Two-electron excitations in atomic calcium. III. High-lying resonances above the 4p threshold

A theoretical prediction of the photoionization spectrum of atomic calcium from the 4p threshold up to nearly the 6s threshold is obtained by an eigenchannel R-matrix calculation of the final-state multichannel quantum-defect parameters in LS coupling. The calculation shows how strong interactions are distributed among a large number (10--16) of channels and predicts an intricate spectrum of overlapping Rydberg series. Further calculations show that the 1P0 Wannier-ridge states up to 11s11p obey a two-electron Rydberg formula, but that they are unusually difficult to excite directly from the ground state