Magneto-Optical Structure of the MgOFBand

The paramagnetic Faraday-rotation pattern of the MgO $F$ center has been investigated, using the ESR-optical double-resonance method, at 12 kG and 1.8°K. The rotation is centered precisely upon the familiar 4.95-eV absorption band, in agreement with the extrapolated position predicted from a previous observation on the extreme low-energy tail of the band. The $p$-state spin-orbit parameter $\gamma$ is given as - (12±3) ${\mathrm{cm}}^{-1\mathrm{}}$. The pattern is highly symmetrical, consistent with a very nearly Gaussian absorption band. The width of the pattern is about 30% less than the width which would correspond to a Gaussian absorption band with rigid magneto-optic shift, if the $F$ band is assumed to be Gaussian with the accepted half-width of 0.48 eV. This implies that noncubic electron-lattice interaction is present: The third moment of the circular dichroism (computed from the observed rotation) in fact has a value such as to indicate that noncubic modes are strongly dominant. The same rotation spectrum is observed in samples of both neutron-irradiated and additively colored MgO. However, in the latter there are curious discrepancies between the apparent $F$-center densities as gauged by the absorption band intensity, the ESR signal strength, and the Faraday rotation amplitude. This is discussed in terms of a postulated near coincidence of the $F$ and $F^{\prime }$ bands, and the occurrence of $F^{\prime }-F$ conversion during the measuring operation; the connection with other observations on the $F-F^{\prime }$ relationship in MgO is indicated.