Forward-scattering arrangement

Fig. 3.5-4 b shows the intensity of the Raman radiation of a forward-scattering arrangement. At an optimum thickness , the Raman radiation has a maximum, which increases as the elastic scattering coefficient decreases. The exciting radiation which emerges from the sample (Fig. 3.5-4 d) has a lower intensity if the elastic scattering coefficient r is higher. 

It is particularly important to know the intensity of the Raman radiation relative to the unshifted radiation scattered in the same direction for forward scattering (0° arrangement) ... 

Thus, the optimum sample arrangement for Raman spectroscopy of crystal pow ders with a low absorption coefficient is a forward-scattering (0°) arrangement of coarse crystallites with an optimum thickness in a multiple scattering arrangement. These are the conditions for the investigation of colorless samples in the visible range of the spectrum. 

Figure 13.17b shows the experimental arrangement used by EZEKIEL et al. [13.31] to measure subnatural linewidths in I2 vapor. An intensity-modulated pump beam and a continuous dye laser probe pass the iodine cell three times. The transmitted probe beam yields the forward scattering signals (detector 1) the reflected beam the backward scattering. A lock-in amplifier monitors the change in transmission due to the pump beam, which yields the probe signal with width The authors achieved a linewidth as narrow as 80 kHz on a transition with a natural linewidth y = 141 kHz. The theoretical limit of y = 16.5 kHz could not be reached because of dye laser frequency jitter. 

The aperture and lens arrangement shown in Fig. 16.1 is needed to ensure that the particles are illuminated with a parallel beam of light (i.e., to ensure that all rays are parallel) and that only the attenuated parallel light reaches the detector. Failure to include the lens and aperture at the detector allows forward-scattered light from the particles to reach the detector, in which case Eq. 16.7 does not hold. The extinction produced by a particle is a function of the particle extinction efficiency Q. ... 

A variety of choices are available for the PSG section of this experiment. As before, the selection is based on the Mueller matrix components of the sample that are sought. A convenient arrangement that has been used for samples subject to uniaxial deformation is described in detail in reference 22 and collects the Raman scattered light in the forward direction. The PSG section of the instrument consists of a polarizer oriented at zero degrees, and a photoelastic modulator at 45°. Following the sample, the PSA section consists of a polarizer oriented at 45°. The signal measured at the photomultiplier tube was shown to have the form ... 

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