2S + 1 is the known as multiplicity. Indicates the degeneracy of the level due to spin.
If S = 0 => multiplicity is 1: singlet term.
If S = 1/2 => multiplicity is 2: doublet term.
If S = 1 => multiplicity is 3: triplet term and so on.
In the following we will discuss the effect of spin-orbit interaction on terms to terms transitions.
Singlet levels
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