Positron annihilation in copper, silver and indium has been studied by means of the Doppler broadening method. High resolution germanium detectors have been used to observe the changes in the annihilation line profiles due to the trapping of positrons by metallic defects. Positron trapping by thermally generated vacancies have been investigated. The smaller temperature effects which occur in addition to the dominant vacancy trapping effect have been discussed and taken into account in deducing mono-vacancy formation energies. The plausible hypothesis of metastable seIf-trapping of positrons has also been discussed. The best values of the mono-vacancy formation energies are estimated to be 1.26 eV for copper and 0.95 eV for silver. For indium a precise value could not be deduced from the data. From the analysis of the line shapes, the probabilities of positron annihilation with core and conduction electrons at different temperatures have been calculated. The results of the measurements on plastically deformed copper indicate that the specific trapping rate of positrons by dislocations is constant in the range 77-293 K. On extending the range of measurements down to 4.2 K, a significant decline in the specific trapping rate has been observed. Alsco included is a chapter on the annihilation of positrons in liquid helium-3 and helium-4. Analysis of the line shapes as well as the deconvolution of the spectra indicate no significant difference in the amount of Positronium formation in liquid helium-3 and helium-4.

A selection of free-standing fine-wire grids with different wire spacings have been wound for use as spectroscopic components at millimetre and submillimetre wavelengths. The performance of the grids has been investigated extensively and found to be in good overall agreement, both qualitatively and quantitatively, with calculations made using a least-squares method developed by Beunen. Where discrepancies occur they are attributed to the effect of uneven spacing of the wires. A microcomputing system based on the Z80 8-bit microprocessor has been built to provide data acquisition and Fourier transformation for a Fourier spectrometer. Measurements have been made of the far infrared optical constants of KC1 and KBr crystals by dispersive Fourier transform spectroscopy at temperatures in the range 7-500K using instruments equipped with both mylar and wire grid beam dividers. The results have been used to calculate the dielectric functions and the an harmonic self energy functions of the q = 0 transverse optic modes in these crystals. The frequencies of the transversive optic (TO) and longitudinal optic (LO) modes determined from the dielectric functions are in good agreement with the accepted values, and in the case of KBr, the frequency-dependence of the an harmonic self-energy functions determined at room temperature is in good agreement with calculations made by Bruce.

A quartz crystal resonator has been used to excite shear waves at a frequency of 20.5 MHz in He films above and below the superfluid transition and just above the liquid-gas critical point. The wave has a viscous penetration depth 6-20 nm and the transverse acoustic impedance Z = R - iX of the film was found from changes in the quality factor and resonant frequency of the crystal. The thickness of a He I film was swept at constant temperature by creating a small temperature difference between the He film on the crystal and bulk liquid helium below it. Calculations of the impedance of a homogeneous film as a function of d/6 using transmission line theory show the film's thickness d could be swept from 1.5 to >60 nm . The impedance of six superfluid films of constant thickness in the range 14-23 nm has been measured for 0.4 T T . From the impedance in the ballistic limit, wI >> 1, the average probability of the quantum evaporation of a ⁴He atom by a roton incident upon the liquid-vapourinterface is estimated to be ~0.35 . A resonance, the temperature of which was dependent on film thickness, was observed in the superfluid film and has tentatively been identified with the resonance in the A/4 Kelvin mode of vortices pinned to the crystal surface. The transverse acoustic impedance Z of helium has also been measured 49 mK above the liquid-gas critical point for pressures up to 2000 torr. In the highly compressible critical region, the impedance shows the effects of the large density gradients that develop close to the crystal surface under van der Waals1 forces. At low pressures, the transition to non-hydrodynamic behaviour is observed, and it is estimated that a fraction 0.2 of He atoms incident upon the crystal are diffusely scattered from it.

This work investigates the use of the perovskite-type compound La1-xSrxCoO3 (x = 0.3, 0.5, 0.6, 0.7) as a catalyst for the reaction SO2 + 2CO → 1/x Sx + 2CO2. Adsorption of SO2 and CO separately on the degassed catalysts at several temperatures, was followed by TPD (temperature programmed desorption), with mass spectroscopic detection of the desorbed species. Trends in the mass changes on adsorption are explained in terms of the changes in reducibility of La1-xSrxCoO3 on changing x. Computation of the SO2 TPD results, which showed two and sometimes three adsorbed species, yielded desorption energies of the range 30-200 kJ mol⁻¹. Several SO2 and CO mixtures were allowed to flow over La1-xSrxCo03 at various temperatures and flow rates. The removal of SO2 and production of CO2 were followed using gas chromatography and sulphur was collected in a cold trap. LaO.7Srg 3COO3 at 550°C removed SO2 from a stoichiometric mixture of SO2 and CO very efficiently without producing COS. COS production, if it occurs, may be problematical in a commercial SO2 removal application. XRD (X-ray diffraction) revealed that the fresh compounds all had the perovskite-type structure, those exposed to either SO2 or CO retained this structure and those exposed to SO2 and CO together were no longer La, Srx CoO3 XRD of the used catalysts showed theI -x X 3)presence of SrSO4 on those exposed to only SO2 and the presence of sulphides of La, Sr and Co, on those exposed to SO2 and CO. XPS (X-ray photoelectron spectroscopy) of catalysts exposed to SO2 and CO indicated sulphidation, with some sulphate species also present. The results may be explained in terms of the relative oxidizibilitiesof the SO2, CO, SO2 reduced catalyst, 'CO reduced catalyst', 'reactionreduced catalyst' and La1-x Sr Co031x x 3The continuation of the operation of the material for the removal of SO2 suggests that in this instance the active catalyst is no longer a perovskite-type oxide, but possibly a metal sulphide.

