19.8: Alternative Techniques
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Fourier transform (FT) Raman
Unlike dispersive Raman spectroscopy, which obtains a spectrum by diffraction of the different wavelengths, Fourier transform (FT) Raman creates an interference pattern that can be analysed to recover the spectrum. It has the advantage of being faster than the dispersive technique, but is limited in resolution and choice of laser wavelength.
Stimulated Raman scattering is a non-linear phenomenon that results in a much larger Raman signal than standard scattering (4 to 5 orders of magnitude greater). It can be triggered with a strong laser pulse. Only the strongest Raman active mode is excited at first, but scattering from it can be strong enough to excite the second mode, which in turn can excite the third and so on in a cascade effect.
Another effect that greatly increases the magnitude of scattering is Resonance Raman (RR). This occurs when the energy of the incident radiation is close to that of an electronic excitation energy (i.e. the band gap). Tunable lasers can be used to achieve this. The Raman scattering of vibrational modes around this excited state is then greatly enhanced by resonance effects.
Surface Enhanced Raman Spectroscopy (SERS)
Surface Enhanced Raman Scattering (SERS) is a process that can occur when samples are adsorbed on gold or silver surfaces. It results in a vast increase in the Raman effect, and is therefore useful for spectroscopy. Though the mechanism is not very well understood, it is believed to be a combination of chemical enhancement of polarisability by bonds formed between the sample and the surface, and electromagnetic resonance of small gold or silver particles. This effect can be combined with Resonance Raman for Surface Enhanced Resonance Raman Spectroscopy (SERRS), which results in very strongly enhanced signals, up to 1014 times more intense than standard Raman scattering.
Coherent Anti-Stokes Raman Spectroscopy (CARS)
Coherent Anti-Stokes Raman Spectroscopy (CARS) is a technique involving two lasers. One is of a fixed frequency ν1 and the other is tunable to a lower frequency ν2. When combined they result in coherent radiation at frequency
along with a number of other frequencies.
If there is a Raman active mode with characteristic frequency νm, then when the second laser is tuned such that
then the coherent emission emerges in a high intensity narrow beam with frequency
This is the anti-Stokes frequency, hence the name.
Raman optical activity – compares polarisations
Raman optical activity is a technique which compares the different polarisations of Raman scattered light from chiral molecules such as those found in biology, in order to determine more about their structure. This can also be combined with amplifying techniques such as resonance and surface enhancement.