The Work Function of Copper

The work functions and aging characteristics of fourteen copper surfaces have been determined by measurement of their contact differences of potential with respect to barium reference surfaces of known work function. Measurement was by the retarding potential method in tubes sealed from the pumps and gettered with vaporized barium. The copper surfaces were prepared by subjecting Hilger's "spectroscopic standard" copper to forty vacuum fusions followed by fractional distillation, redistillation of the fractions, and condensation of the vapor on glass. The barium films were prepared by a similar, standardized technique which yields surfaces reproducible and constant to 0. 01 ev or better. The time interval between the deposition and initial measurement of the metal films was of the order of ten seconds, obtained conveniently with a tube in which the target is returned to the measuring position automatically after deposition of a metal film upon it. The copper surfaces, formed from successive fractions of the distillate, showed marked variability of the initial work function and marked drift toward lower work function in films 1-4, progressive improvement in films 5-8 and, finally, good reproducibility and negligible drift in films 9-14 inclusive. The range of variation in films 1-8 coincides almost exactly with the range of divergent values reported in the literature. Copper evidently retains dissolved gases, in free or combined form, with extraordinary tenacity and these contaminants, evaporated with the copper and reabsorbed by the condensing film, appear to be responsible for the variations observed in films 1-8. The work function value given by films 9-14 is 4. 460. 03 ev, and this value is to be taken as representative of the present work.