P. F. (Paul Frazer) Williams Publications

Ultraviolet and Pulsed Electric Field Treatments Have Additive Effect on Inactivation of E. coli in Apple Juice

T. K. Gachovska et al. — Wed, 07 Jan 2009 13:52:36 PST

Apple juice inoculated with Escherichia coli ATCC 23472 was processed continuously using either ultraviolet (UV), high-voltage pulsed electric field (PEF), or a combination of the PEF and UV treatment systems. Apple juice was pumped through either of the systems at 3 flow rates (8, 14, and 20 mL/min). E. coli was reduced by 3.46 log CFU/mL when exposed in a 50 cm length of UV treatment chamber at 8 mL/min (2.94 s treatment time with a product temperature increase of 13 °C). E. coli inactivation of 4.87 log CFU/mL was achieved with a peak electric field strength of 60 kV/cm and 11.3 pulses (average pulse width of 3.5 μs, product temperature increased to 52 °C). E. coli reductions resulting from a combination treatment of UV and PEF applied sequentially were evaluated. A maximum E. coli reduction of 5.35 log CFU/mL was achieved using PEF (electrical field strength of 60 kV/cm, specific energy of 162 J/mL, and 11.3 pulses) and UV treatments (length of 50 cm, treatment time of 2.94 s, and flow rate of 8 mL/min). An additive effect was observed for the combination treatments (PEF and UV), regardless of the order of treatment (P > 0.05). E. coli reductions of 5.35 and 5.30 log CFU/mL with PEF treatment (electrical field strength of 60 kV/cm, specific energy of 162 J/mL, and 11.3 pulses) followed by UV (length of 30 cm, treatment time of 1.8 s, and flow rate of 8 mL/min) and UV treatment followed by PEF (same treatment conditions), respectively. No synergistic effect was observed.

Enhanced transition temperature in a quantum confined high-T_c superconductor

L. Menon et al. — Thu, 29 May 2008 12:02:31 PDT

A slight enhancement of the onset transition temperature T_c has been observed in quantum dots of the high-T_c superconductor yttrium–barium–copper oxide (Y₁Ba₂Cu₃O_7-δ). The dots were formed when YBCO particles (diameter ≤600 nm) were exposed to an RF plasma to produce a Coulomb crystal. The plasma damages the particles and causes 20–25 nm sized isolated islands of the correct 1-2-3 stoichiometry to segregate within each particle. We believe that quantum confinement of carriers within these islands leads to discretization of the quasi-particle density-of-states which can account for the slight increase in the transition temperature.

Self-assembled networks with neural computing attributes

S. Bandyopadhyay et al. — Thu, 29 May 2008 11:52:55 PDT

Two-dimensional arrays of vertical quantum wire Esaki tunnel diodes, laterally connected to their nearest neighbors by resistive/capacitive connections, constitute a powerful and versatile neuromorphic architecture that can function as classical Boolean logic circuits, associative memory, image processors, and combinatorial optimizers. In this paper, we discuss the basic philosophy behind adopting this architecture for nanoelectronic circuits and report on our experimental progress towards synthesizing this system.

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

Won J. Yi et al. — Thu, 29 May 2008 11:48:09 PDT

Empirical data on streamer formation and propagation in near-atmospheric pressure N₂ and N₂/O₂mixtures 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 behavior, indicating that photoionization processes play an important role.

COULOMB CRYSTALS AND GLASSES FOR SELF-ASSEMBLY OF NANOPARTICLES

P. F. Williams et al. — Thu, 29 May 2008 11:43:13 PDT

Under proper conditions, small particles may be suspended in a regularly-spaced array called a Coulomb crystal. In this paper we discuss the application of this phenomenon to the self-assembly of nanoparticles in a lattice-lie array on a substrate. Issues associated with depositing the particles, and the size and lattice spacing of the particles are discussed.

Electrochemical self-assembly of porous alumina templates

V. Sadasivan et al. — Thu, 22 May 2008 08:33:18 PDT

Porous alumina templates are fabricated by anodization of Al in an acid. The characteristics of the template, such as the pore diameter, interpore separation, and the periodicity of the porous structure depend on the conditions of anodization and the acid that is used as electrolyte. The variation of pore diameter and interpore separation as a function of anodization parameters are described. The use of porous alumina templates as masks to assemble nanoarrays is an important application and the deposition of Au nanodots on a surface of silicon is discussed.

