State Selective Dynamics of TiO2 Charge Carrier Trapping and Recombination

Time-resolved pump-probe photoemission spectroscopy has been used to study the dynamics of charge carrier recombination and trapping on hydroxylated rutile TiO₂(110). Two types of pump excitation were employed, one in the infrared (0.95 eV) and the other in the UV (3.5 eV) region. With IR excitation, electrons associated with defects are excited into the bottom of the conduction band from the polaronic states within the band gap, which are retrapped within 45±10 fs. Under UV excitation, the electrons in these band gap states and valence band electrons are excited into the conduction band. In addition to the fast polaron trapping observed with IR excitation, we also observe a long lifetime (about 1 ps) component to both the depletion of hot electrons at the bottom of the conduction band and the refilling of the band gap states. This points to a band gap state mediated recombination process with a ps lifetime.