Singly ionized calcium (${\mathrm{Ca}}^{+}$)

A calcium ion with one electron removed, leaving a closed 18-electron core and one valence electron. It is distinct from the divalent calcium ion used in much biological terminology. Its bound electronic levels and ionization continuum support the Ca II optical series.

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]