INTERACTION OF THERMAL H-ATOMS WITH NI(100)-H SURFACES - THROUGH SURFACE PENETRATION AND ADSORBED HYDROGEN ABSTRACTION

Considerable quantities, > 3 monolayers, of hydrogen (deuterium) were absorbed in subsurface sites at Ni(100) surfaces via impact of thermal H (D) atoms. Competitive to a direct transition (penetration) into the bulk with an initial probability of 4.5 X 10(-2) impinging H (D) atoms abstract adsorbed D (H) with an initial abstraction probability R = 0.25 and cross section of 1.6 Angstrom(2). The filling of subsurface sites obeys a Langmuir-type rate law. An adsorbed H (D) monolayer does not affect the probability for penetration into the bulk. Above 150 K subsurface H (D) move to the surface and either recombine with adsorbed H (D) followed by recombinative desorption or get adsorbed at the surface if they encounter an empty site. The desorption kinetics from subsurface sites is in quantitative agreement with thermodynamic bulk Ni/H data. Isotope effects were neither observed for bulk penetration nor for abstraction.