A number of monomeric and oligomeric hydroxypyridone mono- and poly-methine bridged oxonol and merocyanine dyes have been prepared and their solution and solid-state properties have been investigated. One particular and commercially important feature of the solution chemistry of the oxonol dyes is the reaction with sulphite ion leading to bleaching of the dye. The mechanism of this process has not previously been investigated. A detailed kinetic and spectroscopic study supports the previously proposed mechanism in which the nucleophile adds to the methane bridge in a Michael-type fashion. The reaction intermediates have been detected spectroscopically for several nucleophiles but in general are too unstable to be isolated. Oxonol and merocyanine dyes readily undergo molecular aggregation in both solution and the solid-state. One previous X-ray diffraction analysis of an oxonol dye has been carried out which revealed that the anions formed planar molecular stacks. Such an arrangement is well suited to intermolecular charge transfer and hence electronic conduction. The main objective of the current work has been to investigate how this stacking might be influenced by variation of substituents and counter-ions with a view to modifying the conduction properties. X-ray structure studies reveal that variation of the cation leads to three different types of solid-state structure: herringbone; uniform; and dimeric. The electrical properties have been investigated in detail by d.c. conductivity and a.c. dielectric spectroscopy measurements. They suggest that in general, the simple hydroxypyridone trimethine oxonols are semi-insulating (10⁻⁸ < δ < 10⁻¹² Ω⁻¹ cm⁻¹) though some exceptions have been found. Any observable d.c. conduction is essentially ionic in nature. The merocyanines and pentamethine oxonols are semi-conducting (10⁻⁴ < δ < 10⁻⁸ Ω⁻¹ cm⁻¹) as a consequence of added electronic contributions to the conduction mechanism. The most highly conducting oxonol salt, containing the tetrathiafulvalene (TTF) radical cation has a d.c. conductivity, δ ≃ 10⁻³ Ω⁻¹ cm⁻¹ (compacted powder, room temperature). In this case electronic conduction dominates.

A microprocessor control system for automatic data acquisition in a photon spectroscopy system was developed. The control process performed Doppler broadening measurements over the temperature range 4.2-600K with simultaneous recording when necessary of pressure. After work on the chalcogenide semiconductor (Pb,Ge)Te, Doppler broadening studies concentrated on the phenomenon of gas condensation on cooled graphite substrates. This class of adsorbent, of proven homogeneity was found ideal to explore the interaction of positrons at surfaces. By measurement of positronium, adlayer formation of methane, krypton, argon, nitrogen and oxygen was followed. All gases produced peaks in positronium emission with an investigation of the substrate further revealing an underlying thermal variation. Sound evidence in the form of accurate simultaneous pressure measurements and isotherms was found for a coverage of one half at the maximum in positronium. These studies of gas physisorption were conducted under a variety of temperatures and with pressures embracing atmospheric to under one Torr. Although results for the gases were of the same form each produced different maximum levels of positronium which appeared to relate to substrate-adsorbate binding energy. Maximum emission in the case of argon showed a temperature dependence however this requires verification. Oxygen was a special case as it yielded 12[percent] para-positronium at half-coverage. The coupling of this to a low 1.5[percent] ortho-positronium emission was attributed to chemical quenching. A convolution procedure based on the superposition of an additional para-positronium derived narrow gaussian to the usual gaussian and inverted parabola components was employed for an estimate of the positronium energy. This was used to attack the question of whether positronium is bound at the surface or is created flying.

