Browsing by Author "Oro, David Michael"

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    Absolute calibration of a retarding-potential Mott polarimeter
    (1992) Oro, David Michael; Dunning, F. B.
    Mott polarimeters are used in many areas of physics to determine the spin polarization of electrons in studies of processes dependent on the electron's intrinsic spin angular momentum and associated spin magnetic moment. Mott polarimeters work by measuring the left-right asymmetry in the spatial distribution of electrons scattered from high-Z nuclei. The polarization of the incident electrons is related to the measured asymmetry via a parameter known as the effective asymmetry (Sherman) function, S$\sb{\rm eff}$, which must be determined through calibration. This work describes a novel technique for calibrating a Mott polarimeter that makes use of electrons of accurately known polarization obtained through Penning ionization reactions involving electron spin polarized He(2$\sp3$S) metastable atoms. This technique has been used to calibrate a compact retarding-potential Mott polarimeter, and the values of S$\sb{\rm eff}$ are presented for both gold and thorium target foils under a variety of operating conditions.
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    The dynamics of atom-surface interactions involving helium(2(1)S), helium(2(3)P) and electron-spin-polarized helium(2(3)S) atoms
    (1994) Oro, David Michael; Dunning, F. B.
    For several years Metastable Atom Deexcitation Spectroscopy (MDS) has been employed as a probe of surface electronic structure offering unparalleled surface specificity. In MDS a thermal-energy beam of rare-gas metastable atoms is directed at the surface under study, and the energy distribution of electrons ejected as a result of metastable atom deexcitation is measured. However, correct interpretation of the data requires detailed knowledge of the dynamics of the deexcitation process. In the present work spin-labeling techniques, specifically the use of electron-spin-polarized metastable He(2$\sp3S$) atoms, coupled with spin analysis of the ejected electrons, are used to probe the dynamics of He(2$\sp3S$) deexcitation at a variety of surfaces. Such measurements, coupled with studies of the deexcitation of He(2$\sp1S$) and He(2$\sp3P$) atoms at Cu(100) and Au(100) show that each species deexcites exclusively through resonance ionization followed by Auger neutralization. The data also provide the first direct confirmation of spin correlation in the Auger neutralization of ions outside a paramagnetic surface. Two proposed models for spin correlation are discussed and potential experimental tests for distinguishing between them are suggested. Studies of SPMDS at surfaces comprising layers of Ar or Xe atoms frozen onto a cryogenically cooled substrate are described and exhibit behavior similar to that observed in gas phase Penning ionization studies indicating that ejection results, in part, from surface Penning ionization (SPI). For Xe, however, additional features are observed and can be attributed to resonance ionization of the incident excited atoms followed by neutralization of the resulting He$\sp+$ ions through an interaction involving neighboring Xe atoms in the film. These results provide a rare example of a surface at which the rates for resonance ionization and Auger deexcitation are comparable. Also, the data show that the electron yield from both films is substantially higher than that from atomically clean metal surfaces. Potential experimental applications of the large spin correlation in He(2$\sp3S$) deexcitation at Cu and Au, and of deexcitation at Xe(100) and Ar(100) films are discussed.