Resonance Raman spectroscopy

Resonance Raman (RR) spectroscopy is a name given to Raman spectroscopy when the incident laser frequency is close in energy to an electronic transition of a compound or crystal under examination. The frequency coincidence (or resonance) can lead to greatly enhanced intensity of the Raman scattering, which facilitates the study of compounds present at low concentrations.

Raman scattering is usually extremely weak, of the order of 1 in 10 million photons that hit a sample are scattered with the loss (Stokes) or gain (anti-Stokes) of energy because of changes in vibrational energy of the molecules in the sample. Resonance enhancement of Raman scattering requires that the wavelength of the laser used is close to that of an electronic transition. In larger molecules the change in electron density can be largely confined to one part of the molecule, a chromophore, then 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 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.

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. Although the technique measures the energy required to change the vibrational state of a molecule as does infrared (IR) spectroscopy, the two methods are actually complementary.

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