The effects of alkylidyne and atomic oxygen overlayers on the dynamics of adsorption of methane on Pt(111)

The dynamics of the trapping of methane on p(2 x 2)-oxygen, -ethylidyne, -butylidyne and -isobutylidyne overlayers on Pt(111) at 50 K was investigated using supersonic molecular-beam techniques. Since each of these adsorbates forms the same structure, trapping dynamics could be studied as a function of the internal structure of the adsorbate. Trapping probabilities on these surfaces at a fixed incident energy and angle increase in the order oxygen < ethylidyne < isobutylidyne < butylidyne, and each of the adsorbate-covered surfaces enhances trapping compared with the clean surface. On the butylidyne-covered surface the initial molecular trapping probability decreases from 0.88 to 0.38 as the incident energy is increased from 8 to 24 kJ mol(-1), compared with a decrease in trapping probability from 0.3 to zero over the same energy range on the clean surface. The angular dependence of the trapping probability indicates that the overlayers lead to an increase in the static corrugation in gas-surface potential. The generally higher values of the trapping probabilities on the alkylidyne-covered surfaces and the weaker angular dependence they exhibit suggest that they have a more corrugated potential than the clean surface. The further increase in trapping probabilities for methane on Pt(lll) covered by the analogous alkanes suggests that the additional frustrated translations of the alkanes may also enhance trapping into the extrinsic precursor. The change in the adsorption probability with methane coverage on the oxygen-covered surface is described by the modified Kisliuk model [J. Phys. Chem. 95 (1991) 2461; J. Chem. Phys. 92 (1990) 1397]. The trapping probability increases with methane coverage, indicating that trapping into the extrinsic precursor state is more efficient in the presence of methane. (C) 2000 Elsevier Science B.V. AU rights reserved.