Raman electronic paramagnetic resonance (Raman-EPR) of Cr$^{3+}$ in ruby*

We have observed the Raman-EPR of the Zeeman split $^{4}$A$_{2}$ ground state of the Cr$^{3+}$ ion in Al$_{2}$O$_{3}$:Cr, i.e., ruby, exploiting the resonance conditions associated with the R$_{1}$ line. Employing a tunable dye laser with a photon energy E$_{L}$ in the vicinity of the Zeeman components of the R$_{1}$ luminescence, we observe the Stokes and anti-Stokes Raman transitions with shifts corresponding to the intra $^{4}$A$_{2}$ ground state levels split by the external magnetic field (\textbf{B}). The proximity of the incident and the scattered radiation to the Zeeman components of R$_{1}$ leads to selective dramatic resonance enhancements of the intensities of EPR transitions brought about as a function of \textbf{B} and E$_{L. }$The microscopic mechanism for the resonance enhancement involves the `in resonance' and `out resonance' conditions fulfilled by the virtual transitions from the sublevels of $^{4}$A$_{2}$ ground state to the sublevels of $^{2}$E by the incident and the scattered radiation in a two step process. Raman-EPR of the Zeeman sublevels of $^{2}$E excited state of R$_{1}$ is also observed.