Molecular dynamics of nonadiabatic processes at surfaces: Chemisorption of H/Al(1 1 1)

Time-dependent density functional theory for the electronic degrees of freedom has been combined with Ehrenfest dynamics for the nuclei to simulate electron–hole pair excitation due to electronic friction during the chemisorption of hydrogen atoms on an Al(1 1 1) surface. The H-atoms are assumed to be spin-unpolarized in the simulations. Trajectories starting with a hydrogen atom at rest above either the on-top or the fcc-hollow site evolve in qualitatively very different ways: at the fcc-hollow position the H-atom acquires sufficient kinetic energy in the chemisorption well to penetrate into the Al-substrate, thereby increasing the coupling of the motion of the H-atom to the substrate electrons. The electronic excitation spectra, however, are roughly characterized by an exponential decay with similar fictitious temperature parameters of the order of 103 K for both kinds of trajectories. The energy dissipation into electron–hole pairs and the nonadiabatic contribution to the force acting on the hydrogen atom have been calculated along the trajectories.