Emission and Absorption Spectra

2.3.3a Emission and Absorption Spectra

When a sample of atoms is heated or subjected to an electric discharge, it absorbs energy and its electrons are excited to higher energy states. As these excited atoms fall back to lower, more stable states, they release the excess energy as radiation of specific wavelengths. Recording the wavelengths (or frequencies) of this emitted radiation produces an emission spectrum — a fingerprint of exactly which energy transitions occurred inside the atom.

An absorption spectrum is essentially the mirror image of an emission spectrum. A continuous beam of radiation (containing all wavelengths) is passed through a sample of the substance. The sample absorbs radiation only at the specific wavelengths that match transitions its electrons can undergo, leaving dark gaps at exactly those wavelengths in an otherwise bright, continuous spectrum — much like a photographic negative of the corresponding emission spectrum.

The systematic study of emission and absorption spectra is called spectroscopy. Because each element's electrons occupy a unique set of allowed energy levels, every element produces its own distinctive pattern of spectral lines — as individual as a fingerprint. This makes spectroscopy an extremely powerful tool for chemical analysis: matching the lines from an unknown sample against the known spectrum of an element quickly confirms the element's identity, without needing to isolate or chemically test the substance at all. …