Molecular Beam Investigations of Surface Chemical Reactions and Dynamics

Citation

Mullins, Charles Buddie (1990) Molecular Beam Investigations of Surface Chemical Reactions and Dynamics. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/64s4-0182. https://resolver.caltech.edu/CaltechETD:etd-03122007-135158

Abstract

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 S_o was studied as a function of incident kinetic energy E_i, incident angle θ_i and surface temperature T_s. 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 ζ_o 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 E_icos^0.5θ_i. The initial dissociative chemisorption of ethane on Ir(110)-(1x2) occurs via a trapping-mediated mechanism at low E_i and a direct mechanism at high kinetic energies. In the trapping-mediated regime S_o decreases rapidly with increasing T_s. 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 E_d-E_c 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 T_s = 80, 190 and 273 K. The trapping probability decreases with increasing E_i in a manner that depends on both θ_i and T_s. The angular scaling law governing the trapping is a function of T_s such that ζ_o scales with E_icos^1.5θ_i at 80 K, E_icos^1.0θ_i at 190 K and E_icos^0.5θ_i at 273 K. These results suggest that parallel momentum dissipation becomes increasingly more important to the trapping dynamics as the surface temperature is increased.

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