Raman Effect
Definition
Inelastic scattering of monochromatic light by a molecule. The scattered photon is shifted in wavelength by an amount equal to a vibrational quantum, giving a fingerprint of the molecule's bond vibrations.
- Type
- Inelastic light scattering
- Discoverer
- C.V. Raman, 1928
- Cause
- Energy exchange with molecular vibrational modes
- Output
- Wavelength-shifted scattered photons
- Application
- Basis of Raman spectroscopy for molecular identification
Common questions
How is the Raman effect different from simple light reflection?+
Reflection and elastic (Rayleigh) scattering return photons at the same wavelength as the incident light. The Raman effect involves a small fraction of photons exchanging energy with molecular vibrations, so they return shifted to a different wavelength, and that shift pattern encodes the molecule's bond structure.
Why is the Raman effect useful for examining trace evidence in place?+
Because it needs no sample preparation or dissolution and works through transparent packaging or on a microscope stage, it lets an examiner identify a fibre, ink, or drug particle non-destructively, preserving the sample for further testing.
Related terms
- Mutual Exclusion Rule
- For a centrosymmetric molecule (one with a centre of inversion), a vibration is either IR active or Raman active, never both. This...
- Polarisability
- The ease with which the electron cloud of a molecule deforms in an external electric field. A vibration is Raman active only...
- SERS
- Surface-enhanced Raman scattering. A nanorough gold or silver surface enhances the Raman signal of molecules adsorbed on it by factors of 10^6...
- SORS
- Spatially offset Raman spectroscopy. The collection optics are offset from the laser spot so the detector reads photons that have travelled through...
- Stokes and Anti-Stokes Lines
- Stokes lines (longer wavelength, lower energy than the laser) come from molecules starting in the ground vibrational state; anti-Stokes lines (shorter wavelength,...