An alternative theory for charge exchange at arbitrary energies is introduced. The method is a Schwinger-type variational unification of the leading second-order Born perturbation approximations and the efficient $L^2$-expansion theories. This new variational principle is devised in accordance with the correct boundary conditions for three charged particles. The present theory exhibits a number of advantages over all the existing methods in regard to both the conceptual and computational points of view.

• Received 10 January 1989

DOI:https://doi.org/10.1103/PhysRevA.39.6134