Raman and infra-red spectra of rock-salt

Rasetti succeeded in recording a Raman spectrum with rock-salt, using the 2537 A. radiations of the mercury arc as exciter. But as neither his preliminary report nor his subsequent paper with Fermi gives quantitative details of the spectrum, it appeared desirable to investigate the subject afresh. In seeking to record the Raman effect with rock-salt, the presence of a disturbing effect became evident, namely, a luminescence exhibited by the crystal attended by the development of a fugitive coloration under the action of the ultra-violet rays. As the Raman spectrum is itself feeble, it is partially masked or even completely overpowered by the continuous spectrum of the luminescence. Ultimately, however, a specimen of rock-salt was found which was non-luminescent. Using this specimen, and with a fine slit which made prolonged exposures necessary, a spectrogram was obtained in which much more detail could be seen than in the picture reproduced with Rasetti and Fermi's paper. As is evident from its microphotometric record here reproduced, the spectrum consists of a series of Raman lines distinctly resolved from each other. The frequency shifts from the parent radiation of the more prominent of them are 135, 184, 235, 278∗, 314∗ and 350∗ cm-1, while there are feebler lines with frequency shifts 202 and 258 cm-1. Further, there are indications that the three lines marked with an asterisk possess imperfectly resolved components