Femtosecond-laser induced dynamics of CO on Ru(0001): Deep insights from a hot-electron friction model including surface motion

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 × 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.

Strong Anisotropic Interaction Controls Unusual Sticking and Scattering of CO at Ru(0001)

“…Details on the PES are reported in Refs. [31,32], but we mention that DFT calculations were carried out using the RPBE functional [34] with D2 [35] van der Waals correction, which yields E ads = 1.69 eV, close to the experimental value. In our calculations, we also allow for the possibility of energy transfer from the molecule to the surface via (i) excitation of surface atom motion and (ii) electron-hole pair excitations at the surface region.…”

Femtosecond laser induced desorption of H2,D2 , and HD from Ru(0001): Dynamical promotion and suppression studied with ab initio molecular dynamics with electronic friction

“…The Ru parameters entering the 2TM are those of Refs. [9,11,38] and are summarized in the caption of Fig. 1.…”

“…Electronic temperature calculated from the 2TM using the implementation and the same material parameters of Refs [9,11,38]. for Ru(0001) and a Gaussian laser pulse of 800 nm wavelength, 130 fs FWHM, and fluence F = 60 J/m 2 .…”

Energy loss and surface temperature effects in ab initio molecular dynamics simulations: N adsorption on Ag(111) as a case study

“…Still, there are processes also occurring at this time scale for which the use of thermostats can be relevant as, for example, reactions induced by femtosecond laser pulses. In this respect, it has recently been shown that the desorption of O 2 from Ag(110) [52,53] and, in particular, the desorption of CO from Ru(0001) [54] is induced to a great extent by the hot surface lattice created by the laser. Thus, we anticipate that the use of thermostats together with the AIMDEF scheme, which is also adapted for describing time-dependent electronic temperatures [55], can be a powerful tool to simulate and understand surface femtochemistry.…”

“…The random force R[T e ,η i (r i )] (third term) describes the effect of e-h pair excitations and deexcitations for nonzero electronic temperatures T e and it is commonly approximated by a Gaussian white noise of variance Var[R(T e ,η i )] = (2k B T e η i )/ t, where k B is the Boltzmann constant and t is the integration time step. The R(T e ,η i ) term is crucial when describing desorption processes induced by femtosecond laser pulses [47][48][49][50][51][52][53][54][55], but it can safely be neglected in simulations of common molecular beam experiments, which are usually performed at surface temperatures of a few hundred Kelvin for which the e-h pair distribution hardly deviates from its ground state at 0 K. Regarding the surface atoms, the constant surface temperature conditions are simulated by thermalizing all or part of the mobile surface atoms (see below) with the Nosé-Hoover thermostat [43,44]. In the Nosé-Hoover thermostat, the real system is coupled to a dimensionless dynamical variable s that together with its conjugate momentum p s act as a thermal bath, i.e., ensure a canonical distribution in the real system.…”