Investigations on the ultrafast electron injection mechanism from the dye alizarin to wide band gap semiconductor colloids in aqueous medium are presented, combined with detailed studies on population, depopulation, and relaxation phenomena in trap states and their influence on the injection process. Because of the very strong electronic coupling between dye and semiconductor in an alizarin/TiO 2 system, a very fast electron injection from the excited dye to the conduction band of TiO 2 is expected. Our measurements show an injection time τ inj < 100 fs, suggesting that the electron transfer follows an adiabatic mechanism. Furthermore, we present experiments over a wide spectral range on the recombination reaction of the electron in the conduction band of the semiconductor colloid and the dye cation to the ground state. We find highly multiphasic recombination dynamics with time constants from 400 fs to the nanosecond time scale. The nonexponential character of the recombination reaction is attributed to fast relaxation processes. The crucial contribution of surface trap states and their influence on the observed dynamics was investigated with alizarin adsorbed on the insulating substrate ZrO 2 . Since the conduction band edge lies far above (≈1 eV) the S 1 state of alizarin, the electron injection into this band is completely suppressed. Despite this fact our spectroscopic investigations show that on ultrafast time scales the formation of an alizarin cation occurs. This observation, is explained by fast electron injection into surface trap states near the docking site on the colloid. For the alizarin/ZrO 2 system the time scale for the injection into these traps is determined to be faster than 100 fs. The relaxation processes in the traps and the repopulation of the S 1 state occur within 450 fs, the subsequent ground-state relaxation takes 160 ps. The ultrafast injection dynamics into the traps, recorded for alizarin/ ZrO 2 , underlines the importance of surface states for the initial charge separation also for systems with a lower band edge such as TiO 2 . We show that in the dye/ZrO 2 system the process of electron injection is not suppressed but "stopped" after the ultrafast transition into trap states. It is therefore a valuable system for probing the electron dynamics in surface states.

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Electron transfer from organic dye molecules to semiconductor-colloidal systems is among the fastest reported charge-separation reactions. We present investigations on alizarin complexing the surface of TiO 2 semiconductor colloids in solution. Because of the very strong electronic coupling between the sensitizer and the semiconductor in the alizarin/TiO 2 system, very fast electron injection from the photoexcited dye to the conduction band of TiO 2 occurs. The real-time observation of the injection process is achieved by transient absorption spectroscopy using a 19-fs excitation pulse provided by a pump pulse from a noncollinear optical parametric amplifier and a probe pulse from a quasi-chirp-free supercontinuum. An injection time τ inj of 6 fs can be unambiguously derived in three different ways from the experimental data: (i) analysis of individual transients at spectral positions without contributions from subsequent reactions (relaxation, recombination); (ii) global fitting procedure for 31 wavelengths over a wide spectral range; and (iii) calculation of the S* state and comparison to the "nonreactive" system alizarin/ZrO 2 . The spectral signature of the 6-fs kinetic component can be assigned to electron transfer from the excited dye molecule to the TiO 2 colloid. Even for this strongly coupled system, we propose a localized excitation with a subsequent adiabatic electron transfer reaction that is, to our knowledge, the fastest electron-transfer reaction that has been directly measured by transient spectroscopy.