Table of contents

Polarized UV - visible absorption spectroscopy was used to investigate the in-plane orientation of the hemicyanine molecules in Y-type Langmuir - Blodgett multilayers. We demonstrated for the first time that the dipping-induced molecular alignment was enhanced with increasing the layer number due to the interlayer interactions. Polarized UV nanosecond pulses could be used to control the photo-induced reorientation of the hemicyanine molecules. The thermal-isotropic process was accompanied by dissociation of the H-aggregate in the hemicyanine LB multilayers.

The luminescence of during and irradiation was measured in the 190 - 820 nm wavelength range. The luminescence evolution with the ion fluence exhibits two behaviours : (i) at low fluence, the amount of centres increases; (ii) at high fluences, these defects are completely (F centres) or partially ( centres) annihilated. This phenomenon results from two concomittant mechanisms : a conversion between F and defects and a destruction of both luminescent species resulting from the radiation-induced damage. By using a simple model we have determined the cross sections associated with creation and annihilation of the centres. The irradiated samples were also investigated by cathodoluminescence and Auger electron spectroscopy. A higher concentration of structural defects and centres is evidenced at the sample area previously irradiated by ions, leading to an unsteady regime of the surface potential under electron excitation.

High-frequency capillary waves at a liquid gallium surface have been studied by means of quasi-elastic light-scattering spectroscopy. The observed frequencies and damping constants of waves differed greatly from the classical theoretical treatment of a liquid Ga surface as that of a simple liquid. This effect was explained in terms of the presence of the surface layer, possessing visco-elastic properties which were, in turn, extracted, for the first time, from the fit of experimental spectra with an appropriate theoretical form. A negative value of the surface dilational viscosity has been derived for all wavenumbers studied and the effect of the visco-elasticity on the strong coupling and mixing between the capillary and dilational mode on the surface of liquid Ga has been demonstrated.

We present high-resolution measurements of the temperature-dependence of mode frequency for three monocrystalline microwave sapphire dielectric resonators at liquid helium temperatures. In two resonators the susceptibility is independent of frequency and displays a non-Curie law behaviour. This can be interpreted as arising from a van Vleck temperature-independent contribution to the paramagnetic susceptibility of the and ions involved. We also report measurements of the combined dependence of thermal expansion and permittivity on temperature in the range 2 - 20 K and show that this is about 50% larger in all three resonators than the value calculated from the presently accepted thermal expansion coefficient of sapphire.

A fully time-dependent collisional - radiative model is employed to calculate relevant population densities in a recombining carbon/hydrogen z-pinch plasma. In particular, the dependence of the small signal gain G on the maximum electron temperature and cooling rate, as well as the influence of Lyman- reabsorption, are studied. Although in conditions typical for dynamic z-pinches the maximum electron temperature and cooling rates would, in principle, be sufficiently high, gain on the Balmer- transition is strongly reduced by Lyman- reabsorption. In order to investigate vacuum spark capillary discharges, the system of rate equations is coupled with balance equations of the plasma energy and the total number of heavy particles. The resulting set of equations is solved self-consistently. Results are presented that show the systematic dependence of the small signal gain on electrical input power, wall material, and capillary geometry. High gain coefficients could be achieved by modelling high-voltage discharges with short ringing periods through capillaries containing boron or carbon. While the maximum achievable gain coefficient for lithium is rather poor the duration of population inversion would be long enough (a few tens of nanoseconds) to make multi-pass operation possible.

We report on the properties of a special RF discharge characterized by a trapezoidal shape of the driving voltage pulse. This driving mode offers an advantage in comparison to the common sinusoidal voltage - in allowing the separation in time of effects which are connected with the time derivative of the driving voltage (displacement current, -mechanism) from effects connected with the amplitude of the driving voltage (production of electrons at the electrodes, -mechanism).

