bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical simulation of streamer–cathode interaction
Authors: Odrobina Igor , Černák Mirko
Published: July 2002

A self-consistent fluid model has been used to analyze streamer arrival at the cathode and its transformation to the stationary cathode fall in a positive point–to–plane corona discharge in N2 at 26.7 kPa. The model is based on a description of the electron and the ion kinetics by one-dimensional continuity equations coupled with Poisson’s equation. The ions and electrons are assumed to be limited to a cylindrical channel with fixed radius and the field is computed using the method of disks. The computed current induced by the streamer–cathode interaction with a small cathode probe is compared with that measured experimentally. The cathode probe signal consists of an initial sharp current spike due to the displacement current followed, some 20 ns later, by a lower current hump due to the ion arrival at the cathode. The current signal is relatively insensitive to changes in the secondary electron emission coefficients. The results obtained indicate that the intense ionization and associated light flash experimentally observed near the cathode at the streamer arrival are not, as generally accepted, due to an intense electron emission but due to a sudden increase in the multiplication factor and a release of electrostatic energy accumulated in the streamer channel–cathode system.
Semiconducting YBaCuO thin films for uncooled infrared bolometers
Authors: Shan P. C. , Çelik-Butler Z. , Butler D. P. , Jahanzeb A.
Published: July 2002

We have fabricated and measured the performance of semiconducting YBaCuO thin-film thermometers on silicon for uncooled bolometric applications. Our YBaCuO thermometers have revealed a change in resistance with respect to temperature (dR/dT) as high as 8.2×103 Ω/K at T=294 K and the temperature coefficient of resistance as high as 3.1% K−1 over a 60 K range around room temperature which implies an excellent bolometric response. At 294 K the thermometers had a noise voltage of 0.75 μV/Hz1/2 at the frequency of 30 Hz and the bias current of 1 μA. Using the data above, we calculated that semiconducting YBaCuO bolometers would have a responsivity of 5.5×104 V/W and a detectivity D* as high as 1.3×108 cm Hz1/2/W for 1 μA bias current if integrated with a typical thermal isolation structure.
Degradation of the thermal oxide of the Si/SiO2/Al system due to vacuum ultraviolet irradiation
Authors: Afanas’ev V. V. , de Nijs J. M. M. , Balk P. , Stesmans A.
Published: July 2002

The generation of point defects in Si/SiO2/Al capacitors due to exposure to vacuum ultraviolet (VUV) (10 eV) radiation has been investigated by studying the electron and hole trapping properties and electron-spin-resonance spectroscopy for exposures ranging from 1014 up to 1019 photons cm−2 absorbed in the oxide. At low VUV exposures, the generation of hydroxyl groups and electron traps with cross section ≥10−16 cm2 is observed; however, for exposures larger than 1017 cm−2 these centers are subsequently eliminated. For exposures larger than 1018 cm−2 the oxide network is gradually destroyed; oxygen atoms are removed from their network positions and decorated with hydrogen atoms, thus producing water molecules and trivalent silicon centers in comparable numbers. At an exposure of 1019 cm−2, of the order of 1014 cm−2 of oxygen atoms are removed, without any indication that the process would saturate. A tentative model is presented in which the degeneration process takes place at regular network sites; a defect precursor is not invoked. It is proposed that neutralization of self-trapped hydrogen/hole pairs at the oxygen atoms plays a crucial role in this process.
Luminescence in hydrogenated amorphous carbon films grown by dc saddle-field glow-discharge decomposition of methane
Authors: Gaspari F. , Kruzelecky R. V. , Lim P. K. , Sidhu L. S. , Zukotynski S.
Published: July 2002

