Analysis of the raman spectrum of single crystals of naphthalene

Summary

The paper reports the results of an exhaustive study and evaluation of 24 spectrograms concerning the Raman effect in a single crystal of naphthalene recorded for various settings of the crystal with respect to the directions of incidence and observation, and also for various directions of vibration in the incident and scattered radiations. The number and symmetry of the modes of oscillation expected of the molecule and of the crystal have been considered by application of the group theory. The experimental studies have revealed, in agreement with theory, the existence of 6 low-frequency lattice oscillations instead of the 4 reported, earlier. They fall into 3 groups of close doublets, the frequencies being 38, 47; 72, 78; and 107, 121. Out of these, the lines 47, 78 and 107 are shown to be due to oscillations symmetric and the other lines 38, 72 and 121 to oscillations antisymmetric to the elements of symmetry of the monoclinic lattice. Considerations of frequency and intensity indicate that these three pairs of lines roughly correspond to rotational oscillations of the molecule about its normal breadth and length réspectively. From a comparison of the data for the melt and the crystal, the frequencies of the 24 normal modes of oscillation of the molecule indicated by theory have been identified. Polarisation studies have revealed that some of the internal oscillations split up into symmetric and antisymmetric components. This splitting was clearly observable for at least two of the lines 512 and 1576 cm.^-1, in other cases, however, they were on the limit of observation or even too small to be detected. It is concluded that the polarisation behaviour of the internal oscillations also is controlled entirely by the symmetry properties of the crystal as in the case of the lattice oscillations.