Raman spectroscopy
/ RAH-mun /
Throw a ball at a wall and it usually bounces straight back with the same speed. But once in a great while it could come back slightly faster or slower because the wall itself was set wobbling, taking or giving a little energy. Light does this with molecules: most photons bounce off unchanged, but a tiny fraction leave with their energy nudged up or down because they set a bond vibrating. Raman spectroscopy listens to exactly those rare, energy-shifted photons.
Raman spectroscopy measures the small shifts in energy of light that has been scattered by a sample, rather than absorbed by it. A laser of a single colour is shone on the molecule; nearly all the light scatters back unchanged, but a faint sliver comes back shifted by an amount equal to one of the molecule's vibrational energies. Plotting those shifts gives a vibrational spectrum, much like infrared but obtained through scattering, and governed by a different selection rule.
Raman is valuable precisely because it complements infrared: a vibration is Raman-active if it changes the molecule's polarizability (how easily its electron cloud distorts), so symmetric vibrations that are invisible to infrared often show up brightly here. It also works through glass and water, needs no special sample preparation, and can probe tiny spots — though the effect is inherently weak, which historically made it demanding until lasers and sensitive detectors arrived.
Airport and pharmacy scanners often use a handheld Raman device pressed against a sealed bottle: a laser passes through the glass, the scattered light returns shifted by the contents' vibrations, and the instrument matches that pattern to a library to confirm whether the liquid inside is a known medicine or something dangerous — all without opening the bottle.
Reading the faint energy-shifted scattered light reveals a molecule's vibrations.
Infrared and Raman often obey a 'rule of mutual exclusion' in molecules with a centre of symmetry: a vibration active in one is silent in the other. This is why the two are run together — between them they catch nearly all the vibrations.