Hydrogen transport in nickel (111)

The intricate dynamics of hydrogen on a nickel (111) surface is investigated. The purpose is to understand the unique recombination reaction of subsurface with surface hydrogen on the nickel host. The analysis is based on the embedded diatomics in molecules many-body potential surface. This potential enables a consistent evaluation of different hydrogen pathways in the nickel host. It is found that the pathway to subsurface-surface hydrogen recombination involves crossing a potential-energy barrier. Due to the light mass of the hydrogen the primary reaction route at low temperature occurs via tunneling. A critical evaluation of tunneling dynamics in a many-body environment has been carried out, based on a fully quantum description. The activated transport of subsurface hydrogen to a surface site, the resurfacing event, has been studied in detail. It is shown that the tunneling dynamics is dominated by correlated motion of the hydrogen and the nickel hosts. A fully correlated quantum-dynamical description in a multimode environment was constructed and employed. The "surrogate Hamiltonian method" represents the nickel host effect on the hydrogen dynamics as that of a set of two-level systems. The spectral density, which is the input of the theory is obtained via classical molecular-dynamics simulations. The analysis then shows that the environment can both promote and hinder the tunneling rate by orders of magnitude. Specifically for hydrogen in the nickel host, the net effect is suppression of tunneling compared to a frozen lattice approximation. The added effect of nonadiabatic interactions with the electron-hole pairs on the hydrogen tunneling rate was studied by an appropriate "surrogate Hamiltonian" with the result of a small suppression depending on the electron density of nickel. The resurfacing rates together with surface recombination rates and relaxation rates were incorporated in a kinetic model describing thermal-desorption spectra. Conditions for a thermal-programmed-desorption peak which manifest the subsurface-surface hydrogen recombination were found.