Direct Observation of Surface-Mediated Electron−Hole Pair Recombination in TiO₂(110)

The kinetics of surface and bulk electron−hole pair recombination have been measured separately on TiO 2 (110) in ultrahigh vacuum under well-controlled surface conditions for the first time. Using 100 eV incident electrons, excitation of electron−hole pairs occurs within ∼6 Å of the surface, and the rate of stimulated desorption of O 2 was measured as a means of determining the charge carrier recombination kinetics, which are found to be mediated by the surface and to be first-order in charge carrier concentration. Comparison with a previous O 2 photodesorption experiment, where excitation by 3.4 eV photons was used for electron−hole pair generation in a deeper region below the surface (∼100 Å), shows that bulk recombination in TiO 2 occurs by second-order kinetics in charge carrier concentration.