Thermionic effects caused by vapours of alkali metals

Abstract

Early in 1923 it was shown* that a tungsten filament heated to 1200° K or more in saturated caesium vapour converts all caesium atoms which strike it into caesium ions. Thus when the filament is surrounded by a negatively charged cylinder a positive ion current flows from the filament, which is inde pendent of the filament temperature (above 1200° K) and independent of the applied potential, if this is sufficient to overcome the space charge effect of the positive ions. At lower voltages the currents follow the 3/2 power law, and the currents are smaller than the corresponding electron currents obtainable from the same filament in the ratio of the square roots of the masses-of the electrons and caesium ions.The reason that the caesium atoms lose their valence electrons so readily upon contact with the filament, is merely that the electron affinity of tungsten (Richardson work function) is 4 '53 volts, while the electron affinity of a caesium atom (ionising potential) is only 3-88 volts. Experiments showed in fact that if the work function for the filament is lowered to 2 • 69, by allowing a monatomic layer of thorium atoms to accumulate on the surface (by diffusion from the interior of a thoriated tungsten filament), the positive ion emission becomes negligible.The positive ions must be attracted to a tungsten surface because of the electron image force. Thus it is that below about 1200° K the caesium ions evaporate so slowly from a tungsten surface that this becomes partly covered by adsorbed caesium ions. The presence of these ions, however, lowers the electron affinity of the surface, so that when about 20 per cent, of the surface is covered, the work function falls below the ionising potential of the caesium. With more caesium on the surface the caesium atoms which strike the filament no longer escape in the form of ions but remain in the atomic state. Thus the positive ion currents disappear below about 1100° K. But the lowering of the electron affinity raises the electron emission, and when the surface becomes more completely covered by caesium as the temperature is lowered, the electron *

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