Table of contents

The electron-beam wobbling method of expanding the vertical exposure are of synchrotron light has been investigated. Displacement and tilt angle of the stored beam caused by a steering magnet were measured using a silicon photodiode array with a compact electron storage ring. It was demonstrated that the experimental results agreed well with theoretical calculations. The effects of electron-beam wobbling on the stored beam were also examined. As a result, it was confirmed that even at a displacement of +or-5-6 mm, which corresponds to a value actually suitable for the stored beam, the tune shift was extremely small and there was no disturbance of the stable conditions of the stored beam. Furthermore, an effective method of simultaneous exposure at every beamline is also discussed.

Output characteristics of a XeCl laser at very high discharge current density (j=14 kA cm^-2) were studied experimentally. At an estimated electron number density of about 1*10¹⁶ cm^-3, electron quenching did not dominate over XeCl* (B or C) formation. The XeCl active medium exhibits very high gain characteristics at high current density. High power laser output was extracted from the active volume and a volatile dependence on HCl partial pressure was observed. The rapid build-up of gain was attributed to fast excimer formation resulting from increased number of densities of ionic precursors (Xe⁺ and Cl^-). Electron-HCl kinetics processes that contribute to the enhancement of the formation of atomic ions were also discussed.

In most applications of lasers for material processing a plasma is detected above the hot surface of the workpiece. Since in certain ranges of temperature and density plasmas are able to absorb a significant part of the incident laser power, the physical processes which supply this energy into the workpiece are of special interest. In this paper the energy supply of a collisional plasma with a low degree of ionisation is considered in the steady state in a one-dimensional geometry. The dominant energy transport process is found to be the release of kinetic and ionisation energy of charged particles being absorbed and then recombining at the wall. Finally, a numerical example is given for the case of laser welding involving the presence of a plasma whose electron temperature and density are predicted.

The process of vaporisation of a light-absorbing metal under laser irradiation is investigated from a theoretical point of view. The tools are the Euler equations, the stationary heat-flow equation and the kinetic equations to calculate the gas dynamical properties of the edge of the Knudsen layer. Using the one-dimensional deflagration theory, it is shown that the local sound velocity is an upper bound for the metal vapour velocity at the phase boundary. Furthermore, it is shown that for an absorbed intensity I_a, which is greater than a certain threshold intensity I_c, the vaporisation process is independent of the ambient gas. This intensity I_c is calculated for aluminium and iron. On the assumption that a stationary limit of the vaporisation process is reached, the solution of the Euler equations is equivalent to the solution of a Riemann problem.

Using an arc-driven shock tube, the turbulent features in the ion density fluctuations of an argon plasma behind collisional ionising shock fronts are determined. Fluctuation power spectra show many discrete modes and follow S(K_z) alpha K_z^-n with 1.2<n<2.2 for shock wave Mach numbers between 15 and 71. The overall turbulent properties are found to be almost frozen with the post-shock flow and to have particle-like dynamical behaviour.

A theory is presented of the electrical nature of capacitatively coupled RF discharges of the type commonly encountered in microcircuit fabrication. Using arguments bearing on ion and electron fluxes associated with the cathode and anode, an expression is developed relating the ratio of the DC self-bias, V_DC, and the applied RF voltage, V₀, to the relative electrode areas, and a formula is given for the plasma potential. Good agreement is achieved for a range of experimental data of V₀, V_DC and plasma potential and the observed relationship in the work of Coburn and Kay (1972) between the ratio of the sheath voltages and the electrode area ratio is convincingly reproduced. The theory promises considerable predictive power which may be of value for the design of RF reactors, especially with regard to the control of ion impact energies at substrate surfaces.

Low-frequency ( approximately 500- approximately 2000 Hz) oscillations are detected in a double-plasma device in association with double layers in which the potential drop somewhat exceeds the ionisation potential of the gas. The oscillations are interpreted as ion-acoustic waves excited by an ionisation instability. A simple theory of the instability is presented and its results are compared with observations.

