Dynamics of molecular CH 4 adsorption on Pt(111)

Abstract

The dynamics of molecular methane adsorption on Pt(111) were probed with supersonic molecular beam techniques. Initial trapping probabilities were directly measured 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 temperature ( T s) of 100 K. The incident methane molecules were rotationally and vibrationally cold. The initial trapping probability decreases with increasing incident translational energy ( E T) and decreasing angle of incidence (θ i) and varies smoothly with incident normal energy ( E n = E T cos 2 θ i ), indicating a low corrugation of the molecule-surf ace interaction potential. The dependence of the initial trapping probability on incident normal translational energy agrees quantitatively with both a modified hard cube model and Leuthäusser's theory at incident normal translational energies below 8 kJ/mol. At higher incident normal translational energies the observed initial trapping probabilities are higher than the values predicted by both models. Energy loss mechanisms other than surface phonon excitations may partially account for this discrepancy. A rapid decrease in the apparent adsorption probability as the surface temperature approaches 140 K is caused by the competitive influence of desorption. The temperature at which the apparent adsorption probability goes to zero agrees well with the desorption temperature measured independently by temperature programmed desorption. In accordance with the aforementioned models, the measured in-plane angular distributions suggest that the trapping probability is relatively independent of surface temperature in the range of 160 to 500 K. The relatively low intensity of methane found near the surface normal in the angular distributions may be partially explained by a wider than cosine angular distribution for the trapped-desorbed channel, which is consistent with the observation that the trapping probability increases with angle of incidence. Comparison of our initial trapping probability versus normal translational energy data to previous mean translational energy measurements of methane molecules desorbing from Pt(111) at the surface normal suggests that detailed balance applies for the non-equilibrium situation involving a collimated monoenergetic molecular beam of methane incident on a Pt(111) surface.