Browsing by Author "Dunning, F. B."
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Item A low-energy, electron-spin-polarized helium(+) ion source for use in surface studies(1998) Lancaster, James Campbell; Dunning, F. B.A spin polarized He$\sp+$ ion source has been developed to study the dynamics of ion-surface interactions. The He$\sp+$ ions are produced by Penning ionization in collisions between electron-spin-polarized He(2$\sp3$S) metastable atoms contained in a weak rf-excited discharge. The ions are extracted from the discharge and focused onto clean or adsorbate-covered surfaces using a series of electrostatic lenses. The impact energy can be varied from $\rm{<}10eV$ to $\rm{\ge}650eV$ and typical beam polarizations are 10-15%. Information about ion/surface interactions, and the dependence of those interactions on the incident ion energy, is obtained by measuring the energy distribution and polarization of electrons ejected from the surface as a result of Auger neutralization.Item A semiclassical model of Rydberg atom collisions(1991) Durham, Mark Allen; Dunning, F. B.A semiclassical model of Rydberg atom collisions with neutral molecules that result in electron attachment has been devised and implemented as a Monte Carlo computer program. Comparisons between model calculations and experimental data have been used to investigate the reactions:$$\eqalign{&{\rm K}(n{\rm d})+{\rm SF}\sb6\to{\rm K}\sp{+}+{\rm SF}\sb6\sp{-*},\cr&{\rm K}(n{\rm d})+\rm CF\sb3I\to K\sp{+}+CF\sb3I\sp{-*}\to K\sp{+}+CF\sb3+I\sp-,\cr&{\rm K}(n{\rm d})+\rm CCl\sb4\to K\sp{+}+CCl\sb4\sp{-*}\to K\sp{+}+CCl\sb3+Cl\sp-.}$$These comparisons provide information about the distribution of excess energy of reaction between internal and translational degrees of freedom and about the lifetimes of the intermediate negative ions, forming the basis of one of the few techniques that can be used to measure lifetimes of collisionally produced excited species on a picosecond time scale.Item A study of ultra-low energy electron - hydrogen fluoride scattering using high-n Rydberg atoms: Possible role of dipole-supported states(1996) Hill, Shannon Bradley; Dunning, F. B.A detailed experimental and theoretical study of the interaction of very-low-energy electrons with the polar target HF is presented. This interaction is investigated experimentally by measuring rate constants for ionization and state-changing in collisions between K(np) Rydberg atoms with $90\sbsp{\sim}{<}n\sbsp{\sim}{<}400$ and HF. The data are found to be consistent with the results of rotational close-coupling calculations that include possible effects associated with dipole-supported real or virtual states. The value of this state is effectively the only free parameter in the theoretical model. Comparison with the data suggests that for low energy electron-HF scattering there is a virtual state which, for $J=0,$ has an energy of 1-1.5 meV. The present work points to the importance of dipole supported states in electron-polar molecule scattering.Item A study of ultra-low-energy electron-molecule collisions using very-high-n Rydberg atoms(1993) Ling, Xuezhen; Dunning, F. B.In the present work very-high-n Rydberg-atoms $(n \sim 100 - n \sim 400)$ are used to probe electron-molecule collisions at ultra-low electron energies. Based on the essentially-free-electron model, for sufficiently high n, Rydberg atom-molecule collisions can be described in terms of a binary interaction between the target molecule and the essentially-free Rydberg electron. Since the average kinetic energy of the Rydberg electron is ultra-low $\rm({\sim}85\mu eV - {\sim}1.4meV),$ analysis of the very-high-n Rydberg-atom collision data can provide information on electron-molecule scattering at electron energies corresponding to electron temperatures of only $\sim$1$\sp\circ$K, which are far below those accessible using any alternate approach. Rate constants for destruction of very-high-n Rydberg atoms in collisions with various target molecules have been measured. This study focuses on Rydberg electron transfer to an electron-attaching molecule which results in negative ion formation via the reactions $$\rm K({\it np\/}) + XY \to K\sp+ + (XY)\sp{-*}\ or\ \to K\sp+ + X\sp- + Y\eqno(1)$$and on rotational energy transfer from polar molecules which leads to Rydberg atom ionization $$\rm K({\it np\/}) + XY({\it