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Chemistry · Glossary

What is Raman and Rayleigh scattering?

Also known as: Rayleigh scattering · Raman scattering · Stokes line · anti-Stokes line

Definition 6.19 University Chemistry — Year 3 · Chapter 6 — Rotational and Vibrational Spectroscopy

When light of frequency ν0\nu_0 crosses a sample, most of the scattered light keeps the frequency ν0\nu_0: Rayleigh scattering. A small fraction is shifted by the rotational or vibrational frequencies of the molecules: Raman scattering. Lines at lower frequency are Stokes lines (the molecule gained energy), at higher frequency anti-Stokes lines (it lost energy).

Left: the rotational Raman spectrum of nitrogen computed at 300\, K: Stokes lines at positive shifts (right), anti-Stokes lines at negative shifts (left, slightly weaker), with the 2:1 alternation of intensities; the grey line at zero marks the much stronger Rayleigh line. Right: energy scheme of Rayleigh and Stokes scattering through a virtual level (not a real state of the molecule).
Left: the rotational Raman spectrum of nitrogen computed at 300 K300\,\mathrm{K}: Stokes lines at positive shifts (right), anti-Stokes lines at negative shifts (left, slightly weaker), with the 2:1 alternation of intensities; the grey line at zero marks the much stronger Rayleigh line. Right: energy scheme of Rayleigh and Stokes scattering through a virtual level (not a real state of the molecule).

Examples

Example 6.22 (Nitrogen, seen by Raman)

NX2\ce{N2} has no dipole and no infrared or microwave spectrum, but its polarisability is anisotropic: its rotational Raman lines start at 6B0=11.94 cm−16B_0 = 11.94\,\mathrm{cm}^{-1} from the exciting line and are spaced by 7.96 cm−17.96\,\mathrm{cm}^{-1}. Their intensities alternate 2:1 between even and odd JJ: the two X14X2214N\ce{^{14}N} nuclei are identical, and nuclear-spin statistics (Chapter 11) give even-JJ levels twice the weight of odd ones.

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