The Franck–Condon principle explains how molecules glow

Person: James Franck
Lived: 1882–1964
Field: Atomic physics
From: German-American

The story

When a molecule absorbs or emits light, both its electrons and its vibrating atoms can change state at once. Why some of these transitions are bright and others faint was a puzzle in the 1920s, when physicists were only beginning to apply the new quantum ideas to molecules.

James Franck, then a professor at Göttingen, was studying how light can break molecules apart. In 1925, trying to explain his observations of iodine molecules splitting under light, he proposed a simple physical picture, which he set out in a 1926 report in the Transactions of the Faraday Society. Electrons jump between energy levels so quickly that the much heavier atomic nuclei have no time to move during the jump. On his diagrams of molecular energy, a transition could therefore be drawn as a vertical line, with the nuclei frozen in place. Transitions are most likely when the molecule’s shape before and after is similar.

The American physicist Edward Condon, one of the young scientists who gathered around Franck at Göttingen, extended the idea in a 1926 Physical Review article, A Theory of Intensity Distribution in Band Systems, and later put it into precise quantum-mechanical form. In that form, the brightness of a transition depends on how much the vibration patterns of the starting and final states overlap. In the same issue of the journal, Raymond Birge used both men’s names together for the first time.

The result, known as the Franck–Condon principle, became a basic rule of spectroscopy and quantum chemistry. It explains the intensity of vibronic transitions, the simultaneous changes in electronic and vibrational energy that shape the spectra of molecules. It applies equally to absorption and to fluorescence, which is why the two spectra of many molecules look like mirror images of each other.

When the Royal Society elected Franck a Foreign Member in 1964, it cited his contribution to the interpretation of molecular spectra. The principle has since been applied to a wide range of related problems, from molecules dissolved in liquids to vibrations in solid crystals.

Why it matters

Chemists, astronomers and biologists use the Franck–Condon principle to read molecular spectra, from identifying molecules in space to studying how plants and eyes capture light.

About James Franck

Nobel physicist whose electron-collision experiment proved atoms have quantised energy levels; he resigned in protest in 1933 and later led the Franck Report against a surprise atomic attack.

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James Franck

1882–1964 · Atomic physics

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