Which Laser for Raman Spectroscopy?
Raman spectroscopy is a non-destructive analytical technique that yields detailed information about the chemical structure, crystallinity, and phase of a sample of matter. It can also be used to investigate various molecular interactions based on the scattering of incident light. Lasers have proven incredibly valuable as light sources for Raman scattering due to their high intensity and practical monochromaticity. But not all Raman spectroscopy lasers are created equally.
So, what parameters should you consider when selecting a laser for Raman spectroscopy?
Considering Excitation Wavelength
Molecules typically scatter most incoming laser light at the same wavelength as that of the source. This is known as Rayleigh scattering. A much smaller portion of the beam – as little as 0.0000001% – is scattered at a different wavelength. This is Raman scattering, and it is this comparatively weak phenomenon that provides insights into the analyte’s chemical structure.
Raman Intensity
The intensity of Raman scattering is tied directly to the excitation wavelength. Generally, Raman spectroscopy lasers with longer wavelengths in the near-infrared (NIR) region correlate to a relatively weak scattering signal. Conversely, short excitation wavelengths typically give stronger signals. An ultraviolet (UV) excitation channel may yield a Raman intensity that is several orders of magnitude greater than that of a NIR laser, for instance.
Excitation wavelength-dependent Raman scattering means that NIR Raman spectroscopy lasers usually require longer acquisition times and greater accumulation numbers. Each of these values decreases exponentially for lasers with visible and sub-visible excitation channels.
However, molecules excited by laser light on the UV-visible spectrum often emit fluorescence that is much stronger than the Raman effect. This is known as fluorescence background, and it is a common pain point for Raman spectroscopy laser sources.
Fluorescence Background
Background fluorescence is unwanted noise that can arise from a variety of sources, including the excited sample, the substrate, and the optical elements. Fluorescence essentially drowns out weaker signals and makes it difficult to acquire clear Raman spectra, particularly over longer acquisition scans where background fluorescence can saturate the detector.
Fluorescence is an intrinsic process based on absorption. Fewer molecules absorb in the visible compared to the ultraviolet regions ― likewise for near-infrared compared to visible light. Raman spectroscopy lasers with longer wavelengths are subsequently preferred for samples known to suffer from high fluorescence background. However, the trade-off in Raman intensity is significant, so there is usually a compromise.
Typical Excitation Wavelengths
There are benefits and drawbacks to choosing either short or long-wave excitation signals, which are typically dictated by sample type. High energy lasers can damage sample materials, but lower energy light sources require longer exposure times which can prove just as damaging. Consequently, the most common lasers used to acquire detailed Raman spectra without damaging samples are 532nm and 785nm light sources.
Raman Spectroscopy Light Sources from Laser Quantum
Laser Quantum offers a series of lasers suitable for Raman spectroscopy light sources. Their foremost product offering is the ventus 532, an industry-leading laser for Raman spectroscopy that is unrivalled in terms of its size-to-power ratio. This is their most compact 532 nm laser and it is optimised for success in the scientific industry.
To watch Laser Quantum’s Raman spectroscopy animation video, click here.
To request more information or a quotation for these or other Laser Quantum products, contact IL Photonics.