Ultrafast dynamics of solvated electrons in polar liquids

The temporal evolution of the optical absorption of solvated electrons in a pure water jet between 5 and 70 degrees C has been investigated in two pulse femtosecond experiments. A 60 fs (FWHM) UV pulse at 270 nm directly ionized pure water and the subsequent absorption was probed by a white light continuum at 12 different wavelengths in the range between 450 and 1000 nm. Due to the thickness of the waterjet the time resolution is limited to about 150 fs. The transient absorption contains not only information on the temporal evolution of the absorption spectrum but also data on the time dependence of the concentration of the solvated electrons. We have used the optical sum rules to separate the temporal evolution of the absorption spectrum from the concentration of the electrons in the time interval between 300 fs and 100 ps. At ultra-short times the absorption spectra are displaced to the red and undergo a substantial blue-shift during the first few picoseconds. After about 5 ps the absorption spectrum of thermally equilibrated solvated electrons is recovered. Within our time resolution the data show no evidence of transient electronically excited states of solvated electrons. We interpret the temporal evolution of the absorption spectrum using the optical sum rules and deduce the time dependent decrease of the mean squared dispersion in position, [Delta r(2)(t)], of the electrons. Certainly, [Delta r(2)(t)] is related to the solvation process of electrons in polar fluids and therefore contains information on the solvation dynamics.