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 elec trons at the liquid-solid interface of aqueous CdS colloids on the fem tosecond time scale. The observed transient absorption is attributed t o electrons trapped at the liquid-solid interface. We show that electr on 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, whi le a single-exponential decay (50 ps) is observed at low intensities. The slow, 50-ps decay is attributed primarily to geminate electron-hol e recombination, which dominates the decay dynamics at low excitation intensities. The fast 2-3-ps decay observed at high intensities is ass igned to nongeminate recombination, suggesting that nongeminate recomb ination plays an important role at high pump powers. The decay dynamic s 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 concentrat ion of the hole scavenger, OH-), in support of the interpretation of t rapped electron-hole recombination for the observed decay.