Second-order Raman spectra of crystals

Abstract

Using the smaller Hilger quartz spectrograph and the λ2536.5 radiations of the mercury arc as exciter, tests were made to determine the minimum exposure times required to bring out the various features observed in a fully exposed Raman spectrum of diamond. The line with a frequency shift of 1332 cm^-1 appeared on the plate with an exposure of only five seconds. Half a minute was needed to record the Brillouin doublet centred about the exciting radiation, the latter itself being filtered out by a mercury vapour screen. An exposure of half an hour sufficed to record the second-order Raman spectrum, while the anti-Stokes frequency shift of 1332 cm^-1 appeared after forty-five minutes. An exposure of three hours results in heavily over-exposing the second-order Raman spectrum, but is useful in bringing out clearly the weak terminal line having a frequency shift 2015 cm^-1, which is liable to be overlooked with smaller exposures. (This line and its more intense neighbour at 2176 cm^-1 are of special interest as they appear with great intensity in the infra-red absorption spectrum of diamond.) Longer exposures up to ten hours reveal no new features. It is very significant that no hint or trace of the continuous spectrum of elastic vibrations forming the so-called 'acoustic branches' of the lattice spectrum of the crystal appears on the plate even with the longest exposures. This will be evident from a comparison of Fig. 1 (a) and 1 (b), the former being the spectrum of the mercury arc and the latter the spectrum of diamond recorded with six hours exposure.