Excitation of 1 S and 3 S Metastable Helium Atoms to Doubly Excited States

We present spectra of triplet and singlet metastable helium atoms resonantly photoexcited to doubly excited states. The first members of three dipole-allowed 1,3 P o series have been observed and their relative photoionization cross sections determined, both in the triplet (from 1s2s 3 S e ) and singlet (from 1s2s 1 S e ) manifolds. The intensity ratios are drastically different with respect to transitions from the ground state. When radiation damping is included the results for the singlets are in agreement with theory, while for triplets spin-orbit interaction must also be taken into account.