Resonance Raman spectroscopy
Resonance Raman spectroscopy (RR spectroscopy or RRS) is a variant of Raman spectroscopy in which the incident photon energy is close in energy to an electronic transition of a compound or material under examination. This similarity in energy (resonance) leads to greatly increased intensity of the Raman scattering of certain vibrational modes, compared to ordinary Raman spectroscopy. Resonance Raman spectroscopy has much greater sensitivity than non-resonance Raman spectroscopy, allowing for the analysis of compounds with inherently weak Raman scattering intensities, or at very low concentrations. It also selectively enhances only certain molecular vibrations (those of the chemical group undergoing the electronic transition), which simplifies spectra. For large molecules such as proteins, this selectivity helps to identify vibrational modes of specific parts of the molecule or protein, such as the heme unit within myoglobin. Resonance Raman spectroscopy has been used in the characterization of inorganic compounds and complexes, proteins, nucleic acids, pigments, and in archaeology and art history. Raman spectroscopy#Theory In Raman scattering, photons collide with a sample and are scattered with a loss of energy: The scattered photons are lower in energy (have a longer wavelength) than the incident photons. This loss of energy is caused by excitation of the sample to a higher vibrational energy level (if the sample was initially in an excited vibrational state, the scattered photon may be higher in energy than the incident photon; this is anti-Stokes Raman scattering). For most materials, Raman scattering is extremely weak compared to Rayleigh scattering, in which light is scattered without loss of energy. Raman-scattered light, which contains information about vibrational transitions, is therefore difficult to observe for many substances. Resonance Raman spectroscopy takes advantage of an increase in the intensity of Raman scattering when the incident photons match the energy of an electronic transition.
