The Emission of Positive Ions from Tungsten and Molybdenum

Abstract

An examination of the positive ion emission from tungsten and molybdenum has been made in which it was sought to determine the following points: (1) The nature of the ions emitted at various temperatures; (2) the temperature variation of the positive ion current; (3) the theory of positive ion emission with regard to where and how the ions are formed; (4) the positive ion work function for these metals; (5) whether the work function, determined by experiment, checks with that calculated by a simple cyclic process involving the thermionic work function, the ionizing potential, and the latent heat of evaporation of the metal.The mass spectrum for tungsten and molybdenum filaments taken at moderate temperatures (1700\ifmmode^\circ\else\textdegree\fi{} to 2000\ifmmode^\circ\else\textdegree\fi{}K) has shown that the emitted ions consist of sodium, the two isotopes of potassium, and aluminum. At high temperatures these impurities disappear and finally both tungsten and molybdenum filaments yield positive ions of their own metal. The latter confirm a report by Wahlin. The temperature variation of the positive ion current at high temperature yields a value of 6.55 volts for the positive ion work function of tungsten and 6.09 volts for that of molybdenum. These values disagree widely from the values 10.88 volts and 9.26 volts calculated from the simple cyclic process mentioned above. This suggests that the ions are formed as a by-product of an irreversible recrystallization of the metal. Theoretical considerations show that the ions are emitted from the metal and are not formed after a neutral atom evaporates.