Table of contents

Measurements of laser-relevant properties of neodymium doped sol-gel derived silica hosts are described. Samples with Nd as the only dopant reveal anomalously short fluorescence lifetimes characteristic of concentration quenching TEM measurements revealing clumping of the neodymium. Co-doping with aluminium allows better dispersion of the neodymium throughout the gel-silica matrix, but fluorescence lifetimes and efficiencies are still lower than expected. Phonon quenching by residual OH^- ions is suspected, absorption, FTIR and Raman spectroscopy being used to support this theory.

A new scheme for the measurement of picosecond and femtosecond single light pulses is proposed. The scheme is based on the generation of an acoustic wave with duration proportional to the light pulse but 10³-10⁴ times longer. Both volume and surface acoustic waves are considered.

A model is presented for the measurement of periodic acoustic fields using laser Doppler anemometry utilising the photon counting correlation method of signal analysis. The model agrees well with previous experimental work and yields, as limiting cases, earlier results (published previously) and results obtained from considering the process to be one of simple frequency modulation.

Phase-geometry-dependent third-order optimised bounds, optical potential bounds and self-consistent effective thermal conductivity of multiphase composites have been computed in the framework of multiple scattering theory. The optical potential bound is the strictest of them and depends considerably on phase geometry. Self-consistent results always lie between the bounds. The results obtained indicate a distinct improvement over other existing calculations. Finally an application of the method to the calculation of thermal conductivity of a technologically important composite has been considered.

A simple incremental kinematic model (IKM) has been used to calculate the variations of indentation pressure P_m and shape of the plastic field with depth, in bilayers, the indentation angle being 35 degrees . For films softer or harder than the substrate, and for an adhesive or frictionless contact, the model's predictions were compared with experimental results at various scales of test. The IKM describes quite well the shape of the plastic fields observed directly in millimetre scale tests on plasticine blocks. In agreement with its predictions, the measured hardness remained equal to the intrinsic hardness of the film until it was fully penetrated, whatever the ratio of film to substrate hardnesses, the friction or the scale of the indent (in plasticine or in thin metallic bilayers). However, for depths greater than the film thickness, the calculated variations of P_m did not fit the observations, except in the few cases where the bilayers were fully plastic.

In a joint experiment between the IFAM-CNR, Pisa, and the Institute of Physics, Academy of Sciences, Tiblisi, the authors studied the converging motion and the collapse of annular and spherical shock waves. The radial converging motion of the waves was followed by holographic interferometry and shadowgraphy. The experimental measurements indicated the stability of the imploding shock wave in both annular and spherical configurations and showed that its amplitude, and consequently velocity, increases steeply as the wave approaches the axis or centre of symmetry, giving evidence for the cumulative nature of the motion. A one-dimensional simulation code for spherically symmetric converging shock waves is used to reproduce the radial change of the velocity of the waves in both cases. The simulation results are compared with the analytical treatment of the problem.

Electric double layers have been produced in a double-plasma device by the injection of an ion beam into a target plasma. A longitudinal magnetic field is present, of sufficient strength to magnetise the electrons. The double layers occur if the electron density in the target plasma is not sufficient to neutralise the injected space charge. The strength and axial position of the double layers can be controlled by varying the plasma density in the target, the ion-beam density, the neutral pressure, or the magnetic field strength. The double layers seem to form at a position where the ion and electron fluxes satisfy, approximately, the Langmuir condition. The electric fields in the double layers are compared to the values predicted by the original Langmuir theory, later modified by Andrews and Allen to include the effects of reflected particles and initial velocities.

A collisional-radiative model is developed to calculate the recombination and ionisation coefficients for sulphur and fluorine in order to study the kinetics of an SF₆ arc plasma at high temperature (T>6000 K). The values obtained for the recombination coefficient of sulphur exceed that of fluorine by a factor of ten. Atom-atom collisions as well as radiative absorption of resonance lines have been taken into account. Lastly ion-recombination relaxation times are compared with the relaxation times of evolution of the electron density in an extinction Sf₆ arc.

