Asymptotic approach to the theory of excitation of multiply-charged ions by electron impact

The problem of excitation of the positive ions by electron impact is discussed using analytic properties of the Green function for the Coulomb field. The solution obtained has the correct asymptotic expansion in powers of (1/Z) n , where Z-1 is the ion charge. Illustrative calculations for the 2s-3s and 2p-3s transitions in O VI show large resonance contributions to the excitation cross sections.