Abstract: A method combining the relativistic multichannel theory with the multichannel quantum defect theory is presented to calculate electron impact excitation. The collision strengths and rate coefficients of U⁹¹⁺ ion are computed with this method. Comparison with the results of R-matrix calculation shows that a) there exists another group of resonances near the excitation threshold; b)with the present method resonance structure can be fully resolved; c)the effective collison strength for 1s-2s transition at low temperiture is almost 20% enhanced by the resonances near the threshold.

Key words: collision strength, effective collision strength, relativistic multi-channel theory, quantum defect theory