Sulfur pair in silicon: ${}_{}{}^{33}{}_{}{}^{}\mathrm{S}$ electron-nuclear double resonance

Sulfur pairs in silicon are studied by electron paramagnetic resonance (EPR) and by ${}_{}{}^{33}{}_{}{}^{}\mathrm{S}$ electron-nuclear double resonance. The trigonal symmetry and electron spin S=(1/2 are experimentally established. For magnetic field B parallel to the [111] pair axis, the EPR intensity is strongly reduced. This gives the spectrum its isotropic appearance. A value for the g anisotropy is reported. The ${}_{}{}^{33}{}_{}{}^{}\mathrm{S}$ nucleus experiences a large quadrupole effect. The nearly isotropic hyperfine interaction is consistent with an even-parity ground state. Two possible models for the valence electronic structure are discussed. The g anisotropies of ${\mathrm{S}}_2^{+}$ and ${\mathrm{Se}}_2^{+}$ are qualitatively explained.