R-3 Repository :: Browsing by Author "Walters, G. King"

Browsing by Author "Walters, G. King"

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3.4 centimeter cyrogenic parametric amplifier system for radio astronomy
(1970) Halpain, Joe Lee; Walters, G. King; Blackwell, L. A.
A low-noise microwave receiver was designed and fabricated to provide the necessary sensitivity to detect the helium 3 line. In order to achieve ultra-low noise amplification in the input states of this receiver, cascaded non-degenerate cryogenically cooled parametric amplifiers were used. These amplifiers were followed by a tunnel diode amplifier and a mixer-preamplifier which provides a total system gain of approximately 60 db. The parametric amplifiers were cooled to the cryogenic temperature by a closed-cycle gaseous helium refrigerator. The ambient operating temperature provided by the refrigerator was approximately 18°K. The parametric amplifiers were pumped with a Ka-band klystron which provides a signal-to-idler ratio of approximately 0.38. Gain stability was achieved by using waveguide ferrite current variable attenuators which were used to control the pump power as required to maintain a constant varactor current. The system was designed to consist of two major assemblies. One of the assemblies is packaged in a 30 x 30 x 60 inch weather proof enclosure for mounting at the focal point of the National Radio Astronomy Observatory 140 foot radio telescope. This enclosure includes a thermoelectric cooling and heating system to maintain a constant temperature inside the assembly under all weather condition. The second assembly was packaged in a standard relay rack type cabinet. This unit contains all of the receiver control, monitoring and special remote test circuits. The two assemblies were designed to operate with connecting cable lengths of up to 500 feet. During the first observing period, the noise temperature performance of the receiver was about three times that expected, i.e., 100°K. However, the stability of the system was sufficient to allow long integration periods so that usable data was obtained. To correct the noise temperature performance, the parametric amplifiers were redesigned, particularly in the area of thermal conductivity. In addition, the manufacturer and package style of the varactor was changed to terminate the high failure rate that had occurred in the diodes. In addition to the low-noise temperature and excellent gain stability achieved by this receiver, several new concepts were utilized in the design of the system which greatly improved its mechanical and thermal properties. One of these is a special non-contacting choke flange on the input line to the parametric amplifiers which allows the input line to be short and unrestricted in location. The system will be utilized again in the summer of 1970. The present performance of the system should provide data which is far superior to that obtained in the last observing period since for the same signal-to-noise ratio, the integration time should be reduced by a factor of four or more.
A high intensity polarized electron source with on-line polarization analysis
(1977) Hodge, Lee Allen; Walters, G. King
This thesis describes the design, construction and operation of an intense source of polarized electrons suitable for application to most proposed scattering experiments. Polarized electrons are produced in a flowing helium afterglow apparatus by chemiionization reactions involving optically pumped helium 2 S metastables. These electrons are electrostatically extracted from the flow tube and formed into a beam. The electron spin polarization is measured by a unique "on-line" Mott scattering polarization analyzer designed and constructed as an integral source component. In this thesis, applications for a polarized electron beam are reviewed and a comparison of the best available sources is made. The operation of the optically pumped flowing helium afterglow source is described in detail and current and polarization performance characteristics are reported: .1 pA at 5% polarization; 1 juA at 45%; 5 juA at 3% and 1 JJA at 2%. Measurements of the beam emittance show it to be very low (2 mrad-cm at 4 eV). Significant advantages of this source are the ease of electron spin reversal or modulation without a change in beam trajectory and the novel "on-line" Mott detector which allows simultaneous polarization analysis and low energy beam use. Several proposed experiments are suggested with a report on progress to date.
A method of polarization analysis of electrons from optically pumped He_
(1967) McCusker, Michael Vance; Walters, G. King
An experiment in progress to extract a polarized beam of electrons from an optically pumped helium source gas is described. Possible ionization mechanisms in helium gas are outlined and the methods of optical pumping are shown to provide a method for identifying electrons arising from those mechanisms involving the 23s1 metastable state. Calculations show that assuming conservation of spin, electrons extracted from the source gas should have an appreciable polarization. Apparatus for extraction of the electrons and for their acceleration to 120 KeV is described. The procedure for a Mott scattering analysis of the polarization of the electron beam is explained and a scattering chamber which has been constructed for this purpose is described and shown to be adequate for these measurements.
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.
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.
A study of local exchange in continuum processes
(1980) O'Connell, James Kevin; Lane, Neal F.; Walters, G. King; Huang, Huey W.
