@article{TremblayFuechselSaalfrank2012, author = {Tremblay, Jean Christophe and F{\"u}chsel, Gernot and Saalfrank, Peter}, title = {Excitation, relaxation, and quantum diffusion of CO on copper}, series = {Physical review : B, Condensed matter and materials physics}, volume = {86}, journal = {Physical review : B, Condensed matter and materials physics}, number = {4}, publisher = {American Physical Society}, address = {College Park}, issn = {1098-0121}, doi = {10.1103/PhysRevB.86.045438}, pages = {13}, year = {2012}, abstract = {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.}, language = {en} } @article{KuleszaTitovDalyetal.2016, author = {Kulesza, Alexander Jan and Titov, Evgenii and Daly, Steven and Wlodarczyk, Radoslaw and Megow, J{\"o}rg and Saalfrank, Peter and Choi, Chang Min and MacAleese, Luke and Antoine, Rodolphe and Dugourd, Philippe}, title = {Excited States of Xanthene Analogues: Photofragmentation and Calculations by CC2 and Time-Dependent Density Functional Theory}, series = {ChemPhysChem : a European journal of chemical physics and physical chemistry}, volume = {17}, journal = {ChemPhysChem : a European journal of chemical physics and physical chemistry}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1439-4235}, doi = {10.1002/cphc.201600650}, pages = {3129 -- 3138}, year = {2016}, abstract = {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.}, language = {en} } @article{TitovSaalfrank2016, author = {Titov, Evgenii and Saalfrank, Peter}, title = {Exciton Splitting of Adsorbed and Free 4-Nitroazobenzene Dimers: A Quantum Chemical Study}, series = {The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment \& general theory}, volume = {120}, journal = {The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment \& general theory}, publisher = {American Chemical Society}, address = {Washington}, issn = {1089-5639}, doi = {10.1021/acs.jpca.5b10376}, pages = {3055 -- 3070}, year = {2016}, abstract = {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.}, language = {en} } @article{KirschWirthTongetal.2014, author = {Kirsch, Harald and Wirth, Jonas and Tong, Yujin and Wolf, Martin and Saalfrank, Peter and Campen, Richard Kramer}, title = {Experimental characterization of unimolecular water dissociative adsorption on alpha-alumina}, series = {The journal of physical chemistry : C, Nanomaterials and interfaces}, volume = {118}, journal = {The journal of physical chemistry : C, Nanomaterials and interfaces}, number = {25}, publisher = {American Chemical Society}, address = {Washington}, issn = {1932-7447}, doi = {10.1021/jp502106t}, pages = {13623 -- 13630}, year = {2014}, abstract = {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.}, language = {en} } @article{LoncaricAlducinSaalfranketal.2016, author = {Loncaric, Ivor and Alducin, Maite and Saalfrank, Peter and Inaki Juaristi, J.}, title = {Femtosecond laser pulse induced desorption: A molecular dynamics simulation}, series = {Nature climate change}, volume = {382}, journal = {Nature climate change}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0168-583X}, doi = {10.1016/j.nimb.2016.02.051}, pages = {114 -- 118}, year = {2016}, abstract = {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.}, language = {en} } @article{WeigelDobryakovKlaumuenzeretal.2011, author = {Weigel, A. and Dobryakov, A. and Klaum{\"u}nzer, Bastian and Sajadi, M. and Saalfrank, Peter and Ernsting, N. P.}, title = {Femtosecond stimulated raman spectroscopy of flavin after optical excitation}, series = {The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces \& biophysical chemistry}, volume = {115}, journal = {The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces \& biophysical chemistry}, number = {13}, publisher = {American Chemical Society}, address = {Washington}, issn = {1520-6106}, doi = {10.1021/jp1117129}, pages = {3656 -- 3680}, year = {2011}, abstract = {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.}, language = {en} } @article{VazhappillyKlamrothSaalfranketal.2009, author = {Vazhappilly, Tijo and Klamroth, Tillmann and Saalfrank, Peter and Hernandez, Rigoberto}, title = {Femtosecond-laser desorption of H-2 (D-2) from Ru(0001) : quantum and classical approaches}, issn = {1932-7447}, doi = {10.1021/Jp810709k}, year = {2009}, abstract = {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.}, language = {en} } @article{ScholzFlossSaalfranketal.2016, author = {Scholz, Robert and Floss, Gereon and Saalfrank, Peter and F{\"u}chsel, Gernot and Loncaric, Ivor and Juaristi, J. I.}, title = {Femtosecond-laser induced dynamics of CO on Ru(0001): Deep insights from a hot-electron friction model including surface motion}, series = {Physical review : B, Condensed matter and materials physics}, volume = {94}, journal = {Physical review : B, Condensed matter and materials physics}, publisher = {American Physical Society}, address = {College Park}, issn = {2469-9950}, doi = {10.1103/PhysRevB.94.165447}, pages = {17}, year = {2016}, abstract = {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.}, language = {en} } @article{LoncaricAlducinSaalfranketal.2016, author = {Loncaric, Ivor and Alducin, Maite and Saalfrank, Peter and Juaristi, J. I.}, title = {Femtosecond-laser-driven molecular dynamics on surfaces: Photodesorption of molecular oxygen from Ag(110)}, series = {Physical review : B, Condensed matter and materials physics}, volume = {93}, journal = {Physical review : B, Condensed matter and materials physics}, publisher = {American Physical Society}, address = {College Park}, issn = {1098-0121}, doi = {10.1103/PhysRevB.93.014301}, pages = {9}, year = {2016}, abstract = {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.}, language = {en} } @article{WirthSchachtSaalfranketal.2016, author = {Wirth, Jonas and Schacht, Julia and Saalfrank, Peter and Paulus, Beate}, title = {Fluorination of the Hydroxylated alpha-Al2O3 (0001) and Its Implications for Water Adsorption: A Theoretical Study}, series = {The journal of physical chemistry : C, Nanomaterials and interfaces}, volume = {120}, journal = {The journal of physical chemistry : C, Nanomaterials and interfaces}, publisher = {American Chemical Society}, address = {Washington}, issn = {1932-7447}, doi = {10.1021/acs.jpcc.5b10975}, pages = {9713 -- 9718}, year = {2016}, abstract = {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.}, language = {en} }