The energy gain and loss of electrons in this RF discharge are studied in a collision-dominated helium discharge with special emphasis on the -mechanism. Experimental results of space- and time-resolved plasma-induced emission spectroscopy are compared and combined with the results of a particle-in-cell Monte Carlo collision simulation. It is demonstrated that the electrons gain energy during the temporal change of the discharge voltage at high neutral particle pressure (400 Pa) in the vicinity of both sheaths and that they dissipate energy in these regions by inelastic collisions. At a lower neutral particle pressure (100 Pa) the electrons gain energy in the vicinity of the expanding sheath and lose energy at the contracting sheath. In this case the energy loss by inelastic collisions takes place in the bulk plasma.

The two-dimensional radial and azimuthal electron temperature and density distributions in a magnetized ring discharge (B = 100 - 190 mT, P(He) = 3 - 6 Pa, are obtained from measured intensity ratios of the HeI lines 447.15 nm, 471.31 nm, 492.19 nm and 587.56 nm, using a calibrated ICCD camera system. Dynamical plasma emission imaging (DPEI) represents a new diagnostic tool for studying not only the stationary parameters but also the electron temperature and density oscillations induced by a coherent low-frequency instability. The phase shift between temperature and density oscillations, as well as the radial profiles of the oscillation amplitudes, may be determined. These features characterize a prominent type of drift instability arising in an inductively coupled rf discharge with an axial magnetic field superimposed where the radial electron temperature and density gradients are in opposite directions. Numerical simulations recurring to the measured plasma densities and temperatures help to clarify the anomalous transport leading to a degradation of plasma confinement.

A new kinetic scheme of the non-equilibrium gas discharge plasma is employed to describe the characteristics of a low-pressure cascaded arc plasma in pure hydrogen. The presented kinetic model includes the dynamics of the following plasma components: vibrationally excited hydrogen molecules in the ground electronic state , hydrogen atoms in the ground and electronically excited states (H(n), n=1,2,...) and electrons, positive and negative hydrogen ions . It has been shown that the low-pressure hydrogen cascaded arc plasma is characterized by high degrees of ionization and dissociation, and can generate high-density fluxes of vibrationally excited hydrogen molecules and negative ions.

Thermoluminescent dosemeters of LiF:Mg,Ti (TLD 100), LiF:Mg,Cu,P (GR 200A) and (TLD 200) were used as detectors for absolute brightness measurements of x-ray emission from a laser-produced plasma. The influence of very short (nanosecond) exposure of the dosemeters by x-ray radiation from a keV spectral range is discussed. Thermoluminescent glow curves of (TLD 200) irradiated by x-ray radiation from radionuclide standards and laser-produced Al or Cu plasmas are analysed applying deconvolution based on general TL kinetics. The dependence of the ratios of areas of low-temperature single glow peaks on x-ray photon energy from 1 to 55 keV is shown.

The present paper reports the Townsend first ionization coefficient in tetraethoxysilane (TEOS) vapour and oxygen mixtures determined experimentally by the steady state Townsend method for E/N from 350 to 2000 Td . This gas mixture is used as a process gas when silicon dioxide films are deposited by using plasma enhanced chemical vapour deposition at low substrate temperatures. The result shows that remarkable synergism for the ionization coefficient exists in the mixtures. The maximum value of the coefficient reaches about 140% of that estimated by a linear interpolation between the values in pure TEOS vapour and pure oxygen. It is also shown that the fractional percentage pressure of the mixture which gives the maximum values of the ionization coefficient linearly decreases with increasing E/N in the present measurement.

The objective of this paper is the formulation of general macroscopic models that can be used in numerical simulation of spark discharge processes in inert gases. The governing equations for a non-polar single-substance continuum material body exposed to thermo-mechanical and electromagnetic actions are formulated in the Eulerian, as well as in the Lagrangian, description. One-dimensional, cylindrically symmetric models are derived in both descriptions. The previously given model of Plooster is reviewed and corrected. Results from simulations are presented, verifying the fact that significant dissimilarities concerning amplitude and speed of the shock will appear when the correct governing equations are not used in the simulation.

