Low-Energy Electron and Positron Scattering by para-Difluorobenzene

Abstract

The Schwinger multichannel method is employed in calculations of the elastic integral and differential cross sections for collisions of low-energy electrons and positrons with para-difluorobenzene (1,4-C 6 H 4 F 2 ). The present calculations involving electron scattering have been performed using both static-exchange and staticexchange plus polarization levels of approximation. Our results indicate the presence of three resonances of π*-character in the low-energy region and a fourth resonance, of σ*-character, at higher energy levels. With respect to the positron scattering, the calculations were conducted using the static plus polarization approximation at three different polarization levels. The present calculated electron and positron cross sections show a reasonable agreement with the total cross sections measured by Makochekanwa et al. (J. Phys. B, 2004, 37, 1841. It is also important to highlight that our computed integral cross section for electron scattering indicates the presence of a Ramsauer−Townsend minimum, while the integral cross section for positron scattering indicates the presence of a virtual state.

Virtual and bound states in low-energy positron scattering by atoms and molecules via modified effective range theory

“…Such a discrete level may weakly bind (when negative) or almost bind (when positive) a particle with a target, enhancing the strength of the interaction. For instance, the concept of virtual state (i.e., positive 'quasi-discrete' energy level) has been used to explain the unexpectedly high rise of the integral elastic cross section as the scattering energy decreases to zero in electron collisions with CO 2 [4], SF 6 [5] and other nonpolar molecules [6,7]. The shallow energy level is also vital in low-energy positron scattering by many atoms and molecules [8,9].…”