Adsorption mechanisms of translationally-energetic O2 and N2: direct dissociation versus direct molecular chemisorption

A direct dissociation mechanism has been traditionally assigned to molecular beam data that exhibit an increase in the initial adsorption probability with increasing kinetic energy. Yet, recent experiments of nitrogen and oxygen adsorption provide support for an alternative high kinetic energy pathway in which incident energy assists in surmounting barriers to molecular chemisorption on a surface as the first step to dissociation. Moreover, systems for which the experimental evidence supports such a mechanism also demonstrate that molecularly chemisorbed intermediates can be spectroscopically observed at low temperatures and coverages from exposure to a gas in thermal equilibrium at room temperature. Likewise, such observations have not been measured for systems which are consistent with direct dissociation. A consideration of this trend regarding the existence of molecularly chemisorbed states and the implications for the dominant, dissociative chemisorption pathway at high kinetic energy is presented for a number of gas surface systems.