Polarisability
Definition
The ease with which the electron cloud of a molecule deforms in an external electric field. A vibration is Raman active only if it changes the polarisability, the Raman analogue of IR's dipole-change rule.
- Field
- Vibrational spectroscopy
- Definition
- Ease of electron cloud deformation in an E field
- Governs
- Raman activity of a vibrational mode
Common questions
How does the polarisability rule differ from the rule governing IR activity?+
IR activity requires a vibration to change the molecule's dipole moment, while Raman activity requires the vibration to change polarisability instead. Because these are different conditions, some vibrations are IR-active but Raman-silent or the reverse, which is why the two techniques are used together as complementary tools.
Why does this rule make Raman useful for certain forensic samples that IR struggles with?+
Symmetric, non-polar bonds such as those in many polymers and carbon-based materials often change polarisability strongly while barely changing dipole moment, giving a strong Raman signal where IR gives a weak one, which is one reason Raman suits certain trace and polymer comparisons.
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...
- Raman Effect
- Inelastic scattering of monochromatic light by a molecule. The scattered photon is shifted in wavelength by an amount equal to a vibrational...
- 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,...