ABSTRACT
The interaction of ionizing radiation-fast electrons, a-particles, x-and ')'-rays, and ultraviolet (UV) and vacuum ultraviolet (VUV) photons-with molecular solids and liquids causes the formation of short-lived electron-hole pairs that, in such media, thermalize and, eventually, localize yielding radical ions and/or trapped (solvated) electrons and holes (see Chaps. 1-5). The distinction between the radical anions and the solvated electrons is arbitrary. For the time being, it will be assumed that "radical ions" have an excess electron or electron deficiency in the valence orbitals of a single solvent molecule ("molecular ions" or "monomer ions") or a small group of such molecules ("dimer ions" or "multimer ions") that do not share charge with neutral solvent molecules that "solvate" them. Naturally, the excess electron in a radical anion is indistinguishable from other valence electrons in this anion. By contrast, in the "solvated electron" (also' known as "cavity electron," see Chap. 7), the electron density resides mainly iIi interstitial sites between the solvent molecules ("solvation cavity") that are polarized by the negative charge at its center (thereby forming the outer shell of a "negative polaron"). The underlying assumption of this visualization is that the solvated electron is a single-electron state whose properties can be given by a band model in which the valence electrons in the solvent and the excess electron in the cavity are wholly separately treated [1]-in the exact opposite way to how the electronic structure of the solvent radical ion is viewed. An additional assumption is usually made that the excess electron interacts with (rigid, flexible, or polarizable) solvent molecules by means of an empirical classical potential. Both of these simplifying assumptions find little support in structural studies of "trapped electrons" in vitreous molecular solids using magnetic resonance spectroscopy [2].
