AUTOIONIZATION IN ELECTRON-HELIUM COLLISIONS - AN (E, 2E) INVESTIGATION

In this (e, 2e) study into the n = 2 autoionization resonances of heli um we present results for the triple differential cross sections (TDCS ) at an incident energy of 80 eV. The scattered-electron angle is 30 d egrees and the range of ejected-electron scattering angles are between -25 degrees and -135 degrees. The measured coincidence ejected-electr on spectra are analysed in terms of the Shore-Balashov parametrization to obtain the direct TDCS f(r) and the resonance parameters a(r) and b(r) for the (2s2)S-1, (2p(2))D-1 and (2s2p)P-1 resonances as a functi on of the ejected-electron momentum. As in our previous studies (1995 J. Phys. B: At. Mel. Phys. 28 728, 1997 J. Phys. B: At. Mol. Phys. 30 3267) these derived parameters are compared with the results of a calc ulation based within the distorted-wave Born approximation (DWBA) fram ework. The post-collision-interaction (PCI) related energy shift Delta epsilon(r), was also determined in the present experiments. Given the somewhat lower beam energy of this work compared to our earlier inves tigations (94.6-99.6 eV), we had anticipated that we would see larger PCI effects and that our DWBA calculation would prove to be too simpli stic to provide a realistic description of the reaction mechanism. in fact, the calculated parameters (f(r), a(r), b(r)) agree quite well wi th the experimental results, both indicating strong correlations betwe en the resonance amplitudes and the direct ionization amplitudes. Furt hermore, Delta epsilon(r) was, to within the uncertainties in the data , found to be zero across the entire range of ejected-electron momenta studied.