A united-atom approximation, employing one-centre wavefunctions, has been adopted in the investigations of the electronic distribution and physical properties of a number of XHn molecules. Calculations have been carried out on HF, H2O, NH3, NH4+, CH4, HCl, H2S, HS-, S--, PH3, PH+4, PH-2 and SiH4; and physical properties which have been computed include molecular energy, bond length, x-ray scattering factor, molar diamagnetic susceptibility, 'breathing' force constant, proton affinity and bond energy. The results of these calculations are, where possible, compared with existing experimental data. In general, the predictions of this simple approach are pleasing. Calculations have been preformed by Moccia on almost all of the systems mentioned above, using a one-centre expansion approximation with an extended set of basis functions. The reported results of Moccia have been analysed and as a result it has been possible to calculate the radial electron density distribution, the x-ray scattering factor, and the molar diamagnetic susceptibility of all the molecules he has considered. The results obtained from Moccia's work, together with the results of a similar analysis by Banyard and March of the work of Ellison and Shull on H2O, have boon compared with the united- atom predictions and with experiment. The comparison gives some indication of the relative merits of the different approximate treatments. A brief review of density matrix theory is presented with particular emphasis on its application to molecular problems. This review includes a discussion of the introduction of the density matrix, and a short account of McWeeny's formulation of the LCAO-MO-SCF theory in density matrix form. Finally, the Waller and Hartree theory of x-ray scattering by free molecules has been developed in terms of the one- and two-particle reduced density matrices.

A semi-theoretical investigation has been made of a new effect arising in the multiple scattering of electrons in a gas. A statistical analysis has been made of the shapes of electron tracks in a Wilson expansion chamber by determining the distributions of certain features of the trajectories, these features being correlated with the shapes of the tracks. These distributions have been compared with similar distributions for simulated tracks of electrons undergoing: A) Kandom collisions, B) Non-random collisions of a particular type, and in the light of these results an indication has been made as to a probable explanation of the nature of the effect. No attempt has been made to formulate any complete theoretical explanation. It should be mentioned that although this account is divided into chapters there is extensive cross referring owing to the nature of the investigation. The general presentation is: SECTION 1.a) Analysis of the expansion chamber tracks (Preliminary theory and analysis) b) Simulation and analysis of electron tracks in a square lattice 2 c) Simulation and analysis of random tracks. SECTION 2. a) angular distributions. b) "Heads-tails" distributions. c) Area distributions. d) Discussion. The analyses mentioned in section 2 are described in detail in each chapter of section 1. The results of each analysis then appear and are discussed in the appropriate chapter of section 2. The appendices include a brief description of the apparatus used, details of the computer programmes employed in the analysis and an account of an investigation into proton channelling in crystals which was made possible by one of the existing computer programmes.

The absorption of a radio wave propagated through the ionosphere at oblique incidence, depends fundamentally, on the height distributions of electron density and the coUisional frequency of the electrons with the neutrail gas atoms. The vciriation of the electron density with height will also determine the paths along which the radio energy flows. Experimental observations have been made of the absorption losses and of the various modes of propagation over four oblique incidence paths, of lengths between 500 and 1000 km. The seasonal, diurnal and anomalous variations in these data have been discussed. The experimental observations have also been used to test the validity of the theoretical results of prediction and ray tracing analyses. The prediction of path parameters, such as mode structure and absorption, is important in the design of broadcasting circuits and the basic equations and assumptions of these methods are discussed in detail in an attempt to determine the accuracy of these procedures. The predicted conditions for the four test circuits have been compared with experimentally observed values. At oblique incidence, the absorption can be calculated by use of ray tracing; methods. A computer program, based on the Booker equations, was written so that the absorption on the oblique paths could be evaluated for a range of electron density models. This program was compared with other standard ray tracing methods for both experimental and analytical models of the ionosphere and the results were found to be in excellent agreement. The ray tracing results were also compared with the experimental data in order to establish the validity of the ionospheric models. Composite models consisting of published D region distributions and experimentally derived E and F regions were found to give results in good agreement with experiment. Absorption data for several oblique and vertical incidence paths was available 'for 1966 and a correlation analysis of these results has been carried out. Marked similarities were evident; a result which is important when predicting path parameters for one oblique circuit from measurements made on another. Absorption variations during ionospheric disturbeoices have also been studied and compared with steep incidence VLF results. The enhanced electron density during these periods appears, in most cases, to be located at heights above 75-80 km. This study has revealed that ray tracing and prediction methods, when applied to oblique incidence circuits of up to.;000 km. in length, can yield resultswhich are consistent with experimental observations of absorption and angle of arrival for such circuits.

