Disclaimer - I am employed at Ondax Inc.
Indeed Low-frequency Raman spectroscopy was not trivial and tests the limits of filters that are based on thin film technology. Lately, it has become increasingly common to use low-frequency Raman to detect crystalline polymorphs of active pharmaceutical ingredients in the pharmaceutical industry due to developments in filter technology.
The classical way to get below a 100 cm-1 is using a triple spectrograph in subtractive mode, as a few people have already suggested. The advantage of using this method is tunability, it might be used with different excitation wavelengths. However, a major drawback is that triple spectrometers have notoriously low throughput. This can prohibit spectra from being collected in a reasonable amount of acquisition time. This is often limiting, especially in industrial or process environments.
One way to circumvent the throughput problem is using Ondax's THz-Raman systems. These systems allow measurements down to 10 cm-1 using an ultra-stabilized laser, patented VHG gratings and carefully designed optical assembly. The fiber output from the THz-Raman system is coupled to a spectrograph and the spectral resolution is governed by the choice of spectrograph. There is a much higher throughput using THz-Raman instrumentation and other 'soft advantages' when compared to a triple spectrograph. Attached are links to a few publications that utilize THz-Raman systems for low-frequency Raman spectroscopy.
In summary, there are pros and cons to either path and researchers should carefully consider and weigh them according to their application.
Article Direct High-Resolution Imaging of Crystalline Components in ...
Article Investigating Crystallinity Using Low Frequency Raman Spectr...
When I worked with Raman measurements, I put the spectrometer grid in 50 cm-1 to prevent the CCD device was damaged. So I think you can get Raman measurements below 100 cm-1.
It is definitely possible. However, you need special equipment to get closer to the Rayleigh line. There are dedicated spectrometer, sometimes called Terahertz-Raman Systems with which you can get down as far as 5 cm-1 or even down to 0.1 cm-1. See e.g.
http://www.nature.com/nphoton/journal/v9/n1/full/nphoton.2014.283.html
Yes it possible but you should use proper equipment. As usual this is a spectrometers with several grating like Horiba Jobin-Yvon T64000 ( http://www.horiba.com/scientific/products/raman-spectroscopy/raman-spectrometers/triple-raman-spectrometers/details/triple-raman-spectrometers-140/ ) or U1000 ( http://www.horiba.com/fileadmin/uploads/Scientific/Documents/Raman/HSC-T64000_U1000-2013-V1.pdf ).
For single grating spectrometers it is possible with Brag Notch Filter (for example http://www.optigrate.com/BragGrate_Notch.html )
I completely agree with A.S. Krylov. Optigrate is the way to go for good Bragg Gratings dedicated to a grating spectrometer.
On a different topic, we have long preferred not to use slow throughput spectrographs as they are not entirely necessary to achieve desirable S/N contrasts near the Rayleigh peak. For diamnd-anvil cell HP research, we can get down to ~ 10 wavenumbers using fast spectrographs, confocal imaging, and the correct filters. But again, our micro-Raman systems (including micro-FTIR) are all custom designed/built for high-performance.
In my lab I have Horiba Jobin-Yvon T64000 - on this instrument is possible go down up to 3-5 cm-1 at any excitation wavelength without of any filters (Depends on sample). This instrument no so fast in measurements but it specially designed for extra low wavenumbers.
I totally agree with Alexander S Krylov about special spectrometer Horiba Jobin-Yvon T64000.
Also you can use Notch Filters at your spectrometer to work in region from 30 cm-1. We use such scheme.
I fully agree with the colleagues. In our lab we have implemented a Bragg notch filter stage (from Ondax and/or Optigrate) into a Renishaw RM1000 Raman spectrometer (single stage) and can easily get down to 5 cm-1 from the laser line both on the Stokes and anti Stokes side.
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