On the formation and properties of positronium and its interactions with gaseous atoms

Abstract

A presentation of the formation and properties of the positronium atom are given in the early part of the thesis from a more or less historical viewpoint. This is followed by the major part of the thesis dealing with the scattering of orthopositronium by helium atoms in two approximations. The first is the static approximation including exchange and the elastic scattering cross sections and the corresponding phase shifts are calculated. The second calculation allows for the long range van der Waal's forces acting between the neutral atoms. The evaluation of the long range potential in the adiabatic approximation using a Rayleigh-Ritz method is presented and the elastic cross sections and phase shifts are re-computed with the potential included in the analysis. Little difference is found in the values from the two approximations - except at very low energies. A survey is presented of the quenching phenomena exhibited by positronium in gases and the quenching of orthopositronium in helium is studied in detail. Measurements of the quenching rate of orthopositronium in helium determine the parameter Ƶeff the effective number of electrons per atom in a singlet state relative to the positron. This number may be calculated from the wave function for the orthopositronium-helium atom system. Since we have computed this wave-function in two approximations for the scattering problem we have utilised the results to calculate Ƶeff. At thermal energies we find Ƶeff = 0.037 and Ƶeff = 0.048 in the static exchange case and long range polarisation case respectively. These results are considerably smaller than the most recent, measured values of Ƶeff ~ 0.1-0.25, and the possible reasons for the discrepancy are discussed. The presence of a resonance in positron-atom scattering can be viewed as the attachment of a positron to the atom or as the binding of a positronium atom to the positive ion. The process is likely to be important in various scattering problems and the simplest case, that of positron attachment to a hydrogen atom, is considered here. The method used is that of the projection operator in conjunction with a Rayleigh-Ritz variational technique. The evidence for a positron-hydrogen atom bound state is presented as a result of the calculation and comparison made with the results of other workers.