Polarized Raman spectra of naphthalene and anthracene single crystals

Abstract

The polarized Raman spectra of single crystals of naphthalene and anthracene have been measured by the use of an argon ion gas laser as an exciting light source. Pronounced effects of polarization have been found for the Raman lines in the lattice and intramolecular vibrational regions which have never been observed in the experiments with usual mercury light source, proving the advantages of lasers for the polarization measurements. Naphthalene and anthracene crystals have the same space group of C 2 h 5 with two molecules in a unit cell. Under the assumption of an oriented gas model, the relative intensifies of the non-totally symmetric intramolecular vibrations in various polarized spectra are predicted only from their crystal structural data. From a comparison between the predicted and observed results, the non-totally symmetric Raman lines have been classified definitely into specific molecular symmetry species. The assignments thus obtained agree very well with those given in the literature. All the six lattice vibrational Raman lines have been resolved. Their observed polarization behaviour permits complete symmetry assignments and the determination of their approximate vibrational modes. The vibrational modes responsible for the lattice vibrational Raman lines have been found to be described fairly well by the rotational oscillations about the three principal axes of the molecule. The effects of temperature on the lattice vibrational Raman lines have also been studied in the temperature range between 300 and 4°K. The frequencies obtained at various temperatures have been used to evaluate the mean square amplitudes of the rotational oscillations of the molecule in the crystal. The results indicate a large amplitude of the rotational motion around the axis normal to the molecular plane, but it decreases rapidly with decreasing temperature. By using the spectral data available and the observed values of thermodynamic functions, the average frequency of the acoustic vibrations has been estimated for naphthalene crystal. The estimated values are 22 cm −1 at 293°K, 28 cm −1 at 195°K, 39 cm −1 at 77°K and 41 cm −1 at 15°K.