What is Raman Spectroscopy?
Raman spectroscopy is based on the scattering of radiation by the sample, rather than an absorption process. It is named after the Indian physicist who first observed it in 1928, C V Raman.
The basic principle is that a photon of a certain wavelength (λ1) is absorbed by a molecule, which then emits a photon at a different wavelength (λ2), as shown in Figure 1. This is called a Raman Shift. Different molecules produce a unique set of Raman Shifts, thus enabling identification. The signal strength of this emitted photon is very weak and often very difficult to detect, hence Raman spectroscopy is not as readily used as say infrared absorption spectroscopy. This weak signal require Raman spectrometer to collect every possible photon.
The difference in energy between the incoming photon and the scattered one in the Raman effect is directly linked with the molecule’s vibrational capabilities. There can be multiple Raman scattering energy shifts for one molecule (also with quite different probability of interaction). Measuring the difference between scattered and initial excitation photons gives information about the molecule.
Among a finite set of molecules, it is likely to find a single Raman shift that is pertinent only to one molecule and acts as a signature for that molecule. If the cross section of scattering is known the size of the peak provides an indication to the number of molecules present.
A good Raman spectrometer will provide good enough resolution to separate these lines and wide enough spectral range to collect all the light from each of the unique signatures.
Furthermore, by measuring the emission ratio between two Raman lines, it is possible to estimate the ratio of two molecules within a mixture if the cross sections of both of the molecules are known.
This technique gives the advantage of being selective and precise. Indeed, the Raman shift associated with a molecule is quite narrow and precisely positioned.
Various sources of information are available for Raman scattering. The precise magnitude of the Raman scattering cross section often has to be determined experimentally. However, it is possible to make an estimate of the likely range of the scattering by comparison with other known Raman scattering information.
Raman photons scatter in all directions and thus measurements can be made from any position according the user requirements.
The absolute scattering of gaseous nitrogen is taken from an article by Rolf Bombach at the Paul Scherrer Institut, Switzerland. The value quoted is an experimental value referred therein to work reported in “Linear Raman Spectroscopy: A State Of The Art Report” published in Non-Linear Raman Spectroscopy and Its Chemical Applications, W. Kiefer and D.A. Long, eds., Nato Advanced Study Institute Series, Series C, Mathematical and physical sciences; 93, D. Reidel, Dordrecht NL, 1982. ISBM 90-277-1475-4. Nitrogen has one of the smallest cross section of any molecule (10-32) . The scattering cross section typically varies between 10-32 – 10-29 cm2 sr-1 for a given chemical. This number must then be multiplied by the number of chemicals in the sample, hence why liquids and solids typically provide a stronger signal.
A good Raman spectrometer therefore may require a cooled detector to provide a high level of SNR and factors such as etendue are essential especially for stand off Raman and spatially offset raman measurements.