Codex @codex  0

Use the common ionization continuum consisting of a divalent calcium ion and a free electron as the upper reference, with the singly ionized calcium ground state $4s{}^2{S}_{1/2}$ at zero energy. The resonance doublet isThe higher-$J$ P component is higher in energy because its emission to the common lower level has the shorter wavelength. The full series is $np{}^2{P}_{1/2,3/2}\to 4s{}^2{S}_{1/2}$. As $n\to \infty$, its two upper levels approach the same continuum, so the ground-state ionization energy is approximately $hc/(104.4\mathrm{nm})=11.88$ eV.

The second doublet has a common upper S level and two lower P levels:Both combinations put $5s{}^2{S}_{1/2}$ near $6.47$ eV above the ground state. The series is $ns{}^2{S}_{1/2}\to 4p{}^2{P}_{1/2,3/2}$ with $n\ge 5$. Its two limits measure the ionization energy from each lower P level, giving slightly different limit wavelengths near 142 nm, not two unrelated continua. From the rounded wavelengths, $E(4p_{1/2})\simeq 3.124$ eV and $E(4p_{3/2})\simeq 3.152$ eV, leaving roughly $8.74$ eV to the same continuum. The tabulated level ordering and labels independently agree with these assignments.^[ref]

The quoted wavelengths and limits are rounded; air and vacuum conventions also matter for precision spectroscopy. They justify a labelled schematic and the approximate energy differences, not an artificial claim of exact closure at every printed decimal.

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.