Investigating the dynamics of ion-surface interactions using electron-spin-polarized positively charged helium ions
Lancaster, James Campbell
Dunning, F. B.
Doctor of Philosophy
Spin-labeling techniques, specifically using electron-spin-polarized 4He+ ions coupled with energy-resolved measurements of the polarization of ejected electrons, are providing significant insight into surface electronic states and the dynamics of the neutralization of charged particles at clean and adsorbate-covered metal surfaces. The electronic structure of surfaces and the process by which charged particles electrically interact with such surfaces are of fundamental interest, yet only partially understood. A powerful technique for studying surface states and interactions has been Ion Neutralization Spectroscopy, in which noble gas ions are directed into a surface, where they are neutralized by electrons from the surface. Energy conservation causes other electrons to be emitted, which are collected and their energy distributions analyzed. Because neutralization takes place outside the surface, this technique serves as a sensitive probe of that part of the material. The current studies expand this technique by analyzing spin-aspects of the interaction. Spin-polarized He+ ions are produced in a radio-frequency driven discharge and directed at selected surfaces. Emitted electrons are analyzed with a retarding grid energy analyzer to determine their energy distributions and a mott polarimeter to measure their spin polarization. Correlating the spins of the outgoing electrons and incoming ions provides previously unavailable information about the dynamics of this reaction. Analysis of the energy distributions and polarization of electrons emitted from Au(100), Cu(100), and Al(100) indicate that neutralization occurs at distances closer to the surface than previously believed, and that for the period during which the ion is close to the surface, its presence causes a spin-dependent perturbation in the local density of electronic states---in essence, it locally magnetizes the surface. Further, the data indicate that surface plasmon excitation, a prominent feature in several theoretical models, does not appear to play a significant role in ion neutralization. Polarization data collected on alkali-covered surfaces clarify the dynamics by which the neutralization process takes place, while CO2 surface studies reveal that the ions undergo a previously unexamined neutralization mechanism which should apply to a broad range of van der Waals solids.
Condensed matter physics; Atomic physics