METHYL HALIDE PHOTOCHEMISTRY ON IODINE-COVERED PT(111)

The photon-driven dissociation of carbon-halogen bonds in methyl halid es adsorbed on iodine-covered Pt-(111) has been examined as a function of iodine coverage and of wavelength. For wavelengths which are not a bsorbed by gas-phase molecules, the photodissociation yield of monolay er amounts of adsorbed methyl halides decreases with increasing iodine coverage. For those wavelengths strongly absorbed in the gas phase, t he photodissociation yield of the adsorbed phase increases with iodine coverage. These variations are analyzed using two model reaction path s, direct photon absorption and substrate-mediated electron attachment , exciting the methyl halide. We present evidence that, even when both paths are operative, a rising yield with increasing iodine atom cover age marks the dominance of direct photon absorption. From the phenomen ological equations describing the latter reaction path, we conclude th at the probability of electron attachment decreases more rapidly with iodine coverage than the probability of metal quenching.