The atomic work function of tungsten for potassium

Abstract

In a recent paper some experiments on the rate of evaporation of positive ions of the alkali metals from a hot tungsten surface were described, and, from the temperature coefficient of the evaporation rate the positive ion work function, the work required to remove a positive ion from the surface, was derived. In the experiments described here the method has been extended to investigate the rate of evaporation of potassium atoms from a tungsten surface, and the atomic work function has been measured. Theory . Consider a hot tungsten surface on which an atomic beam of potassium is incident, and let a negative field be maintained at the surface to prevent positive ions from evaporating. Under these conditions potassium can leave the surface only in the atomic state and the surface concentration will thus go on increasing until the rate of evaporation is equal to the supply in the incident beam, when an equilibrium state will be set up. The equilibrium surface concentration N a ∞ will be given by N a ∞ = Q/ a , where a is the atomic evaporation rate for unit surface concentration, and Q is the intensity of the incident beam. The corresponding ionic surface concentration is N p ∞ = f N a ∞ / g = f Q/ ag , where f and g are quantities such that f N a ∞ is the number of adsorbed atoms which are converted into ions and g N p ∞ is the number of adsorbed ions which are converted into atoms on unit area of the surface per second. We shall, therefore, have f / g = ½ exp(—I s e / k T), where I s is the surface ionization potential, and the numerical factor ½ arises from the different weights to be attributed to the potassium atom and ion.