Two different one-dimensional models describing the evolution of sparking in air are solved numerically using the artificial viscosity method and a flux-corrected transport algorithm. The influence of the solution technique, especially the treatment of the shock wave, is studied. The breakdown stage of the spark is included in the initial conditions. The electrical energy input is calculated from a prescribed current waveform coupled to the calculation of plasma conductivity. The effect of varying power input on the evolution of the plasma kernel is studied. It is shown that shock wave reflection from a rigid wall close to the spark gap influences the growth of the plasma kernel. The results indicate that the Eulerian description of motion can be used to model flow with shock waves with accuracy equal to that of the Lagrangian description.

A theory of x-ray diffraction (XRD) in the presence of optically induced superlattices has been developed both in the case of thin, non-absorbing and thick crystals. The main features of this process have been analysed. Experimental requirements to observe coherent influence of the optically induced superlattices on the process of XRD were formulated. It was shown that the resonance influence of the optically induced dynamical gratings on XRD can be observed under experimentally feasible conditions.}

The mass density, the specific heat at constant pressure and the electrical and thermal conductivities of thermal plasmas formed in - tungsten vapour mixtures were calculated as a function of temperature, for molar concentrations of tungsten in the range 0 - 50% and for pressures 0.1 - 1 MPa. These properties are not really disturbed by the presence of tungsten for proportions lower than 10% except for the electrical conductivity, which increases with the proportion of tungsten at temperatures lower than 13000 K. The results given in this paper show that, for tungsten proportions under 10%, the properties of pure can be used, except for the electrical conductivity. For this last property, the values are very close to those of an - Cu mixture of equivalent proportions.

The annealing effects of As-ion-implanted (211)B CdTe films grown by molecular beam epitaxy have been studied by resonant Raman scattering and photoluminescence spectroscopy. The Raman intensity ratio of 2LO to 1LO phonon modes and the normalized bound-exciton emission intensity indicate that better removal of the implantation-induced damage is obtained with an increase of the annealing temperature beyond , whereas the lattice perfection drops sharply when is higher than . With an increase of in the studied range, more and more As atoms occupy the Te sites as acceptors and the samples behave in a p-type manner with a decreasing compensation coefficient and an increasing hole concentration.

Measurements of the direct current resistivity, , on ceramic zinc stannate polycrystalline samples that had been annealed in a reducing atmosphere at temperatures of C, C, C and C were made as a function of temperature T from room temperature down to the liquid nitrogen temperature. In this temperature range, the data showed that the main contribution to the conductivity comes from carriers that hop directly between localized states executing variable-range hopping processes. Analysis of the data showed that the resistivity - temperature characteristics were well described by the Mott law, confirming the absence of a Coulomb gap at the Fermi level and suggesting that the density of states at the Fermi level is constant. The variation in the characteristic hopping temperature with the annealing temperature showed that was thermally activated with an activation temperature of .

The generation of fast rise-time electromagnetic shock waves in nonlinear transmission lines containing ceramic ferroelectric dielectrics is investigated. The theory of shock wave evolution is explored and used to determine the desired characteristics of the nonlinear dielectric. The rise-time which would be expected in a barium - strontium titanate based ceramic dielectric is estimated from the material properties. This is compared with the actual rise-time obtained in a parallel plate transmission line made from blocks of the ceramic. Shock waves have been generated with amplitudes of 10 kV and 5 kA and a rise-time of 410 ps. The problems associated with measuring the rise-time of the shock waves are discussed.

This paper presents the results of pulsed characterization experiments conducted on vertical GaAs optically activated switches. The voltage hold-off characteristics, optically activated switching properties and lifetime performance of the switches have been examined. An all solid state system has been used to assess the characteristics of epilayered and switches fabricated from liquid encapsulated Czochralski (LEC) and vertical gradient freeze (VGF) GaAs. Non-epilayered devices have also been fabricated from the former material. A range of voltages have been applied to the different switches, up to a maximum of 6.5 kV for the reverse biased VGF devices. The longest lifetime recorded was shots for a LEC device, in which case no damage was visible to the switch. Lifetimes of shots were also observed for the forward-biased devices. A simple model of the voltage dropped across the switch during closure has been developed which assisted in the reduction of the switch `on' resistance. Lifetime and post mortem studies suggest that, in these experiments, the switch closure was due to a single filament. It is concluded that operating a switch with more than one filament, or ensuring that a single filament does not occur in the same position twice, is an effective method for increasing device lifetime.