J\/}) \to K\sp+ + {\it e}\sp- + XY({\it J\/}-1)\eqno(2)$$ The n dependence of the rate constants for Rydberg-atom destruction depends on the reactions involved. In reaction (1) the rate constant is independent of n whereas in reaction (2) it increases nearly linearly with n. When both reactions are possible, the measured n dependence can be explained in terms of contributions from each process. The essentially-free-electron model suggests that the n dependence of the rate constants for Rydberg-atom destruction reflects the energy dependence of the cross sections of the corresponding free electron-molecule collision processes which are $$\eqalignno{&e\sp- + \rm XY \to (XY)\sp{-*}\ or\ \to X\sp- + Y&(1\sp\prime)\cr &e\sp- + \rm XY({\it J\/}) \to {\it e}\sp- + XY({\it J\/}-1)&(2\sp\prime)\cr}$$Analysis of the data therefore provides the behavior of cross sections for these processes at ultra-low electron energies.Item 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.Item An intense source of spin-polarized electrons based on an optically-pumped flowing helium afterglow(1989) Rutherford, George Henry; Dunning, F. B.The optimization of an optically-pumped, flowing helium afterglow for use as a high-current dc polarized electron source with high electron spin polarization is described. The atomic electron spins of He (2$\sp3\rm S\sb1$) metastable atoms created by a microwave discharge are aligned in the presence of a weak magnetic field by circularly polarized radiation from a recently developed LNA laser. The polarized outer 2s electrons are liberated in spin-conserving Penning ionization reactions with CO$\sb2$ and are extracted from the flow tube for polarization analysis. Polarizations approaching 90% were obtained at currents below.1 $\mu$A, falling gradually to 70% at 20 $\mu$A. Comparisons with other sources, chiefly photoemission from GaAs, suggest that for most applications requiring dc currents less than 50 $\mu$A, the flowing afterglow source is superior. Possible future improvements to the present source are also discussed.Item An ultra-compact retarding-potential Mott polarimeter(2007) Neufeld, Dennis D., II; Dunning, F. B.Many areas of study, from particle-surface interactions to magnetic properties of surfaces and thin films, can benefit from electron spin analysis. These experiments frequently employ Mott polarimeters. In the past, Mott polarimeters were bulky, and operated at high voltages (∼150 kV). Modern retarding-potential Mott polarimeters operate in the 20 kV range, and are small enough to be mounted inside a vacuum system. A new, ultra-compact Mott polarimeter is described here which occupies a significantly smaller volume than previous designs. This polarimeter, when equipped with a thorium target and operating with an accelerating potential of 25 kV obtained a figure of merit of ∼1.3 x 10-4, which is similar to the best values achieved by previous Mott polarimeters. Thus this design is well suited for a variety of applications involving angle- and/or energy-resolved polarization measurements.Item Angular dependence of xenon Rydberg atom ionization at conducting and semiconducting surfaces(2008) Dunham, Hardin R.; Dunning, F. B.Ionization of xenon atoms excited to the lowest states in the n=17 and n=20 Stark manifolds at atomically flat Au(111) surfaces and at heavily-doped n-type and p-type Si(100) surfaces having robust native oxide layers is examined over a range of incident angles. Ionization is characterized by resonant surface ionization with contributions from local fields at the surface associated with surface charging or surface inhomogeneities. For oxidized semiconducting surfaces the local fields are a factor of two to three times larger than for conducting surfaces leading to markedly different results.Item Application of LNA lasers to helium optical pumping(1990) Lynn, James Gregory; Dunning, F. B.Electron spin-polarized helium 2$\sp3$S$\sb1$ metastable atoms are a valuable probe of spin-dependent phenomena. Helium metastable atoms can be polarized by illumination with circularly-polarized 1.083 $\mu$m 2$\sp3$S$\sb1 \leftrightarrow$ 2$\sp3$P$\sb{0,1,2}$ resonance radiation in a process known as optical pumping. Metastable atom polarizations obtained in this manner have been limited by the low intensity of optical pumping radiation available from helium discharge