Raman and Infrared Spectroscopy

Abstract

Raman spectroscopy and infrared spectroscopy are adverted as complementary techniques. Both methods provide information about vibrational and rotational transitions in molecule and serve as a good tool for material identification even though principles of these methods are different. However, Raman spectroscopy is not so popular for the chemical analysis of materials as is infrared spectroscopy. Infrared spectroscopy is an absorption technique dealing with the fact that molecules absorb specific frequencies that define their structure. The output data are absorption spectra. On the other hand Raman spectroscopy provides emission spectra originated during interaction of the monochromatic excitation laser beam with molecules, which emit on for the matter characteristic frequencies. This elemental difference is shown in Fig. 1. Fig. 1 Differences of light-matter interaction of infrared and Raman spectroscopy Certain substances have high-quality both Raman and IR features, but for instance centrosymmetric molecules modes give a rise either Raman or infrared spectra but not both. Strong infrared absorptions usually appear as weak Raman signal and vice versa. Infrared spectroscopy has more intensive signal for small polar molecules and single bonded chemical compounds like C-C, C-O, C-H etc. Large absorption bands of water caused by containment of high absorptivity values functional groups disallow infrared measurement of water solutions. On the contrary presence of water in a sample can be determined with ease. Intensive spectra of polyatomic inorganic chemical compounds and majority of organic matters yields Raman spectroscopy. Raman spectrum of water is minimal thereby substances dissolved in water can be identified without problems. A demonstration of infrared and Raman spectrum of styrene-butadiene rubber can be seen in Fig. 2. Fig.2 Infrared and Raman spectrum of styrene-butadiene rubber