Table of contents

Recent publications have shown that electron impact collisions are more important than Penning collisions in populating the upper level of the 441.6 nm transition of He-Cd⁺ lasers operated as positive column discharges. The authors present evidence that this is not the case in hollow cathode discharges.

The authors present measurements of the variation of the longitudinal sound velocity with pressure in shock loaded Ti-6Al-4V specimens. The experimental technique is based on monitoring the stress-time history inside the target with thin manganin stress transducers. The resulting values can be represented by the following linear relation between the sound velocity C_L (mm mu s^-1) and shock stress sigma (kbar): C_L=6.13+0.0049 sigma . This relation is in good agreement with published hydrostatic data for alpha titanium in the 0-9 kbar range.

A new formula is compared with that of Onsager, Kirkwood and Frohlich (OKF) for 39 associated and non-associated liquids. The new expression is found to be generally superior to the older one and often yields excellent estimates of the static dielectric constant with no disposable parameters such as the uncertain Kirkwood correlation factor g needed in the OKF equation.

Current density-voltage measurements have been performed on pure ZnO and ZnO containing 2% by weight of Sn. Both types of sample showed ohmic conduction at low fields followed by a power-law dependence with exponent of approximately 3 for the pure samples and approximately 2 for samples containing Sn. The results were interpreted in terms of space-charge-limited conduction controlled by an exponential distribution of traps. The form of the dominant trapping distribution differed between the two types of sample. An analysis of the trapping parameters indicated that the trapping density was significantly higher for the samples containing Sn. Simple extensions to existing theory were sufficient to predict the transition voltage between ohmic and space-charge limited conduction, and to account for the higher values observed for the pure samples than for those containing Sn.

For original paper see ibid., vol.16, p.915 (1983). The accuracy of the measurement technique used by Kent and Meyer and the reliability of their data on dielectric constant epsilon ' and loss tangent epsilon " are discussed.

Measurements are reported on the energy distribution of the ion beam produced by Ga liquid metal ion sources, for a range of emission currents, i approximately 1-50 mu A. The full width at half-maximum (FWHM). Delta E_1/2, of the energy distribution curves varies from approximately 4.5-5 eV at approximately 1 mu A to approximately 35 eV at 50 mu A. Three regions can be detected in the Delta E_1/2 versus i curve. In the first region, between approximately 0.6 and 2 mu A, Delta E_1/2 varies very slowly with current. In the second region, above approximately 2 mu A and extending up to approximately 25 mu A, the curve rises steeply with current, in fairly good agreement with the predicted Delta E_1/2 varies as i^2/3 relation. For i>or approximately=25 mu A the curve deviates markedly from this relation, and tends to flatten. Although a tentative explanation for the deviation from theory at high currents may be given in terms of gross instabilities of the anode, the deviation at very low currents (where Delta E_1/2 is almost constant with i) is not understood.

A simple method is suggested for measuring the series resistance of a solar cell which utilises only a single I-V curve at a moderate light intensity level.

The effect on the output of an electron beam sustained CO₂ laser due to LIMP (laser induced medium perturbation) is examined. An approximate theory is developed which leads to basic agreement with the experimental observations of beam spreading and temporal pulse structure.

An investigation of the gain of a CW atomic mercury laser using a tunable dye laser is reported. Several isotopes are seen to be simultaneously excited and found to lase in multiple isotope mixtures. The nitrogen pressure dependence of the gain is found to be in good agreement with rate equation analyses, provided a pressure dependent loss term due to mercury dimers is taken into account. The pressure broadening of the laser transition by nitrogen has been measured to be 21.1 MHz mbar^-1.

Attractive molecular forces give rise to singular tensile stresses and to a discontinuity of displacement at the edge of contacts. These boundary conditions are those of fracture mechanics and correspond to adding a rigid body displacement to the classical elastic solution. The adhesive contact of sectionally smooth-ended punches (sphere or cone) is studied. Stress distribution, shape of the deformed surface, and relation between load and penetration are given either for equilibrium or kinetic conditions. For a flat-ended sphere, the adherence of a flat punch and the JKR theory for sphere are obtained as particular cases; for zero surface energy systems, the solution reduces to that of Ejike. The case of a sphere with a spherical cap of different radius and that of a rounded cone are also examined. Experimental verification has been performed with a glass ball (R=2.19 mm and flats between 100 and 240 mu m radius) on polyurethane. Radii of contact, penetration and kinetics of detachment agree with the theory within 2%.

The water absorption sorptivity of brick ceramic falls as the initial water content rises. New results and reanalysed literature data are in agreement with unsaturated flow theory. The approximate formulae of Philip (1957) and of Brutsaert (1976) are compared with experimental data, establishing the value of the Brutsaert formulae.

