J. Phys. Chem. 1993,97, 11489-11492
11489
Excitation Energy Dependence of the Photoionization of Liquid Water Martin U. Sander, Klaus Luther, and Jiirgen Troe' institut fur Physikalische Chemie, Uniuersitat Gattingen, Tammannstrasse 6, 0-37077 Gattingen, Germany, and Max-Planck-institut f i r Biophysikalische Chemie, Am Fassberg, 0-37077 Gatzingen, Germany Received: June 23, 1993' 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 H3O+ions or '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 electrontransfer mechanism; in contrast to earlier conclusions, "quasi-free" electrons apparently play only a minor role at these excitation energies.
1. Introduction The past 40 years have seen great advances in the experimental and theoretical investigation of the formation and decay of hydrated electrons (e,
