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Excitation cross sections in new channels remain finite above threshold because of the degeneracy of the levels in the two-particle Coulomb problem. Below the thresholds there are series of resonances distributed according to the geometrical progression law. In cases with one complex eigenvalue lambda , the S-matrix elements in the vicinity of the threshold continuously trace circles on a complex plane: one circle below the threshold, the other above. The properties of the circle are investigated. The logarithmic angular dependence is obtained for the elastic scattering amplitude on excited states at any threshold and for any mass.
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M Gailitis (1982) studied this question.
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