Raman effect, physical principles

Progress in the Raman spectroscopic study of carbohydrates became possible during the past few years owing to the introduction of laser sources. Before discussing the results of laser-Raman spectroscopy applied to carbohydrates, we shall give a brief recapitulation of the physical principles of the Raman effect. Experimental techniques of infrared spectroscopy have been described in previous reviews,116,17 but no such description has been given for the Raman method. That is why the Description Section, which follows, will include the physical fundamentals of the method, as well as the sampling techniques. 

Infrared and Raman spectroscopy are related by the fact that both permit the detection of bond vibrations. Like IR spectroscopy, the spectral bands are reported in cnT1. An important difference is that the wavelength and intensity of inelastically scattered light is measured in the Raman spectroscopic method. The Raman effect causes the scattered radiation to shift according to the energies of molecular vibrations. Although Raman spectroscopy involves a physical principle different from that in IR spectroscopy, the two techniques are complementary. 

Infrared and Raman spectroscopy are often grouped together, since both techniques provide information on the vibrational modes of a compound. However, since the two spectroscopic techniques are based on different physical principles the selection rules are different. Infrared spectroscopy is an absorption phenomenon, while the Raman spectroscopy is based on a scattering phenomenon (Raman and Krishnan 1928). In general, infrared energy is absorbed by polar groups, while radiation is more effectively scattered in the Raman effect by symmetric vibrations and nonpolar groups (Colthup et al. 1990 Ferraro and Nakamoto 1994). For most molecules other... 

X HE VIBRATIONAL SPECTRUM of any material consists of two parts the infrared (IR) and Raman spectra. IR spectroscopy is sensitive to the changes in dipole moment that occur during the vibrations of atoms that are forming chemical bonds. Raman spectroscopy detects the polarizability tensor changes of the electron clouds that surround these atoms. These apparent differences in the physical principles of both effects have led to the development of these two distinctly different techniques. IR and Raman spectra complement each other. Because they are sensitive to the vibrations of atoms, they are called vibrational spectra. 

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