Polarization study of the extreme-ultraviolet emission from helium following electron impact

Experimental results are presented on the degree of linear polarization of the extreme-ultraviolet emission of neutral and ionized helium following electron-impact excitation and ionization excitation of helium. The polarization of the photon emission from the decay of He (1snp)¹P^o states with wavelengths of 517 to 584 Å has been extended to electron-impact energies of 1500 eV, and compared with theory for electron-impact excitation of neutral helium. In addition, the polarization of a number of the He⁺ decays with wavelengths between 256 and 1640 Å have been measured. Particular attention has been paid to the (2p)²P^o→(1s)²S and (3p)²P^o→(1s)²S decays with wavelengths of 304 and 256 Å, respectively. These have been measured from threshold (66 and 73 eV, respectively) to 1500 eV using a characterized molybdenum/silicon multilayer mirror polarimeter whose reflection and polarization characteristics have been optimized at 304 Å He⁺(2p)²P^o results are compared with a recent threshold alignment measurement and distorted-wave Born-approximation calculation for the (e^-,2e^-) reaction of ionization excitation. Very good agreement with the threshold theoretical predictions is obtained. These results indicate that near threshold, partial waves with L>0 contribute substantially to the two-electron wave function of the escaping electrons. Measurements on the unresolved multiplets at 1215 and 1640 Å, made using a more conventional reflection-type polarization analyzer, indicate that relative fine-structure cross sections are quite different than predicted by some theories.