Browsing by Author "Dunning, F. Barry"

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    A computer controlled, narrow linewidth, broadly tunable, continuous wave laser
    (1979) Ketkar, Suhas Narayan; Dunning, F. Barry; Tittel, Frank K.; Estle, Thomas L.
    A continuous wave, tunable, single frequency dye ring laser has been developed. This laser is capable of giving output powers, in a single longitudinal mode, in excess of 7 mV, using different dyes and Second Harmonic Generation or Sum Frequency Mixing schemes, a wavelength coverage from 26 nm to 8 nm can be achieved. Using a PDP 11/3 minicomputer and a CAMAC dataway, control of a linear as well as a ring laser, oscillating in a single longitudinal mode, has been accomplished. The various optical elements in the dye laser cavity and the nonlinear mixing device are synchronously tracked. The laser can be operated both as a low resolution (4 GHz bandwidth) and a high resolution (.1 GHz bandwidth) visible and ultraviolet radiation source. Using Boyd and Kleinman theory for Second Harmonic Generation of focussed Gaussian beams,theoretical estimates for the wavelength and angle bandwidths are made. Some of the results have been experimentally verified.
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    A polarized helium ion beam for surface electronic structure studies
    (1980) Harrison, Allen Ray; Walters, G. King; Dunning, F. Barry; Rorschach, Harold E.
    This thesis describes the development of a spinpolarized He+ ion beam suitable for use as a- probe of surface electronic structure of metals. The method would represent an extension of ion neutralization spectroscopy (INS), and should provide new information on surface magnetism, electron correlation effects, and the dynamics of the ion neutralization process. Production of the polarized helium ion beam involves optically pumping helium metastable atoms in an rf electric discharge with subsequent extraction of ions created by metastable-metastable collisions. The principles of this technique and details of the experimental apparatus are presented. Ion currents of up to 1~ A were attained and optical pumping absorption measurements suggest a beam polarization of 8 to 1%. For a moderately low beam energy, about 15 eV, an ion density at the target surface of roughly 2 X 1 A-cm- with an energy spread of about 3 eV was attained. A specific experiment is proposed in which the polarized He+ beam would be used to probe the surface magnetic properties of nickel.
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    A spin-polarized He (2³S₁) metastable atom source for surface research
    (1981) Onellion, Marshall; Walters, G. King; Trammell, George T.; Dunning, F. Barry
    Surface science has a paucity of probes, particularly for spin-dependent studies. This thesis describes a 2 3S1 He probe, recently developed in this laboratory, that can be used for such investigations. There are three interaction mechanisms such a probe can undergo: resonance ionization/ Auger neutralization, Auger deexcitation, or surface Penning ionization. All are discussed and compared. The important concepts introduced include electronic transitions, factors affecting the interactions, variation of atomic energy levels, and the convolution problem. After introducing these concepts, and describing the resulting physical phenomena, the experimental apparatus is examined. The thesis delineates the rationale for a multi-chamber design and the problems encountered. One diagnostic, a Stern-Gerlach magnet used as a polarization analyzer, is particularly useful and is described in detail. By using the analyzer, we found the beam is approximately 5 percent polarized. Finally, several problems related to the first experiments are discussed.
