbation theory study. The vibrational population decay is non-exponential, exhibiting pronounced recurrences due to finite bath size. The dependence of the lifetimes of the vibrational levels on the bath size and on the coupling details is investigated. Y1 - 2006 U6 - https://doi.org/10.1016/j.cplett.2006.11.067 SN - 0009-2614 VL - 433 SP - 91 EP - 96 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Füchsel, Gernot A1 - Klamroth, Tillmann A1 - Dokic, Jadranka A1 - Saalfrank, Peter T1 - On the electronic structure of neutral and ionic azobenzenes and their possible role as surface mounted molecular switches JF - The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces & biophysical chemistry N2 - We report quantum chemical calculations, mostly based on density functional theory, on azobenzene and substituted azobenzenes as neutral molecules or ions, in ground and excited states. Both the cis and trans configurations are computed as well as the activation energies to transform one isomer into the other and the possible reaction paths and reaction surfaces along the torsion and inversion modes. All calculations are done for the isolated species, but results are discussed in light of recent experiments aiming at the switching of surface mounted azobenzenes by scanning tunneling microscopes. Y1 - 2006 U6 - https://doi.org/10.1021/jp060969v SN - 1520-6106 VL - 110 IS - 33 SP - 16337 EP - 16345 PB - Soc. CY - Washington ER - TY - JOUR A1 - Saalfrank, Peter T1 - Quantum dynamical approach to ultrafast molecular desorption from surfaces JF - Chemical reviews Y1 - 2006 U6 - https://doi.org/10.1021/cr0501691 SN - 0009-2665 SN - 1520-6890 VL - 106 IS - 10 SP - 4116 EP - 4159 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Paramonov, Guennaddi K. A1 - Saalfrank, Peter T1 - Time-evolution operator method for non-Markovian density matrix propagation in time and space representation : application to laser association of OH in an environment N2 - An efficient method for the numerical solution of a non-Markovian, open-system density matrix equation of motion in coordinate representation is developed. We apply the scheme to model simulations of the laser-assisted O+H -> OH association reaction in an environment. The suggested approach is based on the application of the time-evolution operator to the "closed-system" part of the overall Hamiltonian and transformation of the open-system equation of motion to the Heisenberg picture suitable for numerical propagation. A dual role of the system-environment coupling with respect to the infrared (ir) laser-driven association of OH is demonstrated: the association probability is increased due to the coupling at relatively weak laser fields, but decreased at strong laser fields. Moreover, at a certain strength of the ir laser field, the association probability does not depend on the strength of the system-bath coupling at all. Y1 - 2009 UR - http://pra.aps.org/ U6 - https://doi.org/10.1103/Physreva.79.013415 SN - 1050-2947 ER - TY - JOUR A1 - Tremblay, Jean Christophe A1 - Saalfrank, Peter T1 - Selective subsurface absorption of hydrogen in palladium using laser distillation N2 - A theoretical model for the selective subsurface absorption of atomic hydrogen in a Pd(111) surface by infrared (IR) laser pulses is presented. The dynamics of the adsorbate is studied within the reduced density matrix approach. Energy and phase relaxation of the hydrogen atom are treated using the semigroup formalism. The vibrational excitation leading to subsurface absorption is performed using rationally designed pulses as well as IR laser pulses optimized on- the-fly. It is shown that dissipation can be used as a tool to transfer population to an otherwise inaccessible state via a mechanism known as "laser distillation." We demonstrate that when the reaction path is generalized from a reduced one-dimensional to full three-dimensional treatment of the system, the laser control strategy can prove very different. Y1 - 2009 UR - http://jcp.aip.org/ U6 - https://doi.org/10.1063/1.3212695 SN - 0021-9606 ER - TY - JOUR A1 - Götze, Jan A1 - Saalfrank, Peter T1 - Serine in BLUF domains displays spectral importance in computational models N2 - The BLUF (blue-light sensing using flavine) domain of the AppA photoreceptor protein from Rhodobacter sphaeroides was modelled by using quantum chemical chromophore plus amino acid models at the (TD-)B3LYP/6-31G* level of theory. The models were based on NMR structures, and further refined by CHARM force field molecular dynamics simulations. The goal is to explain the total redshift by about 10 nm in the UV/Vis spectra of BLUF domains after illumination, and to relate it to structural changes. For this purpose UV/Vis spectra of the available NMR structures were calculated and related to geometrical features. In particular, the hydrogen network embedding the central chromophore is discussed. Specifically, the position of a conserved glutamine, Q63, is found to be important in agreement with findings from previous works. Additionally, however, we find a systematic dependence also on the geometry of a conserved serine, S41. Based on a series of calculations with known structures and with artificial structural models, we argue that indeed the light-induced switching of both Q63 and S41 is necessary to explain the full similar to 10 nm redshift in the light (signalling) state of serine containing BLUF domains. Following or accompanying the double switching, two structurally highly important residues W104 and M106 exchange places, but do not affect the overall UV/ Vis properties of the chromophore. Y1 - 2009 UR - http://www.sciencedirect.com/science/journal/10111344 U6 - https://doi.org/10.1016/j.jphotobiol.2008.10.003 SN - 1011-1344 ER - TY - JOUR A1 - Dokic, Jadranka A1 - Gothe, Marcel A1 - Wirth, Jonas A1 - Peters, Maike V. A1 - Schwarz, Jutta A1 - Hecht, Stefan A1 - Saalfrank, Peter T1 - Quantum chemical investigation of thermal cis-to-trans isomerization of azobenzene derivatives : substituent effects, solvent effects, and comparison to experimental data N2 - Quantum chemical calculations of various azobenzene (AB) derivatives have been carried out with the goal to describe the energetics and kinetics of their thermal cis -> trans isomerization. The effects of substituents, in particular their type, number, and positioning, on activation energies have been systematically studied with the ultimate goal to tailor the switching process. Trends observed for mono- and disubstituted species are discussed. A polarizable continuum model is used to study, in an approximate fashion, the cis -> trans isomerization of azobenzenes in solution. The nature of the transition state(s) and its dependence on substituents and the environment is discussed. In particular for push-pull azobenzenes, the reaction mechanism is found to change from inversion in nonpolar solvents to rotation in polar solvents. Concerning kinetics, calculations based on the Eyring transition state theory give usually reliable activation energies and enthalpies when compared to experimentally determined values. Also, trends in the resulting rate constants are correct. Other computed properties such as activation entropies and thus preexponential rate factors are in only moderate agreement with experiment. Y1 - 2009 UR - http://pubs.acs.org/journal/jpcafh U6 - https://doi.org/10.1021/jp9021344 SN - 1089-5639 ER - TY - JOUR A1 - Klinkusch, Stefan A1 - Klamroth, Tillmann A1 - Saalfrank, Peter T1 - Long-range intermolecular charge transfer induced by laser pulses : an explicitly time-dependent configuration interaction approach N2 - In this paper, we report simulations of laser-driven many-electron dynamics by means of the time-dependent configuration interaction singles (TD-CIS) approach. The method is capable of describing explicitly time-dependent phenomena beyond perturbation theory and is systematically improvable. In contrast to most time-dependent density functional methods it also allows us to treat long-range charge-transfer states properly. As an example, the laser-pulse induced charge transfer between a donor (ethylene) and an acceptor molecule (tetracyanoethylene, TCNE) is studied by means of TD-CIS. Also, larger aggregates consisting of several donors and/or acceptors are considered. It is shown that the charge distribution and hence the dipole moments of the systems under study are switchable by (a series of) laser pulses which induce selective, state-to-state electronic transitions. Y1 - 2009 UR - http://xlink.rsc.org/jumptojournal.cfm?journal_code=CP U6 - https://doi.org/10.1039/B817873a SN - 1463-9076 ER - TY - JOUR A1 - Klinkusch, Stefan A1 - Saalfrank, Peter A1 - Klamroth, Tillmann T1 - Laser-induced electron dynamics including photoionization : a heuristic model within time-dependent configuration interaction theory N2 - We report simulations of laser-pulse driven many-electron dynamics by means of a simple, heuristic extension of the time-dependent configuration interaction singles (TD-CIS) approach. The extension allows for the treatment of ionizing states as nonstationary states with a finite, energy-dependent lifetime to account for above-threshold ionization losses in laser-driven many-electron dynamics. The extended TD-CIS method is applied to the following specific examples: (i) state-to-state transitions in the LiCN molecule which correspond to intramolecular charge transfer, (ii) creation of electronic wave packets in LiCN including wave packet analysis by pump-probe spectroscopy, and, finally, (iii) the effect of ionization on the dynamic polarizability of H-2 when calculated nonperturbatively by TD-CIS. Y1 - 2009 UR - http://jcp.aip.org/ U6 - https://doi.org/10.1063/1.3218847 SN - 0021-9606 ER - TY - JOUR A1 - Nacci, Christophe A1 - Foelsch, Stefan A1 - Zenichowski, Karl A1 - Dokic, Jadranka A1 - Klamroth, Tillmann A1 - Saalfrank, Peter T1 - Current versus temperature-induced switching in a single-molecule tunnel junction : 1,5 cyclooctadiene on Si(001) N2 - The biconformational switching of single cyclooctadiene molecules chemisorbed on a Si(001) surface was explored by quantum chemical and quantum dynamical calculations and low-temperature scanning tunneling microscopy experiments. The calculations rationalize the experimentally observed switching driven by inelastic electron tunneling (IET) at 5 K. At higher temperatures, they predict a controllable crossover behavior between IET-driven and thermally activated switching, which is fully confirmed by experiment. Y1 - 2009 UR - http://pubs.acs.org/journal/nalefd U6 - https://doi.org/10.1021/Nl901419g SN - 1530-6984 ER - TY - JOUR A1 - Vazhappilly, Tijo A1 - Klamroth, Tillmann A1 - Saalfrank, Peter A1 - Hernandez, Rigoberto T1 - Femtosecond-laser desorption of H-2 (D-2) from Ru(0001) : quantum and classical approaches N2 - The femtosecond-laser-induced, substrate-mediated associative desorption of molecular hydrogen and deuterium from a Ru(0001) surface in the so-called DIMET limit is studied theoretically. Two widely used models, a "quantum nonadiabatic" approach and a "classical adiabatic" one are employed and compared to each other. The quantum model is realized by the Monte Carlo wave packet (MCWP) method in the framework of open-system density matrix theory: The classical approach is realized with the help of (frictional) Langevin dynamics with stochastic forces. For both models the same ground-state potential energy surface is used and the same two-temperature model adopted to describe the hot- electron-driven desorption dynamics. Apart from these common features both models are different. Still, both account well for the main experimental findings (Wagner et al. Phys. Rev. B 2005, 72, 205404). In particular, an isotope effect in desorption probabilities, the energy content of the desorbing molecules, and the scaling of these observables with laser fluence are reproduced and explained. The similarity of the results obtained with both models is traced back to the fact that, in the present case, the photodynamics takes place dominantly in the ground electronic state because the electronically excited state is rapidly quenched. The short lifetime of the excited state has also the effect that photoreaction cross sections are typically very small. An IR+vis hybrid scheme, by which the adsorbate is vibrationally excited by IR photons prior to the heating of metal electrons with the vis pulse, is shown to successfully promote the reaction even for strongly coupled adsorbate-surface systems. Y1 - 2009 UR - http://pubs.acs.org/journal/jpccck U6 - https://doi.org/10.1021/Jp810709k SN - 1932-7447 ER - TY - JOUR A1 - Schmidt, Roland A1 - Hagen, Sebastian A1 - Brete, Daniel A1 - Carley, Robert A1 - Gahl, Cornelius A1 - Dokic, Jadranka A1 - Saalfrank, Peter A1 - Hecht, Stefan A1 - Tegeder, Petra A1 - Weinelt, Martin T1 - On the electronic and geometrical structure of the trans- and cis-isomer of tetra-tert-butyl-azobenzene on Au(111) N2 - Near edge X-ray absorption. ne structure and X-ray photoelectron spectroscopy have been employed to follow the reversible trans to cis isomerization of tetra-tert-butyl-azobenzene (TBA) adsorbed on Au(111). For one monolayer the molecules adopt an adsorption geometry characteristic of the trans-TBA isomer. The azo-bridge (N = N) is aligned nearly parallel to the surface and the phenyl rings exhibit a planar orientation with a small tilt angle <= 4 degrees with respect to the surface normal. Illumination of the molecular layer at 455 nm triggers the trans to cis isomerization which is associated with a pronounced change of the geometrical and electronic structure. The N1s to pi* transition of the central azo-bridge shifts by 0.45 +/- 0.05 eV to higher photon energy and the transition dipole moment (TDM) is tilted by 59 +/- 5 degrees with respect to the surface normal. The pi-system of one phenyl ring is tilted by about 30 degrees with respect to the surface normal, while the second ring plane is oriented nearly perpendicular to the surface. This reorientation is supported by a shift and broadening of the C-H resonances associated with the tert-butyl legs of the molecule. These findings support a configuration of the photo-switched TBA molecule on Au(111) which is comparable to the cis-isomer of the free molecule. In the photo-stationary state 53 +/- 5% of the TBA molecules are switched to the cis configuration. Thermal activation induces the back reaction to trans-TBA. Y1 - 2010 UR - http://pubs.rsc.org/en/content/articlehtml/2010/cp/b924409c U6 - https://doi.org/10.1039/B924409c SN - 1463-9076 ER - TY - JOUR A1 - Schwarze, Thomas A1 - Mickler, Wulfhard A1 - Dosche, Carsten A1 - Flehr, Roman A1 - Klamroth, Tillmann A1 - Löhmannsröben, Hans-Gerd A1 - Saalfrank, Peter A1 - Holdt, Hans-Jürgen T1 - Systematic investigation of photoinduced electron transfer controlled by internal charge transfer and its consequences for selective PdCl2 coordination N2 - Fluoroionophores of fluorophore-spacer-receptor format were prepared for detection of PdCl2 by fluorescence enhancement. The fluorophore probes 1-13 consist of a fluorophore group, in alkyl spacer and a dithiomaleonitrile PdCl2 receptor. First, varying the length of the alkylene spacer (compounds 1-3) revealed, dominant through-space pathway for oxidative photoinduced electron transfer (PET) in CH2-bridged dithiomaleonitrile fluoroionophores. Second. fluorescent probes 4-9 containing two anthracene or pyrene fragments connected through CH2 bridges to the dithiomaleonitrile unit were synthesized. Modulation of the oxidation potential (E-Ox) through electron-withdrawing or -donating groups on the anthracene moiety regulates file thermodynamic driving force for oxidative PET (Delta G(PET)) in bis(anthrylmethylthio)maleonitriles and therefore the fluorescence quantum yields (Phi(f)), too. The new concept was confirmed and transferred to pyrenyl ligands, and fluorescence enhancements (FE) greater than 3.2 in the presence of PdCl2 were achieved by 7 and 8 (FE=5.4 and 5.2). Finally, for comparison, monofluorophore ligands 10-13 were synthesized. Y1 - 2010 UR - http://onlinelibrary.wiley.com/doi/10.1002/chem.200902281/pdf U6 - https://doi.org/10.1002/chem.200902281 SN - 0947-6539 ER - TY - JOUR A1 - Schwarze, Thomas A1 - Mickler, Wulfhard A1 - Dosche, Carsten A1 - Flehr, Roman A1 - Klamroth, Tillmann A1 - Löhmannsröben, Hans-Gerd A1 - Saalfrank, Peter A1 - Holdt, Hans-Jürgen T1 - Systematic investigation of photoinduced electron transfer controlled by internal charge transfer and its consequences for selective PdCl2 coordination N2 - Fluoroionophores of fluorophore-spacer-receptor format were prepared for detection of PdCl2 by fluorescence enhancement. The fluorescent probes 1-13 consist of a fluorophore group, an alkyl spacer and a dithiomaleonitrile PdCl2 receptor. First, varying the length of the alkylene spacer (compounds 1-3) revealed a dominant through-space pathway for oxidative photoinduced electron transfer (PET) in CH2-bridged dithiomaleonitrile fluoroionophores. Second, fluorescent probes 4-9 containing two anthracene or pyrene fragments connected through CH2 bridges to the dithiomaleonitrile unit were synthesized. Modulation of the oxidation potential (EOx) through electron-withdrawing or -donating groups on the anthracene moiety regulates the thermodynamic driving force for oxidative PET (GPET) in bis(anthrylmethylthio)maleonitriles and therefore the fluorescence quantum yields (f), too. The new concept was confirmed and transferred to pyrenyl ligands, and fluorescence enhancements (FE) greater than 3.2 in the presence of PdCl2 were achieved by 7 and 8 (FE=5.4 and 5.2). Finally, for comparison, monofluorophore ligands 10-13 were synthesized. Y1 - 2010 UR - http://www3.interscience.wiley.com/journal/26293/home SN - 0947-6539 ER - TY - JOUR A1 - Tremblay, Jean Christophe A1 - Krause, Pascal A1 - Klamroth, Tillmann