On the relaxation kinetics following the absorption of light by solvated electrons in polar liquids:: roles of the continuous spectral shifts and of the stepwise transition

The relaxation kinetics that is observed to follow the absorption of 780-nm light by fully solvated electrons in water and in various alcohols is re-analysed. The present investigation aims at assessing the respective roles played in such three-pulse experiments by (i) the stepwise transition between a weakly bound and a strongly bound electron-solvent configuration and (ii) the continuous spectral shifts of the transient spectra of those two species. In particular, the suggestion of Walhout et al. [Walhout, P.K., Alfano, J.C., Kimura, Y., Silva, C., Reid, P.J., Barbara, P.F., 1995, Direct pump/probe spectroscopy of the near-IR band of the solvated electron in alcohols. Chem. Phys. Lett., 232, 135.] that the stepwise transition is as short (<1 ps) in the alcohols as in water is examined. Simulated kinetic traces are generated with the hybrid model of electron solvation using the parameters that were previously extracted from fits to the standard pump/probe measurements of solvent photoionisation and electron solvation kinetics. The ability of the simulations to reproduce the main features of the experimental three-pulse results indicates that these measurements are consistent with previous studies of electron solvation in polar media. As a consequence, it is not necessary to invoke a sub-picosecond stepwise transition to explain the fast signal recovery observed in three-pulse experiments. This component of the measured relaxation kinetics can be attributed to the ultrafast trapping of the electrons that were ejected from their original trap site when they absorbed the pump light. As for the characteristic times of the stepwise transition and of the continuous spectral shifts, they are both comparable to the rotation times of the solvent molecules. The usual interpretation. according to which the solvation of electrons is governed by solvent molecular motions, can thus be maintained. (C) 1999 Elsevier Science Ltd. All rights reserved.