Observation of Triplet Doubly Excited States in Single Photon Excitation from Ground State Helium

We have observed, for the first time, $\mathrm{LS}$-forbidden triplet doubly excited states, in single photon excitation of ground state helium, below the second ionization threshold. These states are identified as ${}_{}{}^3{}_{}{}^{}D_{}^o{}_{}{}^{}$ and ${}_{}{}^3{}_{}{}^{}P_{}^o{}_{}{}^{}$ and their excitation is due to spin-orbit interaction that mixes them with the optically allowed ${}_{}{}^1{}_{}{}^{}P_{}^o{}_{}{}^{}$ states. This observation is possible due to the very high efficiency in detecting metastable atoms created after the fluorescence decay of the doubly excited states, and the new capabilities of third generation synchrotron vacuum ultraviolet sources with high resolution beam lines.