bluuebunny/crossref_metadata_2025 huggingface dataset:
Internal noise of low-frequency preamplifiers
Authors: Smith Steven W.
Published: March 2003

Experimental measurements of low-frequency preamplifiers show that a type LM394 bipolar input stage has less internal input noise than the popular PAR-113 commercial amplifier for source resistances under 10 kΩ. A type 2N6483 JFET design shows similar input noise to the PAR-113. The input current noise of the JFET design is insignificant compared with the Nyquist noise of the source resistance.
Simple, inexpensive modification to increase the range of the Gilson respirometer
Authors: Rahn Edward
Published: March 2003

A method is described to increase the range of the Gilson respirometer to 2.0 ml, by the addition of the 2.0-ml Gilmont syringe.
Vibrotactile instrumentation designed to generate pulses for the study of temporal resolution of the lingual sensory modality
Authors: Petrosino Linda , Fucci Donald
Published: March 2003

An instrumentation package has been designed to obtain vibrotactile fusion thresholds from the lingual cutaneous sensory modality. This instrumentation can be employed to acquire fusion thresholds through the use of a two-pulse stimulus with varying interpulse onset times. The pulse signal, the generating and monitoring system, and the apparatus for tongue positioning is described. The acquisition of lingual fusion thresholds provides information concerning central nervous system processing of tactile sensory information necessary for speech production.
Friction tester for the lubrication properties of ophthalmic solutions
Authors: Kalachandra S. , Shah D. O.
Published: March 2003

A novel and useful device is described to measure the coefficient of sliding friction and the scuff load of ophthalmic solutions for polymeric surfaces. Instead of the conventional electric continuity method, a strain gauge bridge is employed to measure the stability. Vertical forces in the range of 1–10 g are applied to a spherical stylus which runs in a track of lubricating solution applied to a rotating plate of the same or different material as the stylus. The plate rotates at constant speed with respect to the stylus. The horizontal force resulting from friction has been of the order of 0.01–0.10 g. The device is capable of resolving small differences in the lubricating quality of ophthalmic solutions having a low coefficient of friction. The usefulness of the device has been demonstrated by measuring the lower coefficients of friction of an ophthalmic solution in the above-mentioned range of small vertical forces. Reproducible data have been obtained for the ophthalmic test solution.
Fast gas injection system for plasma physics experiments
Authors: Bates S. C. , Burrell K. H.
Published: March 2003

A system has been developed for fast, feedback controlled injection of hydrogen and various other gases into a vacuum system. Gas flow is through short (about 30-mm length), submillimeter diameter tubing at pressures in the fluid flow regime. The flow is controlled by a piezoelectric valve and measured by a miniature pressure sensor mounted in the body of the valve. Flow rates from a single valve of up to 500 Torr liter/s for hydrogen have been obtained. Response time of the flow is a rise time of 0.5 ms (with a 3-ms ringing decay time) and a fall time of 0.3 ms. The flow rate is basically a linear function of the measured pressure. A model has been developed to explain the observed functional dependence of flow rate on pressure, gas type, and tube radius.
Laser-induced volatilization and ionization of microparticles
Authors: Sinha M. P.
Published: March 2003

We have developed a method for the laser-induced volatilization and ionization of individual microscopic particles on a continuous, real-time basis. A beam of particles is produced by the expansion of an aerosol into a vacuum through a capillary nozzle and skimmer system. The particle in the beam is then hit by a high-energy Nd-YAG laser pulse for its volatilization and ionization. Mass spectral measurements using a quadrupole mass spectrometer on the ions thus generated from potassium biphthalate particles (1.96 μm in diameter) have been made. The combined ionization efficiency for K+ ion is found to be ∼10−6. The advantages of laser-induced volatilization and ionization along with the real-time capability of the present method make it very useful for the chemical analysis of aerosol particles by mass spectrometry. For the synchronization of the particle with the YAG laser pulse, the velocity of the particles in the beam has also been determined. This was accomplished by measuring the time-of-flight of monodisperse aerosol particles between two He–Ne laser beams. The velocities of different size particles are found to decrease with their increasing diameters and the velocity distribution for a given size is sharply peaked. The characteristic velocity may be used for the aerodynamic size measurement of aerosol particles and for relating the aerodynamic size to the optical size. Volatilization and ionization of microparticles in the beam eliminates any possible substrate-sample matrix interferences in the analysis. The method may also find applications in other spectroscopic and chemical kinetic studies.
Comments on the ‘‘New method to measure plasma potential with emissive probes’’ [M. A. Makowski and G. A. Emmert, Rev. Sci. Instrum. 5
4, 830 (1983)]
Authors: Cartier Steven L. , Merlino Robert L.
Published: March 2003

A caution is raised concerning the use of the ‘‘droop method’’ [M. A. Makowski and G. A. Emmert, Rev. Sci. Instrum. 54, 830 (1983)] to measure plasma potentials with emissive probes. We show that other effects may be responsible for the observed droop upon which the method is based.
Reply to comments of Cartier and Merlino [Rev. Sci. Instrum. 5
5, 1002 (1984)]
Authors: Makowski M. A. , Emmert G. A.
Published: March 2003