A1 - Saalfrank, Peter T1 - Time-dependent response of dissipative electron systems N2 - We present a systematic study of the influence of energy and phase relaxation on dynamic polarizability simulations in the linear response regime. The nonperturbative approach is based on explicit electron dynamics using short laser pulses of low intensities. To include environmental effects on the property calculation, we use the time- dependent configuration-interaction method in its reduced density matrix formulation. Both energy dissipation and nonlocal pure dephasing are included. The explicit treatment of time-resolved electron dynamics gives access to the phase shift between the electric field and the induced dipole moment, which can be used to define a useful uncertainty measure for the dynamic polarizability. The nonperturbative treatment is compared to perturbation theory expressions, as applied to a simple model system, the rigid H-2 molecule. It is shown that both approaches are equivalent for low field intensities, but the time-dependent treatment provides complementary information on the phase of the induced dipole moment, which allows for the definition of an uncertainty associated with the computation of the dynamic polarizability in the linear response regime. Y1 - 2010 UR - http://pra.aps.org/ U6 - https://doi.org/10.1103/Physreva.81.063420 SN - 1050-2947 ER - TY - JOUR A1 - Schwarze, Thomas A1 - Dosche, Carsten A1 - Flehr, Roman A1 - Klamroth, Tillmann A1 - Löhmannsröben, Hans-Gerd A1 - Saalfrank, Peter A1 - Cleve, Ernst A1 - Buschmann, Hans-Jürgen A1 - Holdt, Hans-Jürgen T1 - Combination of a CT modulated PET and an intramolecular excimer formation to quantify PdCl2 by large fluorescence enhancement N2 - The [6.6](9,10)anthracenophane 1 (Scheme 1) is a selective fluoroionophore for the detection of PdCl2 with a large fluorescence enhancement factor (I/I-0 > 250). Y1 - 2010 UR - http://pubs.rsc.org/en/content/articlehtml/2010/cc/b919973j U6 - https://doi.org/10.1039/B919973j SN - 1359-7345 ER - TY - JOUR A1 - Schwarze, Thomas A1 - Dosche, Carsten A1 - Flehr, Roman A1 - Klamroth, Tillmann A1 - Löhmannsröben, Hans-Gerd A1 - Saalfrank, Peter A1 - Cleve, Ernst A1 - Buschmann, Hans-Jürgen A1 - Holdt, Hans-Jürgen T1 - Combination of a CT modulated PET and an intramolecular excimer formation to quantify PdCl2 by large fluorescence enhancement Y1 - 2010 UR - http://www.rsc.org/ej/CC/2010/b919973j.pdf SN - 1359-7345 ER - TY - JOUR A1 - Füchsel, Gernot A1 - Klamroth, Tillmann A1 - Tremblay, Jean Christophe A1 - Saalfrank, Peter T1 - Stochastic approach to laser-induced ultrafast dynamics : the desorption of H-2/D-2 from Ru(0001) N2 - The desorption of molecular hydrogen and deuterium induced by femtosecond-laser pulses is studied theoretically for the so-called DIMET (Desorption Induced by Multiple Electronic Transitions) process. These investigations are based on nonadiabatic classical Monte Carlo trajectory (CMCT) simulations on a ground and an excited state potential energy surface, including up to all six adsorbate degrees of freedom. The focus is on the hot-electron mediated energy transfer from the surface to the molecule and back, and the energy partitioning between the different degrees of freedom of the desorbing molecules. We first validate for a two-mode model comprising the desorption mode and the internal vibrational coordinate, the classical Monte Carlo trajectory method by comparing with Monte Carlo wavepacket (MCWP) calculations arising from a fully quantum mechanical open-system density matrix treatment. We then proceed by extending the CMCT calculations to include all six nuclear degrees of freedom of the desorbing molecule. This allows for a detailed comparison between theory and experiment concerning isotope effects, energy partitioning (translational, vibrational, and rotational energies and their distributions), and the dependence of these properties on the laser fluence. The most important findings are as follows. (i) CMCT agrees qualitative with the MCWP scheme. (ii) The basic experimental features such as the large isotope effect, the non-linear increase of yield with laser fluence, translationally hot products (in the order of several 1000 K) and non-equipartitioning of translational and internal energies (E-trans > E- vib > E-rot) are well reproduced. (iii) Predictions concerning a strong angular dependence of translational energies at large observation angles are also made. Y1 - 2010 UR - http://xlink.rsc.org/jumptojournal.cfm?journal_code=CP U6 - https://doi.org/10.1039/C0cp00895h SN - 1463-9076 ER - TY - JOUR A1 - Tremblay, Jean Christophe A1 - Monturet, Serge A1 - Saalfrank, Peter T1 - Electronic damping of anharmonic adsorbate vibrations at metallic surfaces N2 - The nonadiabatic coupling of an adsorbate close to a metallic surface leads to electronic damping of adsorbate vibrations and line broadening in vibrational spectroscopy. Here, a perturbative treatment of the electronic contribution to the lifetime broadening serves as a building block for a new approach, in which anharmonic vibrational transition rates are calculated from a position-dependent coupling function. Different models for the coupling function will be tested, all related to embedding theory. The first two are models based on a scattering approach with (i) a jellium-type and (ii) a density functional theory based embedding density, respectively. In a third variant a further refined model is used for the embedding density, and a semiempirical approach is taken in which a scaling factor is chosen to match harmonic, single-site, first-principles transition rates, obtained from periodic density functional theory. For the example of hydrogen atoms on (adsorption) and below (subsurface absorption) a Pd(111) surface, lifetimes of and transition rates between vibrational levels are computed. The transition rates emerging from different models serve as input for the selective subsurface adsorption of hydrogen in palladium starting from an adsorption site, by using sequences of infrared laser pulses in a laser distillation scheme. Y1 - 2010 UR - http://prb.aps.org/ U6 - https://doi.org/10.1103/Physrevb.81.125408 SN - 1098-0121 ER - TY - JOUR A1 - Bleger, David A1 - Dokic, Jadranka A1 - Peters, Maike V. A1 - Grubert, Lutz A1 - Saalfrank, Peter A1 - Hecht, Stefan T1 - Electronic decoupling approach to quantitative photoswitching in linear multiazobenzene architectures JF - The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces & biophysical chemistry N2 - A strategy to optimize the photoswitching efficiency of rigid, linear multiazobenzene constructs is presented. It consists of introducing large dihedral angles between azobenzene moieties linked via aryl-aryl connections in their para positions. Four bisazobenzenes exhibiting different dihedral angles as well as three single azobenzene reference compounds have been synthesized, and their switching behavior has been studied as well as experimentally and theoretically analyzed. As the dihedral angle between the two azobenzene units increases and consequently the electronic conjugation decreases, the photochromic characteristics improve, finally leading to individual azobenzene switches operating independently in the case of the perpendicular ortho,ortho,ortho',ortho'-tetramethyl biphenyl linker. The electronic decoupling leads to efficient separation of the absorption spectra of the involved switching states and hence by choosing the appropriate irradiation wavelength, an almost quantitative E -> Z photoisomerization up to 97% overall Z-content can be achieved. In addition, thermal Z -> E isomerization processes become independent of each other with increasing decoupling. The electronic decoupling could furthermore be proven by electrochemistry. The experimental data are supported by theory, and calculations additionally provide mechanistic insight into the preferred pathway for the thermal Z,Z -> Z,E -> E,E isomerization via inversion on the inner N-atoms. Our decoupling approach outlined herein provides the basis for constructing rigid rod architectures composed of multiple azobenzene photochromes, which display practically quantitative photoswitching properties, a necessary prerequisite to achieve highly efficient transduction of light energy directly into motion. Y1 - 2011 U6 - https://doi.org/10.1021/jp2044114 SN - 1520-6106 VL - 115 IS - 33 SP - 9930 EP - 9940 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Weigel, A. A1 - Dobryakov, A. A1 - Klaumünzer, Bastian A1 - Sajadi, M. A1 - Saalfrank, Peter A1 - Ernsting, N. P. T1 - Femtosecond stimulated raman spectroscopy of flavin after optical excitation JF - The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces & biophysical chemistry N2 - In blue-light photoreceptors using flavin (BLUF), the signaling state is formed already within several 100 ps after illumination, with only small changes of the absorption spectrum. The accompanying structural evolution can, in principle, be monitored by femtosecond stimulated Raman spectroscopy (FSRS). The method is used here to characterize the excited-state properties of riboflavin and flavin adenine dinucleotide in polar solvents. Raman modes are observed in the range 90-1800 cm(-1) for the electronic ground state S-0 and upon excitation to the S-1 state, and modes >1000 cm(-1) of both states are assigned with the help of quantum-chemical calculations. Line shapes are shown to depend sensitively on resonance conditions. They are affected by wavepacket motion in any of the participating electronic states, resulting in complex amplitude modulation of the stimulated Raman spectra. Wavepackets in S-1 can be marked, and thus isolated, by stimulated-emission pumping with the picosecond Raman pulses. Excited-state absorption spectra are obtained from a quantitative comparison of broadband transient fluorescence and absorption. In this way, the resonance conditions for FSRS are determined. Early differences of the emission spectrum depend on excess vibrational energy, and solvation is seen as dynamic Stokes shift of the emission band. The ne state is evidenced only through changes of emission oscillator strength during solvation. S-1 quenching by adenine is seen with all methods in terms of dynamics, not by spectral intermediates. Y1 - 2011 U6 - https://doi.org/10.1021/jp1117129 SN - 1520-6106 VL - 115 IS - 13 SP - 3656 EP - 3680 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Luo, Ying A1 - Utecht, Manuel Martin A1 - Dokic, Jadranka A1 - Korchak, Sergey A1 - Vieth, Hans-Martin A1 - Haag, Rainer A1 - Saalfrank, Peter T1 - Cis-trans isomerisation of substituted aromatic imines a comparative experimental and theoretical study JF - ChemPhysChem : a European journal of chemical physics and physical chemistry N2 - The cis-trans isomerisation of N-benzylideneaniline (NBA) and derivatives containing a central C=N bond has been investigated experimentally and theoretically. Eight different NBA molecules in three different solvents were irradiated to enforce a photochemical trans (hv) -> cis isomerisation and the kinetics of the thermal backreaction cis (Delta)-> trans were determined by NMR spectroscopy measurements in the temperature range between 193 and 288 K. Theoretical calculations using density functional theory and Eyring transition-state theory were carried out for 12 different NBA species in the gas phase and three different solvents to compute thermal isomerisation rates of the thermal back reaction. While the computed absolute rates are too large, they reveal and explain experimental trends. Time-dependent density functional theory provides optical spectra for vertical transitions and excitation energy differences between trans and cis forms. Together with isomerisation rates, the latter can be used to identify "optimal switches" with good photochromicity and reasonable thermal stability. KW - density functional calculations KW - imines KW - isomerization KW - photochemistry KW - thermochemistry Y1 - 2011 U6 - https://doi.org/10.1002/cphc.201100179 SN - 1439-4235 VL - 12 IS - 12 SP - 2311 EP - 2321 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Kröner, Dominik A1 - Ehlert, Christopher A1 - Saalfrank, Peter A1 - Holländer, Andreas T1 - Ab initio calculations for XPS chemical shifts of poly(vinyl-trifluoroacetate) using trimer models JF - Surface science N2 - X-ray photoelectron spectra (XPS) of the polymer poly(vinyl-trifluoroacetate) show C(1s) binding energy shifts which are unusual because they are influenced by atoms which are several bonds away from the probed atom. In this work, the influence of the trifluoroacetate substituent on the 1s ionization potential of the carbon atoms of the polyethylene chain is investigated theoretically using mono-substituted, diad and triad models of trimers representing the polymer. Carbon 1s ionization energies are calculated by the Hartree-Fock theory employing Koopmans' theorem. The influence of the configuration and conformation of the functional groups as well as the degree of substitution are found to be important determinants of XPS spectra. It is further found that the 1s binding energy correlates in a linear fashion, with the total electrostatic potential at the position of the probe atom, and depends not only on nearest neighbor effects. This may have implications for the interpretation of high-resolution XP spectra. KW - Ab initio quantum chemical methods and calculations KW - X-ray photoelectron spectroscopy KW - Insulating films Y1 - 2011 U6 - https://doi.org/10.1016/j.susc.2011.05.021 SN - 0039-6028 VL - 605 IS - 15-16 SP - 1516 EP - 1524 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Füchsel, Gernot A1 - Klamroth, Tillmann A1 - Monturet, Serge A1 - Saalfrank, Peter T1 - Dissipative dynamics within the electronic friction approach the femtosecond laser desorption of H-2/D-2 from Ru(0001) JF - Physical chemistry, chemical physics : a journal of European Chemical Societies N2 - An electronic friction approach based on Langevin dynamics is used to describe the multidimensional (six-dimensional) dynamics of femtosecond laser induced desorption of H-2 and D-2 from a H(D)-covered Ru(0001) surface. The paper extends previous reduced-dimensional models, using a similar approach. In the present treatment forces and frictional coefficients are calculated from periodic density functional theory (DFT) and essentially parameter-free, while the action of femtosecond laser pulses on the metal surface is treated by using the two-temperature model. Our calculations shed light on the performance and validity of various adiabatic, non-adiabatic, and Arrhenius/Kramers type kinetic models to describe hot-electron mediated photoreactions at metal surfaces. The multidimensional frictional dynamics are able to reproduce and explain known experimental facts, such as strong isotope effects, scaling of properties with laser fluence, and non-equipartitioning of vibrational, rotational, and translational energies of desorbing species. Further, detailed predictions regarding translations are made, and the question for the controllability of photoreactions at surfaces with the help of vibrational preexcitation is addressed. Y1 - 2011 U6 - https://doi.org/10.1039/c0cp02086a SN - 1463-9076 VL - 13 IS - 19 SP - 8659 EP - 8670 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Utecht, Manuel Martin A1 - Klamroth, Tillmann A1 - Saalfrank, Peter T1 - Optical absorption and excitonic coupling in azobenzenes forming self-assembled monolayers a study based on density functional theory JF - Physical chemistry, chemical physics : a journal of European Chemical Societies N2 - Based on the analysis of optical absorption spectra, it has recently been speculated that the excitonic coupling between individual azobenzene-functionalized alkanethiols arranged in a self-assembled monolayer (SAM) on a gold surface could be strong enough to hinder collective trans-cis isomerization-on top of steric hindrance [Gahl et al., J. Am. Chem. Soc., 2010, 132, 1831]. Using models of SAMs of increasing complexity (dimer, linear N-mers, and two-dimensionally arranged N-mers) and density functional theory on the (TD-) B3LYP/6-31G* level, we determine optical absorption spectra, the nature and magnitude of excitonic couplings, and the corresponding spectral shifts. It is found that at inter-monomer distances of about 20 angstrom and above, TD-B3LYP excitation frequencies (and signal intensities) can be well described by the frequently used point-dipole approximation. Further, calculated blue shifts in optical absorption spectra account for the experimental observations made for azobenzene/gold SAMs, and hint to the fact that they can indeed be responsible for reduced switching probability in densely packed self-assembled structures. Y1 - 2011 U6 - https://doi.org/10.1039/c1cp22793a SN - 1463-9076 VL - 13 IS - 48 SP - 21608 EP - 21614 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Tremblay, Jean Christophe A1 - Klinkusch, Stefan A1 - Klamroth, Tillmann A1 - Saalfrank, Peter T1 - Dissipative many-electron dynamics of ionizing systems JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - In this paper, we perform many-electron dynamics using the time-dependent configuration-interaction method in its reduced density matrix formulation (rho-TDCI). Dissipation is treated implicitly using the Lindblad formalism. To include the effect of ionization on the state-resolved dynamics, we extend a recently introduced heuristic model for ionizing states to the rho-TDCI method, which leads to a reduced density matrix evolution that is not norm-preserving. We apply the new method to the laser-driven excitation of H(2) in a strongly dissipative environment, for which the state-resolve lifetimes are tuned to a few femtoseconds, typical for dynamics of adsorbate at metallic surfaces. Further testing is made on the