18.4: Other Types of Raman Spectroscopy

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Traditional Raman spectroscopy has several limitations, perhaps the most important of which is that the probability of Raman scattering is much less than that for Rayleigh scattering, which leads to low sensitivity with detection limits often as large as 0.1 M. Here we briefly describe two forms of Raman spectroscopy that allow for significant improvements in detection limits.

Resonance Raman Spectroscopy (RRS)

If the wavelength of the source is similar to the wavelength needed to move the species from its ground electronic state to its first electronic excited state (not the virtual excited state shown in Figure 18.1.2), then the lines associated with the symmetric fundamental vibrations increase in intensity by a factor of \(10^2\) to \(10^6\). The improvement in sensitivity results in a substantial reduction in detection limits as low as \(10^{-8} \text{ M}\). The use of a tunable laser makes it possible to adjust the wavelength of light emitted by the source to maximize the intensity of scattering.

Surface-Enhanced Raman Spectroscopy (SERS)

For reasons that are poorly understood, the intensity of Raman scattering lines is enhanced when the scattering species is absorbed to the surface of colloidal particles of metals such as Ag, Au, or Cu, or to the surface of etched metals. The phenomenon is not limited to just a few lines—as is the case for RRS—and results in a \(10^3\) to \(10^6)\) improvement in the intensity of scattering. If a tunable laser is used for the source, allowing for both RRS and SERS, detection limits of \(10^{-9} \text{ M}\) to \(10^{-12} \text{ M}\) are possible.