Doubly excited resonance structures in He photoionization from 1s2s ^1,3S metastable states

The resonant structures corresponding to selected He sp, 2n^± and 2pnd ^1,3P^o autoionization series below the He⁺ N=2 threshold, following photoionization from 1s2s ^1,3S metastable states, are examined in detail. Theoretical photoionization cross sections, calculated using a B-spline-based configuration-interaction approach with a length-velocity agreement better than 1%, are presented. A sign change in Fano profile parameters q in 1s2s ¹S→sp,2n^{+ 1}P transitions along the 2n⁺ autoionization series is identified and analyzed.