Calcium-ion optical series assignments

In singly ionized calcium, $np{}^{2}P\to 4s{}^{2}S$ resonance doublets, $ns{}^{2}S\to 4p{}^{2}P$ doublets and $np{}^{2}P\to 3d{}^{2}D$ three-component groups approach the same ionization continuum from different lower terms. A three-line group need not have spin multiplicity three. The electric-dipole rule excludes $P_{1/2}\to D_{5/2}$.^[ref]