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- surface chemistry (2)
- dissipative dynamics (1)
- isotope effects (1)
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- photochemistry (1)
- quantum control (1)
- quantum dynamics (1)
- ultrafast reactions (1)

#### Institute

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.

In this contribution, we study using first principles the co-adsorption and catalytic behaviors of CO and O2 on a single gold atom deposited at defective magnesium oxide surfaces. Using cluster models and point charge embedding within a density functional theory framework, we simulate the CO oxidation reaction for Au1 on differently charged oxygen vacancies of MgO(001) to rationalize its experimentally observed lack of catalytic activity. Our results show that: (1) co-adsorption is weakly supported at F0 and F2+ defects but not at F1+ sites, (2) electron redistribution from the F0 vacancy via the Au1 cluster to the adsorbed molecular oxygen weakens the O2 bond, as required for a sustainable catalytic cycle, (3) a metastable carbonate intermediate can form on defects of the F0 type, (4) only a small activation barrier exists for the highly favorable dissociation of CO2 from F0, and (5) the moderate adsorption energy of the gold atom on the F0 defect cannot prevent insertion of molecular oxygen inside the defect. Due to the lack of protection of the color centers, the surface becomes invariably repaired by the surrounding oxygen and the catalytic cycle is irreversibly broken in the first oxidation step.

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.

In the present work, ideas for controlling photochemical reactions in dissipative environments using shaped laser pulses are presented. New time-local control algorithms for the stochastic Schrodinger equation are introduced and compared to their reduced density matrix analog. The numerical schemes rely on time-dependent targets for guiding the reaction along a preferred path. The methods are tested on the vibrational control of adsorbates at metallic surfaces and on the ultrafast electron dynamics in a strong dissipative medium. The selective excitation of the specific states is achieved with improved yield when using the new algorithms. Both methods exhibit similar convergence behavior and results compare well with those obtained using local optimal control for the reduced density matrix. The favorable scaling of the methods allows to tackle larger systems and to control photochemical reactions in dissipative media of molecules with many more degrees of freedom.

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.

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.

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.

In this work, we present theoretical simulations of laser-driven vibrational control of NO adsorbed on a gold surface. Our goal is to tailor laser pulses to selectively excite specific modes and vibrational eigenstates, as well as to favor photodesorption of the adsorbed molecule. To this end, various control schemes and algorithms are applied. For adsorbates at metallic surfaces, the creation of electron hole pairs in the substrate is known to play a dominant role in the transfer of energy from the system to the surroundings. These nonadiabatic couplings are included perturbatively in our reduced density matrix simulations using a generalization of the state-resolved position-dependent anharmonic rate model we recently introduced. An extension of the reduced density matrix is also proposed to provide a sound model for photodesorption in dissipative systems.

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.