= Solution
The limit near 121.8 nm lies about $hc/(121.8\ \mathrm{nm})\simeq10.18$ eV above its lower term. Subtracting from the approximately $11.88$ eV ground-state <ionization energy> puts that lower term near $1.70$ eV: the $3d\,{}^2D_{3/2,5/2}$ pair. The first upper levels are the already identified $4p\,{}^2P_{1/2,3/2}$ pair. Their allowed decays are
$$
\boxed{\begin{array}{c|c}
\lambda&\text{emission assignment}\\\hline
849.8\ \mathrm{nm}&4p\,{}^2P_{3/2}\to3d\,{}^2D_{3/2}\\
854.2\ \mathrm{nm}&4p\,{}^2P_{3/2}\to3d\,{}^2D_{5/2}\\
866.2\ \mathrm{nm}&4p\,{}^2P_{1/2}\to3d\,{}^2D_{3/2}
\end{array}}
$$
The fourth possible pairing, $P_{1/2}\to D_{5/2}$, has $|\Delta J|=2$ and is forbidden as an <electric dipole transition>. The three lines therefore arise from \b[<doublet terms>], not a spin-triplet term: “triplet series” counts three <spectral lines>.
For example, the first two <wavelengths> together with the $4p_{3/2}$ <energy> give $E(3d_{3/2})\simeq1.693$ eV and $E(3d_{5/2})\simeq1.700$ eV. The longer 854.2 nm <wavelength> from the same upper level places its lower D level higher. The 866.2 nm transition independently returns the lower D <energy> within the rounding precision. Increasing the upper <principal quantum number> gives $np\,{}^2P\to3d\,{}^2D$ triplets with two nearby lower-term ionization limits near 121.8 nm. The arrows are included in <1/b>; their emission direction is \b[P to D], not the reverse. The independent tabulation confirms the D-term position and ordering.<Energy Levels of Singly Ionized Calcium (Ca II) by National Institute of Standards and Technology>[3d entries at13650.20 and13710.89 inverse centimetres]{ref}
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