The role of one electron orbitals in Many Body Theory and the Generalized Sudden Approximation is reviewed. Various approximations to the exact Hartree Fock static exchange potentials are discussed and a new, screened exchange potential is derived. Phase shifts for the elastic scattering of electrons from Helium, Neon and Argon and Photoionization cross sections for Lithium, Carbon, Oxygen, Neon and Argon were calculated and compared with the exact static exchange results. The screened exchange potential is the best approximation for Photoionization and the Hara exchange potential is the best for Scattering. Ground state eigenvalues, ionization energies and oscillator strengths have been calculated for the 2t2-la1 transition in Methane. The calculations were carried out using the Xalpha Multiple Scattering Method. The ground state eigenvalues were correctly predicted and the oscillator strengths lie within experimental limits.
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.
Dynamic polarization in polymerized alkynes
(1964) McAdams, Harley Hobbard; Walters, G. King
A measurement Is reported of the proton polarization achieved in a polymer sample through the solid effect. The sample is a polymerized alkyne; the paramagnetism arises from free radicals trapped in the polymer structure. An enhancement factor of 12 was observed at 1.8°K. Growth and decay of the polarization was observed to consist of one component with time constant less than 1 second, another with time constant of 8 seconds.
Dynamics of conversion of atomic helium(triplet-2S) atoms to molecular helium(odd triplet-a-sigma-plus) molecules in ternary collisions (helium)
(1990) Zhao, Xin-xin; Walters, G. King
The temperature dependence of conversion of He(2$\sp3$S$\sb1$) metastable atoms to He$\sb2$(a$\sp3\Sigma\sp+\sb{\rm u}$) metastable molecules in the three-body reaction$$\rm He(2\sp3S\sb1)+2He(1\sp1S\sb0) \to He\sb2(a\sp3\Sigma\sp+\sb{u})+He(1\sp1S\sb0)$$has been investigated over the temperature range 65K-700K. This reaction is thermally activated as a consequence of a long range repulsive barrier in the He(2$\sp3$S$\sb1)$-He(1$\sp1$S$\sb0$) interaction potential. The data reveal that there are two reaction channels with distinctly different activation energies. The temperature dependence of the measured rate coefficient $\delta$(T) is accurately described by$$\rm \delta(T) = \lbrack 87.4\ T\ exp({-}750/T){+}4.1T\ exp({-}200/T)\rbrack \times 10\sp{-37}\ cm\sp{-6}sec\sp{-1}.$$The first activation energy, 750 $\pm$ 70K(63 $\pm$ 6meV), is equal to the known He(2$\sp3$S$\sb1)$-He(1$\sp1$S$\sb0$) repulsive barrier height. The second activation energy is 17 $\pm$ 2 meV.
Effects of adsorbates and temperature on electron spin polarization in low energy electron diffraction from tungsten (001)
(1978) Riddle, Thomas Wayne; Walters, G. King
The electron spin polarization and intensity of the specularly reflected () low energy electron diffraction (LEED) beam at an angle of incidence = 13° have been measured as a function of up to about 1 Langmuir of exposure to O2 and CO. Data have been acquired in the energy range 26-141 eV at 5 volt intervals except near sharp features in the polarization versus energy profile where smaller intervals were used. Data are presented in the form of polarization versus energy and intensity versus energy profiles for four selected exposures in the range measured. The sensitivity of polarization to exposure to the gases CO and 2 is comparable to the sensitivity of intensity. Polarization was found particularly sensitive to exposure in one small energy interval. The polarization as the intensity was found not to always monotonically decrease in magnitude with increasing exposure but may show more interesting behavior. A tendency for the complementary measurement to remain constant at energies where the other varies rapidly with exposure was observed. Differences for both polarization and intensity for the two gases are noticeable but not dramatic. The temperature dependence of electron spin polarization and intensity was also measured for this same clean surface over the range 1-3°C in the energy interval 5-11 ev, which includes the polarization versus energy profile. The effect of temperature on polarization was found to be greatest at energies for which the intensity is low and the polarization, and rate of change of polarization with energy, is large. The results may be Interpreted in terns of a shift of polarization features toward lower energies as a result of thermal expansion when the crystal is heated.
