We present the results from EXOSAT and IUE observations of five BL Lac type objects, Mrk 421, Mrk 501, 1218+304, Mrk 180 and 0414+009. We find that at least for the relatively well studied sources Mrk 421 and Mrk 501, the behaviour in the X-ray and ultra-violet differs significantly. In the majority of cases, the X-ray spectra are well represented by a simple power-law model over the 1-6 keV band. In the case of the high state observations of Mrk 421, however, a marked downward curvature of the spectrum is required above ~3 keV, which can be modelled as an exponential decrease. Similar convex spectra are also suggested, although not statistically required, in a number of other cases. We found no evidence in any of the observations for a hardening of the X-ray spectra above a few keV as has been previously reported. In all cases we find low energy cut-offs in the X-ray spectra consistent with absorption in the line-of-sight gas column density through out own galaxy implying that the intrinsic column density of cold gas in these objects is small (< 1 X 1020 cm-2). All five sources exhibit significant X-ray variability with minimum variability timescales in the range ~3 hr to ~7 days. Furthermore, the X-ray flux and spectral index for each object appears to be correlated in the sense that the X-ray spectrum hardens as the source brightens. In contrast, the ultra-violet spectra are generally consistent with a simple power-law of index ~1, and exhibit only slow drifts in flux on a timescale ~weeks. The ultra-violet to X-ray continuum can therefore be modelled as a power-law of index ~1 below about 0.1 keV, above which the source steepens. The spectral variability in the X-ray band can then be described in terms of a 'pivoting' of the high-energy continuum about the break-point. We have investigated two specific SSC models for the continuum emission. It was found that whilst a simplistic homogeneous disk-shaped emission region can provide an acceptable explanation of single epoch snap-shots of the multi-waveband spectrum, it experiences some difficulty accounting for the high energy spectral variability. In contrast, by allowing a variable radial dependence in the upper cut-off in the synchrotron emitting electron population in an inhomogeneous relativistic jet model, we have demonstrated that the multi-waveband spectrum of the most demanding source, Mrk 421, at all epochs can be reproduced. Thus we conclude that a jet model, broadly following the current paradigm for BL Lac type objects can give an acceptable explanation of present multi-waveband measurements.

This work is concerned with the study of the coma and gas tail of Hailey's Comet. The thesis begins with a summary of the relevant background material. Theories of origin are discussed, after which the known properties of cometary comae, nuclei and tails are described. Narrow-band photometry of Hailey's Comet both pre- and post-perihelion is documented. The observations of the OH band at 3080A are discussed first, as it is a product of H 2O. Production rates and lifetimes against photodissociation are calculated using both the Haser Model and the Average Random Walk Model. The variation of production rates of CN, C2 and C3 are discussed in the light of current knowledge of Hailey's nucleus. Photodestruction lifetimes of these trace species are found using a Monte-Carlo modelling technique. The analysis of large-scale image of the plasma tail of Hailey is described, resulting in the determination of the solar wind velocity at the comet. The dynamics of the ions in the tail are demonstrated using a simple analytical approximation to the magnetic field structure. Observations of discrete plasma formations within the tail allow the measurement of the internal velocity field, from which an estimate of the tail magnetic field strength is derived.

Myosin from the striated adductor muscle of scallop (Pecten maximus) folds into a compact 10S conformer, as has been characterized for smooth muscle and non-muscle myosins. The 10S conformer of scallop myosin is favoured at physiological ionic strengths in the absence of Ca2+ and in the presence of nucleotide triphoshate. The folded transition is accompanied by the trapping of the nucleotide at the active site to give a species with a half-time of about an hour at 20C. Ca2+ binding to the specific, regulatory sites on a myosin head promotes unfolding to the extended 6S conformer and activates product release by around 60-fold. The unfolding transition, however, remains much slower than the contraction-relaxation cycle of scallop striated muscle and could not play a role in the regulation of these events. The turnover of nucleotide by the 10S conformer is an order of magnitude slower than the turnover by native scallop filaments. The latter have very similar kinetic properties to that of scallop heavy meromyosin suggesting that the myosin-linked regulatory system requires only the head and neck domain to function properly. Thus there is no evidence, from nucleotide turnover measurements, for an intermolecular interaction occurring between the neck and tail regions of neighbouring myosin in the filament equivalent to that observed as an intramolecular interaction in the 10S conformer. The role of the 10S conformer in striated muscle may therefore be associated with events which occur on a slower time-scale than the contraction-relaxation cycle, such as transport of myosin molecules from their site of synthesis to the myofibril during growth and development of the muscle. Removal of either one or both of the regulatory light chains from scallop myosin prevents, or at least disfavours, formation of the folded 10S conformer. Readdition of the native regulatory light chains, or those from other other molluscan species, restores to the myosin its ability to fold. Labelling the reactive heavy chain thiol of myosin also disfavours formation of the 10S conformer and allows separation of the modified protein from the native molecules.

