Which experiment showed in 1914 that atoms absorb energy in discrete amounts?

The Franck–Hertz experiment showed in 1914 that atoms absorb energy in discrete amounts.

James Franck and Gustav Hertz accelerated electrons through mercury vapor and measured the electrons’ energy after collisions. At particular accelerating voltages, the current dropped sharply because electrons had transferred a fixed amount of energy to mercury atoms. The repeated drops demonstrated that atoms possess discrete excitation energies rather than a continuous range.

The experiment provided important evidence for the emerging quantum theory and supported Niels Bohr’s model of quantized atomic energy levels. Franck and Hertz initially interpreted their observations in terms of excitation of mercury atoms; later quantum mechanics supplied the more complete theoretical framework.

The experiment is sometimes described as directly measuring atomic energy levels, which is broadly correct, but it did not observe electrons orbiting like planets. Modern quantum mechanics describes atomic electrons with orbitals and stationary states. Franck and Hertz received the 1925 Nobel Prize in Physics for their discovery of the laws governing electron impact on atoms.