Mechanism for the desorption of molecularly and dissociatively adsorbed methane on Pt(111) probed by pulse-laser heating

Abstract

Dynamics of the thermal desorption of CD4 molecularly and dissociatively adsorbed on Pt(111) has been studied by laser‐induced thermal desorption (LITD). CD4adsorbates were prepared at the surface temperature of 55 K and the coadsorbates of CD3 and D were prepared by UVphotolysis of CD4adsorbates. The thermal desorption was induced by irradiating the surface with pulses of a frequency‐doubled Nd:YAG laser and time‐of‐flight distributions of CD4 in each desorption channel were measured as a functions of the desorption angle (θ). The desorption of molecularly adsorbed CD4 shows a broad angular distribution (cos2.5 θ) and its translational energy distribution can be approximately described by the Maxwell–Boltzmann distribution with the temperature close to the estimated maximum surface temperature. In contrast, associatively formed CD4desorbs sharply along the surface normal (cos31 θ) and its average translational energy is almost constant at 0.43 eV in the range of θ=0°–25°. The experimental results are compared with the calculated ones obtained from the statistical model developed by Ukraintsev and Harrison [J. Chem Phys. 101, 1564 (1994)]. Although the statistical model apparently succeeds in describing some of adsorption/desorption features by fitting the data of sticking coefficient measurements with molecular beams, it fails to predict the LITD data for the reverse process when their fitted data are applied in total.