laser-induced intramolecular charge transfer in a quinone derivative as a model for a molecular switch. A modified scheme to treat ionizing states is proposed to reduce the computational burden associated with the density matrix propagation, and it is thoroughly tested and compared to the results obtained with the former model. The new approach scales favorably (similar to N(2)) with the number of configurations N used to represent the reduced density matrix in the rho-TDCI method, as compared to a N(3) scaling for the model in its original form. Y1 - 2011 U6 - https://doi.org/10.1063/1.3532410 SN - 0021-9606 VL - 134 IS - 4 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Wirth, Jonas A1 - Monturet, Serge A1 - Klamroth, Tillmann A1 - Saalfrank, Peter T1 - Adsorption and (photo-) electrochemical splitting of water on rutile ruthenium dioxide JF - epl : a letters journal exploring the frontiers of physics N2 - In this work, the adsorption and splitting of the water molecule by light and/or an external potential is investigated in the frame of (photo-) electrochemical cells using a rutile ruthenium dioxide anode. With the help of periodic density functional calculations, the adsorbed structures of H(2)O and some radicals involved in the splitting process (O, OH, OOH) are obtained and compared with the available experimental results. On the basis of these electronic-structure calculations, we use a method to calculate the stability of the reaction intermediates and conclude on the thermodynamical possibility of the water splitting reaction at the surface. We demonstrate that a moderate overpotential of 0.64 V is required for the reaction to take place at the RuO(2)(110) surface. Y1 - 2011 U6 - https://doi.org/10.1209/0295-5075/93/68001 SN - 0295-5075 VL - 93 IS - 6 PB - EDP Sciences CY - Mulhouse ER - TY - JOUR A1 - Bouakline, Foudhil A1 - Lüder, Franziska A1 - Martinazzo, Rocco A1 - Saalfrank, Peter T1 - Reduced and exact quantum dynamics of the vibrational relaxation of a molecular system interacting with a finite-dimensional bath JF - The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment & general theory N2 - We investigate the vibrational relaxation of a Morse oscillator, nonlinearly coupled to a finite-dimensional bath of harmonic oscillators at zero temperature, using two different approaches: Reduced dynamics with the help of the Lindblad formalism of reduced density matrix theory in combination with Fermi's Golden Rule, and exact dynamics (within the chosen model). with the multiconfiguration time-dependent Hartree (MCTDH) method. Two different models have been constructed, the situation where the bath spectrum is exactly resonant with the anharmonic oscillator transition frequencies, and the case for which the subsystem is slightly off-resonant with the environment. At short times, reduced dynamics calculations describe the relaxation process qualitatively well but fail to reproduce recurrences observed with MCTDH for longer times. Lifetimes of all the vibrational levels of the Morse oscillator have been calculated, and both Lindblad and MCTDH. results show the same dependence of the lifetimes on the initial vibrational state quantum number. A prediction, which should be generic for adsorbate systems is a striking, sharp increase of lifetimes of the subsystem vibrational levels close to the dissociation This is contradictory with harmonic/linear extrapolation laws, which predict a monotonic decrease of the lifetime with initial vibrational quantum number. Y1 - 2012 U6 - https://doi.org/10.1021/jp304466u SN - 1089-5639 VL - 116 IS - 46 SP - 11118 EP - 11127 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Floss, Gereon A1 - Granucci, Giovanni A1 - Saalfrank, Peter T1 - Surface hopping dynamics of direct trans -> cis photoswitching of an azobenzene derivative in constrained adsorbate geometries JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - With ongoing miniaturization of electronic devices, the need for individually addressable, switchable molecules arises. An example are azobenzenes on surfaces which have been shown to be switchable between trans and cis forms. Here, we examine the "direct" (rather than substrate-mediated) channel of the trans -> cis photoisomerization after pi pi* excitation of tetra-tert-butyl-azobenzene physisorbed on surfaces mimicking Au(111) and Bi(111), respectively. In spirit of the direct channel, the electronic structure of the surface is neglected, the latter merely acting as a rigid platform which weakly interacts with the molecule via Van-der-Waals forces. Starting from thermal ensembles which represent the trans-form, sudden excitations promote the molecules to pi pi*-excited states which are non-adiabatically coupled among themselves and to a n pi*-excited and the ground state, respectively. After excitation, relaxation to the ground state by internal conversion takes place, possibly accompanied by isomerization. The process is described here by "on the fly" semiclassical surface hopping dynamics in conjunction with a semiempirical Hamiltonian (AM1) and configuration-interaction type methods. It is found that steric constraints imposed by the substrate lead to reduced but non-vanishing, trans -> cis reaction yields and longer internal conversion times than for the isolated molecule. Implications for recent experiments for azobenzenes on surfaces are discussed. KW - AM1 calculations KW - bismuth KW - configuration interactions KW - excited states KW - gold KW - isomerisation KW - organic compounds KW - photochemistry KW - van der Waals forces Y1 - 2012 U6 - https://doi.org/10.1063/1.4769087 SN - 0021-9606 VL - 137 IS - 23 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Wirth, Jonas A1 - Saalfrank, Peter T1 - The chemistry of water on alpha-alumina kinetics and nuclear quantum effects from first principles JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - Water adsorption on an alumina (alpha-Al2O3) surface is studied here from first principles using periodic density functional theory in the generalized gradient approximation. Two different coverage regimes, low and high, are considered. For the low-coverage regime (with a coverage of 1/4 with respect to the number of coordinatively unsaturated Al sites), possible reactions at the surface such as dissociation, rotation, and diffusion of water and its fragments are investigated, using first principles thermodynamics and kinetics. A microkinetic model is set up with rates calculated from Eyring's transition state theory in order to cover a wide range of time scales. Special emphasis of this study is on the magnitude of quantum effects and on anharmonic corrections, particularly for reactions and dynamics. These have often been neglected in the past for water/alumina systems but can influence the system. This is particularly true for processes involving hydrogen atoms, where, for example, tunneling corrections to reaction rates are found to be important even at room temperature. For a higher-coverage regime (with a coverage of 2 ML), hydrogen dynamics becomes even more complex and is characterized, e.g., by concerted atom motion, strong anharmonicity, and delocalization. In this regime, classical molecular dynamics becomes questionable as well as quantum mechanical treatments based on the harmonic approximation. Y1 - 2012 U6 - https://doi.org/10.1021/jp310234h SN - 1932-7447 VL - 116 IS - 51 SP - 26829 EP - 26840 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Zenichowski, Karl A1 - Dokic, Jadranka A1 - Klamroth, Tillmann A1 - Saalfrank, Peter T1 - Current versus temperature-induced switching of a single molecule - open-system density matrix theory for 1,5-cyclooctadiene on Si(100) JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - The switching of single cyclooctadiene molecules chemisorbed on a Si(100) surface between two stable conformations, can be achieved with a scanning tunneling microscope [Nacci , Phys. Rev. B 77, 121405(R) (2008)]. Recently, it was shown by quantum chemical and quantum dynamical simulations that major experimental facts can be explained by a single-mode model with switching enforced by inelastic electron tunneling (IET) excitations and perturbed by vibrational relaxation [Nacci , Nano Lett. 9, 2997 (2009)]. In the present paper, we extend the previous theoretical work in several respects: (1) The model is generalized to a two-mode description in which two C2H4 units of COD can move independently; (2) contributions of dipole and, in addition, (cation and anion) resonance-IET rates are considered; (3) the harmonic-linear vibrational relaxation model used previously is generalized to anharmonic vibrations. While the present models highlight generic aspects of IET-switching between two potential minima, they also rationalize specific experimental findings for COD/Si(100): (1) A single-electron excitation mechanism with a linear dependence of the switching rate on tunneling current I, (2) the capability to switch both at negative and positive sample biases, and (3) a crossover temperature around similar to 60 K from an IET-driven, T-independent atom tunneling regime, to classical over-the-barrier isomerization with exponential T-dependence at higher temperatures for a bias voltage of +1.5 V and an average tunneling current of 0.73 nA. Y1 - 2012 U6 - https://doi.org/10.1063/1.3692229 SN - 0021-9606 VL - 136 IS - 9 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Klaumünzer, Bastian A1 - Kröner, Dominik A1 - Lischka, Hans A1 - Saalfrank, Peter T1 - Non-adiabatic excited state dynamics of riboflavin after photoexcitation JF - Physical chemistry, chemical physics : a journal of European Chemical Societies N2 - Flavins are chromophores in light-gated enzymes and therefore central in many photobiological processes. To unravel the optical excitation process as the initial, elementary step towards signal transduction, detailed ultrafast (femtosecond) experiments probing the photo-activation of flavins have been carried out recently [Weigel et al., J. Phys. Chem. B, 2011, 115, 3656-3680.]. The present paper contributes to a further understanding and interpretation of these experiments by studying the post-excitation vibrational dynamics of riboflavin (RF) and microsolvated riboflavin, RF center dot 4H(2)O, using first principles non-adiabatic molecular dynamics. By analyzing the characteristic atom motions and calculating time-resolved stimulated emission spectra following pi pi* excitation, it is found that after optical excitation C-N and C-C vibrations in the isoalloxazine rings of riboflavin set in. The Franck-Condon (vertically excited) state decays within about 10 fs, in agreement with experiment. Anharmonic coupling leads to Intramolecular Vibrational energy Redistribution (IVR) on the timescale of about 80-100 fs, first to (other) C-C stretching modes of the isoalloxazine rings, then by energy spread over the whole molecule, including low-frequency in-plane modes. The IVR is accompanied by a red-shift and broadening of the emission spectrum. When RF is microsolvated with four water molecules, an overall redshift of optical spectra by about 20 nm is observed but the relaxation dynamics is only slightly affected. For several trajectories, a tendency for hydrogen transfer from water to flavin-nitrogen (N-5) was found. Y1 - 2012 U6 - https://doi.org/10.1039/c2cp40978j SN - 1463-9076 VL - 14 IS - 24 SP - 8693 EP - 8702 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Füchsel, Gernot A1 - Tremblay, Jean Christophe A1 - Klamroth, Tillmann A1 - Saalfrank, Peter T1 - Selective excitation of molecule-surface vibrations in H2 and D2 dissociatively adsorbed on Ru(0001) JF - Israel journal of chemistry N2 - In this contribution we report about the selective vibrational excitation of H2 and D2 on Ru(0001) as an example for nonadiabatic coupling of an open quantum system to a dissipative environment. We investigate the possibility of achieving state-selective vibrational excitations of H2 and D2 adsorbed on a Ru(0001) surface using picosecond infrared laser pulses. The systems behavior is explored using pulses that are rationally designed and others that are optimized using a time-local variant of Optimal Control Theory. The effects of dissipation on the laser-driven dynamics are studied using the reduced-density matrix formalism. The non-adiabatic couplings between adsorbate and surface are computed perturbatively, for which our recently introduced state-resolved anharmonic rate model is used. It is shown that mode- and state-selective excitation can be achieved in the absence of dissipation when using optimized laser pulses. The inclusion of dissipation in the model reduces the state selectivity and the population transfer yield to highly excited states. In this case, mode activation is most effectively realized by a rational pulse of carefully chosen duration rather than by a locally optimized pulse. KW - dissipative dynamics KW - photochemistry KW - quantum control KW - surface chemistry Y1 - 2012 U6 - https://doi.org/10.1002/ijch.201100097 SN - 0021-2148 VL - 52 IS - 5 SP - 438 EP - 451 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Götze, Jan Philipp A1 - Saalfrank, Peter T1 - Quantum chemical modeling of the kinetic isotope effect of the carboxylation step in RuBisCO JF - Journal of molecular modeling N2 - Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), the most important enzyme for the assimilation of carbon into biomass, features a well-known isotope effect with regards to the CO2 carbon atom. This kinetic isotope effect alpha = k (12)/k (13) for the carboxylation step of the RuBisCO reaction sequence, and its microscopic origin, was investigated with the help of cluster models and quantum chemical methods [B3LYP/6-31G(d,p)]. We use a recently proposed model for the RuBisCO active site, in which a water molecule remains close to the reaction center during carboxylation of ribulose-1,5-bisphosphate [B. Kannappan, J.E. Gready, J. Am. Chem. Soc. 130 (2008), 15063]. Alternative active-site models and/or computational approaches were also tested. An isotope effect alpha for carboxylation is found, which is reasonably close to the one measured for the overall reaction, and which originates from a simple frequency shift of the bending vibration of (CO2)-C-12 compared to (CO2)-C-13. The latter is the dominant mode for the product formation at the transition state. KW - Cluster model KW - Dark reactions KW - Densityfunctional theory KW - Isotope effect KW - Photosynthesis KW - Quantum chemistry KW - RuBisCO Y1 - 2012 U6 - https://doi.org/10.1007/s00894-011-1207-0 SN - 1610-2940 VL - 18 IS - 5 SP - 1877 EP - 1883 PB - Springer CY - New York ER - TY - JOUR A1 - Tremblay, Jean Christophe A1 - Füchsel, Gernot A1 - Saalfrank, Peter T1 - Excitation, relaxation, and quantum diffusion of CO on copper JF - Physical review : B, Condensed matter and materials physics N2 - We investigate the effect of intermode coupling and anharmonicity on the excitation and relaxation dynamics of CO on Cu(100). The nonadiabatic coupling of the adsorbate to the surface is treated perturbatively using a position-dependent state-resolved transition rate model. Using the potential energy surface of Marquardt et al. [J. Chem. Phys. 132, 074108 (2010)], which provides an accurate description of intermode interactions, we propose a four-dimensional model that represents simultaneously the diffusion and the desorption of the adsorbate. The system is driven by both rational and optimized infrared laser pulses to favor either selective mode and state excitations or lateral displacement along the diffusion coordinate. The dissipative dynamics is simulated using the reduced density matrix in its Lindblad form. We show that coupling between the degrees of freedom, mediated by the creation and annihilation of electron-hole pairs in the metal substrate, significantly affects the system excitation and relaxation dynamics. In particular, the angular degrees of freedom appear to play an important role in the energy redistribution among the molecule-surface vibrations. We also show that coherent excitation using simple IR pulses can achieve population transfer to a specific target to some extent but does not allow enforcement of the directionality to the diffusion motion. Y1 - 2012 U6 - https://doi.org/10.1103/PhysRevB.86.045438 SN - 1098-0121 SN - 1550-235X VL - 86 IS - 4 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Zenichowski, Karl A1 - Nacci, Ch A1 - Fölsch, S. A1 - Dokic, Jadranka A1 - Klamroth, Tillmann A1 - Saalfrank, Peter T1 - STM-switching of organic molecules on semiconductor surfaces: an above threshold density matrix model for 1,5 cyclooctadiene on Si(100) JF - Journal of physics : Condensed matter N2 - The scanning tunnelling microscope (STM)-induced switching of a single cyclooctadiene molecule between two stable conformations chemisorbed on a Si(100) surface is investigated using an above threshold model including a neutral ground state and an ionic excited state potential. Switching was recently achieved experimentally with an STM operated at cryogenic temperatures (Nacci et al 2008 Phys. Rev. B 77 121405(R)) and rationalized by a below threshold model using just a single potential energy surface (Nacci et al 2009 Nano Lett. 9 2997). In the present paper, we show that experimental key findings on the inelastic electron tunnelling (IET) switching can also be rationalized using an above threshold density matrix model, which includes, in addition to the neutral ground state potential, an anionic or cationic excited potential. We use one and two-dimensional potential energy surfaces. Furthermore, the influence of two key parameters of the density matrix description, namely the electronic lifetime of the ionic resonance and the vibrational lifetimes, on the ground state potential are discussed. Y1 - 2012 U6 - https://doi.org/10.1088/0953-8984/24/39/394009 SN - 0953-8984 VL - 24 IS - 39 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Bronner, C. A1 - Leyssner, F. A1 - Stremlau, S. A1 - Utecht, Manuel Martin A1 - Saalfrank, Peter A1 - Klamroth, Tillmann A1 - Tegeder, P. T1 - Electronic structure of a subnanometer wide bottom-up fabricated graphene nanoribbon: End states, band gap, and dispersion JF - Physical review : B, Condensed matter and materials physics N2 - Angle-resolved two-photon photoemission and high-resolution electron energy loss spectroscopy are employed to derive the electronic structure of a subnanometer atomically precise quasi-one-dimensional graphene nanoribbon (GNR) on Au(111). We resolved occupied and unoccupied electronic bands including their dispersion and determined the band gap, which possesses an unexpectedly large value of 5.1 eV. Supported by density functional theory calculations for the idealized infinite polymer and finite size oligomers, an unoccupied nondispersive electronic state with an energetic position in the middle of the band gap of the GNR could be identified. This state resides at both ends of the ribbon (end state) and is only found in the finite sized systems, i.e., the oligomers. Y1 - 2012 U6 - https://doi.org/10.1103/PhysRevB.86.085444 SN - 1098-0121 VL - 86 IS - 8 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Herder, Martin A1 - Utecht, Manuel Martin A1 - Manicke, Nicole A1 - Grubert, Lutz A1 - Pätzel, Michael A1 - Saalfrank, Peter A1 - Hecht, Stefan T1 - Switching with orthogonal stimuli electrochemical ring-closure and photochemical ring-opening of bis(thiazolyl) maleimides JF - Chemical science N2 - The photochemistry as well as electrochemistry of novel donor-acceptor bis(morpholinothiazolyl)maleimides has been investigated. Proper substitution of these diarylethene-type molecular switches leads to the unique situation in which their ring-closure can only be accomplished electrochemically, while ring-opening can only be achieved photochemically. Hence, these switches operate with orthogonal stimuli, i.e. redox potential and light, respectively. The switch system could be optimized by introducing trifluoromethyl groups at the reactive carbon atoms in order to avoid by-product formation during oxidative ring closure. Both photochemical and electrochemical pathways were investigated for methylated, trifluoromethylated, and nonsymmetrical bis(morpholinothiazolyl) maleimides as well as the bis(morpholinothiazolyl) cyclopentene reference compound. With the aid of the nonsymmetrical "mixed" derivative, the mechanism of electrochemically driven ring closure could be elucidated and seems to proceed via a dicationic intermediate generated by two-fold oxidation. All experimental work has been complemented by density functional theory that provides detailed insights into the thermodynamics of the ring-open and closed forms, the nature of their excited states, and the reactivity of their neutral as well as ionized species in different electronic configurations. The particular diarylethene systems described herein could serve in multifunctional (logic) devices operated by different stimuli (inputs) and may pave the way to converting light into electrical energy via photoinduced "pumping" of redox-active meta-stable states. Y1 - 2013 U6 - https://doi.org/10.1039/c2sc21681g SN - 2041-6520 VL - 4 IS - 3 SP - 1028 EP - 1040 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Nest, Mathias A1 - Ludwig, M. A1 - Ulusoy, I. A1 - Klamroth, Tillmann A1 - Saalfrank, Peter T1 - Electron correlation dynamics in atoms and molecules JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - In this paper, we present quantum dynamical calculations on electron correlation dynamics in atoms and molecules using explicitly time-dependent ab initio configuration interaction theory. The goals are (i) to show that in which cases it is possible to switch off the electronic correlation by ultrashort laser pulses, and (ii) to understand the temporal evolution and the time scale on which it reappears. We characterize the appearance of correlation through electron-electron scattering when starting from an uncorrelated state, and we identify pathways for the preparation of a Hartree-Fock state from the correlated, true ground state. Exemplary results for noble gases, alkaline earth elements, and selected molecules are provided. For Mg we show that the uncorrelated state can be prepared using a shaped ultrashort laser pulse. Y1 - 2013 U6 - https://doi.org/10.1063/1.4801867 SN - 0021-9606 SN - 1089-7690 VL - 138 IS - 16 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Füchsel, Gernot A1 - Schimka, Selina A1 - Saalfrank, Peter T1 - On the role of electronic friction for dissociative adsorption and scattering of hydrogen molecules at a Ru(0001) surface JF - The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment & general theory N2 - The role of electronic friction and, more generally, of nonadiabatic effects during dynamical processes at the gas/metal surface interface is still a matter of discussion. In particular, it is not clear if electronic nonadiabaticity has an effect under "mild" conditions, when molecules in low rovibrational states interact with a metal surface. In this paper, we investigate the role of electronic friction on the dissociative sticking and (inelastic) scattering of vibrationally and rotationally cold H-2 molecules at a Ru(0001) surface theoretically. For this purpose, classical molecular dynamics with electronic friction (MDEF) calculations are performed and compared to MD simulations without friction. The two H atoms move on a six-dimensional potential energy surface generated from gradient-corrected density functional theory (DFT), that is, all molecular degrees of freedom are accounted for. Electronic friction is included via atomic friction coefficients obtained from an embedded atom, free electron gas (FEG) model, with embedding densities taken from gradient-corrected DFT. We find that within this model, dissociative sticking probabilities as a function of impact kinetic energies and impact angles are hardly affected by nonadiabatic effects. If one accounts for a possibly enhanced electronic friction near the dissociation barrier, on the other hand, reduced sticking probabilities are observed, in particular, at high impact energies. Further, there is always an influence on inelastic scattering, in particular, as far as the translational and internal energy distribution of the reflected molecules is concerned. Additionally, our results shed light on the role played by the velocity distribution of the incident molecular beam for adsorption probabilities, where, in particular, at higher impact energies, large effects are found. Y1 - 2013 U6 - https://doi.org/10.1021/jp403860p SN - 1089-5639 VL - 117 IS - 36 SP - 8761 EP - 8769 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Füchsel, Gernot A1 - Tremblay, Jean Christophe A1 - Klamroth, Tillmann A1 - Saalfrank, Peter T1 - Quantum dynamical simulations of the femtosecond-laser-induced ultrafast desorption of H2 and D2 from Ru(0001) JF - ChemPhysChem : a European journal of chemical physics and physical chemistry N2 - We investigate the recombinative desorption of hydrogen and deuterium from a Ru(0001) surface initiated by femtosecond laser pulses. We adopt a quantum mechanical two-state model including three molecular degrees of freedom to describe the dynamics within the desorption induced by electronic transition (DIET) limit. The energy distributions as well as the state-resolved and ensemble properties of the desorbed molecules are analyzed in detail by using the time-energy method. Our results shed light on the experimentally observed 1) large isotopic effects regarding desorption yields and translational energies and 2) the nonequal energy partitioning into internal and translational modes. In particular, it is shown that a single temperature is sufficient to characterize the energy distributions for all degrees of freedom. Further, we confirm that quantization effects play an important role in the determination of the energy partitioning. KW - quantum dynamics KW - laser chemistry KW - isotope effects KW - surface chemistry KW - ultrafast reactions Y1 - 2013 U6 - https://doi.org/10.1002/cphc.201200940 SN - 1439-4235 VL - 14 IS - 7 SP - 1471 EP - 1478 PB - Wiley-VCH CY - Weinheim ER - TY - GEN A1 - Banerjee, Shiladitya A1 - Saalfrank, Peter T1 - Vibrationally resolved absorption, emission and resonance Raman spectra of diamondoids BT - a study based on time-dependent correlation functions N2 - The time-dependent approach to electronic spectroscopy, as popularized by Heller and coworkers in the 1980's, is applied here in conjunction with linear-response, time-dependent density functional theory to study vibronic absorption, emission and resonance Raman spectra of several diamondoids. Two-state models, the harmonic and the Condon approximations, are used for the calculations, making them easily applicable to larger molecules. The method is applied to nine pristine lower and higher diamondoids: adamantane, diamantane, triamantane, and three isomers each of tetramantane and pentamantane. We also consider a hybrid species “Dia = Dia” – a shorthand notation for a recently synthesized molecule comprising two diamantane units connected by a C[double bond, length as m-dash]C double bond. We resolve and interpret trends in optical and vibrational properties of these molecules as a function of their size, shape, and symmetry, as well as effects of “blending” with sp2-hybridized C-atoms. Time-dependent correlation functions facilitate the computations and shed light on the vibrational dynamics following electronic transitions. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 238 KW - adamantane KW - models KW - molecules KW - states KW - thermochemistry Y1 - 2013 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-94542 SP - 144 EP - 158 ER - TY - JOUR A1 - Banerjee, Shiladitya A1 - Saalfrank, Peter T1 - Vibrationally resolved absorption, emission and resonance Raman spectra of diamondoids: a study based on time-dependent correlation functions JF - Physical chemistry, chemical physics : a journal of European Chemical Societies Y1 - 2014 U6 - https://doi.org/10.1039/c3cp53535e SN - 1463-9076 SN - 1463-9084 VL - 16 IS - 1 SP - 144 EP - 158 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Ehlert, Christopher A1 - Unger, Wolfgang E. S. A1 - Saalfrank, Peter T1 - C K-edge NEXAFS spectra of graphene with physical and chemical defects: a study based on density functional theory JF - Physical chemistry, chemical physics : a journal of European Chemical Societies N2 - Recently, C K-edge Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra of graphite (HOPG) surfaces have been measured for the pristine material, and for HOPG treated with either bromine or krypton plasmas (Lippitz et al., Surf. Sci., 2013, 611, L1). Changes of the NEXAFS spectra characteristic for physical (krypton) and/or chemical/physical modifications of the surface (bromine) upon plasma treatment were observed. Their molecular origin, however, remained elusive. In this work we study by density functional theory, the effects of selected point and line defects as well as chemical modifications on NEXAFS carbon K-edge spectra of single graphene layers. For Br-treated surfaces, also Br 3d X-ray Photoelectron Spectra (XPS) are simulated by a cluster approach, to identify possible chemical modifications. We observe that some of the defects related to plasma treatment lead to characteristic changes of NEXAFS spectra, similar to those in experiment. Theory provides possible microscopic origins for these changes. Y1 - 2014 U6 - https://doi.org/10.1039/c4cp01106f SN - 1463-9076 SN - 1463-9084 VL - 16 IS - 27 SP - 14083 EP - 14095 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Wirth, Jonas A1 - Neumann, Rainer A1 - Antonietti, Markus A1 - Saalfrank, Peter T1 - Adsorption and photocatalytic splitting of water on graphitic carbon nitride: a combined first principles and semiempirical study JF - Physical chemistry, chemical physics : a journal of European Chemical Societies N2 - Graphitic carbon nitride, g-C3N4, is a promising organic photo-catalyst for a variety of redox reactions. In order to improve its efficiency in a systematic manner, however, a fundamental understanding of the microscopic interaction between catalyst, reactants and products is crucial. Here we present a systematic study of water adsorption on g-C3N4 by means of density functional theory and the density functional based tight-binding method as a prerequisite for understanding photocatalytic water splitting. We then analyze this prototypical redox reaction on the basis of a thermodynamic model providing an estimate of the overpotential for both water oxidation and H+ reduction. While the latter is found to occur readily upon irradiation with visible light, we derive a prohibitive overpotential of 1.56 eV for the water oxidation half reaction, comparing well with the experimental finding that in contrast to H-2 production O-2 evolution is only possible in the presence of oxidation cocatalysts. Y1 - 2014 U6 - https://doi.org/10.1039/c4cp02021a SN - 1463-9076 SN - 1463-9084 VL - 16 IS - 30 SP - 15917 EP - 15926 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Knie, Christopher A1 - Utecht, Manuel Martin A1 - Zhao, Fangli A1 - Kulla, Hannes A1 - Kovalenko, Sergey A1 - Brouwer, Albert M. A1 - Saalfrank, Peter A1 - Hecht, Stefan A1 - Bleger, David T1 - ortho-Fluoroazobenzenes: visible light switches with very long-lived Z isomers JF - Chemistry - a European journal N2 - Improving the photochemical properties of molecular photoswitches is crucial for the development of light-responsive systems in materials and life sciences. ortho-Fluoroazobenzenes are a new class of rationally designed photochromic azo compounds with optimized properties, such as the ability to isomerize with visible light only, high photoconversions, and unprecedented robust bistable character. Introducing sigma-electron-withdrawing F atoms ortho to the N=N unit leads to both an effective separation of the n -> pi* bands of the E and Z isomers, thus offering the possibility of using these two transitions for selectively inducing E/Z iso-merizations, and greatly enhanced thermal stability of the Z isomers. Additional para-electron-withdrawing groups (EWGs) work in concert with ortho-F atoms, giving rise to enhanced separation of the n -> pi* transitions. A comprehensive study of the effect of substitution on the key photochemical properties of ortho-fluoroazobenzenes is reported herein. In particular, the position, number, and nature of the EWGs have been varied, and the visible light photoconversions, quantum yields of isomerization, and thermal stabilities have been measured and rationalized by DFT calculations. KW - azobenzenes KW - photochromism KW - photoswitches KW - substituent effects KW - visible light Y1 - 2014 U6 - https://doi.org/10.1002/chem.201404649 SN - 0947-6539 SN - 1521-3765 VL - 20 IS - 50 SP - 16492 EP - 16501 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Füchsel, Gernot A1 - Tremblay, Jean Christophe A1 - Saalfrank, Peter T1 - A six-dimensional potential energy surface for Ru(0001)(2x2):CO JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - We present a new global ground state potential energy surface (PES) for carbon monoxide at a coverage of 1/4, on a rigid Ru(0001) surface [Ru(0001)(2x2):CO]. All six adsorbate degrees of freedom are considered. For constructing the PES, we make use of more than 90 000 points calculated with periodic density functional theory using the RPBE exchange-correlation functional and an empirical van der Waals correction. These points are used for interpolation, utilizing a symmetry-adapted corrugation reducing procedure (CRP). Three different interpolation schemes with increasing accuracy have been realized, giving rise to three flavours of the CRP PES. The CRP PES yields in agreement with the DFT reference and experiments, the atop position of CO to be the most stable adsorption geometry, for the most accurate interpolation with an adsorption energy of 1.69 eV. The CRP PES shows that diffusion parallel to the surface is hindered by a barrier of 430 meV, and that dissociation is facilitated but still activated. As a first "real" application and further test of the new potential, the six-dimensional vibrational Schrodinger equation is solved variationally to arrive at fully coupled, anharmonic frequencies and vibrational wavefunctions for the vibrating, adsorbed CO molecule. Good agreement with experiment is found also here. Being analytical, the new PES opens an efficient way towards multidimensional dynamics. (C) 2014 AIP Publishing LLC. Y1 - 2014 U6 - https://doi.org/10.1063/1.4894083 SN - 0021-9606 SN - 1089-7690 VL - 141 IS - 9 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Saalfrank, Peter A1 - Juaristi, J. I. A1 - Alducin, Maite A1 - Blanco-Rey, Maria A1 - Muino, R. Diez T1 - Vibrational lifetimes of hydrogen on lead films : an ab initio molecular dynamics with electronic friction (AIMDEF) study JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - Using density functional theory and Ab Initio Molecular Dynamics with Electronic Friction (AIMDEF), we study the adsorption and dissipative vibrational dynamics of hydrogen atoms chemisorbed on free-standing lead films of increasing thickness. Lead films are known for their oscillatory behaviour of certain properties with increasing thickness, e.g., energy and electron spill-out change in discontinuous manner, due to quantum size effects [G. Materzanini, P. Saalfrank, and P.J.D. Lindan, Phys. Rev. B 63, 235405 (2001)]. Here, we demonstrate that oscillatory features arise also for hydrogen when chemisorbed on lead films. Besides stationary properties of the adsorbate, we concentrate on finite vibrational lifetimes of H-surface vibrations. As shown by AIMDEF, the damping via vibration-electron hole pair coupling dominates clearly over the vibration-phonon channel, in particular for high-frequency modes. Vibrational relaxation times are a characteristic function of layer thickness due to the oscillating behaviour of the embedding surface electronic density. Implications derived from AIMDEF for frictional many-atom dynamics, and physisorbed species will also be given. (C) 2014 AIP Publishing LLC. Y1 - 2014 U6 - https://doi.org/10.1063/1.4903309 SN - 0021-9606 SN - 1089-7690 VL - 141 IS - 23 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Banerjee, Shiladitya A1 - Saalfrank, Peter T1 - Vibrationally resolved absorption, emission and resonance Raman spectra of diamondoids : a study based on time- dependent correlation functions Y1 - 2014 UR - http://pubs.rsc.org/en/content/articlehtml/2014/cp/c3cp53535e U6 - https://doi.org/10.1039/C3CP53535E ER - TY - GEN A1 - Ehlert, Christopher A1 - Unger, Wolfgang E. S. A1 - Saalfrank, Peter T1 - C K-edge NEXAFS spectra of graphene with physical and chemical defects BT - a study based on density functional theory N2 - Recently, C K-edge Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra of graphite (HOPG) surfaces have been measured for the pristine material, and for HOPG treated with either bromine or krypton plasmas (Lippitz et al., Surf. Sci., 2013, 611, L1). Changes of the NEXAFS spectra characteristic for physical (krypton) and/or chemical/physical modifications of the surface (bromine) upon plasma treatment were observed. Their molecular origin, however, remained elusive. In this work we study by density functional theory, the effects of selected point and line defects as well as chemical modifications on NEXAFS carbon K-edge spectra of single graphene layers. For Br-treated surfaces, also Br 3d X-ray Photoelectron Spectra (XPS) are simulated by a cluster approach, to identify possible chemical modifications. We observe that some of the defects related to plasma treatment lead to characteristic changes of NEXAFS spectra, similar to those in experiment. Theory provides possible microscopic origins for these changes. