FEMTOSECOND STUDIES OF PHOTOINDUCED ELECTRON DYNAMICS AT THE LIQUID-SOLID INTERFACE OF AQUEOUS CDS COLLOIDS

We report the direct measurements of the dynamics of photoinduced electrons at the liquid-solid interface of aqueous CdS colloids on the femtosecond time scale. The observed transient absorption is attributed to electrons trapped at the liquid-solid interface. We show that electron trapping due to surface states or defect sites occurs in less than 100 fs. The trapped electrons then decay by a double exponential with time constants of 2-3 ps and 50 ps at high excitation intensities, while a single-exponential decay (50 ps) is observed at low intensities. The slow, 50-ps decay is attributed primarily to geminate electron-hole recombination, which dominates the decay dynamics at low excitation intensities. The fast 2-3-ps decay observed at high intensities is assigned to nongeminate recombination, suggesting that nongeminate recombination plays an important role at high pump powers. The decay dynamics are also found to be sensitive to the solvent environment. The decay is slower when hole scavengers such as I- are added to the liquid and faster when the pH of the solution is lowered (decrease in concentration of the hole scavenger, OH-), in support of the interpretation of trapped electron-hole recombination for the observed decay.