Fluorescence spectroscopy is a powerful tool for obtaining information on microscopic processes in discharge (corona) plasmas in dense media, such as high pressure supercritical gases and even liquids. Spectroscopic observations of the light emitted from an ionization zone near a tip electrode can be used to determine structural information of the local environment of the emitting atoms or molecules. The spectra observable are sensitive to the immediate surroundings of the emitting species, making them useful for the study of cold non-equilibrium plasmas at varying pressures and temperatures. The aim of this project is to further our understanding of helium (4He) in different phases, providing information to help understand the nature of the interaction between the helium atoms within this environment. In this work, spectra have been recorded for a corona discharge in gaseous and liquid helium. The experimental conditions covered a wide region of thermodynamic states in two different designs. The first experiment was carried out at cryogenic temperatures at 3.8, 4.0, 4.5 and 5.0 K under a range of pressure (0.1-5.0 bar). The second set of experiments were carried out at room temperature with range of pressures 0.1-30 bar. Several thousand spectra were recorded in the visible and near-infrared regions. Analyses were conducted on spectra including the atomic 3s 1S→2p 1P and 3s 3S→2p 3P transitions. These transitions showed line shifts, spectral broadening and intensity changes that were dependent on the magnitude of the pressure and temperature, and therefore on the thermodynamic phase. In addition to atomic lines, rotationally-resolved molecular emission bands arising from the δ3Σu+→b3Πg and D1Σu+→BΠg transitions of the electrically excited helium dimer (excimer), He2∗, were recorded. Remarkably, and similarly to the analysis of atomic lines, the molecular (excimer) emission showed sharp and distinct lines at low temperature in the region below the saturated vapor pressure (SVP) of helium. However, lineshifts, linewidths and line intensities of the excimers increased strongly upon an increase in pressure, indicating that solvated He2∗ in the liquid phase exhibits hindered rotation. The rotation of excimers within the liquid phase was attributed to the location of the excimer within a large bubble. A further interesting observation was the abrupt changes of lineshift and linewidths with pressures that occurred by crossing the SVP curve for both atomic and molecular transitions. A model was developed to explain this in which clusters form around excimers and excited helium atoms before the helium liquefies. In a separate application of the corona discharge, it was employed to make plasmas from a mixture of pure air and perfume. The idea here was collecting spectra of this mixture with and without helium to explore and understand some of the emitted species generated in this mixture of plasmas at atmospheric pressure and to the first time.

The design and construction of a vacuum reflectometer for the study of the total reflection of soft X-rays is described. The instrument, which uses a plane mica monochromator and a proportional counter detector, was used to obtain curves of reflecting power versus glancing angle for a number of characteristic wavelengths in a region previously neglected by experimenters. New data is given for reflection by optical flats of glass and stainless steel. These results are used to test (a) the Fresnel equations for reflection, modified to take account of absorption and (b) the theories of X-ray dispersion. From the discrepancies between theory and experiment, conclusions are drawn regarding the surface structure of the specimens. Some results are also given for evaporated films of copper and ytterbium fluoride. From the latter, it would seem that the Drude-Lorentz dispersion theory holds near an X-ray absorption line.

In this thesis data from the Sweden And Britain Radar Experiment is examined in a statistical investigation of the variation of the radar backscatter amplitude due to variations in the ionospheric convection. The response of the radar backscatter is examined over an eight year interval from 1982 until 1989 and a strong dependence of the variation of the backscatter amplitude on flow direction and magnitude determined during the solar cycle. The results are discussed with reference to three geophysical parameters which control the relationship between the backscatter intensity and Doppler velocity, namely the ambient electron density, the electron density fluctuation amplitude and the dimensions of the scattering layer. Data from the SABRE system is further utilised in two studies involving three distinct types of transient perturbations of the ionospheric convection pattern. Substorm Associated Radar Auroral Surges (SARAS) are examined in a statistical manner to obtain a more complete description of the feature. The results of this examination are utilised in a discussion of a possible magnetospheric source and generation mechanism for SARAS. The auroral radar signature of an o band is characterised with the aid of the complementary and coincident information provided by Viking UV images. Characterisation of the VHF radar signature resulting from the passage of this o band structure allows a link between a second o band and a radar flow burst signature to be made. It is suggested that a flow burst signature is observed coincident with the western (trailing edge) of the luminous region of an eastward drifting o band. A preliminary investigation of the field aligned current systems associated with the o band structure is also presented, the results of which compared favourably with models pertaining to the electrodynamics of an o band system.

No description available.