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - paper 173 KW - absorbtion fine-structure KW - graphite Y1 - 2014 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-74429 SP - 14083 EP - 14095 ER - TY - GEN A1 - Wirth, Jonas A1 - Neumann, Rainer A1 - Antonietti, Markus A1 - Saalfrank, Peter T1 - Adsorption and photocatalytic splitting of water on graphitic carbon nitride BT - a combined first principles and semiempirical study N2 - Graphitic carbon nitride, g-C₃N₄, is a promising organic photo-catalyst for a variety of redox reactions. In order to improve its efficiency in a systematic manner, however, a fundamental understanding of the microscopic interaction between catalyst, reactants and products is crucial. Here we present a systematic study of water adsorption on g-C₃N₄ by means of density functional theory and the density functional based tight-binding method as a prerequisite for understanding photocatalytic water splitting. We then analyze this prototypical redox reaction on the basis of a thermodynamic model providing an estimate of the overpotential for both water oxidation and H⁺ reduction. While the latter is found to occur readily upon irradiation with visible light, we derive a prohibitive overpotential of 1.56 eV for the water oxidation half reaction, comparing well with the experimental finding that in contrast to H₂ production O₂ evolution is only possible in the presence of oxidation cocatalysts. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - paper 172 KW - augmented-wave method KW - hydrogen KW - initio molecular-dynamics KW - oxidation KW - photooxidation KW - reduction KW - simulations KW - tight-binding KW - transition KW - visible-light Y1 - 2014 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-74391 SP - 15917 EP - 15926 ER - TY - JOUR A1 - Wirth, Jonas A1 - Neumann, Rainer A1 - Antonietti, Markus A1 - Saalfrank, Peter T1 - Adsorption and photocatalytic splitting of water on graphitic carbon nitride BT - a combined first principles and semiempirical study JF - physical chemistry, chemical physics : PCCP N2 - Graphitic carbon nitride, g-C₃N₄, is a promising organic photo-catalyst for a variety of redox reactions. In order to improve its efficiency in a systematic manner, however, a fundamental understanding of the microscopic interaction between catalyst, reactants and products is crucial. Here we present a systematic study of water adsorption on g-C₃N₄ by means of density functional theory and the density functional based tight-binding method as a prerequisite for understanding photocatalytic water splitting. We then analyze this prototypical redox reaction on the basis of a thermodynamic model providing an estimate of the overpotential for both water oxidation and H⁺ reduction. While the latter is found to occur readily upon irradiation with visible light, we derive a prohibitive overpotential of 1.56 eV for the water oxidation half reaction, comparing well with the experimental finding that in contrast to H₂ production O₂ evolution is only possible in the presence of oxidation cocatalysts. KW - initio molecular-dynamics KW - augmented-wave method KW - visible-light KW - tight-binding KW - transition KW - oxidation KW - photooxidation KW - simulations KW - reduction KW - hydrogen Y1 - 2014 U6 - https://doi.org/10.1039/c4cp02021a SN - 1463-9076 SN - 1463-9084 VL - 2014 IS - 16 SP - 15917 EP - 15926 ER - TY - JOUR A1 - Ehlert, Christopher A1 - Unger, Wolfgang E. S. A1 - Saalfrank, Peter T1 - C K-edge NEXAFS spectra of graphene with physical and chemical defects BT - a study based on density functional theory JF - physical chemistry, chemical physics : PCCP N2 - Recently, C K-edge Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra of graphite (HOPG) surfaces have been measured for the pristine material, and for HOPG treated with either bromine or krypton plasmas (Lippitz et al., Surf. Sci., 2013, 611, L1). Changes of the NEXAFS spectra characteristic for physical (krypton) and/or chemical/physical modifications of the surface (bromine) upon plasma treatment were observed. Their molecular origin, however, remained elusive. In this work we study by density functional theory, the effects of selected point and line defects as well as chemical modifications on NEXAFS carbon K-edge spectra of single graphene layers. For Br-treated surfaces, also Br 3d X-ray Photoelectron Spectra (XPS) are simulated by a cluster approach, to identify possible chemical modifications. We observe that some of the defects related to plasma treatment lead to characteristic changes of NEXAFS spectra, similar to those in experiment. Theory provides possible microscopic origins for these changes. KW - absorbtion fine-structure KW - graphite Y1 - 2014 U6 - https://doi.org/10.1039/c4cp01106f SN - 1463-9076 SN - 1463-9084 VL - 2014 IS - 16 SP - 14083 EP - 14095 ER - TY - JOUR A1 - Kirsch, Harald A1 - Wirth, Jonas A1 - Tong, Yujin A1 - Wolf, Martin A1 - Saalfrank, Peter A1 - Campen, Richard Kramer T1 - Experimental characterization of unimolecular water dissociative adsorption on alpha-alumina JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - alpha-Al2O3 surfaces are common in both engineered applications and the environment. Much prior work indicates that their properties, e.g., reactivity, polarity, and charge, change dramatically on interaction with water. Perhaps the simplest question that can be asked of alpha-Al2O3/water interaction is how a single water molecule interacts with the most stable alpha-Al2O3 surface: the alpha-Al2O3(0001). Over the last 15 years, a series of theoretical studies have found that water dissociatively adsorbs on alpha-Al2O3(0001) through two channels. However, to our knowledge no experimental evidence of these dissociation pathways has appeared. By combining sample preparation via supersonic molecular beam dosing, sample characterization via coherent, surface specific vibrational spectroscopy and electronic structure theory, we report the first experimental observation of reaction products of each, theoretically predicted, dissociation channel. These results thus overcome a 15 year old experiment/theory disconnect and make possible a variety of intriguing experiments that promise to provide significant new insights into water/Al2O3 and water/oxide interaction more generally. Y1 - 2014 U6 - https://doi.org/10.1021/jp502106t SN - 1932-7447 VL - 118 IS - 25 SP - 13623 EP - 13630 PB - American Chemical Society CY - Washington ER - TY - GEN A1 - Schulze, Michael A1 - Utecht, Manuel Martin A1 - Moldt, Thomas A1 - Przyrembel, Daniel A1 - Gahl, Cornelius A1 - Weinelt, Martin A1 - Saalfrank, Peter A1 - Tegeder, Petra T1 - Nonlinear optical response of photochromic azobenzene-functionalized self-assembled monolayers N2 - The combination of photochromic and nonlinear optical (NLO) properties of azobenzene-functionalized self-assembled monolayers (SAMs) constitutes an intriguing step towards novel photonic and optoelectronic devices. By utilizing the second-order NLO process of second harmonic generation (SHG), supported by density-functional theory and correlated wave function method calculations, we demonstrate that the photochromic interface provides the necessary prerequisites en route towards possible future technical applications: we find a high NLO contrast on the order of 16% between the switching states. These are furthermore accessible reversibly and with high efficiencies in terms of cross sections on the order of 10−18 cm2 for both photoisomerization reactions, i.e., drivable by means of low-power LED light sources. Finally, both photostationary states (PSSs) are thermally stable at ambient conditions. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 196 Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-81198 ER - TY - JOUR A1 - Ehlert, Christopher A1 - Kröner, Dominik A1 - Saalfrank, Peter T1 - A combined quantum chemical/molecular dynamics study of X-ray photoelectron spectra of polyvinyl alcohol using oligomer models JF - Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy N2 - X-ray photoelectron spectroscopy (XPS) is a powerful tool for probing the local chemical environment of atoms near surfaces. When applied to soft matter, such as polymers, XPS spectra are frequently shifted and broadened due to thermal atom motion and by interchain interactions. We present a combined quantum mechanical QM/molecular dynamics (MD) simulation of X-ray photoelectron spectra of polyvinyl alcohol (PVA) using oligomer models in order to account for and quantify these effects on the XPS (C1s) signal. In our study, molecular dynamics at finite temperature were performed with a classical forcefield and by ab initio MD (AIMD) using the Car-Parrinello method. Snapshots along, the trajectories represent possible conformers and/or neighbouring environments, with different C1s ionization potentials for individual C atoms leading to broadened XPS peaks. The latter are determined by Delta-Kohn Sham calculations. We also examine the experimental practice of gauging XPS (C1s) signals of alkylic C-atoms in C-containing polymers to the C1s signal of polyethylene. We find that (i) the experimental XPS (C1s) spectra of PVA (position and width) can be roughly represented by single-strand models, (ii) interchain interactions lead to red-shifts of the XPS peaks by about 0.6 eV, and (iii) AIMD simulations match the findings from classical MD semi-quantitatively. Further, (iv) the gauging procedure of XPS (C1s) signals to the values of PE, introduces errors of about 0.5 eV. (C) 2014 Elsevier B.V. All rights reserved. KW - Simulation of polymer XPS KW - Delta-Kohn Sham method KW - Thermal broadening effects KW - Interchain interactions KW - Classical MD KW - Poly vinyl alcohol Y1 - 2015 U6 - https://doi.org/10.1016/j.elspec.2014.12.007 SN - 0368-2048 SN - 1873-2526 VL - 199 SP - 38 EP - 45 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Banerjee, Shiladitya A1 - Stueker, Tony A1 - Saalfrank, Peter T1 - Vibrationally resolved optical spectra of modified diamondoids obtained from time-dependent correlation function methods JF - Physical chemistry, chemical physics : a journal of European Chemical Societies N2 - Optical properties of modified diamondoids have been studied theoretically using vibrationally resolved electronic absorption, emission and resonance Raman spectra. A time-dependent correlation function approach has been used for electronic two-state models, comprising a ground state (g) and a bright, excited state (e), the latter determined from linear-response, time-dependent density functional theory (TD-DFT). The harmonic and Condon approximations were adopted. In most cases origin shifts, frequency alteration and Duschinsky rotation in excited states were considered. For other cases where no excited state geometry optimization and normal mode analysis were possible or desired, a short-time approximation was used. The optical properties and spectra have been computed for (i) a set of recently synthesized sp(2)/sp(3) hybrid species with CQC double-bond connected saturated diamondoid subunits, (ii) functionalized (mostly by thiol or thione groups) diamondoids and (iii) urotropine and other C-substituted diamondoids. The ultimate goal is to tailor optical and electronic features of diamondoids by electronic blending, functionalization and substitution, based on a molecular-level understanding of the ongoing photophysics. Y1 - 2015 U6 - https://doi.org/10.1039/c5cp02615f SN - 1463-9076 SN - 1463-9084 VL - 17 IS - 29 SP - 19656 EP - 19669 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Schulze, Michael A1 - Utecht, Manuel Martin A1 - Moldt, Thomas A1 - Przyrembel, Daniel A1 - Gahl, Cornelius A1 - Weinelt, Martin A1 - Saalfrank, Peter A1 - Tegeder, Petra T1 - Nonlinear optical response of photochromic azobenzene-functionalized self-assembled monolayers JF - Physical chemistry, chemical physics : a journal of European Chemical Societies N2 - The combination of photochromic and nonlinear optical (NLO) properties of azobenzene-functionalized self-assembled monolayers (SAMs) constitutes an intriguing step towards novel photonic and optoelectronic devices. By utilizing the second-order NLO process of second harmonic generation (SHG), supported by density-functional theory and correlated wave function method calculations, we demonstrate that the photochromic interface provides the necessary prerequisites en route towards possible future technical applications: we find a high NLO contrast on the order of 16% between the switching states. These are furthermore accessible reversibly and with high efficiencies in terms of cross sections on the order of 10(-18) cm(2) for both photoisomerization reactions, i.e., drivable by means of low-power LED light sources. Finally, both photostationary states (PSSs) are thermally stable at ambient conditions. Y1 - 2015 U6 - https://doi.org/10.1039/c5cp03093e SN - 1463-9076 SN - 1463-9084 VL - 17 IS - 27 SP - 18079 EP - 18086 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Lorenz, Ulf A1 - Saalfrank, Peter T1 - Measures for the non-Markovianity of a harmonic oscillator coupled to a discrete bath derived from numerically exact references JF - The European physical journal : D, Atomic, molecular, optical and plasma physics N2 - System-bath problems in physics and chemistry are often described by Markovian master equations. However, the Markov approximation, i.e., neglect of bath memory effects is not always justified, and different measures of non-Markovianity have been suggested in the literature to judge the validity of this approximation. Here we calculate several computable measures of non-Markovianity for the non-trivial problem of a harmonic oscillator coupled to a large number of bath oscillators. The Multi Configurational Time Dependent Hart ree nietliod is used to provide a numerically converged solution of the system-bath Schrodinger equation, from which the appropriate quantities can be calculated. In particular, we consider measures based on trace-distances and quantum discord for a variety of initial states. These quantities have proven useful in the case of two-level and other small model systems Tpically encountered in quantum optics; but are less straightforward to interpret for the more complex model systems that are relevant for chemical physics. Y1 - 2015 U6 - https://doi.org/10.1140/epjd/e2014-50727-8 SN - 1434-6060 SN - 1434-6079 VL - 69 IS - 2 PB - Springer CY - New York ER - TY - JOUR A1 - Matis, Jochen Rene A1 - Schoenborn, Jan Boyke A1 - Saalfrank, Peter T1 - A multi-reference study of the byproduct formation for a ring-closed dithienylethene photoswitch JF - Physical chemistry, chemical physics : a journal of European Chemical Societies N2 - Photodriven molecular switches are sometimes hindered in their performance by forming byproducts which act as dead ends in sequences of switching cycles, leading to rapid fatigue effects. Understanding the reaction pathways to unwanted byproducts is a prerequisite for preventing them. This article presents a study of the photochemical reaction pathways for byproduct formation in the photochromic switch 1,2-bis-(3-thienyl)-ethene. Specifically, using single-and multi-reference methods the post-deexcitation reaction towards the byproduct in the electronic ground state S-0 when starting from the S-1-S-0 conical intersection (CoIn), is considered in detail. We find an unusual low-energy pathway, which offers the possibility for the formation of a dyotropic byproduct. Several high-energy pathways can be excluded with high probability. Y1 - 2015 U6 - https://doi.org/10.1039/c5cp00987a SN - 1463-9076 SN - 1463-9084 VL - 