Raman scattering is an inelastic scattering process that occurs when light interacts with molecular vibrations, phonons, or other low-frequency excitations in a material. This phenomenon is named after the Indian physicist C.V. Raman, who, along with his colleague, discovered it in 1928. In simple terms, when a monochromatic light source, typically a laser, shines on a sample, most of the light is elastically scattered, meaning it retains its original energy (or wavelength).
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Raman scattering is inelastic scattering of light in which the outgoing photon has a different frequency because matter gains or loses internal energy. Molecular vibration is a particularly useful recipient: a visible photon can reveal an internal energy spacing normally associated with infrared frequencies. Rayleigh scattering supplies the much stronger, approximately unchanged-frequency reference. The distinction concerns the transferred energy, rather than merely a change of direction or the colour of the illuminating beam.
The elementary bookkeeping is conservation of energy:Here is the Planck constant, and are the incident and scattered photon frequencies, and are initial and final internal molecular energies. This expression neglects translational recoil and Doppler broadening; those can matter in sufficiently precise gas measurements. A positive internal energy change gives Stokes Raman scattering, and a negative one gives Anti-Stokes Raman scattering. The energy difference determines a possible line position, but it does not by itself establish that a transition is optically allowed or predict its strength.
The spontaneous molecular process developed here has a connection to the Raman transition used in atomic measurement methods: two photon interactions transfer an internal energy difference. Coherently driving that process with two applied light fields is a different experimental regime from collecting weak spontaneous Raman scattering beside an illuminating beam.