Photodetachment study of He^- quartet resonances below the He(n = 3) thresholds

The photodetachment cross section of He^-(1s2s2p ⁴P) has been measured in the photon energy range 2.9-3.3 eV in order to investigate doubly excited states. Measurements were made channel specific by selectively detecting the residual He atoms left in a particular excited state following detachment. Three Feshbach resonances were found in the He(1s2p ³P^o)+e^-(∈p) partial cross section: a ⁴S resonance below the He(1s3s ³S) threshold and two ⁴P resonances below the He(1s3p ³P^o) threshold. The measured energies of these doubly excited states are 2.959 260(6), 3.072(7) and 3.264 87(5) eV. The corresponding widths are found to be 0.20(2), 50(5) and 0.61(5) meV. The measured energies agree well with recent theoretical predictions for the 1s3s4s ⁴S, 1s3p^{2 4}P and 1s3p4p ⁴P states, respectively, but the widths deviate noticeably from calculations for 1s3p^{2 4}P and 1s3p4p ⁴P states.