In their 1914 experiment, James Franck and Gustav Hertz demonstrated that in collisions between accelerated electrons and gaseous atoms, energy was only transferred from the electrons to the atoms if the electron had gained an energy before the collision equal to or greater than the energy required to excite the atom from its ground state to its next lowest energy level. If it had more energy than needed, the electron only transferred the amount corresponding to the energy difference between the two atomic energy levels. That is, atoms have discrete, quantized energy states, providing early evidence for the validity of quantum theory. Franck and Hertz won the 1925 Nobel Prize in recognition of this result.
Review the introduction to the Franck-Hertz effect provided by the excellent HyperPhysics website. There are 5 key panels on this website that you should read:
In addition, familiarize yourself with the more detailed description in (Melissinos 2003) on pages 10 - 19.