The growth and atomic structure of indium on the 7x7 reconstructed (111) surface of silicon has been studied using X-ray scattering and synchrotron radiation. A correlation is seen between features in the X-ray reflectivity curves and breaks in the Auger signal versus time plot as indium is grown. The oscillation of the X-ray signal indicates that below 400C two consecutive pseudomorphic indium layers are formed before three dimensional islanding occurs. Kinematic analysis predicts the shape of the X-ray signal versus time curve and yields the perpendicular displacement of the indium layers from the substrate. It is proposed that the first layer of indium atoms are bonded vertically above the top most layer of silicon atoms in T1 sites. The structure of the ?3 x ?3 R30 reconstruction induced by the adsorption of ? of a monolayer of In has been determined. The Patterson function obtained from structure factor intensities at zero perpendicular momentum transfer, indicates lateral displacement in the silicon surface atoms. The indium adatoms are shown to occupy the 4-fold coordinated T4 sites above the second layer silicon atoms. Elastic strain energy minimisation has been used to show that relaxations extend down to the first six layers of the bulk. The Patterson function for the 4x1 adsorbate phase indicates that the saturation coverage is 1 monolayer with each surface unit cell having four indium atoms rather than two as indicated by Scanning Tunnelling microscopy images. The results show that the indium atoms sit in a complicated arrangement on the Si substrate near T1, T4 and H3 sites. Intensity profiles of one fractional order rod and the (00L) reflectivity give information concerning displacement of the indium adatoms normal to the surface.

No description available.

Stoichiometric amorphous GaP films have been prepared by RF sputtering on to substrates held at temperatures from 20°C to 200 °C. Some room-temperature deposited samples were annealed at higher temperatures (200-400 °C) in vacuum. Structural, optical and electrical characterisation of these films was carried out by means of a variety of experimental techniques. Electron microscopy experiments, using both transmission electron microscope (TEM) and scanning electron microscope together with energy dispersive X-ray analyser (SEM-EDAX) have revealed that, while on the macroscopic scale the samples are homogeneous, continuous and smooth, on the microscopic scale they contain structural inhomogeneities, namely voids, and a small degree of compositional non-uniformity. Extended X-ray absorption fine structure (EXAFS) spectroscopy has provided information on the atomic structure of the films, in particular on the short-range order around both Ga and P atoms. It was found that the a-GaP network is four-fold coordinated and chemically ordered, i.e. wrong bonds, Ga-Ga and P-P bonds, do not exist. In addition the technique revealed very little disorder in the bond length but wide distributions in the bond angles, the latter being more pronounced around P atoms than around Ga atoms. Information on the short-range atomic structure has been obtained from infrared (IR) spectroscopy through the identification of the vibrational modes of the bonds. The main finding of these measurements was that the a-GaP network is chemically ordered, which is consistent with the EXAFS work. From a combination of conventional reflection-transmission (R-T) and the most recently developed photothermal deflection spectroscopy (PDS) techniques, the absorption coefficient in the range of 10 cm-l < 10.;5 cm-1, the refractive index and values ofthe optical gap were determined. The results are explained in terms of the nature and distribution in energy of the density of states (DOS) in the forbidden gap. The DOS in the conduction band is also extracted from the X-ray absorption near edge structure (XANES) data, which is combined with the DOS-valence band information taken from the literature to obtain full details of the electronic structure of a-GaP. D.c conductivity measurements at high- and low-temperatures showed a continuously varying activation energy. This was interpreted as a transport mechanism taking place in a band tail but in which the conduction path moves downwards in the tail as T is lowered.

