An (e, 2e) study of helium autoionization experiment and theory

Abstract

Results are presented for the triple differential (e, 2e) cross sections in helium in the region encompassing the (2s2)1S, (2s2p)3P, (2p2)1D and (2s2p)1P resonances at an incident energy of 94.6 eV for a scattered electron angle of 30 degrees and for ejected-electron scattering angles in the-range -25 to -135 degrees . The measured coincidence ejected-electron spectra are compared against those calculated in an equivalent local form of the distorted-wave impulse approximation with the resonant scattering amplitudes being evaluated by a six-state momentum-space coupled channels method. In each case the calculation is folded with the experimental energy resolution. The data are analysed in terms of the Shore-Balashov parametrization to obtain the direct (e, 2e) cross section fr and the resonance parameters ar and br for the (2s2)1S, (2p2)1D and (2s2p)1P resonances as a function of ejected-electron momentum. These derived parameters are compared against the results of a calculation where configuration interaction expansions for the resonances and helium ground state, which employed hydrogenic and multiconfiguration Hartree-Fock orbitals, respectively, were used. Here the distorted-wave Born approximation was employed for the (e, 2e) cross section calculation. The calculated parameters agree quite well with the experimental results with both indicating strong correlations between the resonance amplitudes and the direct ionization amplitudes, Finally we report and discuss our results of an experimental investigation into post-collision interaction effects for the present kinematical conditions.

A study of autoionization phenomena in helium using (e, 2e) spectroscopy

“…A number of studies have been conducted on the autoionizing states of helium, with a thorough summary of this work being found in Samardzic et al (1995). Subsequent to those discussed in Samardzic et al (1995), however, we also note the following more recent investigations.…”

“…A number of studies have been conducted on the autoionizing states of helium, with a thorough summary of this work being found in Samardzic et al (1995). Subsequent to those discussed in Samardzic et al (1995), however, we also note the following more recent investigations. Raeker et al (1995) have used a distorted-wave Coulomb-Born no-exchange method, where the fast ionizing electron is described by distorted waves and the slow ejected electron and its interaction with the residual ion by an R-matrix expansion, to calculate the resonance a r and b r parameters and the direct triple differential cross section (TDCS) as a function of the ejected-electron angle.…”

“…Finally we note that Kuchiev and Sheinerman (1989) commented, consistent with the view of , that experiments which record the presence of PCI effects outside the near-threshold region have been inadequate and that it would be desirable to have systematic experimental studies of the phenomenon away from near threshold. In particular, it would be useful to further investigate, in the coincident ejected-electron spectra, whether the observed strong dependence for the PCI energy-shift effect on the kinematics of the process being studied (Samardzic et al 1995) is indeed found under further (different) kinematical conditions. The present experimental work encompasses the results of just such a study.…”

Recent Applications of (e, 2e) Techniques

“…Fig. 2 provides some examples of the observed coincidence ejected electron spectra (Samardzic et al 1995). We see a series of resonance profiles superimposed upon a background of direct ionisation events.…”

“…The reaction has also been used to study the correlations between resonance and direct ionisation amplitudes in the autoionising region of helium e.g. Weigold et al (1975b), Pochat et al (1982), Kuchiev andSheinerman (1989, 1994), Lower and Weigold (1990), McDonald andCrowe (1992, 1993), McCarthy and Shang (1993), Kheifets (1993), and Samardzic et al (1994and Samardzic et al ( , 1995.…”