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.
Modern ultrafast spectroscopic techniques provide new opportunities to study chemical reaction dynamics in liquids and hold the possibility of obtaining much of the same detailed information available in gases. Vibrational energy transfer studies are the most advanced of the investigations and demonstrate that it is possible to observe state-specific pathways of energy flow within a vibrationally excited molecule (intramolecular vibrational relaxation) and into the surrounding solvent molecules (intermolecular energy transfer). Energy transfer in liquids and gases share many common aspects, but the presence of the solvent also alters the relaxation in both obvious and subtle ways. Photodissociation is amenable to similarly detailed study in liquids, and there are informative new measurements. Bimolecular reactions have received the least attention in state-resolved measurements in liquids, but the means to carry them much further now exist. Studying photodissociation and bimolecular reaction of molecules prepared with initial vibrational excitation in liquids is a realistic, but challenging, goal.
Two-photon absorption ͑2PA͒ spectroscopy in the range from 7 to 10 eV provides new insight on the electronic structure of liquid water. Continuous 2PA spectra are obtained via the pump-probe technique, using broadband probe pulses to record the absorption at many wavelengths simultaneously. A preresonance enhancement of the absolute 2PA cross section is observed when the pump-photon energy increases from 4.6 to 6.2 eV. The absorption cross section also depends on the relative polarization of the pump and probe photons. The variation of the polarization ratio across the spectrum reveals a detailed picture of the 2PA and indicates that at least four different transitions play a role below 10 eV. Theoretical polarization ratios for the isolated molecule illustrate the value of the experimental polarization measurement in deciphering the 2PA spectrum and provide the framework for a simple simulation of the liquid spectrum. A more comprehensive model goes beyond the isolated molecule picture and connects the 2PA spectrum with previous one-photon absorption, photoelectron, and x-ray absorption spectroscopy measurements of liquid water. Previously unresolved, overlapping transitions are assigned for the first time. Finally, the electronic character of the vertical excited states is related to the energy-dependent ionization mechanism of liquid water.
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