Specimens of liquid sulphur were obtained by melting orthorhombic sulphur crystals or ultrapure granules of sulphur. Between the melting point (119°C) and 160°C, liquid sulphur is composed of puckered S8 ring molecules bound by weak van der Waal forces as in the solid. Above 160°C, polymerisation of ring molecules takes place, Drift mobility techniques have been used to study the charge transport during melting and in the liquid up to about 200°C. The specimen cell consisted of two parallel quartz discs separated by glass spacers, 50 to 700 microns thick. Transparent electrodes were applied to the inner surfaces of the discs. Charge carriers were generated close to one of the electrodes by a fast ultraviolet light pulse (~ 10 microsecond duration), and carriers of one polarity were drawn across the specimen in the applied field. Their transit time was measured giving a value for the drift mobility. The results showed that the charge transport is due to both negative and positive charges which move in the liquid by means of phonon-assisted random hopping between neighbouring molecules. From a detailed discussion of the results it is concluded that we are dealing here with an electronic, rather than an ionic charge transport. The electron and hole mobilities at the melting point are 1.0 x 10-4 and 5.5 x 10-5 cm 2 / volt sec respectively. The Electron mobility which is a hopping transport in the solid, shows a drop of a factor of 30 during change of state. This has been attributed to a decrease of overlap J by a factor of 5.5. Between 119°C and 160°C the electron mobility increases in the liquid with increasing temperature. with an activation energy of 0.15 + 0.03 ev. Holes have the same activation energy within a larger experimental error. Above 160°C both electron hole mobilities decrease slowly with increasing temperature. This has been attributed to a decrease in the hopping probability resulting from a loss of ring molecules by polymerisation. Dark current measurements on liquid sulphur suggested that in less pure specimens impurity ions set up a space charge near each electrode of opposite polarity to that of the electrode which modifies the current in accordance with Thomson and model of conduction in gases. The measurement of the efficiency of carrier generation in liquid sulphur has been carried out at an applied field of 33 K volt/cm, considerably less than the saturation value. The results show that the ultraviolet photoconductivity reaches a peak at about 2600 A with a generation efficiency of 10-2. The recombination lifetime of generated carriers has been estimated to be about 10-7 sec. The visible emission of solid sulphur under x-ray excitation has been studied. The observed emission has an estimated efficiency between 10-4--10 -06. Three emission peaks at 4900, 5300 and 5700 A have been detected. The emission is thought to be due and transitions from a molecular excited state, 2.7 ev above the ground state, to the singlet ground state of the S8 molecule.

This thesis describes a number of experimental studies relating to the surface properties of orthorhombic sulphur in the near and vacuum ultraviolet, in the energy range 3-14 e.v. The optical absorption coefficient using a thin film technique at 80°K has been measured up to photon energies of 6.2 e.v. This modifies previous results taken over a more limited energy range and correlates well with a band model of the absorption processes in this region proposed by Spear and Adams. An examination of surface transits has shown anisotropy in charge separation in the excited region and is identified with an anomalous photovoltaic effect first described Ruppel and Grant. Further observations on the photovoltaic response are made and possible causes are discussed. Photoconductivity and photoemission measurements have been made over the entire energy range and are correlated to produce a tentative band model. This confirms the essential correctness of the previous work by Cook and Spear and adds new data on intraband structure and the nature of the higher lying conduction bands. The model predicts the onset of high mobility electron generation at 4.9 e.v. The behaviour of an insulating solid under photoemission has proved to be very different from that of a metal. These characteristics have been analysed from a theoretical standpoint with results that are consistent with the observed phenomena. Theoretical calculations include those made on the nature of the potential and field distribution in a surface cell which explains some of the experimental results in this thesis and certain reported anomalies by other authors, and on the charge distribution during carrier transits when subject to the diffusive action of stochastic trapping and release.

