The detailed dynamics of a H atom on the (111) surface of Ni were investigated. The analysis was based upon an embedded diatomics molecular many-body potential surface, which permitted a consistent evaluation to be made of various H pathways in the Ni host. It was found that the pathway to sub-surface/surface H recombination involved the crossing of a potential energy barrier. Due to the low mass of H, the main reaction route at low temperatures involved tunnelling. A critical evaluation of tunnelling dynamics, in a many-body environment, was carried out using a full quantum description. The activated transport of sub-surface H to a surface site was studied in detail. It was shown that the tunnelling dynamics were dominated by the correlated motion of H and Ni. A fully correlated quantum-dynamic description for a multi-mode environment was developed. A so-called surrogate Hamiltonian method was used to represent the effect of the Ni host upon H dynamics in terms of a set of 2-level systems. This analysis showed that the environment could enhance or suppress the tunnelling rate by several orders of magnitude. For the case of H in a Ni host, the net effect was expected to be the suppression of tunnelling, as compared with the frozen-lattice approximation. The effect of non-adiabatic interactions with electron-hole pairs, upon the H tunnelling rate, was also studied. It was predicted that there would be a slight suppression; depending upon the electron density of Ni.

R.Baer, Y.Zeiri, R.Kosloff: Physical Review B, 1997, 55[16], 10952-74