What is low frequency analysis, and why is it needed?

Low frequency analysis refers to the low Raman shift (low wavenumber, cm-1) region of the spectrum.  Most standard Raman spectrometers will allow analysis down to 100-200 cm-1 which allows the standard ‘fingerprint’ spectral range to be detected with ease. However, there are certain materials which exhibit interesting spectral features below 100 cm-1.

Low frequency analysis

Raman systems configured for low frequency analysis allow measurements below 100 cm-1 so that researchers can investigate and characterise these additional spectral features.  Research systems can allow low frequency analysis down to 30-50 cm-1 (for standard single monochromator instruments) and even 4-5 cm-1 (for triple monochromator instruments).

For most routine analyses, the standard Raman range from 100 cm-1 upwards is sufficient for identification and characterisation.

However, there are certain materials which exhibit spectral features below 100 cm-1, and being able to measure these peaks is vital for full characterisation – indeed, in some cases, analysing these low frequency features is the only method to distinguish different materials.  Examples where low frequency analysis is important include:

  • Polymorphs of pharmaceutical materials
  • Crystal lattice modes
  • Longitudinal acoustic modes in polymers
  • Certain metal oxide and halide species
  • Semiconductor superlattices
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