Paramagnetic Resonance and Fluorescence ofEr3+at Cubic Sites in ZnSe

${\mathrm{Er}}^{3+}$ has been found at two distinct sites having cubic symmetry in ZnSe:Er, Cu. One of these is believed to be a simple Zn substitutional site, and in it the ${\mathrm{Er}}^{3+}$ ground-state transforms as the ${\Gamma }_7$ representation of $T_d$ symmetry. The lowest excited state, at an energy of 6.5±1 ${\mathrm{cm}}^{-1\mathrm{}}$, has ${\Gamma }_8$ symmetry. Electron-paramagnetic-resonance transitions have been seen at 14.1 GHz for both of these states. Because of their proximity, an applied magnetic field appreciably mixes these states, inducing an anisotropy into the ${\Gamma }_7$ resonance which has been measured at 9.3, 14.1, and 35.0 GHz. The $g$ factor of the ${\Gamma }_7$ obtained by extrapolating to zero magnetic field, is 5.950±0.005. This implies that the appropriate Landé factor $g_J$ is 1.190±0.001. The resonance of the ${\Gamma }_8$ state, when fitted to the calculations of Lea, Leask, and Wolf, gives a value of their parameter $x=-0.71\mathrm{}$. We also find $B_4=-(1.9\mathrm{}\pm 0.3)\times {10}^{-3\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $B_6=+(3.3\mathrm{}\pm 0.5)\times {10}^{-6\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $A_4〈r^4〉=-(43\mathrm{}\pm 6)$ ${\mathrm{cm}}^{-1\mathrm{}}$, and $A_6〈r^6〉=+(1.6\mathrm{}\pm 0.2)$ ${\mathrm{cm}}^{-1\mathrm{}}$ for this center. The second ${\mathrm{Er}}^{3+}$ defect, having cubic symmetry, has also been seen in resonance. It has a ${\Gamma }_8$ ground state and shows a strong line fluorescence, primarily from the ${}_{}{}^4{}_{}{}^{}S_{\frac{3}{2}}^{}{}_{}{}^{}$ state, which has a very broad excitation band. The crystal-field splittings of the