The temporal behaviour of electron swarm transport in an RF field has been investigated for the electron non-conservative case in CH₄, H₂ and their mixture by using the first three terms in Legendre polynomials with a fourier expansion in time for the electron velocity distribution from the time-dependent Boltzmann equation. Calculations have been made for reduced RMS electric fields E_R/N in the range 15 to 70 Td and reduced angular frequencies omega /N in the range 10⁵/N₀ to 10¹⁰/N₀ cm² s^-1, where N₀ is the number density at 1 Torr and 273 K. The temporal modulation of the isotropic part of the velocity distribution is essentially determined by the ratio between omega ^-1 and an energy relaxation time tau _e. It is shown that in molecular gases with rotational and vibrational collisions at very low energy, the asymptotic form of the energy distribution at very high frequency field differs from a Maxwellian as in atomic gases. The phenomenon of negative differential conductivity in CH₄ has been studied as a parameter of omega /N. The gas mixture effect in an RF field is also discussed.

Electromigration (EM) in metals is used to produce directed particle fluxes within the material and can be used to change the composition of a given material or to purify it. EM experiments were performed on sintered YBa₂Cu₃O_{7- delta} (YBCO) rods at 200 degrees C and 350 degrees C and the results were subsequently analysed by magnetic susceptibility and X-ray diffraction. Explorative trials showed that oxygen does migrate under the influence of the field in this material and motion is directed towards the cathode. Another remarkable feature during electromigration was that towards the cathode. Another remarkable feature during electromigration was that a decomposition reaction occurred at the anode, resulting in the formation of YBCO (tetragonal), CuO, Y₂O₃ and BaCO₃ phases. It appears that the effects of electromigration will have a bearing on the stability of this material during use.

An analysis of the effective continuous media (ECM) approach has been made for the prediction of effective thermal conductivity of moist porous materials. The validity of the expression has been tested for different types of soils saturated with different liquids having variations in liquid content and temperature. A transient method, utilising a line heat source, has been employed to measure the effective thermal conductivity. A comparison has been made between the experimental and predicted values. The model is also tested on other reported thermal conductivity values. In all the cases the developed model provides a prediction close to the effective thermal conductivity. The results are also compared with models previously developed.

The meniscus shape, triple-line contour and contact angle of an essentially axisymmetric drop of liquid on a slightly heterogeneous solid surface have been studied recently. The general solution involves a Fourier representation of the field of inhomogeneities which means that event isolated perturbations lead to a series representation. It is shown that in these circumstances, the series solution approximates closely to a logarithmically decaying 'spur' form when the drop diameter is large compared to the local perturbation. An equivalence is obtained with an equation previously presented. Adjacent heterogeneities near the triple line interact and produce deformation in the zone between them such that the liquid front may not return to its unperturbed position.

DC resistivities of glass-ceramics in the system BaO-Fe₂O₃-B₂O₃-X₂O₃ (X=Sb,As) containing alpha -Fe₂O₃ crystalline phase have been investigated over the temperature range 85-350 K. The specimens exhibit fairly high room-temperature conductivities of the order of 10^-4 Omega ^-1 cm^-1. The resistivity variation as a function of temperature is explained on the basis of a small polaron hopping conduction mechanism between the iron ion sites of different valence states in the glass phase. Mott's low-temperature T^-1/4 law and the recently formulated Triberis and Friedman high-temperature T^-1/4 variation are found to be obeyed by the present glass system. The density of localised states in the system calculated using the above models is found to be of the order of 10²⁰ eV^-1 cm^-3. However, the T^-2/5 law of Triberis and Friedman gives a value for the density of localised states which is consistent with the TM ion concentration within the glass phases of the glass-ceramic system.

The electrical conductivity of plasma copolymerised films from allylamine and ethylene increases with an increase in the flow-rate of allylamine during the polymerisation, while the optical gap evaluated from optical absorption decreases. Deconvolution of infrared spectra showed that the ratio of sp² to sp³ carbons is not related to the nitrogen content of the film. This fact shows that the conductivity increase is not due to the development of conjugated carbon double bonds. It is deduced that the increase in conductivity is mainly caused by two different effects of the introduced nitrogen, i.e. the reduction in optical gap and the increase in unpaired electrons, both arising from the interaction between nitrogen atoms and conjugated carbon double bonds.

Optical characterisation has been performed for single crystal 2H-SnS₂ synthesised by the chemical vapour transport method at low temperatures. The optical absorption spectra measured in the normal-incidence geometry at room temperature show an indirect transition with an energy gap E_g=2.17+or-0.01 eV. Photoluminescence spectra at liquid helium temperature indicate two series of phonon replica of excitons with phonon energies LO₁=62 meV and LO₂=70 meV, where the zero phonon lines are 2.310 eV and 2.224 eV respectively.

