Degeneracy molecular spectroscopy

In molecular spectroscopy the term symmetry species is usually substituted for the rather unwieldy expression irreducible representation Furthermore, the word degeneracy, as employed in spectroscopy, is often used to specify their dimension. 

Divergent couplings ai e a nuisance for the computational treatment of the nuclear dynamics. In cases of exact or near degeneracy of electronic potential-energy surfaces it is therefore preferable to introduce an alternative electronic representation, the so-called diabatic (or quasi-diabatic) representation, which avoids singular coupling elements. The basic concept of diabatic states has been introduced in early descriptions of atomic collision processes and vibronic-coupling phenomena in molecular spectroscopy. ... 

The E X E ]T effect, where a doubly degenerate vibrational mode lifts the degeneracy of a doubly degenerate electronic state, is presumably the most extensively investigated vibronic-coupling problem in molecular and solid-state spectroscopy, see [26-28] for reviews. 

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