Application Note
Understanding Raman Spectroscopy
A foundational guide to inelastic scattering and its application in molecular identification.
Raman spectroscopy is a non-destructive chemical analysis technique which provides detailed information about chemical structure, phase and polymorphy, crystallinity, and molecular interactions. It is based upon the interaction of light with the chemical bonds within a material.
The Scattering Phenomenon
When light interacts with molecules in a gas, liquid, or solid, the vast majority of the photons are scattered elastically. This is known as Rayleigh scattering. In Rayleigh scattering, the energy (and therefore the wavelength) of the scattered photons is identical to that of the incident photons.
However, a very small fraction of the scattered light (approximately 1 in 10 million photons) is scattered inelastically. In this process, the scattered photons have a different energy than the incident photons. This is the Raman effect, named after Sir C.V. Raman who discovered it in 1928.
Stokes vs. Anti-Stokes
- Stokes Shift: The molecule absorbs energy, resulting in a scattered photon with lower energy (longer wavelength). This is the most common and widely measured Raman signal.
- Anti-Stokes Shift: The molecule loses energy (if it was already in an excited vibrational state), resulting in a scattered photon with higher energy (shorter wavelength). These signals are much weaker at room temperature.
Why is Raman Useful?
The energy shift between the incident and scattered light is directly related to the vibrational modes of the chemical bonds in the molecule. Because every molecule has a unique set of vibrational modes (a structural "fingerprint"), the Raman spectrum is unique to that specific molecule.
| Raman Shift (cm⁻¹) | Vibrational Mode | Typical Functional Group |
|---|---|---|
| 2800 - 3100 | C-H stretching | Alkanes, Alkenes, Aromatics |
| 1600 - 1700 | C=C stretching | Alkenes, Aromatics |
| 400 - 1500 | Skeletal vibrations | "Fingerprint region" (highly specific) |
Instrumentation Challenges
Because the Raman signal is inherently very weak, instrumentation requires:
- A high-power, highly stable monochromatic light source (laser).
- Excellent Rayleigh rejection filters to prevent the intense elastic scatter from blinding the detector.
- A high-sensitivity detector (such as our Spectra-VIS or Photon-UV lines, depending on the laser excitation wavelength).
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