Electron exchange model potential: Application to positronium helium scattering

The formulation of a suitable nonlocal model potential for electron exchang e is presented, checked with electron-hydrogen and electron-helium scatteri ng, and applied to the study of elastic and inelastic scattering and ioniza tion of orthopositronium (Ps) by helium. The elastic scattering and the n=2 excitations of Ps are investigated using a three-Ps-state close-coupling a pproximation. The higher (n greater than or equal to 3) excitations and ion ization of Ps atoms are treated in the framework of the Born approximation with present exchange. Calculations are reported of phase shifts and elasti c, Ps excitation, and total cross sections. The present target elastic tota l cross section agrees well with experimental results at thermal to medium energies. [S1050-2947(99)04201-8].