The dynamics of neutralization of electron-spin-polarized (4)He(+) ions at surfaces
Bixler, David Lynn
Dunning, F. B.
Doctor of Philosophy
Ion Neutralization Spectroscopy (INS) is a well known probe of surface electronic structure with unparalleled surface specificity. In INS, a beam of singly-charged low-energy rare gas ions is directed at a target surface, and electrons ejected as a result of ion neutralization are analyzed. The use of spin-labeling techniques in INS provides a powerful tool for understanding the nature of the ion neutralization process. In the current research a spin polarized He + ion source has been developed and used to study the dynamics of ion-surface interactions. The He + ions are produced in a weak rf-excited discharge by Penning ionization reactions between helium metastable atoms. The He(2 3 S) metastable atoms are electron-spin-polarized by optical pumping on the D 1 transition with 1083nm radiation. Spin conservation in Penning ionization reactions produces spin-polarized ions, which are extracted from the discharge and focused onto the target surface using a series of electrostatic lenses. The impact energy can be varied from ∼10eV to ∼600eV with beam currents of ∼0.3nA and typical beam polarizations of 10–15%. Measurements of the polarization of electrons ejected from an Au(100) surface as a result of Auger neutralization of polarized ions reveal a net spin correlation which increases dramatically at high electron energies. The net correlation is explained as resulting from interference effects between different neutralization channels; and the increase at high electron energies is explained by considering the local perturbation in the surface electronic structure induced by the presence of the polarized He + ion. Electrons ejected during neutralization of polarized He + ions at a Xe film were also studied. These reveal a low-energy feature in the electron energy distribution (EED) that was previously observed with metastable deexcitation spectroscopy (MDS). The presence of this feature in the EED for ion neutralization supports the hypotheses put forth to explain the previous results. Improvements to the polarized He + ion source are currently being explored and a number of new targets are being considered.