An optical technique for measurement of semiconductor surface electric fields

Harshad P. Sardesai et al. — Wed, 16 Apr 2008 07:35:37 PDT

We present an optical technique for the measurement of semiconductor surface electric fields. The measurement technique uses the Kerr electro-optic effect in nitrobenzene, a phase sensitive interferometer, and associated data acquisition units to measure the surface electric fields between the contacts of a planar semiconductor device. This technique was used to measure the surface fields on silicon devices used in pulsed power applications, but has the potential for use as an electric field probe for any device having high surface electric fields, both pulsed and dc. The measurement technique showed a temporal resolution of 100 ns, which can be easily reduced to a few nanoseconds using superior data acquisition and detection systems. The spatial resolution was about 50 pm for devices that had a typical contact separation of about 500 pm (power devices). This technique can be applied to measure the surface fields on devices commonly used in microelectronic applications. In this paper we discuss in detail the measurement technique and present the results obtained for silicon photoconductive power switches.

Time-resolved two-dimensional imaging of ground-state species using laser-induced fluorescence

R. A. Dougal et al. — Wed, 16 Apr 2008 07:31:43 PDT

Laser-induced fluorescence has been employed to obtain two-dimensional images of the spatial distribution of a vapor in the ground state with nanosecond resolution. The method has a wide range of potential applications, as it can be used to observe any species which has energy levels accessible to tunable laser systems. In the application we describe, aluminum vapor was observed jetting from spark gap electrodes and diffusing throughout the gap volume.

Simple powerful tunable single-mode and mode-locked TEA CO₂ laser

R. A. Dougal et al. — Wed, 16 Apr 2008 07:25:56 PDT

A simple method for constructing a single longitudinal-mode CO₂ laser that is tunable within individual rotational transitions is presented. Useful power levels, typically 0.3 J per pulse in a single-mode beam, are obtained from a single-stage device. Mode-locked output can also be obtained from the laser. In addition, design and construction of a local oscillator for making absolute in situ frequency measurements is described.

Some Comments on the Plasmon Spectrum of Tetrathiafulvalene Tetracyano-p-quinodimethane (TTF- TCNQ)

P. F. Williams et al. — Wed, 16 Apr 2008 07:21:21 PDT

Our random-phase-approximation model calculation of the high-frequency dielectric response of a quasi-one-dimensional metal is generalized to the case of n conducting strands per unit cell. For a model of the two-band system tetrathiafulvalene tetracyano-p-quinodimethane we obtain good agreement with recent experiments, and also predict an acoustic-plasmon branch, Some implications for the physics of the material are discussed.

Reply to “Comment on 'Resonance Raman Scattering and Collision-Induced Redistribution Scattering in I₂’ ”

D. L. Rousseau et al. — Wed, 16 Apr 2008 07:15:22 PDT

We demonstrate that neither laser broadening nor inelastic collisional broadening plays a dominant role in the data of our recent experiments as has been suggested, and again we conclude that our results can only be explained by redistribution emission resulting from phase-changing processes.

Comment on “Two-Magnon Resonant Raman Scattering in MnF₂”

D. L. Rousseau et al. — Wed, 16 Apr 2008 07:04:24 PDT

We have not been able lo reproduce the recently reported resonance enhancement of Raman scattering from magnons in MnF₂ by Amer, Chiang, and Shen. Fifteen crystals ranging from extremely pure samples to ones with high concentrations of impurities, in addition to one supplied by Amer, Chiang, and Shen, were investigated. By observing the off-resonance scattering, we conclude that if an enhancement of greater than a factor of 2 were present, it would have been seen in our resonance spectra.

Resonance Raman Scattering and Collision-Induced Redistribution Scattering in I₂

D. L. Rousseau et al. — Wed, 16 Apr 2008 06:59:32 PDT

We report the first experimental observation of broad-banded re-emission resulting from quasi-elastic collisional transfer processes. The data may be interpreted by the general theory of scattering of monochromatic light in gases. The connection between this theory and one derived for solids is discussed.

