On the electronic structure of liquid water: Facts and reflections

Estimations of the water reorganization energy following excess electron (e(exc)(-)) production and of interband transition energies using magneto-optical Faraday rotation measurements in liquid water have been recently reported. In such a context, the electronic structure of liquid water is presently revisited. The existence of an Urbach tail extending below the water conduction band and its efficient participation in the e(exc)(-) trapping seem to be well substantiated. From thermodynamic considerations, a value V-o similar to-0.75 eV for the energy of the bottom of the conduction band with respect to vacuum, and a value U-t similar to 0.5 eV for the Urbach tail extension are now proposed. Reported semiempirical determinations of the energy gap E-g are presented and discussed, suggesting the average value E-g = 8.7 +/- 0.5 eV. On the whole, the main terms underlying the electronic structure of liquid water would thus have to be revised downwards; also the present values turn out to be close to the corresponding ones estimated for crystalline or amorphous ice. (C) 1997 Elsevier Science B.V.