Types of Light Scattering

12 Raman Scattering and Other Multi-photon Processes 

The Liouville-space diagrams in Fig. 12.3 help to clarify the main physical distinction between Raman scattering and ordinary fluorescence. Both processes require four interactions with a radiation field, and therefore four steps in Liouville space [1]. There are six possible pathways with four steps between the initial state whose population is indicated by a,a at the lower-left comer of Fig. 12.3 A and the final state b,b) at the upper right the three paths shown in Fig. 12.3B-D and their complex conjugates. Ordinary fluorescence occurs by paths B and C, whereas 

Experimentally, Raman scattering differs from fluorescence in several ways. Eirst, Raman emission lines are much narrower than fluorescence emission spectra. 

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In practice, depending on the angle between the incident and scattered light paths, one can make several types of light scattering measurements ... 

Filter and store 2-butanone or other reference solvent ( 150 ml) (depending on the type of light scattering device used) as described earlier. 

As discussed above, continuum resonance occurs when the excitation laser energy is higher than the dissociation limit of an excited, bound electronic state or directly with purely repulsive states. Continuum resonance Raman spectra of gaseous molecules are very sensitive to the position and shape of the potential functions involved in this type of light scattering as well as to the electronic transition moments between ground and excited states. Since it is possible to calculate the relevant spectra using both the KHD... 

Laser excitation for fluorescence detection has received much research interest, but as of yet there is no commercially available instrument. Fluorescence intensity increases with excitation intensity, and it is generally assumed that laser excitation would then offer improved limits of detection. However, as Yeung and Synovec have shown, various types of light scattering, luminescence from the flow cell walls, and emission from impurities in the solvent all increase with source intensity as well, yielding no net improvement in signal-to-noise ratio (53). Where laser excited fluorescence may prove useful is for the design of fluorescence detectors for microbore packed and open tubular LC columns, where the laser source can be focused to a small illuminated volume for on-column detection. 

There are two types of light scattering detector the evaporative light scattering detector and the liquid light-scattering detector. The... 

Fig. 2. Four types of light scattering processes used in molecular spectroscopy. Raman scattering takes place if the incident light corresponds to a transparent region of the molecular absorption spectrum the intermediate state is then virtual in that it does not closely resemble any particular molecular state. Preresonance Raman scattering takes over when the incident beam approaches an electronic absorption band, so that the corresponding electronic state dominates the intermediate state. It turns into resonance Raman scattering when the intermediate state is dominated by a few (ro-)vibronic levels in the vicinity of the incident light frequency. Ultimately the resonance fluorescence limit is reached when the incident beam coincides with a single sharp level of the electronic manifold.

Rgure 6.15 Different types of light-scattering patterns are obtained from sphemlitic polyethylene using (a) V and (b) Hy polarization (54). Note the twofold symmetry of the Vy pattern, and the fourfold symmetry of the Hy pattern. This provided direct experimental evidence that spherulites were anisotropic. 

Fig. 4.42. Types of light-scattering patterns observed with polyethylenes. Reproduced with permission from [3]. Copyright 1979, Faraday Discuss. Chem. Soc.

Figure 6.9. Royco corporation manufactures several different photozone stream counters which exploit different types of light scattering, a) Top view of the optical arrangement of the Model 203 photozone stream counter, b) Optical arrangement Model 218 photozone stream counter. c) Optical arrangement Model 345 photozone stream counter.

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