S-wave collisions of positrons with helium

S-wave elastic collisions of positrons with helium have been studied using the method of models in an optical-potential formulation that gives lower bounds to the phaseshifts within the accuracy of the helium-atom model. Four simple approximations to the helium ground-state wavefunction are used in a trial function with up to 84 correlation terms. The annihilation rate calculated from what is considered to be the best model treated (of analytic Hartree-Fock type with the correct polarizability) is in good agreement with the most recent experiment. The scattering results are compared with those of other recent calculations. In a related calculation involving the eigenvalues of the correct positron-helium Hamiltonian in the basis of the correlation functions, a stabilized eigenvalue, which may suggest a resonance, is found in the inelastic region (above the positronium formation threshold and just below the first singlet excited state of helium). (author)