Following a brief introduction, including a background of X-ray Astronomical studies, basic crystal structure concepts, and X-ray reflection theory, practical methods of measuring the three principal spectrometric parameters are discussed. Of the three parameters, the diffraction profile F.W.H.M. (WC), the peak reflection coefficient (P), and the integrated reflection coefficient (RC) the latter is considered the most important in practice. Several methods (both single crystal and two crystal) by which unambiguous measurements of R can be obtained are developed and described. Particular attention is paid to the form of the correction factor relating the two crystal integrated coefficient (RCC) to the single crystal parameter. Measurements of the three principal spectrometric parameters as functions of wavelength for three of the acid phthalates are described and the results compared with values computed from the Perfect Crystal and Ideal Mosaic models. Observations of a previously reported enhancement in RC at the OK absorption edge wavelength in K.A.P. are confirmed, and new observations of a similar phenomenon in R.A.P. and A.A.P. are described. In addition, an anomalous reflection at the wavelength of the CK edge by the 001 planes of A.A.P. is reported. An explanation in terms of anomalous dispersion theory is given which explains the observations at least semi-quantitatively. The problem of adequately testing thetheory quantitatively is discussed. A catalogue of the three principal parameters as functions of wavelength for Gypsum (020), S.H.A. (110), A.D.P. (101), E.D.d.T. (020), and P.E.T. (002), is given and both experimental and theoretical considerations are discussed. Computed values for Pyrolytic Graphite (002), LiF (200), and LiH (200), are also included. The shortcomings of the work are discussed and the author's recommendations for possible continuations of the studies are noted.

Using a high-resolution curved crystal spectrometer of the Johann type with a fine focus x-ray source, the Mv'Iv absorption spectra of metallic cerium, cerium silicide, cerium fluoride, cerium oxide and xenon have been presented. Using the same spectrometer with a normal, point emission source, the Malpha, beta emission spectrum of cerium oxide has also been examined. For metallic cerium, by comparing the MV and MIV absorption lines obtained at 22 K with those obtained at room temperature, the mechanism of the y > alpha phase transition in cerium has been discussed. Cerium oxide was found to exhibit an anomalous MV, IV absorption spectrum. Two extra peaks, one connected with MV and the other with MIV have been observed to occur in the spectrum, A suggestion has been made to account for these features. Measurements on the MV, IV absorption spectra of oxidized metallic absorbers have indicated that cerium oxide (CeO2) results when the metal is oxidized in air, but oxidation in oxygen results in either Ce2O3 or a metallic sample covered with a protective layer of Ce2O 3. The measurement of MV, IV absorption in solid xenon has been performed and the resulting spectrum has been compared with that for the gas. The spectrum is dominated by two delayed, strong peaks arising from the transitions 3d > ef as for the gas. In the solid, in the region of strong peaks there are also observed remarkable modulations in the absorption coefficient which are discussed as EXAFS.

The structures of molten nickel chloride, nickel bromide and nickel iodide have been investigated by applying the technique of neutron diffraction to isotopically enriched samples. The three partial structure factors and corresponding radial distribution functions which characterize the structure have been determined from the experimental data. Both steady state and pulsed neutron sources have been used for the diffraction experiments; the latter employing time-of-flight techniques. A high temperature furnace was designed for the pulsed neutron experiments and this is described. The steady state experiments used existing furnaces. The data analysis and correction procedures for both types of experiment are discussed. In conjunction with earlier studies of molten divalent chlorides, the results show that whilst the gross structural features of these 2:1 molten salts are consistent with a rigid ion model, the electronic structure of the cation appears to play an important role in determining the detailed structure. The results emphasize the importance of ionicity and charge transfer effects and the need to take these into account in model interaction potentials. A model, based on observations of the solid state, and consistent with the results presented in this thesis is discussed in which the net transfer of charge is proportional to the electronegativity of the anion. The nickel ion in molten nickel iodide appears to be considerably more mobile than in the other nickel halides. Some suggestions are made for further work to determine the mobility of the nickel ion and to test the proposed model for these systems.

