Absolute Raman scattering coefficients

These equations have been employed for the determination of absolute Raman scattering coefficients of crystal powders (D Orazio and Schrader, 1974, 1976). Experiments have confirmed the consequences of this theory (Klosowski and Steger, 1979). 

Fig. 3.5-13 demonstrates the influence of sample and instrumental parameters on the intensities of the observed Raman lines, excited with the Nd YAG laser at 1064 nm. The lower abscissa shows the absolute wavenumber scale, the upper abscissa represents the Raman shift. The relative intensity of Raman radiation is calculated by Eq. 3.5-9, taking into account the absorption coefficient a cm ) of water (Fig. 3.5-3) and a constant Raman scattering coefficient (assumed to be equal to 1 cm ). The elastic scattering coefficients as parameters, r = 0, 10, 100, 500 cm , describe the properties of a liquid, a coarse, a medium, and a fine powder, respectively. The traces show the relative output voltage of a Germanium detector. 

This example illustrates that measurements of absolute scattering coefficients and quantitative analyses are somewhat more difficult in NIR FT Raman spectroscopy than in the visible region, where absorption can usually be neglected. In any case, it is necessary to carefully calibrate (D Orazio and Schrader, 1974, 1976) the spectral sen.sitivity of the instrument. 

If an optical frequency co is close to one-photon electronic resonance with an intermediate state /, the terms including the denominator -co — 0 will be dominant and we obtain a good approximation by retaining only these (resonant nonlinear Raman spectroscopy). In the absence of such additional electronic resonances, we can neglect the damping coefficient for all denominators whose absolute value is far from zero and write them in terms of real Raman scattering tensor components for the transition from... 

The quantity (da/df2)j is called absolute differential Raman scattering cross section. It is often termed absolute Raman intensity as well. From q. (8.45) it is clear that (da/dn)i depends on several factors such as Vq and V and the absolute temperature T. In order to operate with comparable quantities that are independent fi-om the experimental conditions, the so-called "standard intensity" or "scattering coefficient" Sj is commonly used [73,253,260-263]... 

On the basis of transferability properties for intensity parameters in the hydrocarbon series, Martin [309] has predicted depolarization ratios, scattering coefficients and absolute differential Raman scattering cross sections for propane in the gas-phase employing electro-optical parameters determined for ethane. A good correspondence between the predicted and experimental data has been achieved. A survey of calculated Raman spectra of other hydrocarbons is presented in the book of Gribov and Orville-Thomas [155]. Gussoni and co-workers [300,319] have predicted the Raman spectrum of polyethylene and perdeutero-polyethylene by transferring electro-optical parameters evaluated for methane and cyclohexane. 

---