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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.
== 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. 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.
Infrared spectroscopy involves measuring the direct absorption of photons with the appropriate energy to excite molecular bond vibrations. 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 vibrational transitions that are observed in IR spectroscopy are not observed in Raman spectroscopy, and vice versa. RR spectroscopy is an extension of conventional Raman spectroscopy that can provide increased sensitivity to specific (colored) compounds that are present at low (micro to millimolar) in an otherwise complex mixture of compounds.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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