Insights into Acetone Photochemistry on Rutile TiO2(110). 2. New Photodesorption Channel with CH3 Ejection along the Surface

Angle-resolved photon stimulated desorption (PSD) combined with infrared reflection-adsorption spectroscopy and temperature programmed desorption reveal two distinct channels in the photochemistry of acetone on rutile TiO2(110) surface. During UV irradiation of co-adsorbed oxygen and acetone molecules, methyl radicals (CH3) are ejected in two different directions: i) normal to the surface and ii) off-normal at .~. +- 66° to the surface normal in the azimuth (i.e. perpendicular to the rows of bridging oxygen and Ti atoms). Both components are relatively narrow and non-cosine, indicating non-thermal evolution of CH3 radicals. The direction of the “off-normal” PSD component is consistent with orientation of the C-CH3 bonds in the n2-acetone diolate—a photoactive form of acetone chemisorption on the oxidized TiO2(110) surface proposed earlier from experimental and theoretical studies. The direction of the “normal” PSD component requires an orientation of a C-CH3 bond which is not consistent with the n2-acetone diolate structure. The angular distribution of the CH3 PSD depends on the acetone coverage. The “off-normal” PSD component dominates at lower acetone coverage (< 0.2 ML), but does not increase at higher coverages in accord with the acetone diolate peak intensity in the infrared reflection-absorption spectra. The “normal” PSD component grows with the acetone coverage up to 0.6 ML. The newly discovered “normal” PSD channel is tentatively assigned to a photo-produced n2- acetone enolate as a potential precursor based on the H/D exchange experiments