A Differential Method Applied to the Surface Ionization of Sodium Halides on Tungsten

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Abstract
A differential molecular‐beam method for the study of the surface ionization of salts was developed. A reference filament and a working filament were placed in the paths of molecular beams emitted from the same furnace orifice; and the ratios of the positive ion currents collected from the two surfaces were measured with a center‐tap galvanometer for different temperatures of the working filament. This method was applied to the study of the ionization of sodium halides on tungsten and tungsten‐oxygen surfaces. The results were not in close agreement with the simple theory of the ionization of a gaseous sodium atom at a tungstensurface. The energy quantity, (I—Φ)ε, (I being the ionization potential of the alkali atom, and Φ, the work function of the surface) was about 0.3 v.e. less than that demanded by theory. To explain this defect an extension was made of the simple theory, in which it was assumed that a small fraction of the surface was covered with halogen atoms even to high temperatures. The experimental results could then be explained upon the assumption that from two to four percent of the tungstensurface (the exact value depending upon the particular halide and tungstensurface involved) held adsorbed halogen atoms up to 2600°K.

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