Spectroscopic measurements are carried out on inductively coupled radio-frequency (RF) argon plasma over the pressure range 120 to 760 Torr. The electron number density is obtained by two methods: H_beta profile and the absolute intensity of the argon continuum. Number densities of excited argon levels are deduced from absolute line intensities. The electron density measured in the hottest zone of the plasma decreases from 10¹⁶ to 6*10¹⁵ cm^-3 with the decrease of pressure from 760 to 120 Torr. Comparison between the different number densities shows that the atmospheric plasma is in local thermodynamic equilibrium (LTE) whereas weak departures from equilibrium are observed at pressures below 300 Torr.

A set of electron gas-atom collision cross sections of krypton, giving good agreement between calculated and experimental values of drift velocity and Townsend first ionisation coefficient is obtained for the electron energy range 1< epsilon <30 eV by Boltzmann equation analysis. It is found that slight modification near the onset of excitation cross section alters the calculated Townsend first ionisation coefficient significantly; the difference of up to 100% between the values of the experimental Townsend first ionisation coefficient by Kruithof and by Dutton et al. can be accounted for by this modification.

The authors present a study about the temporal evolution of the electric field intensity during the plasma formation process. The experimental measurements have been performed in argon gas at low pressure and using pulsed microwave power at 2.45 GHz to produce the discharge. From the electric field radial distribution of the surface wave and with experimental measurements of the electron density and electric field outside the discharge tube, the temporal evolution of the electric field within the plasma is obtained. This field reaches a maximum during the creation process, in the first moments of the plasma formation. This fact justifies the maximum of the light intensity emitted by the plasma observed experimentally.

Low-pressure oxygen discharges created by surface waves are characterised. Three discharge characteristics, the electron-neutral collision frequency nu , the maintaining electric field E_eff and the power required to maintain one electron theta , are deduced from the measurement of the electron density. They are independent of the microwave power and are studied as functions of the tube diameter (8-52 mm), the pressure (0.05-2 Torr) and the excitation frequency (390 or 210 MHz). The concentrations of ground state and singlet molecular oxygen are measured by VUV absorption and the concentration ratio of singlet to ground state molecular oxygen is about 10%, whatever the microwave power and the pressure. The ground state atomic oxygen concentration is measured downstream from the discharge by VUV absorption. The dissociation rate increases with the microwave power but remains less than 5%. A power balance model in the discharge, i.e. calculation of theta , is performed and yields a relation between theta /N and E_eff/N (n being the total neutral density), which is in good agreement with the experimental results. Finally, similarity laws are shown in the form theta /N and E_eff/N (Nr), where r denotes the tube radius.

A Schlieren optical system has been used to measure changes in neutral density in a point-plane gap, during and following positive streamers in air. The results show that energy is transferred to the air molecules by two processes. Electron-neutral elastic collisions during the streamer generate a shock-front which moves radially outwards at around 350 m s^-1. Following the streamer current pulse,which lasts for around 0.1 mu s, positive ions drifting towards the negative plane transfer their energy to the neutrals over a period of tens of mu s. This positive ion transfer generates the corona wind and the schlieren technique allows the wind speed to be measured close to the point. Corona wind speeds were found to be in the range 8 to 12 m s^-1.

The director orientation of the ferroelectric liquid crystal molecules within a surface-stabilised is measured by means of Diamant capacitance bridge. It turns out that the polarisation versus voltage curve has a strongly frequency-dependent hysteresis loop. For example, the width of the hysteresis loop (i.e. the coercive voltage) increases with increasing frequency f like f¹2/, over a large range of frequencies. A simple mathematical model, which considers the director orientation to be uniform throughout the display, explains these experimental facts.

Microindentation hardness studies have shown that nitrogen bombardment of boron-coated nickel surfaces greatly improves the microhardness when compared with pure nickel. The nitrogen dose and energy and the boron thickness are important in determining the effect on hardness. The combination of these parameters determines the distribution of radiation damage relative to the boron-nickel interface and hence the degree of both intermixing and formation of boron nitride. The maximum useful nitrogen dose is determined by the onset of blistering of the surface layer.

