Advanced Topic 920 of 1102 Analytical, Lab & Spectroscopy
Raman Spectroscopy
Uses inelastic scattering of light to probe vibrational modes.
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In-depth explanation of Raman Spectroscopy
Raman spectroscopy measures the inelastic scattering of monochromatic light, usually from a laser, by molecules. Most scattered light has the same energy as the incident light, but a small fraction is shifted in energy by an amount equal to a vibrational energy of the molecule. A vibration is Raman active when it changes the polarizability of the molecule, so the technique complements infrared spectroscopy, because symmetric stretches that are weak in the IR are often strong in Raman. Water is a weak Raman scatterer, so the technique suits aqueous samples and measurements through glass. It requires little or no sample preparation and can analyse solids, liquids and gases.
Going deeper: Raman Spectroscopy
Key ideas: Raman shift is reported in cm-1 relative to the laser line, and Stokes lines (lower energy) are stronger than anti-Stokes at room temperature. Symmetric, nonpolar bonds such as C=C, S-S, C-C and C-S give strong signals, in contrast to polar bonds in IR. The mutual exclusion rule says that in a molecule with a centre of inversion, no vibration is both IR and Raman active.
Common mistakes and connections: Fluorescence from impurities can swamp the weak Raman signal, so a longer wavelength laser (785 or 1064 nm) is often chosen. High laser power can burn coloured samples. Students think Raman and IR show the same peaks, but selection rules differ. Surface-enhanced Raman (SERS) boosts the signal enormously with metal nanoparticles. Related to IR, symmetry and group theory.
Visualization of Raman Spectroscopy
Carbon tetrachloride shows Raman lines at 218, 314 and 459 cm-1; the strong 459 line is the symmetric C-Cl stretch, weak in the infrared. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Raman spectroscopy identifies mineral and artists' pigments without destroying the sample.
Frequently asked questions about Raman Spectroscopy
What is Raman Spectroscopy?
Uses inelastic scattering of light to probe vibrational modes.
Is Raman Spectroscopy a beginner, intermediate or advanced topic?
Raman Spectroscopy is a advanced topic in Analytical, Lab & Spectroscopy. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Raman Spectroscopy?
Study these earlier topics in Analytical, Lab & Spectroscopy first: Infrared Spectroscopy, Characteristic IR Absorptions, Fingerprint Region.
What comes after Raman Spectroscopy?
The next topics in Analytical, Lab & Spectroscopy are NMR Spectroscopy and Chemical Shift.
Can I practise Raman Spectroscopy with quizzes?
Yes. The interactive lesson includes a self-check question on Raman Spectroscopy, and the Analytical, Lab & Spectroscopy quiz draws on every topic in this area.
Where Raman Spectroscopy fits in Analytical, Lab & Spectroscopy
Raman Spectroscopy is topic 44 of 99 in Analytical, Lab & Spectroscopy.