What is spectroscopy?

The physics of the measurement

Atoms and molecules can occupy only discrete energy levels. When electromagnetic radiation strikes a sample, a photon whose energy exactly matches the gap between two levels can be absorbed, raising the system to a higher state; the system can later emit a photon as it returns to a lower state. Because energy and wavelength are linked (E = hν), each transition corresponds to a specific wavelength. Plotting absorbed or emitted intensity against wavelength produces a spectrum whose pattern of lines and bands is characteristic of the substance, allowing both identification and quantitative analysis.

Types of spectroscopy

Spectroscopic methods are usually grouped by the region of the electromagnetic spectrum they exploit. Ultraviolet–visible (UV–Vis) spectroscopy probes electronic transitions; infrared spectroscopy probes molecular vibrations; and microwave and radio regions probe rotations and nuclear spin states, the last underpinning NMR spectroscopy.

They are also classified by mechanism — absorption, emission, fluorescence, or scattering (as in Raman spectroscopy). Atomic emission and atomic absorption methods characterise elements, while molecular methods characterise bonds and functional groups.

Uses in research

Spectroscopy is one of the most widely used analytical approaches in science. Chemists use it to determine molecular structure and monitor reactions; astronomers use stellar spectra to measure composition and motion; and environmental and materials scientists use it to quantify trace species. Coupled with separation methods, spectroscopic detectors quantify components after chromatography. Because results depend on standardised wavelengths and reference data, reproducible spectroscopy relies on well-characterised calibration and shared spectral databases.