Dynamics of the interaction of ethane with lr(110)-(1 x2)

Abstract

Experimentally determined values of the initial adsorption probability of ethane on Ir(110)-(1X2) are presented which probe the dynamics of the interaction. The data were obtained from supersonic molecular beam measurements with an incident kinetic energy Eiranging between 1.2 and 24 kcal/mol, surface temperatures Tsbetween 77 and 550 K, and incident angle θibetween 0° and 45°. Experimentally determined values of the initial trapping probability ξ0of ethane into a physically adsorbed state at Ts= 77 K as a function of Eiand θiand experimentally determined values of the initial probability of dissociative chemisorption S0as a function of Ei,θi,and Tsare presented. The value of ξ0is found to decrease with increasing Eiconsistent with the fact that an increasingly larger fraction of the incident kinetic energy must be dissipated in order for the molecule to physically adsorb. The initial trapping probability has a relatively weak dependence on θisuch that the value of ξ0is found empirically to scale as Eicos0.5θi. Two distinct mechanisms of dissociative chemisorption on the bare surface are revealed. At low Eia temperature-dependent trapping-mediated chemisorption mechanism dominates, while at relatively high Eia temperature-independent direct mechanism dominates. For Eiless than 13.4 kcal/mol, the value of S0decreases rapidly with increasing Ts, consistent with a trapping-mediated mechanism. For a surface temperature of 154 K, S0decreases with increasing Eifor 1.2≤Ei≤13.4 kcal/mol, in a manner similar to that for the molecular trapping probability. The data in the low Eiregime also support quantitatively 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— Ecis 2.2 ± 0.2 kcal/mol. In the trapping-mediated chemisorption regime, the value of S0is found to be rather insensitive to incident angle, scaling with Eicos0.5θijust as for trapping of molecular ethane into a physically adsorbed state. For a normal energy Eicos2θigreater than 8 kcal/mol, chemisorption via a direct mechanism becomes significant and increases with increasing Ei. Values of S0in the direct chemisorption regime scale with normal energy and are independent of Tsover the range from 350 to 1350 K. © 1990, American Vacuum Society. All rights reserved.