Isotopic Shift in the Electron-Spin-Resonance Absorption Spectrum ofGd3+in Thorium Oxide

Abstract

The fine-structure spectrum of trivalent gadolinium occurring as a cation substitution in thorium oxide has been reinvestigated to determine whether this crystal-ion system exhibits fine-structure splittings which are dependent upon isotopic species. Such a dependence has been observed; however, the magnitude of the effect is found to be just within the limits of detectability. For this crystal-ion system, three spin-Hamiltonian parameters are found to be sufficient to characterize the fine-structure spectrum, an isotopic $g$ value, and two crystal-field-splitting energy parameters, $b_4$ and $b_6$. At 77°K the relative shift in $b_4$ between ${}_{}{}^{155}{}_{}{}^{}\mathrm{Gd}$ and ${}_{}{}^{157}{}_{}{}^{}\mathrm{Gd}$ is $\frac{\delta b_4}{b_4}=4.2\mathrm{}\times {10}^{-5\mathrm{}}$, the value of $b_4$ associated with the heavier isotope being greater in magnitude. No shifts in the $g$ value or in $b_6$ could be detected. When differences in the fine-structure parameter $b_4$ are measured using the average spectral component positions due to the $I=0\mathrm{}$ isotopes as references, shifts for ${}_{}{}^{155}{}_{}{}^{}\mathrm{Gd}$ and ${}_{}{}^{157}{}_{}{}^{}\mathrm{Gd}$ are obtained which follow a simple mass-difference-ratio relationship.