Electrochromism is extensively investigated for practical application of display and memory devices. To develop the material, reliable information on the optical and electronic properties of the solid film is indispensable. Here, we propose valence-differential spectroscopy that can selectively extract the spectral components related to the oxidized/reduced metal site. We applied the spectroscopy to Co2+–Fe2+delta and Co2+delta–Fe2+ cyanide films with finely control of averaged valence (delta) of the transition metal by external electric pulses. The spectroscopy revealed transition energy E, width Gamma, and oscillator strength f of the spectral components related to the transition metal.
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