Alternative Empirical Notation
Electronic states are also often identified by an empirical single-letter label. The ground state is labelled X, excited states of the same multiplicity (i.e., having the same spin quantum number) are labelled in ascending order of energy with capital letters A,B,C...; excited states having different multiplicity than the ground state are labelled with lower-case letters a,b,c... In polyatomic molecules (but not in diatomic) it is customary to add a tilde (e.g. X~, a~) to these empirical labels to prevent possible confusion with symmetry labels based on group representations.
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