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)($2\ifmmode\times\else\texttimes\fi{}2$):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.
Highlights
Femtosecond-laser (FL) induced reactions of molecules at metal surfaces are interesting for applications and for fundamental reasons [1,2]
In its most simple variant, the electronic damping can be derived from the substrate electron density in which the adsorbate atoms are embedded [the so-called local density friction approximation (LDFA) [17]], and stochastic forces arising from a time-dependent electronic temperature which is determined from the two-temperature model (2TM) [18]
In the following molecular dynamics with electronic friction (MDEF)(-generalized Langevin oscillator (GLO)) simulations, often a large number of trajectories were propagated over long time periods
Summary
Femtosecond-laser (FL) induced reactions of molecules at metal surfaces are interesting for applications and for fundamental reasons [1,2]. In its most simple variant, the electronic damping can be derived from the substrate electron density in which the adsorbate atoms are embedded [the so-called local density friction approximation (LDFA) [17]], and stochastic forces arising from a time-dependent electronic temperature which is determined from the two-temperature model (2TM) [18] This methodology has been successfully used elsewhere to describe FL induced desorption of diatomic molecules from metal surfaces [19,20,21]. The combined method, called MDEFGLO in what follows, includes electronic friction, hot-electron driven processes, surface motion, and, importantly, all (six) molecular degrees of freedom of CO relative to the surface as dynamical variables To account for the latter, we make use of a recently developed ground-state potential energy surface (PES) which is based on gradient-corrected density functional theory.
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