Browsing by Author "Ratliff, John Martin"

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    Energy dependent electron polarization studies of penning ionization channels
    (1985) Ratliff, John Martin; Walters, G. King; Dunning, F. Barry; Mutchler, Gordon S.
    Penning ionization reactions have been studied in a flowing afterglow apparatus to explore the contributions of autoionizing channels in reactions of spin-polarized He(23S) with alkali atoms. Spin polarization analysis of the Penning electrons has allowed us to identify a significant autoionizing channel for rubidium, and no such contribution for cesium and potassium. Using the results of this study, a model of the reaction process was constructed to predict the Penning electron polarization produced in He(23S) -- polarized rubidium reactions. An attempt to measure this polarization was made, but it became clear in the course of the experiment that success is possible only with a substantially upgraded apparatus. A simultaneous energy and polarization measurement of the electrons produced in reactions of polarized He(23S) with 2 has also been attempted. This initial experiment was unsuccessful but indicates that results can be obtained, again given an improved apparatus.
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    Studies of electron exchange collisions and polarized electron production in a flowing helium afterglow
    (1989) Ratliff, John Martin; Walters, G. King
    A flowing helium afterglow apparatus has been used to study thermal-energy electron exchange collisions between spin-polarized electrons and O$\sb2$ or NO molecules. Penning ionization of CO$\sb2$ by spin-polarized He(2$\sp3$S) metastable atoms is used to produce electrons which retain the spin orientation of the metastables, and which rapidly thermalize in the CO$\sb2$. The reactant gas (O$\sb2$ or NO), when introduced into the flowstream, causes a decrease in the electron spin-polarization. The electrons are then extracted from the flowtube for measurement of the polarization they retain. The rate constant for the reaction e$\sp{-}(\uparrow$) + X $\to$ e$\sp {-}(\downarrow$) + X can then be determined, given the amount of polarization decrease, reactant-gas density, and reaction time. The rates are found to be k(O$\sb2$) = (8 $\pm$ 3.5) x 10$\sp{-11}$ cm$\sp3$/sec and k(NO) = (9 $\pm$ 4) x 10$\sp{-11}$ cm$\sp3$/sec. An upper limit to the electron attachment rate for formation of an excited negative ion is derived from these measurements, and the contribution of exchange to the total scattering is discussed. In addition, a new, non-invasive technique for measuring electron-drift velocity in the flowtube is describe. Modifications of the afterglow apparatus and use of laser radiation for He(2$\sp3$S) spin-orientation enable it to produce an electron beam having moderate to high current and high spin polarization. Polarizations of 80% are achieved for currents up to 1$\mu$A, with 60% polarization retained at 25$\mu$A. This compares favorably with other polarized electron sources, making the afterglow apparatus a candidate for use as a beam source in high-energy electron accelerators.