Vibronic Transitions in Ruby and MgO:V2+

Experiments are described on the low-temperature optical fluorescence of ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$:${\mathrm{Cr}}^{3+}$ and Mg${\mathrm{O}:}^{2+}$, in which the vibronic side-band intensity near the sharp $R$ line is measured as a function of the energy separation $\hslash \omega$ from the no-phonon $R$ line. The results are in reasonable agreement with Vredevoe's theory, which predicts for low $\omega$ an intensity proportional to ${\omega }^3$ for ${\mathrm{Al}}_2$O:C${\mathrm{r}}^{3+}$, and to ${\omega }^5$ for MgO:${\mathrm{V}}^{2+}$.