Abstract
A new spectroscopy for solution, designated Photoelectron Emission Spectroscopy, has been developed. Since this technique is applicable to almost any solution and provides the ionization potential of solutes, the ionization potentials can be measured for solutes which give neither a reproducible oxidation potential by electrochemistry nor the ionization potential in a vacuum by photoelectron spectroscopy. Since photoionization is a one-electron oxidation reaction, the ionization potential in solution is very similar to the oxidation potential in character. Therefore, the ionization potential measured by the present method is designated as the optical oxidation potential. In this review the optical oxidation potential values are given for organic molecules, metal complexes and halide anions, and are compared with the electrochemical oxidation potentials and the results by other spectroscopies. In order to make proceed an oxidation reaction at an electrode, there is a limit for the electrode potential, i.e. no higher potential is necessary to accelerate the reaction, which is the optical oxidation potential proposed here, while there exists a lower limit known as the standard oxidation potential. The difference between these two oxidation potentials provides a kinetically important parameter, the reorganization energy for electron-transfer reaction. The present spectroscopic method has proved to be a technique which can provide a new measure of the reducing power of solvated species and new insights into the solvation energy and solvation structure. Its application to the analysis of an air/solution interface is also described.
