Table of contents

A demonstration of the thermodynamic compensation law is given; this law is shown to be a consequence of experimental approximations. It has been shown that the Meyer-Nelder rule is a consequence of experimental methods that consider a straight-line approximation of the In(X) versus 1/T plot of measurements of a physical quantity X in a limited temperature range. The ability of the Meyer-Neldel rule to give meaningful results is linked to the quality of the measurements and approximations made, but it appears that no genuine physical effect or fundamental relationships or universal quantities underlie its applicability.

The results of a photodissociation study using KrF, ArF, F₂ and two-photon KrF and ArF laser excitation at 5.0, 6.4, 7.9, 10.0 and 12.8 eV respectively to pump the ground state of HgF₂ are reported. These are used to determine spectroscopic properties and in the evaluation of a mercurous fluoride, HgF(B² Sigma to X² Sigma ), dissociation laser to extend the visible tuning range of mercury halide lasers into the red. Photolytic UV pumping of the heated HgF₂ vapour within a fluorinated glass cell was chosen to provide an inert cavity. Theoretical potential curves and dissociation energies are given for HgF₂ and low HgF levels extrapolated from previous work on other mercury halide compounds and it has been deduced from this study that energies between 7.85 and 9.9 eV are required to pump the HgF(B² Sigma ) state. No HgF(B to X) emission, via dissociation of the parent molecule HgF₂, has been observed here or elsewhere, and this, coupled with the involatility, the corrosive nature and high pump energies required shows HgF* is not suitable as a laser medium.

In this paper, a large spectroscopic study of a hollow cathode discharge produced in a concentric cylinder perforated tube is presented. First, the N_Cd cadmium ground state and the He(2³S) metastable concentrations are determined from laser absorption measurements. Then, the thermal electron energy T_e is estimated to be 0.5 eV from LIF observations, which is confirmed by probe measurements. The n_e^t thermal electron density (5*10¹² to 2*10¹³ cm^-3 for 10¹²<N_Cd<10¹⁴ cm^-3 at 150 mA) is determined from H beta Stark broadening and from the He(3³P) total quenching rate measured by LIF on the He 388.9 nm line. Moreover, for several experimental conditions, many Cd, Cd⁺, He and He⁺ excited state population densities are deduced from calibrated emission spectroscopy. The population inversions between the 6G and 4F levels and between the 6F and 6D levels are observed. The voltage-current characteristics versus cadmium density and helium pressure are also recorded and related to the atomic excitation in the discharge. From the behaviour of the helium and cadmium excited state population, for different experimental conditions, the part played in excitation professes by the three electron groups (thermal, secondary, primary electrons) can be discriminated. Consequently an estimate of the electron energy distribution function is deduced by combining the absolute population measurement and rate equation for several He or Cd⁺ excited states chosen to probe a particular range of electron energy. Finally, the populating channels of several Cd⁺ excited states are investigated. It is determined that the mixed 9S, 9P, 8D, 6F , 6G and 6H levels are mainly populated by a charge transfer reaction with a total rate coefficient of 4.23*10^-10 cm³ s^-1. The 5F, 5G and 7D levels are essentially populated by radiative cascades and the 4F, 6D and 7P levels are populated by radiative cascades and by electron excitation from the Cd⁺ ground state.

Modified Rayleigh-Gans-Debye (RGD) approximations have been used as approximations to exact theory for fibre size measurements from the positions of the minima in the scattering curve. Size parameters in the range 2.0<or=x<or=20.0 are of interest. Percentage errors in size parameters obtained using the RGD approximation and modified RGD approximations have been tabulated for relative refractive indices m=1.05 and m=1.15.

Multiple-beam interferometric methods of the Fizeau-type in transmission and on reflection were utilised for the determination of the effect of ion-beam irradiation on the refractive indices and birefringence of Dralon fibres. Nitrogen gas was used to produce a continuous ion-beam current density of N₂⁺ ions equal to 100 mu A cm^-2 at a reduced pressure of 5.6*10^-4 Torr. Fibre samples which are anisotropic were exposed to the ion beam for the time intervals: 1.2, 1.8, 2.4, 3.6 and 4.8*10³ s. Empirical formulae are suggested for the measured variation of optical properties with time of exposure. The wavelength dependence of refractive indices of the irradiated fibres was studied interferometrically. Illustrations are given by microinterferograms.

