Theory of Hydrogen Quantum Diffusion

Doctoral thesis, 1996

Atomic hydrogen adsorbed on a metal surface is one of the simplest possible examples of chemisorption, yet it is very challenging. The adsorption and the very elementary process of surface diffusion, a single atomic jump, have been investigated theoretically using several methods. The system that has been studied is hydrogen on the (001) surface of nickel, a system which has been subject to extensive research in the past, experimental as well as theoretical. The adsorption geometry and potential energy surface have been calculated using a pseudopotential plane-wave method, which is a first-principles approach to the problem. The potential energy surface hereby obtained has been exploited to develop a model potential which accurately reproduces the first-principles data. The model potential has in turn been employed to address the following problems, which are still impossible to treat using first-principles methods. The vibrational excitation energies and bandwidths for hydrogen and deuterium, by solving the Schrödinger equation. The relaxation energies by self-consistently relaxing the surface and solving the Schrödinger equation for hydrogen and deuterium on it. The diffusion coefficients for hydrogen and deuterium at different temperatures using the path-centroid formulation of quantum transition-state theory. The isotope effect in hydrogen surface diffusion. The adsorption geometry and the vibrational excitation energies agree quantitatively with experimental data. The diffusion constant is consistent with experimental data at high temperature but not with the data at low temperature. Regarding the isotope effect we find no signs of unusual behavior, a result which is in disagreement with experimental findings. The experimental situation is, however, unclear. Very recent low temperature diffusion experiments seem strongly to contradict earlier results. This contradiction has made it even more important to perform quantitative calculations in the quantum tunneling regime.