The refractive index and extinction coefficient of purple K3Sb film prepared on a quartz substrate were determined from the transmittance data in the photon-energy region 0.5–6.2 eV through the Kramers–Kronig (KK) dispersion analysis. The interference effect due to the internal reflection of the light in the substrate was eliminated by taking the average value over the substrate thickness. The refractive index in the low-photon-energy region is nearly equal to 2.0, e.g., 1.97 at 0.5 eV and 2.17 at 0.9 eV. The static refractive index evaluated from the KK integral of the imaginary part of complex dielectric constant up to 10 eV is 1.98. The spectrum of the refractive index shows some structures corresponding to the structure in the transmission spectrum; the peak at 1.05 eV corresponds to the start of the fundamental absorption, the sharp decrease from the shoulder at 2.1 eV to the minimum at 2.75 eV corresponds to the first strong absorption band, and the well-defined peak at 3.35 eV corresponds to the sharp increase of the absorption. Calculated reflectivity using the refractive index and extinction coefficient obtained from the KK dispersion analysis explains quite well the features of the measured reflectivity in the 2.7–5.8-eV range.
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