Electron beam excitation studies of helium
(1967) Dennis, Wiley Sam; Walters, G. King
A comparison is made of relative number densities of excited states produced in Helium gas excited by an RF discharge and by the high energy (200kev, maximum) electron beam, it being found that the 33D states are relatively much more numerous in the discharge; a likely explanation is that free electrons in the discharge are energetic enough (~35ev), due to energy supplied by the RF field, to excite 3 JD states directly, while beam electrons and secondary electrons which they eject have, respectively, too much and too little energy to produce such states. Since triplet states cannot be produced directly by the high energy beam, the mechanism of their production is investigated. Electron-ion recombination is eliminated due to lack of a recombination spectrum and due to an observed linear dependence of triplet line intensities on beam current. Excitation by ejected secondary electrons is eliminated on the basis of a calculated cross section for the process of 10-28 cm2 or less. Excitation Transfer from singlet states is concluded to produce the triplets; pressure dependence of the 5016A (3 P-2 S) line is found to follow closely the predictions of the Two Level Transfer Model. A modification of this model which takes account of the fact that triplet states are not produced directly by the beam allows the prediction of the pressure dependence of the 5876A (3 D—2 P) line. Certain technical questions are investigated. It is found that light output of the chamber is more than ample (160kev, 0.02 m.a.) and that x-ray induced noise in the EMI photomultiplier is not excessive. The 0.000125 inch Havar foils used are found to carry 0.3 m.a. at 160kev. The beam is found to be scattered by the foil and then by the A1 back plate of the chamber, causing electrons to strike the walls of the chamber and the glass to fluoresce and become brown tinted with use. A modified chamber design to rectify this situation is suggested. The possible uses of the technique of high energy electron beam excitation of gases and liquids are discussed. Mobility studies of ions and excited species in gases and liquids should be possible. The fact that the high energy beam excites only those states with the same spin multiplicity as the ground state (singlets in Helium) should facilitate the measurement of excitation transfer cross sections between states of different multiplicity. Finally, the process of optical pumping of Helium and the uses of beam excitation therein are dismissed. The relaxing effect of the beam’s magnetic field gradient upon the optically polarized He3 nuclei is investigated, and it is concluded that the application of a small external field will be necessary but that such a field should not affect the beam optics substantially.
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.
Electron spin polarization in low energy electron diffraction
(1975) Kalisvaart, Maarten; Walters, G. King
Substantial electron spin polarization has been observed in low energy electron diffraction (LEED) from the (1) surface of tungsten. The electron polarization is determined by directing a fraction of the elastically diffracted electrons contained within a given LEED beam into a Mott analysis system. An overview of the theoretical background of LEED is given with emphasis on the formalisms developed to calculate the intensities of the beams. The theoretical predictions of electron spin polarization in LEED are reviewed. A short description is given of the experimental technique, and the results of both polarization and intensity measurements on the beam are presented. Polarizations ranging from -35% to +37% have been observed and are shown to depend sensitively on both electron energy and angle of incidence. No similarity was found between the observed LEED polarizations and those predicted theoretically for elastic scattering from free tungsten atoms.
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.
Energy transfer reactions in electron beam excited mixtures of xenon and argon gases
(1976) Bonnifield, Thomas Dyer; Walters, G. King
The time dependence of the vacuum ultraviolet emissions from mixtures of xenon and argon gases is measured following excitation by a low intensity electron beam. The xenon concentration is varied from 8.4 x 113 cm-3 to 2.76 x 1 cm while the argon density is varied between 5.1 x 1 cm and 2.9 x 1 cm . The second molecular continuum of argon is observed to be quenched in electronic excitation transfer to xenon atoms forming the xenon state with a rate constant of (4.39 - .5) x 1 cm /sec. At the higher xenon concentrations the second molecular continuum of xenon is formed by destruction of xenon atoms in collisions with a ground state xenon atom and a ground state argon atom at the rate of (2.15 -+. 25)-x312 cm /sec. The first molecular continuum of xenon is observed to be collision induced radiation from the xenon P level as well as radiation from high vibrational levels of the Ou molecule of xenon. The collision induced radiation rate constant is found to be (3.2-.7) x 1 cm/sec. The rate of formation of xenon molecules radiating in the first continuum is (2.1+- .2) x 1-31c m6 /sec. Collisional de-excitation of the xenon P^ level occurs with a rate constant of (1.5 +- . 3)-x114 c3m /sec.
Extraction of a helium ion beam from an optically polarized helium source gas
(1965) Stockwell, Norman David; Walters, G. King
Optical pumping techniques can be used to produce nuclear polarization in He3 gas excited by a weak electric discharge. By optically polarizing He3 to a polarization P0 and extracting ions from the gas, the resulting ion beam is expected to retain a polarization between P0 and P0 /2 depending on the experimental conditions. Such a polarized beam would have great utility in nuclear physics. An ion source using this principle has been built by straight-forward modifications of normal rf ion-source geometry to provide the higher pressure and lower flow rate needed for the optical pumping. The dependences of ion current upon source gas pressure and cleanli-ness, electrical discharge intensity, the axial magnetic field, and the extraction voltage are measured. He4 rather than He3 has been used to date and ion-beam polarization cannot be measured with the present apparatus. However, under discharge and flow conditions which appear suitable for He3 polarization, beam current of 1 /ua are attained. Methods for measuring beam polarization are proposed. Some future experiments in atomic physics are considered.