An improved method of generating a cylindrical blast wave is described which overcomes the disadvantages inherent in the converging-diverging nozzle technique previously used by the authors (1981). A diverging detonation vessel was constructed from two sheets of glass and two Tufnol blocks which form a variable-angle sector. An exploding wire, placed at the apex of the sector, was used to generate blast waves of variable strength. The trajectory of the shock wave and the density field behind the front was recorded by a streak-Schlieren interferometer. It is shown that the observed velocity profile simulates the behaviour of that found in freely propagating detonations satisfactorily. Moreover it is confirmed that the post-shock density decay parameter has a constant value of 4 which is in accord with previous studies; it is suggested that this is a universal value in detonation cells.

Ion swarm transport is simulated by a Monte Carlo technique in the case of a constant mean free time collision model with isotropic scattering after the ions have been released into a gas with a delta -function distribution in space and a steady-state velocity distribution. The time behaviour of the transport coefficients is investigated for ion-molecule mass ratios m/M of 0.1, 1.0 and 10 under the condition that the field energy is one hundred times greater than the thermal energy. In particular, discussion is given for the skewness and excess of the density distribution. It is also shown that these higher-order coefficients become increasingly important with decreasing m/M.

The effects of the two-step ionisation and photoionisation from excited states of helium and argon gases are studied using a more realistic theoretical model for cascade ionisation. Two sets of experimental data were investigated, covering the wavelength range 720-840 nm with laser light pulse duration 18 ns and 40 ns at two different pressures, 760 Torr and 1.064*10⁴ Torr. These investigations show the importance of the photoionisation of excited atoms at atmospheric pressure, while at higher pressure the collisional ionisation processes dominate.

The effect of wall material and wall temperature on negative ion production in a hydrogen plasma has been studied. Under the same plasma conditions, within the combined uncertainties of the measurements, the same negative ion densities were found for Pyrex, stainless steel, copper and molybdenum. However, there seems to be a trend for stainless steel walls to produce somewhat higher negative ion densities. Contamination of the walls with hydrocarbons was found to reduce the negative ion density. It was also found that cooling the Pyrex walls leads to an increase in the negative ion density.

The authors present a theoretical model describing the evaporation and expansion of vapours from metal surfaces subject to high energy density. The model links the surface temperature of the metal with the evaporation rate and the hydrodynamic parameters of the vapour near the surface. These parameters serve as boundary values for the hydrodynamic equations describing the gas flow, which are solved by a 1-D Lagrangian simulation code. A criterion for the transition from hydrodynamic flow to molecular flow far from the metal is presented, with the aid of which they predict the vapour flux and velocity distribution as a function of the surface temperature. Good agreement between their predictions and empirical results is found.

Band-shaped aluminium implants on plane carbon targets were irradiated at 0.1-2*10¹⁵ W cm^-2 by 600 ps laser pulses of 0.527 mu m wavelength. The resulting blow-off plasmas were diagnosed by X-ray spectroscopy using entrance slits of various dimensions, orientations and positions with respect to the target plane. The authors studied the longitudinal and radial variations of line intensities and Stark widths of He-like and H-like Rydberg series. The effect of time integration was also qualitatively studied by varying implant thickness. Results showed a tenfold decrease of instrumental broadening due to the small lateral expansion of the plasma eliminating radial spatial deconvolution and minimising opacity effects. Comparison with computer modelling showed that n>or=4 Al¹²⁺ Lyman series emissions peaked in a 20 mu m width region close to the target and that electron densities deduced from Stark broadening agree well with the simulations.

Laser fluorescence spectroscopy is a useful technique to determine the density and the flux of metallic atom impurities in the edge region of Tokamaks. A new scheme to measure the Doppler profile of impurity atoms by a single laser shot is proposed. The frequency of dye-laser emission is rapidly scanned across the Doppler profile during the laser action in the order of microseconds. A flashlamp-pumped dye laser is tuned by a piezoelectrically scanned etalon, and a scanning range of 60 pm is obtained with a resolution of 3 pm at lambda approximately=600 nm. Fluorescence line profiles of Na atoms are determined with a resolution of less than 1 pm. Several problems of Tokamak measurements are briefly discussed.

This paper gives the Laue distribution of time delay of electrical breakdown (t_d) for five pressure values of nitrogen in gas-filled diodes. Distributions were obtained for 20% overvoltage, and passive time ( tau ) 5 s and 7.2*10³ s. The breakdown in nitrogen is induced by long-lived metastable states formed in the previous gas ionisation. The results show that the Laue distributions also hold for such groups of data when t_d varies between a fraction of a second and several hundreds of seconds.

