The form of the threshold phenomena for the excitation of mesic atoms by electrons

For the excitation of mesic atoms ( mu p) by electrons the threshold behaviour changes its type on the threshold when the mesic atom principal quantum number reaches N=9. For N<9, the excitation cross sections start from zero on the threshold and increase with energy following a power law. If N>or=9, the excitation cross sections take finite values immediately above the threshold while below the threshold there exist a resonance series distributed according to the geometrical progression law.