Refraction index and oscillator strength in MoO₃ photocolored films

Amorphous MoO₃ films were grown onto glass substrates by the thermal evaporation technique. Color centers in the films were generated by UV-VIS light irradiation (photochromism) in air at room temperature. As-grown films looked pale yellowish to naked eye and acquired a bluish tone after the coloration process. Refraction index and extinction coefficient were calculated on as-grown and colored samples by using macroscopic optical differential reflectance measurements fitted with the transfer matrix theoretical model. The coloration efficiency parameter and the refraction index values of MoO₃ films allowed us to calculate the oscillator strength (f) value equal to 0.22 ± 0.02 by means of the optical absorption spectra. This value was obtained by fitting the experimental results to an optional expression of Smakula's formula, derived from a semi-classical theoretical model.