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, I₂, 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. For 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.