4 research outputs found
The "0.4 eV" Shape Resonance of Electron Scattering from Mercury in a Franck-Hertz Tube
The alternative version of the Franck-Hertz experiment with mercury, in which
a two-grid tube is used as a combination of electron gun, equipotential
collision space, and detection cell, was analyzed recently in considerable
detail. In particular, it was inferred that, at optimal pressure, the formation
of peaks in the anode current at inelastic thresholds is mediated inside the
detection cell by the large variation, a maximum at 0.4 eV, in the cross
section for elastic scattering. This variation is due to a shape resonance in
the electron-mercury system and is observable persuasively at the onset of
anode current as a sharp peak followed by a clear minimum. In the present
paper, the passage of electrons through the second grid to anode region is
analyzed in terms of kinetic theory. The discussion is based on a simplified
expression for the electron current derivable from an approximate form of the
Boltzmann transport equation that maintains the spatial density gradient but
omits elastic energy losses. The estimated range of pressure underlying this
kind of idealization is in good agreement with experiment. An explicit solution
is obtained by constructing an analytic expression for the momentum transfer
cross section of mercury using a recent theory of generalized Fano profiles for
overlapping resonances. This solution is used in order to model successfully
the formation of peaks at the threshold of anode current and at excitation
potentials, and to explain the dependence of the observed profiles on the
pressure and on the sign and magnitude of the potential across the detection
cell
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