We report a picosecond laser study of the transient absorption of hydrated electrons generated by the 3-5 eV multiphoton ionization of liquid water. The geminate kinetics indicate that e aqis produced by at least three different mechanisms over this energy range. Power dependence of the signal amplitude shows a two-photon threshold for 4.0 eV excitation and a three-photon threshold absorption at 3.47 eV, consistent with two-or three-photon excitation of the A ˜(1 B 1 ) lowest excited state. For (three-photon) excitation in the range 3.02-3.47 eV very little (e15%) geminate recombination is observed while for the (two-photon) excitation at shorter wavelengths significant recombination (g55%) is observed. In the region of 3.85-4.54 eV, photonenergy-independent kinetics indicate that e aqis produced via two-photon excitation of the A ˜state followed by an ionization process in which the electrons do not obtain any excess kinetic energy. For photon energies in the range of 4.75-5.05 eV, the escape fraction increases slightly, consistent with two-photon excitation of higher energy states. Simulation with a diffusion model shows that the electron is ejected at least 25 Å farther into the bulk for the 3.02-3.47 eV photon energies relative to two-photon ionization in the 3.67-5.0 eV range. We conclude that the larger distances result from a (3 + 1)-photon resonance-enhanced multiphoton ionization (REMPI) process, made possible by visible/near-UV absorption of the water excited states. Possible mechanisms of the water ionization are discussed. A new mechanism is proposed to explain the production of solvated electrons from excitation of the A ˜(1 B 1 ) state of liquid water, well below the Born-Oppenheimer ionization threshold. On the basis of the gas phase properties of this state, we assume a direct dissociation to give OH radical and H atoms, with the excess energy almost entirely transferred to kinetic energy of the H atoms: H 2 O* f OH + H(hot). It is proposed that the hot H atoms immediately react with an adjacent water molecule to form hydronium ion and a solvated electron, in a process analogous to the thermal reaction of H atoms with water at elevated temperatures: H(hot) + H 2 O f H 3 O + + e aq -.

Transient absorption measurements monitor the geminate recombination kinetics of solvated electrons following two-photon ionization of liquid water at several excitation energies in the range from 8.3 to 12.4 eV. Modeling the kinetics of the electron reveals its average ejection length from the hydronium ion and hydroxyl radical counterparts and thus provides insight into the ionization mechanism. The electron ejection length increases monotonically from roughly 0.9 nm at 8.3 eV to nearly 4 nm at 12.4 eV, with the increase taking place most rapidly above 9.5 eV. We connect our results with recent advances in the understanding of the electronic structure of liquid water and discuss the nature of the ionization mechanism as a function of excitation energy. The isotope dependence of the electron ejection length provides additional information about the ionization mechanism. The electron ejection length has a similar energy dependence for two-photon ionization of liquid D 2 O, but is consistently shorter than in H 2 O by about 0.3 nm across the wide range of excitation energies studied.

Time resolved transient absorption spectroscopy has been used to determine quantum yields for electron photodetachment in 193 nm and (where possible) 248 nm laser excitation of miscellaneous aqueous anions, including hexacyanoferrate(II), sulfate, halide anions (Cl -, Br -, and I -), pseudohalide anions (OH -, HS -, CNS -), and several common inorganic anions for which no quantum yields have been reported heretofore:SO 3 2-, NO 2 -, NO 3 -, ClO 3 -and ClO 4 -. Molar extinction coefficients for these anions and photoproducts of electron detachment from these anions at the excitation wavelengths were also determined. These results are discussed in the context of recent ultrafast kinetic studies and compared with the previous data obtained by product analyses. We suggest using electron photodetachment from the aqueous halide and pseudohalide anions as actinometric standard for time-resolved studies of aqueous photosystems in the UV.

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, e aq -, 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 o C, 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).

A simple conductivity jump method was used to measure the escaped solvated electron yield following twophoton excitation of water with Raman-shifted light from an amplified mode-locked Nd:YAG laser. Between 7.8 and 9.3 eV, the quantum efficiency for the escape yields changes from 1.9% to 22%, with an almost exponential dependence on the excitation energy. Quantum efficiency in D 2 O is smaller and resembles the H 2 O behavior at 0.35 eV lower energy. The quantum yield measured for one-photon excitation near the water absorption edge at 6.4 eV is a surprisingly large 1.3%. We propose that the mechanism for low energy photoionizaton of water is best described as a dissociative proton-coupled electron transfer to a preexisting trap.