Development of a Hyperthermal State-selected Molecular Oxygen Beam and Its Application to the Study of O 2 Adsorption on Pt(111)

A single spin-rotational state-selected [(J,M)=(2,2)] O2 beam allows us to conduct a spinand alignment -controlled O2 chemisorption experiment. We have recently expanded its available translational energy range to 0.10.9 eV. In this study, the beam has been used for the analysis of O2 chemisorption on Pt(111). Although this system has been investigated intensively due to its technological importance, the origin of the low O2 sticking probability and its unusual energy dependence has remained unclear. The present results indicate that, at low translational energy (E0) conditions, direct activated chemisorption occurs only when the O2 axis is nearly parallel to the surface. At high energy conditions (E0>0.5 eV), however, the sticking probability for the parallel O2 decreases with E0 while that of the perpendicular O2 increases, accounting for the nearly energy-independent O2 sticking probability determined previously by a randomly oriented O2 beam.