Angular distribution of the fluorescence of helium doubly photo- excited states converging on the He+(N = 2) ionization threshold

The asymmetry parameter β for fluorescence photons emitted during the radiative decay of the sp2,n+(1Po), sp2,n−(1Po), 2pnd(1Po) and 2pnd(3Do) series of helium doubly excited states, formed by the absorption of linearly polarized synchrotron radiation, has been measured. The results are compared with recent theoretical calculations ( ˇ Zitnik et al 2002 Phys. Rev. A 65 032520). At high n the β parameter is shown to increase and oscillate due to the excitation of triplet states, optically accessible from the helium ground state due to spin– orbit effects.