Application of hyperthermal spin- and alignment-controlled O₂ beam to surface reaction analysis

Clarification of reactivity of species contained in plasma to solid surfaces is considered necessary for understanding the plasma-surface interaction. Concerning the molecular species with kinetic energies of 10 meV -a few eV, their interaction with surfaces has been investigated intensively due to its importance in thermal gas-surface reactions. Molecular beams with well-defined kinetic energies and internal states have often been used for such purpose. O₂ is involved in many technologically important processes such as heterogeneously catalyzed reactions, corrosion, and semiconductor processes. Since O₂ is a linear molecule with an electron spin, the alignment of the molecular axis and/or the spin state during its collision with surfaces need to be considered for understanding the reaction mechanism. A spin-rotational state-selected O₂ beam developed by us permits the experimental clarification of such effects [1]. We have recently applied this technique to the study of O₂ chemisorption on flat and stepped Pt surfaces, which is technologically important as the initial step of O₂ reduction reaction in fuel cell and catalytic oxidative purification of car exhaust gas. Our studies have clarified that the activation energy for O₂ chemisorption depends strongly on its alignment relative to the surface; molecules with its axis nearly parallel to the surface mainly contribute the O₂ chemisorption and subsequent CO oxidation reaction at kinetic energy < 0.2 eV, but O₂ molecules with unfavorable alignment also react at high energy conditions. Catalytic CO oxidation reaction induced by atomic oxygen will also be presented for comparison. [1] M. Kurahashi, Prog. Surf. Sci., 91, 29 (2016).