The possible dependence of the extrapolated ranges of electrons on the detector characteristics has been investigated by two methods. These experiments were performed under well standardised geometrical conditions, employing electron beams (0.25 - 0.75 MeV) derived from a Van de Graaff electrostatic generator. The possible effects of poor geometry and small bias voltages on range values have also been investigated, but the results show that these effects could be kept very small in normal transmission measurements. The range values obtained in five metals (Be, Al, Cu Ag and Au) and in four dielectrics (glass, mica, sulphur and polystyrene) by direct current collection method and in A1, Ag and mica by an ionisation method, show that the latter method yields larger range values. A comparison with other results showed that the range values obtained with the Geiger counter are also greater than those obtained by the current collection method, The ionisation-in-depth results in Al showed good agreement with the other results, and with Spencer's theory for equivalent source geometry. A universal transmission curve was obtained for each element using the true range values calculated from the Bethe-Bloch stopping power formula, and its intercept was found to be a smooth function of the atomic number for both metals and insulators. Electron bombardment conductivity has been studied primarily in sulphur and in selenium for comparison. Large current gains were observed in sulphur in confirmation of von Hippel's theory of the conductivity of the elements of Group VIb of the periodic table. The single crystals of sulphur showed as much bombardment conductivity as selenium, but much greater conductivity than the polycrystalline specimen, as was noted by Joffe (1928) and as would be expected for a more perfect crystal lattice. The dependence of electron bombardment conductivity and the current distribution in bombarded dielectrics on the depth of penetration has been studied and the effects of space- charge on those phenomena investigated and discussed.

New and improved measurements have been made of the absorption of ultra soft x-radiation by light gases and plastic foils. A short introduction to the place of x-ray absorption in the historical development of x-ray research is followed by a discussion of the reasons for interest in absorption in the ultra soft x-ray region A review of the few x-ray absorption measurements that have been made in this region by other workers together with an outline of the present investigation complete the first chapter. Two different absorption spectrometers were designed and are described in Chapter 3. The gas absorption measurements were confined to a grazing incidence concave diffraction grating instrument, while a small radius curved crystal spectrometer was employed for the plastic investigation. No natural crystals occur possessing lattice spacings sufficiently large to permit Bragg reflection of the wavelengths of interest (up to 85 A). Artificial fatty acid crystals constructed by a method introduced in the thirties by Langmuir and Blodgett, were developed to fill this gap. The examination and development of these crystals is described in the fourth chapter. The experimental techniques used, involved the production of characteristic emission line radiation and the use of non integrating x-ray detection methods and are detailed in Chapter 5. In the concluding chapter the results of the investigation are compared with other published absorption data and the theoretical treatments of Hall and Bearden. These wave mechanical treatments are shown to be inadequate for the L shell, however, better agreement is obtained between the Bearden K-shell theory and experiment.

This thesis presents the results of an investigation into the hole mobility and its temperature dependence in single crystal specimens of the layer compound Pbl2 in a direction parallel to the c-axis. Thin specimens, between 20 and 70ms thick, were cleaved from, a larger boule crystal and fitted with evaporated metal electrodes. The temperature dependence of the electrical conductivity along this axis has also been studied, as well as the photoconductive response at 290K. The hole drift mobility was investigated by the use of fast electron beam techniques. A 301ceV excitation pulse of l0nsea duration was focussed on to the specimen and generated free carriers near the top electrode. A measurement of the transit time in a pulsed applied field led to a value for the hole drift mobility At room temperature was found to lie between 1.8 and 5cm2volt-1 sec-1. For most specimens the temperature dependence of mobility above about 250K can be represented by an equation of the form /.;cc exji t/feT,where 6 represents an energy that varies between 0.025 to 0.150eV for the crystals investigated. At a particular temperature, it is found that the low mobility specimens correspond to the highest values of C .. On the basis of the results it is concluded that the hole transport takes place in a narrow band and that the mobility is determined by optical mode scattering. At temperatures below 250K, however, specimens with high show the presence of another transport mechanism in which becomes essentially temperature independent. Attempts have been made to fit the mobility results to two transport theories: Holstein's small polaron theory and the model developed by Fivaz and Mooser for transport in layer structures. Possible reasons for the range of observed optical phonon energies are discussed. Measurements of the temperature dependence of the dark conductivity revealed a trapping level 0.55eV below the bottom of the conduction band, which hitherto had not been detected. In addition a defect trapping level 0.26eV above the valence band was found in agreement with previous workers. At temperatures greater than 400K to l4-50K,.the electrical properties are determined by an ionic conduction mechanism with an activation energy of 1.3eV. This appears to be connected with the creation and movement of Pb2+interstitials. The room temperature photoconductive response shows the absorption edge to be located at about 2.4eV. On the basis of the present results and those of other workers, an energy level scheme for PbI2 is proposed which can account for the majority of the observed features.

