Ionization-excitation of He atoms by electron impact: alignment of )

Abstract

We have measured the alignment of after simultaneous ionization and excitation by electron impact via the non-isotropic angular distribution of photons of the decay . The range of incident electron energy was between and . Within experimental errors we obtained a constant value of except in the range - where the alignment has values between -1.2 and -1.3. These larger negative values can be explained by the decay of two-electron excitations into . The alignment value of -0.92(14) indicates that in the investigated range of energy 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 theoretical value of obtained by a combined DWBA + R-matrix calculation.