Surface photochemistry and dynamics of weakly bound adsorption systems

The photo-induced processes of methane adsorbed on Pt(lll) and Pd(lll) surfaces have been studied by post-irradiation temperature-programmed desorption and angle-resolved time-of-flight measurements. Methane adsorbs weakly on those metals. Although gaseous methane does not show any appreciable absorption cross sections at 6.4 eV, methane weakly adsorbed on those metals is photodissociated to produce methyl and hydrogen by the irradiation of 6.4-eV photons. The incident angle dependence of cross sections of the photochemistry obtained with linearly polarized light indicates that direct electronic excitation of methane adsorbate plays an important role in the photochemistry of methane. We interpreted that the photochemistry is induced via the electronic transition from the ground state localized at methane to the excited state of the methane-substrate atom complex where the first excited Rydberg-like state of methane significantly mixed with substrate empty states. Photofragments of methane, H and CH3, further react with preadsorbed methyl and hydrogen species, respectively. In particular, methane is desorbed via associative recombination between a "hot" hydrogen and a methyl adsorbate. The average translational energy of the desorbed methane is 0.26 eV and 0.53 eV for Pd(lll) and Pt(lll), respectively, This difference can be explained by the difference in the surface electronic structure between Pd(lll) and Pt(111).