Energy and Charge Transfer in Hyperthermal Ion - Scattering
Abstract
I have investigated the energy and charge transfer between hyperthermal (10 eV to 500 eV) alkali ions and copper surfaces, using a combination of theoretical and experimental techniques. Experiments were performed with a monochromatic, well-collimated ion beam incident on a single crystal copper surface maintained in an ultra-high -vacuum scattering chamber. Energy- and angle-resolved scattering spectra were taken for K^+ and Na^+ scattered from Cu(110) and Cu(100). Classical scattering calculations show that the energy transfer from ion to surface is determined largely by the repulsive part of the ion-surface interaction potential. Approximating this repulsive potential as a sum of ion-surface-atom pair potentials, computed within the Hartree-Fock approximation, provides very good agreement between calculated and measured scattered ion energy distributions. The measured energy distributions also provide evidence for a small attractive ion-surface interaction, on the order of 3 eV at the ion's closest approach to the surface, which can be approximated as a modified image potential. The combination of repulsive pair potentials and an attractive image potential has been tested against scattering data from 100 eV to 400 eV for both K^+ and Na^+ scattered from Cu(110), and from 10 eV to 100 eV for Na^+ scattered from Cu(100), and provides excellent agreement in these cases. I have also studied the neutralization of hyperthermal K ^+ scattered from cesiated copper surfaces. Standard models of resonant neutralization predict a relatively abrupt transition from zero to nearly complete neutralization of the scattered K^+ as the work function is lowered. Our observation of a more gradual decrease in the ion survival probability is due to the effect of the adsorbate-induced local electrostatic potential on the potassium ionization energy. A simple model of the adsorbate potential accounts for the differences between the measured neutralization and the predictions of the standard model. Finally, neutralization of alkalis into excited states is considered and measurements of the photon yield of 400 eV Li^+ scattered from Cu(100) are discussed in terms of the radiative decay of these excited states.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- August 1990
- Bibcode:
- 1990PhDT........55G
- Keywords:
-
- ENERGY TRANSFER;
- Physics: Condensed Matter; Engineering: Materials Science