The literature on the internal friction of martensite, the theory of microplasticity, and the microplasticity of iron-based alloys is reviewed. Low-frequency internal friction, strain relaxation and microplasticity tests were conducted on a 0.4% carbon martensite. Modifications to the Ke pendulum made strain relaxation and microplasticity tests possible. Amplitude-dependent damping at temperatures of 20 to 130°C was explained by coherent precipitation of ? -carbide. Koster type experiments at temperatures of 125-188°C support the pinning model of Beaulieu. The peak in the temperature range 210-250°C at a frequency of 1Hz observed in bainite was shown to be analogous to the 200-250°C peak present in martensite. The background damping is discussed, a method for its removal presented and the width of the isolated peaks analysed to determine their width parameter B. The separate effect of initial stress and testing temperature on the strain relaxation at constant stress for 0.4% carbon martensite at temperatures up to 255°C are interpretted by thermally activated unpinning of dislocations. Curves at 110 and 137°C were used to determine the parameter B, and these values in conjunction with those obtained from the peak width suggest that the distribution in the relaxation time is in ?. The internal friction of martensite was analysed in terms of the current theories for the cold-worked peak and it was found to be consistent with the theories proposed by Schoeck, and Ino and Sugeno. Microplasticity tests on martensite between 20 and 387°C show that, depending upon testing conditions, reversible and irreversible bowing of dislocations can occur. Attempts were made to explain this bowing, and hence the internal friction of martensite, in terms of the double kink mechanism of microplasticity. An unidentified peak at 300°C for 1 Hz was discovered in an Fe - 28% Cr. alloy quenched from 1100°C.

The atmospheres of the terrestrial planets are generally supposed to have been formed by secondary degassing processes. It is proposed instead, in this thesis, that the atmospheres formed as a necessary consequence of the final stages of planetary accretion. When the planetary embryos reached a critical size, the accretional material impacting the surface began to vaporize and some remained in this gaseous state. As the gases accumulated, further incoming material was decelerated, thus limiting the rate of atmosphere growth. When reasonable assumptions are made, concerning the nature of the impacting material, an acceptable model for the early terrestrial atmospheres results. Our knowledge of the noble gas abundances for the present Martian and terrestrial atmospheres does not preclude the possibility of atmosphere production from the impact of mainly primitive Cl meteorite type material. Subsequent development of these primitive atmospheres into those which currently exist is possible. The evolution of the Earth's atmosphere is discussed in detail, with emphasis on the development of the carbon dioxide cycle. The primitive CO2,-H 2O atmosphere would not have produced a runaway greenhouse condition on Earth, such as occurs on Venus, due to the rapid solution of carbon dioxide in water which condensed onto the surface. The creation of an atmospheric dust veil is proposed as a critical factor in maintaining surface temperatures within the range possible to permit the formation of bodies of surface water. The development of the carbon dioxide cycle through three main stages is proposed. Carbon dioxide partial pressures in the atmosphere remained in excess of the current value throughout the Precambrian, to maintain higher surface temperatures throughout this period: present values were not approached until the advent of deep sea organisms. Methods of regulating the carbon dioxide reservoir in the atmosphere are discussed for each stage and the importance of subduction processes in the present situation is stressed. Finally, the proposed evolutionary histories for the atmospheres of the Earth, Mars and Venus are outlined.