The orthorhombic form of sulphur is a typical molecular crystal and is composed of a structure of S8 rings. Specimens have been grown from solution in CS2 both in a thin platelet habit and in the form of thicker double pyramids. Drift nobility techniques have been used to study the charge transport between 180K and 370K. The results show that the charge is carried by the generated holes having a room temperature lattice mobility of about 10 cm2 sec-1v-1 and a lifetime of 10-5 sec. At the lower temperatures the hole drift mobility is an activated process, and during transit the generated holes interact with a level of centres 0.19 ev above the valence band. With increasing temperature a transition to a lattice controlled transport takes place. On the basis of the infra-red transmission spectrum it is suggested that the above centres are associated with solvent molecules. The range of generated electrons is estimated to be less than 10-8 cm2/volt; a predominant electron trap is situated 0.9 ev below the conduction band. Measurements on the photoconductivity in orthorhombic Sulphur revealed for the first time a response in the ultra-violet. The photocurrent reaches a peak at 2,550 A.U. with unit quantum efficiency and it is likely that this is an intrinsic photo- conductive effect. The photoconductivity in the visible region, which has been reported before, (15-16) has a maximum at 4,600 A.U. with a quantum efficiency several orders of magnitude less than that in the ultra-violet. It is suggested that this photocurrent is generated by the interaction between molecular excitons. The absorption spectrum has a sharply rising edge at 4,500 A.U. reaching an absorption coefficient of 2 x 103 /cm at 3,600 A.U. There is a slight window in the ultra-violet with a minimum at 2,700 A.U. The absorption edge shows distinct dichroism, moving to longer wavelengths as the E-vector becomes parallel to the c-axis. The total shift of the edge corresponds to an energy difference of 0.05 ev.

A detailed investigation into the thermodynamic properties of binary mixtures containing hexafluorobenzene is described. The excess enthalpies of mixing of hexafluorobenzene with eight amines and two hydrocarbons are presented and the excess heat capacity determined in the case of the amines. Measurements were made using a twin version of the "Larkin-McGlashan" Calorimeter. The extension to a twin calorimeter together with the advantages which ensue are fully discussed. An apparatus is described for the precise measurement of vapour pressure. The results obtained in the case of pure hexafluorobenzene are compared with other precise measurements and satisfactory agreement is found. Computer programs have been developed to calculate the excess Gibbs Function of Mixing GE from experimental vapour pressure measurements. The computer programs were tested using reliable literature data and the results were judged to be satisfactory. An apparatus to measure the volume change on mixing is described and the technique was checked by studying the benzene- cyclohexane system. The excess volume of mixing is reported for hexafluorobenzene + N,N-dimethylaniline. Preliminary investigations are reported into the use of N.M.R. and solid liquid phase diagrams in interpretting the behaviour of specifically interacting media. The results of all the thermodynamic data are discussed in terms of complex formation and the possibility of considerable stabilisation by charge transfer forces is considered. The work on aqueous solutions described in Appendix I centres around the properties of acetonitrile water mixtures. The excess volume of mixing for this system is presented at two temperatures as also are the partial molar volumes. The results are discussed in terms of solute - solvent and solvent - solvent interactions. Also presented is an analysis of the maximum density properties of aqueous solutions. Particular emphasis is placed upon alcohol water mixtures and visual correlations are explained thermodynamically. In this department some studies of the behaviour of cadmium chloride in formamide were carried out by Dr. B.L. Muju. The work done by the present author in interpretting these data is summarised in Appendix II.

Since their discovery during the last century, the asteroids, or minor planets, have remained one of the principal anomalies in the solar system. The purpose of the present work is to construct a theoretical model of the asteroid system, from which something of the origin and evolution of the asteroids might be learned. First, the history of the discovery of the asteroids is outlined together with their major physical properties, and the probable connexion between asteroids and meteorites is discussed. In seeking an explanation of the asteroid system the early theory of a single disrupted planet must be dismissed. Yet of the various hypotheses put forward, a process of fragmentation seems by far the most likely. Recent theories based on fragmentation are therefore reviewed. Our own model is then presented. In describing the results of collisional fragmentation it is easiest to treat the distribution of asteroid masses. Accordingly, equations describing the change with time of the asteroid mass distribution have been derived. Solutions, both analytical and numerical, are presented here, and their consequences discussed. Special attention is given to the important subsidiary problem of the break-up of individual asteroids.

No description available.