Final state distributions of methyl photoproducts from the photooxidation of acetone on TiO ₂(110)

The UV photooxidation of acetone on a reduced TiO ₂(110) surface was investigated using a combination of photodesorption and thermal desorption measurements and pump-probe laser detection of gas-phase products. In agreement with earlier studies, acetone adsorbed on TiO ₂ does not undergo a UV photoreaction unless codosed with molecular oxygen. The only gas-phase photoproducts are methyl radicals originating from fragmentation of the active acetone surface species and photodesorbed molecular oxygen. Postirradiation TPD measurements show that acetate is the primary surface product remaining after photooxidation. The dependence of the methyl radical formation rate on oxygen and thermal pretreatment of the TiO ₂ surface is consistent with the formation of an acetone-oxygen (diolate) complex involving adsorbed acetone and oxygen adatoms. Pump-delayed-probe laser techniques were used to measure the velocity and translational energy distributions of methyl radicals resulting from fragmentation of the acetone diolate. The observed translational energy distributions are well described by empirical fits involving two components with average energies of 0.19 eV ("fast") and 0.03 eV ("slow"). The latter are found to be insensitive to surface temperature or preannealing conditions, suggesting that the "fast" and "slow" components represent different final states of methyl radicals originating from fragmentation of a single photoactive species. The methyl kinetic energy distributions were also found to be independent of UV pump energy which is consistent with a substrate-induced process involving thermalized charge carriers, electrons or holes, which transfer to the acetone diolate to induce fragmentation. The results are discussed in terms of probable substrate-induced photoreaction mechanisms and analogous molecular photofragmentation processes.