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

Abstract The geminate recombination of hydrated electrons after two-photon excitation of liquid H 2 O and D 2 O at 10 eV two-photon energy was followed by time-resolved absorption spectroscopy. The recombination was found to proceed faster and with a higher yield in D 2 O than in H 2 O. This reversal of the H/D isotope effect, as compared to earlier results with excitation energies of 7.7–9.3 eV, allows one to assign the prevailing photoionization mechanisms of liquid water: up to 9.3 eV, a concerted electron-transfer process mainly involves preformed acceptor sites and probably, at the lowest photon energies, a charge-transfer-to-solvent-like transition. The dynamics of these processes is not influenced by proton/deuteron substitution. Between 9.3 and 9.5 eV, the ionization mechanism begins to switch over to a delayed autoionization process, critically depending on either a proton or a hydrogen atom transfer. The vertical photoionization energy of liquid water is conjectured to be around 11 eV. Only at that energy, fast above-threshold autoionization can set in, which is again independent from proton motions.