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    A Study of Heavy-Rydberg Ion-Pair Formation through Rydberg-Atom-Molecule Collisions
    (2018-08-30) Buathong, Sitti; Dunning, F. Barry
    Thermal collisions between potassium Rydberg atoms with intermediate values of quantum number n and target molecules that capture free low-energy electrons are investigated. The velocity, binding energy, and angular distributions of heavy-Rydberg ion-pair states formed through such collisions are measured. The experimental data are analyzed with the aid of semiclassical Monte Carlo simulations to investigate the dynamics of electron attachment to a variety of target species. Multiple reaction channels involving short- and long-lived intermediates are seen for electron capture by 1,1,1-C2Cl3F3 and CBrCl3 (where dissociation of CBrCl3 intermediates involves production of Br- and Cl- ions). Electron attachment to BrCN produces long-lived BrCN-* intermediates whose dissociation results in the creation of rotationally-hot CN- anions. Electron capture by Fe(CO)5 leads to formation of Fe(CO)5-* intermediates which can fragment producing Fe(CO)4- anions, either directly or after partial redistribution of the excess energy within the intermediate has occurred. Electron transfer in Rydberg atom collisions with C2Cl4 provide no evidence of formation of long-lived C2Cl4 ions, only Cl- ions are seen. Moreover, strong Rydberg atom scattering is observed following collisions of K(12p) Rydberg states with CH3NO2. This finding is consistent with a theory which predicts that collisions between Rydberg atoms with intermediate values of n and polar molecules can lead to strong resonant quenching of Rydberg states, i.e., n and l changing, through formation of short-lived K+...CH3NO2- ion-pair states. Finally, the improvements in sensitivity to electron capture reaction dynamics that might be achieved using different Rydberg species and a crossed molecular beam are examined. Simulations are performed to demonstrate that the use of a hyperthermal helium Rydberg atomic beam and a crossed beam of target molecules such as CCl4, BrCN and CF3I can provide new insights into dissociative electron attachment reactions. The calculations show that not only can much detailed information on translational energy release distributions and intermediate lifetimes be obtained, but also on the electron radial probability density distributions in the Rydberg atoms themselves.
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    A theoretical treatment of the stark effect on excited states of sodium
    (1982) Copel, Margery Ann; Dunning, F. Barry; Stebbings, Ronald F.;Dodds, Stanley A.
    Calculated energy levels of highly excited sodium atoms in an electric field are presented. The energy levels have been computed in the low ( to .4 V/cm) and intermediate (.2 to .4 V/cm) field regions for the 34^3/2,5/2 states, and in the high (up to 6 V/cm) field strength regions for the n = 15 states. At low field strength, all the 34^3/2,5/2 states are found to decrease in energy with increasing field strength. The method used to calculated the energy levels is outlined. The results are compared with second order perturbation theory and with experimental data obtained using the method of quantum beats to measure the energy difference between states.
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    Adiabetic electric field ionization of Sodium Rydberg atoms
    (1982) McMillian, Gary Benton; Stebbings, Ronald F.; Dunning, F. Barry; Lane, Neal F.
    Utilizing the technique of high resolution selective field ionization it is possible to observe ionization structure as narrow as 2 V/cm for Na(nS,nD) atoms. Field ionization structure this narrow is characteristic of atoms evolving predominantly adiabatically in a rapidly increasing electric field. By increasing the slew rate of the applied field it is possible to induce departures from the adiabatic path at specific avoided level crossings. These departures result in multiple ionization thresholds which may be explained by arguments based on a simple quantum mechanical model of adiabatic field ionization.
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    Analysis of the Dynamics of Heavy-Rydberg Ion Pair Formation Through Studies of Electron Capture Reactions
    (2017-09-06) Kelley, Michael John; Dunning, F. Barry
    Scattering of K(np) Rydberg atoms by electron-attaching molecules can result in the formation of positive-negative ion pairs through electron transfer reactions. These reactions can involve the dissociation of a transient intermediate negative ion into a smaller negative ion and a neutral fragment, or they can result in the parent molecule forming a metastable negative ion, with the excess energy being distributed among its internal modes. For Rydberg atoms of intermediate n, it is possible for these ion pairs to orbit each other at large separation, weakly bound by their mutual electrostatic attraction, forming what are known as heavy-Rydberg states. Measurements of the binding energy and velocity distributions of the ion pairs are used to probe the dynamics of electron transfer reactions. The data are analyzed using a semiclassical Monte Carlo collision code to explore the lifetime of the intermediate, the nature of its dissociation, the distribution of the excess energy of reaction, and the branching ratio of the various reaction channels. Results obtained using a variety of attaching targets including CF3I, CCl4, SF6, CH2Br2, 1,1,1-C2Cl3F3, CCl3Br, Fe(CO)5, and CH3NO2 have demonstrated both the wide variety of different behaviors that can accompany electron capture as well as the different behaviors of the product ion-pair states themselves. Remarkably, despite previous work showing that it was possible for CH3NO2 to form long-lived valence-bound negative ions through Rydberg electron transfer, the present work showed that such collisions do not result in the formation of long-lived bound ion-pair states. Rather, the data show that collisions lead to very strong Rydberg atom scattering through formation of transient ion-pair states. These results are consistent with theoretical work that predicts significant collisional quenching of Rydberg atoms by highly polar molecules and highlight a new mechanism for Rydberg atom scattering.
