Photochemistry of methane on deuterium-covered Pt(111) surfaces

The photochemistry of methane adsorbed on deuterium-precovered Pt(111) surfaces has been investigated by temperature-programmed desorption and X-ray photoelectron spectroscopy before and after irradiation with 193 nm photons. Two different deuterium adlayers are used: ordered D(1 x 1) and disordered D on Pt(111). Methane adsorbates on these D-covered surfaces at 40 K undergo either desorption or dissociation to methyl and hydrogen adsorbates. The H-D exchange reaction to produce CH3D adsorbates also takes place. This reaction is due to photoinduced recombination between CH3 photofragments and precovered D adatoms. As has been observed in methane on the bare Pt(111) surface, the photoreactions are quenched when about half of the methane adsorbates is depleted. The total depletion cross sections of methane on the D-covered surfaces are smaller than that on the bare surface. The fraction for the desorption (dissociation) channel increases (decreases) when the surface is congested with deuterium adatoms. When the surface is annealed after the photon irradiation, methyl adsorbates recombine either with photochemically produced H adatoms or pre-adsorbed D adatoms and desorb as CH4 or CH3D, respectively. The ratio of the desorption yield of CH4 to CH3D is larger than that expected from the kinetic isotope effect of the associative recombination. This result is discussed in terms of the non-uniform distribution of the photo-products and the restricted mobility of H (D) adatoms under the presence of CH3 adsorbates.