Resonance Fluorescence and Resonance Raman Scattering: Lifetimes in Molecular Iodine

P. F. Williams et al. — Wed, 16 Apr 2008 06:53:49 PDT

We report a significantly shortened scattering lifetime in molecular iodine when the frequency of a single-mode 5145-Å incident laser is tuned away from resonance with a discrete transition. This observation of a shortened scattering time is indicative of what has been conventionally called a Raman process. Off-resonance lifetime measurements, as reported here, may provide a means of determining the relative contribution to the absorption line-width of homogeneous and inhomogeneous broadening.

Discrete and Diffuse Emission Following Two-Photon Excitation of the E State in Molecular Iodine

D. L. Rousseau et al. — Wed, 16 Apr 2008 06:47:28 PDT

We studied the fluorescence spectrum of E →B transitions in molecular iodine following two-photon absorption. Discrete re-emission lines terminating in B-state levels ranging from ν = 0 to the dissociation limit were observed in addition to a series of broad diffuse lines. These diffuse lines represent the first observation of Condon "internal diffraction” resulting from a single vibronic state, and therefore the variation of the bands directly displays the variation of the Franck-Condon factor.

Dependence of the Electronic Transition Moment on Internuclear Separation in Na₂: A Quantum Mechanical Treatment

P. F. Williams et al. — Tue, 15 Apr 2008 11:47:45 PDT

We present a quantum mechanical treatment of laser-induced atomic fluorescence from molecular sodium. It is shown that such fluorescence data may be analyzed with an ν-centroid approximation and an accurate determination of the electronic transition moment may be made.

Resonant Raman Scattering from Iodine: Spectral Changes with Excitation Frequency

P. F. Williams et al. — Tue, 15 Apr 2008 11:43:13 PDT

We have recorded high-resolution resonant Raman spectra of molecular iodine which show very pronounced systematic spectral band-shape variations with incident laser frequency. This frequency dependence and the presence of the multiple overtones in resonant Raman scattering from iodine may be explained by a theory in which the scattering intensity depends on Franck-Condon overlap integrals. We have obtained semiquantitative agreement between the observed spectra and spectra calculated from this theory.

Raman Scattering from Localized Vibrational Modes in GaP

D. T. Hon et al. — Tue, 15 Apr 2008 11:38:44 PDT

High-frequency localized vibrational modes of impurities in a III-V compound have been observed by Raman scattering. Three lines from the GaP samples closely correspond to some loccal modes previously reported in infrared studies, and depolarization measurements tend to confirm their proposed assignments. We describe several attractive features of this method for the study of semiconductor impurities.

Self-consistent dielectric response of a quasi-one-dimensional metal at high frequencies

P. F. Williams et al. — Tue, 15 Apr 2008 11:30:19 PDT

We present the results of a calculation of the frequency- and wave-vector-dependent longitudinal dielectric function of a quasi-one-dimensional electron gas. The electrons are taken to be localized to the chains and both the tight-binding and free-electron extremes are considered along the chain axis. Local-field effects are included. Dispersion curves for plasmons and single-particle-excitation spectra are presented. We find that the plasmon modes are not Landau damped and that for long wavelengths these modes have eigenfrequencies ranging continuously from the usual three-dimensional plasma frequency for propagation along the chain axis to zero for propagation perpendicular to it. Finally, we discuss the effects these excitations should have on the optical properties. The absorption in the free-electron extreme contains both single-particle and plasmon contributions throughout the optical spectrum. In the tight-binding limit, the plasmon contributions persist to frequencies larger than the single-particle bandwidth. In no event is the absorption of the Drude form.

Optical excitation of plasmons in a quasi-one-dimensional conductor

P. F. Williams et al. — Tue, 15 Apr 2008 11:26:01 PDT

We have carefully determined the optical spectrum of K₂Pt(CN) ₄Br_0.3 - 3H₂O near the reflectivity edge. Our data reveal, in addition to the expected monotonic background, a broad peak in ε₂ (ω) above a threshold coinciding with the zero of ε₁ (ω) near 16000 cm^-1. We assign this band to indirect excitation of plasmons. Details of the excitation of plasmons are discussed with particular attention being paid to the problems presented by the one-dimensional character of the material. It is concluded that the electronic structure of this material cannot be adequately described by either a free-electron model or a one-band tight-binding model.