Early-Time Dynamics of the Photoexcited Hydrated Electron

Employing photon echo techniques, we investigate the early relaxation dynamics of the equilibrated hydrated electron within the first 200 fs upon photoexcitation. The use of 5-fs laser pulses provided unprecedented temporal resolution of our measurements. We show that even for extremely short pulse durations the signals obtained in photon echo spectroscopy, can be described in the conventional way, provided care is taken of the spectral filtering effect and experimental beam arrangement. We next conclude that the absorption spectrum of the hydrated electrons is primarily homogeneously broadened. The comparison of two pulse photon echo experiments on pure water and on hydrated electrons allows us to measure the pure dephasing time of ∼1.6 fs. The line shape of the absorption spectrum is described excellently by an extended Lorentzian contour with a spectral width fully determined by the pure dephasing time. From the polarization-dependent transient grating experiments we establish that the polarization anisotropy of the hydrated electron falls to the zero value within 5 fs after initial excitation. A prominent role of a non-Condon effect due to strong coupling of the electron to neighboring water molecules is suggested. Based on the observed isotopic effect we concluded that the initial relaxation dynamics are determined by the inertial response of the water molecules which is librational in its origin. The microscopic picture of the early dynamics of the hydrated electron based on the experimental results is presented. Finally, we develop a theoretical model based on wave packet dynamics, which is capable of capturing the subtle features of the experimental data.