Ultrafast electron injection and recombination dynamics of dye sensitised TiO2 particles

We report studies of electron injection and recombination dynamics of the electronically excited laser dye fluorescein 27 adsorbed on nanometer-sized TiO2 colloidal particles in aqueous solution. Using femtosecond pump–probe spectroscopy, we show that electron injection occurs in ∼300 fs. This time scale for the electron injection process was confirmed recording the signal of the stimulated emission decay of the dye and the simultaneous rise of the absorption of the injected electron. In addition, our results provide strong evidence that the subsequent recombination process is controlled by cooling of the hot semi-oxidised radical anion initially prepared by ultrafast electron injection.