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

Dynamics of the thermal desorption of CD₄ molecularly and dissociatively adsorbed on Pt(111) has been studied by laser-induced thermal desorption (LITD). CD₄ adsorbates were prepared at the surface temperature of 55 K and the coadsorbates of CD₃ and D were prepared by UV photolysis of CD₄ adsorbates. 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 (cos^2.5 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 CD₄ desorbs 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.