Simulations of collision-induced absorption of hydrogen on Ni(111)

Experiments by Ceyer and co-workers [Faraday Discuss. Chem. Soc. 91, 437 (1991)] have demonstrated that hydrogen atoms adsorbed on the Ni(111) surface can be driven below the surface under the impact of a hyperthermal (> 2 eV) rare gas atom beam. We have modeled these experiments using classical molecular dynamics (MD) simulations, with the goal of elucidating the mechanism of this collision-induced absorption (CIA) process. The simulations favor a mechanism involving direct impact of the rare gas atom with an adsorbed hydrogen atom. The MD results are consistent with experiment in showing that the CIA efficiency increases with rare gas atomic mass for Ar, Kr, and Xe; interestingly, they predict a reversal of this trend when the rare gas is changed from Ne to He. These results are interpreted in terms of a crossover from a light collider regime of very efficient direct impulsive collisions to a massive collider regime of direct collisions strongly coupled to substrate dynamics and relaxation. The simulated CIA cross sections scaled approximately with normal incident collision energy, consistent with experiment. A hydrogen isotope effect, in which CIA was enhanced for deuterium with respect to hydrogen, was found in the simulations where none was observed experimentally. We show that this discrepancy may come from quantum effects, due to zero-point energies and to energy dissipation by electron-hole pair excitations, which tend to counteract and approximately cancel the isotopic difference observed in a purely classical simulation. (C) 1999 American Institute of Physics. [S0021-9606(99)70246-1].