DOUBLY-EXCITED RESONANCE STRUCTURES IN HE PHOTOIONIZATION FROM 1S2S S-1,S-3 METASTABLE STATES

The resonant structures corresponding to selected He sp, 2(+/-) and 2p nd P-1,3(0) autoionization series below the He+ N=2 threshold, followi ng photoionization from 1s2s S-1,S-3 metastable states, are examined i n detail. Theoretical photoionization cross sections, calculated using a B-spline-based configuration-interaction approach with a length-vel ocity agreement better than 1%, are presented. A sign change in Fano p rofile parameters q in 1s2s S-1-->sp,2n(+ 1)P transitions along the 2n (+) autoionization series is identified and analyzed.