Raman spectra of crystals and their interpretation
An examination of the spectrograms of the Raman effect in rock salt obtained by Rasetti reveals the existence of nine distinct Raman lines both on the Stokes as well as on the antistokes sides. The frequency shifts of these lines have been estimated. From the microphotometer record reproduced in Rasetti’s paper, the relative intensities of the lines as well as the intensity ratios of the Stokes to the antistokes lines have also been evaluated. These nine frequencies have been identified as due to the nine distinct modes of vibration of the sodium chloride lattice to be expected on the basis of the Raman dynamics of crystal lattices. Though these vibrations are forbidden in the Raman effect as fundamentals, they are allowed as octaves and appear as such in the spectrum. It is pointed out that the Born theory of crystal dynamics is not capable of offering any satisfactory explanation for the observed facts.