Magnetic Circular Dichroism of theR2Band in KCl and KF

Circular dichroism has been observed in both the zero-phonon line and sidebands of the $R_2$ transition in KCl and KF. The magnetic field was of magnitude 8 kG and the temperature range was $7{}_{}{}^{\circ }{}_{}{}^{}\mathrm{K}>T>\mathrm{}1.3\mathrm{}{}_{}{}^{\circ }{}_{}{}^{}\mathrm{K}$. The zero-phonon-line dichroism was analyzed by a method of moments. It is shown theoretically that the changes in zeroth moment depend on the strain in the sample, whereas the changes in first moment do not. Experimentally the strain dependence of the zeroth moment is measured by applying a variable external stress simultaneously with the magnetic field. Quite good agreement is found with theory in a non-parameter fit. An estimate of the ground-state splitting in the internal strain field of the crystal is made (${\delta }_0=1.5\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$). The first-moment data yield values of the reduced angular momentum $\Lambda b$ (0.06±0.01 for KCl and 0.11±0.01 for KF) and of the reduced spin-orbit coupling $\Lambda \lambda b$ (-0.32±0.03 ${\mathrm{cm}}^{-1\mathrm{}}$ for KCl and -0.68±0.07 ${\mathrm{cm}}^{-1\mathrm{}}$ for KF). The strength of the Jahn-Teller interaction as defined by the parameter $k^2$ is deduced ($k^2=3.2\mathrm{}$ for KCl and 2.3 for KF). A study of the well-resolved sideband peaks in KF enables a self-consistent assignment of phonon symmetries for the peaks in strain- and magnetic-field-induced dichroism to be made. The good agreement between theory and experiment in all measurements further confirms the Van Doorn $F_3$ model of the $R$ center and Silsbee's analysis based on a dynamic Jahn-Teller effect in the ground state.