Raman spectroscopy

Raman spectroscopy shines a single-colour laser on a material and listens for the faint colour shift in the light that scatters back — and from that shift it reads the chemical bonds inside. Bonds behave like tiny springs joining atoms, each with its own natural vibration frequency; Raman is a way of plucking those springs with light and hearing their notes, so it can tell polyethylene from polystyrene, or graphite from diamond, without touching or destroying the sample.

Almost all the laser light bounces off unchanged, but about one photon in a million gives a little energy to a molecular vibration (or takes some) and comes back shifted in wavelength. That shift, measured in wavenumbers (cm^-1), equals the vibration's energy and so fingerprints the bond: a carbon-carbon double bond, an O-H group, a benzene ring each sit at their own Raman shift. The plotted spectrum is a row of peaks; the pattern names the molecule or phase.

Raman is a favourite for polymers and carbon materials — it distinguishes plastics, detects crystallinity, and famously separates graphite, diamond, graphene and amorphous carbon by their signature peaks. It needs little or no sample prep and can work through glass or water. Honest limits: the Raman signal is inherently weak, so strong fluorescence from a sample can swamp it, and it probes bonds (molecular structure), not which elements are present — for that you use EDS. It is complementary to FTIR, which reads a different subset of vibrations.