Question on Russell-Saunders term Symbols

[luiz1]

Hello Forum,
I'm reading a paper on the low-lying electronic states of Mn and can't quite figure out the notation. For example, we are told the [Ar]3d54s2 lowest energy configuration gives rise to a 'a 6S5/2' ground state. I understand that the 6S5/2 in terms of the (2S+1)L(J) Russell-Saunders term notation. My question is: what does the 'a' stand for? Similarly, we have excited states of 'y 6P' and 'z 6P'. Again, what do the 'y' and 'z' stand for?

Any help would be appreciated.

Regards,
L

 

[DrClaude]

The ground electronic state is designated X. All excited electronic states that can be reached (electric-dipole-allowed transitions) from the ground state are labelled A, B, C, ..., in order of increasing energy. States that can't be reached (forbidden transitions) are labelled a, b, c, ..., again in order of energy.