Radio-Frequency Spectrum ofHe−3andHe−4

All independent energy intervals have been measured in the metastable $1s2s2p{}_{}{}^4{}_{}{}^{}P$ state of the helium negative ions ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{-}{}_{}{}^{}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{-}{}_{}{}^{}$. These results are of sufficient precision to provide a critical test of any proposed wave functions for this state and, when adequate wave functions become available, can be used to test relativistic and radiative corrections in three-electron atomic systems.