17 IS - 21 SP - 14088 EP - 14095 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Schulze, Michael A1 - Utecht, Manuel Martin A1 - Hebert, Andreas A1 - Rück-Braun, Karola A1 - Saalfrank, Peter A1 - Tegeder, Petra T1 - Reversible Photoswitching of the Interfacial Nonlinear Optical Response JF - The journal of physical chemistry letters N2 - Incorporating photochromic molecules into organic/inorganic hybrid materials may lead to photoresponsive systems. In such systems, the second-order nonlinear properties can be controlled via external stimulation with light at an appropriate wavelength. By creating photochromic molecular switches containing self-assembled monolayers on Si(111), we can demonstrate efficient reversible switching, which is accompanied by a pronounced modulation of the nonlinear optical (NLO) response of the system. The concept of utilizing functionalized photoswitchable Si surfaces could be a way for the generation of two-dimensional NLO switching materials, which are promising for applications in photonic and optoelectronic devices. Y1 - 2015 U6 - https://doi.org/10.1021/jz502477m SN - 1948-7185 VL - 6 IS - 3 SP - 505 EP - 509 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Floss, Gereon A1 - Saalfrank, Peter T1 - The Photoinduced E -> Z Isomerization of Bisazobenzenes: A Surface Hopping Molecular Dynamics Study JF - The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment & general theory N2 - The photoinduced E -> Z isomerization of azobenzene is a prototypical example of molecular switching. On the way toward rigid molecular rods such as those for opto-mechanical applications, multiazobenzene structures have been suggested in which several switching units are linked together within the same molecule (Bleger et al., J. Phys. Chem. B 2011, 115, 9930-9940). Large differences in the switching efficiency of multiazobenzenes have been observed, depending on whether the switching units are electronically decoupled or not. In this paper we study, on a time-resolved molecular level, the E -> Z isomerization of the simplest multiazobenzene, bisazobenzene (BAB). Two isomers (ortho- and para-BAB), differing only in the connectivity of two azo groups on a shared phenyl ring will be considered.To do so, nonadiabatic semiclassical dynamics after photo-excitation of the isomers are studied by employing an "on-the-fly", fewest switches surface hopping approach. States and couplings are calculated by Configuration Interaction (CI) based on a semiempirical (AM1) Hamiltonian (Persico and co-workers, Chem. Eur. J. 2004, 10, 2327-2341). In the case of para-BAB, computed quantum yields for photoswitching are drastically reduced compared to pristine azobenzene, due to electronic coupling of both switching units. A reason for this (apart from altered absorption spectra and reduced photochromicity) is the drastically reduced lifetimes of electronically excited states which are transiently populated. In contrast for meta-connected species, electronic subsystems are largely decoupled, and computed quantum yields are slightly higher than that for pristine azobenzene because of new isomerization channels. In this case we can also distinguish between single- and double-switch events and we find a cooperative effect: The isomerization of a single azo group is facilitated if the other azo group is already in the Z-configuration. Y1 - 2015 U6 - https://doi.org/10.1021/acs.jpca.5b02933 SN - 1089-5639 VL - 119 IS - 20 SP - 5026 EP - 5037 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Martinez-Mesa, Aliezer A1 - Saalfrank, Peter T1 - Semiclassical modelling of finite-pulse effects on non-adiabatic photodynamics via initial condition filtering: The predissociation of NaI as a test case JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - Femtosecond-laser pulse driven non-adiabatic spectroscopy and dynamics in molecular and condensed phase systems continue to be a challenge for theoretical modelling. One of the main obstacles is the "curse of dimensionality" encountered in non-adiabatic, exact wavepacket propagation. A possible route towards treating complex molecular systems is via semiclassical surface-hopping schemes, in particular if they account not only for non-adiabatic post-excitation dynamics but also for the initial optical excitation. One such approach, based on initial condition filtering, will be put forward in what follows. As a simple test case which can be compared with exact wavepacket dynamics, we investigate the influence of the different parameters determining the shape of a laser pulse (e.g., its finite width and a possible chirp) on the predissociation dynamics of a NaI molecule, upon photoexcitation of the A(0(+)) state. The finite-pulse effects are mapped into the initial conditions for semiclassical surface-hopping simulations. The simulated surface-hopping diabatic populations are in qualitative agreement with the quantum mechanical results, especially concerning the subpicosend photoinduced dynamics, the main deviations being the relative delay of the non-adiabatic transitions in the semiclassical picture. Likewise, these differences in the time-dependent electronic populations calculated via the semiclassical and the quantum methods are found to have a mild influence on the overall probability density distribution. As a result, the branching ratios between the bound and the dissociative reaction channels and the time-evolution of the molecular wavepacket predicted by the semiclassical method agree with those computed using quantum wavepacket propagation. Implications for more challenging molecular systems are given. (C) 2015 AIP Publishing LLC. Y1 - 2015 U6 - https://doi.org/10.1063/1.4919780 SN - 0021-9606 SN - 1089-7690 VL - 142 IS - 19 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Wirth, Jonas A1 - Hatter, Nino A1 - Drost, Robert A1 - Umbach, Tobias R. A1 - Barja, Sara A1 - Zastrow, Matthias A1 - Rück-Braun, Karola A1 - Pascual, Jose Ignacio A1 - Saalfrank, Peter A1 - Franke, Katharina J. T1 - Diarylethene Molecules on a Ag(111) Surface: Stability and Electron-Induced Switching JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - Diarylethene derivatives are photochromic molecular switches, undergoing a ring-opening/-closing reaction by illumination with light. The symmetry of the closed form is determined by the WoodWard Hoffinann rules according to which the reaction proceeds by corirotatory rotation -in that case. Here, we show by a cOrnbined approach of scanning tunneling microscopy (STM) and density functional theory (DFT) calculations that the Open isomer of 4,4'-(4,4'-(perfluorocydopent-1-ene-1,2-diyl)bis(5-methyl-thiophent-4,2,4-dipyridine) (PDTE) retains its open form upon adsorption on a Ag(111) surface. It caribe switched into a closed form, which we identify as the digrotatOly cydization product, by controlled manipulation 'With the STM tip, Evidence of an electric-field dependent switching-process 'is interpreted on the basis of a Simple electroStatic Model, which suggests that the reaction proceedS via an "upright" intermediate state. This pathway thus strongly differs from the switching reaction in solution. Y1 - 2015 U6 - https://doi.org/10.1021/jp5122036 SN - 1932-7447 VL - 119 IS - 9 SP - 4874 EP - 4883 PB - American Chemical Society CY - Washington ER - TY - GEN A1 - Lorenz, Ulf A1 - Saalfrank, Peter T1 - Comparing thermal wave function methods for multi-configuration time-dependent Hartree simulations (vol 140, 044106, 2014) T2 - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr Y1 - 2015 U6 - https://doi.org/10.1063/1.4938051 SN - 0021-9606 SN - 1089-7690 VL - 143 IS - 22 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Schulze, Michael A1 - Utecht, Manuel Martin A1 - Moldt, Thomas A1 - Przyrembel, Daniel A1 - Gahl, Cornelius A1 - Weinelt, Martin A1 - Saalfrank, Peter A1 - Tegeder, Petra T1 - Nonlinear optical response of photochromic azobenzene-functionalized self-assembled monolayers JF - Physical chemistry, chemical physics : PCCP ; a journal of European Chemical Societies N2 - The combination of photochromic and nonlinear optical (NLO) properties of azobenzene-functionalized self-assembled monolayers (SAMs) constitutes an intriguing step towards novel photonic and optoelectronic devices. By utilizing the second-order NLO process of second harmonic generation (SHG), supported by density-functional theory and correlated wave function method calculations, we demonstrate that the photochromic interface provides the necessary prerequisites en route towards possible future technical applications: we find a high NLO contrast on the order of 16% between the switching states. These are furthermore accessible reversibly and with high efficiencies in terms of cross sections on the order of 10−18 cm2 for both photoisomerization reactions, i.e., drivable by means of low-power LED light sources. Finally, both photostationary states (PSSs) are thermally stable at ambient conditions. Y1 - 2015 U6 - https://doi.org/10.1039/c5cp03093e SN - 1463-9076 SN - 1463-9084 VL - 27 IS - 17 SP - 18079 EP - 18086 PB - Royal Society of Chemistry CY - Cambridge ER - TY - GEN A1 - Matis, Jochen René A1 - Schönborn, Jan Boyke A1 - Saalfrank, Peter T1 - A multi-reference study of the byproduct formation for a ring-closed dithienylethene photoswitch N2 - Photodriven molecular switches are sometimes hindered in their performance by forming byproducts which act as dead ends in sequences of switching cycles, leading to rapid fatigue effects. Understanding the reaction pathways to unwanted byproducts is a prerequisite for preventing them. This article presents a study of the photochemical reaction pathways for byproduct formation in the photochromic switch 1,2-bis-(3-thienyl)-ethene. Specifically, using single- and multi-reference methods the post-deexcitation reaction towards the byproduct in the electronic ground state S0 when starting from the S1–S0 conical intersection (CoIn), is considered in detail. We find an unusual low-energy pathway, which offers the possibility for the formation of a dyotropic byproduct. Several high-energy pathways can be excluded with high probability. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 215 Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-89594 SP - 14088 EP - 14095 ER - TY - JOUR A1 - Matis, Jochen René A1 - Schönborn, Jan Boyke A1 - Saalfrank, Peter T1 - A multi-reference study of the byproduct formation for a ring-closed dithienylethene photoswitch JF - Physical chemistry, chemical physics : PCCP ; a journal of European Chemical Societies N2 - Photodriven molecular switches are sometimes hindered in their performance by forming byproducts which act as dead ends in sequences of switching cycles, leading to rapid fatigue effects. Understanding the reaction pathways to unwanted byproducts is a prerequisite for preventing them. This article presents a study of the photochemical reaction pathways for byproduct formation in the photochromic switch 1,2-bis-(3-thienyl)-ethene. Specifically, using single- and multi-reference methods the post-deexcitation reaction towards the byproduct in the electronic ground state S0 when starting from the S1–S0 conical intersection (CoIn), is considered in detail. We find an unusual low-energy pathway, which offers the possibility for the formation of a dyotropic byproduct. Several high-energy pathways can be excluded with high probability. Y1 - 2015 U6 - https://doi.org/10.1039/C5CP00987A SN - 1463-9076 SN - 1463-9084 IS - 17 SP - 14088 EP - 14095 PB - The Royal Society of Chemistry CY - Cambridge ER - TY - GEN A1 - Banerjee, Shiladitya A1 - Stüker, Tony A1 - Saalfrank, Peter T1 - Vibrationally resolved optical spectra of modified diamondoids obtained from time-dependent correlation function methods N2 - Optical properties of modified diamondoids have been studied theoretically using vibrationally resolved electronic absorption, emission and resonance Raman spectra. A time-dependent correlation function approach has been used for electronic two-state models, comprising a ground state (g) and a bright, excited state (e), the latter determined from linear-response, time-dependent density functional theory (TD-DFT). The harmonic and Condon approximations were adopted. In most cases origin shifts, frequency alteration and Duschinsky rotation in excited states were considered. For other cases where no excited state geometry optimization and normal mode analysis were possible or desired, a short-time approximation was used. The optical properties and spectra have been computed for (i) a set of recently synthesized sp2/sp3 hybrid species with C[double bond, length as m-dash]C double-bond connected saturated diamondoid subunits, (ii) functionalized (mostly by thiol or thione groups) diamondoids and (iii) urotropine and other C-substituted diamondoids. The ultimate goal is to tailor optical and electronic features of diamondoids by electronic blending, functionalization and substitution, based on a molecular-level understanding of the ongoing photophysics. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 211 Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-86826 ER - TY - JOUR A1 - Banerjee, Shiladitya A1 - Stüker, Tony A1 - Saalfrank, Peter T1 - Vibrationally resolved optical spectra of modified diamondoids obtained from time-dependent correlation function methods JF - Physical chemistry, chemical physics : PCCP ; a journal of European chemical societies N2 - Optical properties of modified diamondoids have been studied theoretically using vibrationally resolved electronic absorption, emission and resonance Raman spectra. A time-dependent correlation function approach has been used for electronic two-state models, comprising a ground state (g) and a bright, excited state (e), the latter determined from linear-response, time-dependent density functional theory (TD-DFT). The harmonic and Condon approximations were adopted. In most cases origin shifts, frequency alteration and Duschinsky rotation in excited states were considered. For other cases where no excited state geometry optimization and normal mode analysis were possible or desired, a short-time approximation was used. The optical properties and spectra have been computed for (i) a set of recently synthesized sp2/sp3 hybrid species with C[double bond, length as m-dash]C double-bond connected saturated diamondoid subunits, (ii) functionalized (mostly by thiol or thione groups) diamondoids and (iii) urotropine and other C-substituted diamondoids. The ultimate goal is to tailor optical and electronic features of diamondoids by electronic blending, functionalization and substitution, based on a molecular-level understanding of the ongoing photophysics. Y1 - 2015 U6 - https://doi.org/10.1039/C5CP02615F SN - 1463-9084 SN - 1463-9076 VL - 17 IS - 29 SP - 19656 EP - 19669 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Tong, Yujin A1 - Wirth, Jonas A1 - Kirsch, Harald A1 - Wolf, Martin A1 - Saalfrank, Peter A1 - Campen, Richard Kramer T1 - Optically probing Al-O and O-H vibrations to characterize water adsorption and surface reconstruction on alpha-alumina: An experimental and theoretical study JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - Oxide/water interfaces are ubiquitous in a wide variety of applications and the environment. Despite this ubiquity, and attendant decades of study, gaining molecular level insight into water/oxide interaction has proven challenging. In part, this challenge springs from a lack of tools to concurrently characterize changes in surface structure (i.e., water/oxide interaction from the perspective of the solid) and O-H population and local environment (i.e., water/oxide interaction from the water perspective). Here, we demonstrate the application of surface specific vibrational spectroscopy to the characterization of the interaction of the paradigmatic alpha-Al2O3(0001) surface and water. By probing both the interfacial Al-O (surface phonon) and O-H spectral response, we characterize this interaction from both perspectives. Through electronic structure calculation, we assign the interfacial Al-O response and rationalize its changes on surface dehydroxylation and reconstruction. Because our technique is all-optical and interface specific, it is equally applicable to oxide surfaces in vacuum, ambient atmospheres and at the solid/liquid interface. Application of this approach to additional alumina surfaces and other oxides thus seems likely to significantly expand our understanding of how water meets oxide surfaces and thus the wide variety of phenomena this interaction controls. (C) 2015 AIP Publishing LLC. Y1 - 2015 U6 - https://doi.org/10.1063/1.4906346 SN - 0021-9606 SN - 1089-7690 VL - 142 IS - 5 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Scholz, Robert A1 - Floss, Gereon A1 - Saalfrank, Peter A1 - Füchsel, Gernot A1 - Loncaric, Ivor A1 - Juaristi, J. I. T1 - Femtosecond-laser induced dynamics of CO on Ru(0001): Deep insights from a hot-electron friction model including surface motion JF - Physical review : B, Condensed matter and materials physics N2 - A Langevin model accounting for all six molecular degrees of freedom is applied to femtosecond-laser induced, hot-electron driven dynamics of Ru(0001)(2 x 2): CO. In our molecular dynamics with electronic friction approach, a recently developed potential energy surface based on gradient-corrected density functional theory accounting for van der Waals interactions is adopted. Electronic friction due to the coupling of molecular degrees of freedom to electron-hole pairs in the metal are included via a local density friction approximation, and surface phonons by a generalized Langevin oscillator model. The action of ultrashort laser pulses enters through a substrate-mediated, hot-electron mechanism via a time-dependent electronic temperature (derived from a two-temperature