Final state distributions of methyl radical desorption from ketone photooxidation on TiO₂(110)

In this work, we report on product energy distributions for methyl radicals produced by UV photooxidation of a set of structurally related carbonyl molecules, R(CO)CH₃ (R = H, CH₃, C₂H ₅, C₆H₅), adsorbed on a TiO₂(110) surface. Specifically, laser pump-probe techniques were used to measure the translational energy distributions of methyl radicals resulting from α-carbon bond cleavage induced by photoexcited charge carriers at the TiO₂ surface. Photoreaction requires the presence of co-adsorbed oxygen and/or background oxygen during UV laser (pump) exposure, which is consistent with the formation of a photoactive oxygen complex, i.e., η²-bonded diolate species (R(COO)CH₃). The methyl translational energy distributions were found to be bimodal for all molecules studied, with "slow" and "fast" dissociation channels. The "fast" methyl channel is attributed to prompt fragmentation of the diolate species following charge transfer at the TiO₂ surface. The average translational energies of the "fast" methyl channels are found to vary with R-substituent and correlate with the mass of the remaining surface fragments, RCO_x (x =1 or 2). By comparison, the average energies of the "slow" methyl channels do not show any obvious correlation with R-substituent. The apparent correlation of the "fast" methyl translation energies with surface fragment mass is consistent with a simple two-body fragmentation event isolated on the diolate molecule with little coupling to the surface. These results also suggest that the total available energy for methyl fragmentation does not vary significantly with changes in R-substituent and is representative of exit barriers leading to "fast" methyl fragments.