Surface photochemistry: 6. CH sub 3 Br on Pt(111)

The surface photochemistry of monolayer and submonolayer CH{sub 3}Br on Pt(111) has been studied by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) to diagnose the results of continuous low-intensity ultraviolet irradiation. For wavelengths of 300 nm or less, and surface temperatures near 100 K, evidence is presented for the nonthermal cleavage of the C-Br bond and the formation of adsorbed Br atoms and CH{sub 3} groups. The wavelength dependent is strong and red-shifted compared to the gas-phase electronic excitation spectrum of CH{sub 3}Br. By quantitative XPS, the maximum photolysis yield of retained Br from one monolayer of CH{sub 3}Br is 60 {plus minus} 5%. While there is some slow thermal desorption, there is no detectable photodesorption of the parent during irradiation. Excitation mechanisms and product formation channels are discussed.