How did the Franck-Hertz experiment support the idea of discrete atomic energy states?

The Franck-Hertz experiment, conducted in 1914 by James Franck and Gustav Hertz, provided the first direct experimental verification of the existence of discrete atomic energy states, a core postulate of the Bohr model proposed by Niels Bohr in 1913.12

In their experiment, Franck and Hertz directed a stream of low-energy electrons through a gas within an electron tube. They observed that as they gradually increased the energy of the bombarding electrons, the gas atoms would not absorb energy until a specific critical threshold was reached.13

Below this threshold, the electrons passed through the gas with minimal disturbance. However, once the electrons reached the precise energy required to promote an atomic electron to a higher energy level, the bombarding electrons lost their kinetic energy in collisions, resulting in a sudden drop in the electron current. This demonstrated that atoms can only absorb energy in discrete amounts, confirming that atomic electrons exist in specific, quantized states rather than continuous energy levels.132