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In this section I review briefly the nature of the vibrational and rotational levels of interest and the corresponding selection rules. My purpose is to make the literature, especially that on which the catalogue of laser lines is based, more accessible. Therefore, although physical principles are discussed, the emphasis is on simple formulae and nomenclature. I restrict the discussion to material commonly encountered in the context of molecular lasers — very good broader accounts of the subject of molecular spectroscopy are widely available [2.1-4]. 

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