A numerical solution of a system of equations describing DC and microwave diffusion-controlled discharge plasmas has been carried out. It is shown that a knowledge of the energy dependence of the electron momentum transfer cross section makes it possible to predict a priori the dissimilarities in the electron distribution functions in the two types of discharge: differences in electron mean energies in the case of equal power consumptions in inert gas plasmas; information on differences in electron distribution function 'tails' in the case of equal electron mean energies in inert gas plasmas and in the case of equal power consumptions in molecular plasmas.

A magnetogasdynamic arc model due to Kovitya and Lowke has been combined with a collisional-radiative model of the argon atom to show how intense radiation from an arc can produce excited-state densities in the boundary of the arc which are far in excess of the densities corresponding to thermodynamic equilibrium. Quantitative predictions based on this model have been compared with measurements recently reported by Farmer and Haddad. There are encouraging similarities between theory and experiment, although it is suggested that the theory could be further improved by including the effect of resonance radiation which is scattered in the arc boundary.

Rayleigh scattering of argon-ion laser radiation has been used to obtain number densities in the outer regions (T<9000 K) of a free-burning arc. Excellent agreement is found between the measured argon atom densities and those predicted by theoretical models (e.g. Kovitya and Cram, 1986). A considerable discrepancy exists between the number densities derived from these scattering measurements and those obtained from argon line emission measurements. This difference is shown to be consistent with the non-equilibrium theory of Cram and co-workers for the outer regions of these arcs.

For pt.III see Appl. Phys. Lett., vol.45, p.24-5 (1984). Measured temperature profiles are presented for free-burning argon arcs at pressures from 1 to 5 atm and comparisons are made with prediction based on the theoretical models of Kovitya and Cram and Hsu and Pfender. Evidence is presented for departures from local thermodynamic equilibrium (LTE) in the region near the cathode of the arc in agreement with previous results and it is also suggested that the assumption of LTE in the outer regions of the arc may not be valid.

Using the Poynting vector formalism under certain assumptions an analytical expression can be derived for the electrode lead currents induced by plasma moving in an inhomogeneous electric field between a number of electrodes. Its limitations are made explicit and its validity is experimentally checked in a needle-to-plane set-up where laser induced breakdown has been studied.

A computer simulation technique for the study of electron swarm parameters in gases is presented. The method enables calculation of the electron energy distribution function (EDF) as well as various average electronic properties in electrical discharges. The simulation is applied to He-CO mixtures and the results are in fair agreement with experimental values and with other computational works. A strong directional anisotropy in the EDF is observed at high relative concentrations of CO, especially at electron energies for which the inelastic collision cross section is relatively big and at high values of the reduced electric field (E/N). The calculations include the influence of elastic, inelastic and both superelastic vibrational collisions (SVC) and superelastic electronic collisions (SEC). The results show that the SEC create different plateaus in the EDF, affecting only the tail of the EDF. The role of SEC becomes more important as E/N is lowered, the effect being more evident in the absence of SVC.

Langmuir films of poly( gamma -benzyl-L-glutamate) (molecular weight=28000) at a variety of surface pressures both below and above the monolayer-bilayer transition have been deposited onto infrared-transparent chalcogenide glass discs using a horizontal touching method. Polarised infrared spectra were obtained on the resultant films (about 60 monolayers thick) and the dichroic ratios of the four strongest absorptions (3292, 1733, 1655 and 1549 cm^-1) were measured. At very low pressures (2.5 mN m^-1) the alpha -helices in the monolayer are randomly orientated. At moderate pressures (5-10 mN m^-1) where the film is a monolayer, the helices are oriented predominantly perpendicularly to the compressing barrier, but in a collapsed bilayer (25 mN m^-1) they are predominantly parallel to the barrier. The results, which are not confused by the direction of withdrawal as in conventional Langmuir-Blodgett deposition, demonstrate clearly an interesting reorientation effect in these films.

