Table of contents

The assumption that images formed in the scanning transmission electron microscope with an annular detector may be interpreted in terms of incoherent imaging theory is shown to have limited validity.

The results of gas breakdown in argon are presented where breakdown was caused by picosecond pulses from a Nd:YAG laser. Laser pulsewidths were 30, 55, 140 or 270 ps. Using this data with that previously known, a graph of the variation of breakdown threshold with laser pulse duration leads to confirmation that the predicted behaviour of multiphoton ionization processes expected for very short laser pulses is not observed.

A simple analytical study is given for the relativistic equations of motion of an electron in a uniform magnetic field and a circularly polarized microwave electric field. This study allows, in an easy way, for the investigation of the effect of the different factors on the energy exchange between the electric field and the electron, and the state of motion of the electron with respect to the electric field. The jump of electron energy with the change of initial phase of the electron motion is studied in detail. Optimum conditions for energy exchange are derived.

The iodine photodissociation laser oscillator has been investigated over a range of operating conditions. Flashlamp pumping sources have been compared with special emphasis on microsecond excitation. The model for the laser in which the laser output is proportional to the pumping energy in excess of threshold and collisional quenching of the upper-level population is negligible. Laser output energy densities up to 33 J litre^-1 (for laser gas CF₃I) and 110 J litre^-1 (for n- and i-C₃F₇I) are obtainable without pyrolysis; the peak stimulated emission cross section for 100 Torr of n-C₃F₇I is 0.9+or-0.2*10^-18 cm².

Thermalization parameters for water poisoned with non-1/ nu absorbers, Cd, Gd and Sm have been calculated using the neutron scattering kernel recently proposed by the authors (see abstr. A43583 of 1974). Calculated values of the parameters are compared with the available experimental data and with values based on other scattering kernels. For the first time a reasonable agreement between the theoretical and recent experimental results is obtained. Time-dependent spectra in infinite assemblies have also been calculated and reaction rates as functions of time have been computed. These are in close agreement with the measurements of Fujino et al. (1971).

In electron energy distribution measurements with Langmuir probes, in low pressure discharges, distortions appear if secondary discharge parts such as anode glow or a positive column exist between the plasma surrounding the probe and the reference electrode. For a current-independent instance Rp of these discharge parts it is possible to evaluate a factor (R_s+R_p/R_s)³ to correct the relation between the current amplitude of the first harmonic and the second derivative. I_{2 omega} (U=U₀ sin omega t) = -¹/₄U₀²((R_s+R_p)/R_s)³d²I/dU² cos 2 omega t. U₀ is the amplitude of the applied fundamental wave, and R_s is the resistance of the probe-plasma transition.

Electron warm motion across a uniform electric field in the presence of elastic, excitation, ionisation and attachment collisions with the gas has been simulated. The motion has been analyzed to give swarm parameters for drift velocity (v), mean energy, radial (D_r) and longitudinal (D_L) diffusion coefficient, ionisation and attachment coefficients. The results have been compared with solutions of Boltzmann's equations. In the presence of ionisation and for a fixed mean energy, the Monte Carlo results for v and D_L increased as the proportion of energy going into ionisation increased. In the presence of attachment decreasing values of both v and D_L are observed.

Measurements and calculations of the plasma temperature distribution and of the visible emission spectrum have been carried out for high-pressure sodium arcs with a diameter of 7.6 mm. Results are presented for sodium vapour pressures ranging from 50 to 500 Torr and for power inputs per unit arc length of 2800 and 5800 W m^-1, which demonstrate an overall agreement between measured and calculated data. The axial temperature lies between 3600 and 4100K.

Evidence is discussed which suggests that there are significant differences between the processes occurring when the films are <5-10 nm thick and those occurring when they are >10 nm thick. For the former, electron emission can occur by tunnelling at an average electric field of about 10⁹ V m^-1 whereas for the latter some evidence suggests that emission occurs when conducting filaments are electroformed through the film at an average field of 10⁷-10⁸ V m^-1. The effects of UV irradiation on charge production and differences in crater rims between these two film thicknesses, and possible differences in positive ion current in the cathode fall region are discussed.