Existing techniques have been reviewed for the production of thin electron-transparent films of alkali halides for study by electron microscopy and a new technique has been developed. A novel apparatus has been designed and built by means of which samples prepared from the vapour phase in high vacuum may be transported to, and examined in an electron microscope without exposure to the atmosphere. Special specimen holders and a couple of heaters have been designed so that the effects of varying temperatures on the growth and subsequent behaviour of alkali halide crystals may be studied. Other factors examined are the pressure during growth, the effects of atmospheric moisture after growth and the electron irradiation in the electron microscope. The surface of the alkali halide crystals have been examined by decoration with evaporated gold and subsequent examination in the electron microscope and their crystallographic structures investigated by electron diffraction. Thicknesses of the crystals have been measured by extinction contour technique. The principal materials studied are CsBr, CsCl, CsF and Csl, as over growth on electron-transparent crystals of Csl and on two different types of mica: muscovite and biotite. These are the first studies of alkali halides on Csl and on biotite. The main results of investigations are: 1. CsI may be prepared as a flat, electron-transparent single crystal suitable for electron microscopic studies in situ. 2. Small crystallites are produced by vapour deposition of alkali halides in the early stages and after exposure to the atmosphere large single crystals are formed. 3. CsBr, CsCl, CsF and CsI grow epitaxially on Csl at room temperature. 4. CsF may grow in bcc form in thin layers in certain circumstances, instead of the usual feeform in bulk. 5. No significant difference was observed in the growth of alkali halides on muscovite and biotite.

<p><!-- Fragment document type declaration subset: ArborText, Inc., 1988-1999, v.4002 --> <formattedtext>The Role of TrkB and BDNF in Governing the Steroid-Mediated Regulation of Male Copulatory Behavior in the Syrian Hamster (<i>Mesocricetus auratus</i>)</formattedtext>

<p>Spin coherence is important for the fields of electron spin resonance (ESR), nuclear magnetic resonance (NMR), magnetic resonance imaging (MRI), and quantum devices. For donor spins in silicon, coherence both quantifies their potential as qubits and measures environmental processes. By understanding those processes, we can construct experiments which remove them to obtain longer coherence times. Silicon crystals are uniquely suited to this task, benefiting from decades of advancements in purification. The two most well-known decoherence mechanisms for donors in silicon are 29Si atoms and the donor spins themselves. Although well studied for electron spins, these mechanisms are less understood for nuclear spins. Using crystals with controlled concentrations of 29Si and 31P donors, I evaluate the limitations imposed on 31P nuclear spins. I find that nuclear spin echo decay times vary linearly with 29Si concentration. The non-exponential decays shown here establish a range of 29Si flip-flop rates, with some being fast compared to experiment timescales and others being slow. Furthermore, when compared to measurements of ionized nuclear spins, the echo decays here imply a "frozen core" picture where the donor electron spin protects the nuclear spin by detuning neighboring 29Si atoms. In studying spin coherence relative to 31P concentration, I find that nuclear spin echo experiments can measure donor electron spin flip-flop rates. A stochastic model reproduces the experiments by fitting a local Zeeman frequency linewidth. However, experiments in more lightly doped crystals (<10</p><p>15 P/cm</p><p>3) suggest that coherence is not limited by flip-flops. The source of decoherence in these crystals is unknown, but the experiments serve as an upper bound on electric field noise. Magnetic field fluctuations are known to obstruct spin coherence measurements. One popular method for removing that noise is dynamical decoupling via repeated pi rotations. However, these sequences also elongate echo decays for ensemble spins known to be decohered instead by instantaneous diffusion. This result suggested that cumulative rotation errors might artificially inflate decay times. After demonstrating that such effects were insignificant in practice, I find instead that pi rotation errors reduce the dipole-dipole coupling between spins.

abstract: Understanding the interplay between the electrical and mechanical properties of single molecules is of fundamental importance for molecular electronics. The sensitivity of charge transport to mechanical fluctuations is a key problem in developing long lasting molecular devices. Furthermore, harnessing this response to mechanical perturbation, molecular devices which can be mechanically gated can be developed. This thesis demonstrates three examples of the unique electromechanical properties of single molecules. First, the electromechanical properties of 1,4-benzenedithiol molecular junctions are investigate. Counterintuitively, the conductance of this molecule is found to increase by more than an order of magnitude when stretched. This conductance increase is found to be reversible when the molecular junction is compressed. The current-voltage, conductance-voltage and inelastic electron tunneling spectroscopy characteristics are used to attribute the conductance increase to a strain-induced shift in the frontier molecular orbital relative to the electrode Fermi level, leading to resonant enhancement in the conductance. Next, the effect of stretching-induced structural changes on charge transport in DNA molecules is studied. The conductance of single DNA molecules with lengths varying from 6 to 26 base pairs is measured and found to follow a hopping transport mechanism. The conductance of DNA molecules is highly sensitive to mechanical stretching, showing an abrupt decrease in conductance at surprisingly short stretching distances, with weak dependence on DNA length. This abrupt conductance decrease is attributed to force-induced breaking of hydrogen bonds in the base pairs at the end of the DNA sequence. Finally, the effect of small mechanical modulation of the base separation on DNA conductance is investigated. The sensitivity of conductance to mechanical modulation is studied for molecules of different sequence and length. Sequences with purine-purine stacking are found to be more responsive to modulation than purine-pyrimidine sequences. This sensitivity is attributed to the perturbation of &pi-&pi stacking interactions and resulting effects on the activation energy and electronic coupling for the end base pairs. / Dissertation/Thesis / Doctoral Dissertation Physics 2014