The X-ray diffraction patterns of vanadium pentoxide (V2O5) films, deposited by flash evaporation on glass slides upheld at 400 °C, reveal orthorhombic \(\alpha\) -V2O5with a prominent (001)-peak. The normal-incidence transmittance-wavelength \((T_{\exp } (\lambda ) - \lambda )\) spectra of the {air/flash-evaporated V2O5 film/glass slide/air} samples had been collected in the \(\lambda\) -range 400–1100 nm. There \(T_{\exp } (\lambda ) - \lambda\) spectra exhibit a steep drop at, \(\lambda \approx 550\,{\text{nm}}\) above which few broad, maxima/minima are displayed. The point-wise unconstrained minimization approach (PUMA) was implemented to fit the \({T}_{\text{exp}}\left(\lambda \right)-\lambda\) data to a theoretical transmittance formula, \({T}_{\text{theor}}\left(\lambda ,{n}_{s}\left(\lambda \right);n\left(\lambda \right), \kappa \left(\lambda \right), d\right)\) . The analysis yielded a value for the thickness \((d\) ) of the \(\alpha\) -V2O5 films and for the spectral dispersion of its refractive index \(n(\lambda )\) and extinction coefficient \(\kappa \left(\lambda \right)\) , with \({n}_{s}\left(\lambda \right)\) , the refractive index of glass slides was pre-calculated at each \(\lambda\) . The extracted \(n\left(\lambda \right)-\lambda\) data fit well a two-constant Wemple-DiDomenico (WDD) equation in the film’s transparent and weak absorption regions. The \(\alpha \left(h\nu \right)\) , the absorption coefficient was calculated from \(\alpha \left(h\nu \right)\equiv 4\pi \kappa \left(\lambda \right)/\lambda\) at each photon energy ( \(h\nu\) ). In this work, the \(\alpha \left(h\nu \right)\) - \(h\nu\) data covering the band-tail and strong absorption regions, together with their overlapped region, were analyzed by a three-parameter Dilogarithm function \({\text{Li}}_{2} {\text{(h}}\nu {; }\alpha_{0} {,}\Gamma_{{\text{U}}} {\text{,E}}_{{\text{g}}} {)}\) to determine simultaneously \(\alpha_{0} ,\Gamma_{U}\) (Urbach-tail energy), and \({E}_{\text{g}}\) (bandgap energy). For comparison, the \(\alpha \left(h\nu \right)-h\nu\) data were fitted to the Tauc and Urbach \(\alpha \left(h\nu \right)\) -formulas in their limited spectral regions to find \({E}_{\text{g}}\) and \({\Gamma }_{\text{U}}\) separately, a procedure that involves complexity in picking properly the individual spectral range. Such problematic features are overcome in the three-parameter \(\alpha \left(h\nu \right)\) -Dilogarithm model.
