Critical potentials of mercury with a Franck-Hertz tube

A classic experiment, where a four-electrode Franck-Hertz tube is used to show various energy states (critical potentials) to which a neutral mercury atom can be raised by electron impact, is upgraded and elucidated thanks to a novel arrangement of applied inter-electrode potentials. By that means, a suitably located plasma region, the working field-free scattering chamber, can be created dynamically and maintained thereafter under appropriate conditions of cathode emission current and mercury vapour pressure. Concurrently, a more efficient mechanism for displaying excitation peaks becomes operant in the adjoining electron collection cell, by means of the window for slow-electron transmission that is available in mercury below the resonance peak in the cross section at about 0.4 eV. The resultant enhanced resolution in current-voltage curves permits the identification and measurement within 0.05 eV of the leading features in the known electron impact excitation spectrum of mercury.SCOPUS: ar.j