On the Photoionization Mechanism of Liquid Water

Abstract

Experimental data on the photoionization of liquid water are reviewed and reanalyzed. It is suggested that, near to the ionization threshold at 6.5 eV and up to about 8 eV, an optical Charge Transfer (CT) mechanism leads to the formation of hydrated electrons with a low quantum yield. The absorption band of liquid water at the longest UV wavelengths is assigned to this CT transition superimposed on an intramolecular transition into the R3s state. At excitation energies where the R3p states become attainable, i.e. somewhere between 6.5 and 8 eV, a photo‐induced electron transfer mechanism sets in which is characterized by high quantum yields of hydrated electrons and by average electron‐ion separations below those expected for intermediately “quasi‐free” electrons. This mechanism is dominant up to at least 10 eV, i.e. up to the highest excitation energies for which subpicosecond studies are currently available. In this energy range, autoionization generates only a minor part of all hydrated electrons produced. Only at excitation energies approaching the gas‐phase ionization potential, autoionization and direct photoionization are supposed to become dominant. If any autoionization channels are open below excitation energies of 10 eV, then the band gap energy of water, which is currently believed to be about 8.9 eV, has to be located as high as 10–12 eV.

Electrons in condensed media

“…Actually in LRG, almost all ionized electrons can be collected by a modest external field, a fact which is of great importance in the operation of large ionization chambers (Doke, 1981 By contrast, even the primary ionization and excitation processes in polar liquids seem to be different from their gas-phase counterparts, in view of recent experimental and theoretical findings (Sander et al, 1993a, b;Bartels and Crowell, 2000;Mozumder, 2002a). Accordingly, the low-energy processes, down to $6.5 eV, giving a small yield of e À aq ; are due to neither direct nor autoionization, but to optical charge transfer or to photoinduced electron transfer as the energy is increased (Sander et al, 1993a). A concerted proton-coupled electron transfer mechanism, first proposed by Keszei and Jay-Gerin (1992), has support in the experiments of Bartels and Crowell (2000) up to $9.3 eV, while Sander et al (1993a, b) suggest that quasi-free electrons may not be produced until the band gap energy ($10-12 eV) is reached.…”

Wavelength dependence of nanosecond infrared laser-induced breakdown in water: Evidence for multiphoton initiation via an intermediate state

“…1(b) and (c)]. This indicates that excitation into the CB becomes possible in an "autoionization" process, which involves reorganization of the excited water molecule into a form with lower ionization potential [30,31,41,45,48]. Above the threshold for vertical ionization, where nuclear position remain unchanged, the electron ejection length finally reaches a value of about 4 nm [30,49] [ Fig.…”

Note: Deep UV-pump THz-probe spectroscopy of the excess electron in water

“…It is generally accepted that there are at least two photoionization pathways for the excess electron. 2,3,5,6 That is, if the excitation energy is large enough (>9.8-9.9 eV), 3 the electron can reach the conduction band of water directly and vertically. At energies below, a concerted nuclear rearrangement or a coincidentally pre-arranged minimum on the potential surface are required for the detachment process to occur.…”