Predominance of the second-order, two-step mechanism in the electron impact double ionization of helium at intermediate impact energy

Abstract

The (e,3-1e) four-fold differential cross sections (4DCS) are measured for the double ionization of helium in coplanar asymmetric geometry for a wide range of ejected electron energies and at an incident energy of about 600 eV. The experimental angular distributions of the 4DCS are characterized by large angular shifts of the forward and backward lobes with respect to the momentum transfer direction or its opposite, respectively. This validates our previously published results [

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

“…The experimental results in Fig. 1(a) to (c) and in 1(f) have been previously published in [16]. They are reproduced here for the completeness of the discussion.…”

“…This analysis is described in details in [16], hence it will only briefly be summarized here. TS2 is a two-step process: in the first step, the slowest 'c'-electron is ejected in an (e,2e) ionising process of the He atom, where the corresponding scattered electron (called 'a*') appears with the highest probability at the Compton scattering angle, ±θ a* , corresponding to the Bethe ridge [26,27] in this work, from the equal (case (f)) to the highly unequal (case (e)) sharing.…”

“…A coplanar geometry is used, where all electrons are observed in the collision plane defined by the incident and scattered momentum vectors, k 0 and k a , respectively. The fast, forward-scattered electron (indexed 'a') is detected at the scattered energy E a = 500 eV and at two symmetrical scattering angles, θ a = + (6°± 3°) and − (6°± 3°) as set by input slits at the entrance to the electrostatic lenses associated with the 'a'- Table 1 where for completeness we also include the data sets published in [16] as they are also used in the discussion given in the present paper.…”

“…[We briefly recall (see e.g. [16][17][18]) that first order or first Born mechanisms (in the Born series) involve only one single interaction of the projectile with the target. This is the case for the so-called 'shake-off' (SO) and 'two-step 1' (TS1) mechanisms.…”

“…(In the absence at the time of elaborate non-first order calculations, the experimental data in [16] were only compared to first order theoretical results.) Our objective is to add further evidence from both experiment and theory that at the impact energy of the present work (~600-700 eV) DI of Helium atom is dominated by non-first order mechanisms such as TS2.…”