Positron-helium collisions: Positronium formation using the distorted-wave approximation

Accurate Hylleraas-type correlated helium wave functions are used to predict positronium (Ps) formation cross sections in positron-helium collisions within the frame work of the distorted-wave approximation at intermediate and high energies of positron impact. Exponential correlated atomic target wave functions taking into account up to N=30 basis terms are utilized. Reliable total cross sections for the ground and excited 2s-state Ps formation are reported at intermediate and high energies. The present distorted-wave results are in conformity with the existing theoretical and experimental values available in the literature for intermediate and high-energy positrons. Surface plots of the DWA differential cross section reveal rich structures due to constructive and destructive interference between angular momentum states of the moving Ps.