Measurement of the hyperfine structure of33S−

A measurement of the hyperfine structure of a stable atomic negative ion has been made using optical probing of microwave transitions. Hyperfine-split Zeeman resonances were observed using state-selective photodetachment to probe the ${}_{}{}^2{}_{}{}^{}\mathrm{P}_{3/2}^{}{}_{}{}^{}$ state of ${}_{}{}^{33}{}_{}{}^{}\mathrm{S}_{}^{\mathrm{-}}{}_{}{}^{}$ ions which were stored in a Penning ion trap. The results for the dipole and quadrupole hyperfine constants are a=91.49(9) MHz and b=26.24(23) MHz. The potential for further application of the technique is discussed.