Dynamics of the interaction of ethane with Ir(110)-(1×2)

Abstract

Experimentally determined values of the initial adsorption probability of ethane on Ir(110)-(1×2) are presented which probe the dynamics of the interaction. The data were obtained from supersonic molecular beam measurements with an incident kinetic energy Ei ranging between 1.2 and 24 kcal/mol, surface temperatures TS between 77 and 550 K, and incident angle θi between 0° and 45°. Experimentally determined values of the initial trapping probability ζ0 of ethane into a physically adsorbed state at TS=77 K as a function of Ei and θi and experimentally determined values of the initial probability of dissociative chemisorption S0 as a function of Ei,θi, and TS are presented. The value of ζ0 is found to decrease with increasing Ei consistent 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 θi such that the value of ζ0 is found empirically to scale as Ei cos0.5 θi. Two distinct mechanisms of dissociative chemisorption on the bare surface are revealed. At low Ei a temperature-dependent trapping-mediated chemisorption mechanism dominates, while at relatively high Ei a temperature-independent direct mechanism dominates. For Ei less than 13.4 kcal/mol, the value of S0 decreases rapidly with increasing TS, consistent with a trapping-mediated mechanism. For a surface temperature of 154 K, S0 decreases with increasing Ei for 1.2≤Ei≤13.4 kcal/mol, in a manner similar to that for the molecular trapping probability. The data in the low Ei regime 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−Ec is 2.2±0.2 kcal/mol. In the trapping-mediated chemisorption regime, the value of S0 is found to be rather insensitive to incident angle, scaling with Ei cos0.5 θi just as for trapping of molecular ethane into a physically adsorbed state. For a normal energy Ei cos2 θi greater than 8 kcal/mol, chemisorption via a direct mechanism becomes significant and increases with increasing Ei. Values of S0 in the direct chemisorption regime scale with normal energy and are independent of TS over the range from 350 to 1350 K.