Energy of the First Excited Electronic Level (s) ofFe2+in CaO

The electron-spin recovery time $\tau$ has been observed for low concentrations of ${\mathrm{Fe}}^{2+}$ ions in CaO in fields of about 1800 G from 1.3 to 4.1 K; line broadening at 8.9 GHz has been measured from 7 to 15 K. The temperature dependence of $\tau$ and the linewidth give the energy of the first excited state (s) to be 23.9 ± 1.5 ${\mathrm{cm}}^{-1\mathrm{}}$, indicating strong orbital reduction, probably due to dynamic Jahn-Teller distortions. The observed bottleneck generated at 9, 18, and 700 GHz by pulse saturation at 9 GHz is consistent with earlier observation of intrinsic spin-lattice relaxation of ${\mathrm{Fe}}^{2+}$ at similar concentrations in MgO.