The concern of Cartier and Merlino regarding ac pickup in the emissive probe technique is not believed to be a problem since the sheath potential is low when the probe is in the emissive condition. This is consistent with both their and our data.
Neutron scattering instrumentation at reactor based installations
Authors: Pynn Roger
Published: March 2003

During the past decade neutron scattering techniques have been applied to an increasingly wide range of scientific problems. Concurrently, a number of substantial improvements of neutron scattering instrumentation have occurred to stimulate this trend. In this article several such developments which have occurred at reactor-based installations are described. Individual spectrometer components which are discussed in some detail include: neutron-optical devices such as guide tubes, supermirrors and multilayer systems; neutron monochromators with optimum reflectivity, mosaic and focusing characteristics; position-sensitive detectors of several types; and equipment required for neutron polarization analysis. Several novel spectrometers which have enhanced the role of neutron scattering during the past ten years are also described. These include spectrometers for small-angle scattering, backscattering, and neutron spin echo. An extensive bibliography is included which covers both early and more recent developments.
Automation of length measurements which involve analysis of interference patterns
Authors: Michel A.
Published: March 2003

Calibration of gauges having parallel plane ends against the wavelength of light involves the use of interference patterns which have to be observed visually by an operator. In this paper an automatic method of measurement is described and compared to the visual method.
Simple technique to obtain a position modulated scan in scanning electron microscopes
Authors: Dixon A. E. , Williams D. F.
Published: March 2003

A simple technique is described which, without modification of the beam column of a scanning electron microscope, adds position modulation to the electron beam scan. This modulation can be added either perpendicular or parallel to the normal electron beam scan, which in the latter case yields a spatial derivative when a lock-in amplifier is used. Applications of this technique to EBIC and other contrast mechanisms are illustrated.
Hollow anode ion–electron source
Authors: Miljević V.
Published: March 2003

A new type of ion–electron source, based on a gas discharge in a diode with concave cathode and a special type of hollow anode, is presented. In this type of discharge, the maximum plasma density is localized at the exit aperture. Constant hydrogen ion currents in excess of 1 mA have been obtained with a 1-mm-diam exit aperture and an extraction voltage of 3 kV. The power supplied to the source, whose volume is less than 0.5 cm3, is only a few watts.
Stress analysis of a beveled diamond anvil
Authors: Bruno M. S. , Dunn K. J.
Published: March 2003

A finite element stress analysis has been performed on a brilliant cut high-pressure diamond anvil. The analysis includes the presence of a metal gasket. A perfectly cohesive interface is assumed to exist between the diamond and metal. Different configurations of the anvil face were studied. The stress distribution resulting from various beveled angles has been analyzed. It has been found that for a flat anvil, with a center normal pressure of about 210 kbar, an octahedral shearing stress of about 90 kbar is present near the center and monotonically increases radially to about 208 kbar along the outer edge. When the anvil surface is beveled, the octahedral shearing stress at the outer corner decreases significantly. The optimum beveled angle necessary to minimize these stresses seems to lie in the neighborhood of 15°. The assumptions made and other stress considerations are discussed.
Digital low noise dc power supply for tunneling spectrometers
Authors: Eib N. K. , Furukawa T. , Mittal K. L.
Published: March 2003

Digital control of a mercury battery low-noise dc power supply for an electron tunneling spectrometer is described. Advantages are reduced data accumulation time, fully automated spectrometer operation, and signal processing flexibility. Results are illustrated with an inelastic electron tunneling spectroscopic (IETS) study of n-propyltrichlorosilane [CH3–(CH2)2–Si–Cl3] adsorbed from vapor on a thin dc plasma-grown aluminum oxide film.
Rapid response SiC thin-film thermistor
Authors: Nagai Takeshi , Yamamoto Kazushi , Kobayashi Ikuo
Published: March 2003

The thermistor consisted of a rf-sputtered SiC film deposited on one surface of an alumina ceramic plate. The thermistor element was brazed to the inner surface of a cylindrical metal housing at the closed end. The thermistor can detect temperatures with a thermal time constant of about 0.6 s.
Multiple sample holder for electron bombardment heating
Authors: du Plessis J.
Published: March 2003

A sample holder is described by which samples may be alternatively heated in front of a cylindrical mirror analyzer by electron bombardment. Damage to the specimen and high secondary electron yield are eliminated.
Array-processor-based Fourier-transform ion cyclotron resonance mass spectrometer
Authors: Hays John D. , Dunbar Robert C.
Published: March 2003

A Fourier-transform ion cyclotron resonance spectrometer (Fourier-transform mass spectrometer) is described which takes advantage of the speed and flexibility of a substantial array processor. Using this array processor, a preprogrammed instruction stack is read sequentially to generate the sequence of rf excitation waveforms applied to the transmitter plates, and the ion response signal is accumulated, processed, and transformed. Control, programming, storage, output, and display functions are handled by the host minicomputer system. Broadband capability allows data acquisition up to 3.5 MHz, giving the ability to acquire extended mass spectra in one pass. The rapid transforming capability of the processor makes it convenient to acquire frequency-domain spectra in real time.
Multiformat electronic cell sorting system: I. Theoretical considerations
Authors: Tyrer Harry W. , Kunkel-Berkley Christine
Published: March 2003

