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Raman spectroscopy is a powerful tool for chemical identification in a range of fields, from winemaking to quality control in pharmaceutical manufacturing and forensic analysis. The highly versatile nature of Raman spectroscopy and its ability to measure gas, solution and solid phase samples, have meant its use has become increasingly commonplace in industry.
Raman spectroscopy works by probing the vibrational modes of molecules. Each molecule has its own unique fingerprint, with vibrations at characteristic frequencies, that can be compared to database spectra for chemical identification. A simple Raman spectrometer has three main components, a light source for sample excitation, a sampling apparatus and a detector.
The Light Source
Of these components, the quality and properties of the light source are particularly important for obtaining high-quality Raman spectra.
Many modern Raman spectrometers use lasers as their excitation source. Diode-pumped, solid-state lasers are a popular choice as they are often turn-key systems capable of delivering high-intensity, highly monochromatized, continuous wave radiation and can emit in a range of different wavelengths. As the observed Raman spectrum is highly dependent on the excitation parameters, it is crucial to pick a source that is suitable for the application at hand.
Resolving the Problem
A highly monochromatized, narrow-bandwidth excitation source is desirable in a Raman spectrometer as it is one of the limiting factors in determining the smallest energy differences that can be resolved i.e. the energy resolution of the instrument.
High energy resolution is of key importance for chemical identification of complex mixtures such as petroleum, where there are many structurally-similar chemical species where the vibrational modes used for chemical fingerprinting may be separated in energy by less than a few wavenumbers. Another example for which high resolution is essential is for performing lineshape analysis on crystalline samples to assess the degree of crystallinity. Without a sufficiently narrowband excitation source, the observed lineshape is limited not by the sample, but broadened by the laser bandwidth.
As well as the narrow linewidth, an ideal source must have good spectral stability, where there are minimal fluctuations in the central excitation wavelength. Spectral instabilities will also contribute to linewidth broadening.
Raman scattering, the process at the heart of Raman spectroscopy, is an inherently inefficient process with weak transitions, particularly in comparison to absorption spectroscopy. This, in turn, leads to small signal levels with long acquisition times, which is not always desirable if fast screening of compounds is required, or for obtaining meaningful signal levels on very dilute chemical species.
One way of compensating for this inefficiency is to use laser sources with high output powers that can be used to achieve high excitation intensities. The observed Raman signal intensity is directly proportional to the excitation intensity so acquisition times can be reduced by simply upping the excitation intensity.
Another advantage of using lasers rather than lamp sources, such as helium-neon lamps, is the ability to tightly focus their output beam. By reducing the spot size of the beam, the excitation power density increases, again, leading to improved signal levels. This can also give improved spatial resolution for Raman microscopy experiments. Some care must be taken that the power density does not become sufficiently high that it does not lead to sample degradation, which is particularly problematic in the case of many solid samples.
Choosing the best excitation wavelength for a Raman experiment is not a trivial task. There are many factors that need to be considered when attempting to record a spectrum with an optimum signal to noise ratio. For example, for solid samples particularly, the wavelength-dependence of the penetration depth must be considered as well.
Although the Raman signal intensity is proportional to the fourth power of the excitation frequency, shorter wavelength excitation often means greater amounts of competition from larger, unwanted fluorescence signals which can obscure the Raman peaks of interest. As the optimum wavelength for comprising between these factors is sample-dependent, care must be taken to choose a laser source capable of generating wavelengths appropriate for the samples of interest.
Overall, an appropriately chosen, high-quality laser source in a Raman spectrometer can lead to reduced acquisition times and better resolving and chemical identification power.