Raman strength correction

Figure 1. Rotational—vibrational line strength correction factors for pure rotational Raman scattering (fM)0 and for O-, S-, and Q-branch vibrational Raman scattering (foh fots, and folQ). The value J is the rotational quantum number of the initial level (O), Stokes (A), anti-Stokes.

Figure 3. Calculated band profiles of Stokes vibrational Raman scattering from Nt at 2000 K assuming a triangular slit function with FWHM = 5.0 cm 1. The bottom curve includes the isotropic part of the Q-branch only. The top curve is a more exact calculation including O- and S-branch scattering, the anisotropic part of the Q-branch and line-strength corrections owing to centrifugal distortion. The base lines have been shifted vertically for clarity.

The application of Raman spectroscopy becomes more challenging when samples exhibit significant fluorescence (e.g., sediment samples which are brown in color). Other difficulties occur when hydrate samples contain occluded gas (e.g., the vC-H peak for methane gas overlaps with that for methane in the small cage of structure I hydrate). In this case, care must be given to assignment of the spectra (Hester, 2007). The latter example illustrates the strength of combining Raman and NMR spectroscopy to ensure correct interpretation of the data. 

Fluorescence is an extremely sensitive technique but it is not suitable as a general method to estimate natural DOC content due to the reason that it is impossible to find a reference material that would be common for all different natural waters. Characteristic for different fluorescence studies of NOM/DOM is that they may occasionally be somewhat surprising, contradictory, or laboriously explicable. The main reason for this incoherence is that fluorescence measurements are affected by many environmental factors, e.g., type of solution, pH, ionic strength, temperature, redox potential of the medium, and interactions with metal ions and organics. Several corrections are required to obtain a reliable and comparable spectrum, e.g., instrumental factors, Raman water peak, scattering effects (primary and secondary inner filter effects [31,32]), arbitrary fluorescence units should be standardized (dihydrate of quinine sulfate), etc. 

The results reported in Figs. 28.10 and 28.11 show that the use of ECC theory allows the dynamics of polyene systems to be treated in a unified way, giving a correct prediction of the frequency shifts of the relevant Raman bands with conjugation length. Moreover, it gives a measure (through F, ) of the extent and strength... 

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