The UV photodesorption of molecular oxygen from a reduced TiO 2 (110) single-crystal surface was investigated as a function of photon excitation energy, substrate temperature, and preannealing conditions. A pump-delayedprobe method using pulsed lasers for UV excitation (pump) and VUV ionization (probe) were used in conjunction with time-of-flight mass spectrometry to measure velocity distributions of the desorbing O 2 molecules. The measured velocity distributions exhibit three distinct features, two of which are attributed to prompt desorption resulting in "fast" velocity distributions and one "slow" channel whose average kinetic energy tracks the surface temperature. The latter is assigned to trapping-desorption of photoexcited O 2 * which are trapped in the physisorption well prior to thermal desorption. The velocity distributions show no dependence on photon energy over the range studied (3.45-4.16 eV), consistent with a substrate-mediated, hole-capture desorption mechanism. The observed prompt desorption channels have mean translational energies of ∼0.14 and ∼0.50 eV and are attributed to the photodesorption of two distinct initial states of chemisorbed oxygen. The identities of the chemisorbed initial states associated with oxygen vacancy or interstitial defect sites are discussed in light of previous experimental and theoretical studies of oxygen on reduced TiO 2 (110) surfaces.

The UV photooxidation of acetone on a reduced TiO2(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 TiO2 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 TiO2 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.

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)CH(3) (R = H, CH(3), C(2)H(5), C(6)H(5)), adsorbed on a TiO(2)(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 TiO(2) 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., η(2)-bonded diolate species (R(COO)CH(3)). 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 TiO(2) 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.

The UV photooxidation of 2-butanone on TiO 2 (110) was studied using pump−probe laser methods and time-of-flight (TOF) mass spectrometry to identify the gas-phase photoproducts and probe the dynamics of the photofragmentation process. A unique aspect of this work is the use of coherent VUV radiation for single-photon ionization detection of gas-phase products, which significantly reduces the amount of parent ion fragmentation as compared to electron impact used in previous studies. The pump−probe product mass spectra showed ions at mass 15 (CH 3 + ) and mass 29 (C 2 H 5 + ), which are associated with the primary α-carbon bond cleavage of the adsorbed butanone−oxygen complex, as well other C 2 H x + (x = 2−4) fragments, which could originate from ethyl radical secondary surface chemistry or dissociative ionization. Using two different VUV probe energies, it was possible to show that the fragment ions at mass 27 (C 2 H 3 + ) and mass 28 (C 2 H 3 + ) are not due to secondary reactions of ethyl radicals on the surface, but rather from dissociative ionization of the ethyl radical parent ion (mass 29). Another photoproduct at mass 26 (C 2 H 2 + ) peak is also observed, but its pump−probe delay dependence indicates that it is not associated with nascent ethyl radicals. Pump-delayed-probe measurements were also used to obtain translational energy distributions for the methyl and ethyl radical products, both which can be empirically fit to "fast" and "slow" components. The ethyl radical energy distribution is dominated by the "slow" channel, whereas the methyl radical has a much larger contribution from "fast" fragments. The assignment of the C 2 H x (x = 3, 4) fragments to ethyl (C 2 H 5 ) dissociative ionization was also confirmed by showing that all three products have the same translational energy distributions. The origin of the "fast" and "slow" fragmentation channels for both methyl and ethyl ejection is discussed in terms of analogous neutral and ionic fragmentation processes in the gas phase. Finally, we consider the possible energetic and dynamical origins of the higher yield of ethyl radical products as compared to that for methyl radicals.