Work-Function Measurements on Monolayer Films of Uranium on (100), (110), and (113) Oriented Faces of Tungsten Single Crystals by Photoelectric and Contact Potential Difference Techniques

Abstract

Uranium was evaporated on to (100), (110), and (113) oriented faces of tungsten single crystals at pressures of less than 5×10−10 Torr. The work function of each of the tungsten surfaces was monitored by photoelectric and contact potential difference (Kelvin and electron-beam) techniques as uranium was adsorbed on to it. The curves of work function versus coverage were fitted to the Gyftopoulos and Levine theory of monolayer adsorption. The constants of the curves gave a value for the density of uranium atoms in a monolayer on each tungsten surface: For the (100) surface one uranium atom is adsorbed for each unit cell in the two-dimensional lattice of surface tungsten atoms, on the (110) surface there is one uranium atom for every two unit cells, and on the (113) surface there are four uranium atoms to each unit cell. For film thicknesses of about two monolayers, the photoelectric technique gave work function values in excellent agreement with those from the Kelvin method, the values being 3.73±0.02 eV for the U/W100 system, 3.90±0.03 eV for U/W110, and 3.67±0.03 eV for U/W113. The electron-beam values were higher than those obtained by the other two methods, probably due to an increase in the electron reflection coefficient as uranium is adsorbed on to tungsten.