lamps, until now the only practical source of 1.083 $\mu$m light. However, recent advances in the field of solid-state near-infrared tuneable lasers have resulted in the development of several materials that exhibit laser action at 1.083 $\mu$m. The most promising of these is a neodymium-doped lanthanum magnesium hexaluminate, known as LNA. During the course of this work several LNA lasers have been designed and fabricated for specialized applications in helium optical pumping. Unstabilized single- and multi-mode lasers have been used to optically pump helium metastable atoms in a flowing helium afterglow, which serves as the basis for a highly competitive polarized electron source that provides electron polarizations approaching 90%. A frequency-stabilized single-mode LNA laser has been developed to optically pump a beam of metastable helium atoms for use in surface studies, and metastable polarizations resulting from its use exceed 96%. Although not designed specifically for other applications, the stabilized laser is ideally suited for application in laser cooling of helium metastable atoms and for helium spectroscopic applications. For example, laser absorption spectroscopy has been used to observe the decay of helium 2$\sp3$S$\sb1$ atoms as they are converted into He$\sb2$(a $\sp3\Sigma\sbsp{\rm u}{+}$) molecules in three-body reactions in a high-pressure helium afterglow, revealing interesting phenomena possibly related to quantum mechanical tunneling. The present work shows that LNA lasers have an important future in helium optical pumping.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 Associative ionization in collisions of potassium Rydberg atoms with molecules(1988) Kalamarides, Alexander A.; Dunning, F. B.The formation of long-lived ($\tau$ $\gg$ 10 $\mu$sec) product ions through associative ionization reactions of the type: K(nd) + BC $\to$ (KBC)$\sp+$ + e$\sp-$ has been investigated for 8 $\leq$ n $\leq$ 18, using a variety of polyatomic target molecules (denoted BC). Although such reactions clearly involve the Rydberg core ion, the presence of the Rydberg electron is also essential to stabilize the collision product. Rate constants for these reactions have been measured with the mixed-gas technique and are compared with those predicted by ion-molecule reaction theories. Studies with a number of different target molecules have been undertaken, so as to investigate the factors that influence the rate constants for associative ionization. In particular, it is observed that the rate constants increase with both the dipole moment and complexity of the target, and they also depend strongly on the principal quantum number, n, of the Rydberg state involved.Item Chaotic ionization of a Rydberg atom subjected to alternating kicks(2012) Ye, Shuzhen; Dunning, F. B.Quasi-one-dimensional Rydberg atoms exposed to alternating positive and negative electric field pulses (kicks) are an example of a chaotic atomic system. Chaotic ionization is predicted in this system via a phase space turnstile mechanism, and we have explored this experimentally. Turnstiles form a general transport mechanism for numerous chaotic systems, and this study is the first to explicitly illuminate their relevance to atomic ionization. Two experiments are presented. In the first we show that the ionization of the electron depends not only on the initial electron energy, but also on the phase space position of the electron with respect to the turnstile--that part of the electron packet inside the turnstile ionizes quickly, after one period of the applied field, while that part outside the turnstile ionizes after multiple kicking periods. In the second experiment we show the signature of the turnstile manifests itself in the step-function-like behavior of the ionization fraction as a function of the kick strength. This behavior persists for different values of kicking periods and starting electron energies.Item Collisional formation of heavy Rydberg ion-pair states(2010) Cannon, Matt; Dunning, F. B.The formation of heavy Rydberg ion-pair states through electron transfer in collisions between K(np) Rydberg atoms and molecules that attach low-energy electrons is investigated. At low n, collisions with a wide variety of target species can lead to the formation of hound ion-pair states, a small fraction of which (under apropriate