model), causing random forces acting on the molecule. The model is applied to laser induced lateral diffusion of CO on the surface, "hot adsorbate" formation, and laser induced desorption. Reaction probabilities are strongly enhanced compared to purely thermal processes, both for diffusion and desorption. Reaction yields depend in a characteristic (nonlinear) fashion on the applied laser fluence, as well as branching ratios for various reaction channels. Computed two-pulse correlation traces for desorption and other indicators suggest that aside from electron-hole pairs, phonons play a non-negligible role for laser induced dynamics in this system, acting on a surprisingly short time scale. Our simulations on precomputed potentials allow for good statistics and the treatment of long-time dynamics (300 ps), giving insight into this system which hitherto has not been reached. We find generally good agreement with experimental data where available and make predictions in addition. A recently proposed laser induced population of physisorbed precursor states could not be observed with the present low-coverage model. Y1 - 2016 U6 - https://doi.org/10.1103/PhysRevB.94.165447 SN - 2469-9950 SN - 2469-9969 VL - 94 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Kulesza, Alexander Jan A1 - Titov, Evgenii A1 - Daly, Steven A1 - Wlodarczyk, Radoslaw A1 - Megow, Jörg A1 - Saalfrank, Peter A1 - Choi, Chang Min A1 - MacAleese, Luke A1 - Antoine, Rodolphe A1 - Dugourd, Philippe T1 - Excited States of Xanthene Analogues: Photofragmentation and Calculations by CC2 and Time-Dependent Density Functional Theory JF - ChemPhysChem : a European journal of chemical physics and physical chemistry N2 - Action spectroscopy has emerged as an analytical tool to probe excited states in the gas phase. Although comparison of gas-phase absorption properties with quantum-chemical calculations is, in principle, straightforward, popular methods often fail to describe many molecules of interest-such as xanthene analogues. We, therefore, face their nano-and picosecond laser-induced photofragmentation with excited-state computations by using the CC2 method and time-dependent density functional theory (TDDFT). Whereas the extracted absorption maxima agree with CC2 predictions, the TDDFT excitation energies are blueshifted. Lowering the amount of Hartree-Fock exchange in the DFT functional can reduce this shift but at the cost of changing the nature of the excited state. Additional bandwidth observed in the photofragmentation spectra is rationalized in terms of multiphoton processes. Observed fragmentation from higher-lying excited states conforms to intense excited-to-excited state transitions calculated with CC2. The CC2 method is thus suitable for the comparison with photofragmentation in xanthene analogues. KW - density functional calculations KW - CC2 calculations KW - multiphoton processes KW - photofragmentation KW - xanthenes Y1 - 2016 U6 - https://doi.org/10.1002/cphc.201600650 SN - 1439-4235 SN - 1439-7641 VL - 17 SP - 3129 EP - 3138 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Boese, Adrian Daniel A1 - Saalfrank, Peter T1 - CO Molecules on a NaCl(100) Surface: Structures, Energetics, and Vibrational Davydov Splittings at Various Coverages JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - In this work, we study the adsorption of CO from low to high coverage at a defect-free NaCl(100) surface by means of duster and periodic models, using highly accurate wave function-based QM:QM embedding as well as density functional theory. At low coverages, the most accurate methods predict a zero-point-corrected adsorption energy of around 13 kJ/mol, and the CO molecules are found to be oriented perpendicular to the surface. At higher coverages, lower-energy phases with nonparallel/upright, tilted orientations emerge. Besides the well-known p(2 x 1)/antiparallel phase (T/A), we find other tilted phases (tilted/irregular, T/I; tilted/spiral, T/S) as local minima. Vibrational frequencies for CO adsorbed on NaCl(100) and Davydov splittings of the C-O stretch vibration are also determined. The IR spectra are characteristic fingerprints for the relative orientation of CO molecules and may therefore be used as sensitive probes to distinguish parallel/upright from various tilted adsorption phases. Y1 - 2016 U6 - https://doi.org/10.1021/acs.jpcc.6b03726 SN - 1932-7447 VL - 120 SP - 12637 EP - 12653 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Titov, Evgenii A1 - Granucci, Giovanni A1 - Goetze, Jan Philipp A1 - Persico, Maurizio A1 - Saalfrank, Peter T1 - Dynamics of Azobenzene Dimer Photoisomerization: Electronic and Steric Effects JF - The journal of physical chemistry letters N2 - While azobenzenes readily photoswitch in solution, their photoisomerization in densely packed self-assembled monolayers (SAMs) can be suppressed. Reasons for this can be steric hindrance and/or electronic quenching, e.g., by exciton coupling. We address these possibilities by means of nonadiabatic molecular dynamics with trajectory surface hopping calculations, investigating the trans -> cis isomerization of azobenzene after excitation into the pi pi* absorption band. We consider a free monomer, an isolated dimer and a dimer embedded in a SAM-like environment of additional azobenzene molecules, imitating in this way the gradual transition from an unconstrained over an electronically coupled to an electronically coupled and sterically hindered, molecular switch. Our simulations reveal that in comparison to the single molecule the quantum yield of the trans -> cis photoisomerization is similar for the isolated dimer, but greatly reduced in the sterically constrained situation. Other implications of dimerization and steric constraints are also discussed. Y1 - 2016 U6 - https://doi.org/10.1021/acs.jpciett.6b01401 SN - 1948-7185 VL - 7 SP - 3591 EP - 3596 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Loncaric, Ivor A1 - Alducin, Maite A1 - Saalfrank, Peter A1 - Inaki Juaristi, J. T1 - Femtosecond laser pulse induced desorption: A molecular dynamics simulation JF - Nature climate change N2 - In recent simulations of femtosecond laser induced desorption of molecular oxygen from the Ag(110) surface, it has been shown that depending on the properties (depth and electronic environment) of the well in which 02 is adsorbed, the desorption can be either induced dominantly by hot electrons or via excitations of phonons. In this work we explore whether the ratios between the desorption yields from different adsorption wells can be tuned by changing initial surface temperature and laser pulse properties. We show that the initial surface temperature is an important parameter, and that by using low initial surface temperatures the electronically mediated process can be favored. In contrast, laser properties seem to have only a modest influence on the results. (C) 2016 Elsevier B.V. All rights reserved. KW - Laser induced desorption KW - Molecular dynamics with friction KW - Local density friction approximation KW - Generalized Langevin oscillator model Y1 - 2016 U6 - https://doi.org/10.1016/j.nimb.2016.02.051 SN - 0168-583X SN - 1872-9584 VL - 382 SP - 114 EP - 118 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Titov, Evgenii A1 - Saalfrank, Peter T1 - Exciton Splitting of Adsorbed and Free 4-Nitroazobenzene Dimers: A Quantum Chemical Study JF - The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment & general theory N2 - Molecular photoswitches such as azobenzenes, which undergo photochemical trans <-> cis isomerizations, are often mounted for possible applications on a surface and/or surrounded by other switches, for example, in self-assembled monolayers. This may suppress the isomerization cross section due to possible steric reasons, or, as recently speculated, by exciton coupling to. neighboring switches, leading to ultrafast electronic quenching (Gahl et al., J. Am. Chem. Soc. 2010, 132, 1831). The presence of exciton coupling has been anticipated from a blue shift of the optical absorption band, compared to molecules in solution. From the theory side the need arises to properly analyze and quantify the change of absorption spectra of interacting and adsorbed switches. In particular, suitable methods should be identified, and effects of intermolecule and molecule surface interactions on spectra should be disentangled. In this paper by means of time-dependent Hartree-Fock. (TD-HF), various flavors of time-dependent density functional theory (TD-DFT), and the correlated wave function based, coupled cluster (CC2) method we investigated the 4-nitroazobenzene molecule as an:example: The low-lying singlet excited states in the isolated trans monomer and dieter as well as their composites with a silicon pentamantane nanocluster, which serves also as a crude model for a silicon surface, were determined. As most important results we found that (i) HF, CC2, range-separated density functionals, or global hybrids with large amount of exact exchange are able to describe exciton (Davydov) splitting properly, while hybrids with small amount of exact exchange fail producing spurious charge transfer. (ii) The exciton splitting in a free dimer would lead to a blue shift of the absorption signal; however, this effect is almost nullified or even overcompensated by the shift arising from van der Waals interactions between the two molecules. (iii) Adsorption on the Si "surface" leads to a further, strong red shift for the present system. (iv) At a next-nearest neighbor distance (of similar to 3.6 angstrom), the exciton splitting is similar to 0.3 eV, with or without "surface", suggesting a rapid quenching of the molecular pi ->pi* excitation. At larger distances, exciton splitting decreases rapidly. Y1 - 2016 U6 - https://doi.org/10.1021/acs.jpca.5b10376 SN - 1089-5639 VL - 120 SP - 3055 EP - 3070 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Loncaric, Ivor A1 - Alducin, Maite A1 - Saalfrank, Peter A1 - Juaristi, J. I. T1 - Femtosecond-laser-driven molecular dynamics on surfaces: Photodesorption of molecular oxygen from Ag(110) JF - Physical review : B, Condensed matter and materials physics N2 - We simulate the femtosecond-laser-induced desorption dynamics of a diatomic molecule from a metal surface by including the effect of the electron and phonon excitations created by the laser pulse. Following previous models, the laser-induced surface excitation is treated through the two temperature model, while the multidimensional dynamics of the molecule is described by a classical Langevin equation, in which the friction and random forces account for the action of the heated electrons. In this work we propose the additional use of the generalized Langevin oscillator model to also include the effect of the energy exchange between the molecule and the heated surface lattice in the desorption dynamics. The model is applied to study the laser-induced desorption of O-2 from the Ag(110) surface, making use of a six-dimensional potential energy surface calculated within density functional theory. Our results reveal the importance of the phonon mediated process and show that, depending on the value of the electronic density in the surroundings of the molecule adsorption site, its inclusion can significantly enhance or reduce the desorption probabilities. Y1 - 2016 U6 - https://doi.org/10.1103/PhysRevB.93.014301 SN - 1098-0121 SN - 1550-235X VL - 93 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - White, Alec F. A1 - Heide, Chiara Josephine A1 - Saalfrank, Peter A1 - Head-Gordon, Martin A1 - Luppi, Eleonora T1 - Computation of high-harmonic generation spectra of the hydrogen molecule using time-dependent configuration-interaction JF - Molecular physics N2 - Here we apply and expand the knowledge developed in the case of the H atom to describe high-harmonic generation (HHG) for the H-2 molecule by using time-dependent configuration interaction with single excitations. The implications of using a finite atomic orbital basis set and the impact of a heuristic lifetime model which addresses ionisation losses are discussed. We also examine the influence of the angular momentum of the basis on the computed HHG spectra. Moreover, we discuss the impact of adding diffuse functions and ghost atoms in different geometrical configurations around the molecule. The effects of these additional centres on the HHG spectra are correlated with the physical interpretation of this nonlinear optical phenomenon as given by the three-step model, relating the maximal radial extent of the electron as predicted by the model to the radial extent of the Gaussian basis sets. [GRAPHICS] . KW - High-harmonic generation KW - strong field KW - time-dependent configuration interaction Y1 - 2016 U6 - https://doi.org/10.1080/00268976.2015.1119900 SN - 0026-8976 SN - 1362-3028 VL - 114 SP - 947 EP - 956 PB - Springer CY - Abingdon ER - TY - JOUR A1 - Schönborn, Jan Boyke A1 - Saalfrank, Peter A1 - Klamroth, Tillmann T1 - Controlling the high frequency response of H-2 by ultra-short tailored laser pulses: A time-dependent configuration interaction study JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - We combine the stochastic pulse optimization (SPO) scheme with the time-dependent configuration interaction singles method in order to control the high frequency response of a simple molecular model system to a tailored femtosecond laser pulse. For this purpose, we use H-2 treated in the fixed nuclei approximation. The SPO scheme, as similar genetic algorithms, is especially suited to control highly non-linear processes, which we consider here in the context of high harmonic generation. Here, we will demonstrate that SPO can be used to realize a "non-harmonic" response of H2 to a laser pulse. Specifically, we will show how adding low intensity side frequencies to the dominant carrier frequency of the laser pulse and stochastically optimizing their contribution can create a high-frequency spectral signal of significant intensity, not harmonic to the carrier frequency. At the same time, it is possible to suppress the harmonic signals in the same spectral region, although the carrier frequency is kept dominant during the optimization. (C) 2016 AIP Publishing LLC. Y1 - 2016 U6 - https://doi.org/10.1063/1.4940316 SN - 0021-9606 SN - 1089-7690 VL - 144 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Ehlert, Christopher A1 - Holzweber, Markus A1 - Lippitz, Andreas A1 - Unger, Wolfgang E. S. A1 - Saalfrank, Peter T1 - A detailed assignment of NEXAFS resonances of imidazolium based ionic liquids JF - Physical chemistry, chemical physics : a journal of European Chemical Societies N2 - In Near Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy X-Ray photons are used to excite tightly bound core electrons to low-lying unoccupied orbitals of the system. This technique offers insight into the electronic structure of the system as well as useful structural information. In this work, we apply NEXAFS to two kinds of imidazolium based ionic liquids ([C(n)C(1)im](+)[NTf2](-) and [C(4)C(1)im](+)[I](-)). A combination of measurements and quantum chemical calculations of C K and N K NEXAFS resonances is presented. The simulations, based on the transition potential density functional theory method (TP-DFT), reproduce all characteristic features observed by the experiment. Furthermore, a detailed assignment of resonance features to excitation centers (carbon or nitrogen atoms) leads to a consistent interpretation of the spectra. Y1 - 2016 U6 - https://doi.org/10.1039/c5cp07434g SN - 1463-9076 SN - 1463-9084 VL - 18 SP - 8654 EP - 8661 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Wirth, Jonas A1 - Schacht, Julia A1 - Saalfrank, Peter A1 - Paulus, Beate T1 - Fluorination of the Hydroxylated alpha-Al2O3 (0001) and Its Implications for Water Adsorption: A Theoretical Study JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - Fluorination of the hydroxylated alpha-Al2O3 (0001) surface is studied using periodic density functional theory calculations. On the basis of a hypothetical reaction substituting surface hydroxyl groups with fluorine atoms, we find surface fluorination to be strongly exergonic but kinetically hindered. Fluorinated surface areas turn out to be rather hydrophobic as compared to hydroxylated areas, suggesting fluorination as a potential route for tuning oxide surface properties such as hydrophilicity. Y1 - 2016 U6 - https://doi.org/10.1021/acs.jpcc.5b10975 SN - 1932-7447 VL - 120 SP - 9713 EP - 9718 PB - American Chemical Society CY - Washington ER - TY - GEN A1 - Ehlert, Christopher A1 - Holzweber, Markus A1 - Lippitz, Andreas A1 - Unger, Wolfgang E. S. A1 - Saalfrank, Peter T1 - A detailed assignment of NEXAFS resonances of imidazolium based ionic liquids N2 - In Near Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy X-Ray photons are used to excite tightly bound core electrons to low-lying unoccupied orbitals of the system. This technique offers insight into the electronic structure of the system as well as useful structural information. In this work, we apply NEXAFS to two kinds of imidazolium based ionic liquids ([CnC1im]+[NTf2]- and [C4C1im]+[I]-). A combination of measurements and quantum chemical calculations of C K and N K NEXAFS resonances is presented. The simulations, based on the transition potential density functional theory method (TP-DFT), reproduce all characteristic features observed by the experiment. Furthermore, a detailed assignment of resonance features to excitation centers (carbon or nitrogen atoms) leads to a consistent interpretation of the spectra. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 315 KW - ray absorption-spectroscopy KW - fine-structure KW - spectra KW - simulations KW - molecules KW - dynamics KW - graphene KW - surface KW - salts Y1 - 2016 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-394417 SP - 8654 EP - 8661 ER - TY - GEN A1 - Wirth, Jonas A1 - Kirsch, Harald A1 - Wlosczyk, Sebastian A1 - Tong, Yujin A1 - Saalfrank, Peter A1 - Kramer Campen, Richard T1 - Characterization of water dissociation on α-Al2O3(1102) BT - theory and experiment N2 - The interaction of water with α-alumina (i.e. α-Al2O3) surfaces is important in a variety of applications and a useful model for the interaction of water with environmentally abundant aluminosilicate phases. Despite its significance, studies of water interaction with α-Al2O3 surfaces other than the (0001) are extremely limited. Here we characterize the interaction of water (D2O) with a well defined α-Al2O3(1[1 with combining macron]02) surface in UHV both experimentally, using temperature programmed desorption and surface-specific vibrational spectroscopy, and theoretically, using periodic-slab density functional theory calculations. This combined approach makes it possible to demonstrate that water adsorption occurs only at a single well defined surface site (the so-called 1–4 configuration) and that at this site the barrier between the molecularly and dissociatively adsorbed forms is very low: 0.06 eV. A subset of OD stretch vibrations are parallel to this dissociation coordinate, and thus would be expected to be shifted to low frequencies relative to an uncoupled harmonic oscillator. To quantify this effect we solve the vibrational Schrödinger equation along the dissociation coordinate and find fundamental frequencies red-shifted by more than 1500 cm−1. Within the context of this model, at moderate temperatures, we further find that some fraction of surface deuterons are likely delocalized: dissociatively and molecularly absorbed states are no longer distinguishable. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 320 Y1 - 2016 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-394497 SP - 14822 EP - 14832 ER - TY - JOUR A1 - Wirth, Jonas A1 - Kirsch, Harald A1 - Wlosczyk, Sebastian A1 - Tong, Yujin A1 - Saalfrank, Peter A1 - Campen, Richard Kramer T1 - Characterization of water dissociation on alpha-Al2O3(1(1)over-bar02): theory and experiment JF - Physical chemistry, chemical physics : a journal of European Chemical Societies N2 - The interaction of water with a-alumina (i.e. alpha-Al2O3) surfaces is important in a variety of applications and a useful model for the interaction of water with environmentally abundant aluminosilicate phases. Despite its significance, studies of water interaction with alpha-Al2O3 surfaces other than the (0001) are extremely limited. Here we characterize the interaction of water (D2O) with a well defined alpha-Al2O3(1 (1) over bar 02) surface in UHV both experimentally, using temperature programmed desorption and surface-specific vibrational spectroscopy, and theoretically, using periodic-slab density functional theory calculations. This combined approach makes it possible to demonstrate that water adsorption occurs only at a single well defined surface site (the so-called 1-4 configuration) and that at this site the barrier between the molecularly and dissociatively adsorbed forms is very low: 0.06 eV. A subset of OD stretch vibrations are parallel to this dissociation coordinate, and thus would be expected to be shifted to low frequencies relative to an uncoupled harmonic oscillator. To quantify this effect we solve the vibrational Schrodinger equation along the dissociation coordinate and find fundamental frequencies red-shifted by more than 1500 cm(-1). Within the context of this model, at moderate temperatures, we further find that some fraction of surface deuterons are likely delocalized: dissociatively and molecularly absorbed states are no longer distinguishable. Y1 - 2016 U6 - https://doi.org/10.1039/c6cp01397j SN - 1463-9076 SN - 1463-9084 VL - 18 SP - 14822 EP - 14832 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Loncaric, Ivor A1 - Fuchsel, Gernot A1 - Juaristi, J. I. A1 - Saalfrank, Peter T1 - Strong Anisotropic Interaction Controls Unusual Sticking and Scattering of CO at Ru(0001) JF - Physical review letters N2 - Complete sticking at low incidence energies and broad angular scattering distributions at higher energies are often observed in molecular beam experiments on gas-surface systems which feature a deep chemisorption well and lack early reaction barriers. Although CO binds strongly on Ru(0001), scattering is characterized by rather narrow angular distributions and sticking is incomplete even at low incidence energies. We perform molecular dynamics simulations, accounting for phononic (and electronic) energy loss channels, on a potential energy surface based on first-principles electronic structure calculations that reproduce the molecular beam experiments. We demonstrate that the mentioned unusual behavior is a consequence of a very strong rotational anisotropy in the molecule-surface interaction potential. Beyond the interpretation of scattering phenomena, we also discuss implications of our results for the recently proposed role of a precursor state for the desorption and scattering of CO from ruthenium. Y1 - 2017 U6 - https://doi.org/10.1103/PhysRevLett.119.146101 SN - 0031-9007 SN - 1079-7114 VL - 119 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Lorenz, U. A1 - Saalfrank, Peter T1 - A novel system-bath Hamiltonian for vibration-phonon coupling BT - Formulation, and application to the relaxation of Si-H and Si-D bending modes of H/D:Si(100)-(2 x 1) JF - Chemical physics : a journal devoted to experimental and theoretical research involving problems of both a chemical and physical nature N2 - We present a rigorous method to set up a system-bath Hamiltonian for the coupling of adsorbate vibrations (the system) to surface phonons (the bath). The Hamiltonian is straightforward to derive and exact up to second order in the environment coordinates, thus capable of treating one- and two-phonon contributions to vibration-phonon coupling. The construction of the Hamiltonian uses orthogonal coordinates for system and bath modes, is based on an embedded cluster approach, and generalizes previous Hamiltonians of a similar type, but avoids several (additional) approximations. While the parametrization of the full Hamiltonian is in principle feasible by a first principles quantum mechanical treatment, here we adopt in the spirit of a QM/MM model a combination of density functional theory (“QM”, for the system) and a semiempirical forcefield (“MM”, for the bath). We apply the Hamiltonian to a fully H-covered Si(100)-(2 × 1) surface, using Fermi’s Golden Rule to obtain vibrational relaxation rates of various H–Si bending modes of this system. As in earlier work it is found that the relaxation is dominated by two-phonon contributions because of an energy gap between the Si–H bending modes and the Si phonon bands. We obtain vibrational lifetimes (of the first excited state) on the order of 2 ps at K. The lifetimes depend only little on the type of bending mode (symmetric vs. antisymmetric, parallel vs. perpendicular to the Si2H2 dimers). They decrease by a factor of about two when heating the surface to 300 K. We also study isotope effects by replacing adsorbed H atoms by deuterium, D. The Si–D bending modes are shifted into the Si phonon band of the solid, opening up one-phonon decay channels and reducing the lifetimes to few hundred fs. Y1 - 2016 U6 - https://doi.org/10.1016/j.chemphys.2016.06.004 SN - 0301-0104 SN - 1873-4421 VL - 482 SP - 69 EP - 80 PB - Elsevier Science CY - Amsterdam ER - TY - JOUR A1 - Goulet-Hanssens, Alexis A1 - Utecht, Manuel A1 - Mutruc, Dragos A1 - Titov, Evgenii A1 - Schwarz, Jutta A1 - Grubert, Lutz A1 - Bleger, David A1 - Saalfrank, Peter A1 - Hecht, Stefan T1 - Electrocatalytic Z -> E Isomerization of Azobenzenes JF - Journal of the American Chemical Society N2 - A variety of azobenzenes were synthesized to study the behavior of their E and Z isomers upon electrochemical reduction. Our results show that the radical anion of the Z isomer is able to rapidly isomerize to the corresponding E configured counterpart with a dramatically enhanced rate as compared to the neutral species. Due to a subsequent electron transfer from the formed E radical anion to the neutral Z starting material the overall transformation is catalytic in electrons; i.e., a substoichiometric amount of reduced species can isomerize the entire mixture. This pathway greatly increases the efficiency of (photo)switching while also allowing one to reach photostationary state compositions that are not restricted to the spectral separation of the individual azobenzene isomers and their quantum yields. In addition, activating this radical isomerization pathway with photoelectron transfer agents allows us to override the intrinsic properties of an azobenzene species by triggering the reverse isomerization direction (Z -> E) by the same wavelength of light, which normally triggers E -> Z isomerization. The behavior we report appears to be general, implying that the metastable isomer of a photoswitch can be isomerized to the more stable one catalytically upon reduction, permitting the optimization of azobenzene switching in new as well as indirect ways. Y1 - 2017 U6 - https://doi.org/10.1021/jacs.6b10822 SN - 0002-7863 VL - 139 IS - 1 SP - 335 EP - 341 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Rietze, Clemens A1 - Titov, Evgenii A1 - Lindner, Steven A1 - Saalfrank, Peter T1 - Thermal isomerization of azobenzenes: on the performance of Eyring transition state theory JF - Journal of physics : Condensed matter N2 - The thermal Z -> E (back-) isomerization of azobenzenes is a prototypical reaction occurring in molecular switches. It has been studied for decades, yet its kinetics is not fully understood. In this paper, quantum chemical calculations are performed to model the kinetics of an experimental benchmark system, where a modified azobenzene (AzoBiPyB) is embedded in a metal-organic framework (MOF). The molecule can be switched thermally from cis to trans, under solvent-free conditions. We critically test the validity of Eyring transition state theory for this reaction. As previously found for other azobenzenes (albeit in solution), good agreement between theory and experiment emerges for activation energies and activation free energies, already at a comparatively simple level of theory, B3LYP/6-31G* including dispersion corrections. However, theoretical Arrhenius prefactors and activation entropies are in qualitiative disagreement with experiment. Several factors are discussed that may have an influence on activation entropies, among them dynamical and geometric constraints (imposed by the MOF). For a simpler model-Z -> E isomerization in azobenzene-a systematic test of quantum chemical methods from both density functional theory and wavefunction theory is carried out in the context of Eyring theory. Also, the effect of anharmonicities on activation entropies is discussed for this model system. Our work highlights capabilities and shortcomings of Eyring transition state theory and quantum chemical methods, when applied for the Z -> E (back-) isomerization of azobenzenes under solvent-free conditions. KW - thermal isomerization Y1 - 2017 U6 - https://doi.org/10.1088/1361-648X/aa75bd SN - 0953-8984 SN - 1361-648X VL - 29 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Malyar, Ivan V. A1 - Titov, Evgenii A1 - Lomadze, Nino A1 - Saalfrank, Peter A1 - Santer, Svetlana T1 - Photoswitching of azobenzene-containing self-assembled monolayers as a tool for control over silicon surface electronic properties JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - We report on photoinduced remote control of work function and surface potential of a silicon surface modified with a photosensitive self-assembled monolayer consisting of chemisorbed azobenzene molecules (4-nitroazobenzene). Itwas found that the attachment of the organic monolayer increases the work function by hundreds of meV due to the increase in the electron affinity of silicon substrates. The change in the work function on UV light illumination is more pronounced for the azobenzene jacketed silicon substrate (ca. 250 meV) in comparison to 50 meV for the unmodified surface. Moreover, the photoisomerization of azobenzene results in complex kinetics of thework function change: immediate decrease due to light-driven processes in the silicon surface followed by slower recovery to the initial state due to azobenzene isomerization. This behavior could be of interest for electronic devices where the reaction on irradiation should be more pronounced at small time scales but the overall surface potential should stay constant over time independent of the irradiation conditions. Published by AIP Publishing. Y1 - 2017 U6 - https://doi.org/10.1063/1.4978225 SN - 0021-9606 SN - 1089-7690 VL - 146 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Melani, Giacomo A1 - Nagata, Yuki A1 - Wirth, Jonas A1 - Saalfrank, Peter T1 - Vibrational spectroscopy of hydroxylated alpha-Al2O3(0001) surfaces with and without water BT - an ab initio molecular dynamics study JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - Using gradient- and dispersion-corrected density functional theory in connection with ab initio molecular dynamics and efficient, parametrized Velocity-Velocity Autocorrelation Function (VVAF) methodology, we study the vibrational spectra (Vibrational Sum Frequency, VSF, and infrared, IR) of hydroxylated alpha-Al2O3(0001) surfaces with and without additional water. Specifically, by considering a naked hydroxylated surface and the same surface with a particularly stable, "ice-like" hexagonal water later allows us to identify and disentangle main spectroscopic bands of OH bonds, their orientation and dynamics, and the role of water adsorption. In particular, we assign spectroscopic signals around 3700 cm(-1) as being dominated by perpendicularly oriented non-hydrogen bonded aluminol groups, with and without additional water. Furthermore, the thin water layer gives spectroscopic signals which are already comparable to previous theoretical and experimental findings for the solid/(bulk) liquid interface, showing that water molecules closest to the surface play a decisive role in the vibrational response of these systems. From a methodological point of view, the effects of temperature, anharmonicity, hydrogen-bonding, and structural dynamics are taken into account and analyzed, allowing us to compare the calculated IR and VSF spectra with the ones based on normal mode analysis and vibrational density of states. The VVAF approach employed in this work appears to be a computationally accurate yet feasible method to address the vibrational fingerprints and dynamical properties of water/metal oxide interfaces. Published by AIP Publishing. Y1 - 2018 U6 - https://doi.org/10.1063/1.5023347 SN - 0021-9606 SN - 1089-7690 VL - 149 IS - 1 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Goulet-Hanssens, Alexis A1 - Rietze, Clemens A1 - Titov, Evgenii A1 - Abdullahu, Leonora A1 - Grubert, Lutz A1 - Saalfrank, Peter A1 - Hecht, Stefan T1 - Hole Catalysis as a General Mechanism for Efficient and Wavelength-Independent Z -> E Azobenzene Isomerization JF - CHEM N2 - Whereas the reversible reduction of azobenzenes has been known for decades, their oxidation is destructive and as a result has been notoriously overlooked. Here, we show that a chain reaction leading to quantitative Z -> E isomerization can be initiated before reaching the destructive anodic peak potential. This hole-catalyzed pathway is accessible to all azobenzenes, without exception, and offers tremendous advantages over the recently reported reductive, radical-anionic pathway because it allows for convenient chemical initiation without the need for electrochemical setups and in the presence of air. In addition, catalytic amounts of metal-free sensitizers, such as methylene blue, can be used as excited-state electron acceptors, enabling a shift of the excitation wavelength to the far red of the azobenzene absorption (up to 660 nm) and providing quantum yields exceeding unity (up to 200%). Our approach will boost the efficiency and sensitivity of optically dense liquid-crystalline and solid photo-switchable materials. Y1 - 2018 U6 - https://doi.org/10.1016/j.chempr.2018.06.002 SN - 2451-9294 VL - 4 IS - 7 SP - 1740 EP - 1755 PB - Cell Press CY - Cambridge ER - TY - JOUR A1 - Hänsel, Marc A1 - Barta, Christoph A1 - Rietze, Clemens A1 - Utecht, Manuel Martin A1 - Rueck-Braun, Karola A1 - Saalfrank, Peter A1 - Tegeder, Petra T1 - Two-Dimensional Nonlinear Optical Switching Materials BT - Molecular Engineering toward High Nonlinear Optical Contrasts JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - Combining photochromism and nonlinear optical (NLO) properties of molecular switches-functionalized self-assembled monolayers (SAMs) represents a promising concept toward novel photonic and optoelectronic devices. Using second harmonic generation, density functional theory, and correlated wave function methods, we studied the switching abilities as well as the NLO contrasts between different molecular states of various fulgimide-containing SAMs on Si(111). Controlled variations of the linker systems as well as of the fulgimides enabled us to demonstrate very efficient reversible photoinduced ring-opening/closure reactions between the open and closed forms of the fulgimides. Thus, effective cross sections on the order of 10(-18) cm(-2) are observed. Moreover, the reversible switching is accompanied by pronounced NLO contrasts up to 32%. Further molecular engineering of the photochromic switches and the linker systems may even increase the NLO contrast upon switching. Y1 - 2018 U6 - https://doi.org/10.1021/acs.jpcc.8b08212 SN - 1932-7447 SN - 1932-7455 VL - 122 IS - 44 SP - 25555 EP - 25564 PB - American Chemical Society CY - Washington ER -