Photoluminescence in hydrogenated amorphous carbon thin films deposited using the dc saddle-field glow-discharge technique onto glass and single-crystal silicon substrates was studied. Samples prepared using positive substrate bias exhibited strong broad-band photoluminescence at room temperature. The luminescence spectrum had a major peak at 1.9 eV and two smaller peaks at 2.3 and 2.6 eV. Samples prepared using grounded or negatively biased substrates exhibited only weak photoluminescence near 2.6 eV. Infrared spectroscopy indicates that the luminescence at 1.9 and 2.3 eV is related to the presence of C–OH bonds, whereas the photoluminescence at 2.6 eV appears to be an intrinsic property of the a-C:H films.
Emission and storage properties of LiTaO3:Tb3+ phosphor
Authors: Nikl M. , Morlotti R. , Magro C. , Bracco R.
Published: July 2002

The emission properties of a new phosphor based on LiTaO3:Tb3+ are described. It is shown to be a storage phosphor, i.e., after previous x-ray exposure, Tb3+ emission in green-red region of spectrum can be photostimulated by light in 400–800 nm spectral range. Emission centers (Tb3+ ions), electron storage centers (oxygen vacancies) and competing nonradiative recombination centers (oxygen ion and Li vacancy couples), are recognized as the key elements in the energy transfer processes and overall emission efficiency of LiTaO3:Tb3+.
Oxidation states and magnetism of Fe nanoparticles prepared by a laser evaporation technique
Authors: Jönsson B. J. , Turkki T. , Ström V. , El-Shall M. S. , Rao K. V.
Published: July 2002

Nanoparticles of iron and iron oxide have been prepared in a thermal diffusion cloud chamber using pulsed laser evaporation. SEM/TEM studies of these particles reveal a size distribution with a mean diameter of about 60 Å. This is consistent with the mean particle size estimated from the magnetic data. The oxidation levels of these nanoparticles prepared at different partial oxygen pressures were investigated using FTIR. All the samples are found to exhibit superparamagnetism with blocking temperatures ranging from 50 K to above room temperature. Magnetic anisotropy constants are calculated from the frequency dependence of the blocking temperatures are found to be one quarter of magnitude higher than is known for the bulk.
Irregular grain structure in micromagnetic simulation
Authors: Porter D. G. , Glavinas E. , Dhagat P. , O’Sullivan J. A. , Indeck R. S. , Muller M. W.
Published: July 2002

A randomized microstructure based on the Voronoi diagram is proposed for micromagnetic models. Simulations illustrate variability of extrinsic magnetic properties with microstructure, medium noise dependence on medium properties, and jitter dependence on trackwidth.
Extraction of slow oxide trap concentration profiles in metal–oxide–semiconductor transistors using the charge pumping method
Authors: Maneglia Y. , Bauza D.
Published: July 2002

The charge pumping technique is shown to be a very efficient method for studying the slow traps in the oxide of metal–oxide–semiconductor (MOS) transistors. The trap concentration is obtained by varying the gate pulse frequency, the other measurement parameters being kept constant. The concentrations measured on virgin devices are in agreement with those generally obtained on state-of-the-art MOS transistors using noise measurements. On virgin and stressed devices, they also agree with those measured using a drain current transient technique. The concentration profiles show an increase of the trap density near the oxide–semiconductor interface.
Effect of temperature on dielectric properties of ice in the range 5–39 GHz
Authors: Matsuoka Takeshi , Fujita Shuji , Mae Shinji
Published: July 2002

The relative complex dielectric permittivity, ε*=ε′−iε″, of ice has been measured in the frequency range 5–39 GHz and in the temperature range 190–265 K. The cavity resonator method at 5 and 10 GHz and the open resonator method at 33 and 39 GHz were used to determine the low dielectric loss of ice. The real part of permittivity ε′ was independent of the frequency. The temperature dependence of ε′ was observed and is discussed in terms of contributions from anharmonic effects to infrared polarizability. The ε″ obtained bridges the gap of previous results between 200 and 258 K. We discuss the frequency and temperature dependence of the effect of the infrared absorption band on ε″. The ε″ variation with frequency increased as the temperature decreased at 5–39 GHz. It is possible that absorption takes place at frequencies below the infrared region.
Spalling of Cu6Sn5 spheroids in the soldering reaction of eutectic SnPb on Cr/Cu/Au thin films
Authors: Liu Ann A. , Kim H. K. , Tu K. N. , Totta Paul A.
Published: July 2002

