R-matrix calculations of polarisation effects in low-energy positron-molecule collisions

The study of the interaction of positrons with atoms and molecules has become increasingly popular, because more and more experimental activities have become feasible. Although exchange effects are absent, the polarisation effects, caused by the attractive nature between the positron and the target electrons, make the positron-molecule collisions more difficult to handle than the corresponding electron collisions. This thesis gives the calculations of positron collisions with polar molecule H2O and non-polar molecules, H2 and C2H2 at energies below the positronium formation threshold. All calculations were carried out using the modified version of the UK molecular R-matrix code. Due to the large permanent dipolar nature of water molecule, the three models tested give very similar results. However, for positron collision with non-polar molecules, the polarisation effects can be very important in the calculation. The molecular R-matrix with pseudostates (MRMPS) method has been employed to analyse the positron collision with non-polar molecules, and found to lead to an excellent representation of target polarisation. C2H2 is the simplest molecule that has very enhanced annihilation parameter Zeff , which can be determined by the total scattering wavefunction. So a new sub-code was developed for calculating Zeff based on the UK R-matrix polyatomic code and employed to treat positron collisions with H atom, H2 and C2H2 molecules. It has been found that Zeff values are also sensitive to the degree of polarisation included in calculations and are greatly improved by use of the MRMPS method.