The improvement of the Schwinger multichannel method for positron-atom/molecule scattering

Author(s)

Sergio d'Almeida Sanchez

• Document type: Doctoral Thesis
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Instituto de Física Gleb Wataghin
• Defense date: 2008-03-28

Examining board members

Marco Aurélio Pinheiro Lima; Lee Mu Tao; Maria Cristina Andreolli Lopes; Amir Ordacgi Caldeira; Luiz Marco Brescansin

• Advisor: Marco Aurélio Pinheiro Lima

Abstract

In this work, three modifications in the Schwinger Multichannel Method (SMC) for positron-molecule(atom) scattering are presented: the inclusion of a complex potential to simulate the absorption of particules, the inclusion of a real delta potential, whose physical origin can be found in the relativistic corrections, and the improvement in the basis set used in the method with the inclusion of functions that describes the f -type atomic orbitals. The results for the complex potential show that its in uence is very small when compared to the Coulomb potential and the new data obtained for the annihilation parameter are not different from the ones calculated without its presence. It was also verified that one can not use the absorption potential to calculate the annihilation parameter directly in the SMC method due to numerical precision problems. We also tested the in uence of the complex potential which shows that, although it affects the scattering cross sections, it does not modify the balance between the atractive and the repulsive potential experienced by the positron, i.e., there is no competition between the real scattering potential and the complex potential. All the later does is simulate the absorption of particules. A real delta potential, however, competes directly with the Coulomb potential. Its use in the selection of basis set to obtain a better description of the annihilation process is impraticable, since it modifies the problem we are studying. The last modification of the method, the inclusion of the f -type orbital, brought important results and significantly enhanced the quality of our cross section data for the Helium atom and the H2 molecule. Its use on bigger molecules and in the electron scattering problem is an important step to be taken (AU)