Raman spectroscopy is a non-destructive light scattering technique spanning a wide range of scientific and industrial applications. Most often, Raman spectroscopy is used to characterize or identify the chemical composition and structure of an unknown material. Incident laser light in the UV, visible or NIR, is scattered inelastically from molecular vibrational modes. In a typical experiment [see Figure 2], a frequency-doubled Nd:YAG laser emitting monochromatic green radiation at λex = 532.2 nm (18,790 cm-1 see Eq 1) will scatter from a molecule with vibrational modes of up to ~4,000 cm-1.
The frequency difference (measured in relative cm-1) between the incident and scattered photons is termed the Raman shift. The majority of inelastically scattered photons are found at positive Raman shifts, corresponding to lower energies and longer wavelengths. This is referred to as Stokes scattering. The scattered photons are analyzed by a spectrometer to produce a Raman spectrum. The Stokes spectrum from 0 to 4,000 relative cm-1, or λRS = 532.2 to 676 nm (see Eq 2) in this case, gives a complete picture of the Raman-active modes of the molecule under investigation. Unknown molecules can be identified by their Raman spectrum, which is often termed a molecular fingerprint.
Equation 1: (cm-1) ex = 107/λex(nm)
Equation 2: rel cm-1 = 107/λex(nm) - 107/λRS(nm)
Stokes and Anti-Stokes scattering
See Figure 1, b) and c). In Stokes, the energy of the molecule increases and the Raman scattered photons are red-shifted. In Anti-Stokes, energy of the molecule decreases and the Raman scattered photons are blue-shifted. Molecules giving rise to anti-Stokes scattering must have been originally in a vibrationally excited state, which represents a small fraction of thermally equilibrated molecules. See equation 3, Boltzmann population, , with kBT = 207 cm-1 at room temperature.
An Anti-Stokes Raman spectrum can be used to measure the vibrational temperature of a sample.
Figure 2: Principle of Raman Spectroscopy
Figure 3: Confocal Stokes and Anti-Stokes spectra of L-cystine, acquired with Acton TriVista 555 Stokes
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