Radiative and relativistic effects in the decay of highly excited states in helium

A combination of R-matrix multichannel quantum defect theory (MQDT), optical potential, and frame-transformation methods was used to derive He(2lnl?) photofluorescence cross sections with LS-forbidden characteristics. As a result, new high-resolution measurements were reproduced to a remarkable degree. The principal effect of the spin-orbit operator was quantified as an oscillatory perturbation below threshold.