Resonance Raman Spectroscopy
Many substances, especially colored ones, may absorb laser beam energy and generate strong fluorescence, which contaminates Raman spectrum. This is one of the central problems in Raman spectroscopy, especially when UV lasers are used. Nevertheless, it was discovered that instead of fluorescence, some types of colored molecules could produce strong Raman scattering at certain conditions. This effect was called Resonance Raman.
Resonance Raman Effect occurs when excitation laser frequency is chosen in such a way that it crosses the frequencies of electronic excited states and resonates with them. The intensity of Raman bands, which originate from electronic transitions between those states, are enhanced three-to-five orders of magnitude. Not all the bands of spontaneous Raman spectrum are enhanced. The so-called chromophoric group responsible for the molecules' coloration experiences the highest level of enhancement because the chromophoric group normally has the highest level of light absorption.
The highest intensity of Resonance Raman signal is obtained when laser frequency equals the first of the second electronic excited state (figure below). Therefore, tunable lasers are the most appropriate choice. But even when the frequency of the laser does not exactly match the desired electronic excited states, impressive enhancement of Raman signal occurs.
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