Electron impact excitation and uv emission in the Franck–Hertz experiment

With the Franck–Hertz experiment described here the inelastic collision of electrons with mercury atoms and the ultraviolet emission of the excited atoms can be observed simultaneously. The Franck–Hertz tube is a four electrode tube with a concentric system and a quartz window. An electron current stabilization allows the observation of a stepwise increase in light intensity by means of a photomultiplier. The emission spectrum shows the Hg 2536‐Å line and two molecular emission bands arising from Hg dimer and trimer transitions. The light emission from the trimer state reveals the influence of geometrical factors of the tube at different accelerating potentials.