New features of the Franck-Hertz experiment

The fundamental properties of the signal structure in Franck-Hertz experiments are analyzed. The central result is that the spacings between the minima in Franck-Hertz curves are not equidistant but increase linearly with the number of minima. This increase is especially pronounced at low atomic pressure. We suggest that the increase of the spacings is caused by the additional acceleration of electrons over their mean free path after the excitation energy is reached. Our model accurately estimates the lowest excitation energies of mercury (4.67eV) and neon (16.6eV) atoms and the mean free path of electrons in standard Franck-Hertz experiments. These results contradict the usual assumption that the spacings between successive minima or maxima are equal. We demonstrate that a standard Franck-Hertz apparatus can be upgraded to do more advanced experiments.