Transient spectra, formation, and geminate recombination of solvated electrons in pure water UV-photolysis: an alternative view

The temporal evolution of the optical absorption of solvated electrons in a neat water jet has been investigated in two pulse femtosecond experiments. A 90 fs (FWHM) UV pulse at 267 nm directly ionized the neat water and the subsequent absorption has been probed by a white light continuum at 10 different wavelengths in the range between 450 and 1000 nm. Due to the thickness of the water jet 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 formation and geminate recombination 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 solvated electrons in the time interval between 300 and 100 ps after to photoionization pulse. At ultrashort times the absorption spectra are found in the infrared and undergo a substantial blue-shift during the first few picoseconds. After about 5 ps the absorption spectrum of thermally equilibrated solvated electrons is obtained. 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 (〈Δr2(t)〉) of the electrons. Unmistakably, 〈Δr2(t)〉 is related to the solvation process of electrons in polar fluids and therefore contains the solvation dynamics. In addition, we clearly see for the first time the delayed formation of solvated electrons followed by geminate recombination.