Dynamics of metal electron excitation in atom-surface collisions: A quantum wave packet approach

Electron-hole pair excitations upon atom impact on a metal surface are studied in a framework of a one-dimensional independent-electron model. The method employed treats electron dynamics quantum mechanically and the atom motion classically, and the two are coupled through the time-dependent self-consistent field (TDSCF) approximation. A variational method is used to calculate the time evolution of the electronic wave packet. Calculations were carried out for the colliders. He, Ar and H; the surface parameters were chosen to model Li. Some of the results obtained are: (1) Electron excitation by H is much more efficient than for a rare-gas collider. Experimental search for hole-pair excitations should thus be best pursued with H as a collider. (2) At 0 K surface temperature ΔE/E, the fraction of collision energy converted to hole-pair excitations, decreases as the collision energy increases for energies up to ≈ 1 eV. At collision energy E = 0.01 eV, the fraction of energy transferred is ≈ 0.2% for He and ≈ 10% for H. (3) Atom trapping due to energy transfer to electrons occurs with high probability (50-100%) at sufficiently low collision energies. Ar trapping takes place at energies below 1 K and H trapping below 20 K. (4) The calculations show a pronounced transition from atom de-excitation to atom excitation by electron-hole pairs as surface temperature increases. (5) Perturbation theory is tested against the present method. It breaks down mainly for trapping and for temperature effects.