### Refine

#### Year of publication

#### Document Type

- Article (45)
- Postprint (2)
- Doctoral Thesis (1)

#### Keywords

- surface chemistry (2)
- Chelates (1)
- DFT (1)
- Electron dynamics (1)
- Ligand design (1)
- Palladium (1)
- Renewable resources (1)
- S li-gands (1)
- STM (1)
- UV (1)

We report time-dependent configuration interaction singles calculations for the ultrafast laser driven many- electron dynamics in a polyatomic molecule, N-methyl-6-quinolone. We employ optimal control theory to achieve a nearly state-selective excitation from the S-0 to the S-1 state, on a time scale of a few (approximate to 6) femtoseconds. The optimal control scheme is shown to correct for effects opposing a state-selective transition, such as multiphoton transitions and other, nonlinear phenomena, which are induced by the ultrashort and intense laser fields. In contrast, simple two-level pi pulses are not effective in state-selective excitations when very short pulses are used. Also, the dependence of multiphoton and nonlinear effects on the number of states included in the dynamical simulations is investigated.

Recent progress towards a quantum theory of laser-induced desorption and related phenomena is reviewed, for specific examples. These comprise the photodesorption of NO from Pt(111), the scanning tunnelling microscope and laser- induced desorption and switching of H at Si(100), and the electron stimulated desorption and dissociation of CO at Ru(0001). The theoretical methods used for nuclear dynamics range from open-system density matrix theory over nonadiabatically coupled multi-state models to electron-nuclear wavepackets. Also, aspects of time-dependent spectroscopy to probe ultrafast nonadiabatic processes at surfaces will be considered for the example of two-photon photoemission of solvated electrons in ice layers on Cu(111)

Correlated many-electron dynamics : application to inelastic electron scattering at a metal film
(2005)

The multiconfiguration time-dependent Hartree-Fock and the time-dependent configuration interaction singles method are applied to the correlated many-electron dynamics of a one-dimensional jellium model system. We study the scattering of an initially free electron at a model film in the framework of both approaches. In particular, both methods are compared with regard to how they describe the underlying physical processes, namely inelastic electron scattering, inverse photoemission, and electron impact ionization

Recent experiments using time- and angle-resolved two-photon photoemission (2PPE) spectroscopy at metal/polar adsorbate interfaces succeeded in time-dependent analysis of the process of electron solvation. A fully quantum mechanical, two-dimensional simulation of this process, which explicitly includes laser excitation, is presented here, confirming the origin of characteristic features, such as the experimental observation of an apparently negative dispersion. The inference of the spatial extent of the localized electron states from the angular dependence of the 2PPE spectra has been found to be non-trivial and system-dependent. (C) 2005 American Institute of Physics

In this paper we report on time dependent configuration interaction singles (TD-CIS) calculations aimed at simulating two-photon-photoelectron emission (2PPE) spectra of metal films, the latter treated within a one-dimensional jellium model. The method is based on a many-electron approach in which electron-electron-scattering is approximately accounted for and no artificial lifetimes have to be assumed for excited electrons. This contrasts with one-electron models where lifetimes and "dissipation" have to be introduced. The driving force for the photoelectron ejection in 2PPE experiments is the electric field of two laser pulses that are generally separated by a delay time, Delta t. To compute energy- and time-resolved 2PPE signals P(E, Delta t), a new scheme based on the time-energy method is proposed to analyze electronic wave packets in asymptotic regions of the potential

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

The adsorption of molecules to the surface of carbon nanostructures opens a new field of hybrid systems with distinct and controllable properties. We present a microscopic study of the optical absorption in carbon nanotubes functionalized with molecular spiropyran photoswitches. The switching process induces a change in the dipole moment leading to a significant coupling to the charge carriers in the nanotube. As a result, the absorption spectra of functionalized tubes reveal a considerable redshift of transition energies depending on the switching state of the spiropyran molecule. Our results suggest that carbon nanotubes are excellent substrates for the optical readout of spiropyran-based molecular switches. The gained insights can be applied to other noncovalently functionalized one-dimensional nanostructures in an externally induced dipole field.

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.

The optical switching of molecular ensembles in a dissipative environment is a subject of various fields of chemical physics and physical chemistry. Here we try to switch arrays of molecules from a stable collective ground state to a state in which all molecules have been transferred to another stable higher-energy configuration. In our model switching proceeds through electronically excited intermediates which are coherently coupled to each other through dipolar interactions, and which decay incoherently within a finite lifetime by coupling to a dissipative environment. The model is quite general, but parameters are chosen to roughly resemble the all-trans -> all-cis isomerization of an array of azobenzene molecules on a surface. Using analytical and optimal control pulses and the concept of "laser distillation," we demonstrate that for various aggregates (dimers up to hexamers), controlled and complete switching should be possible.