The authors report absorption and resorption current measurements on low-loss polypropylene over ranges of 10 mu s to 25 h, 0.5 to 2 MV cm^-1, and -175 to 100 degrees C. While the resorption transients always decay hyperbolically, above 20 degrees C the absorption transients develop a conduction tail which is insensitive to contact metal. The current is resolved into volume and surface components, whose variation with time, field and temperature is illustrated. TSD measurements show no significant charge injection from Al contacts, but electron-beam-induced depolarisation experiments disclose heterocharge. Very deep electron traps (depth >or approximately=0.9 eV) and, tentatively, hole traps approximately=0.75 eV deep are identified by TSD on electron-bombarded films. All the observations point to an ionic mechanism, with a threshold related to glass transition, for the conduction tail.

A shock wave in water was generated by an electric spark in the first focus of a truncated ellipsoidal reflector. From schlieren pictures of the reflector second focus region, taken at the moment the shock wave has reached it, radial pressure profiles of the focused shock wave were evaluated. The accuracy of the method used was tested by ray tracing through a semi-spherical layer inhomogeneity and by inverse reconstruction of the refractive index. The method was found to be useful for a qualitative indication of the pressure distribution at the second focus but laborious and inaccurate for a quantitative evaluation of the pressure.

M.H. Pitkethly and M.S. Bader (see ibid., vol.20, p.315-22 (1987)) used the Weibull two-parameter distribution P_f(L)=(1-exp(-L( sigma / sigma ₀)^w)) to model the probability of failure of a fibre of length L under an applied stress sigma where omega and sigma ₀ are the Weibull shape and scale parameters, respectively. The authors considered that these parameters depend on the specific material and the length of the fibre being tested. This interpretation leads to the determination of a set of constants sigma ₀ and omega for each fibre length with the same material. In this paper, it is shown that a unique set of parameters allows Weibull two-parameter distribution to model the probability of failure of one type of fibre.

The characterise the surface condition of thin solid foils (x approximately=1000 AA) by ion impact, a novel combination of different independent methods of control has been used: (1) Auger electron spectroscopy (AES); (2) secondary electron spectroscopy (SES); (3) measurement of the total electron coefficient gamma ; and (4) detection of elastically scattered projectiles. Surfaces of various elemental foils (6<or=Z_T<or=83) have been carefully prepared by heavy-ion- (Kr⁺-) induced sputtering. Coverage could be removed from the target surfaces (better than 0.3 remaining monolayers) and the roughness could be reduced considerably.

A spectrometer with different cells for photo-acoustic or thermal detection of optical absorption at liquid helium temperature is described. Three types of detectors can be used: a microphone a carbon-glass thermometer or the (semiconducting) sample itself. The first tests are made in the near-infrared range on III-V materials doped with chromium (GaP) and vanadium (GaAs, GaP and InP). They demonstrate the high sensitivity of the thermometer.

The electrical behaviour of a quantum Hall effect device is described in terms of an equivalent circuit. This circuit is able to model the principal electrical characteristics of an ungated Hall bridge device. The effects of electrical loading of the Hall terminals and some anomalous effects related to contact resistances are investigated experimentally using a GaAs/AlGaAs modulation-doped device and are described in terms of the equivalent circuit.

A model for the mechanism controlling transient photoconductivity (TPC) decay is proposed. From this model, the temperature dependence of the TPC response time is derived. This theoretical result allows the authors to explain the experimentally observed temperature dependence of the TPC response times, supporting thereby the proposed model for the TPC decay mechanism. The influence of the presence of iodide on the temperature dependence of the TPC response time is obvious. For AgBr:I emulsions at least two distinct linear parts in the Arrhenius plot were found, a first one resulting in a high activation energy associated with the activation energy for interstitial motion and a second one leading to a low activation energy corresponding to the trap depth of the shallow electron traps. Pure AgBr shows only one activation energy, presumably because interstitial motion remains possible at temperatures down to 40 K.

