Photochemistry in CH₃I Adlayers on TiO₂(110) Studied with Postirradiation Thermal Desorption

The reaction dynamics of photofragments produced by UV photolysis of neat CH 3 I films, adsorbed on defect-free TiO 2 (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. CH 4, C 2 H 6, I 2, CH 2 I 2, and C 2 H 5 I are detected in PITPD. CH 4 is a product of translationally hot methyl radicals reacting with CH 3 I. Desorption of the CH 4 trapped in the CH 3 I films is limited by diffusion in the films. The I-containing products detected in PITPD come predominantly from reactions of trapped I, CH 2 I, and CH 3 radicals, which occur during the TPD process. The desorption of I-containing products in PITPD follows desorption-limited kinetics on the TiO 2 (110) surface. For C 2 H 6 detected in PITPD, it is not clear whether the dominant reaction pathway involves hot or cold radicals. Electrons produced in the TiO 2 substrate upon UV irradiation do not induce significant dissociation of CH 3 I adsorbed on the (110) surface and thus do not contribute significantly to fragment reactions.