In this paper, we report simulations of laser-driven many-electron dynamics by means of the time-dependent configuration interaction singles (TD-CIS) approach. Photoionization is included by a heuristic model within calculations employing standard Gaussian basis sets. Benzo[g]-N-methyl-quinolinium-7-hydroxylate (BMQ7H) serves as a test system to generate predefined wave packets, i.e. a superposition between the ground and fifth excited state, in a large molecule. For this molecule, these two states have a very similar geometry, which enables us to use the fixed nuclei approximation. Furthermore, this geometric stability would also prevent a dephasing of the electron wave packet due to nuclear dynamics in an experimental realization of our simulations. We also simulate the possible detection of such a wave packet by ultra short probe laser pulses, i.e. pump-probe spectra.

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.

There is a demand for new and robust PdII extractants due to growing recycling rates. Chelating dithioethers are promising substances for solvent extraction as they form stable square-planar complexes with PdII. We have modified unsaturated dithioethers, which are known to coordinate PdII, and adapted them to the requirements of industrial practice. The ligands are analogues of 1,2-dithioethene with varying electron-withdrawing backbones and polar end-groups. The crystal structures of several ligands and their palladium complexes were determined as well as their electro- and photochemical properties, complex stability and behaviour in solution. Solvent extraction experiments showed the superiority of some of our ligands over conventionally used extractants in terms of their very fast reaction rates. With highly selective 1,2-bis(2-methoxyethylthio)benzene (4) it is possible to extract PdII from a highly acidic medium in the presence of other base and palladium-group metals.

Selective excitation of molecule-surface vibrations in H2 and D2 dissociatively adsorbed on Ru(0001)
(2012)

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.

We report explicitly time-dependent coupled cluster singles doubles (TD-CCSD) calculations, which simulate the laser-driven correlated many-electron dynamics in molecular systems. Small molecules, i.e., HF, H(2)O, NH(3), and CH(4), are treated mostly with polarized valence double zeta basis sets. We determine the coupled cluster ground states by imaginary time propagation for these molecules. Excited state energies are obtained from the Fourier transform of the time-dependent dipole moment after an ultrashort, broadband laser excitation. The time-dependent expectation values are calculated from the complex cluster amplitudes using the corresponding configuration interaction singles doubles wave functions. Also resonant laser excitations of these excited states are simulated, in order to explore the limits for the numerical stability of our current TD-CCSD implementation, which uses time-independent molecular orbitals to form excited configurations.

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.

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.

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.

The formation of CuCl nanoplatelets from the ionic liquid precursor (ILP) butylpyridinium tetrachlorocuprate [C4Py](2)[CuCl4] using ascorbic acid as a reducing agent was investigated. In particular, electron paramagnetic resonance (EPR) spectroscopy was used to evaluate the interaction between ascorbic acid and the Cu(II) ion before reduction to Cu(I). EPR spectroscopy suggests that the [CuCl4](2-) ion in the neat IL is a distorted tetrahedron, consistent with DFT calculations. Addition of ascorbic acid leads to the removal of one chloride from the [CuCl4](2-) anion, as shown by DFT and the loss of symmetry by EPR. DFT furthermore suggests that the most stable adduct is formed when only one hydroxyl group of the ascorbic acid coordinates to the Cu(II) ion.

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.

Motivated by recent atomic manipulation experiments, we report quantum chemical calculations for chemi- and physisorption minima of chlorobenzene on the Si(111)-7x7 surface. A density functional theory cluster approach is applied, using the B3LYP hybrid functional alongside Grimme's empirical dispersion corrections (D3). We were able to identify chemisorption sites of binding energies of 1.6 eV and physisorption energies of 0.6 eV, both in encouraging agreement with the trend of experimental data. The cluster approach opens up the possibility of a first-principles based dynamical description of STM manipulation experiments on this system, the interpretation of which involves both the chemi- and physisorbed states. However, we found that special care has to be taken regarding the choice of clusters, basis sets, and the evaluation of the dispersion corrections.

Laser-induced condensed phase reactions are often interpreted as nonequilibrium phenomena that go beyond conventional thermodynamics. Here, we show by Langevin dynamics and for the example of femtosecond-laser desorption of hydrogen from a ruthenium surface that light adsorbates thermalize rapidly due to ultrafast energy redistribution after laser excitation. Despite the complex reaction mechanism involving hot electrons in the surface region, all desorption product properties are characterized by equilibrium distributions associated with a single, unique temperature. This represents an example of ultrahot chemistry on the subpicosecond time scale.

