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

The asymmetry parameter P for fluorescence photons emitted during the radiative decay of the sp2,n+(P-1(o)), sp2,n-(P-1(o)), 2pnd(P-1(o)) and 2pnd(D-3(o)) 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 (Zitnik et al 2002 Phys. Rev. A 65 032520). At high n the beta 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.