500px|thumb|alt=Energy level diagram showing scattering and fluorescence|Energy level diagram showing relationship between Rayleigh, Raman, and resonance Raman scattering and fluorescence.

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. Resonance Raman spectroscopy has been used in the characterization of inorganic compounds and complexes, proteins, nucleic acids, pigments, 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 cross-section when the incident photons match the energy of an electronic transition. If the energy of the photon striking the sample is equal or close to that of an electronic transition in the sample, certain Raman-active vibrational modes&mdash;those producing nuclear displacement in the same direction as the electronic transition&mdash;will exhibit greatly enhanced scattering, up to 10<sup>6</sup>-fold compared to nonresonance Raman. Resonance enhancement is most apparent in the case of π-π* transitions and least for metal centered (d–d) transitions. This method has been used to examine the dynamics of excited electronic states, binding of oxygen or other gases to heme-containing proteins, and protein dynamics.

  • Resonance hyper-Raman spectroscopy: Excitation of the sample occurs by two-photon absorption, rather than by absorption of a single photon. This arrangement allows for excitation of modes that are forbidden in ordinary resonance Raman spectroscopy, with intensity enhancement due to resonance, and also simplifies collection of scattered light. It is especially useful for molecules that are both polar and polarizable.
  • Surface-enhanced resonance Raman spectroscopy: A hybrid of RRS and surface-enhanced Raman scattering. The sample is applied to conducting nanoparticles and a laser matching the surface plasmon resonance of the nanoparticles is used as source. If the wavelength of the surface plasmon matches that of an electronic transition in the sample, the Raman scattering will be greatly enhanced compared to ordinary RRS.
  • Resonance Raman microscopy: A microscope is used to focus the source laser onto a particular point in the sample, and spectra are collected for many such points. The Raman intensity at different points can then be assembled into a microscopic image of the sample. By appropriate choice of source wavelength, a microscopic map of the distribution only of a component of interest can be made.

Applications

300px|thumb|alt=Example of resonance and nonresonance Raman spectra|Resonance (top) and nonresonance (bottom) Raman spectra of [[molybdenum sulfide|MoS<sub>2</sub> on silicon. Note that use of a source wavelength of 633 nm, near an electronic transition, causes appearance of bands that are too faint to be visible with a 532 nm source. Figure courtesy of David Tuschel.[https://www.spectroscopyonline.com/view/exploring-resonance-raman-spectroscopy] ]]

Because of its selectivity and sensitivity, resonance Raman spectroscopy is typically used to study molecular vibrations in compounds that would have very weak and/or complex Raman spectra in the absence of resonance enhancement. Like ordinary Raman spectroscopy, resonance Raman is compatible with samples in water, which has a very weak scattering intensity and little contribution to spectra. However, the need for a source laser with a wavelength matching that of an electronic transition in the analyte of interest somewhat limits the applicability of the method. The resonance Raman spectra of other polyene pigments, such as spheroidene and retinal, have been used to identify differences in chromophore conformation in photoactive proteins. Resonance Raman spectroscopy has been used in archaeology to identify dyes and pigments in cultural artifacts, and the ability of RRS to distinguish different modern inks and dyes has found application in forensic science. This method has been used to examine gas binding in hemeproteins and the catalytic cycle of various enzymes. Using ultraviolet laser excitation, it is possible to selectively excite the sidechains of aromatic amino acids (phenylalanine, tyrosine, and tryptophan) to deduce the local environment and hydrogen-bonding interactions by these residues. With shorter-wavelength ("deep") ultraviolet excitation, it is also possible to excite the peptide bonds of a protein in order to examine secondary structure. Protein folding and denaturation have been examined using deep-UV resonance Raman spectroscopy of the polypeptide backbone, with excitation wavelengths shorter than 200 nm.

Nanomaterials

Resonance Raman spectroscopy has also been used to characterize the structure and photophysical properties of nanoparticles. Using lasers tuned to the visible and near-infrared electronic transitions of carbon nanotubes, it is possible to enhance structure-sensitive vibrational bands of the nanotubes.

See also

  • Scattering
  • Rayleigh scattering
  • X-ray Raman spectroscopy
  • Coherent anti-Stokes Raman spectroscopy
  • Tip-enhanced Raman spectroscopy
  • Vibronic spectroscopy
  • Depolarization ratio

References

Further reading

  • https://www.spectroscopyonline.com/view/exploring-resonance-raman-spectroscopy
  • http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Vibrational_Spectroscopy/Raman_Spectroscopy/Raman%3A_Interpretation
  • http://www.horiba.com/us/en/scientific/products/Raman-spectroscopy/Raman-academy/Raman-faqs/what-is-polarised-Raman-spectroscopy/