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.

The new pi-conjugated 1,2,3-triazol-1,4-diyl fluoroionophore 1 generated via Cu(I) catalyzed [3 + 2] cycloaddition shows high fluorescence enhancement factors (FEF) in the presence of Na+ (FEF = 58) and K+ (FEF = 27) in MeCN and high selectivity towards K+ under simulated physiological conditions (160 mM K+ or Na+, respectively) with a FEF of 2.5 for K+.

Tetrahalidocuprates(II) show a high degree of structural flexibility. We present the results of crystallographic and electron paramagnetic resonance (EPR) spectroscopic analyses of four new tetrabromidocuprate(II) compounds and compare the results with previously reported data. The cations in the new compounds are the sterically demanding benzyltriphenylphosphonium, methyltriphenylphosphonium, tetraphenylphosphonium, and hexadecyltrimethylammonium ions; they were used to achieve a reasonable separation of the paramagnetic Cu(II) ions for EPR spectroscopy. X-Ray crystallography shows that in all four complexes the [CuBr4](2-) units have a distorted tetrahedral coordination geometry which is in agreement with DFT calculations. The EPR hyperfine structure was not resolved. This is due to the exchange broadening resulting from still incomplete separation of the paramagnetic Cu(II) centres. Nevertheless, the principal values of the electron Zeemann tensor (g(parallel to) and g(perpendicular to)) of the complexes could be determined. A correlation of structural (X-ray) parameters with the spin density at the copper centres (DFT) is well reflected in the EPR spectra of the bromidocuprates. This enables the correlation of X-ray and EPR parameters to predict the structure of tetrabromidocuprates in physical states other than the crystalline state. As a result, we provide a method to structurally characterize [CuBr4](2-) in, for example, ionic liquids or in solution, which has important implications for e.g. catalysis or materials science.

We present a novel laser pulse control for the chiroptical switch 1-(2-cis-fluoroethenyl)-2-fluoro-3,5-dibromobenzene mounted on adamantane, where the latter imitates a linker group or part of a solid surface. This molecular device offers three switching states: a true achiral "off"-state and two chiral "on"-states of opposite handedness. Due to the alignment of its chiral axis along the surface normal several defined orientations of the switch have to be considered for an efficient stereocontrol strategy. In addition to these different initial conditions, coupled torsional degrees of freedom around the chiral axis make the quest for highly stereoselective laser pulses a challenge. The necessary flexibility in pulse accomplished by employing the iterative stochastic pulse optimization method we presented recently. Still, the complexity of the system dictates a combined treatment by fast molecular dynamics and computationally intensive quantum dynamics. Although quantum effects are found to be of importance, the pulses optimized within the classical treatment allow us to turn on the chirality of the switch, achieving high enantioselectivity in the quantum treatment for all orientations at the same time.

High conductivity and a tunability of the band gap make quasi-one-dimensional graphene nanoribbons (GNRs) highly interesting materials for the use in field effect transistors. Especially bottom-up fabricated GNRs possess well-defined edges which is important for the electronic structure and accordingly the band gap. In this study we investigate the formation of a sub-nanometer wide armchair GNR generated on a Au(111) surface. The on-surface synthesis is thermally activated and involves an intermediate non-aromatic polymer in which the molecular precursor forms polyanthrylene chains. Employing angle-resolved two-photon photoemission in combination with density functional theory calculations we find that the polymer exhibits two dispersing states which we attribute to the valence and the conduction band, respectively. While the band gap of the non-aromatic polymer obtained in this way is relatively large, namely 5.25 +/- 0.06 eV, the gap of the corresponding aromatic GNR is strongly reduced which we attribute to the different degree of electron delocalization in the two systems.

We present and discuss the results of crystallographic and electron paramagnetic resonance (EPR) spectroscopic analyses of five tetrachloridocuprate(II) complexes to supply a useful tool for the structural characterisation of the [CuCl4](2-) moiety in the liquid state, for example in ionic liquids, or in solution. Bis(benzyltriethylammonium)-, bis(trimethylphenylammonium)-, bis(ethyltriphenylphosphonium)-, bis(benzyltriphenylphosphonium)-, and bis(tetraphenylarsonium) tetrachloridocuprate(II) were synthesised and characterised by elemental, IR, EPR and X-ray analyses. The results of the crystallographic analyses show distorted tetrahedral coordination geometry of all [CuCl4](2-) anions in the five complexes and prove that all investigated complexes are stabilised by hydrogen bonds of different intensities. Despite the use of sterically demanding ammonium, phosphonium and arsonium cations to obtain the separation of the paramagnetic Cu(II) centres for EPR spectroscopy no hyperfine structure was observed in the EPR spectra but the principal values of the electron Zeeman tensor, g(parallel to) and g(perpendicular to), could be determined. With these EPR data and the crystallographic parameters we were able to carry out a correlation study to anticipate the structural situation of tetrachloridocuprates in different physical states. This correlation is in good agreement with DFT calculations.

