Distorted-wave theory for positron-hydrogen collisions

The scattering amplitude for positron-hydrogen collisions has been conveniently derived within the framework of distorted-wave theory by approximating the distortion potential in a particular channel as the average of perturbation of that particular channel over the bound states. A real advantage of this method is that a few straightforward calculations yield the expression of scattering amplitude as a function of partial wave. The method has been successfully applied to study 1s-2s excitation of hydrogen atom by positron impact and Ps(ns) (n=1,2,3) formation in positron-hydrogen collisions in the intermediate and high energy range. Further to bring the scattering amplitudes in tracktable forms we have formulated an effective and efficient method for evaluating general three-denominator Lewis integral. The results compare nicely with other theoretical and experimental results available in the literature.