IONIZATION-EXCITATION OF HE ATOMS BY ELECTRON-IMPACT - ALIGNMENT OF HE-2)((2P P)

We have measured the alignment A(20)(P-2) of He+(2p P-2) after simulta neous ionization and excitation by electron impact via the non-isotrop ic angular distribution of photons of the decay He+(2p) --> He+(1s) gamma(304 Angstrom). The range of incident electron energy E(0) was be tween E(He+, n = 2) = 65.4 eV and E(He+, n = 3) = 72.9 eV. Within expe rimental errors we obtained a constant value of A(20)(P-2) = -0.92(14) except in the range E(0) = 69-70.5 eV where the alignment has values between -1.2 and -1.3. These larger negative values can be explained b y the decay of two-electron excitations 3l3l' into He+(2p). The alignm ent value of -0.92(14) indicates that in the investigated range of ene rgy E(0) not only two-electron partial waves with L = 0 but also with L > 0 contribute to the ionization-excitation cross section. Our value of -0.92(14) agrees reasonably well with a nearly constant theoretica l value of approximate to-0.67 obtained by a combined DWBA + R-matrix calculation.