Claudia R.C. de Carvalho

Progress with the Schwinger multichannel method in positron- molecule scattering

We report calculations for low-energy positron‐C2H2 and ‐N2 scattering obtained through the Schwinger multichannel method (SMC). Integral and diAerential cross-sections and the annihilation parameter, Zeff , are presented. For nitrogen, evidence of a Ramsauer‐Townsend minimum and the possibility of overcorrelation in the scattering configuration space are discussed. Annihilation probability densities (APD) calculated for He and H2 seem to bring indirect support to a model previously proposed to explain why acetylene, unlike nitrogen, presents very high low-energy annihilation rates. ” 2000 Elsevier Science B.V. All rights reserved.

Annihilation dynamics of positrons in molecular environments

Abstract We present recent results for positron–molecule scattering obtained with the Schwinger multichannel method. Our calculations include integral, differential and momentum transfer cross sections and the annihilation parameter Z eff for C 2 H 2 and C 2 H 4 molecules. We discuss a mechanism for annihilation of positrons in low-energy positron–molecule scattering. We interpret the annihilation process through virtual electronic excitation of the target combined with electronic cloud relaxation and virtual positronium formation. We study the influence of the first electronic state of the target on the cross section and on the annihilation parameter in e + –C 2 H 2 scattering.

The Schwinger multichannel method (SMC) calculations for Zeff were off by a factor of Z

Abstract Our previously reported Z eff values were in accidental agreement with experiment. They were actually underestimated by a factor of Z . In order to full understand the nature of the discrepancies with experimental data, we carried out several tests for positron–He collisions. We first tried many different (Cartesian Gaussian) basis sets in a single-excitation configuration–interaction (CI) framework. Then, we included single- and double-excitations of the target in the CI expansion (CISD) of the scattering wave function. Finally, we tried to mimic the virtual positronium cusp. It seems that cusp description should indeed be further improved.

On the Contribution of Polarization - Correlation Forces to High Annihilation Rates in Positronmolecule Collisions

We present calculated integral cross section and annihilation parameter (Z eff) for positron collisions against the H 2 molecule. The results were obtained with the Schwinger multichannel method (SMC) [ J. S. E. Germano and M. A. P. Lima, Phys. Rev. A 47, 3976 (1993) ]. Our model takes correlation-polarization forces into account and completely disregards nuclear motion and real positronium formation. The results indicate that the occurrence of an electronic Feshbach resonance should lead to high annihilation rates.