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 el ectron-impact excitation and ionization excitation of helium. The polarizat ion of the photon emission from the decay of He (1snp) P-1 degrees states w ith wavelengths of 517 to 584 Angstrom has been extended to electron-impact energies of 1500 eV, and compared with theory for electron-impact excitati on of neutral helium. In addition, the polarization of a number of the Hedecays with wavelengths between 256 and 1640 Angstrom have been measured. P articular attention has been paid to the (2p) P-2 degrees --> (1s) S-2 and (3p) P-2 degrees --> (1s) S-2 decays with wavelengths of 304 and 256 Angstr om, respectively. These have been measured from threshold (66 and 73 eV, re spectively) to 1500 eV using a characterized molybdenum/silicon multilayer mirror polarimeter whose reflection and polarization characteristics have b een optimized at 304 Angstrom He+ (2p) P-2 degrees results are compared wit h a recent threshold alignment measurement and distorted-wave Born-approxim ation calculation for the (e(-),2e(-)) reaction of ionization excitation. V ery good agreement with the threshold theoretical predictions is obtained. These results indicate that near threshold, partial waves with L > 0 contri bute substantially to the two-electron wave function of the escaping electr ons. Measurements on the unresolved multiplets at 1215 and 1640 Angstrom, m ade using a more conventional reflection-type polarization analyzer, indica te that relative fine-structure cross sections are quite;different than pre dicted by some theories. [S1050-2947(99)08208-6].