The trajectory of an electrically charged droplet falling in air and electrostatically deflected is analyzed for the influence of droplet size, air frictional drag, and electrical charge. An analytical solution for the trajectory was obtained (assuming a linear velocity component) and programmed into a computer to generate graphs of the variation in trajectory. The experimental work verified that the droplet deflection is proportional to charge and the loss of velocity in the direction of deflection is predicted from the equations obtained here. These studies were used in designing a cell sorting device which places biological cells in defined locations on a microscope slide.
Generation of a modulated intense relativistic electron beam with a frequency tunable by a magnetic field
Authors: Friedman M. , Serlin V.
Published: March 2003

A new mechanism capable of strongly modulating an intense relativistic electron beam using two cavities was found recently. The frequency of the modulated intense relativistic electron beam (IREB) depends on the time it takes an electron to transit the distance between the two cavities. This transit time can be varied by changing the distance between the two cavities or by changing the propagation velocity of the electron beam. It is shown, in this paper, that by varying the intensity of a magnetic field located between the two cavities the drift velocity of the electrons in the IREB is changed, influencing the frequency of the IREB modulation. In a typical experiment a modulated IREB with power ∼5×109 W was generated. The frequency of modulation was varied by ∼34% using an auxiliary small magnetic field coil.
Densimetry in compressed fluids by combining hydrostatic weighing and magnetic levitation
Authors: Masui R. , Haynes W. M. , Chang R. F. , Davis H. A. , Sengers J. M. H. Levelt
Published: March 2003

A magnetic suspension densimeter is described that has been built for measuring the density of compressed liquids at pressures up to 15 MPa in the temperature range 20°–200°C with an uncertainty of 0.1%. The densimeter combines the principle of magnetic levitation of a buoy with that of liquid density determination by hydrostatic weighing. To accomplish this, the support coil is suspended from an electronic balance, and the balance readings are recorded (1) with the buoy at rest, and (2) with the buoy in magnetic suspension. Details are given of the construction of the cell, coil, buoy, and thermostat. The procedure is described by which cell and buoy are aligned so that the suspended buoy does not touch the cell wall. Test data on the densities of seven different liquids were obtained at room temperature. They agree with reliable literature values to within 0.1%. In a separate experiment, the bulk thermal expansion coefficient of the buoy material was determined. This experiment and its results are also given here.
High-field magnet for low-temperature low-field cryostats
Authors: Israelsson U. E. , Gould C. M.
Published: March 2003

A small superconducting magnet with a large highly confined field has been designed for ultralow-temperature cryostats. The magnet provides a central field of 3 T, with a homogeneity of 0.1% over 7.3 mm and negligible hysteresis, while the field at a radial distance of 2 cm is zero. The confinement is accomplished by using the magnet wire itself to simulate the current distribution of a perfect type I superconducting shield.
Microprocessor-based controller for automatic amorphous silicon solar cells characterization
Authors: Cibils Roberto M. , Buitrago Román H.
Published: March 2003

This work describes an innovation of traditional systems used in C–V and G–V curve measurements. Although this system has been specifically developed to be used in the characterization of hydrogenated amorphous silicon (a-Si:H), it is useful when information about conductance (G) or capacitance (C) frequency dependence is needed. The highlights of the new system are signal heterodination and microprocessor-based digital control over current range and modulation frequency. The outstanding advantages of this system are significant reduction of operation time and more reliable measurements. Additionally, there is no waste of computer time in the control of data-acquisition operations. The system has an excellent signal-to-noise ratio, a nearly infinite line signal rejection, and a broad frequency range of operation (20 to 105 Hz).
Computer menu tablet employing Hall-effect switches
Authors: Blackburn James A. , Vik S.
Published: March 2003

A menu tablet comprised of a 16×16 array of Hall-effect switches is described. Activation of any desired cell is simply accomplished by the proximity of a small permanent magnet. A suitable 8-bit parallel output port is included, along with a data-ready strobe line. The device is potentially useful as a peripheral for desktop and minicomputer installations.
(e, 2e) spectrometer for investigating the spectral momentum density of thin films
Authors: Ritter A. L. , Dennison J. R. , Dunn J.
Published: March 2003

An (e, 2e) spectrometer has been constructed for detecting scattered and recoiling electrons from a thin solid film. The cross section for observing the two electrons in coincidence is proportional to the spectral momentum density of the target. In this spectrometer the energy of the incident electron beam is 25 keV and the beam current is approximately 40 μA. The energy resolution (FWHM) is ≤4 eV. The momentum resolution (FWHM) can be varied from 0.2 to 1.0 Å−1. Preliminary coincidence data from an amorphous carbon film are presented.
Microcomputer photoacoustic data-acquisition system for multiphoton absorption
Authors: Adams H. M. , Selkirk E. B. , Goodale J. W. , Evans D. K. , McAlpine Robert D.
Published: March 2003

A simple and inexpensive microcomputer data-acquisition system is described for the investigation of the absorption of infrared photons using a photoacoustic detector. The system produces a graphic display of the results during an experiment and permits the operator to correct procedural errors.
High-resolution surface calorimeter
Authors: Marx R.
Published: March 2003

