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Raman spectroscopy has been used by scientists for years and advancements to it are always being made. One such advancement is hyper Raman spectroscopy. Hyper Raman spectroscopy can provide information on molecules that is not possible with traditional Raman spectroscopy. In this article, we look at what Raman and hyper Raman spectroscopy are, and how modifications are being made to further improve hyper Raman spectroscopy.
Raman and Hyper Raman Spectroscopy
Raman spectroscopy is a type of vibrational spectroscopy that relies on Raman scattering to deduce the vibrational, rotational and other low-frequency modes of different molecules. Once the spectrometer has isolated these modes, it can then back out the chemical structure of the molecule being analyzed.
As mentioned, Raman spectroscopy in all forms is reliant on the Raman scattering of a molecule. Raman scattering is a form of inelastic light scattering. This type of inelastic scattering occurs because the incident photons (produced by a monochromatic laser source) become scattered into photons with a different frequency (these are the ones which are detected). This change in frequency causes a measurable difference in the energy and wavelength between the detectable photon and the incident photon. This is also known as the Raman effect.
Raman spectroscopy has become a valuable tool across many areas of science and new advancements are always being made to improve the sensitivity of the technique and in the observation of new vibrational modes. There are many different molecules and materials that can be analyzed with Raman spectroscopy, although most of them are of a crystalline nature because the lattices provide a much better light scattering environment.
So how does hyper Raman spectroscopy differ from standard Raman spectroscopy? Hyper Raman spectroscopy is a modified version of Raman spectroscopy and many of the basic operations are the same. Where hyper Raman spectroscopy differs is in that the scattered light is at frequencies which are lower than twice the frequency of the incident light. This approach means that two incident photons become converted into a single photon of scattered light and a phonon. The signal produced by hyper Raman spectroscopy is weak, however, it can be used to provide vibrational information on some silent modes which are supressed by conventional Raman spectroscopy because of issues with molecular symmetry.
Hyper Raman Spectroscopy Developments
There have been a couple of interesting and beneficial developments with hyper Raman spectroscopy: plasmon enhancement and the combination of resonance Raman and hyper Raman spectroscopies, both of which are detailed below.
Resonance Hyper Raman Spectroscopy
Resonance Raman spectroscopy is a different mode of Raman spectroscopy that is used to measure incident photons with energies close to an electronic transition within a material. Since then, it has been adapted and combined with hyper Raman spectroscopy to produce resonance hyper Raman spectroscopy. The main reason for this combination is because hyper Raman spectroscopy is a weak signal mode and has often struggled with analyzing molecules in a dilute solution.
Resonance hyper Raman spectroscopy is seen to be a non-linear Raman method, and as a result, is more sensitive because the amount of energy emitted is lower than the incident energy. It has also been identified as a way of measuring low concentrations (with a strong signal) of analyte within a liquid medium through the push-pull mechanisms exhibited by molecules with both large dipole moments and large polarizabilities. This enhanced method can also provide information about excited states for both one and two photon transitions.
Surface Enhancement Through Plasmons
This advancement doesn’t come from modifications to the instrument itself, rather, it is do with the placement of the material of interest onto a plasmonic surface, the most common being the use of gold nanostructures. The introduction of these plasmonic substrates enhances the Raman scattering of the molecule of interest, which increases the overall sensitivity of the technique. There is a dedicated Raman technique to this type of analysis, known as surface-enhanced Raman spectroscopy (SERS). This is essentially being applied to hyper Raman spectroscopy through plasmonic substrates in order to enhance the weak hyper Raman signal.
On the analysis side, again, it is an approach that can exploit the push-pull mechanisms of donor-acceptor molecules by reducing the amount of symmetry exhibited by the molecule and by reducing the second harmonic generation, i.e. the background noise. Plasmon-enhanced Raman spectroscopy has been extensively used for a wide range of molecules, including chromophores, biologics and organics.
Sources and Further Reading
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