Far-infrared EPR of ${\mathrm{Al}}_2$${\mathrm{O}}_3$: ${\mathrm{Mn}}^{3+}$ and $\gamma$-irradiated ruby

The far-infrared spectra of four samples of Mn-doped ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ were measured in the frequency range from 3 to 30 ${\mathrm{cm}}^{-1\mathrm{}}$ in applied magnetic fields up to 55 kG using the techniques of Fourier-transform spectroscopy. Several absorption lines, which are attributed to ${\mathrm{Mn}}^{3+}$ ions, were observed for fields oriented both parallel and perpendicular to the $c$ axis. Polarization measurements established that the lines were due to electric dipole transitions. The temperature dependence of the lines was studied in the range from 1.3 to 20 K. Both groundand excited-state transitions were studied. The ground state was found to be a singlet. Ground-state transition intensities were found to be magnetic field dependent. A crystal-field theory was developed which successfully describes the low-lying states. The Jahn-Teller interaction, which is known to be important for this ion, is included in the theory only in the approximation that its effect is to quench the strengths of various terms in the Hamiltonian. The empirically determined trigonal field and spin-orbit quenching parameters are approximately 0.6 and 0.1, respectively. Excellent agreement with experiment was obtained for the groundstate transitions. The less good agreement obtained for excited-state transitions reveals weaknesses in the model. A $\gamma$-irradiated ruby was also studied and very weak absorption lines believed due to ${\mathrm{Cr}}^{2+}$ and ${\mathrm{Cr}}^{4+}$ were observed.