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    Chaotic ionization of a stationary electron state via a phase space turnstile
    (American Physical Society, 2013) Burke, Korana; Mitchell, Kevin A.; Ye, Shuzhen; Dunning, F. Barry; Rice Quantum Institute
    The ionization of a highly excited Rydberg atom subjected to a periodic sequence of electric field impulses, or “kicks,” is chaotic. We focus on the dynamics of a single kicking period in order to isolate the ionization mechanism. Potassium Rydberg atoms, prepared in a quasi-one-dimensional state, are exposed to a sequence of ionization kicks, and the total fraction of ionized atoms is then measured. These experimental data are compared to a one-dimensional classical model. The classical analysis reveals that the ionization process is governed by a phase space turnstile—a geometric structure associated with chaotic transport in diverse systems. The turnstile geometry is reflected in the experimental data. Previous work explored the dependence of the turnstile geometry on the kicking period. The present work explores the dependence on the kicking strength. In particular, increasing the kicking strength allows us to observe the stretching of the turnstile lobe as it penetrates the region of phase space occupied by the electronic state, leading to a sharp rise in the total ionization fraction. This work thus highlights the importance of phase space geometry in organizing chaotic transport in atomic Systems.
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    Collisional ionization in RB (NS,ND) -SF [backward], collisions
    (1985) Lu, Fan; Dunning, F. Barry; Walters, G. King; Jeys, Thomas H.
    Tbe thereotical work of Matsnzawa based on the essentially free electron model suggests that the rate constant for collisional ionization of a high rydberg atom is essentially the same as that for attachment of free electron haying the same momentum distribution. In the present work we test the essentially free electron model by comparing rate constants measured for the following reaction with those expected on the basis of free electron studies. The data are in good agreement indicating that studies of Rydberg atom collisions can provide information on very-low-energy electron interaction.
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    Collisions involving XE(nf) Rydberg atoms: mℓ [enclosed by vertical lines]--changing and the influence of an external electric field
    (1982) Slusher, Morgan Paul; Dunning, F. Barry; Stebbings, Ronald F.; Corcoran, Marjorie D.
    Thermal energy collisions between xenon Rydberg atoms in n = 31, l > 3 states and CO2 are reported. The rate constant is derived for those collisions that result in a change in m^l. The value so obtained is similar to that obtained previously for Xe(nf) - CO2 collisions that result in a change in l. It is argued that these observations imply that collisions result in simultaneous changes in l and m^l. In addition, the influence of an applied electric field on state-changing in collisions between laser-excited Xe(nf) atoms and xenon target gas is described. The presence of the field- results in marked changes in both the rate constant for collisional state-changing and in the final state distribution. The changes can be accounted for by use of energy transfer arguments based on the "essentially-free" electron model, although the degree of spatial overlap between wavefunctions associated with the initial and final states may also be important.
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    Collisions of Xenon(nf) Rydberg atoms with electron attaching targets
    (1984) Zollars, Byron George; Stebbings, Ronald F.; Dunning, F. Barry; Dodds, Stanley A.
    The destruction of Xenon(nf) Rydberg atoms in collisions with 1,1,1-C2C13JF3 and C5F8 takes place by transfer of the Rydberg electron to the target, resulting in a positive and negative ion. Rate constants for this reaction have been measured for Xe(nf) over the range 26 < n < 43, and the negative ion products have been identified. The cross sections derived from these measurements are in good agreement with recent free electron attachment cross sections.
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    Cross sections for double ionization of atomic oxygen by electron impact
    (1981) Ziegler, Daniel L.; Stebbings, Ronald F.; Dunning, F. Barry; Brooks, Phillip R.
