ON THE PHOTOIONIZATION MECHANISM OF LIQUID WATER

Experimental data on the photoionization of liquid water are reanalyze d. It is suggested that, near to the ionization threshold at 6.5 eV an d up to about 8 eV, an optical Charge Transfer (CT) mechanism leads to the formation of hydrated electrons with a low quantum yield. The abs orption band of liquid water at the longest UV wavelengths is assigned to this CT transition superimposed on an intramolecular transition in to the R 3 s state. At excitation energies where the R 3 p states beco me attainable, i.e. somewhere between 6.5 and 8 eV, a photo-induced el ectron transfer mechanism sets in which is characterized by high quant um yields of hydrated electrons and by average electron-ion separation s below those expected for intermediately ''quasi-free'' electrons. Th is mechanism is dominant up to at least 10 eV, i.e. up to the highest excitation energies for which subpicosecond studies are currently avai lable. In this energy range, autoionization generates only a minor par t of all hydrated electrons produced. Only at excitation energies appr oaching 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 g ap energy of water, which i s currently believed to be about 8.9 eV, h as to be located as high as 10-12 eV.