The adsorption of monolayer films of Uranium and Zirconium on Tungsten

Abstract

The form and magnitude of the potential change at the surface of a metal is of paramount importance in electron and atom interactions in this region. The theoretical activity in the study of these surface potentials has increased in recent years, stimulated mainly by the plethora of data now available as well as the upsurge in the technological applications of surface phenomena. However there has been a need for a few accurate measurements of the dependence of the surface potential, that is the variation in work function, on the condition of well-defined surfaces. This thesis not only fills this gap but assesses the many phenomenological theories presently available. New lines of enquiry, both in the development of these theories and in the form of future experiments capable of unambiguous interpretation, are suggested. The change in work function on adsorption of uranium and zirconium on single crystal and polycrystal tungsten surfaces has been exhaustively investigated, the measurement technique being, for the most part, the Zisman vibrating capacitor modification of the Kelvin method. The majority of the work concerned films of up to one atomic layer, evaporated on to (110) and (100) oriented tungsten crystals, The use of ultra-high vacuum techniques, essential to the achievement of the extreme cleanliness required for this work, is described in some detail. Pressures of below 1 x 10 torr were routinely obtained, and allowed measurements to be made on surfaces that were atomically clean. The way in which the work function was affected through contamination by residual gases has also been studied. In the course of this work a relationship was established which enabled the work function change due to adsorption of a wide range of materials to be predicted. For each orientation of the tungsten substrate a unique number exists which, together with the adsorbate electronegativity, makes this calculation possible. The relevant adsorption theories concerning work function changes in bimetallic adsorption systems are discussed and assessed. Despite recent activity in fundamental many-body concepts no theory is yet sufficiently developed to allow comparison with experiments. It is concluded that even the phenomenological theories reviewed in this work cannot as yet adequately predict work function/coverage data.

More information

Identifiers

Catalogue record

Export record