Using a cheap and easily reproducible glass-spray system, the authors have successfully fabricated and characterised tin oxide (SnO₂) thin films of varying constituents and properties. The properties of such films, when optimised, compared favourably with those from well established and widely reported systems and procedures. They have further the dependence of electrical properties of the fabricated films on the concentration of working chemicals, thickness of the fabricated films, pressure of the carrier gas used and the degree of degradation under local atmospheric conditions. The dependence of optical properties of the films on some of these variables is also investigated. Doping procedure and optimum concentration of dopants are explained. Results of the authors' numerous investigations revealed that the resistivity remains fairly constant at its optimum low value over a wide range of film thickness. Resistivity of the film decreases from about 154 Omega cm for undoped film to as low as 50 Omega cm for films well doped with 0.7 g antimony pentachloride, in 1 ml of working solution. This doping does not decrease the optical qualities of the film. Possible applications of tin oxide films in the fabrication of silicon solar cells are also discussed.

Plasma treatment has greatly suppressed high-field current and electroluminescence in poly-p-xylylene (PPX) thin films prepared by Gorham's method. The breakdown strength of plasma-treated PPX (P-PPX) was about 10 MV cm^-1 and higher than that of PPX or poly-2-chloro-p-xylylene (PCPX). In addition, the dielectric loss of P-PPX was almost the same as that of PPX and it was low compared to that of PCPX. The ESCA spectra showed that there is some oxidation on P-PPX surfaces. It was suggested that the oxidised P-PPX surface layer suppressed hole injection from anode metals in P-PPX. These results suggest that plasma treatment suppressed collision ionisation in PPX and hole injection from anode metals more effectively than chlorine atoms in PCPX.

Magnetic measurements on sintered Y-Ba-Cu-O superconductor were carried out at 77 K using tubular samples. The magnetisation in the field range 0-150 G strongly depends on the sintering conditions of the sample and the external peak field value. The magnetic critical current density varies with field as given by J_c(H)=J_c(0) exp(-H/H₀). In the field range 0-150 G, J_c(0) and H₀ correspond to the Josephson weak links and are strongly dependent on sample preparation conditions. On the other hand, in the field range above 150 G these parameters correspond to the superconducting grains and are independent of sample preparation conditions.

The dielectric relaxation of 1,2,4-butanetriol in its supercooled liquid state has been studied at pressures up to 1 GPa. The frequency of the applied field is varied within the range from 1 mHz to 10 MHz. The automated and computer controlled dielectric spectroscopy measuring system is presented in detail. It is found that neither the dielectric relaxation spectra nor the static susceptibility can be accurately described within the theoretical framework which only considers the behaviour of non-interacting dipoles. Instead, the dielectric loss is described by the many-body correlated cluster theory of Dissado and Hill (1983). A relationship between the correlation parameters and the static susceptibility is discussed. The results are compared with previous findings on the homologues glycerol and 1,2,6-hexanetriol.

Li₂B₄O₇ activated with a Cu impurity exhibits the 'unique' characteristics of linear thermoluminescence (TL) effects, a rare feature very similar to that of LiF:Mg, Cu, P, another phosphor of this category. Its thermoluminescence is investigated for its trap distribution characteristics. The analysis, based on a fractional glow technique, shows that TL processes in Li₂B₄O₇:Cu involve a number of trap groups with activation energies in the range of 0.9 to 1.73 eV distributed 'conspicuously' in the 80-355 degrees C glow temperature region, strikingly dissimilar to that of other common TL phosphors. The main glow peak region of 170-260 degrees C with a peak at 220 degrees C has been linked to the presence of predominantly deep trapping levels having energy in the region of 1.65+or-0.08 eV. The adjoining lower- and higher-temperature TL contributions are ascribed to the occurrence of shallower levels with considerably lower activation energies of thermal stimulation. These thermoluminescence characterisation data, in terms of the competing TL trap distribution model proposed earlier, satisfactorily explain the 'unique' behaviour of this material. The common and exceptionally rare TL features of Li₂B₄O₇:Cu and LiF:Mg,Cu,P are attributed to the physical characteristics of Cu as a dopant. It is suggested that doping with Cu introduces predominantly deeper trapping levels in lattices like Li₂B₄O₇ and LiF.

