From macro-measure of Franck-Hertz curves to micro-collisions between electrons and argon atoms

A Franck-Hertz experiment is adopted in our undergraduate laboratory by using a cylindrical tetrode tube filled with argon vapor. The Franck-Hertz curves, i.e. the variation of the anode current via the accelerating voltage, are experimentally measured, and the key roles of the filament voltage, the control voltage, and the retarding voltage played in the experiments are studied. Diagrams of the micro-elastic/inelastic collisions between the electrons and argon atoms, which are of significance for students to build a connection between the macroscopic measurements and microscopic collisions occurring inside the Franck-Hertz tube, are schematically presented based on the experimental results. The peak/valley interval in the experimental Franck-Hertz curves of argon vapor increases as the accelerating voltage increases. The results further suggest that the lowest excitation energy of an argon atom cannot be exactly derived from the experimental results since all the four sub energy levels within the first excitation energy state of the argon atom are involved in the inelastic electron-atom collisions.