Gary D. Fletcher

Great interest exists in the study of the scattering of electrons by hydrogen atoms; such scattering is the most fundamental atomic collision process. Furthermore, measurements of electron spin dependence in such collisions yield additional information about the interaction, beyond that obtained from spin-averaged differential cross section measurements. This thesis is a report on measurements of the spin dependence in low energy collisions of electrons and hydrogen atoms for two channels: 90(DEGREES) elastic scattering and total impact ionization. The basic quantity measured, as a function of incident electron energy, is the asymmetry. (DIAGRAM, TABLE OR GRAPHIC OMITTED…PLEASE SEE DAI). where (sigma)(,(UPARR)(DARR)((UPARR)(UPARR))) corresponds to the appropriate cross section {the 90(DEGREES) elastic cross section or the total ionization cross section} for incident electron and atomic electron spins antiparallel (parallel). This asymmetry can be related to the interference between the direct and exchange scattering amplitudes; equivalently, it can be expressed in terms of the singlet and triplet cross sections. The measurements of the interference between the direct and exchange amplitudes as a function of energy is the new information gained from this experiment. The measurements were performed using crossed beams of polarized electrons and polarized hydrogen atoms. The source of polarized electrons was based on the Fano effect and provided typically 2 nA of electrons in the interaction region with a polarization of 0.61((+OR-)5%) to 0.75((+OR -)5%). The atomic hydrogen beam was produced by thermal dissociation in a tungsten oven and state selected in a sextupole magnet, with a beam in the interaction region of density 10(‘9) atoms/cm(‘3) and polarization 0.50(2). Frequent reversal of both the electron and hydrogen polarizations allowed the measurement of the above asymmetry. Elastic asymmetry measurements were made for ten incident electron energies from 4.4 to 30.3 eV. The asymmetry was found to vary from +0.003(46) to -0.343(65). While most of the theoretical predictions agree with these measurements at low energy (< 10 eV), only one close coupling calculation (BUR63, SCO65) agrees well at higher energies. Ionization asymmetry measurements were simultaneously performed for seven energies from 14.4 to 30.3 eV. An extensive check of systematic effects led to a revision of previous ionization results. Both the present results and the corrected earlier results (ALG77b) agree over their common energy range, while disagreeing with theoretical predictions below an energy of about twice the ionization energy, and exhibit no strong energy dependence near threshold. In the course of these asymmetry measurements a careful study of the Mott scattering electron polarization measurements was done. A better understanding of the backgrounds associated with this Mott apparatus was obtained, and revised results for earlier electron polarization measurements were determined.