ALKYL HALIDE PHOTOCHEMISTRY ON AG(111) .2. METHYL-BROMIDE

The surface photochemistry of methyl bromide (CH3Br) on Ag(111) at 100 K has been studied. While absorbed CH3Br does not thermally dissociate, UV photodissociation occurs readily. Upon the C-Br bond cleavage, all Br and a small portion of CH3 are retained at the surface. There is no photodesorption of molecular CH3Br. In post-irradiation TPD, all the retained CH3 fragments recombine to form C2H6 below 300 K and surface Br desorbs as AgBr above 700 K. The photodissociation threshold for the first layer (chemisorbed) is approximately 3.0 eV and the second layer (physisorbed) is > 3.5 eV, both lower than in the gas phase (approximately 4.5 eV). The cross section for the photodissociation of 1 ML CH3Br as a function of wavelength is estimated. Compared to the gas phase, the photodissociation cross section for chemisorbed CH3Br is much larger while that for physisorbed CH3Br is slightly larger. The average effective photolysis rate decreases with increasing initial CH3Br coverage. Comparison with photochemistry CH3Br on Pt(111) is made.