The dark-current source of a semiconductor field-assisted photocathode has been theoretically analysed and calculated. Under a given temperature of operation, a principal source of dark-current emission from the cathode is the impact ionisation component of hot holes in the Schottky barrier range. A specific expression for the multiplication factor of the impact ionisation is obtained. According to the theoretical calculation, the emission intensity of dependence of dark-current on the applied bias, doping concentration and operating temperature has been predicted. It may be helpful to design a field-assisted photocathode and choose an optimum operation condition.

It is shown that pulsed laser-induced fluorescence in combination with pulsed supersonic expansion is an appropriate technique for sensitive and selective analysis of gas mixtures containing different small polycyclic hydrocarbons that may play an important role in the sprouting of stored potato tubers. The vibronic excitation spectra of 1,4- and 1,6-dimethylnaphthalene are presented.

The Townsend first ionisation coefficient alpha and the electron attachment coefficient eta were measured in CF₂Cl₂/CO₂ gas mixtures using the steady-state Townsend method over the range 35<or=E/p<or=140 V cm^-1 Torr^-1 with the mixture ratios being 10/90, 25/75, 50/50, 75/25, and 90/10. The limiting E/p values derived from the pre-breakdown current growth measurements were in agreement with the results of static breakdown experiments. It is interesting to note that although the effective ionisation coefficient alpha in CF₂Cl₂ is quite close to that measured in SF₆, the dielectric strength of the CF₂Cl₂/CO₂ mixture is apparently lower than that of SG₆/CO₂ with the same CO₂ content.

The mechanism of electron temperature determination in a positive column of glow discharge plasmas was explained in terms of the balance of charged particle creation against the loss to the wall, in which Friedman's modification of Bohm's sheath criterion was taken into account. The essence is to correctly estimate the term of wall loss from a fluid-dynamic point of view. The reason for applying a fluid-dynamic description to the low ion-neutral collisionality region is also discussed.

In order to characterise the laser ablation process from high-T_c superconductors, the time evolution of plasma produced by a Q-switching Nd:YAG laser from a GdBa₂Cu₃O₇ superconducting sample has been studied using spectroscopic and ion-probe techniques. It has been observed that there is a fairly large delay for the onset of the emission from oxide species in comparison with those from atoms and ions of the constituent elements present in the plasma. Faster decay occurs for emission from oxides and ions compared with that from neutral atoms. These observations support the view that oxides are not directly produced from the target, but are formed by the recombination process while the plasma cools down. Plasma parameters such as temperature and velocity are also evaluated.

Low resistance ohmic contacts were manufactured on a natural semiconducting diamond (type IIb), by implanting boron ions at a temperature of 200 degrees C. This was followed by annealing at 1200 degrees C and removal of the top ion-damaged layer by etching in a hot acids solution. The thin, highly doped surface which remained could then be contacted with metals which need not interact chemically with the diamond surface.

Ytterbium (Yb), was thermally evaporated onto the Y-Ba-Cu-O thin film surface to form an ohmic contact. Specific surface contact resistance in the 10^-6 Omega cm² range was achieved for an as-deposited single layer Yb contact. Specific surface resistance in the 10^-9 Omega cm² range was achieved by using multilayer metal contacts, such as Yb/Cr/Al. These metals used for high-quality ohmic contacts to superconductor thin films are of significance due to their compatibility with semiconductor electronic devices.

The threshold voltage of streamer initiation in cyclohexane does not change much from its boiling point to its melting point. Below the melting point, there is a substantial increase in this voltage. This is thought to be due to the lowering of the injected current by space charge phenomena, the charge carrier mobilities being much lower in the solid state.