Binary-Encounter Model for Direct Ionization of Molecules by Positron-Impact

Abstract

We introduce two models for the computation of direct ionization cross sections by positron impact over a wide range of collision energies. The models are based on the binary-encounter-Bethe model and take into account an extension of the Wannier theory. The cross sections computed with these models show good agreement with experimental data. The extensions improve the agreement between theory and experiment for collision energies between the first ionization threshold and the peak of the cross section. The models are based on a small set of parameters, which can be computed with standard quantum chemistry program packages.

Electron and Positron Impact Ionization of SF6−nHn(n = 0 − 6); {SCln, SFn−1Cl(n = 1 − 6)} and SF5X(X = CN, CFO)

“…The BEB method can also be used to model the direct positron impact ionization of atoms and molecules with the Binary Encounter Dipole (BED) model combined with the Wannier-type threshold law [36] to give a simple analytical formula for ionization [29,30]. The BEB model is the simplified version of the BED [28] model.…”

“…Also, Bartschat et al showed [27] a way to calculate electron impact ionization cross-sections using the R-matrix method for atoms, which is computationally expensive. In this work, we have used the BEB model, where no fitting parameters are used to calculate electron impact TICS, which was originally formulated by Kim et al [28]; for positron impact TICS, we used the various modified BEB models formulated by Fedus et al [29] and Franz et al [30]. In recent years, the BEB method has been very successful in calculating TICS for various molecules ranging from diatomic [31] to organic molecules [32], with applications in plasma and biological sciences.…”

“Atoms” Special Issue (Electron Scattering in Gases—From Cross Sections to Plasma Modeling)

“…In this volume, we present a "cross-section" of different approaches, applications and processes, from industrial plasmas [3,4] to molecules of biological interest [5][6][7]; from experiments at ultra-low temperatures [8,9] to processes relevant for thermonuclear synthesis [10]. Thanks to a global response to the invitation, this volume hosts in an equilibrated manner the experimental aspects [6,8,9], ab-initio theories [7,[10][11][12] and semi-empirical approaches [13][14][15][16], both for electron and positron scattering [11,[15][16][17].…”

“…The ionization of atoms and molecules, at least in the case of electron collisions, has rather vast experimental coverage. This allows us to test different theoretical and semi-empirical approaches, which, in turn, allow us to predict cross sections for species inaccessible for direct measurements, such as highly charged ions [10], molecular metastables [15] or metals [14].…”

“…In turn, H 2 metastables show even higher total ionization cross sections, up to about 10 × 10 −16 cm 2 at their maxima. A similar model, derived from the BEB approach, has been applied by Franz et al [15] for positron ionization of diatomic molecules. In the high energy limit, according to the Born approximation, positron and electron-scattering cross sections should merge.…”