Spectral Intensities for the Emission |$\text{}^{4}$S$\text{}_{3}\text{}_{/}\text{}_{2}$ Γ$\text{}_{8}$〉 → |$\text{}^{4}$I$\text{}_{15}\text{}_{/}\text{}_{2}$ Γ〉in the Cs$\text{}_{2}$NaErCl$\text{}_{6}$

We report explicit vibronic intensity calculations for the |$\text{}^{4}$S$\text{}_{3}\text{}_{/}\text{}_{2}$ Γ$\text{}_{8}$> → |$\text{}^{4}$I$\text{}_{15}\text{}_{/}\text{}_{2}$ Γ>excitations in the Cs$\text{}_{2}$NaErCl$\text{}_{6}$ elpasolite-type systems, based upon new and updated experimental data, obtained from optical absorption measurements made at 10 K, in the energy range from 6,000 up to 26,000 cm$\text{}^{-1}$. Our calculation model is a generalization of the vibronic crystal field-ligand polarization method and the calculation is performed using a minimum set of adjustable parameters - all and each of them have a clear physical meaning. Our strategy was chosen so as to make a significant distinction with previous calculations, performed by other authors, who have worked out a model originated from a supra-parameterized scheme, within the framework of the superposition model of Newman. Throughout the course of the current work, it is shown that our model is suitable to handle this kind of calculations and also that the numerical results obtained are in fairly good agreement with experiment.