INTERMEDIATE STATES IN THE ADSORPTION OF MO ON IR(111) - SUBSTRATE-TEMPERATURE EFFECTS

Measurements of the initial adsorption probability, S-0, of nitric oxi de (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 ki netic 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 independ ent of surface temperature. We propose a trapping-mediated mechanism f or 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 s trongly bound, molecularly chemisorbed state. At high kinetic energies , we propose that adsorption initially occurs directly into the molecu lar chemisorption well. Indeed, electron energy loss spectroscopy meas urements show no evidence for direct dissociation at a low T-s and ind icate that the surface temperature must exceed similar to 400 K for di ssociative chemisorption to occur. The probability of dissociative che misorption of NO on Ir(111) decreases with increasing temperature (in the range 500-900 K in this study) at all kinetic energies investigate d. Here, we propose a model for low kinetic energies that includes bot h a physisorbed species and a molecularly chemisorbed species as precu rsors to dissociation. For the high energy regime, the trapping probab ility into the physically adsorbed state is assumed to be zero, and th us, we model the adsorption occurring directly as a molecularly chemis orbed intermediate with subsequent dissociation at elevated temperatur es. The success of the model is demonstrated by the agreement of kinet ic parameters determined separately for data employing a high kinetic energy beam and a low energy beam. (C) 1996 American Vacuum Society.