Electronic Raman Scattering and Infrared Absorption in the Samarium Monochalcogenides

Electronic transitions between the ${}_{}{}^7{}_{}{}^{}F_J^{}{}_{}{}^{}$ multiplet levels of ${\mathrm{Sm}}^{2+}$ have been observed optically in SmS, SmSe, SmTe, and ${\mathrm{Sm}}_{1-x}{\mathrm{Y}}_x\mathrm{S}$. In SmS, SmSe, and SmTe, the strong temperature dependence of the singlet-triplet energy is well described by the random-phase-approximation results of Wang and Cooper, allowing us to determine both the exchange and the spin-orbit interaction. However, no temperature dependence of the singlet-triplet energy is observed in metallic ${\mathrm{Sm}}_{1-x}{\mathrm{Y}}_x\mathrm{S}$.