This thesis describes, with results to date, the establishment of a continuing programme of investigations into the nature of the solar coronal plasma. The diagnostic technique is that of measurement of optical emission lines in the x-ray, 1-25 u, region. The first chapter discusses the current state of knowledge of some of the properties of the corona, and chapter 2. examines the potential capabilities, and limitations, of x-ray measurement methods for study of coronal problems. A Bragg spectrometric technique is systematically selected in chapter 3. as an optimum measurement method, and a derivation of the necessary equations to describe the operation of the chosen equipment design is given. The properties and selection of Bragg analyser crystals are discussed in chapter 4. The next two chapters present the results and interpretation of the first two sets of observations, and the final chapter presents the current status of the programme. Results of more recent observations are presented and the objectives are discussed of new observations to be made towards the end of 1970. The work described has established a routine capability for the measurement of dozens of emission lines from spatially resolved regions of the solar corona, and has developed methods for characterising those regions of the plasma in terms of particle temperatures, number densities and elemental composition. New observing equipment, at present in preparation, will refine this characterisation by providing improved spatial definition, improved absolute energy flux calibration and adequate resolution to yield true line spectral profiles.

In the first part of this work, a distribution function P(012) for the interelectronic angle in atomic systems is suggested and examined for the ground states of 2- and 4-electronions. For He, several excited states have also been examined and comparisons made between P(012) functions resulting from different correlated wavefunctions. Various expectation values and energies have been evaluated in order to assist our study of the behaviour and relative importance of angular correlation. Some ideas of mathematical statistics have been applied in this examination. The 'positive' correlation effects predicted by Kutzelnigg et al, for the 1p state of He, are shown to be artifacts of insufficiently-flexible wavefunctions. Information concerning the relative shielding of electrons between states, and hence the diffuseness of 'charge clouds' is obtained from the corresponding P(012) functions. In part II, the method of configuration interaction (CI) and its application to the calculation of diatomic molecular wavefunctions is described. The ground states and many excited states of the molecules F2, F2-, F2+, F2++, NF and NF- have been examined using the complete CI and complete valence CI approximations with minimal basis sets of Slater orbitals. Several experimentally-unknown states are included in this study. Potential energy curves have been determined and, where applicable, used to obtain theoretical values of the spectroscopic constants by means of Dunham Analysis. In some instances other energy-related properties are presented. The F2++ system is found to be unbound. Where possible our results are compared with previous calculations and also with experiment. The suitability of molecules for treatment by CI calculations within minimal basis sets is discussed.

Apparatus has been developed to perform measurements of the thermoelectric power and resistivity of liquid metals at high temperatures of the order of 1500°C. Measurements have been performed on the transition metals, iron, cobalt, nickel and palladium in the liquid state, together with some measurements on these metals in the solid state and on liquid alloys of nickel with cobalt and palladium with silver. In most cases the results obtained are in agreement with other experimental data, where available. If use is made of a single site resonant scattering model (the only theory for liquid transition metals for which a number of quantitative calculations of resistivity and thermoelectric power have been reported), approximate expressions can be derived which give the resistivity of liquid nickel --- cobalt alloys in terms of the resistivities of the pure components. It is also possible to express the thermoelectric power of the alloy in terms of the thermoelectric powers and resistivities of the pure components and the resistivity of the alloy. The predictions of these expressions have been compared with the experimental data; the comparison indicates that the theory does not account for the observed transport properties of liquid nickel-cobalt alloys. The conclusion is therefore drawn that by implication the theory is not applicable, in its present form, to pure liquid transition metals.

Travelling Ionospheric Disturbances (T.I.D.s) are perturbations of the plasma in the ionosphere caused by the passage of acoustic gravity waves. These oscillations fall into two distinct classes, labelled large scale (LS) and medium scale (MS), respectively. These TIDs have frequently been observed at high-latitude and their source of generation may be the magnetic activity and/or the weather system within the region. This thesis presents the analysis of TIDs observations taken over a eight year (1981- 1989) period by means of an advanced Incoherent Scatter radars (EISCAT radar system) at F-region high-latitude, employing CP-2 common programme. An eight year synoptic study of TID effects in electron density measured at various heights is presented. A similar investigation of the field aligned ion velocity has also been undertaken. These two data sets have been computed and the wave disturbance in the parameter quantified. Large scale disturbances occured more frequently than those of medium scale and both are observed most frequently during daytime and at the first equinox. There are two dominant azimuths for wave propagation southwest and northeast. A very clear example of wave activity was observed by EISCAT facility on April 25th, 1989 between 0800-1300 UT in the measurements of Ne, Te, Ti, and Vi, extending to hundreds of kilometres height in the ionosphere. The wave trace amplitude shows amplification with height. The relative phase difference between the TID amplitude in Ne/Ne, Te and Ti with Vi has been established. Comparative cases have been presented between the theoretical and the observed values to test Hooke's formula and Hines dispersion relation. It has been noticed that there was no correlation between the three-hourly Kp indices, the hourly AE indices or the international daily sunspot number Ri indices with the average values of the TIDs spectral power. Good correlation was found between the low cloud type occurrence and the MSTIDs main parameters. A method of estimating the horizontal distance of the wave source from the radar position is presented.