We present and discuss the results of crystallographic and electron paramagnetic resonance (EPR) spectroscopic analyses of five tetrachloridocuprate(II) complexes to supply a useful tool for the structural characterisation of the [CuCl4]2− moiety in the liquid state, for example in ionic liquids, or in solution. Bis(benzyltriethylammonium)-, bis(trimethylphenylammonium)-, bis(ethyltriphenylphosphonium)-, bis(benzyltriphenylphosphonium)-, and bis(tetraphenylarsonium)tetrachloridocuprate(II) were synthesised and characterised by elemental, IR, EPR and X-ray analyses. The results of the crystallographic analyses show distorted tetrahedral coordination geometry of all [CuCl4]2− anions in the five complexes and prove that all investigated complexes are stabilised by hydrogen bonds of different intensities. Despite the use of sterically demanding ammonium, phosphonium and arsonium cations to obtain the separation of the paramagnetic Cu(II) centres for EPR spectroscopy no hyperfine structure was observed in the EPR spectra but the principal values of the electron Zeeman tensor, g∥ and g⊥, could be determined. With these EPR data and the crystallographic parameters we were able to carry out a correlation study to anticipate the structural situation of tetrachloridocuprates in different physical states. This correlation is in good agreement with DFT calculations.

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

In this paper we report time-dependent configuration interaction singles calculations modeling the laser- induced current through a metal-insulator-metal (MIM) contact. We compare our results to recent experiments [D. Diesing, M. Merschdorf, A. Thon, and W. Pfeiffer, Appl. Phys. B (to be published)]. We use two jellium slabs separated by a vacuum region in a one-dimensional model to describe the MIM contact. The contact is coupled to ultrashort (fs) laser pulses by the semiclassical dipole approximation. We discuss simulated two-pulse correlation spectra in comparison to experimental results

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

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

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.

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.

Ultrafast electronic excitations of small sodium clusters and the onset of electron thermalization
(2009)

In this paper we report simulations of the ultrafast laser excitation and relaxation of the correlated valence electrons of a Na-8 cluster. The aim is twofold: first, while the total energy stays constant when the exciting laser pulse is over, we observe that the entropy computed from the reduced one electron density matrix rises on a much longer time scale. We discuss whether this can be understood as the onset of the thermalization of a finite system. Second, we describe this process with eight different methods of wavefunction-based electronic structure theory, which have been adapted for an explicitly time-dependent description. Their respective advantages and limitations for the simulation of the excitation and subsequent relaxation are explained.

The incongruous solvation of polyphosphides and phosphanes or the direct reduction of white phosphorus in liquid ammonia leads to the hydrogen polyphosphides catena-dihydrogen triphosphide, P3H23-, and catena-trihydrogen triphosphide, P3H32-, in the crystalline compounds K-3(P3H2)center dot 2.3NH(3) (1), Rb-3(P3H2)center dot NH3 (2), [Rb(18-crown-6)](2)(P3H3)center dot 7.5NH(3) (3), and [Cs(18-crown-6)](2)(P3H3)center dot 7NH(3) (4).

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.

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.

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.

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.

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.

Stochastic approach to laser-induced ultrafast dynamics : the desorption of H-2/D-2 from Ru(0001)
(2010)

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.

Hot localised charge carriers on the Si(111)-7×7 surface are modelled by small charged clusters. Such resonances induce non-local desorption, i.e. more than 10 nm away from the injection site, of chlorobenzene in scanning tunnelling microscope experiments. We used such a cluster model to characterise resonance localisation and vibrational activation for positive and negative resonances recently. In this work, we investigate to which extent the model depends on details of the used cluster or quantum chemistry methods and try to identify the smallest possible cluster suitable for a description of the neutral surface and the ion resonances. Furthermore, a detailed analysis for different chemisorption orientations is performed. While some properties, as estimates of the resonance energy or absolute values for atomic changes, show such a dependency, the main findings are very robust with respect to changes in the model and/or the chemisorption geometry.