Stokes Shift (Raman)
Definition
The Raman-scattered photon at lower energy than the incident photon. The Stokes line is the one routinely measured because it is stronger than the anti-Stokes line.
- Definition
- Raman photon at lower energy than the incident photon
- Relative to anti-Stokes
- Stronger, so routinely measured
- Origin
- Molecules starting in the ground vibrational state
Common questions
Why is only the Stokes shift typically reported in a routine forensic Raman spectrum?+
The Stokes-shifted photons are produced in much greater number than anti-Stokes photons at room temperature, giving a stronger and cleaner signal, so instruments are configured to collect and report the Stokes side of the spectrum for identification work.
How does the Stokes shift value relate to the identity of the material being analysed?+
The magnitude of the shift corresponds to specific vibrational energy levels of the molecule's bonds, so the pattern of Stokes shift peaks across the spectrum functions as a molecular fingerprint used to match or identify an unknown substance against reference libraries.
Related terms
- Atomisation
- Conversion of a sample into free gaseous atoms in the ground state (AAS) or excited state (AES). The mandatory first step of...
- 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...
- Fingerprint Region (IR)
- 1500 to 400 cm⁻¹. The complex lower-wavenumber zone where the whole-molecule pattern is unique enough to identify a substance by overlay.
- Hollow Cathode Lamp (HCL)
- A sealed lamp whose cathode is made of the element being analysed, filled with argon or neon at low pressure. Strikes a...
- ICP Source
- Inductively coupled argon plasma at 6000 to 10000 K. The hottest practical excitation source for AES, giving simultaneous multi-element analysis (ICP-OES, ICP-MS).
- Molar Absorptivity (ε)
- The proportionality constant in the Beer-Lambert law, in L mol⁻¹ cm⁻¹. A property of the analyte at a given wavelength, solvent and...