A variational approach to the scattering of fast electrons by hydrogen and helium atoms

Abstract
The rational Pade approximants are proposed to calculate the cross sections for the elastic scattering of electrons by hydrogen and helium atoms. The relation between these approximants and the Schwinger variational principle is used to evaluate the second perturbative order. The contribution from the terms that correspond to the lowest intermediate states is exactly evaluated and that from all the others is replaced by a single state. The energy of this state is determined by requiring the stationarity of the modulus of the forward scattering amplitude. This approach is a high energy approximation. The theoretical results for elastic e-H scattering are reported. For elastic e-He scattering a fair agreement with the available experimental data is obtained at energies greater than 300 eV.

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