What is the Raman Effect

Raman Spectroscopy

Raman spectroscopy offers numerous advantages. Since lasers are used in the visible region in Raman instruments, flexible quartz fiber optic cables can be used to excite the sample and detect the scattered radiation. These cables can be quite long if necessary. Since visible light is used, glass or quartz can be used to hold the samples. For the investigation of chemical reactions this means that the Raman probe can be inserted into a reaction or Raman spectra can be recorded through a window, for example in an external reaction sample loop or a flow cell. With the latter approach, the possibility of contamination of the sample stream is excluded. Since quartz or high quality sapphire can be used as window material, high pressure cells can be used to record Raman spectra of catalytic reactions. When studying catalysts, operando spectroscopy with the Raman effect is quite useful to study the in situ reactions on catalytic surfaces in real time. Another advantage of Raman spectroscopy is that hydroxyl bonds are not particularly Raman-active, so that Raman spectroscopy is very suitable for aqueous media. Raman spectroscopy is considered non-destructive, even if some samples can be affected by the laser radiation. When choosing this technique, it is important to consider how fluorescent a given sample can be. Raman scattering is a weak phenomenon and fluorescence can be superimposed on the signal, making it difficult to acquire good quality data. This problem can often be alleviated by using an excitation source with a longer wavelength.

As far as reaction analysis is concerned, Raman spectroscopy is sensitive to numerous functional groups, but it turns out to be exceptional in capturing molecular structural information. It provides a unique molecular fingerprint. Since Raman uses the polarizability of bonds and has the potential to measure low frequencies, it is sensitive to crystal lattice vibrations. In this way, the user receives polymorphic information that can be difficult to capture with the FTIR. Thus, Raman spectroscopy can be used very effectively for the investigation of crystallization and other complex processes.