Summary

James Franck's 1914 paper, co-authored with Gustav Hertz, does not present a book but a landmark experimental report. Its central thesis is that atoms absorb energy only in discrete, quantized amounts, directly confirming Niels Bohr's 1913 atomic model. The experiment passed electrons through mercury vapor and measured the energy loss after collisions; a sharp drop in current at 4.9 electronvolts proved that mercury atoms could only absorb that specific energy packet, exciting an electron to a higher orbital. The paper details the apparatus, the observed voltage-current curve, and the conclusion that this energy corresponds to the first excitation potential of mercury. A reader takes away the first unambiguous experimental proof of quantized energy levels in atoms, a cornerstone of quantum mechanics.

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Key concepts

Excitation potential — The minimum energy (in electronvolts) required to raise an atom's electron from its ground state to the first excited state, measured as 4.9 eV for mercury.
Inelastic collision — An electron-atom collision where the electron loses kinetic energy to excite the atom, causing a drop in current in the experiment.
Quantized energy levels — The principle that electrons in atoms can only occupy specific, discrete energy states, not a continuous range.
Franck-Hertz tube — A vacuum tube containing a heated cathode, a grid, and a collector plate, filled with low-pressure mercury vapor to measure electron energy loss.
Bohr model confirmation — The experiment provided direct empirical support for Bohr's postulate that atomic energy levels are quantized, a key step in early quantum theory.