Electron and positron collisions with polar molecules: studies with the benchmark water molecule

Abstract

It is difficult to measure low-energy cross sections for collisions of charged particles with strongly dipolar systems since the magnitude of such cross sections is completely dominated by collisions in the forward direction. Theoretically, it is possible to account for the strong forward scattering using the Born approximation but the procedure for combining Born 'top-up' with the more sophisticated treatments required to treat the scattering in other directions is not unique. This comment describes recent progress in describing both electron and positron collisions with polar molecules taking the important water molecule as a benchmark. Previous calculations on electron water at collision energies below 7 eV are compared with new experiments. Positron water studies up to 10 eV are re-analysed based on given experimental acceptance profiles, which depend on the details of the apparatus and method used in the measurements. It is suggested that theory is capable of giving reliable results for elastic and rotationally inelastic electron/positron collisions with strongly dipolar species.

Cross-section calculations for positron scattering from pyrimidine over an energy range from 0.1 to 10000 eV

“…More recently, Baluja et al [31] extended the R-matrix code to polyatomic targets for positron collisions for energies below the positronium (Ps) formation. A few targets have been studied since then [6,9,36]. Only a brief summary of the method is therefore given in the present work.…”

“…A widespread procedure to avoid these undesirable divergences is based on using the first Born approximation (FBA) for a charged particle in a point-dipole potential [41][42][43], since the contributions from all the partial waves, both individually and as summed quantities [44], can be calculated analytically within the FBA. Based on this approximation a variety of Born "top-up" procedures [45,46] have been developed in the past [6]. Among them we have chosen the frame-transformation method, implemented by Sanna and Gianturco [45] in the POLYDCS code, which is in turn based on the multipole-extracted adiabatic-nuclei (MEAN) procedure proposed by Norcross and Padial [42].…”

“…Hence, the formula given by Eq. (2) can be understood as a short-range correction to the original Born approximation [6]. The present cross sections were calculated assuming the molecule is a symmetric top.…”

“…However studies regarding positron scattering are scarcer and the majority of them have been restricted to a limited energy domain, either at low [6] or high energies [7]. Moreover, the computational analysis of positron scattering with polyatomic molecules is still fairly limited to relatively simple molecular structures [7][8][9].…”

Detection of HF emission from the Orion Bar

“…We have calculated electron impact excitation rates for the ΔJ = 1 transitions of HF in the Born approximation, and scaled the values to the results of the R-matrix calculations by Thümmel et al (1992). These rates should be accurate to within a factor of ≈2, similar to the well-studied e-H 2 O system with a similar dipole moment (Zhang et al 2009), and sufficient for our purposes. Calculations with RADEX indicate that for T = 100 K and n e = 10 cm −3 , the observed HF line intensity is reproduced for N(HF) ≈ 1 × 10 15 cm −2 .…”