A differential calorimeter for measuring heat capacities of adsorbed systems is described. The essential feature of the technique is a heat link which maintains two adsorption cells in near thermal equilibrium. The coupling of two cells facilitates the elimination of the background. Heat capacity measurements of adsorbents are suitable to identify phases, to detect phase boundaries, and to study singular behavior at phase transitions in detail. Some results are shown to demonstrate the sensitivity and usefulness of the method.
Calibration apparatus for long-stem and capsule-type Pt resistance thermometers between the triple points of Ar and In
Authors: Ancsin J. , Phillips J. M.
Published: March 2003

An apparatus has been developed capable of primary calibration of long-stem and capsule-type Pt resistance thermometers at the triple point of Ar, using sealed cells, as well as secondary calibration at any temperature between the triple points of Ar (83.798 K) and of In (156.629 °C).
Experimental facility for nanosecond time-resolved, low angle x-ray diffraction experiments using a laser-produced plasma source
Authors: Forsyth J. M. , Frankel R. D.
Published: March 2003

A low angle x-ray diffraction camera that is coupled to a nanosecond laser-produced plasma x-ray source is described. The system includes a grazing incidence toroidal x-ray collector, a digital, intensified, television based x-ray detector, and environmentally controlled sample chamber. The camera is designed for use in time-resolved x-ray diffraction studies. Preliminary kinetics x-ray scattering results from the purple membrane of the Halobacterium halobium are presented.
Pulse fluorometry using simultaneous acquisition of fluorescence and excitation
Authors: Birch D. J. S. , Imhof R. E. , Dutch A.
Published: March 2003

We report a new method of measuring fluorescence lifetimes which uses the single-photon technique and two detection channels with matched impulse response for simultaneous acquisition of fluorescence and excitation (SAFE). This differential arrangement is shown to correct automatically for variations in the optical pulse profile during the measurement, thus eliminating a common source of error. It can be used to improve precision and sensitivity with any pulsed light source such as a flashlamp, laser, or synchrotron. A routing system separates photomultiplier coincidences from the dual detection channels into different memory segments of a multichannel analyzer (MCA) using a single time-to-amplitude converter (TAC). Comprehensive data are presented on tuning the single-photon response of the Philips range of XP2020Q photomultipliers. Results obtained using a coaxial flashlamp to excite a dilute solution of PPO in ethanol give a lifetime of 1.63±0.02 ns in good agreement with that obtained using conventional fluorometry. The method is also useful in the study of dual emissions such as in monomer–excimer systems and in the measurement of time-resolved emission anisotropy.
Synchronization of a picosecond mode-locked dye laser oscillator–amplifier with a streak camera system
Authors: Quitevis E. L. , Kenney-Wallace G. A.
Published: March 2003

We report a scheme for synchronizing a mode-locked oscillator–amplifier dye laser with a streak camera detection system. A detailed description of the electronics is given. Operational constraints imposed by existing subcomponents of the apparatus are discussed and directions for future refinement outlined.
Balloon-borne chemiluminescent sonde for the measurement of tropospheric and stratospheric nitric oxide
Authors: Kondo Y. , Iwata A. , Takagi M. , Matthews W. A.
Published: March 2003

A new chemiluminescent sonde for the balloon-borne measurement of nitric oxide has been developed. Air is drawn through the reaction chamber by a constant volume pump with a volume flow rate of approximately 300 l/min essentially independent of the altitude, and mixed with ozone, generated on board by silent discharge from pure oxygen. The flow of the ozone/oxygen mixture is regulated at a constant flow rate of 400 STP cm3/min by a mass flow controller. To calibrate the system sensitivity periodically in flight, NO in N2 gas, with a concentration of 2 ppm, is injected at the inlet tube with flow rates of 9 and 18 STP cm3/min controlled also by a mass flow controller. Teflon coated internal surfaces have allowed the instrument to have a quick response and this has led to reliable measurements of NO at low mixing ratios. The overall accuracy in determining the NO mixing ratio is between 5% and 10% depending on the altitude, and the detection limit is 10–20 ppt.
ac high-stability microprocessor-based temperature controller for use at high temperatures
Authors: Hawary Majed M. , Holwerda C. , Van Zytveld J. B.
Published: March 2003

We describe the design, operation, and testing of a compact ac temperature controller capable of controlling furnace temperatures to ±0.03 °C up to 1350 °C. Utilizing a thermocouple sensing element, this microprocessor-based unit supplies 60-Hz furnace current, virtually free of rf noise. Furthermore, this controller will either hold the furnace at the desired temperature or will ramp the furnace temperature up or down linearly with time.
Orientation of Ar(3P2) atoms by laser optical pumping
Authors: Giberson K. W. , Hart M. W. , Hammond M. S. , Dunning F. B. , Walters G. K.
Published: March 2003

A beam of argon metastable atoms with a high degree of electron-spin polarization has been produced by optical pumping using an Oxazine 750 dye laser. The beam is suitable for the study of electron spin and orbital orientation dependences in a variety of collision processes.
Simple collimator for use with diamond-anvil cells in a synchrotron beam
Authors: Spain I. L. , Black D. R. , Menoni C. S.
Published: March 2003

A simple double-slit system is described which allows a synchrotron beam to be collimated onto a small sample (∼150-μm diameter) in a diamond-anvil cell using remote control. The apparatus can be constructed easily and inexpensively, and allows the shutter positions to be monitored accurately using readily available electronic equipment. A desirable feature of the collimator is the relatively small time needed for adjustment.
Current regulator for high-power cw gas lasers
Authors: Berkovitz B. , Shemesh A. , Rosenbluh M.
Published: March 2003

