Photoelectric Work Functions of (100) and (111) Faces of Silver Single Crystals and Their Contact Potential Difference

Abstract

The work functions of the (100) and (111) faces of silver single crystals have been determined photoelectrically by the Fowler method of analysis, and were compared with the contact potential difference measured by the Kelvin null method. The equilibrium value of the work function for the (100) face is 4.81\ifmmode\pm\else\textpm\fi{}0.01 ev, and that for the (111) face is 4.75\ifmmode\pm\else\textpm\fi{}0.01 ev. These values were obtained after heating for more than 2000 hours at various temperatures up to visible red heat. The gas pressure in the tube during the final stages of outgassing was between 1 and 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ mm Hg with the crystals hot. Values of the measured contact potential difference obtained at frequent intervals during the outgassing agreed with the differences of the photoelectric work functions to within \ifmmode\pm\else\textpm\fi{}0.01 volt. After these equilibrium values were obtained the crystals became contaminated by an unidentified impurity from heated tantalum with the result that the photoelectric data no longer agreed with the Fowler theoretical curve or with the measured contact potential difference. The apparatus permitted deposition of silver from the vapor state onto the crystal faces, after two previous distillations of the silver had been made to eliminate gas effects. Presence of the contamination referred to above complicated the results following deposition, but evidence is presented to show that the deposited silver gave a more gas-free surface than heating alone. The rate of return of gas to the surface following the deposit was determined by a comparison of the experimental results with a theory presented by Emslie. The results are explained by postulating that long heating of the crystals produced a stable gas configuration of a monomolecular layer on their faces, and that deposition of a gas-free layer of silver produced a surface free of most, if not all, of this layer of gas. The work function of the gas-free surface is 0.09\ifmmode\pm\else\textpm\fi{}0.03 volt less than that of the underlying structure previous to deposit.