Raman Scattering
Definition
Inelastic scattering of a photon from a molecular vibration; the scattered photon carries a frequency shift equal to the vibrational frequency, producing a chemically diagnostic Raman spectrum independent of excitation wavelength.
- Mechanism
- Inelastic photon-vibration energy exchange
- Shift basis
- Vibrational frequency of the molecule
- Wavelength independence
- Spectrum shape unaffected by excitation wavelength
- Output
- Chemically diagnostic Raman spectrum
Common questions
Why does a fluorescent sample cause problems for Raman scattering measurements?+
Fluorescence emission is far more intense than the weak Raman signal and can swamp it entirely. Analysts work around this by choosing a longer excitation wavelength, using time-gated detection, or switching to a different laser line to avoid the fluorescing region.
How does Raman scattering complement infrared absorption for the same sample?+
The two techniques are governed by different selection rules, so bonds that are weak or silent in one are often strong in the other. Running both on a questioned material, such as a paint chip or fibre, gives more complete confirmation of its chemical identity than either alone.
Related terms
- Absorption
- The purification technique in which a crude antiserum is mixed with antigen preparations from cross-reacting species. Cross-reactive antibodies bind those antigens and...
- Beer-Lambert Law
- A = εbc. Absorbance equals molar absorptivity times path length times concentration. The linear relation between absorbance and concentration that makes UV-Vis...
- Chromophore
- The molecular subunit responsible for a dye's colour, typically a conjugated double-bond system; chemical erasure destroys the chromophore while leaving the decolourised...
- Fluorescence Lifetime
- The average time a fluorophore spends in the S1 excited state before emitting a fluorescence photon; typically 1-10 ns for organic fluorophores,...
- Fluorescence Quantum Yield (Phi)
- The fraction of absorbed photons that result in fluorescence emission (0 to 1); determines the brightness of a fluorescent signal and is...
- Jablonski Diagram
- An energy-level diagram showing the ground state, singlet excited states, and triplet state of a molecule, with arrows indicating absorption, internal conversion,...
- Kubelka-Munk Model
- A diffuse-reflectance theory relating the measured reflectance of a scattering-plus-absorbing layer to its absorption and scattering coefficients; used in forensic paint and...
- Molar Extinction Coefficient (ε)
- The intrinsic absorptivity of a molecule at a given wavelength, in L/(mol·cm); oxyhaemoglobin at 415 nm has ε ≈ 125,000, making it...
- Phosphorescence
- Light emission from the triplet excited state (T1) via a spin-forbidden radiative transition; characterised by emission persisting after the excitation source is...
- Specular Reflection
- Mirror-like reflection from a smooth surface where angle of incidence equals angle of reflection; minimised in forensic photography with crossed polarisers to...
- Stokes Shift
- The wavelength difference between the excitation peak and the emission peak of a fluorophore. A positive Stokes shift (emission at longer wavelength...