How Raman spectroscopy works

Energy transfer actually occurs as a two-photon event  

Accordingly, the selection rules for Raman and IR spectroscopy are different. In Raman spectroscopy, there must be a change in polarizability of the molecule upon excitation, whereas a change in dipole moment is required for IR. A dipole moment is the magnitude 

Using these concepts, it is possible to establish some rough rules to predict the relative strength of Raman intensity from certain vibrations.2 

Under constant experimental conditions, the number of Raman scattered photons is proportional to analyte concentration. Quantitative methods can be developed with simple peak height measurements.5 Just as with IR calibrations, multiple components in complex mixtures can be quantified if a distinct wavelength for each component can be identified. When isolated bands are not readily apparent, advanced multivariate statistical tools (chemometrics) like partial least squares (PLS) can help. These work by identifying all of the wavelengths correlated to, or systematically changing with, the levels of a component. Raman spectra can also be correlated to other properties, such as stress in semiconductors, polymer crystallinity, and particle size because these parameters are reflected in the local molecular environment. 

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Some idea of how Raman spectroscopy works—how light from nonelastic scattering on molecules contains information on the vibratory state of the bonds therein— has been given in Section 2.11. Raman spectroscopy can be used to obtain information on the structure of ions in molten salts, as has been shown in the last three sections. Ha-e, two further molten salt systems that eontain eomplexes and that have been subjected to Raman spectroscopy are described. The first one concerns melts of zinc chloride hydrate. 

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