For Neon gas, the process of absorbing energy from electron collisions produces visible evidence. When the accelerated electrons excite the electrons in neon to upper states, they de-excite in such a way as to produce a visible glow in the gas region in which the excitation is taking place. There are about ten excited levels in the range 18.3 to 19.5 eV. They de-excite by dropping to lower states at 16.57 and 16.79 eV. This energy difference gives light in the visible range. Since the accelerated electrons undergo inelastic collisions with the neon and are then accelerated again, they can undergo a series of such collisions if the accelerating voltage is high enough. The accelerating voltage from the Franck-Hertz apparatus used to produce the picture was capable of producing and accelerating voltage of about 80 volts, so you could get up to four collisions. This can be seen under proper conditions as four bands of light from the de-excitation in the collison regions.
The Franck-Hertz display for neon shown at left was formed by sweeping the accelerating voltage and recording current vs voltage on an x-y plot. The measured separation of the peaks corresponds to about the midpoint of the range of excitation energies of the involved neon transitions.