Final State Distributions of O2 Photodesorbed from TiO2(110)

The UV photodesorption of molecular oxygen from a reduced TiO2(110) single-crystal surface was investigated as a function of photon excitation energy, substrate temperature, and preannealing conditions. A pump-delayed-probe 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 similar to 0.14 and similar to 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 TiO2(110) surfaces.