Dynamics of O Chemisorption on a Flat Platinum Surface Probed by an Alignment-Controlled O Beam.

O adsorption on Pt surfaces is of great technological importance owing to its relevance to reactions for the purification of car exhaust gas and the oxygen reduction on fuel-cell electrodes. Although the O /Pt(111) system has been investigated intensively, questions still remain concerning the origin of the low O sticking probability and its unusual energy dependence. We herein clarify the alignment dependence of the initial sticking probability (S ) using the single spin-rotational state-selected [(J,M)=(2,2)] O beam. The results indicate that, at low translational energy (E ) conditions, direct activated chemisorption occurs only when the O axis is nearly parallel to the surface. At high energy conditions (E >0.5 eV), however, S for the parallel O decreases with increasing E while that of the perpendicular O increases, accounting for the nearly energy-independent O sticking probability determined previously by a non-state-resolved experiment.