Knowledge Bank

Recent developments in Raman spectroscopy have centered on the improvement of experimental techniques. In order to decrease exposure times, the intensity of the incident light has been increased by using reflectors or water-cooled mercury arcs operating at higher currents. A multiple reflection Raman tube has been constructed which has made possible the observation of the rotational spectrum of $CH4$. Also, photoelectric methods of recording Raman spectra have been developed to the point where they have been used successfully for qualitative and quantitative analyses in industrial research. From the theoretical viewpoint, not much has been added to the original work of Placzek. Since his work indicated that overtones and summation tones would occur rarely, he did not consider depolarization factors and relative intensities for these cases in great detail. However, he did indicate how to proceed in these cases and it seems that the work should be extended because overtones and summation tones are common occurrences in Raman spectra. Several methods have been proposed by others for calculating depolarization factors and relative intensities for fundamentals, but they either are too complicated to use for large molecules or, if they are simpler, they have not been tested enough to determine their reliability. Here, it appears, is a very fertile field for future investigation. Considerable experimental and theoretical work has been done by infrared spectroscopists on the anharmonic treatment of polyatomic molecules. Relations involving anharmonic constants can furnish useful checks on assignments of combined Raman and infrared spectral data. These checks can be important because sometimes missing fundamentals are identified through their appearance in summation and difference tones. Also, the resonance denominators which occur in anharmonic calculations can be used to predict cases where Fermi resonance can occur. Finally, it seems that potential constants should be determined in more cases where deuterium substitution is possible. Constants based on a single molecule can be in considerable error so the requirement that the set work for both the original and the deuterium substituted molecule can prevent the more important potential constants from differing too much from their proper values.