Report on Notation for the Spectra of Polyatomic Molecules

A SATISFACTORY standardization of notation . for diatomic molecules and their spectra already eXIsts. l The problem of standardization for polyatomic molecules is much more complicated, because of the greater number of degrees of freedom for rotation and vibration, and because of the considerable variety of types of. symmetry, or symmetry groups, to which polyatomlC molecules belong, as contrasted with only two types (Coov and Dooh) for diatomic molecules. Fortunately, the system used in Herzberg's well-known book on infrared and Raman spectra2 is already very generally accepted, and forms a suitable basis for the standard notation proposed here. However, when one considers the entire range of polyatomic spectroscopy from ultraviolet to microwave and magnetic resonanc~ spectra, there are several points of notation which need clarification, revision, supplementation, or simply codification and affirmation in a form to which spectroscopists of all wavelengths can conveniently refer. The principal need for supplementation, as compared with Herzberg's book, arises from the consideration of molecules in nontotally-symmetric electronic states, a matter which is especially essential for electronic spectra. In some points the problem of notation for polyatomic spectra overlaps that for other related areas; in particular, valence theory, and the states of atoms in crystals. Some consideration has been given in this report to the needs of these areas, and a few recommendations have been made. The problems are rather different for small molecules the high symmetry of which is often very important fo; the classification of vibration modes and quantized states, and large molecules, where rigorous symmetry is often nonexistent or unimportant (though approximate or local symmetry is often important). Some consideration has been given in this report to large molecules, but the major emphasis is on the smaller or more symmetrical molecules .