Photochemistry in CH3I adlayers on TiO2(110) studied with postirradiation thermal desorption

The reaction dynamics of photofragments produced by UV photolysis of neat CH3I films, adsorbed on defect-free TiO2(110), have been investigated using postirradiation temperature programmed desorption (PITPD). At low fluences, the film structure remains frozen, retaining photofragments and reaction products in the film. CH4, C2H6, I-2, CH2I2, and C2H5I are detected in PITPD. CH4 is a product of translationally hot methyl radicals reacting with CH3I. Desorption of the CH4 trapped in the CH3I films is limited by diffusion in the films. The I-containing products detected in PITPD come predominantly from reactions of trapped I, CH2I, and CH3 radicals, which occur during the TPD process. The desorption of I-containing products in PITPD follows desorption-limited kinetics on the TiO2(110) surface. Far C2H6 detected in PITPD, it is not clear whether the dominant reaction pathway involves hot or cold radicals. Electrons produced in the TiO2 substrate upon UV irradiation do not induce significant dissociation of CH3I adsorbed on the (110) surface and thus do not contribute significantly to fragment reactions.