Experimental Determination of the Magnetic Moment of the Negative Sulfur Ion

We describe the measurement of the atomic $g$ factor of the negative sulfur ion in the ${}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ ground state. This measurement uses a new technique for observation of microwave transitions in negative ions. Zeeman transitions in ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}_{}^{-}{}_{}{}^{}$ ions confined in a Penning trap are made observable through state-selective photodetachment. The magnetic field is measured by driving the cyclotron motion of electrons trapped in the same trap. The $g_J$ factor is found to be 1.333 984(28).