Two-center electron-electron correlation within the independent event model

Abstract

The Independent Event Model (IEVM) is used to analyze collision processes for systems with three and four electrons, in situations where up to three active electrons are involved and dynamical correlation effects play a major role. The model is applied to single electron loss and loss-ionization processes. All the possible ionization mechanisms for both collision partners in each exit channel are considered, including antiscreening, which, in the IEVM, can be taken into account in a consistent way along all the other ones, keeping unitarity. As a casestudy, the IEVM is applied to the analysis of the single loss of He + , Li + , and Li 2+ ions by He atoms with the simultaneous single and double ionization of the target. We present plots of the cross sections of single electron loss accompanied or not by the ionization of the target as functions of the projectile energy. The calculations describe well the experimental energy dependence and the high-velocity absolute values for the cross sections. I IntroductionCollisions between dressed ions -considered here as ions which carry electrons -and neutral atoms, when both collision partners have active electrons, present a multiplicity of possible simultaneous exit channels which result in singleor multi-electron transitions within and between the participating systems. Some of these channels include the single or multiple ionization of the projectile and of the target atom, followed -or not -by the capture of one -or more -target electrons by the incoming ion. Particular attention, from both the experimental and theoretical points of view, has been drawn in recent years to the simultaneous ionization of dressed projectiles colliding with neutral noble gas targets, which is governed by two competing mechanisms, the so-called direct loss-ionization and antiscreening modes [1,2,3,4]. The importance of the dynamical electron correlation in this type of collision is now well established, in particular at intermediate and high velocities, where the electron-electron interaction (antiscreening) can dominate the total electron loss cross section [5,6]. This has been evinced experimentally by the measurement either of total cross sections [7,8,9] or of the momentum distributions of the recoil ions, which provide a definite signature for the antiscreening mechanism [10,11,12,13].The simultaneous occurrence of competing processes, mainly when they include two-center electron correlation, renders difficult a comprehensive theoretical description of the collision. Only in the simplest cases, single-channel analyses can be used to describe properly the experimental results. The pioneering works of Bates and Griffing [14] provided the first description of such electron-loss processes within the first-Born approximation. Actually, the behavior of both the screening and the antiscreening modes for light targets is conveniently described by first-order models, such as the plane wave Born approximation (PWBA) [2,3,15,16,17].There are not many options available to treat collision syste...

Absolute cross sections for projectile electron loss accompanied by target multiple ionization in collisions of He+with noble gases

“…The absolute cross sections for the multiple ionization of He, Ne, Ar, Kr and Xe atoms accompanied by the electron loss of the He + projectile are listed in tables A.1-A.5 as functions of the projectile energy. The data for He and Ne targets have already been published in graphical form in [80] and [8], respectively; for the sake of completeness, we present them here in tabular form.…”

“…It should be mentioned that the sum of the partial cross sections presented here have already been used in [24] in their analysis of the antiscreening contribution to the projectile electron loss. Also, the data for the He and Ne targets have already been published in graphical form in previous works (see [80] for He and [8] for Ne). However, here they are also presented in tabular form and analysed using different approaches from those in the articles, in conjunction with the other targets.…”

Excitation of hydrogen atoms in collisions with helium atoms: the role of electron–electron interaction

“…Second, a full treatment requires in principle the inclusion of a dynamic electron-electron interaction (also called electron correlation) [24]. In ion-atom collisions, it has been shown that electron correlation has to be included in many cases in order to explain experimental cross sections [25][26][27][28][29]. For example, at low projectile energies, calculations clearly show that, even if the electron-electron interaction can be treated as a perturbation, its inclusion leads to large cross sections, up to a few 10 −17 cm 2 [27][28][29].…”

Dominant screening process in the projectile electron loss for F- + Ar collisions

“…(ii) The antiscreening process has an energy threshold similar to the one found in electron impact ionization [4,5,13]. The screening process is therefore, in principle, prominent below the antiscreening threshold.…”