Raman spectroscopy is a scattering form of molecular spectroscopy. It is often compared with IR spectroscopy because both provide information about the structure and properties of molecules from their vibrational transitions. IR absorption occurs when the frequency of incoming light equals the vibrational frequency of a particular vibrational mode of the molecule which allows the photon to be absorbed (not scattered). This is a single photon event with respect to the molecule's dipole moment.
In contrast, Raman scattering, or the Raman effect, is a two-photon event involving the change in polarizability of the molecule with respect to its vibrational motion in the form of scattered energy. The interaction of a molecule's polarizability with incoming light induces a dipole moment and the light scattered by this induced dipole contains the Raman scattering along with Rayleigh scattering. Rayleigh scattering corresponds to the light scattered elastically, at the same frequency as that of the incident radiation. Raman scattering is shifted in frequency and energy from that of the incident radiation by the amount of energy gained or lost in the molecule as it scatters energy and relaxes.
These molecular specific transitions, for both FTIR and Raman spectroscopy, when plotted as a spectrum provide a unique pattern or fingerprint for the compound being investigated. Because of symmetry properties of a molecule, vibrations that are seen in the Raman spectrum, may not be seen (or weakly observed) in the IR spectrum and vice versa when interrogating asymmetric molecules. This behavior is summarized in the selection rules that govern these types of interactions. Based on the similar, but unique, molecular information gained by these techniques Raman and IR are considered to be complementary technologies.