Photochemical Charge-Transfer Reactions in Frozen Aqueous Systems. II. Formation and Trapping of Holes

Abstract

Ultraviolet irradiation of frozen aqueous solutions of ceric ions was shown to lead to an electron transfer from the solvent molecules in the hydration shell to the ceric ion. The resulting ``hole'' in the matrix is trapped and exhibits a characteristic electron spin resonance (ESR) spectrum. The quantum yield for the trapping of the holes has been investigated as a function of the concentration of the solute and for different ice matrices. ESR spectra of trapped holes have also been observed on optical bleaching of the color centers which are produced by γ irradiation of frozen aqueous matrices containing H2SO4, H3PO4, or HClO4 at 77°K. Subsequent annealing of these colorless matrices at 120°K restores the original color centers, whereas the trapped holes disappear. These processes can be repeated through many cycles, practically without diminution of the color-center—hole concentrations.