This thesis presents the results of a study of the characteristics and the origins of the background components in the ROSAT Wide-Field Camera. Contributions from six distinct sources have been positively identified: 1) cosmic rays and other high-energy (E ~ 1 MeV) charged particles, 2) low-energy (E ~ 50keV) electrons in the auroral zones, 3) a spacecraft glow phenomenon, similar to that observed on the Space Shuttle, but confined to the far-ultraviolet regime, 4) resonantly scattered Solar EUV line photons, for example He II 304A, 5) a charged particle component from an unidentified source (possible ring current precipitation), and 6) intrinsic noise in the WFC microchannel plate detectors. In addition, the results of a search for positively idenfitiable signatures from the diffuse XUV background are presented. No features have been identified in the background, which places a strong upper limit on the diffuse flux which is nearly an order of magnitude lower than that implied from previous surveys in this waveband. This finding has serious implications for the displacement model of the local interstellar medium. An analysis of the WFC images of the Vela and Cygnus Loop supernova remnants is also presented. The WFC survey data from the Vela SNR are combined with data taken with the ROSAT PSPC in a spatially resolved joint spectral fitting effort, work which is made possible only by the quality of the PSPC data. The results show for the first time that two gas phases are present at distinct temperatures with differing spatial distributions, and that the variations in the X-ray temperature across the remnant inferred in previous (single temperature) analyses are probably due to changes in the emission measure ratio of the two components.

This thesis describes an investigation of the faintest known proper motion members of the Pleiades star cluster. The primary aim of the work is an attempt to determine whether the lowest mass members of the cluster are brown dwarfs. A brief introduction outlines the fundamental properties of brown dwarfs and discusses the relation of research in this area to other branches of astronomy. The second chapter describes the basic principles of astronomical photometry and spectroscopy, the instruments used and the methods of data reduction. Chapter three gives a review of the current theory and observations of brown dwarfs. The results are presented in chapters four, five and six. RIJHK photometry for a sample of 62 objects is presented. The 1,1 - K diagram is shown to be a good discriminant of binary status for low mass cluster objects and using this diagram two single brown dwarf candidates and seven potential brown dwarf containing binaries are identified. FOS optical spectroscopy of 31 objects is used in an investigation of temperature indicators for cool stars. In addition gravity sensitive features indicate that the surface gravity of the Pleiades objects is lower than equivalent main sequence objects. Also discussed are the chromospheric and coronal activity of cluster members, and evidence is presented showing a turnover in both at ~ 0.3M⊕, where the objects become fully convective. CGS4 1-2.5 micron spectra are used in a futher investigation of methods of determining effective temperature and in searching for evidence of low mass companions. One object shows evidence for a low mass companion of ~ 0.035M⊕. Chapter seven discusses the evidence from these results that the faintest members observed are brown dwarfs and evidence from other workers that they are not. This chapter also gives suggestions for future work. Appendices compile the theoretical models and data used in the project.

This thesis is an account of research into various aspects of X-ray detection, with a common thread being the use of synchrotron radiation. An investigation into the so called "X-ray vectorial effect" is described. The aim being to design a detector able to determine the degree of polarisation in astronomical X-ray sources and the angle of the polarisation vector. Two experiments are reported, the first showing marked polarisation sensitivity, but not totally answering doubts. The second experiment demonstrated that the X-ray vectorial effect does not exist and with it the critical importance of accurate calibration of angles of incidence. Monte Carlo modelling of electron transport in photocathodes resulting from X-ray absorption is presented alongside compatible experimental results. An alternative polarimetry technique, also based on utilising the known polarisation sensitivity of photoelectron creation, is then derived and predicted to be competitive with the best current polarimeters and offering greater scope for improvement. A novel efficiency calibration technique for bare Microchannel plates (MCPs) using the Daresbury synchrotron with very low beam current is examined allowing us to conduct photon counting measurements, while utilising the energy tunability of the synchrotron. In quantum efficiency measurements this technique brought out absorption edge fine structure and EXAFS from the constituents of MCP lead glass with the energy resolution required in calibration the MCP spectrometer readout on the AXAF satellite. Complimentary measurements of the quantum efficiency of alkali halide (Csl and KBr) photocathodes are presented over the continuous energy range between 2 and 8 keV and at a range of incidence angles. These alkali halides are generally coated onto the MCP channels to enhance the detector efficiency. Synchrotron radiation was also utilised to examine the radiation damage characteristics of alkali halide photocathodes. Both Csl and KBr are shown to suffer significant degradation of photoelectric efficiency after minutes of irradiation with of order 1010 photons s-1. The way X-ray flux, angle of incidence and energy affect the degree of degradation is investigated. A solid state model of lattice defect production, diffusion and trapping incorporating surface effects and photoelectron emission is described. This first attempt at modelling the physical basis of photoyield degradation is shown to accurately recreate much of the observed behaviour including the shapes of the signal decay curves and recovery out of the synchrotron beam yielding a clear insight into the degradation process and suggesting ways its effects can be reduced.