This paper aims at shedding some light on the beam energy spread versus current curves of liquid metal ion sources (LMISs). Although Knauer's collisionless space-charge broadening model originally gave almost perfect agreement with experiment, deviations were discovered later at low and high currents. To make matters worse, as more metals were investigated, complete disagreement was found in certain cases. Although the low-current deviation from Knauer's model is not well understood and only tentative explanations can be given, the high-current deviation has been explained in terms of a self-sustaining instability that sets in at a critical current, the value of which varies greatly from metal to metal. This instability violates a basic premise of Knauer's model, namely that the ions' paths do not cross after their emission. Calculations show that, for metals for which there is total disagreement with Knauer's model, the instability sets in almost from the onset. An important result of this work is the development of a stability criterion for LMISs. It is shown that a low evaporation field and a high surface tension are two principal factors that improve stability. The stability is also affected, to a lesser extent, by the liquid density and charge-to-mass ratio of the ions.

The presence of current pulses superimposed on the dc level of the ion current in liquid metal ion sources has been known for a long time. This work investigates the behaviour of the pulses for varying temperature and working liquid metal. Attempts are made to explain the differences observed in the threshold current for the appearance of the pulses and also in their terminal, or saturation, frequency. Differences are also found between sources using the same working metal, although the pulse appearance threshold current is the same for a given metal.

The experimental results indicate that a low source operating temperature is desirable if droplet emission is to be minimized.

A multi-layer micro-electrode structure has been developed for the selective manipulation and separation of bioparticles using travelling field dielectrophoresis effects. An important feature is that, in the separation process, the selected particles move in a stationary supporting fluid. Stationary suspensions of viable and non-viable yeast cells were used as a model system to demonstrate the general application of this device for the selective retention or transport of bioparticles in suspended mixtures. The efficiency of this process depends on the dielectric properties of the particles and their suspending medium, and is a sensitive function of the frequency of the travelling field. Apart from their use as particle separators, such micro-electrode devices are also envisaged to form integral components in the development of `biofactory on a chip' technology.

Forward spectrum weighting has been used to generate broad-group cross sections. However, for some applications of perturbation theory, the adjoint spectrum is needed. Recently an approximate method has been proposed for the adjoint spectrum in a lattice cell, based on an equivalence principle. The approximations made can be avoided if the neutron energy dependence is treated in detail, and the neutron transport through the regions of a lattice cell is adequately represented. In this paper the neutron balance adjoint equations are derived for a heterogeneous unit cell based on the collision probabilities method, and the adjoint spectrum is calculated from this system of equations for different intervals of energy. Forward and adjoint spectra comparisons are done for a unit cell having in its inner region. In the numerical applications of the neutron balance adjoint equations the most important resonances of were considered. The results show significant variations between the forward and adjoint spectra, leading to remarkable differences in the effective resonance integrals.

The uptake of zinc by cerebellar rat cultures upon exposure to solutions was comparatively investigated using two well known condensed matter physics techniques: synchrotron photoelectron spectromicroscopy and inductively coupled plasma atomic emission spectroscopy. The objective was to apply a strategy - well known in surface physics - to distinguish between `surface' and `bulk' phenomena. The results clearly demonstrate that exposure significantly enhances the bulk (cell cytoplasm) Zn concentration with respect to the physiological level, whereas the effect on the surface (cell membrane) is negligible.

A dynamical model based on the two-fluid dynamical equations with energy generation and loss is obtained and used to investigate the self-generated magnetic fields in high-temperature dense plasmas such as the solar core. The self-generation of magnetic fields might be looked at as a self-organization-type behaviour of stochastic thermal radiation fields, as expected for an open dissipative system according to Prigogine's theory of dissipative structures.