No description available.

This thesis describes observations of a number of the magnetic cataclysmic variables known as AM Herculis stars (or polars) made in the X-ray and optical wavebands. Chapter 1 provides an introduction to cataclysmic variables (CVs), outlines their different classifications and evolution, and then describes the properties of the AM Her stars in detail. Chapter 2 describes the Ginga and ROSAT X-ray satellites from which much of the data discussed in the remaining chapters were obtained. In chapter 3 the phase resolved Ginga hard X-ray spectrum of AM Her is discussed. We find that simple thermal models do not describe the data well, with the observed iron line emission at lower energies and much stronger than expected from the inferred continuum temperatures. However, if we allow for an extra spectral component due to reflection of X-rays from the white dwarf surface, a consistent physical description of the data can be obtained. The inclusion of the hard reflection spectrum allows a reduction in the required incident continuum temperature, which provides a larger amount of thermal iron line emission at slightly lower energies. Together with the fluorescent line from reflection, this correctly accounts for the observed line energy and strength. The inclusion of the reflection component can also explain the variability of the spectrum and the iron line strength as a function of the white dwarf spin phase. Chapter 4 describes variability studies of AM Her and related systems. To begin with, the results of a search for the expected X-ray counterpart to the optical 1 - 3s quasi-periodic oscillations in the Ginga data of the three systems AM Her, EF Eri, and V834 Cen is presented. No QPO is detected, and the upper limits suggest we are not sensitive enough to the oscillating phenomenon (which is thought to be caused by an oscillating shock). The latter half of this chapter examines the longer timescale variability of AM Her as observed with Ginga, and also reports the results of some simultaneous UBVRI optical coverage. The variability observed in the Ginga band (2 - l0keV) is best described by a shot noise model, which consists of random bursts of emission with an e-folding decay timescale of 70s, allowing the characteristic lengthscale of inhomogeneities in the accreting material to be estimated. The optical red variability is found to correlate well with that observed by Ginga, whereas the optical blue does not. It is argued that multiple emitting blobs, of varying density, can account for the observed correlations. In chapter 5, the results of quasi-simultaneous ROSAT and Ginga observations of the polar QQ Vul are presented. The soft X-ray light curve is complex, at times dominated by a ~ 7.5 min timescale modulation. The detection of a soft X-ray spectral variation, together with soft and hard X-ray light curves which peak at different orbital phases, provides further evidence for the presence of two pole accretion. The data also confirms the soft X-ray excess observed previously in this object. Finally, chapter 6 describes the optical follow up observations of a new source, RE1307+535, which was discovered by the ROSAT WFC all sky survey. The optical photometry of this faint CV show it to be a short period system (Porb = 79.7min), at a distance d ? 750pc. The source is identified as a new polar on the basis of the optical discovery spectrum, and the EUV and optical light curves. The optical spectrum also reveals the presence of a strongly magnetic white dwarf, with B ~ 30-40 MG.

The growth mode and structure of metal overlayers, stabilised by molecular beam epitaxy, on oriented semiconductor and metal substrates, was investigated using surface x-ray diffraction and synchrotron radiation. The growth of the metastable fee phase of Fe on Cu(001) at 300 K followed a layer- by-layer mode with limited atomic intermixing of Fe and Cu at the overlayer-substrate interface. Relaxation to the bulk bcc Fe structure occurred at a critical coverage dependent on the growth conditions. Fe deposition at higher substrate temperatures was characterised by strong Fe-Cu intermixing giving poor layer development. The growth was diffusion limited at low temperature. The vertical morphology of clean Cu(00l) and adsorbate-covered surfaces at specific Fe coverages was determined from kinematical analysis of intensity measurements perpendicular to the surface and showed changes in the surface interlayer spacing with film thickness. The metastable bcc phase of Co grown on GaAs(001) evolved in a predominantly three-dimensional manner with significant disruption of the semiconductor substrate, causing inclusion of As and Ga into the growing metal film. The structure of the Co film, determined from x-ray specular reflectivity measurements, was consistent with bcc Co. The behaviour of Fe deposited on GaAs(001) was similar, although the overlayer formation was better ordered. In deposited on the Si(001)2x1 surface at substrate temperatures between 300 to 623 K exhibited a Stranski-Krastanov growth mode, that is the formation of one or two atomic monolayers (ML), followed by three-dimensional island formation. The metal induced a series of coverage and temperature dependent surface reconstructions. Deposition of In at 300 K caused only minimal substrate disruption, whereas In and Ga interacted strongly with the dimerised Si(001)2x1 surface at a substrate temperature of 373 K. During formation of the Si(001)2x1-In surface reconstruction the dimerised Si structure was partly destroyed and replaced by In dimers.