How low can the low frequency analysis be made?

Many samples provide sufficient information for identification and characterization in the standard 'fingerprint' range, (400-1800 cm-1). Raman can however provide access to the full vibrational (and lattice mode) ranges with general ease.

Bench-top FTIR instruments generally do not operate below this 400 cm-1 threshold (due to the limitation of the optics) and so many inorganics and species that have spectroscopic features below 400 cm-1 are not well analyzed.

Standard Raman instruments tend to utilize holographic notch or steep dielectric edge filters to remove the Rayleigh scatter and to transmit only the Raman signal. Modern instruments can typically now measure down to about 100cm-1; ideal for these types of lower frequency samples (eg. Metal oxides).

However, there are numerous materials which exhibit spectral features below even this 100cm-1 range, such as lattice modes in crystals, LAM modes in polymers, certain metal halides and semiconductor materials (superlattices).

For these types of samples higher performance laser rejection filters are required. These can be grating based designs or filter based designs depending upon the performance that is required.

A high-end research Raman spectrometer is capable of measuring extremely low Raman frequencies, down to a few cm-1 from the laser line. For example, the T64000 Raman spectrometer can operate in such a fashion and provide access to spectral features down to 2-3 cm-1.