Structure and gas adsorption kinetics for monocrystalline surfaces studied with low energy ion scattering

Calculations of blocking cone sizes for the “blocking geometry” of the scattering and recoiling imaging spectrometry (SARIS) technique have been performed and a universal formula for calculating the blocking cone size for arbitrary energies and interacting species has been derived. The results obtained from the formula are compared with experimental SARIS blocking cone data for He+ and Ne+ scattering from a Pt(111) surface in the energy range 3–20 keV. The blocking cones in this low energy range are appreciably asymmetric with respect to the interatomic axis, with differences reaching as much as 15%. Energy distributions of scattered Kr+ and fast recoiled Pt atoms from a Pt(111) surface were measured as a function of exit angle. By using classical trajectory simulations, it is shown that SARIS mapping allows one to probe the kinematics of both scattering and recoiling events, the probability for their occurrence in specific trajectories, and their detection probabilities. The kinetics of O₂ chemisorption at low dose on Ni(111) and the nature of the chemisorption site have been studied at 300 and 500 K using time-of-flight scattering and recoiling spectrometry (TOF-SARS), LEED, and scattering and recoiling imaging code (SARIC) simulations. Variations in the TOF-SARS spectra with different crystal alignments during O₂ dosing provided direct information on the location of oxygen adatoms on the Ni(111) surface at very low coverages as well as site-specific occupation rates and occupancies. The results identify three chemisorption stages as a function of oxygen exposure. The kinetics of isothermal adsorption and migration of atomic hydrogen on a Si(100) surface has been investigated by the TOF-SARS technique. A continuous decrease in saturation coverage with temperature under constant atomic hydrogen exposure has been observed for temperatures in the range 325–750 K. For T_S = 500–650 K, the decrease is described by a kinetic model in which the surface concentration of physisorbed hydrogen atoms is depleted due to the increased rate of migration from precursor sites to primary monohydride sites. According to the kinetic model introduced, the migration constant obeys the Arrhenius equation with low activation energy of 0.71 eV.