Depolarised scattering intensity

Fig. 1. Experimental set up (90° collection optics) for the measurement of laser Raman scattering, illustrating the definition of the depolarisation ratio. ly and Ij are the intensities of light scattered, respectively, parallel and perpendicular to the polarisation of the incident exciting beam...

In the usual Raman experiment, the observations are made perpendicular to the direction of the incident beam, which is plane polarised. The depolarisation ratio is defined as the intensity ratio of the two polarised components of the scattered light which are parallel and perpendicular to the direction of the propagation of the (polarised) incident light. The polarisation of the incident beam is perpendicular to the plane of propagation and observation. For this geometry, the depolarisation ratio is defined as the intensity ratio ... 

Bartoli and Litovitz [6] found values of C of about 0.2% for some typical polarised (A) bands - by making measurements on the 459 cm" band of CCl which has a well-known [18] depolarisation ratio. Calculation of I ((1d) is then straightforward. Polarisation scrambling behind the main entrance slit ensures that the monochromator is equally sensitive to transmission of and lyy scattered light. There have been reports of local heating effects caused by a relatively high powered laser beam. However, we have never found this to be a problem although, of course, it is not possible to physically monitor the microscopic temperature. Stokes/Antistokes intensity ratios which measure the Boltzman population factors (and hence the microscopic temperature) have always corresponded well to the laboratory (bath) temperature even for input powers up to 2w. 

Fig.3 The intensity of the depolarised Kaiyleigh scattering from argon as a function of density at room temperature (ref 34). l/density is plotted the dashed line shows the 2-body intensity.

In Raman spectroscopy, the direction of observation of the radiation scattered by the sample is perpendicular to the direction of the incident beam. Polarised Raman spectra may be obtained by using a plane polarised source of electromagnetic radiation (e.g. a polarised laser beam) and placing a polariser between the sample and the detector. The polariser may be orientated so that the electric vector of the incident electromagnetic radiation is either parallel or perpendicular to that of the electric vector of the radiation falling on the detector. The most commonly used approach is to fix the polarisation of the incident beam and observe the polarisation of the Raman radiation in two different planes. The Raman band intensity ratio, given by the perpendicular polarisation intensity, /j., divided by the parallel polarisation intensity, 7, is known as the depolarisation ratio, p. 

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