| Research: The Role of Adsorbates
in Secondary Emission from Low-Energy Ion Bombardment of Surfaces
Updated: 11.05.01
Research Advisor: Dr. Roy Champion Bombardment of surfaces with photons, ions and electrons is commonly used to determine surface composition and structure or to modify the surface itself. Of practical interest is the dependence of surface characteristics, such as work function or sputtering yields of ions and secondary electrons, on adsorbate coverage. Knowledge of this dependence has application in plasma and low-pressure systems, as well as to understanding of interaction of charged particles with surfaces and adsorbates. We have studied ion and electron emission from low-energy bombardment of various substrates, including aluminum, molybdenum, stainless steel, tungsten, and aluminum (Al). I'm currently working on noble gas ion bombardment of polycrystalline Al; below are recently acquired ion and electron kinetic energy distributions for 470 eV He+ bombardment of an oxygen-adsorbed (~ 500 Langmuir exposure) Al surface:
Positively-charged helium ions are incident at an energy of 470 eV on an oxygen-adsorbed strip of polycrystalline Al and effect emission of secondary particles from the adsorbed surface. I collect the negative sputtering product-- mostly electrons, but some oxygen anions. The product yield (emitted particles per incident sodium ion) varies as I change the energy of the impinging ions and the amount of adsorbate on the surface, as seen in the figure below.
These experiments are carried out under ultra-high vacuum (UHV) on the order of 10^-10 Torr. Using kinetic energy and TOF analyses, the sputtering products may be determined. A model has been proposed [J. Tucek, S. Walton, and R. Champion, "Secondary-electron and negative-ion emission from Al: Effect of oxygen coverage," Phys. Rev. B 53, 14127 (1996)] in which the products are generated by a combination of collision cascade and excitation effects. Publications: W. S. Vogan, R. L. Champion, ""Oxygen adsorption on a Si(100) substrate: effects on secondary emission properties," Surf. Sci. 492 (2001) 83. W. S. Vogan, S. G. Walton, R. L. Champion, "The role of O and Cl adsorbates on the secondary emission properties of tungsten," Surf. Sci. 459 (2000) 14. |
