Resonance Raman Spectroscopy:

In a general Raman spectral measurement the spectrum is acquired through irradiating the sample within a region where it does not have an absorption band. Normally green, red or near-infrared lasers are used for the reasons and the wavelengths are below the first electronic transitions of most molecules. The situation thus changes while the wavelength of the exciting laser lies close to the electronic absorption of a molecule. Under these conditions a resonance occurs that might enhance the intensities of a few (often only one) Raman lines to the tune of 10² to 10⁶. That phenomenon is exploited in Resonance Raman Spectroscopy (RRS) in the incident photon causes an electronic transition of the molecule and the excited state instantly relaxes down to one of the vibrational levels of the electronic ground state as displays in Figure and an intense signal results. A signal enhancement is found to depend on the closeness of the excitation beam to the λ_max of the absorption band.

Figure: A schematic diagram showing energy changes during Resonance Raman Spectroscopy

In addition, since this resonance enhancement is observed only for the Raman bands related with the chromophore, the spectrum is quite easy and simple to analyse. With this method there has been a tremendous improvement in the sensitivity of the technique and today we might go down to a concentration of the order of 10^-8M. Resonance Raman spectroscopy is used extensively in the chemistry of fullerenes, polydiacetylenes and other such molecules which strongly absorb in the visible region.