Ultrafast dynamics of acetone photooxidation on TiO₂(110)

Using light energy to drive chemical reactions on semiconductor surfaces is the basis for technological applications ranging from the removal of organic pollutants to the generation of renewable solar fuels, yet our understanding of the mechanisms has been hindered by the multistep nature of the process and the wide range of time scales over which it occurs (femtoseconds to seconds). In this work, we use ultrafast laser pump-probe techniques to follow the time evolution of substrate-induced photooxidation of acetone on a titania surface. A UV light at 260 nm initiates carrier-induced fragmentation of adsorbed acetone on a TiO₂(110) surface that was pretreated with oxygen. The photoreaction results in the ejection of methyl radicals into the gas-phase that are detected by the probe pulse via resonant multiphoton ionization. The time evolution of the methyl radicals leaving the surface exhibits ultrafast rise times, 300-700 fs, followed by a more gradual rise that plateaus by 10 ps, with faster rates at a low acetone coverage. These results are interpreted in terms of a time-dependent rate expression and a mechanism in which the fragmentation of the acetone surface species is driven by interactions with nonequilibrium, "hot" holes.