Excitation energy dependence of the photoionization of liquid water.

Liquid water was ionized by UV two-photon excitation with intense subpicosecond laser pulses at 248.5 nm (two-photon energy 10 eV). Transient absorption signals, attributed to hydrated electrons, were monitored with probe pulses at 497 nm. It was found that about 30% of the hydrated electrons undergo geminate recombination with H[sub 3]O[sup +] ions or [sup [sm bullet]]OH radicals, which is considerably less than found in previous studies with excitation pulses at 310-312.5 nm (two-photon energy 8 eV) but still more than estimated for electron pulse radiolysis. Up to an excitation energy of 10 eV, the majority of ionization events is suggested to follow a concerted electron-transfer mechanism; in contrast to earlier conclusions, [open quotes]quasi-free[close quotes] electrons apparently play only a minor role at these excitation energies. 39 refs., 2 figs., 1 tab.