Autoionization in electron-helium collisions: an (e, 2e) investigation

In this (e, 2e) study into the n = 2 autoionization resonances of helium we present results for the triple differential cross sections (TDCS) at an incident energy of 80 eV. The scattered-electron angle is 30 degrees and the range of ejected-electron scattering angles are between -25 degrees and -135 degrees. The measured coincidence ejected-electron 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 function 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 calculation based within the distorted-wave Born approximation (DWBA) framework. 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 investigations (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 simplistic to provide a realistic description of the reaction mechanism. in fact, the calculated parameters (f(r), a(r), b(r)) agree quite well with the experimental results, both indicating strong correlations between the resonance amplitudes and the direct ionization amplitudes. Furthermore, Delta epsilon(r) was, to within the uncertainties in the data, found to be zero across the entire range of ejected-electron momenta studied.