CONSTRUCTION AND CHARACTERIZATION OF A SPIN POLARIZED HELIUM ION BEAM FOR SURFACE ELECTRONIC STRUCTURE STUDIES
HARRISON, ALLEN RAY
Doctor of Philosophy
Ion neutralization and metastable de-excitation spectroscopy, INS and MDS, allow detailed analysis of the surface electronic configuration of metals. The orthodox application of these spectroscopies may be enhanced by electronic spin polarization of the probe beams. For this reason, a spin polarized helium ion beam has been constructed. The electronic spin of helium metastables created within an rf discharge may be spacially aligned by optically pumping the atoms. Subsequent collisions between metastables produce helium ions which retain the orientation of the electronic spin. Extracted ion polarization, although not directly measurable, may be estimated from extracted electron polarization, metastable polarization, pumping radiation absorption and current modulation measurements. Ions extracted from the optically pumped discharge exhibit an estimated polarization of about ten per cent at a beam current of a few tenths of a microampere. Extraction of helium ions from the discharge requires that the ions have a high kinetic energy. However, to avoid undesirable kinetic electron ejection from the target surface, the ions must be decelerated. Examination of various deceleration configurations, in particular exponential and linear deceleration fields, and experimental observation indicate that a linear decelerating field produces the best low energy beam to the target surface. Auger analysis of the target surface allows determination of the degree of surface contamination. Although various analog systems for Auger analysis are available, a digital, computer compatible system has been constructed. The technique of ion neutralization spectroscopy and metastable de-excitation spectroscopy allows probing of the surface electronic structure of a metal target. With spin polarization of the probe helium ion beam, the apparatus provides a means to ascertain the existence and properties of surface magnetization. The apparatus not only may perform the highly surface selective ion neutralization and metastable de-excitation spectroscopies upon a common target surface but also provides the additional diagnostic ability achieved with spin polarization of the probe beam.