Complex Fano asymmetry parameters for helium L = 0, 1, 2 autoionizing levels excited by electron impact

Theoretical Fano q-parameters for He autoionizing states excited by electron impact are presented, as extracted from first- and second-order hybrid distorted-wave Born + R-matrix calculations. It is found that the q-parameters for (2s2)1S, (2p2)1D, and (2s2p)1P from the first-order calculations are essentially real quantities, while those from the second-order calculations are complex but with a real part similar to the first-order values. These findings are interpreted in terms of the relative phases of the first- and second-order amplitudes.