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Femtosecond time-resolved near-infrared spectra of TiO 2 and Pt/TiO 2 powders are measured at the wavelength region of 0.9−1.5 μm with the direct absorption method. Broad absorption bands of charge carriers, mainly free and trapped electrons, are observed for TiO 2 and Pt/TiO 2 . The absorption band shape changes with a time constant of 160 fs after the photoirradiation, which most probably reflects the trapping of the generated free electrons. Population decay curves of the carriers after 1 ps, obtained for three different pump light powers, are well-explained by the second-order decay kinetics with a common second-order rate constant, indicating nongeminate recombination of the electron−hole pairs. When Pt is loaded to TiO 2, an additional decaying process of 2.3 ps is observed. This decay component represents the transfer of generated electrons from TiO 2 to Pt, which is consistent with the known increase of overall catalytic activities by the Pt cocatalyst.
Iwata et al. (Wed,) studied this question.