A laser current regulator with an operating range of 10 to 50 mA and overall voltages ranging from 2 to 15 kV is described. The current regulation is better than 0.1% for frequencies from dc to 10 kHz. The regulator is designed for operation with a double discharge laser tube in which the two halves of the discharge share a common cathode and one of the anodes is held at ground potential for reasons of operator safety.
Flow cytometry
Authors: Steinkamp John A.
Published: March 2003

Flow cytometry instrumentation developed from early efforts to count cells and particles in liquid suspension as they passed through a sensing device. Since the mid-1960’s sophisticated instruments have been designed for analyzing cells based on various cytological, biochemical, and functional properties. These instruments have revolutionized automated cell analysis methods in that measurements are made at high speed, multiparameter data is correlated on each cell, statistical precision is high, and cells are separated in high purity from heterogeneous mixtures for identification and functional analysis. Advanced instruments capable of measuring cell volume, surface area, multicolor fluorescence, fluorescence polarization, light scatter within various angular regions, and axial light loss (extinction) at different wavelengths are being used in biomedical research for analyzing and sorting normal and abnormal cell populations. This article reviews the development of flow cytometers, the conceptual basis of flow measurements, and discusses some of the numerous applications of the technology in biology and medicine.
Prediction of flash lamp explosion by stress measurements
Authors: Yoshida Kunio , Kato Yoshiaki , Yoshida Hidetsugu , Yamanaka Chiyoe
Published: March 2003

We have precisely measured stress distributions in the quartz tubes of high-energy linear flash lamps, and determined the correlations between the measured stress and flash lamp explosion. Premature lamp explosion can be ascribed to the initial tensile stress at the outer surface of the tube, whereas normal lamp explosion can be ascribed to the accumulated tensile stress at the inner surface. The upper limits of the stresses in these two cases are given so that they can be used to prevent the lamp explosions by removing marginal tubes from use before explosion.
Apparatus for producing controlled positive or negative corona discharge for semiconductor materials processing
Authors: Modlin Douglas N. , Tiller William A.
Published: March 2003

An apparatus capable of producing current or voltage controlled positive and negative ion beams in 1-atm oxidizing and nonoxidizing ambients between 25° and 950 °C is presented. The apparatus utilizes a point-to-plane corona discharge in the gas phase. Room-temperature experiments revealed that the small differences in ion beam shape between the two polarities is primarily due to the difference in applied voltage required to produce the same current for the two polarities. Due to increased ion mobility and decreased threshold voltage at higher temperatures, a slight broadening of the ion beam current-density profiles for both positive and negative ions is predicted.
Novel x-ray diffractometer/position-sensitive detector system
Authors: Fraass B. A. , Granfors P. R. , Hilleke R. O. , Simmons R. O.
Published: March 2003

The principles, construction, and operation of a versatile, orientable x-ray diffractometer system having a position-sensitive detector are described. The system precisely and rapidly measures Bragg scattering from very weak scatterers such as quantum crystals of helium and hydrogen held in fixed orientation at precisely controlled extreme temperatures and pressures inside fixed bulky apparatus.
Automated pressure regulator
Authors: Waxman M. , Davis H. A. , Horowitz M. , Everhart B.
Published: March 2003

A pressure regulator has been constructed that automatically nulls a sensitive differential pressure indicator of the type used in high-quality PVT experiments, thus permitting at least partial automation of such experiments. Distinguishing features are: high resolution, sufficient rigidity for operation in a pressure range up to 100 MPa, and a control logic that permits nulling of the pressure transducer, even if the initial state is very far from balance.
Fourth-order uniform electric field from two charged rings
Authors: Jungerman J. A.
Published: March 2003

It is shown that an electric field uniform to fourth order can be formed by using two charged rings of equal radii separated by √6 times the ring radius. Calculations of the field distribution and the deviation from uniformity are made. The axial and radial electric fields are calculated using an expansion valid near the origin of coordinates and, more generally, by evaluation of elliptic integrals.
Superconducting heat switch of simple design
Authors: Schuberth Erwin
Published: March 2003

Superconducting heat switches consisting of tin and indium wires pressed between machined silver rods and oriented parallel to the operating magnetic field are described. Their advantages are simplicity of design and physical smallness. Thermal conductivities of such switches in the normal and in the superconducting state are reported.
Simple vacuum pump exhaust filter
Authors: Forman Richard A. , Kratz Harvey D.
Published: March 2003

A simple vacuum pump exhaust filter based upon an automotive air cleaner has been constructed and tested. The major virtues of the filter system are ease of coupling to an external exhaust and the availability of filter elements.
Lattice-gas study of the kinetics of catalytic conversion of NO–CO mixtures on rhodium surfaces
Authors: Bustos V. , Gopinath C. S. , Uñac R. , Zaera F. , Zgrablich G.
Published: March 2003

