DETAILED INVESTIGATION OF THE FEMTOSECOND PUMP-PROBE SPECTROSCOPY OF THE HYDRATED ELECTRON

We recently reported the first pump-probe measurements on the hydrated electron with sufficient time resolution (similar to 35 fs) to direct ly observe the initial processes in the solvation dynamics of this key prototype for condensed-phase dynamics [Silva, C,; Walhout, P. K.; Yo koyama, K.; Barbara, P. F, Phys. Rev. Lett. 1998, 80, 1086]. An unprec edented relaxation process for the hydrated electron was observed that occurs on the 35-80 fs time scale and exhibits a solvent isotope effe ct (tau(D2O)/tau(H2O) similar to 1.4), The new process was assigned to inertial/librational motion of the water surrounding the excess elect ron. The present paper reports a more extensive study of the similar t o 35 fs resolved dynamics of the hydrated electron in H2O and D2O at m ore probe wavelengths and as a function of pump-pulse intensity. The r esults are in agreement with the preliminary report and support the im portance of librational water motion in the relaxation dynamics of the hydrated electron. New high excitation pulse intensity measurements r eveal evidence of a high-intensity, two-photon channel involving eject ion of the hydrated electron from its initial site to a different site in the solvent.