An experimental determination of the ionisation potential for electrons in helium

Abstract

Bohr has developed a theory of atomic structure by applying a system of mechanics based on the quantum theory of radiation to the theory of the nucleus atom proposed by Rutherford. In the case of certain simple atoms he has calculated the minimum voltages through which an electron must fall in order to be able to produce ionisation by collision, and a comparison of the calculated values with those obtained experimentally serves as a check on the validity of the theory. Next to the hydrogen atom, that of helium has the simplest constitution, and this, for the normal atom, consists of two electrons rotating in non-radiating orbits round a doubly charged positive nucleus. The application of Bohr’s formula to helium indicates that the minimum velocity necessary to remove one electron from the helium atom is about 29 volts, but the direct experimental determinations of several observers agree in fixing the value at about 20 volts. Bohr has attempted to explain this by the assumption that the ionisation potential measured does not correspond to the complete removal of the electron from the atom, but only to a transition from the normal state of the atom to some other stationary state, where the one electron rotates outside the other. When the outer electron falls back to its original orbit, radiation of sufficiently high frequency to liberate electrons by photo-electric action from the metal parts of the apparatus is produced, and Bohr has suggested that such a secondary effect has been mistaken for a genuine ionisation of helium by electron collisions. The first direct determination of the minimum ionisation potential for helium was made by Franck and Hertz, using the method originally devised by Lenard. In their experiments, electrons from a glowing filament were accelerated towards a platinum gauze, on the other side of which they encountered an opposing field, which prevented any of them from reaching a collecting electrode. If, however, the electrons on passing through the gauze possessed sufficient energy to ionise the gas present by collisions, the positive ions liberated were driven towards the collecting electrode, and the ionisation was measured by an electrometer in connection with it. The experiment consisted in gradually increasing the potential difference accelerating the electrons until positive ions were detected by the electrometer. The accelerating voltage at which this occurred was increased by the experimentally determined velocity of emission of the electrons from the glowing filament, and the value 20·5 volts was obtained for the ionisation potential.