    Laboratory measurements of the cross sections for double ionization of atomic oxygen by electrons are presented for collision energies from threshold to 4eV. This process displays a maximum cross section of about .5 A^2 at a collision energy of approximately 2eV. These absolute cross sections were obtained from measurements of the ratio of the cross sections for double and single ionization, coupled with absolute cross sections for single ionization previously measured by other investigators. The configuration and operation of the apparatus for these measurements are described. Procedures used to assess the effects of oxygen excited states on the measurements are reported.
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    Cusp particle detection and ion injection source oscillations
    (1984) Frahm, Rudy Allan; Reiff, Patricia H.; Dunning, F. Barry; Freeman, John W.
    The magnetic merging process has been suggested as a means for particle injection into the cusp regions of the earth's magnetosphere. If this process occurs, the injected plasma should reveal a characteristic signature. Electrostatic analyzers are used to detect this plasma. This thesis discusses the design of the electrostatic analyzers which are the central components of the High Altitude Plasma Instrument (HAPI) and Low Altitude Plasma Instrument (LAPI) flown on DE-A and DEHB (Dynamics Explorer satellites) respectively. Also discussed is the calibration of the electrostatic analyzers which I was involved in. This includes the co-authored calibration data acquisition program and my Calibration Data Reduction Routine. Typical satellite instrument output is in spectrogram energy-time format with the particle flux intensity indicated by gray shading. This presentation gives a "visual picture" of how the differential energy flux at each energy changes with time. A presentation of this type using the DE-A HAPI data shows the magnetic merging injection signature: multiple ion "V" structures as the satellite passes through the cusp region when the interplanetary magnetic field has a southward component. An alternative output format which I have helped create produces individual line spectra which allows these "V" structures to be examined in finer detail. If one looks at these "V" structure spectrograms, it is easily noticed that they appear to pulsate with time. I have conducted a harmonic analysis of the peak distribution function per spin from several satellite passes- to determine the periodicity of these pulsations, perhaps related to Kelvin-Helmholtz waves at the magnetopause.
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    Development of a computer controlled continuous wave ultraviolet-visible dye laser spectrometer
    (1979) Pollock, C. R. (Clifford R.); Tittel, Frank K.; Dunning, F. Barry; Rabson, Thomas A.
    The goal of this research was to design and develop a computer-controlled source of coherent, narrowline radiation, tunable in the ultraviolet range from 26 to 4 nm. This was accomplished by using a DEC LSI-11 minicomputer with CAMAC interface to control a single frequency dye laser, a second harmonic generation (SHG) crystal, beam directing optics, and diagnostic equipment. The computer system provided control of initialization, fine and coarse resolution scanning, wavelength calibration, and the acquisition and processing of data. Because of the number of interrelated adjustments necessary to tune a single-frequency ultraviolet source, computer control was needed for both convenient operation and optimum ultraviolet power. A new dye ring laser was designed for high single mode in the visible and ultraviolet regions. The motivation for this research came from the increasing need for a convenient, high-resolution, ultraviolet source of radiation for spectroscopic studies of molecules and atoms. This research has achieved i) a new level in resolution and power for CW ultraviolet radiation sources, and ii) has developed the software to conveniently control a CW visible-ultraviolet spectrometer.
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    Development of a computer controlled Fizeau wavemeter
    (1985) Gray, Damien Francis; Dunning, F. Barry; Jeys, Thomas H.; Mutchler, Gordon S.
    A Fizeau wavemeter bas been developed that has demonstrated an accuracy of 1 part in 1^6 across the visible spectrum and 5 parts in 1^7 across any 1 A interval. The present design is a modified version of that initially proposed by Gardner. The design was optimized by developing a computer model of the instrument. This model suggests that the theoretical accuracy limit for this type of wavemeter is on the order of 1 part in 1^8.
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    Differential cross sections for neutral-neutral and charge-transfer collisions
    (1985) Schafer, David Allen; Stebbings, Ronald F.; Dunning, F. Barry; Mutchler, Gordon S.