conditions) can subsequently dissociate as free ions through internal-to-translational energy transfer. The lifetimes of the ion-pair states can also be influenced by autodetachment of the electron or by neutralization through charge transfer. Two primary methods are employed to investigate the lifetimes of ion-pair states. Measurements of the onsets in the arrival time distribution of the ions provide lifetime information on short time scales (a few microseconds or less) for those ion pairs which undergo dissociation through conversion of internal energy. Additionally, ion pair behavior on longer time scales (> 1 --- 2 micros) can be studied as a function of binding energy through direct field dissociation.Item Comparative studies of negative ion formation in Rydberg atom collisions with attaching and polar targets(2004) Suess, Leonard; Dunning, F. B.Low-energy electron transfer in collisions between a Rydberg atom and a target molecule can result in the formation of negative ions. Depending as to the properties of the target molecule the electron may be bound in a localized molecular orbital (valence-bound) or bound by its dipole potential in a diffuse orbital located at the positive end of the dipole (dipole-bound). Here we examine the properties of valence-bound and dipole-bound negative ions created through electron transfer in collision with Rydberg atoms, in particular, their lifetimes, collisional properties and electric-field-induced detachment characteristics. Intercomparison of the various data also leads to new insights into electron attachment to molecules that can support both valence-bound and dipole-bound negative ion states.Item Creating Strontium Rydberg Atoms(2013-05-28) Zhang, Xinyue; Dunning, F. B.; Killian, Thomas C.; Natelson, DouglasDipole-dipole interactions, the strongest, longest-range interactions possible between two neutral atoms, cannot be better manifested anywhere else than in a Rydberg atomic system. Rydberg atoms, having high principal quantum numbers n>>1 and dipole moments that scale as n^2, provide a powerful tool to examine dipole-dipole interactions. Therefore, we have studied the production and production rates of strontium Rydberg atoms created using two-photon excitation and have explored their properties in two distinct experiments. In the first experiment, very-high-n (n~300) Rydberg atoms are produced in a tightly collimated atomic beam allowing spectroscopic studies of their energy levels and their Stark effects. Simulations using a two-active-electron model, developed by our theoretical collaborators, allow detailed analysis of the results and are in remarkable agreement with the experimental results. The high density of Rydberg atoms achieved ~ 5*10^5 cm^(-3), in this experiment will allow studies of strongly interacting Rydberg-Rydberg systems. The second experiment, in which a cold strontium Rydberg gas is excited in a magneto-optic trap, features an imaging technique offering both spatial and temporal resolution. We use this technique to observe and study the evolution of an ultra-cold strontium Rydberg gas which reveals the importance of Rydberg-Rydberg interactions in the early stages of this evolution. Strongly interacting Rydberg gas provides an opportunity to realize a very strongly-correlated ultra-cold plasma.Item Decay energetics and lifetime measurements of transient negative ions using Rydberg atoms(1997) Parthasarathy, Ramapriya; Dunning, F. B.Dissociative electron attachment to CFCl$\sb3$, CF$\sb2$Br$\sb2$, and 1,1,1- and 1,1,2-C$\sb2$Cl$\sb3$F$\sb3$ is studied by measuring the velocity and angular distributions of negative ions produced through electron transfer in collisions with velocity selected K(np) Rydberg atoms. Data analysis is done with the aid of Monte Carlo simulations. Measurements indicate that the mean lifetimes of the transient ions CFCl$\sb3\sp{-*}$ and CF$\sb2$Br$\sb2\sp{-*}$ are $\sim$3.5 ps and $\sim$4.5 ps respectively and that only a small fraction of the excess energy of reaction appears in translation. The 1,1,1-C$\sb2$Cl$\sb3$F$\sb3$ data show one reaction channel resulting in immediate dissociation with an ion lifetime of $\sim$0 ps, and a second channel associated with a longer lived intermediate, $\tau\ \sim$ 0.1 to 1.0 ps. Only limited energy transfer occurs for the 1,1,2 isomer but the operative reaction channel(s) could