Raman Scattering Tensors for Single Crystals of Naphthalene

Abstract

The Raman scattering tensor for each gerade vibrational mode of crystalline naphthalene is evaluated from polarized Raman spectra of oriented single crystals. For molecular crystals, where the intermolecular interactions are weak, the form of the tensors is determined by the properties and symmetry of the molecule rather than the crystal; consequently, the symmetries of the gerade vibrations are readily identified. A comparison of these data with the phosphorescence and fluorescence spectra allows the fundamental vibrational frequencies to be assigned empirically. Except for one b2g mode and one b3g mode, the empirical assignments support those in the literature made on the basis of force field calculations. It is found that crystal effects on the gerade vibrations are small: frequency shifts are 2–4 cm−1, exciton splittings are less than 1 cm−1, and intensities are described qualitatively by the oriented gas model, although an unambiguous example of crystal-induced Fermi resonance is evident. Disagreement between the observed exciton splittings and some calculated splittings by N. Rich and D. Dows [Mol. Cryst. Liquid Cryst. 5, 111 (1968)] based on an atom–atom potential model may indicate that some finer features of the intermolecular potential are not well represented by such a model. The temperature dependence of the Raman linewidth and line shape is documented.