Understanding Raman Spectroscopy: Principles and Applications

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Raman spectroscopy is a technique used for chemical analysis by exciting molecular vibrations with light. This inelastic scattering of light allows researchers to gather valuable information about samples. The method relies on the Raman effect, discovered by C.V. Raman, which involves the emission of light with different wavelengths due to molecular vibrations. By using monochromatic light sources and laser technology, Raman spectroscopy has become a powerful tool for various scientific applications.

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Table Contents Definition Introduction The Raman Effect How does Raman Spectroscopy work? Applications of Raman Spectroscopy Information provided by Raman spectroscopy Conclusion 2

Definition Raman spectroscopy belongs into the category of vibrational spectroscopy. This means that it analyzes a sample chemically, by using light to create (excite) molecular vibration, and interpreting this interaction afterwards. 3

Introduction It is based on the inelastic scattering of light that occurs when matter is irradiated by light. As the change of wavelength is very small compared to the wavelength of the irradiating light, the change of wavelength is most easily observed when using monochromatic light sources. After this (monochromatic) light has interacted with the sample, a very small part of it has changed its wavelength. This is change is called: the Raman effect. We can now collect that light and can use it gain information about the sample. 4

The Raman effect For a better understanding it is important to know, that when photons (light) "strike" matter, most of the scattered light remains unchanged in its wavelength. For example, if you point a green laser pointer at a wall, you will always see a green dot. The scattered light obviously has the same color and this phenomenon is called Rayleigh scattering. However, also inelastic scattering processes can occur, which then lead to the emission of light with a different wavelength. This usually happens in relation to molecular vibration. This scattering phenomenon, which was predicted by Adolf Smekal in 1923 and discovered by C.V. Raman in 1930, is called the Raman effect. 5

The Raman effect Discovering and understanding the Raman effect opened the door to a new kind of spectroscopy. However, Raman spectroscopy did not really take off until the discovery of the laser, since the use of monochromatic light plays an important role. Thus, the sample is irradiated with a laser and some of the scattered light is analyzed with a spectrograph (dispersive or FT technology). At the end we obtain a Raman spectrum that shows us characteristic signals or "bands" for the material under investigation. 6

How does Raman spectroscopy work? To acquire Raman spectra, you just have to focus the laser onto the sample you want to investigate. That sample however, must not be showing fluorescence to the laser used for excitation. If that is the case, the fluorescence will cover most of the Raman effect, since it is so weak in comparison. After the laser light has irradiated the sample, the scattered light is passed through a filter (to get rid of any light from the excitation laser). Then it is directed onto a grating, which distributes the inelastic parts like a prism and according to wavelength. At the end these rays are directed to a CCD sensor which then outputs a spectrum depending on the intensity. 7

Applications of Raman spectroscopy Raman spectroscopy can be used in all areas where non-destructive (microscopic) chemical analysis and imaging is required. It delivers answers for qualitative and quantitative analytical questions. In general, Raman is easy to use and quickly provides key information to characterize the chemical composition and structure of a sample. Basically, it matters little whether the samples are solid, liquid or gaseous. 8

Applications of Raman spectroscopy Here are some applications of Raman spectroscopy: Pharmaceuticals Geology and mineralogy Semiconductors Material research Life-science 9

Information provided by Raman spectroscopy A Raman spectrum is like a chemical fingerprint that clearly identifies a molecule or material. And just like a human fingerprint, it can be compared with reference libraries to identify the material very quickly or distinguish it from others. Such Raman spectral libraries often contain hundreds of spectra with which the spectrum of a sample is compared to determine the analyte. It provides insights into a sample's: Chemical composition and properties Crystallinity and polymorphism Contaminations and defects Thermal and mechanical exposure 10

Conclusion Raman spectroscopy is a powerful tool to assess internal quality and safety, due to many advantages such as non destructive detection, no sample preparation, and fast measurement. Due to its narrow and sharply resolved bands, Raman spectroscopy shows great potential in qualitative, structural, and quantitative analysis. The industrial application of this technology may be a realistic option in the near future. 12