P. F. Williams

Two‐dimensional studies of streamers in gases

We present the results of two‐dimensional computer simulations of streamer initiation and propagation in atmospheric pressure N2. The simulation algorithm makes use of flux‐corrected transport techniques and was used as a tool to study the solutions of the transport equations under conditions suitable for streamers, for which realistic analytic solutions are not known. We present and discuss conclusions about streamer transport based on the results of these studies. Finally, we present a novel method of checking on the numerical accuracy with which the algorithm solves the transport equations.

Resonance Raman scattering of light from a diatomic molecule

Resonance Raman scattering from a homonuclear diatomic molecule is considered in detail. For convenience, the scattering may be classified into three excitation frequency regions—off‐resonance Raman scattering for incident energies well away from resonance with any allowed transitions, discrete resonance Raman scattering for excitation near or in resonance with discrete transitions, and continuum resonance Raman scattering for excitation resonant with continuum transitions, e.g., excitation above a dissociation limit or into a repulsive electronic state. It is shown that the many differences in scattering properties in these three excitation frequency regions may be accounted for by expressions derived from simple perturbation theory. Scattering experiments from molecular iodine are presented which test and verify the general scattering theories. Spectral measurements, time decay measurements, and pressure broadening measurements were made on I2 in the discrete resonance Raman scattering region; and spect...

Giant photoresistivity and optically controlled switching in self-assembled nanowires

We report the observation of giant photoresistivity in electrochemically self-assembled CdS and ZnSe nanowires electrodeposited in a porous alumina film. The resistance of these nanowires increases by one to two orders of magnitude when exposed to infrared radiation, possibly because of real-space transfer of electrons from the nanowires into the surrounding alumina by photon absorption. This phenomenon has potential applications in “normally on” infrared photodetectors and optically controlled switches.

Experimental study of streamers in pure N2 and N2/O2 mixtures and a ≈13 cm gap

Empirical data on streamer formation and propagation in near-atmospheric pressure N2 and N2/O2 mixtures are presented. The data were obtained primarily from high-speed, high-sensitivity shutter and streak photography of streamers produced in a ≈13 cm gap. The streamer propagation velocity as a function of applied voltage, polarity, total pressure, and O2 concentration are provided. In addition, information on streamer bifurcation, and streamer shape and size is included. The breakdown process has two phases: a fast phase featuring the propagation of streamers, and much slower phase involving the heating of the gas to form the spark channel. Also we found that the addition of O2 significantly alters the streamer characteristics and behaviour, indicating that photoionization processes play an important role.

The Resonance Raman Effect

The application of Raman scattering to molecular systems, as discussed in the previous chapters, yield a great deal of information about ground state structural, spectroscopic, and physical properties. In contrast when the incident frequency is in resonance with a real transition of a system, (bringing about the resonance Raman effect) excited electronic state properties may be studied. In addition due to the phenomena of resonance enhancement, specific parts of complicated molecules may be probed without interference from other parts of the molecule or from solvents. These features make the application of resonance Raman scattering to molecules complementary to non-resonance investigations and they make it a very important tool in the study of properties of molecular systems.

Triggering in trigatron spark gaps: A fundamental study

The trigatron has been in widespread use as a demand‐triggered, high‐voltage switch for more than 40 years. In spite of the popularity and maturity of the technology, there persists an uncertainty over the basic physical mechanism(s) responsible for triggering breakdown in the devices. We present the results of an empirical study of trigatrons that directly demonstrates that breakdown is initiated by a streamer launched from the trigger pin, independent of the trigger spark. We compare our results with those of previous workers, and discuss the generality of our conclusions.

Evidence for excited electronic state interference in resonance raman scattering

Abstract The complex resonance Raman spectra of molecular bromine have been analyzed quantitatively and a clear demonstration of interference in the Raman intensity from the B( 3 11 0 + u ) and 1 17 1u excited states has been found.

Surface flashover of silicon

We have previously reported the results of experiments based on high-speed shutter and streak photography which show clearly that in surface flashover of silicon in a vacuum ambient the current flows primarily in the silicon, not in the ambient. Here we present scanning electron microscope (SEM) photographs of the surface damage resulting from flashover which show that this current is filamentary. Photomicrographs obtained from samples with diffused p/sup +/ and n/sup +/ contacts show that the contacts exert a strong influence over the flashover characteristics. Finally, we report the results of experiments in which the sample was illuminated with a weak pulse of visible light either before or just after the application of the voltage pulse to the sample. These experiments show that flashover can be inhibited by such a light pulse, and shed light on the relationship between the flashover mechanism and electric field inhomogeneities.

Fundamental processes in laser-triggered electrical breakdown of gases

The authors describe the results of an experimental study of the physical mechanisms important for laser triggering of spark gaps in the longitudinal, striking-electrode geometry. The authors present clear, new evidence for the propagation of a streamer in under-volted gaps, and for the interaction of the triggering laser with the streamer head. They find that the propagation conditions of the initial streamer markedly affect the subsequent heating of the incipient channel to form the spark. A model of the laser-triggered breakdown process based on these results is presented and discussed.

Physical mechanism of triggering in trigatron spark gaps

Since the first trigatron spark gap was described by J. D. Craggs, M. E. Haine, and J. M. Meek [J. Inst. Electr. Eng. 93A, 963 (1946)], there has been controversy about the physical mechanism responsible for triggering the devices. In this letter we present experimental evidence that directly shows the sequence of physical events responsible for triggering in the gap we studied, and we present a model for trigatron triggering based on this information. We believe this model to be general and discuss it in light of existing literature. We briefly discuss the implications of the model for the engineering design of trigatron gaps.