Liquid water ionization: mechanistic implications of the H/D isotope effect in the geminate recombination of hydrated electrons

The geminate recombination of hydrated electrons after two-photon excitatio n of liquid H2O and D2O at 10 eV two-photon energy was followed by time-res olved absorption spectroscopy. The recombination was found to proceed faste r and with a higher yield in D2O than in H2O. This reversal of the H/D isot ope effect, as compared to earlier results with excitation energies of 7.7- 9.3 eV, allows one to assign the prevailing photoionization mechanisms of l iquid water: up to 9.3 eV, a concerted electron-transfer process mainly inv olves preformed acceptor sites and probably, at the lowest photon energies, a charge-transfer-to-solvent-like transition. The dynamics of these proces ses is not influenced by proton/deuteron substitution. Between 9.3 and 9.5 eV, the ionization mechanism begins to switch over to a delayed autoionizat ion process, critically depending on either a proton or a hydrogen atom tra nsfer. The vertical photoionization energy of liquid water is conjectured t o be around 11 eV. Only at that energy, fast above-threshold autoionization can set in, which is again independent from proton motions. (C) 2000 Elsev ier Science B.V. All rights reserved.