Once a laser illuminates on a specimen, photons of the laser beam will be absorbed by the sample and pumped to virtual states; it then reemits photons with same or slightly higher/lower frequencies and jump back to ground state. If the frequency is same, it is a Rayleigh scattering process. If the frequency is lower, it is called stock shift; otherwise it is an anti-stock shift. The frequency changes is related to the molecular vibration, rotation, bending. Therefore, similar as IR spectroscopy, Raman can also be adopted as a powerful tool to study the molecular structures.
The process can be schematically shown in the above figure. In terms of light intensity, the Rayleigh scattering >> stock>anti-stock.
About 0.001% of incident photons involve in Raman process. For Raman spectroscopy, one of the key tasks is to separate useful Raman signal from much stronger Rayleigh scattering signal. Pulsed laser (Vis, IR) are used to generate more Raman signal and improve signal/noise ratio.
Raman Vs IR
Both Raman and IR reflect the similar molecular vibration modes. and they can be complementarily used to investigate molecular structures, e.g. for symmetric vibration like in CO2 extension, IR spectroscopy signal is very weak, however Raman can have quite good signal. Examples are shown below for comparison between Raman and IR spectrum.
Some information are from internet but lost the original sources. and more detailed knowledge can be obtained from other sources.