The kinetics of the catalytic reduction of NO by CO on Rh(111) was simulated by using a lattice-gas model and a Monte Carlo algorithm. These simulations were designed to incorporate some new experimental results, which reveal that the formation of a N–NO intermediate is necessary for molecular nitrogen production. The steady-state phase diagram for the overall NO reduction reaction was studied in terms of several parameters representing different reaction schemes. It was found that, under the assumptions made in the model, an Eley–Rideal mechanism which includes a NO(gas)+N(ads)→N2(gas)+O(ads) step is absolutely necessary to be able to sustain a steady-state catalytic regime.
Relative product yields in the one-photon and vibrationally mediated photolysis of isocyanic acid (HNCO)
Authors: Berghout H. Laine , Hsieh Shizuka , Crim F. Fleming
Published: March 2003

Direct observation of all three photofragmentation channels of HNCO gives the relative yields of NH+CO3, H+NCO, and NH+CO1 at nine different photolysis energies for both thermal and vibrationally excited molecules. Each higher energy channel dominates as it becomes accessible, but vibrational excitation changes the relative yield of NH1 markedly. Photolysis of HNCO(3ν1) at the same total energy yields up to 2.5 times more NCO with a corresponding reduction in NH1. The relative yield of NH3 and NCO, however, is indifferent to vibrational excitation. The dependence of the yields on vibrational excitation supports a picture in which direct decomposition on the S1 surface produces NH+CO1 and in which internal conversion to S0 leads to H+NCO, by spin-allowed unimolecular decay, and to NH+CO3, by intersystem crossing and decomposition on T1. The observed vibrational enhancement of the NCO yield is consistent with vibrational excitation impeding the decomposition to NH+CO1 on S1 and, thus, increasing the number of molecules that cross to S0 and decay to H+NCO.
Problematic p-benzyne: Orbital instabilities, biradical character, and broken symmetry
Authors: Crawford T. Daniel , Kraka Elfi , Stanton John F. , Cremer Dieter
Published: March 2003

The equilibrium geometry, harmonic vibrational frequencies, and infrared transition intensities of p-benzyne were calculated at the MBPT(2), SDQ-MBPT(4), CCSD, and CCSD(T) levels of theory using different reference wave functions obtained from restricted and unrestricted Hartree-Fock (RHF and UHF), restricted Brueckner (RB) orbital, and Generalized Valence Bond (GVB) theory. RHF erroneously describes p-benzyne as a closed-shell singlet rather than a singlet biradical, which leads to orbital near-instabilities in connection with the mixing of orbital pairs b1u-ag (HOMO–LUMO), b2g-ag (HOMO-1-LUMO), and b1g-ag (HOMO-2-LUMO). Vibrational modes of the corresponding symmetries cause method-dependent anomalous increases (unreasonable force constants and infrared intensities) or decreases in the energy (breaking of the D2h symmetry of the molecular framework of p-benzyne). This basic failure of the RHF starting function is reduced by adding dynamic electron correlation. However RHF-MBPT(2), RHF-SDQ-MBPT(4), RHF-CCSD, RB-CCD, and RHF-CCSD(T) descriptions of p-benzyne are still unreliable as best documented by the properties of the b1u-, b2g-, and b1g-symmetrical vibrational modes. The first reliable spin-restricted description is provided when using Brueckner orbitals at the RB-CCD(T) level. GVB leads to exaggerated biradical character that is reduced at the GVB-MP2 level of theory. The best results are obtained with a UHF reference wave function, provided a sufficient account of dynamic electron correlation is included. At the UHF-CCSD level, the triplet contaminant is completely annihilated. UHF-CCSD(T) gives a reliable account of the infrared spectrum apart from a CCH bending vibrational mode, which is still in disagreement with experiment.
Curvature effect on surface diffusion: The nanotube
Authors: Shu D. J. , Gong X. G.
Published: March 2003

Using an empirical Tersoff–Brenner many-body potential for the carbon atoms and a model Lennard-Jones interaction between the adatom and carbon atoms, the diffusion of an adatom over curved surfaces of C nanotubes is studied by calculating the potential energy surface and performing molecular dynamics simulation. The average curvature of the surface is found to have an important influence on the diffusion of the adatom. Positive curvature increases the diffusion barrier and corrugates the potential energy surface, while the negative curvature smoothes the potential energy surface, therefore it lowers the diffusion barrier. We also find that nanotube helicity can play an important role on the diffusion path, thus the adatom has different diffusion path for the armchair and zig–zag nanotube. The nature of the curvature effect on the surface diffusion is connected with the strain effect.
An ab initio study of the betaine anion–dipole-bound anionic state of a model zwitterion system
Authors: Rak Janusz , Skurski Piotr , Gutowski Maciej
Published: March 2003

The electron binding to a model zwitterionic molecule, betaine, is studied at the coupled cluster level of theory with single, double, and noniterative triple excitations as well as at the density functional theory level. Our results indicate that there is only one stable conformer of betaine with a dipole moment of 11.5 D. This dipole moment supports a relatively strongly bound anionic state and the vertical electron detachment energy was found to be 2261 cm−1. Furthermore, we predict the excess electron binding energy for the betaine analog with a blocked canonical structure (N,N-dimethylglycine methyl ester) to be less than 100 cm−1. This significant difference in electron binding energies suggests that the gas phase instability of zwitterions of some common naturally occurring amino acids with respect to their canonical forms may be reversed by the excess electron attachment. The calculated Franck–Condon factors for the betaine’s anion/neutral pair suggest that the photoelectron spectrum of the anion will display only a very weak vibrational structure.
The X̃ 2B1, B22, A12, and A22 states of oxygen difluoride cation (F2O+): High-level ab initio calculations and simulation of the ultraviolet photoelectron spectrum of F2O
Authors: Wang De-Chao , Chau Foo-Tim , Mok Daniel Kam-Wah , Lee Edmond P. F. , Beeching Levi , Ogden J. Steven , Dyke John M.
Published: March 2003

