The positive ion work function of tungsten for the alkali metals

Abstract

The evaporation of positive ions of potassium from a hot tungsten surface has been investigated qualitatively by Moon and Oliphant. In their experiments a beam of positive ions, the energy of which could be varied from 0 to 6000 volts, was allowed to fall on a tungsten strip heated to a desired temperature. A collector maintained at a negative potential with regard to the strip collected the positive ions which re-evaporated from it. When an equilibrium state had been reached such that the beam of ions evaporating was equal to the incident beam, the incident beam was suddenly cut off and the decay of potassium ions from the target followed by means of an oscillograph. The results of these experiments, which were of a purely qualitative nature, showed that when the energy of the incident ions was low or the temperature of the target was high the current from the target decayed exponentially with time, but that when the target temperature was low or the energy of the ions high, the decay began to depart appreciably from the exponential form, becoming much more complex in nature. The interpretation of the experiments indicated that an explanation of the evaporation of the fast ions must be sought in processes taking place within the body of the metal, whereas with slow ions the behaviour could probably be explained in terms of phenomena at the surface. The experiments described in the present paper were carried out to investigate from a quantitative point of view the simpler case of the evaporation of slow ions.