An experimental system has been designed with a twofold objective. The system, which has a variable drift length, has been used to measure, for a range of counting gases, drift velocities and longitudinal diffusion coefficients employing the time-of-flight method, and lateral diffusion coefficients using the Townsend method. Several problems have been encountered during the work, these requiring continued modification to be carried out on the system. A detailed description is given of the system and its stages of development. Attempts to measure longitudinal diffusion coefficients were not successful and possible reasons are fully discussed. A modified Boltzmann-Holstein equation has been employed with published cross-section data to calculate numerically the electron energy distribution, and hence electron transport coefficients, in the gas mixtures studied, at non-ionising field values. The Boltzmann-Holstein equation is discussed from first principles and the method of numerical solution is explained. Good agreement has been found between calculations and measurements of drift velocities for methane and a mixture of neon/10% methane and of D/ for Ne/10% CH4. Predictions for argon/methane and argon/carbon dioxide mixture are less accurate; possible reasons are discussed.

Surface photometric observations at near infrared wavelengths (J, H and K wavebands) of the galaxies M31, M82 and NGC 972 are described and the results discussed in the light of other, recent literature. The techniques used in surface photometry of extended objects at near infrared wavelengths are described and, as an introduction, a detailed review of near infrared, extragalactic astronomy is given. R.A. and Dec. scans through the centre of M31 show the two micron surface brightness distribution in the nuclear bulge follows the de Vaucouleurs law and that the V-K gradient within the central +/- 5 arcmins is small, ? (V-K)r=5' < 0.1. Comparisons are made with other published data. Photometric scans through the active, central region of M82 reveal that the two micron emission from the galaxy is concentrated within a very high surface brightness "core" source measuring approximately 25" x 8" (375 x 120 pc). The core is surrounded by a disk measuring 90" by 35" (1350 pc x 525 pc) which though of lower surface brightness is still much brighter than the corresponding region in other normal disk systems. Contour maps of the two regions are presented. The asymmetry in the brightness distribution along the major axis is attributed to obscuration by the major dust lane, from which it is inferred that the extinction produced by the lane is, Av ~ 16. The two micron core is approximately coincident and co-extensive with the diffuse radio emission and the visible "hot-spot" region, and is probably associated with a burst of star formation near the centre of the galaxy. Two micron scanning observations of NGC 972 yield a low resolution contour map of the whole galaxy which is similar in some respects to published blue-waveband maps. In particular, the similarity in the gross distribution of B and K light along the major axis suggests the unusual structure seen at shorter wavelengths cannot be due to dust obscuration as was previously assumed. An exciting possibility arising from this work is that the secondary feature in the southeast of the system may be a small, previously unsuspected, interacting galaxy, which might account for the IO-type appearance of NGC 972. The two micron luminosity of the central region of the galaxy is unusually high for its low mass and young mean spectral type, and is comparable with that of M82. J, H and K slit aperture scans yielded J-H and H-K colour indices which suggest the presence of an extended source of free-free emission near the centre of the galaxy. These are the first reported observations of NGC 972 at infrared wavelengths.

A review of X-ray telescope focal-plane instrumentation is given. The architectures and operating principles of contemporary charge-coupled device (CCD) imagers are discussed. The fabrication of CCDs manufactured by EEV Ltd. is described. An overview of a slow-scan CCD evaluation system is given and the design, construction and operation of a microcomputer controlled programmable CCD readout sequencer is detailed. The technique of on-chip pixel binning is discussed. CCD noise sources and noise measurement techniques are reviewed and constructional details of a low noise analogue CCD signal processor are presented. A description of CCD data processing and image display software is given. The results of X-ray efficiency and energy resolution measurements on standard and specially developed deep-depletion CCDs are presented and accounted for. It is shown that deep-depletion devices can be fabricated to offer energy resolution equal with that of standard devices, together with superior detection efficiencies. Both device types are shown to give spectral resolution comparable with that obtained from other silicon-based detectors. Aspects of CCD operation in the space environment are considered and the relevance of this work to the development of CCD instrumentation for the European Space Agency's XMM satellite is assessed. In conclusion, the important characteristics of standard and deep-depletion CCDs are noted and areas thought to require further investigation are highlighted.