The dynamics of argon and methane trapping on Pt(111) at 30 and 50 K: energy scaling and coverage dependence

The dynamics of argon and methane trapping on clean Pt(111) were investigated using supersonic molecular beam techniques at a surface temperatures of 30 and 50 K, well below the desorption temperatures of 46 and 67 K, respectively. The initial trapping probabilities for both Ar and methane scale with normal incident energy (E(T)cos(2) theta), indicating a smooth gas-surface potential. The trapping probability for Ar decreases from 0.8 to zero as the incident normal energy is increased from 2 to 29 kJ mol(-1). Below 10 kJ mol(-1) incident energy, the results agree with previous experiments [C.B. Mullins et al., Chem. Phys. Lett. 163 (1989) 111] and molecular dynamics simulations [M. Head-Gordon et al., J. Chem. Phys. 94 (1991) 1516] which were conducted above the desorption temperature. The trapping probability for methane decreases from 0.7 to zero as the incident normal energy is increased from 3 to 20 kJ mol(-1). Trapping on the argon- or methane-saturated surface is greatly enhanced compared to the clean surface at all incident energies and angles, and exhibits near total energy scaling (E(T)cos(0.3) theta), indicating a corrugated gas-surface potential. The kinetics of trapping as a function of coverage are quantitatively described by the modified Kisliuk model [H.C. Kang et al., J. Chem. Phys. 92 (1990) 1397; C.R. Arumainayagam et al., J. Chem. Phys. 94 (1991) 1516], which allows for an extrinsic precursor to adsorption. The trapping probabilities of both argon and methane increase with coverage, indicating that trapping into an extrinsic precursor state is more efficient than trapping onto the bare Pt(111) surface. (C) 2000 Elsevier Science B.V. All rights reserved.