  Temporal evolution of transient absorption spectra for pre-solvated electron
(e-) generated by biphotonic (200 nm) ionization of liquid H2O and D2O has been
studied on femto- and pico- second time scales. These spectra were obtained in
the intervals of 50 nm between 500 and 1700 nm. Two distinctive regimes of the
spectral evolution were observed. In both of these regimes, the spectral
profile changes considerably with delay time. For t<1ps, two new features (the
1150 nm band and 1400 nm shoulder) were observed in the spectral region where
O-H overtones appear in the spectra of light water. These two features were not
observed for the e- in D2O. Vibronic coupling to the modes of water molecules
lining the solvation cavity is a possible origin of these features. On the
sub-picosecond time scale, the absorption band of e- progressively shifts to
the blue. At later delay times (t> 1ps), the position of the band maximum is
"locked", but the spectral profile continues to change by narrowing on the red
side and broadening on the blue side; the oscillator strength is constant
within 10%. The time constant of this narrowing is ca. 0.56 ps for H2O and 0.64
ps for D2O, respectively. Vibrational relaxation and time-dependent decrease in
the size and anisotropy of the solvation cavity are suggested as possible
causes for the observed spectral transformations in both of these regimes. The
geminate recombination kinetics for hydrated electrons generated by absorption
of two 200 nm quanta (12.4 eV total energy) in light and heavy water are almost
identical, suggesting that the average separation between the e- and its
geminate partners in D2O is 13% narrower than in H2O. We suggest that
autoionization of water competes with direct ionization even at this high
photoexcitation energy.
