The Hidden Ramsauer-Townsend Effect in Positron Scattering by Rare Gas Atoms

Abstract

The aim of this work is to present and discuss the Ramsauer-Townsend effect in the context of low-energy positron scattering by rare gas atoms. Opposed to what happens in electron-atom scattering, the competition between the static repulsive and polarization attractive potentials makes the phenomenon improbable in positron case. Nonetheless, we show using a semiempirical potential formulation that, while the effect is explicitly observed in the total cross sections for He and Ne, for the heavier noble gases it becomes "hidden" or implicit. We show that as the atomic polarizability increases, the suppression of s-wave goes to higher energies due to the variation of the scattering length with the polarizability. No specific signature for the effect is found in the shape of the differential cross sections but curiously, while the effect is implicit in the total cross section for Ar, it generates a minimum structure in the momentum transfer cross section. The theme is presented for non-specialist audience with emphasis in basic atomic and scattering theories along with practical results, the main objective being to boost the traditional discussions with a new view on the subject.

Semiempirical Calculations on Low-Energy Electron Scattering by Zn and Cd Atoms

“…Following our previous works [16,18] and particularly inspired by the article of O'Connel and Lane [19], we write the effective Hamiltonian for the electron-atom system as…”

“…In our previous investigations on positron-atom scattering [15,16,18], we adopted the model electronic density of Salvat et al [25]. As highlighted by Rabasović et al [26], the model densities given by Salvat et al [25] provide a poor representation of the atomic shell structure.…”

“…where α d is the static dipole polarizability [30]. Here, we adopt the same semiempirical potential as considered in Arretche et al [18]:…”

“…As expected, the potentials for Zn and Cd are rather alike since they exhibit very similar electronic structures. Once the potentials were defined, the cross sections were computed as in Arretche et al [18].…”

Low Energy Positron Scattering by F and F2

“…In this work, we employed three different theoretical approaches in order to solve the scattering equations and to obtain the cross sections. We utilized the method of continued fractions (MCF), the Schwinger variational principle as implemented in the SMC for positrons, and the code POTSCT designed specifically to determine the cross sections for a spherically symmetric potential and adapted for positron-atom scattering as described by Arretche et al…”