Formation and decay of argon and xenon molecules radiating in the vacuum ultraviolet
(1975) Gleason, Robert E.; Walters, G. King
The time dependence of the vacuum ultraviolet emission from dense (1-3 atm) argon and xenon is measured following excitation by a low intensity electron beam. Independent radiation is observed from the lowest excited 1Q and + states of A^ and Xej. The radiative lifetimes of these states have been determined to be 4,2 + .2 nsec (*) and 5. + .5 ^sec (lu) for argon and 5. + 1 nsec (U) and 96 + 5 nsec (lu) for xenon. The time dependence of the AZ>2 U emission indicates that the state is formed in three body collisions with a rate coefficient of 2.8 + .3 x 1 cm /sec. Because of interference from the emission from the 1Q state, a precise determination of the rate of formation of the Xe2 U state is not possible. By considering the pressure dependence of both the decay time and the intensity of the emission from Xe2 *, the production rate of the state has been estimated to be approximately 2 x 1 cra^/sec. Under the excitation conditions of this experiment, the emission from the U state was found to be about 1 - 15Z of that from lu for both gases at pressures of a few atmospheres. The ratio of + to lu emission decreases at lower pressures.
FT-ICR studies of giant carbon fullerenes
(1992) Lee, Maggie Yeuk Mui; Walters, G. King
FT-ICR studies of high mass $\rm (C\sb{>150})$ carbon clusters have brought insight to the controversial structures of carbon fullerenes. Laser vaporization followed by supersonic beam technique produced carbon clusters that directly injected into the bore tube of a 6 Tesla magnet. Mass spectra of the trapped cluster ions reveal the presence of only even-numbered clusters in the low mass regions, thus verifying the predominance of graphite closed shells. It is believed that the larger clusters also exist as aggregates loosely bound to each other on their surfaces and will readily disaggregate upon evaporation. When clusters of size C$\sb{300}$ or larger are fragmented by excimer laser, the products are mostly even-numbered ions resulting from C$\sb2$ loss which satisfy the retention of a fullerenes structure. In addition, there are distributions of multiply-charged high mass positive clusters detected as the photofragments undergo thermionic emission. This fragmentation experiment is currently used to compare the hypotheses of (1) giant fullerenes model where carbon condenses to form empty hollow cages of large radii, (2) aggregation of small clusters by van der Waals forces to form high mass clusters, and (3) "Russian-egg" model where high mass clusters exist as concentric closed shells.
Investigations of surface electronic structure and surface-atom interactions with spin-polarized metastable atom de-excitation spectroscopy
(1989) Hammond, Mark Stuart; Walters, G. King
Metastable Atom De-excitation Spectroscopy (MDS) provides a powerful technique with which to investigate surface electronic structure with unparalleled surface specificity. In this technique a thermal energy beam of noble-gas metastable atoms is directed at the surface to be studied 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 on the incident metastable atoms are polarized and the spin-polarization of the ejected electrons is measured with a Mott polarimeter. Energy-integrated electron spin-polarization (ESP) measurements for Pd(110) and energy-resolved ESP measurements for Cu(100) surfaces under a variety of conditions indicate that metastable atom-surface interactions are more complex than has been assumed in past MDS studies. Spin-polarized MDS has also proven useful in the study of surface magnetism. The present work includes a study of the surface electronic and magnetic structure of the Fe(110) surface. Results indicate an interesting reversal of surface electron spin-polarization for Fe(110) surfaces exposed to oxygen.
Laser annealing of Ni (001)
(1983) McConnell, Robert P.; Walters, G. King; Dunning, F. Barry; Huang, Huey W.
Experimental aspects of laser cleaning and annealing of a Ni(1) surface with a pulsed ruby laser are reported. Effects of applying laser energy densities of .4 J/cm to 1.1 J/cm to an Ar-ion sputter-cleaned surface are determined using Auger Electron Spectroscopy (AES), Low Energy Electron Diffraction (LEED), and Polarized LEED (PLEED) as surface diagnostics. AES profiles, LEED Intensity v. Voltage (I-V) curves, and PLEED Polarization v. Voltage (P-V) curves taken on the laser annealed surface are compared to identical information for the thermally annealed surface. Effects of substrate temperature on laser annealing results are investigated. It is concluded that laser-annealing yields clean, well ordered surfaces for elevated substrate temperature, but not at room temperature.

Browsing by Author "Walters, G. King"

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