BROKEN SYMMETRY OF ADSORBED METHANE AND SELF-LIMITING PHOTOINDUCED DISSOCIATION ON PT(111)

Adsorbed states of methane on Pt(lll) and photoinduced processes by Ar F laser irradiation (193 nm) have been studied by infrared reflection absorption spectroscopy (IRAS). The symmetry of first layer methane is degraded from T-d to C-3v (or lower symmetry), bur that of overlayer methane maintains T, symmetry. The overlayer and first layer methane m olecules desorb from Pt(111) at similar to 40 and similar to 70 K, res pectively. The first layer C-3v, CH4 is photodissociated into CH3 and H species. These reaction products modify the surface in such a way as to make the remaining first-layer CH, molecules photochemically inact ive, and thus the reaction becomes self-limiting. The photoreaction ki netics have been studied with time-resolved IRAS, and the total cross- sections are estimated to be sigma = 2.6 x 10(-19) cm(2) for CH4 and s igma = 1.7 x 10(-19) cm(2) for CD4.