The ultraviolet photoelectron spectrum of F2O was recorded with a higher resolution than previously published. New vibrational structure was observed in the second and third bands. Near state-of-the-art molecular orbital calculations were performed on the X̃ 1A1 state of F2O and the X̃ 2B1, B22, A12, and A22 state of F2O+, and their potential energy functions were computed. Spectral simulations based on Franck–Condon factor calculations including the Duchinsky effect were carried out within the harmonic oscillator model and also with the inclusion of anharmonicity, in order to assist spectral assignment. Based on the computed ionization energies obtained with the coupled cluster and multireference configuration interaction methods with basis sets of up to quintuple zeta quality, the order of the low-lying cationic states of F2O+ has been firmly established. However, the detailed assignment of the overlapping second and third photoelectron bands was only achieved with the aid of spectral simulation. The iterative Franck–Condon analysis (IFCA) procedure was carried out for the first band {F2O+ (X̃ 2B1)←F2O (X̃ 1A1)} in the photoelectron spectrum. With the geometrical parameters of F2O (X̃ 1A1) being fixed at the available experimental values, geometrical parameters of the X̃ 2B1 state of F2O+ were derived. Based on anharmonic Franck–Condon factor calculations, the recommended IFCA geometrical parameters for the ground state of F2O+ are R(FO)=1.323±0.002 Å and ∠FOF=107.3±0.2°.
Determination of optical properties of nitrogen-doped hydrogenated amorphous carbon films by spectroscopic ellipsometry
Authors: Hayashi Y. , Yu G. , Rahman M. M. , Krishna K. M. , Soga T. , Jimbo T. , Umeno M.
Published: March 2003

Nitrogen-doped hydrogenated amorphous carbon films have been deposited on silicon substrates by radio-frequency plasma-enhanced chemical vapor deposition using different N2/CH4 gas ratios from 0 to 3. The real and imaginary parts, n and k, of the complex index of refraction of these films have been determined for wavelengths between 300 and 830 nm by spectroscopic ellipsometry. Excellent agreement has been found between measured and modeled spectra, in which an empirical dielectric function based on classical Lorentz oscillator and Tauc joint density of states, and a linear void distribution along the thickness of the films have been assumed. Decrease in the optical energy gap and increase in the extinction coefficient, k, with increase in nitrogen concentration have been observed. Refractive index, n, increases rapidly with increase in nitrogen concentration up to 6.8 at. % (∼7.0 at. %) and then increases slowly with further increase in nitrogen concentration. For all the samples, n is found to be highest at the film-substrate interface which gradually decreases towards the film surface.
Monte Carlo simulations of Ca–montmorillonite hydrates
Authors: Chávez-Páez M. , dePablo L. , dePablo J. J.
Published: March 2003

Monte Carlo simulations are performed to investigate the role of the divalent cation Ca on the stability and swelling of montmorillonite. Constant stress simulations (NPzzT ensemble) are used to predict the basal spacing as a function of the water content in the interlayer, and constant chemical potential simulations (μVT ensemble) are used to find the more stable basal spacings of the clay–water systems. Two model clays are considered in this work, namely, Otay and Wyoming montmorillonites. It is found that the equilibrium basal spacings for Otay montmorillonite are D1=11.9 Å and D2=14.4 Å. For Wyoming montmorillonite they are D1=12.2 Å and D2=14.7 Å. It is also found that montmorillonites develop one layer of water at D1, and two layers of water at D2. We found that, for all the water contents considered here, the cations are hydrated.
Pressure and temperature dependences of the relaxation dynamics of cresolphthalein-dimethylether: Evidence of contributions from thermodynamics and molecular interactions
Authors: Paluch Marian , Ngai K. L. , Hensel-Bielowka Stella
Published: March 2003

