Aachen PK 2003 – scientific programme

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P: Plasmaphysik

P 17: Poster III

P 17.3: Poster

Thursday, March 20, 2003, 17:30–19:15, Foyer

Kinetics of trapped and non-trapped electrons in the Franck-Hertz experiment — •F. Sigeneger and R. Winkler — Institut für Niedertemperatur-Plasmaphysik, 17489 Greifswald, Germany

The experiment of Franck and Hertz is one of the key experiments of modern atomic theory as it clearly evidences the existence of discrete energy states of atoms. Although this experiment has often been repeated and described in many textbooks most of these explanations are based on simplifying assumptions concerning the energy distribution and the elastic collisions of the electrons. Therefore, the behaviour of the electron gas has been investigated on a rigorous kinetic basis considering mercury vapour in a cylindric Franck-Hertz tube. In the kinetic study the radially inhomogeneous Boltzmann equation is solved considering an accelerating field between the central cathode and the grid and a retarding one between grid and anode. The isotropic distribution of the electrons at the cathode is prescribed by an energy distribution which depends on details of the thermionic emission. Furthermore, the extraction of electrons which are trapped around the grid is modelled by a spatially narrow lifetime term. The electron Boltzmann equation is solved for a series of cathode-grid voltages. By analyzing the dependence of the resultant anode current on the accelerating cathode-grid voltage the famous Franck-Hertz characteristics is obtained. The electron power balance reveals the particular impact of the energy loss in elastic collisions and of the changing importance of the two lowest exciting states of the mercury atom on the maxima positions of the characteristics. The significant changes of the latter with the pressure can be explained by these effects.