Dynamics of molecular methane adsorption on platinum(111). Arumainayagam, Christopher R.; McMaster, Mark C.; Schoofs, Gregory R.; Madix, Robert J. Dep. Chem., Stanford Univ., Stanford, CA, USA. Surf. Sci. (1989), 222(1), 213-46. CODEN: SUSCAS ISSN: 0039-6028. Journal written in English. CAN 112:63300 AN 1990:63300 CAPLUS (Copyright 2001 ACS)

The dynamics of mol. CH₄ adsorption on Pt(111) were probed with supersonic mol. beam techniques. Initial trapping probabilities were measured directly between 0.94 and 0.16 for incident total translational energies between 3.4 and 20.2 kJ/mol and angles of incidence (with respect to the surface normal) between 0° and 45° at a surface temp. (T_s) of 100 K. The incident CH₄ mols. were rotationally and vibrationally cold. The initial trapping probability decreases with increasing incident translational energy (E_T) and decreasing angle of incidence (q_i) and varies smoothly with incident normal energy (E_n = E_T cos²q_i), indicating a low corrugation of the mol.-surface interaction potential. The dependence of the initial trapping probability on incident normal translational energy agrees quant. with both a modified hard cube model and the U. Leuthaeusser (1981, 1983) theory at incident normal translational energies <8 kJ/mol. At higher incident normal translational energies, the obsd. initial trapping probabilities are higher than the values predicted by both models. Energy loss mechanisms other than surface phonon excitations may account partially for this discrepancy. A rapid decrease in the apparent adsorption probability as the surface temp. approaches 140 K is caused by the competitive influence of desorption. The temp. at which the apparent adsorption probability goes to zero agrees well with the desorption temp. measure independently by temp.-programmed desorption. In accordance with the aforementioned models, the measured in-plane angular distributions suggest that the trapping probability is relatively independent of surface temp. in the range of 160 to 500 K. The relatively low intensity of CH₄ found near the surface normal in the angular distributions may be explained partially by a wider than cosine angular distribution or the trapped-desorbed channel, which is consistent with the observation that the trapping probability increases with angle of incidence. Comparison of the initial trapping probability vs. normal translational energy data to previous mean translational energy measurements of CH₄ mols. desorbing from Pt(111) at the surface normal suggests that detailed balance applies for the non-equil. situation involving a collimated monoenergetic mol. beam of CH₄ incident on a Pt(111) surface.