Spectrum and polarization of helium doubly excited triplet states

The beam-foil spectrum of helium from 2050 to 3600 Å 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 Å) from 0° to 80° in 10° increments and for two weaker transitions (2562 and 2818 Å) 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 $\frac{M}{I}$ (a Stokes parameter) at 0° foil tilt for helium at this energy. The lifetime of the 2578-Å 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 $2p3p{}_{}{}^3{}_{}{}^{}D$ levels of 6×${10}^{-6\mathrm{}}$ eV.