Fully Determined (e,3e) Experiments for the Double Ionization of Helium

Abstract

Angular distributions of the two ejected electrons following electron impact double ionization of helium have been measured using a multicoincidence multiangle ͑e, 3e͒ spectrometer at an incident energy ϳ5.5 keV and equal outgoing energies E b E c 10 eV. The measured distributions are similar to photodouble ionization ͑hn, 2e͒ ones, but with evidence of additional nondipolar contributions which tend to fill up the characteristic node at the mutual angle u bc 180 ± . They are also in satisfactory agreement with recent ͑e, 3e͒ calculations. [S0031-9007(98)07708-4] PACS numbers: 34.80.DpThe study of the double ionization (DI) process under photon or charged particle impact has undergone a recent revival due to the spectacular advances in coincidence detection techniques. In these measurements the energy transfer to the target, the energies and momenta of the final electrons are determined. Such a detailed differential investigation is fundamental to unravel the role of the dynamical electron-electron correlation during the collision. The understanding of correlation in multielectron transitions, where the Coulomb force plays a paramount role, is one of the basic unsolved problems of modern atomic physics. Measurement of the coincidence angular distributions of the DI products should provide an essential insight into this problem. The ideal target for such a study is helium, the simplest two-electron system that yields a pure three-body or four-body problem in the final state under photon or electron impact, respectively. In the last few years ͑hn, 2e͒ photodouble ionization (PDI) experiments [1] have provided a wealth of new results in several kinematics and energy sharing conditions from near zero up to 50 eV above the threshold of the process [2]. In contrast, electron impact ͑e, 3e͒ experiments [3] have proved much more difficult to perform. Detailed ͑e, 3e͒ experiments have been recently reported for the outer-shell DI of argon [4] and neon [5], but were unsuccessful for helium. Moreover, because of the low triple coincidence counting rate, the modest energy resolution did not allow us to distinguish among different final states of the doubly charged ion [4,5]. In that sense, these were not kinematically completely determined experiments.In this paper we report the ͑e, 3e͒ cross sections for DI of He. For the first time, a kinematically completely determined experiment (apart from electron spins variables) has been performed, since the He 21 ion is a bare nucleus with no relevant internal structure. These experiments were performed in the so-called dipole scattering regime, characterized by high incident energy and small momentum transfer to the target (i.e., small scattering angle), where the electron impact cross section is expected to converge on the photoionization cross section [6].A complete description of the electron impact ͑e, 3e͒ process in a coplanar geometry is given by the variables represented in the vector diagram in Fig. 1. E j and k j ͑j 0, a, b, or c͒ are the electron energies and momenta. The ...

Experimental and theoretical confirmation of the role of higher order mechanisms in the electron impact double ionization of helium

“…The progress in computational methods, together with the spectacular development of multi-parameter detection techniques, opened up the way to performing complete experiments in which all kinematical parameters (vector momenta and energies) of all involved particles are determined. Electron impact (e,3e) experiments for DI are one of the most fundamental of such processes, though they remain difficult to perform because they necessitate the triple coincidence detection of three particles in the final state (three electrons [1,2] or alternatively the residual ion plus two electrons [3]). …”

(e, 3e) Differential cross section of He (21 S) and He (23 S)

“…The position with multi-electron targets has not changed much since then [3,4]. However, recent experiments [5][6][7] on He have provided a fresh impetus to´e 3eµ studies. Helium is the simplest two-electron atom as the residual He 2· ion is a bare nucleus with no relevant internal structure.…”

Insights from the zero-angular-momentum wave in single and double ionization of He by fast electrons

“…Even the most sophisticated ab initio theories are unable to describe absolute experimental data without the need for a rescaling. The differences are specially important in the high-energy regime, where experimental absolute fivefold differential cross sections are available; see for example, Lahmam-Bennani et al and Taouil et al [2,3].…”