  Charge-transfer-to-solvent (CTTS) reactions of hydroxide induced by 200 nm
monophotonic or 337 nm and 389 nm biphotonic excitation of this anion in
aqueous solution have been studied by means of pump-probe ultrafast laser
spectroscopy. Transient absorption kinetics of the hydrated electron, eaq-,
have been observed, from a few hundred femtoseconds out to 600 ps, and studied
as function of hydroxide concentration and temperature. The geminate decay
kinetics are bimodal, with a fast exponential component (ca. 13 ps) and a
slower power "tail" due to the diffusional escape of the electrons. For the
biphotonic excitation, the extrapolated fraction of escaped electrons is 1.8
times higher than for the monophotonic 200 nm excitation (31% vs. 17.5% at 25
oC, respectively), due to the broadening of the electron distribution. The
biphotonic electron detachment is very inefficient; the corresponding
absorption coefficient at 400 nm is < 4 cm TW-1 M-1 (assuming unity quantum
efficiency for the photodetachment). For [OH-] between 10 mM and 10 M, almost
no concentration dependence of the time profiles of solvated electron kinetics
was observed. At higher temperature, the escape fraction of the electrons
increases with a slope of 3x10-3 K-1 and the recombination and
diffusion-controlled dissociation of the close pairs become faster. Activation
energies of 8.3 and 22.3 kJ/mol for these two processes were obtained. The
semianalytical theory of Shushin for diffusion controlled reactions in the
central force field was used to model the geminate dynamics. The implications
of these results for photoionization of water are discussed.
