Spectrum and polarization of helium doubly excited triplet states

The beam-foil spectrum of helium from 2050 to 3600 {} has been acquired at 160-keV incident ion energy using a multiple-scan, computerized system. Eleven doubly-excited-state transitions, including two not previously observed, have been assigned, and the agreement between experimental and theoretical energy levels is excellent. The Stokes parameters as a function of foil-tilt angle have been measured for the two strongest transitions (2578 and 3013 {}) from 0^∘{} to 80^∘{} in 10^∘{} increments and for two weaker transitions (2562 and 2818 {}) at 0^∘{}, 20^∘{}, 40^∘{}, and 60^∘{}. The polarization patterns are quite different from those of the helium singlets and include the first measurement of negative M/I (a Stokes parameter) at 0^∘{} foil tilt for helium at this energy. The lifetime of the 2578-{} transition has been investigated in detail. The present result, 0.109±{}0.004 ns, agrees with previous measurements and yields an upper limit to the autoionization width of the 2p3p³D levels of 6×{}10^-6 eV.