Structure of the $R_{1}$ and $R_{2}$ bands of isotopes of the Cr$^{3+}$ ion in ruby single crystal at room temperature

Room-temperature absorption and luminescence spectra of the $R_{1}$ and $R_{2}$ bands in a single crystal of Ruby Al$_{2}$O$_{3}$ : Cr$^{3+}$ (0.05%) are analyzed. The luminescence in these bands was induced both by crystal excitation by radiation of a halogen lamp and by selective excitation of two upper levels of the $^2T_{1}$ state by radiation of a tunable laser diode (656–662 nm). Four different shifts of the $R_{1}$ and $R_{2}$ bands and four different distances between these bands the change in which is a multiple of $\sim$ 0.52 cm$^{-1}$ are discovered in spectra of se-lectively excited luminescence bands. Detailed comparative analysis of the obtained spectral bands of luminescence and corresponding absorption spectra allowed determining combined structure of each of the $R_{1}$ and $R_{2}$ bands formed by satellite doublets of these bands associated with all four stable isotopes $^{50}$Cr$^{3+}$, $^{52}$Cr$^{3+}$, $^{53}$Cr$^{3+}$, and $^{54}$Cr$^{3-}$ of chromium ions. Splitting of these doublets varies from 7.04 to 9.14 сm$^{-1}$ depending on the isotope mass.