The bleaching of thermoluminescence (TL) yield to a residual value by light is explained, at least in some cases, to be due to an equilibrium between excitation and de-excitation by the illuminating light. Kinetic differential equations are given governing the process for the simple case of a single trapping state for electrons and a single variety of hole recombination centre. The equations are solved numerically for chosen sets of the relevant parameters starting either from empty traps and centres or from highly populated ones. In agreement with experimental results, the numerical computation predicts that after a long enough illumination the process of filling of the traps converges to the same equilibrium value as that produced by bleaching. It is also found that the equilibrium value is independent of the light intensity, though, of course, this equilibrium is approached faster with more intense light. The computed dependencies of the filling of trapping states as a function of illumination time are shown. The final equilibrium value for the filling of trapping states can also be evaluated analytically for any set of given parameters; these compare very favourably with the calculated results. The change of the results with the variation of the relevant parameters is discussed.

The amorphous silicon film, excited by a picosecond laser, has been investigated by a laser microprobe. The optical properties are measured simultaneously with its Raman spectrum. The sharp Raman peak of this properly annealed film is similar to that of crystalline silicon (c-Si) with a significant drop in reflection. A second pulse of proper fluence can reduce the Raman intensity and increase the transmission greatly and this is ascribed to explosive crystallisation and laser quenching. This reamorphisation phenomenon of the annealed film suggests a feasible method for erasable optical recording.

Pulsed nitrogen laser induced ablation of solid zirconium targets was monitored using laser induced fluorescence. Starting from 'new' surfaces, the density evolution under the influence of different gas atmospheres (oxygen, helium, hydrogen and nitrogen) with pressures up to 10^-3 mbar has been studied. It was observed that even small amounts of gas lead to a large increase in the velocity and the density of the ablated atomic cloud.

The possibility of creating a Carnot cycle for photovoltaic energy conversion is explored. The thermodynamic functions for an electron-hole gas are derived and they are used to show that a conventional Carnot cycle involving two heat reservoirs can be constructed. The additional problems which arise from radiative energy transfers are show to lead to the result that only if the electron-hole plasma is simplified to the two-level case can a reversible cycle occur. Since this cycle requires an infinity of reservoirs it is not a Carnot cycle in the sense of classical thermodynamics.

Third-order nonlinear optical properties of a solution-processible copolymer of 3-methylthiophene and methyl methacrylate are reported. The value of third-order nonlinear susceptibility X⁽³⁾ approximately 7*10^-12 esu of the copolymer is about two orders of magnitude smaller than that of poly(3-dodecylthiophene).

The previously presented model of Bala and co-workers (1989) for the variation of effective thermal conductivity of powders (two-phase system) with porosity has been extended and developed to predict the thermal conductivity of two-phase macroporous systems with different interstitial media. Numerical computations of thermal conductivity with the range of porosity have been done for alumina-helium, alumina-air, silver-helium, silver-air, silver-carbon dioxide and silver under vacuum conditions. Computed values of effective thermal conductivity are in close agreement with the experimental data of Smith and co-workers (1962, 1963) in the porosity range under study.

In view of contradiction in the dielectric strength of the CF₂Cl₂/N₂ gas mixture reported by different investigators, the authors report the Townsend first ionisation coefficient alpha and the electron attachment coefficient eta measured in that gas mixture with various mixing ratios using the steady-state Townsend method over the range 40<or=E/p<or=140 V cm^- Torr^-1. The limiting E/p values derived from pre-breakdown current growth were in good agreement with static breakdown experiments. In contrast to the SF₆/N₂ gas mixture, the dielectric strength of the CF₂Cl₂/N₂ mixture as a function of CF₂Cl₂ concentration is quite close to a straight line similar to that of CF₂Cl₂/CO₂.

The authors present the results of self-consistent two-dimensional numerical simulations of RF glow discharges in non-attaching (N₂) and attaching (SF₆) gases using a single-moment fluid model. A more accurate flux-corrected transport (MAFCT) and a reconstructed fast Fourier transform (RFFT) techniques have been used to solve the space charge equations in the presence of large gradients and dynamic range. The distributions of the normalised electron velocity in a cylindrically symmetric geometry have been found to be considerably different between N₂ and SF₆. The results obtained further illustrate the important role of the sheath effect close to the edge of the radial wall of the reactor.