Absolute differential Raman scattering

The mean value of the absolute differential Raman scattering cross section of the Q-branch of nitrogen was established to be (Schrotter and Klockner 1979)... 

A. Absolute Differential Raman Scattering Cross Section of Nitrogen.212... 

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]... 

Eq. (8.45) shows that for an ordinary Raman experiment the absolute differential Raman scattering cross sections can be expressed in terms of derivatives of the molecular polarizability invariants a and y with respect to normal coordinates. These derivatives contain valuable information about the variation of molecular polarizability with vibrational motion. Gas-phase Raman scattering cross sections are most suited for intensity analysis since at low partial pressure of the sampling gas these quantities are not influenced by effects of intermolecular interactions, thus reflecting properties of individual molecules. 

Direct determination of absolute Raman differential cross sections is quite difficult and tedious work often leading to incorrect results. It is easier to measure cross sections relative to some standard. The absolute differential Raman scattering cross sections of the sample can then be straightforwardly obtained. 

The absolute differential scattering cross section of the standard needs to be determined as precisely as possible. A number of measurements has been performed ovm-the past forty years [260,278-287]. The introduction of lasers in Raman spectroscopy and of computer processing of spectral data has improved highly the accuracy of Raman intensity measurements. As a result, the absolute differential Raman scattering cross section of nitrogen reported frrom different laboratories deviates within a few percent only [260,284,285,287]. 

The absolute differential Raman scattering cross section of the line of the sample can be obtained from the relative value by using the absolute scattering cross section of nitrogen as given in Eq. (8.73). 

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. 

Experimental and calculated spectral parameters for propyne in the gas phase (wavenumbers in cm, absolute differential Raman scattering cross sections (da/dn)j in I0 36 the depolarization ratios pj are dimensionless)... 

It is also interesting to note that circular differential Raman scattering, circular intensity differential (CID), has been reported for a series of optically active sulfoxides and a correlation found between the absolute configuration at sulfur and the differential scattering (240). Thus, all (/ )-alkyl p-tolyl sulfoxides investigated show a common (positive) CID feature in the 300 to 400 cm" region. 

The paramount difficulties in the experimental determination of accurate absolute infrared intensities and differential Raman scattering cross sections seem, however, to persist in the theoretical evaluation of these quantities as well. As we will see, predicted Raman intensities appear to be more consistent with experiment as compared to infrared intensities. 

OPUS differentiates between the Raman spectrum and the single channel spectrum. The single channel spectrum of the Stokes and anti-Stokes Raman scattering for a sulphur sample is shown in Fig. 4.4. Note that the abscissa here is expressed in absolute wavenumbers. Therefore, the exciting laser line appears at vl = 9394 cm and the bands at wavenumbers lower and higher than 9394 cm arise from Stokes and anti-Stokes Raman scattering, respectively. On the other hand, a standard Raman spectrum comprises the spectral range from 0 to 3500 cm. Load the file RAMAN SULPHUR and find out the difference between these two types of spectra. 

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