PHOTO-FIELD EMISSION ENERGY DISTRIBUTIONS OF SINGLE CRYSTAL TUNGSTEN
Abstract
Several experimental methods exist which allow a study of the surface of metals. Two of the most fruitful, photoemission and field emission, have been used for energy band and surface state spectroscopy of both clean and adsorbate covered surfaces. Photoemission gives information on initial states below the Fermi energy and final states above the vacuum level of the metal being studied. By contrast, field emission gives detailed information only on states below the Fermi energy. By combining these two processes the range of energies between the Fermi energy and the vacuum level can be studied. This is called photo-field emission. The major goal of this thesis was two fold. Early measurements of the total photo-current show an oscillatory behavior as the applied electric field is increased for electrons emitted below threshold. The ultimate cause of this oscillatory effect is unknown. Secondly, since the experimental procedure is still new and relatively unexplored, an in-depth exploration of energy distributions of electrons emitted from various faces of the emitter was undertaken in order to estimate the sensitivity of the procedure to various surface phenomena such as surface states. In order to gain a basic understanding of photo-field emission, measurements of the energy distribution of electrons emitted below threshold for an extended range of applied electric field were performed. Tungsten was used as the emitter material because the oscillations were first observed on this material and also the clean surfaces of tungsten are among the most well known surfaces so that structure in experimental data could presumably be identified. The major conclusion of this work is that to the limit of sensitivity of the electron energy analyzer, the energy distributions fit model calculations based on a simple extension of a field emission model which does not predict oscillations in the total photo current. A very large and interesting structure on the W(100) surface was observed which grows out of the expected energy distribution and separates from it by shifting to lower energies as the electric field is increased. This structure is tentatively attributed to a surface resonance on that surface of tungsten.
Degree
Ph.D.
Subject Area
Condensation
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