Experimental results from molecular beam investigations of trapping and dissociative chemisorption phenomena for several gas-surface systems are presented. The dissociative chemisorption of oxygen on Ir(110)-(1x2) in the limit of zero coverage So was studied as a function of incident kinetic energy Ei, incident angle θi and surface temperature Ts. Results from this investigation indicate that two mechanisms account for the initial chemisorption. At low incident kinetic energy (less than 4 kcal/mol) chemisorption mediated by trapping is primarily responsible for the dissociative adsorption while at high energies a direct mechanism can account for the results. In both energy ranges the initial dissociative chemisorption probability is insensitive to incident angle.
The trapping of molecular ethane as well as the dissociative chemisorption of ethane on the clean Ir(110)-(1x2) surface has also been investigated. The initial trapping probability ζo is found to decrease with incident kinetic energy from a value of ~0.98 at 1 kcal/mol to ~0.1 at 16 kcal/mol. These data scale with Eicos0.5θi. The initial dissociative chemisorption of ethane on Ir(110)-(1x2) occurs via a trapping-mediated mechanism at low Ei and a direct mechanism at high kinetic energies. In the trapping-mediated regime So decreases rapidly with increasing Ts. These data quantitatively support a kinetic model consistent with a trapping-mediated chemisorption mechanism. The difference in the activation energies for desorption and chemisorption from the physically adsorbed, trapped state Ed-Ec is 2.2±0.2 kcal/mol. Chemisorption at high kinetic energies, in the direct regime, is independent of surface temperature.
Additionally, the trapping probability of Ar on Pt(111) has been measured as a function of incident kinetic energy, and angle for Ts = 80, 190 and 273 K. The trapping probability decreases with increasing Ei in a manner that depends on both θi and Ts. The angular scaling law governing the trapping is a function of Ts such that ζo scales with Eicos1.5θi at 80 K, Eicos1.0θi at 190 K and Eicos0.5θi at 273 K. These results suggest that parallel momentum dissipation becomes increasingly more important to the trapping dynamics as the surface temperature is increased.