A method for studying the contact surface between a sphere and a plane is presented. The contact consists of a conducting sphere pressed against a metallised insulating strip. A model presented in the appendix allows one to calculate either the effective radius of contact or the electric resistance of an eventual interface layer. A set of experiments involving loading-unloading cycles (from 6 gf to 350 gf) of the sphere is described. Experimental values of the electric resistance are in good agreement with those obtained for the ideal case of a Hertz contact. The discrepancy with the Hertz law is found to be due to topographical defects.

High-field dark conduction in (DC discharged) plasma-polymerised thiophene thin films is reported. A contact-limited Schottky mechanism is found to predominate at fields above 3*10⁵ V cm^-1. The dominance of the Schottky mechanism is confirmed by employing dissimilar-electrode configurations. The metal-polymer interface barrier heights calculated are 1.22 eV for Al, 1.03 eV for Cu and 0.879 eV for Au.

Electrical resistivity and Hall effect coefficient measurements have been made on MnSi_1.73 thin films in the temperature range 100 to 850 K. The temperature dependence of resistivity shows the typical behaviour of a degenerate semiconductor. The energy gap calculated from resistivity and Hall effect data is about 0.45 and 0.42 eV, respectively. The sign of the Hall effect is positive throughout the whole measured temperature region, with an electron to hole mobility ratio less than unity. The hole concentration in the degenerate state was found to be 7.1*10²⁰ cm^-3.

Writing and erasing a holographic image on a PTD (photothermoplastic device) needs the adjustment of a great number of parameters. Only the parameters concerning corona charging are discussed. In conditions described previously, the diffraction efficiency of holograms recorded on a PTD reaches 20%. Such a high value makes PTDS interesting for many applications. However, the main interest in PTDS comes from the possibility of cycling, i.e. erasing and writing new holograms. In optical switching for telecommunication networks, many cycles are needed. With thermoplastics, such as SE 10, an abietic ester which gives high diffraction efficiency, several hundred cycles are made. The corona charging conditions are studied. It is shown that it is useful to choose a good-quality needle (materials, radius of curvature) because its degradation can be observed. This needle quality is discussed. It appears also, that in a corona set-up when an organic product is layered on the counter plate, the corona current can be abnormally high; it can be three or four times higher when a resistive organic material is layered on the metallic conductive counter plate than when it is not. This widely spread phenomenon is discussed. Finally, the acceptance potential that varies with corona conditions and which is a material parameter that varies with sample nature, temperature or layer thickness is presented. Also, the decrease of surface potential with time is shown to be a specific property of each product.

The equivalent dielectric constant of an inhomogeneous medium consisting of columns or layers of regularly spaced dielectric spheres is calculated using the method of Rayleigh as modified by Lam. A regular spacing of columns or layers is assumed, with only interactions from spheres within individual columns or layers taken into account. Interactions between columns (or layers) are ignored. The unit cell structure chosen for each geometry allows independent variation of volume fraction and inter-sphere spacing. For fixed sphere-to-sphere spacing, the plot of dielectric constant versus volume fraction is linear, and the slope is an infinite series of multipolar terms from neighbouring spheres. This series converges quickly except for the case of closely spaced spheres with high relative dielectric constant. The results for columns and layers may be used to bracket the effective dielectric constant of particle beds.

A technique for the measurement of the field dependence of the complex frequency-dependent susceptibility of magnetic fluids together with some results for a colloidal dispersion of magnetite are reported.

An electric pulse discharge has been used to generate dense hydrogen plasmas, and measurements were made of the Balmer recombination continuum between 260 and 400 nm. The results show that the effect of finite electron density on bound-scattering transitions is negligible over the whole range of experimental conditions.

Thin solid films of at least two superconducting orthorhombic phases of yttrium-barium-copper oxide have been formed by RF diode sputtering from the bulk material and subsequently heat treating the deposits. The effect of different film compositions, crystallisation heat treatments and substrate types on crystal structure and electrical characteristics are described.