Two-photon dissociation and ionization of liquid water studied by femtosecond transient absorption spectroscopy

The photodissociation and photoionization of liquid water following two-photon absorption at 266 nm is studied in the spectral range from 213 to 1108 nm with subpicosecond time resolution. Probing in the UV enables the first direct simultaneous observation of the photoproducts e(aq)(-) H-aq, and HOaq. This makes it possible to follow the geminate recombination kinetics between the photoproducts and to determine the relative yields of the dissociation and ionization channels. The concentration of hydrated electrons deduced from the visible and near-infrared transient absorption measurements decays by 40% +/- 2% within the first 90 ps due to recombination with OHaq and H3O+. Analyzing our measurements of the hydrated electron concentration using the independent reaction time approximation results in the relative yields of 82% +/- 3%and 18% +/- 3% for recombination with OHaq and with H3O+, respectively. This is in excellent agreement with the relative yield of 82% +/- 10% for recombination with OHaq determined directly from our ultraviolet transient absorption measurements. The contribution of hydrated electrons from direct ionization is insignificant when liquid water is excited below 9.32 eV and the ionization is likely to occur via dissociation and proton transfer. The transient ultraviolet absorption data shows that if ionization exclusively occurs via dissociation, 65% of the produced H(hot) atoms react with the surrounding solvent molecules to produce hydrated electrons. If proton transfer, on the other hand, is the only process responsible for the ionization, our measurements show that the ratio between dissociation and ionization is 55%. Geminate recombination of OHaq and H-aq fragments following the photodissociation at 9.32 eV is not observed indicating that the translation energy of at least one of the fragments is sufficient to penetrate the water solvent cage. Finally, we have measured the two-photon absorption spectrum of liquid water from 110 to 160 nm, and the spectrum is in good agreement with our ab initio gas-phase calculations of the two-photon absorption cross sections for the transitions involved. (C) 1999 American Institute of Physics. [S0021-9606(99)51107-0].