Magnetic Circular Dichroism Spectra of Divalent Lanthanide Ions in Calcium Fluoride

Abstract

The broad-band spectra of most of the divalent lanthanide ions in calcium fluoride were studied with circularly polarized light in magnetic fields at liquid-helium temperatures. Large field- and temperature-dependent anisotropies were found in the bands of ions having an odd number of electrons, all of whose states have Kramers degeneracy. A variety of effects are seen for ions having an even number of electrons, ranging from zero as in Ca${\mathrm{F}}_2$:${\mathrm{Er}}^{2+}$ to the large, complex effects in Ca${\mathrm{F}}_2$:${\mathrm{Dy}}^{2+}$. The magnetic linear dichroism (MLD) of both ${\mathrm{Dy}}^{2+}$ and ${\mathrm{Eu}}^{2+}$ were also measured. The magnetic circular dichroism (MCD) effects of the photochromic ions ${\mathrm{La}}^{2+}$, ${\mathrm{Ce}}^{2+}$, ${\mathrm{Gd}}^{2+}$, and ${\mathrm{Tb}}^{2+}$ are similar to each other and are only briefly described. This study is primarily concerned with the nonphotochromic ions, whose spectra are due to transitions between the opposite-parity configurations $4f^n$ and $4f^{n-1\mathrm{}}5d$. These ions all have the ground configuration $4f^n$. The magnetic dichroism effects are discussed in terms of the symmetry properties of the lanthanide ion. The large magneto-optic effects are the result of large spin-orbit coupling and predominantly totally symmetric vibronic coupling. Since the MCD and MLD effects are large throughout entire bands, they may be conveniently used in a variety of optical pumping experiments and may serve to detect changes in level populations.