Electronic State Densities of Aquo‐Complex Ions in Water Determined by Electrochemical Tunneling Spectroscopy

An oxide‐covered platinum‐silicide electrode has been utilized for determining electronic state densities of ions in aqueous electrolytes. A photoemission experiment in electrolytes has shown that the barrier height of the oxide layer, which is an essential parameter for the tunneling spectroscopy, is about 3.1 eV on electrodes suitable for the tunneling spectroscopy. The electronic densities of states for Ce^3+/4+ and Fe^2+/3+ ions, both of which are expected to form aquo‐complexes in aqueous electrolytes, have been investigated. A peculiar feature noted in the obtained results is that the electronic state densities for these ions are not a simple gauss‐shape distribution function but a displaced‐gaussian with two characteristic reorganization energies and . Here, is an ordinary reorganization energy characterizing the width of distribution, where as characterizes the energetic displacement of the distribution toward the higher energy directions. The estimated values of and are 0.7–0.8 and 0.4 eV for Ce⁴⁺, whereas 0.4–0.5 and 0.8 eV for Fe²⁺, respectively. The origin of has been attributed to the reorganization of solvated water molecules based on the change in the ionic radii of ions.