The growth and morphology of intermetallic compounds between the solder and substrate play an important role in the solderability and reliability of electronic solder joints. Solder on thin films, as in chip joint, acts as an electrical and mechanical/physical interconnection between the chip and the substrate. We have studied the interfacial reactions between eutectic SnPb (63Sn37Pb, wt%) and Cr/Cu/Au thin films. Our results found here have been compared to the solder reaction on bulk Cu. The eutectic solder has 7° of wetting angle on Cr/Cu/Au thin films rather than 11° on Cu substrate. Sideband around the solder cap was found in both the thin film case and the Cu case. Spalling of Cu6Sn5 compound grains occurred in the thin-film case when the Cu film was consumed but not in the case of bulk Cu. We observed a shape change from hemispherical ‘‘scallops’’ to spheroids before spalling took place. The shape change is assisted by ripening a reaction among the scallops. We have calculated a critical size of the scallop, depending on the Cu film thickness, when the shape change or spalling starts.
Picosecond absorption spectroscopy of surface modified copper sulfide nanocrystals in polymeric film
Authors: Artemyev M. V. , Gurin V. S. , Yumashev K. V. , Prokoshin P. V. , Maljarevich A. M.
Published: July 2002

The transient induced absorption and bleaching are examined in 10 nm size CuS nanocrystals embedded in polyvinylalcohol film. Partial surface oxidation of CuS nanocrystals produces a new near-IR-absorption band peaked at 1100 nm. The surface oxidized shell is supposed to form a midgap electron acceptor level and the near-IR band relates to the electron transfer from the CuS valence band. The near-IR band is bleached easily by 15 ps pump pulse from YAG:Nd laser (1.08 μm). The bleaching is accompanied by the induced absorption at the red side of CuS fundamental edge. Both signals induced decay quickly with the time constant 65–70 ps. The model is proposed where the electron–hole pair excited by near-IR photon (electron in surface shell and hole inside CuS core) creates strong internal electric field, which induces the Stark red shift of CuS fundamental edge.
Origin of infrared bands in neutron-irradiated silicon
Authors: Sarlis N. V. , Londos C. A. , Fytros L. G.
Published: July 2002

Infrared absorption measurements were made of the localized vibrational modes due to defects produced in Czochralski-grown Si material after irradiation with fast neutrons and subsequent thermal treatments. The investigation was focused, in particular, on three satellite bands in the region of the A center, located at 839, 833 and 824 cm−1 respectively, the annealing behavior of which was carefully monitored. Correlation of our results with previous infrared, electron paramagnetic resonance and positron annihilation studies favors attributing these bands to the V2O, V3O2 and V2O2 defects respectively. In addition, semiempirical calculations were carried out for the vibrational frequencies of these defects, and the predicted values are in agreement with the above assignments.
Microstructured magnetic tunnel junctions (invited)
Authors: Gallagher W. J. , Parkin S. S. P. , Lu Yu , Bian X. P. , Marley A. , Roche K. P. , Altman R. A. , Rishton S. A. , Jahnes C. , Shaw T. M. , Xiao Gang
Published: July 2002

We have used a simple self-aligned process to fabricate magnetic tunnel junctions down to submicron sizes. Optical and electron-beam lithographies were used to cover a range of areas spanning five orders of magnitude. The bottom magnetic electrodes (Co or permalloy) in our junctions were exchange biased by an antiferromagnetic layer (MnFe). The top electrodes were made of soft magnetic materials (Co or permalloy). We have consistently obtained large magnetoresistance ratios (15%–22%) at room temperature and in fields of a few tens of Oe. The shape of the field response of the magnetoresistance was varied from smooth to highly hysteretic by adjusting the shape anisotropy of one junction electrode.
Experimental study of the effective secondary emission coefficient for rare gases and copper electrodes
Authors: Auday G. , Guillot Ph. , Galy J. , Brunet H.
Published: July 2002

