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342 IX. Raman Spectroscopy

Here’s the IR spectrum of air. What’s missing?
Raman spectroscopy is primarily
used to measure molecular
vibrations. In that, it is very
similar to IR absorption
spectroscopy. But it can detect
vibrations that don’t show up in
IR spectroscopy.
From A.J. Sommer, “Infrared spectroscopy”, Spectroscopy 10/28/20
Raman and IR spectra of 1-hexyne and 3-hexyne
Normal modes that are
more symmetrical
produce Raman-active
bands that are IRinactive. The bands in
the red boxes are
mainly C≡C stretches.
Raman spectrum of air and hydrogen
From J. Odhner, D. Romanov, & R. Levis Proceedings of SPIE - The International
Society for Optical Engineering (2010) 7582. 10.1117/12.842129.
Raman signals are intrinsically much weaker than IR absorption.
Only 10-9 of the incident photons are scattered by this mechanism.
(Cf. Horiba, “Experiment 1…” (2016))
For this reason, it was a specialty until the invention of lasers.
Lasers are perfect light sources for Raman spectroscopy: intense,
polarized, narrow bandwidth. Nearly all Raman spectroscopy is
now “laser Raman spectroscopy”.
Scattering is the outcome of a collision.
Three types of atomic & molecular scattering: elastic, inelastic,
Three types of interaction btwn matter and electromagnetic waves:
absorption, spontaneous emission, stimulated emission.
The first two are incoherent, the last is coherent. It’s why lasers
(Light Amplification by the Stimulated Emission of Light) produce
coherent light.
Raman scattering vs. Rayleigh scattering
Rayleigh scattering
intensity is proportional
to ν4. It is the reason for
blue skies and red
Fluorescence is isotropic,
incoherent, and delayed.
Elastic scattering:
Light only changes
Inelastic scattering:
Light changes direction
and energy.
Raman spectroscopy relies on Raman scattering, a coherent
inelastic two-photon interaction between molecules and light. This
interaction can only be explained with quantum physics. Raman
& Krishnan made the first experimental observations.
From A.V. Vlasov, et al., Crystals 10(1):38 (January 2020) doi.org/10.3390/cryst10010038
Design of a Raman
Methane doesn’t follow the
mutual exclusion rule. This is
experimental evidence that it
isn’t centrosymmetric.
Benzene follows the
mutual exclusion rule.
This is experimental
evidence that it is
Stimulated Raman scattering. The 421 xpt. Momentum matching
means that SRS results in collimated Raman beams.
Resonance Raman scattering?