FEMTOSECOND STUDY OF THE INTENSITY DEPENDENCE OF ELECTRON-HOLE DYNAMICS IN TIO2 NANOCLUSTERS

The dynamics of charge carrier trapping and recombination as a functio n of pump fluence of 20 angstrom diameter TiO2 particles in aqueous so lutions is measured. Trapping of photogenerated conduction band electr ons occurs in less than 500 fs. The subsequent fluence-dependent decay of trapped electrons has both a fast and a slow component. The early time recombination results are interpreted by second-order kinetics. A global fit to the data predicts a bulk second-order recombination rat e constant of (1.8 +/- 0.7) x 10(-10) cm3 s-1 for trapped electrons wi th holes. The signal at long times is attributed to electrons ejected from the nanocluster and/or deeply trapped electrons produced via thir d-order kinetics, consistent with an Auger process.