Femtosecond laser-induced diffusion and desorption of CO adsorbed on a weak electron-phonon coupling surface: Cu(110).

Abstract

In this work, we perform molecular dynamics (MD) simulations of CO molecules chemisorbed on Cu(110) under femtosecond laser irradiation. We use the two temperature model and a previously developed potential energy surface based on density functional theory total energies (obtained using the nonlocal vdW-DF2 functional) and parameterized using artificial neural networks. We find that laser irradiation induces diffusion parallel to [1̄10] much more efficiently than parallel to [001] due to a significantly smaller energy barrier in the former case (i.e., 0.12 vs 0.49 eV). We also observe photoinduced desorption (an endothermic process characterized by ΔE = 0.6 eV) with a probability that exhibits a power law dependence with laser fluence. At the lowest fluence studied (F = 30 J m-2), for which experimental data are available, the theoretical photoinduced diffusion probabilities both parallel and perpendicular to [1̄10] agree with the measured values, whereas our calculations predict desorption probabilities smaller than those obtained in experiments. Our MD simulations show that (i) the energy exchange with the hot electron bath is the main responsible for photoinduced processes and (ii) phonons tend to reduce the kinetic energy of the adsorbate, as keeping fixed the position of the Cu atoms during the simulations (thereby quenching CO-phonon energy exchange) significantly increases CO diffusion and desorption probabilities. Thus, our study advances the understanding of ultrafast surface dynamics on metal surfaces with weak electron-phonon coupling, and we hope that it will motivate further experimental investigations.