Electron Energy Loss Investigation through the Nobel Prize Winning Franck-Hertz Experiment

Franck-Hertz experiment is a profound way of looking at the quantization of energy levels in an atom. A triode tube filled with mercury vapors is employed to accelerate electrons that in turn excite mercury atoms and energy transfer can be observed only for certain discrete values of energy of the colliding electrons. This implies that energy transfer to and from an atom can only be in fixed amounts, and hence a discrete atomic structure.

You can find an associated lecture on this experiment at the following link:
Modern Physics (2018) - Lecture 33 - Franck-Hertz experiment