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The surface photochemistry of monolayer and submonolayer CH3Br on Pt(lll) 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 CH3 groups. The wavelength dependence is strong and red-shifted compared to the gas-phase electronic excitation spectrum of CH3Br. By quantitative XPS, the maximum photolysis yield of retained Br from one monolayer of CH3Br is 60 ± 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.

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