Paramagnetic-Resonance Spectra ofS2−in Alkali Halides

Abstract

The impurity ion ${\mathrm{S}}_2^{-}$ has been introduced into six alkali-halide crystals: NaI, KCl, KBr, KI, RbBr, and RbI. The center is characterized by extreme $g$ anisotropy; in KBr, for example, the principal $g$ factors were 3.5037, 0.8434, and 0.8388. The first of these corresponds to the S-S bond direction which, in all crystals studied, was parallel to the $〈110〉$ directions of the crystal. The direction of minimum $g$ factor was associated with the direction of the molecular $\pi$ orbital occupied by the unpaired electron. This direction was $〈110〉$ for ${\mathrm{S}}_2^{-}$ in KCl, RbRr, and RbI, but $〈100〉$ for ${\mathrm{S}}_2^{-}$ in NaI, KBr, and KI. Sulfur enriched in the isotope ${\mathrm{S}}^{33}$ was used to investigate the ${\mathrm{S}}^{33}$ hyperfine interaction. The largest ${\mathrm{S}}^{33}$ hyperfine interaction occurred when $H_0$ was parallel to the S-S bond direction, and not, as might have been expected, when $H_0$ was parallel to the direction of the $\pi$ function. This was due to the dominant contribution from the L·I term in the hyperfine Hamiltonian. Hyperfine structure due to lattice nuclei could not be resolved but gave rise to the highly anisotropic linewidths of the resonances.