not be identified. Possible reaction mechanisms are discussed with the aid of ab-initio calculations.Item Design and development of an apparatus to study atom-surface interactions using Rydberg atoms(2000) Haich, Christian Allen; Dunning, F. B.An apparatus has been developed for studying the interaction of Rydberg atoms with surfaces, particularly the perturbation of Rydberg states by a nearby surface and the distance from the surface at which resonance ionization occurs. For this purpose, a source of metastable Xe(3P 0,2) atoms was constructed and characterized. Using single photon laser excitation from the 6s'[1/2]0 state, the nf[3/2]1 Rydberg series for n = 16--20 has been observed with signals well above the background. Measurements of the energy levels of these Rydberg states has identified systematic errors in quoted energy levels from earlier work. The isotopic structure of the 20f Rydberg state was resolved, and the effect of Rydberg excitation in an electric field was investigated. Studies are underway to better understand the behavior of Xe(nf) Rydberg atoms in an electric field. This information will be used in subsequent experiments to study Rydberg-surface interactions.Item Design and testing of a low-energy spin-polarized electron gun and its application to SPEELS(1994) Magugumela, Maurice Todani; Dunning, F. B.A spin-polarized low-energy electron gun with high current transmission is described here and has been applied to SPEELS studies. Current transmission in excess of 40% is routinely achieved. This is a dramatic improvement compared with the $\sim$1% transmission obtained using an earlier electron gun. The high current transmission has allowed SPEELS studies of energy loss mechanisms in low-energy electron scattering from atomically clean metallic surfaces. There are two dominant energy loss mechanisms: dipole scattering which occurs in the vacuum outside the target and impact scattering via electron-hole pair excitation occurring inside the target surface. Preliminary results of SPEELS experiments on Cu(100) and Ag(100) surfaces and on thin films of Mo, Co, and Fe epitaxially grown on a Cu(100) are presented.Item Design and testing of compact Mott polarimeters for use in energy- and angle-resolved polarization measurements(1988) Zhang, Xia; Dunning, F. B.UHV-compatible Mott polarimeters that employ electron accelerating voltages of $\sim$20kV have been designed and tested. The efficiency of the polarimeters, $\sim$3 $\times$ 10$\sp{-5}$, is competitive with those provided by other polarimeters, but the present polarimeters are considerably simpler and more compact. The scattering efficiency of the polarimeters is 2.2 $\times$ 10$\sp{-3}$, which makes it possible to accurately measure electron spin polarizations using input beam currents as low as 10$\sp{-14}$A. One polarimeter design incorporates an in-line retarding potential energy analyzer with an energy resolution $\sim$0.5eV, and is suitable for a wide variety of applications requiring energy- and angle-resolved polarization measurements.Item Determination of transient negative ion lifetimes using Rydberg atoms(1996) Popple, Richard Allen; Dunning, F. B.Potassium Rydberg atoms are used to investigate the lifetimes, on a picosecond timescale, of transient negative ions formed during dissociative electron attachment to CCl$\sb4$ and CF$\sb3$I. The transient negative ions are formed during Rydberg electron transfer reactions of the type$${\rm K}(n{\rm p})+AB\to{\rm K}\sp{+}+AB\sp{-*}\to{\rm K}\sp{+}+A\sp{-}+B$$For low values of principal quantum number $n\ (n\ \sbsp{\sim}{<}\ 20)$ the electrostatic interaction between the ${\rm K}\sp{+}-AB\sp{-*}$ and ${\rm K}\sp{+}+A\sp{-}$ ion pairs perturbs the ion trajectories. As a consequence, the final angular and velocity distributions of the ${\rm K}\sp{+}+A\sp{-}$ ions are dependent on the lifetime of the $AB\sp{-*}$ intermediate ion. The $AB\sp{-*}$ lifetime is determined by comparing product ion distributions, determined using position sensitive detection techniques, with the results of a Monte Carlo simulation that models the kinematics of the reaction. The data show that the lifetime of the excited $\rm CF\sb3\Gamma\sp{*}$ intermediate formed by Rydberg electron capture is short, $\sbsp{\sim}{<}2$ ps, and that the lifetime of the CCl$\sbsp{4}{-*}$ intermediate is $7.5\pm2.5$ ps.