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 1-hexyne 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, reactive 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 sunsets. Fluorescence is isotropic, incoherent, and delayed. Elastic scattering: Light only changes direction 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. Energy→ Energy→ Energy→ From A.V. Vlasov, et al., Crystals 10(1):38 (January 2020) doi.org/10.3390/cryst10010038 Design of a Raman microspectrometer 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 centrosymmetric. Stimulated Raman scattering. The 421 xpt. Momentum matching means that SRS results in collimated Raman beams. Resonance Raman scattering?