Resonance Raman spectroscopy (RR spectroscopy) is a Raman spectroscopy technique in which the incident photon energy is close in energy to an electronic transition of a compound or material under examination. The frequency coincidence (or resonance) can lead to greatly enhanced intensity of the Raman scattering, which facilitates the study of chemical compounds present at low concentrations. Raman scattering is usually extremely weak, since only about 1 in 10 million photons that hit a sample are scattered with a loss (Stokes) or gain (anti-Stokes) in energy from changes in vibrational energy of the molecules in the sample; the rest of the photons are scattered with no change in energy. Resonance enhancement of Raman scattering requires the incident wavelength (usually from a laser) to be close to that of an electronic transition of the molecules. In larger molecules the change in electron density can be largely confined to one part of the molecule, a chromophore, and in these cases the Raman bands that are enhanced are primarily from those parts of the molecule in which the electronic transition leads to a change in bond length or force constant in the excited state of the chromophore. For large molecules such as proteins, this selectivity helps to identify the observed bands as originating from vibrational modes of specific parts of the molecule or protein, such as the heme unit within myoglobin. Overview Raman spectroscopy and RR spectroscopy provide information about the vibrations of molecules, and can also be used for identifying unknown substances. RR spectroscopy has found wide application to the analysis of bioinorganic molecules. The technique measures the energy required to change the vibrational state of a molecule as does infrared (IR) spectroscopy. The mechanism and selection rules are different in each technique, however, band positions are identical and therefore the two methods provide complementary information. Infrared spectroscopy involves measuring the direct absorption of photons with the appropriate energy to excite molecular bond vibrational modes and phonons. The wavelengths of these photons lie in the infrared region of the spectrum, hence the name of the technique. Raman spectroscopy measures the excitation of bond vibrations by an inelastic scattering process, in which the incident photons are more energetic (usually in the visible, ultraviolet or even X-ray region) and lose (or gain in the case of anti-Stokes Raman scattering) only part of their energy to the sample. The two methods are complementary because some