The Strong Potential Approximation with Non-Orthogonality Terms

Non-orthogonality contributions are included in the Strong Potential Approximation (SPA) for electron capture. These non-orthoganality contributions occur because the off-energy-shell Coulomb wave functions, representing the intermediate states of the collision, are not orthogonal to the intial bound state wave functions. As a result of this non-orthogonality an extra term arises in the matrix element between initial and intermediate states. This term may be written as an addition to the interaction potential which is treated in first order perturbation theory in the SPA method. It is shown here that this nonorthogonality contribution is zero in the SPA when peaking approximations are applied.