Intermediate states in the adsorption of MO on Ir(111): Substrate temperature effects

Measurements of the initial adsorption probability, S-0, of nitric oxide (NO) on Ir(111) as a function of incident kinetic energy, E(i), and surface temperature, T-s, are presented. We observe a decrease in S-0 with increasing kinetic energy, E(i), from 0.052 to 1.3 eV. At low kinetic energies, the initial molecular adsorption probability decreases with increasing surface temperature (ranging between 77 and 300 K in this study), while at high kinetic energies, this quantity is independent of surface temperature. We propose a trapping-mediated mechanism for adsorption at low kinetic energies. In this low energy regime, the surface temperature dependence reflects a kinetic competition between desorption from a physically adsorbed state and conversion to a more strongly bound, molecularly chemisorbed state. At high kinetic energies, we propose that adsorption initially occurs directly into the molecular chemisorption well. Indeed, electron energy loss spectroscopy measurements show no evidence for direct dissociation at a low T-s and indicate that the surface temperature must exceed similar to 400 K for dissociative chemisorption to occur. The probability of dissociative chemisorption of NO on Ir(111) decreases with increasing temperature (in the range 500-900 K in this study) at all kinetic energies investigated. Here, we propose a model for low kinetic energies that includes both a physisorbed species and a molecularly chemisorbed species as precursors to dissociation. For the high energy regime, the trapping probability into the physically adsorbed state is assumed to be zero, and thus, we model the adsorption occurring directly as a molecularly chemisorbed intermediate with subsequent dissociation at elevated temperatures. The success of the model is demonstrated by the agreement of kinetic parameters determined separately for data employing a high kinetic energy beam and a low energy beam. (C) 1996 American Vacuum Society.