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.