Critical potentials of mercury with a Franck Hertz tube

Abstract

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.

Double-layer formation in the extended Franck-Hertz experiment

“…Therefore the high-potential spectrum should not appear at all, in accordance with the observations in Ref. ͓10͔,Sec. 8.1.…”

“…This process then starts again and the system enters a phase of self-sustained relaxation oscillations ͑see Ref. ͓10͔,Sec. 8.2͒ described by a "hysteresis" loop in the current-voltage plane.…”

New features of the Franck-Hertz experiment

“…Note that even the energy resolved measurements with more so-phisticated versions of the Franck-Hertz experiment do not resolve the lowest excited state 6 3 P 0 in mercury. 6,12 The determination of the lowest excitation energy of mercury is often performed by measuring the spacings between the maxima in Franck-Hertz curves. To verify this approach we determined the values of E a from the spacings between the maxima.…”

Photoelectric Franck-Hertz experiment and its kinetic analysis by Monte Carlo simulation

“…Franck-Hertz tubes with four electrodes (referred to as "extended FH cells") have also been developed; these experimental arrangements provide the possibility to resolve various excitation levels (critical potentials) of atoms [8][9][10]. In Ref.…”

“…In Ref. [10] the experimental conditions were optimized for the measurements of the critical potentials of mercury, while in a subsequent paper [11], an analytical theory was developed that explained the prominent features, including the double-layer formation, in the extended Franck-Hertz experiment.…”