Spin dependence of penning ionization and production of polarized electrons in a flowing helium afterglow

Abstract

This dissertation describes a method to study the dependence upon electron spin orientation of ionizing (Penning) reactions of the type He(23 Sl) + X ---> He(11S0) + X+ + e- Helium metastable 23S1 atoms are spin polarized by optical pumping. By measuring the spin polarization of both the free electrons and ions produced in such a reaction, the degree of spin angular momentum conservation of the reaction can be obtained. Helium gas flows at near sonic speed through a microwave cavity, in which a discharge is lit, and is exhausted by a high speed pump. The metastable 23S1 atoms in this "flowing afterglow" are optically pumped by the absorption of circularly polarized resonance radiation from a helium lamp. The resulting unequal populations of the mj sublevels of the 23S1 state are then computed from the absorption signal. These populations are used to predict the maximum spin polarization of the electrons produced in the Penning reaction above for two types of reactant species X; helium 23S1 atoms and argon atoms. Electrons are extracted from the afterglow and their spin polarization is measured by Mott scattering from a gold foil at 120 KeV energy. A method of determining the spin polarization of the ions by observing optical emission from excited ion states is described (Schearer, 1970). Polarization measurements of electrons produced in a weak, static discharge (McCusker, 1969) have shown this to be an intense source of polarized electron. It is expected the extraction of electrons from a flowing afterglow will be a significant improvement on this previous technique.