Shallow angle scattering of slow, multiply-charged ions from surfaces

Date of Completion

January 1997


Physics, Atomic




Multiply-charged N, O, S, F, and Kr ions were extracted from Lawrence Livermore National Laboratory's EBIT II at energies ranging from 4.0 keV*q to 11.9 keV*q. After collimation and charge state selection, the ions struck targets of Au(111), Mica, and C(0001) at incident angles ranging from 1.0 to 2.5 degrees. The angular and charge-state distribution measurements of the scattered projectiles are presented. When plotted versus incident angle, the angular distributions show projectile energy gains which are attributed to the image charge interaction. Image charge energy gains were found to be on the order of tens of eV. The production of negative ions was investigated for the cases of $\rm O\sp{q+},\ S\sp{q+},$ and F$\rm\sp{q+}$ scattered from highly-oriented pyrolytic graphite (HOPG). The velocity dependence of S$\sp-$ yields are fit to a Saha-Langmuir-type equation. Negative ion yield is found to be strongly dependent upon the projectile velocity component parallel to the surface.^ Recent developments in the field of multiply-charged ion (MCI) surface collisions are discussed and the operational theory behind the functioning of Lawrence Livermore National Laboratory's Electron Beam Ion Trap (EBIT II) is outlined. Preparation of the HOPG targets used in the experimental work is described in detail. The theory leading to position measurements with Resistive Anode and Wedge-Strip-Zig (WSZ) type position sensitive microchannel plate detectors, as well as the electronics used in these measurements are described. The grazing-incidence geometry of the experiment is explained. Theoretical descriptions of the classical image charge interaction, neutralization and deexcitation mechanisms, charge transfer near conducting surfaces, and the formation of negative ions, as applied to shallow-angle scattering of MCIs are also given. ^