    Absolute differential cross sections for neutral-neutral and charge—transfer collisions involving fast oxygen are presented. Neutral-neutral collisions are measured over the energy range 5-5eV, and between angles of .1 degrees and 5. degrees, while charge transfer measurements are presented for 15eV oxygen ions over the same angular range. A position sensitive detector is used to detect both the primary and scattered particles, yielding absolute differential cross sections. Target chambers with small ratios of length to exit aperture diameter are used and the end effects due to gas streaming out of the aperture are estimated by a Monte Carlo computer simulation. The excited state composition of the oxygen ion beam is considered, and differential cross sections for charge transfer are presented for both ground and excited-state oxygen ions.
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    Dynamics and Dissociation of Collisionally Formed Heavy-Rydberg Ion-Pair States
    (2015-02-27) Wang, Changhao; Dunning, F. Barry; Killian, Thomas C; Brooks, Philip R
    Heavy-Rydberg ion-pair states, molecular quantum systems that are bound by their long-range pure Coulomb attraction, are formed through Rydberg atom collisions with molecules that attach low energy electrons. Collisions of potassium Rydberg atoms K(np) with the attaching species CCl4 and SF6 lead, respectively, to dissociative and non-dissociative electron attachment with subsequent formation of bound ion-pair states K+..Cl- and K+..SF6-. The experimental apparatus used to produce and detect heavy-Rydberg ion-pair states consists of a reaction region and a separate analysis region. It separates a fraction of the product ion pairs from the parent Rydberg atoms, eliminating spurious background signals associated with blackbody-induced and electric-field ionization of Rydberg atoms. A position sensitive detector in the analysis region is employed to measure their velocity, angular, and binding energy distribution and lifetimes. Measurements of the spatial distribution of ion pairs provide information on their velocity and angular distributions, and their binding energy distribution is measured using electric field-induced dissociation. The lifetime of the ion-pair states is influenced by multiple processes including internal-to-translational energy transfer, autodetachment of the electron, and neutralization through charge transfer. The experimental results are compared with the results of a Monte Carlo collision code that models both the initial Rydberg electron attachment and the subsequent evolution and molecular dynamics of the ion pairs. This model highlights the factors such as the kinematics of the Rydberg atom and the attaching particle and energy released in dissociation (in the case of dissociative attachment) that are important in governing ion pair formation. The model calculations are in good agreement with the experimental data.
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    Dynamics of Ion-Pair Formation in K(14p,20p)-SF6, CCl4 Collisions
    (2014-08-14) Kelley, Michael; Dunning, F. Barry; Killian, Thomas C.; Foster, Matthew
    Collisions between K(np) Rydberg atoms and electron attaching molecules can result in the creation of ion pairs through electron transfer. The mutual Coulomb attraction of these ions can lead to the formation of weakly-bound heavy-Rydberg ion-pair states orbiting at large radii. In this work, ion-pair states are created through collisions with SF6 and CCl4 in a small collision cell. These ion-pair states are allowed to drift into an analysis region where electric-field-induced dissociation allows for the velocity and binding energy distributions of these states to be determined. These distributions are analyzed through comparisons with the results of a Monte Carlo collision code. The data reveal that both CCl4 and SF6 form ion-pair states with long lifetimes and broad binding energy distributions, and their velocity distributions are strongly forward-peaked.
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    Electron spin polarization in low energy diffraction from Ni(001) and Ni(001)c (2x2) Te
    (1982) Jamison, Keith Douglas; Walters, G. King; Dunning, F. Barry; Estle, Thomas L.
    Combined measurements in low-energy electron diffraction of both intensity and electron spin polarization are described for Ni(1) and Ni(1)c(2x2)Te. Polarizations and intensities are determined in the energy ranges 2-1V and 25 to 2V, respectively. The beam has been measured at angles of incidence in the range 1° to 18° for azimuthal angles $ = ° and § = 45°. The 1/2 1/2 and 1 beams have been studied at normal incidence. Significant polarizations are observed, even for the low atomic number crystal Ni (1). Pronounced changes in the polarization profiles are induced by the addition of a tellurium overlayer.
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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.