When monochromatic radiation is incident upon a sample then this light will interact with the sample in some fashion. It may be reflected, absorbed or scattered in some manner. It is the scattering of the radiation that occurs which can tell the Raman spectroscopist something of the samples molecular structure.
If the frequency (wavelength) of the scattered radiation is analyzed, not only is the incident radiation wavelength seen (Rayleigh scattering) but also, a small amount of radiation that is scattered at some different wavelength (Stokes and Anti-Stokes Raman scattering). (approx. only 1 x 10-7 of the scattered light is Raman). It is the change in wavelength of the scattered photon which provides the chemical and structural information.
Light scattered from a molecule has several components - the Rayleigh scatter and the Stokes and Anti-Stokes Raman scatter.
Light scattered from a molecule
In molecular systems, these frequencies are principally in the ranges associated with rotational, vibrational and electronic level transitions. The scattered radiation occurs over all directions and may also have observable changes in its polarization along with its wavelength.