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 investigat ed using supersonic molecular beam techniques at a surface temperatures of 30 and 50 K, well below the desorption temperatures of 46 and 67 K, respect ively. The initial trapping probabilities for both Ar and methane scale wit h normal incident energy (E(T)cos(2) theta), indicating a smooth gas-surfac e 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 k J 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 we re conducted above the desorption temperature. The trapping probability for methane decreases from 0.7 to zero as the incident normal energy is increa sed from 3 to 20 kJ mol(-1). Trapping on the argon- or methane-saturated su rface is greatly enhanced compared to the clean surface at all incident ene rgies and angles, and exhibits near total energy scaling (E(T)cos(0.3) thet a), indicating a corrugated gas-surface potential. The kinetics of trapping as a function of coverage are quantitatively described by the modified Kis liuk model [H.C. Kang et al., J. Chem. Phys. 92 (1990) 1397; C.R. Arumainay agam et al., J. Chem. Phys. 94 (1991) 1516], which allows for an extrinsic precursor to adsorption. The trapping probabilities of both argon and metha ne increase with coverage, indicating that trapping into an extrinsic precu rsor state is more efficient than trapping onto the bare Pt(111) surface. ( C) 2000 Elsevier Science B.V. All rights reserved.