Absolute triply differential (e, 2e) cross sections for He in the intermediate energy region with comparison to theory

Absolute triply differential (e, 2e) cross section measurements are presented for He at incident energies E0 = 32.6, 44.6, 64.6, and 104.6 eV with equal energy sharing and the θ12 = π configuration. Results of distorted partial-wave calculations agree with the measurements; those of convergent close-coupling calculations agree with the relative angular distributions, and with absolute values after rescaling by consideration of the singly differential cross section. The (e, 2e) process in He, in the case in which the He+ ion is left unexcited, is a fundamental way of investigating three-body Coulomb dynamics. In the intermediate range of energies from a few eV to the range of 50–100 eV above threshold, the many-body aspects of the process come to the fore, providing a stringent test of electron-impact ionization theories. Interest in triply differential (e, 2e) measurements in both H and He was sparked in the late 1980s by measurements taken at only 4 eV above the ionization thresholds [1, 2]. These showed that the shapes of the angular distributions depend on the target even though the asymptotic Coulomb fields experienced by the three final-state particles are target independent. While theoretical calculations [3–6] were able to replicate reasonably well the experimentally observed (e, 2e) angular distributions, the first absolute measurements, for He [7, 8], agreed with only two of these [4, 6]. The implication was that accurate accounting of target effects on the various electronic partial waves as well as treatment of electron–electron interactions are necessary to obtain agreement with absolute data and that omission of these effects can lead to disagreement with experiment by factors of 2– 200 (see table II of the second reference of [4]). In response to these first absolute measurements for He, theorists have since devoted increased attention to providing not only accurate angular distributions but also accurate absolute cross sections [8–10]. In the intermediate energy region, an absolute measurement for He has been obtained by Gélébart and Tweed [11] at an excess energy of ≈ 75.4 eV, and the relative experimental measurements of Murray et al. [12] have been put on an absolute basis, in part by normalization to the measurements of [11]. However, these latter mea-