Schwinger multichannel method for positron-molecule scattering

Abstract

We present a formulation for low-energy positron-molecule collisions based on the Schwinger multichannel method (SMC) which was previously adapted for electron-molecule scattering. We have preserved very important features of the SMC such as its ability to deal with electronically inelastic scattering and its applicability to molecular targets of arbitrary geometry. Our implementation also allows for the inclusion of a substantial number of closed electronic channels to represent polarization effects, which are very important at low impact energies. As an application, to illustrate the form with which polarization is represented in this formulation, we have calculated elastic integral and differential cross sections for e+-H2 and e+-CH4 collisions. We included all possible single-particle excitations from the target and found substantial disagreement with respect to model potential calculations and attenuated-beam experimental data. We discuss possible reasons for these discrepancies.

Positron scattering from methane

“…It has been well described elsewhere [29][30][31][32][33] and so only the main aspects will be highlighted here. The final working expression to …”

Positron collisions with ethene

“…The experiment was based on a linear transmission technique [3], covering the positron energies from 0.1 to 70 eV. The calculated fixed-nuclei elastic cross sections (not accounting for positronium formation) were obtained with the Schwinger multichannel method [17,18] at very low collision energies, up to 10 eV. The description of polarization effects has been improved with respect to the previously reported calculations by da Silva et al [10,13].…”

“…This method has been described in detail in several publications [17,18], so here we will only discuss those points that are relevant to the present calculations.…”

Recent advances in the application of the Schwinger multichannel method with pseudopotentials to electron-molecule collisions

“…This result is general and independent of the level of correlation used for describing the target states Φ i . A more straightforward way of obtaining a Schwinger multichannel equation was proposed by Germano and Lima [44] for positron-molecule scattering. With the help of the open (P ) and closed channel (Q) projectors, where P + Q = 1, the Schrödinger equation was written as…”