Franck hertz experiment.

The excitation of an atom by any method from one state to another involves transfer of energy in quanta. This quanta either absorbed or emitted corresponds to some line in the spectrum of the atom. With the consideration Franck and Hertz  in 1914 performed the experiment using inelastic collision of accelerated electrons with atoms as a source of excitation. A direct demonstration of existence of discrete stationary states postulated by Bohr theory of atom was first provided by James Franck and Hertz.