The Contact Difference of Potential Between Barium and Zinc The External Work Function of Zinc

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Abstract
The methods of contact potential measurement described in previous reports of this series have been extended to zinc, for which the published work function data are discordant. Measurement was by the retarding potential method with variable anode, in tubes sealed from the pumps and gettered with vaporized barium. Barium was the reference metal and the measured surfaces of both barium and zinc were prepared by fractional distillation followed by revaporization of the middle fractions. In observations on three tubes and fifteen pairs of Ba-Zn surfaces the observed contact P.D., mean value 1.76 v, was reproducible and constant to ±0.02 v. Combination of this value with the work function of barium, now well established at 2.52 ev, assigns the value 4.28±0.02 ev to the work function of zinc at room temperature. The initial measurement on a surface was taken within 8 seconds of its deposition. As a consequence of this small time lapse it becomes possible to answer the fundamental but neglected question of whether a work function measured under optimum vacuum conditions is characteristic of the clean metal or of a gas-contaminated surface in equilibrium with the residual gas. The rate of deposition of oxygen on zinc is estimated from the behavior of a tungsten "test surface" in a comparable vacuum. Since the regimes of measurement and of equilibrium gas film formation overlap and no change of work function with aging is detectable, it is concluded that the observed work function is characteristic of clean zinc. From the point of view of the adsorption processes involved, the measurements show that the mean time of sojourn of oxygen is much less on zinc than on tungsten.