EXCITATION-ENERGY DEPENDENCE OF THE PHOTOIONIZATION OF LIQUID WATER

Liquid water was ionized by UV two-photon excitation with intense subp icosecond laser pulses at 248.5 nm (two-photon energy 10 eV). Transien t absorption signals, attributed to hydrated electrons, were monitored with probe pulses at 497 nm. It was found that about 30% of the hydra ted electrons undergo geminate recombination with H3O+ ions or .OH rad icals, which is considerably less than found in previous studies with excitation pulses at 310-312.5 nm (two-photon energy 8 eV) but still m ore than estimated for electron pulse radiolysis. Up to an excitation energy of 10 eV, the majority of ionization events is suggested to fol low a concerted electron-transfer mechanism; in contrast to earlier co nclusions, ''quasi-free'' electrons apparently play only a minor role at these excitation energies.