Photochemical Charge-Transfer Reactions in Frozen Aqueous Systems. I. Electron Transfer from the Solute to the Matrix

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Abstract
Hydrogen‐atom ESR signals have been detected at 77°K in ultraviolet‐irradiated frozen acid solutions when the latter contain solutes such as I— and Fe2+, which are characterized by charge‐transfer spectra in the ultraviolet. The quantum yield of hydrogen‐atom formation has been studied as a function of the solute concentration and of the nature and concentration of the acid present in the matrix. The results can be explained on the assumption that ultraviolet irradiation leads to a charge transfer, giving in the first instance an excited state in which the electron is transferred, to a greater or lesser extent, to the hydration shell where it is bound in the field of the oriented water molecules around the ion. The electron in the excited state can either revert to the ground state or may react with electron acceptors present in the matrix, where their reaction with, e.g., acids leads to the formation of H atoms. It has been found that the excited ion can apparently also be deactivated by interaction with another like ion in the ground state. This is suggested by the observation that with increasing concentration of the light‐absorbing ions the H‐atom yield decreases.