Raman spectroscopy is an inelastic scattering phenomenon the probes molecular vibrations to provide a molecular fingerprint of materials. Currently, there are six major types of Raman spectroscopy in use today, which are: Spontaneous Raman Spectroscopy, Resonance Raman spectroscopy (RRS), Surface Enhanced Raman spectroscopy (SERS), Coherent Anti-Stokes Raman Scattering (CARS), Stimulated Raman Spectroscopy (SRS) and Spatially Offset Raman Spectroscopy (SORS).
Fourier transform infrared spectroscopy (FTIR) is a form of vibrational spectroscopy that relies on the absorbance, transmittance or reflectance of infrared light. Using this method, light is absorbed in different amounts in a sample at distinct frequencies which correspond to the vibrational frequencies of the bonds in the sample.
Raman and FTIR spectroscopy differ in some key fundamental ways. Raman spectroscopy depends on a change in polarizability of a molecule, whereas IR spectroscopy depends on a change in the dipole moment. Raman spectroscopy measures relative frequencies at which a sample scatters radiation, unlike IR spectroscopy which measures absolute frequencies at which a sample absorbs radiation. FTIR spectroscopy is sensitive to hetero-nuclear functional group vibrations and polar bonds, especially OH stretching in water. Raman on the other hand is sensitive to homo-nuclear molecular bonds. For example, it can distinguish between C-C, C=C and C≡C bonds.
Both methods can be used with microscopic techniques. The key advantage of Raman spectroscopy is that it requires little to no sample preparation while the FTIR method has constraints on sample thickness, uniformity and dilution to avoid saturation. The key advantage to FTIR is the difference of the two methods in dealing with interference. Fluorescence may interfere with the ability of taking Raman spectra, which would not be an issue with FTIR.
Whether you are using Raman spectroscopy or FTIR spectroscopy, both methods have advantages and limitations, but when combined, these two methods become a powerful tool when performing materials characterization.