Browsing by Author "Butler, William Hollis"
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Item Application of time-of-flight velocity selection to metastable de-excitation spectroscopy(1988) Butler, William Hollis; Dunning, F. B.Previous experiments have shown that Metastable De-excitation Spectroscopy (MDS) provides a valuable probe of surface electronic and magnetic structure. Spin labelling of the excited electron can yield additional information about the de-excitation mechanisms. In such experiments, neon ($\sp3$P$\sb2$) and argon ($\sp3$P$\sb2$) metastable atoms are spin-labelled (polarized) by laser optical pumping, and their polarization measured by a Stern-Gerlach analyzer. In the present work, a Time-of-Flight (TOF) velocity selection capability has been implemented in an existing MDS apparatus to permit accurate determination of the metastable atom polarization achieved by optical pumping. The data show that polarizations in excess of 85% and 92% may be obtained for Ne ($\sp3$P$\sb2$) and Ar ($\sp3$P$\sb2$), respectively. TOF techniques are also used to investigate the velocity distribution of metastable atoms in the beam, and to eliminate spurious signals due to photon-surface interactions.Item Spin-polarized metastable de-excitation spectroscopy: A new probe of alkali chemisorption on surfaces(1991) Butler, William Hollis; Dunning, F. B.Metastable (Atom) De-excitation Spectroscopy (MDS) provides a powerful technique with which to investigate surface electronic structure with extreme surface specificity. In this technique a thermal energy beam of noble-gas metastable atoms is directed at the surface under study and the kinetic energy distribution of ejected electrons that result from metastable atom de-excitation is measured. Although the measured distribution contains information about the electronic structure of the outermost surface layer, its detailed analysis requires knowledge of the dynamics of the metastable atom-surface interaction. In the present work, these dynamics have been investigated directly by use of spin-labeling techniques. The electron spins of the incident metastable atoms are polarized and the spin-polarization of the ejected electrons is measured with a Mott polarimeter. Energy resolve electron spin-polarization measurements are reported for a variety of sub-monolayer coverages of cesium on a Cu(100) surface, and for oxygen and cesium co-adsorption on a Cu(100) surface. The Cs/Cu(100) system exhibits large ($\sim$2.8 eV) change in the surface work function. The results of the current work suggest that MDS interactions in both high and low work function regimes are more complex than has previously been supposed. Several additional interactions are suggested to explain the data acquired. The question of the occupancy of the adsorbed cesium valence level at various coverages is also addressed.