= Calcium-ion optical series assignments
In <singly ionized calcium>, $np\,{}^2P\to4s\,{}^2S$ resonance doublets, $ns\,{}^2S\to4p\,{}^2P$ doublets and $np\,{}^2P\to3d\,{}^2D$ 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 $P_{1/2}\to D_{5/2}$.<Energy Levels of Singly Ionized Calcium (Ca II) by National Institute of Standards and Technology>[4s,3d,4p and5s assignments]{ref}
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