The measurement of the de-activation rate of the N₂ vibrational metastables produced by a gas discharge in short gaps at atmospheric pressure indicates that their concentration remains significantly high for some hundreds of microseconds. The creation and destruction of the metastables in air at atmospheric pressure have been analyzed. The theoretical interpretation is consistent with the experimental results. The effect of a pre-existing metastable trail on the streamer propagation has been analyzed. It is suggested that the metastable molecules act as an energy reserve which is transferred to the electrons in superelastic collisions. The balance of the energy exchanges is analyzed in detail.

Calculations have been made of the spark constant of Weizel and Rompe (1947) which is an essential quantity of a theory describing the electric behaviour of low-inductance spark discharges. Modifications proposed earlier have been taken into account, and almost quantitative agreement has been obtained between experimentally and theoretically determined values for Ar, Xe, and air.

In papers on nanosecond gas discharge light sources there has been no theoretical approach capable of describing the dependence of the half-width and the maximum power of the optical pulse on the many parameters which can be varied. On the basis of the theory of Weizel and Rompe (1947) extended to thermal plasmas, the author predicts at least the electric behaviour of the spark discharge in low-inductance circuits normally used for the generation of nanosecond light pulses.

If the cathode fall potential of an abnormal glow discharge with a cold cathode exceeds an upper limit the discharge is converted into a vapour arc. The discharge voltage and current density at transition have been measured in Hg vapour and in Xe. The transition voltage decreases with rising ambient gas density and depends only slightly on the nature of the gas, whereas the transition current density rises linearly with gas density and is 90 times larger in Xe. In the proposed theory of the instability, a current density perturbation changes the gas density distribution causing an increase in total secondary electron emission and leading to a transition to the arc.

The motion of vacuum arc cathode spots under the influence of the self-generated magnetic field has been investigated for copper electrodes at currents up to 6.9 kA. The spots were photographed with a high-speed framing camera. The magnetic flux density was measured. Surface contamination caused large changes in spot velocity. For velocities up to 15 ms^-1 and corresponding flux densities up to 2*10^-2T the velocity is proportional to the flux density. Above 20 ms^-1 and 4*10^-2T the velocity increases relatively slowly with increasing flux density. Velocities of more than 30 ms^-1 were not observed.

Radiative energy losses from an arc in jet air flow have been studied. By confirming the importance of these losses, the author supplements a previous paper (see abstr. A82063 of 1974) in which a large discrepancy in the arc energy balance was attributed to them. For arcs burning to an upstream cathode of sintered copper (tungsten in an M=0.8 air flow, radiation is the major energy loss mechanism, accounting for more than 50% of the column voltage requirements. The presence of upstream electrode vapours in the arc column is responsible for the high values of the radiation loss.

The contact of human fingers with solid surfaces and their subsequent separation have been studied microscopically. Scanning electron microscopy and contact angle measurements have been applied to examine the deposit film. When a finger touches a surface, a film of liquid present on the finger spreads to produce apparent contact over much of the fingerprint ridge area. Lifting of the finger causes the film to recede and leave a deposit which may be either a distribution of droplets on a thin contaminating layer of adsorbed molecules, or a continuous film: which type is dependent on both the composition of the fingerprint and the nature of the substrate.

Measurements of refractive indices and droplet profiles of fingerprint deposits on flat solid surfaces have been made using transmitted light interference microscopy. Changes in the profile with time and relative humidity have been determined. From these measurements inferences are made on the structure of the droplets. The absence of significant variation in contact angle for fingerprints on substrates of different surface energy is explained in terms of a contaminant film over the area of finger ridge contact. The thickness of this film corresponds closely to that expected for a monolayer of contamination.