A relationship between the isobaric, isothermal and isochoric lattice Gruneisen parameters has been examined and analysed for several polymers. Relationships between the various parameters describing the thermo-acoustic and nonlinear properties of polymers are developed and their relations with thermo-acoustic data established. It is shown that the isochoric temperature derivative of the sound speed or bulk modulus is the dominant factor, with a significant contribution to the thermo-acoustic properties. Using this simple model, Beyer's parameter of nonlinearity, thermo-acoustic parameters, molecular constant, Huggins parameter and the lattice Gruneisen parameters are shown to be related to each other. Calculated data on these parameters for eight polymers are presented. The results have been used to develop further understanding of the significance of microscopic factors such as molecular order and intermolecular interactions, upon macroscopic thermo-acoustic properties in polymers. A new dimensionless thermo-acoustic parameter S₀* expressed in terms of volume expansivity only has been introduced, which gives, on an average, a constant value of 1.06 in polymers.

An anomalous photovoltaic effect (APE) has been observed in thin films of zinc selenide obliquely deposited in vacuum for the first time. Photovoltages of up to 70 V have been measured in some samples. The effect is influenced strongly by some experimental parameters. The changes in the photovoltages may be attributed to the film structure which has been investigated during the course of this study.

The theory of dark conduction in dyes assumes thermal excitation from the ground state to an excited state, followed by trapping of the hole near the electrode. Electrical conductivity studies on crystal violet (CV), an organic dye of industrial importance, have been carried out in both dark and light conditions. The present paper deals with the effect of field, temperature and electrode materials on the electrical conduction in CV as well as in CV sensitised with AgI. These studies suggest that conduction is mainly due to the charge carriers injected from the electrode into the dye (Schottky-Richardson mechanism) and the electron transfer mechanism. Photo and dark activation energies in the sensitised dye have been calculated.

The sheet resistance has been measured at room temperature for non-homogeneous thin gold films deposited on a glass substrate. A non-ohmic behaviour is observed when the applied electric field exceeds a certain critical value which varies strongly with surface coverage. An effective two-phase conductivity model gives a good qualitative account of the observed sheet resistance and the critical field as a function of the amount of metal deposited.

Amorphous silicon films with high intrinsic resistivity have been produced by RF sputtering without the addition of hydrogen or halogen. Co-sputtering with tantalum resulted in n-type material with resistivity<1 Omega cm and a shift in the Fermi level of 0.6 eV.

The kinetics of structural relaxation have been studied in the amorphous alloy Fe₄₀Ni₄₀P₁₄B₆ over the temperature range 230-380 degrees C, by the analysis of the magnetoelastic coupling coefficient, Young's modulus and remanent magnetisation. Measurements provide evidence that different mechanisms should be considered in order to explain the observed behaviour. The results are discussed in terms of different theoretical relaxation models previously reported. Comparison of relaxation data with those predicted from models based on an activation energy spectrum shows an overall consistency.

Isothermal decay of TL peak XII recorded at five different temperatures, viz. 680, 693, 701, 711 and 719K, is shown to be non-exponential. The data when fitted into the May and Partridge equation yield the order of kinetics value between 1.3 and 1.4. Activation energy E and frequency factor s are determined independent of each other from isothermal decay data using a recently suggested method. Also the trapping parameters E and s are determined using a new method based on the fundamental TL equations of Antonov-Romanovski. The values obtained by the two methods are 1.83 eV and 1.76 eV for E and 2.8*10¹¹ s^-1 and 2.1*10¹¹ s^-1 for s respectively and these are in good agreement with each other.

In recent papers the authors (see ibid., vol.16, p.425, 1983) have described methods for obtaining trapping parameters from thermoluminescence curves. The methods are applicable when the traps are filled to saturation concentration. Here, the problem of obtaining trapping parameters from TL curves recorded under non-saturation conditions is discussed.

The characteristics of secondary electron emission (SEE) of three-component composites including organic compounds have been investigated. The maximum SEE yield for conductive composites which consist only of polyvinylchloride and electroconductive particles such as NATCNQ and carbon black is low (less than 1). The addition of a third component, such as titanium, magnesium or beryllium oxides, sodium chloride, or the complex of TCNQ with anthracene, has enabled the authors to produce composites characterised by high maximum SEE yields for low energy of primary electrons. The study of the SEE yield from such electron-conductive polymeric composites shows that the SEE yield depends above all on the species and quantity of the third compound. On the basis of the data obtained a channel electron multiplier CEM has been developed. The CEM made of the three-component system is characterised by a gain of 10⁷ to 10⁸ (3 kV) at the maximum output current of 10^-6 A.

The thermal melting model has been widely used to explain the liquid phase epitaxial regrowth during pulsed laser annealing of ion implanted semiconductors. The authors present results of similar calculations regarding regrowth of epitaxial heterojunctions such as the Ge-GaAs system. They calculate the Nd:glass laser ( lambda =1.06 mu m) fluences necessary to melt various thicknesses of the a-Ge layer and study the regrowth velocity. The high solidification rates can be used to produce metastable alloys which are conventionally produced by vapour phase quenching of nonthermal species.