Raman spectroscopy
/ RAH-mun spek-TROS-kuh-pee /
Raman spectroscopy watches how light bounces off a molecule and comes back slightly out of tune. Shine a single pure color of laser light at a sample: almost all of it bounces back unchanged, but a tiny fraction trades a little energy with the molecule's vibrations and returns shifted to a slightly different color. Listening to that handful of off-color photons tells you how the molecule is shaking.
More precisely, the instrument measures the shift between the laser's frequency and the scattered light's frequency. Those shifts match the energies of the molecule's vibrations, so the resulting spectrum is a fingerprint of molecular structure — much like an infrared spectrum, but read out from scattered rather than absorbed light.
It matters because Raman can examine samples through glass or water, needs little preparation, and probes vibrations that infrared often misses, including those of symmetric bonds. Its honest weakness is that the Raman effect is faint, so strong fluorescence from the sample can swamp the weak signal, and sensitive samples can be scorched by the laser.
A pharmacist points a handheld Raman scanner at a sealed glass vial. Without opening it, the device reads the scattered light's shifts and matches them to the spectrum of the expected drug, confirming the contents.
Raman reads structure from scattered light, even through a sealed container.
Because water scatters Raman light only weakly, the method works well for aqueous and biological samples — the very opposite of infrared, where water's strong absorption is a constant nuisance.