A single spin-rotational state-selected [(
J,
M)=(2,2)] O
2 beam allows us to conduct a spin- and alignment -controlled O
2 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 O
2 chemisorption on Pt(111). Although this system has been investigated intensively due to its technological importance, the origin of the low O
2 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 O
2 axis is nearly parallel to the surface. At high energy conditions (
E0>0.5 eV), however, the sticking probability for the parallel O
2 decreases with
E0 while that of the perpendicular O
2 increases, accounting for the nearly energy-independent O
2 sticking probability determined previously by a randomly oriented O
2 beam.
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