Raman Spectroscopy
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.
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
