Energy loss in gas-surface dynamics: Electron–hole pair and phonon excitation upon adsorbate relaxation

We study the effect of electron and phonon degrees of freedom on the relaxation dynamics of adsorption processes in gas-surface systems by using ab initio molecular dynamics that incorporates an electronic friction force (AIMDEF). As representative cases we have chosen three systems with different adsorption energies and adsorbate-to-surface atom mass ratios: H on Pd(1 0 0), N on Ag(1 1 1), and N2N2 on Fe(1 1 0). We show, through inspection of the total energies and trajectories of the hot adsorbates on the surface, that electron–hole (e–h) pair excitations dominate relaxation of the light gas species, while the phonon channel is dominant for the heavy species. In the latter case e–h pairs become more important at the final thermalization stages.