What are the advantages and disadvantages of ultra-violet (UV) lasers for Raman?

Ultra-violet (UV) lasers for Raman spectroscopy typically include laser wavelengths ranging from 244 nm through to 364 nm.

Theoretically UV Raman spectroscopy is no different from standard analysis using visible laser wavelengths.  However, in practice there are a number of practical difficulties and disadvantages which must be considered.

Advantages

  • With certain samples, UV laser excitation can interact in ways not possible when using visible laser sources.  For example, in semiconductor materials the penetration depth of UV light is typically in the order of a few nanometers, and thus UV Raman can be used to selectively analyse from a thin top surface layer (as is commonly found in silicon on insulator SOI materials).  In another example, UV excitation can give rise to specific resonance enhancement with biological moieties, particularly protein, DNA and RNA structures.  Specific analysis of these materials within tissue can be difficult using visible laser wavelengths.
  • Fluorescence suppression can often be assisted using UV lasers, by spectrally separating the Raman and fluorescence signatures.  With visible lasers it is common that Raman and fluorescence are superimposed, and the incomparable strength of the fluorescence is what can perturb (or completely mask) the Raman spectrum.  With UV excitation the Raman spectrum lies close to the laser line, whereas the fluorescence is often slightly removed to higher wavelengths.  Thus, they no longer overlap, and the fluorescence is no longer an issue.
  • Increased sensitivity can result from UV excitation, since Raman scattering efficiency is proportional to λ-4, where λ is the laser wavelength.  Thus, Raman scattering at 325 nm is a factor of 14 more efficient than that at 633nm.

Disadvantages

  • UV Raman still remains a more sophisticated technique which requires greater expertise to handle.  Reasons for this include the fact that the laser beam is now invisible, and that the lasers are larger, more complex, and considerably more expensive.
  • Samples are more prone to burning and degradation from the laser beam since the energy per photon is increased.  However, new techniques such as DuoScan™ optics allow the laser beam to be rapidly rastored over the sample and thus preventing immediate burning.  As an example, cellulose will burn with 325 nm excitation within a few milliseconds, but with DuoScan™ it remains resilient to burning for more than five minutes.
  • Many Raman systems designed for visible and near infra-red analysis are not suitable for UV Raman.  UV Raman requires specific mirror coatings, microscope objectives, diffraction gratings, and CCD detector for optimised results.  Modern systems such as the LabRAM HR can be configured to work efficiently from the UV through to the infra-red without compromise, but nonetheless the additional requirements do come at a cost.
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