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Yesterday I asked this question concerning the existence of terms. This question is on the same topic. I have been looking at some tables (in Yang and Hamilton (2010), pg 199) at the terms that exist for two electrons in the same configuration. I noticed that for example if the state $^1S$ exists the state $^3S$ would not and vice versa. Is this a general rule? I.e. can we say:

For two electrons in the configuration $nl^2$ if the term $^1L$ exists then the term $^3L$ will not and vice versa.

If so can it be proved easily and if not why not?

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The general rule is correct as you have stated it. This is because there is only one way to add two $l$ angular momenta to make a total angular momentum of $L$; this combination will be either even or odd under exchange of the two sectors. Since the total electron state needs to be exchange-antisymmetric, and the singlet and triplet states are respectively antisymmetric and symmetric, only one of the combinations will be possible.

The key part of the argument is that the combination of $l\oplus l=L$ will always give some definite exchange symmetry. The simplest way I can see of proving this is reducing the couplings to the Wigner $3j$ symbols, which have definite symmetries under exchange (cf. the DLMF here), which specify to $$ \begin{pmatrix} l&l&L \\ m_2 & m_1 & -M \end{pmatrix} = (-1)^{l+l+L} \begin{pmatrix} l&l&L \\ m_1 & m_2 & -M \end{pmatrix} $$ i.e. the coupling will have $(-1)^L$ parity under exchange, independently of the value $l$.

This means that you can strengthen your statement to read

the coupling of two electrons from the same shell, $nl^2$, can only form the term $^3L$ if $L$ is odd and the term $^1L$ if $L$ is even.

On the other hand, it is perfectly possible for an atom to exhibit both $^1L$ and $^3L$ terms for any $L$ - you just need to combine electrons from different shells. Thus, you can have both $^1P$ and $^3P$ terms in the $1s2p$ shell of neutral helium, along with a slew of other similar states which you can explore via the NIST energy levels database.

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