When light travels through a solid, liquid or gas some of the light will be scattered, traveling off in directions which differ from that of the incoming light. Most of the scattered light will retain its original frequency — this is known as elastic scattering, Rayleigh scattering being an example. A small proportion of the scattered light will have a frequency less than that of the incoming light and a still smaller proportion will have a higher frequency — this is known as inelastic scattering. Raman scattering is a form of inelastic scattering and is named after Chandrasekkara Venkata Raman, who received a Nobel Prize for his work on the subject in 1930.
Although scattering can be thought of as light simply reflecting off small particles, the reality is more complex. When electromagnetic radiation, of which light is a type, interacts with a molecule, it can distort the shape of the molecule’s electron cloud; the extent to which this happens is known as the polarizability of the molecule and is dependent on the structure of the molecule and the nature of the bonds between its atoms. Following interaction with a light photon, the shape of the electron cloud can oscillate at a frequency related to that of the incoming photon. This oscillation in turn causes the molecule to emit a new photon at the same frequency, resulting in elastic, or Rayleigh, scattering. The extent to which Rayleigh and Raman scattering occur is dependent on the polarizability of the molecule.
Molecules can also vibrate, with the bond lengths between atoms periodically increasing or decreasing by 10%. If a molecule is in its lowest vibrational state, sometimes an incoming photon will push it into a higher vibrational state, losing energy in the process and resulting in the emitted photon having less energy and therefore a lower frequency. Less commonly, the molecule might already be above its lowest vibrational state, in which case the incoming photon might cause it to revert to a lower state, gaining energy which is emitted as a photon with a higher frequency.
This emission of lower and higher frequency photons is the form of inelastic scattering known as Raman scattering. If the spectrum of the scattered light is analyzed, it will show a line at the incoming frequency due to Rayleigh scattering, with smaller lines at lower frequencies, and still smaller lines at higher frequencies. These lower and higher frequency lines, known as Stokes and anti-Stokes lines, respectively, occur at the same intervals from the Rayleigh line and the overall pattern is characteristic of Raman scattering.
Since the frequency intervals at which the Stokes and anti-Stokes lines appear is dependent on the types of molecules with which light is interacting, Raman scattering can be used to determine the composition a sample of material, for example, the minerals present in a piece of rock. This technique is known as Raman spectroscopy, and normally employs a monochromatic laser as the light source. Particular molecules will each produce a unique pattern of Stokes and anti-Stokes lines, enabling their identification.