The relaxation dynamics of a low molecular supercooled liquid, cresolphthalein-dimethylether (KDE), has been investigated in the vicinity of glass transition temperature by dielectric relaxation measurements. Glass transition is approached and studied by the use of isothermal and isobaric paths. Under isobaric condition, the departure of the correlation function of the α-relaxation from exponential decay with time (nonexponentiality) is even less than the intermediate liquids such as glycerol. However, the Tg-scaled temperature dependence of the α-relaxation time, τ, resembles “fragile” glass-formers like orthoterphenyl (OTP). Thus KDE, like propylene carbonate (PC) that has in common a basic chemical structural unit, is an exception to the correlation between “fragility” and nonexponentiality found to hold for most glass formers. The dielectric relaxation measurements with the application of pressure show that KDE has large pressure coefficient of the glass transition temperature, (dTg/dP)P→0, or large activation volume, typically of “fragile” glass-formers. However, unlike “fragile” glass-formers, the change of the scaled activation volume with log τ for KDE is slow, comparable with that of glycerol. The dielectric dispersion of KDE as a function of frequency at different pressure and temperature combinations has an excess high frequency wing but no resolved Johari–Goldstein β-relaxation, resembling that of intermediate liquids. Thus the variable pressure and temperature measurements of the relaxation dynamics show that KDE (and likewise PC) behaves like a “fragile” liquid in some properties and an intermediate liquid in other properties. Previously it has been proposed that there are two distinct factors that determine the molecular dynamics of glass formers, one from thermodynamics and the other from cooperative many-body dynamics. We explain this split character of KDE and PC by the prominence of the thermodynamics factor, possibly due to the unique basic chemical structural unit they have in common. The prominent thermodynamics factor gives rise to properties, such as the Tg-scaled temperature dependence of the α-relaxation time and large (dTg/dP)P→0, that resemble “fragile” liquids. While the cooperative many-body dynamics in KDE and PC is weak as evidenced by low degree of nonexponentiality of the α-relaxation correlation function. Consequently some properties, including the shape of the dielectric dispersion, coincide with that of intermediate liquids. The explanation is supported by the comparison of adiabatic calorimetric data of PC and OTP to show that indeed the thermodynamics factor in PC is like that of a very “fragile” liquid.
Vibration–rotation kinetic energy operators: A geometric algebra approach
Authors: Pesonen Janne
Published: March 2003

The elements of the reciprocal metric tensor g(qiqj), which appear in the exact internal kinetic energy operators of polyatomic molecules can, in principle, be written as the mass-weighted sum of the inner products of measuring vectors associated to the nuclei of the molecule. In the case of vibrational degrees of freedom, the measuring vectors are simply the gradients of the vibrational coordinates. It is more difficult to find these vectors for the rotational degrees of freedom, because the components of the total angular momentum operator are not conjugated to any rotational coordinates. However, by the methods of geometric algebra, the rotational measuring vectors are easily calculated for any geometrically defined body-frame, without any restrictions to the number of particles in the system. In order to show that the rotational measuring vectors produced by the present method agree with the known results, the general formulas are applied to the triatomic bond-z, and to the triatomic angle bisector frame. All the rotational measuring vectors are also explicitly derived for a new triatomic body frame defined in terms of two Jacobi vectors. As a final application, all the rotational measuring vectors are presented for a new N-atomic frame defined in terms of N−1 Jacobi vectors, and for a simple N-atomic frame defined in terms of N nuclear position vectors (N=3,4,5,6,…).
The E1Σg+ state of lithium dimer revised
Authors: Jastrzȩbski W. , Pashov A. , Kowalczyk P.
Published: March 2003

This article describes the use of the inverted perturbation approach method for construction of an accurate potential energy curve of the E1Σg+ “shelf” state in Li2. Contrary to previous reports the applied method provides a potential curve free of unphysical oscillations and reproduces energies of all the levels known from previous experiments. Our results confirm the possibility of describing the E state within the Born–Oppenheimer approximation.
Dynamics of the nuclear magnetic resonance COSY-revamped by asymmetric z-gradients (CRAZED) experiment
Authors: Ramanathan Chandrasekhar , Bowtell Richard
Published: March 2003

The nuclear magnetic resonance COSY-revamped by asymmetric z-gradients experiment has been applied to a wide range of problems from functional magnetic resonance imaging to structure determination in porous media. In this article, we investigate the temporal dynamics of the experiment in the regime where the linear approximation (γμ0M0t≪1) for the signal evolution is no longer valid. Numerical solutions of the nonlinear evolution equations were obtained with and without radiation damping and compared to previous analytical approximations to explore their range of validity. Experiments were performed on cyclohexane at 11.7 T, and the signal dependence on flip angle and modulation gradient strength investigated—under conditions where the effects of radiation damping were minimized. The analytical expressions were found to be valid only when diffusion effects are either negligible or when the characteristic time for diffusional attenuation is small compared to the dipolar demagnetizing time (km2D≫γμ0M0). Excellent agreement was obtained between the experimental data and the numerical solutions of the modified Bloch equations.
Electrolytes at charged interfaces: Pair integral equation approximations for model 2–2 electrolytes
Authors: Eaton Andrew C. , Haymet A. D. J.
Published: March 2003

The structure and thermodynamics for model 2–2 electrolytes at a charged interface have been determined by the so-called “pair” approximation of integral equation theory. In addition to Coulombic interactions, the potential models for the ion–ion and ion–wall interactions employ “soft” continuous potentials rather than “hard”-sphere or “hard”-wall potentials. The solvent is modeled as a structureless dielectric continuum at 25 °C. The structure is calculated using the inhomogeneous Ornstein–Zernike relation, together with the hypernetted chain closure and two choices for the functional relationship between the singlet and pair correlation functions. Both the interfacial density profile and the inhomogeneous pair correlation functions are calculated. Some thermodynamic properties of these systems are also evaluated. The results of the pair approximation are compared with the so-called “singlet” approximation, selected computer simulation results, Gouy–Chapman–Stern predictions, and experimental data. While qualitative agreement is generally found between the two levels of integral equation approximation, measurable quantitative improvements exist for both structural and thermodynamic predictions in the pair approximation.