The kinetics of the interaction of hydrogen with oxygen adsorbed on Cu(ll1) up to 0.5 monolayer is studied with ellipsometry.It is shown that hydrogen dissociates at Cu sites and that an important step is the formation of OH,,,, from OcB,,, and H,,,, with an activation energy of 93 f 12 kJ/mol. The operation of an ionization gauge changes the reaction mechanism in the sense that it facilitates hydrogen dissociation.In this case the overall activation energy is only 5 kJ/mol.

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