Measurements are made for variations of the effective secondary emission coefficient γ with reduced field E/p. Plane-parallel copper electrodes (5 cm diameter) are sustained with a dc voltage (0<V<1 kV) and can be separated by a variable distance (2 mm<d<1 cm). Current–voltage characteristics, Paschen curves, and γ(E/p) variations are shown for various rare gases (neon, argon, krypton, and xenon). γ(E/p) values are deduced from Paschen curves and published α/p(E/p) variations by using the self-sustain condition. Comparisons are made with various experimental or calculated results taken from the literature.
Interdiffusion in GaAs(1-x)Sbx/GaAs superlattices studied with high-resolution x-ray diffraction and secondary ion mass spectroscopy
Authors: Senz S. , Egger U. , Schultz M. , Gösele U. , Ito H.
Published: July 2002

Nominally undoped GaAsSb/GaAs superlattices were annealed at temperatures between 900 and 1100 °C in a closed quartz ampoule. A strong dependence of the interdiffusion coefficients in the GaAs/GaAsSb superlattices on the arsenic vapor pressure was observed by two independent methods: secondary ion mass spectroscopy, and high-resolution x-ray diffraction using dynamic calculations to extract interdiffusion coefficients. The interdiffusion coefficient was low in the Ga-rich regime where an Arrhenius like dependence with an activation energy of 1.5±0.4 eV and a preexponential factor of 7.1×10−12 cm2 s−1 was found. For the As-rich regime the activation energy was 2.0±0.1 eV and the preexponential factor 7.8×10−9 cm2 s−1.
Micromagnetics of mesoscopic epitaxial (110) Fe elements with nanoshaped ends
Authors: Yu J. , Rüdiger U. , Thomas L. , Parkin S. S. P. , Kent A. D.
Published: July 2002

The magnetization reversal and magnetic domain configurations of 0.5-μm-wide epitaxial (110) Fe particles with rectangular and needle-shaped ends and competing magnetic anisotropies have been investigated. Magnetic force microscopy imaging and longitudinal Kerr hysteresis loop measurements in conjunction with micromagnetic simulations have been used to elucidate the basic micromagnetic behavior. End shape is shown to be a determining factor for the nucleation of magnetization reversal and the resulting magnetic domain configurations.
Ion energy distributions in pulsed large area microwave plasma
Authors: Zabeida Oleg , Martinu Ludvik
Published: July 2002

The energy and the flux of ions impinging on surfaces exposed to low pressure plasmas are important factors which determine the chemical structure and the physical properties of the surfaces and of the thin films. In the present work, we use a large area microwave (MW) plasma reactor in which a grounded sample holder is exposed to a MW (2.45 GHz) discharge excited in different gases, such as Ar, N2, and He, at a pressure ranging from 50 to 200 mTorr. A three-grid, differentially pumped ion energy analyzer is used to measure the ion energy distribution functions (IEDF). The use of a pulsed plasma gives rise to a structured IEDF in which the mean ion energy values vary between 2 and 10 eV. The pulse frequency and the duty cycle were found to strongly affect the IEDF and the ion flux. The evolution of the IEDF is analyzed in terms of the pulsed plasma global model, used to derive the characteristic time constants of plasma ignition and plasma decay. It is shown that the ions in the low energy portion of the IEDF originate from the time period between the individual power pulses, and their relative contribution increases with decreasing the duty time. Controlled pulsing thus allows one to selectively adjust the ion energy, and thereby the surface phenomena in materials processing which are primarily influenced by ion bombardment.
An approach to the defect structure analysis of lithium niobate single crystals
Authors: Safaryan F. P. , Feigelson R. S. , Petrosyan A. M.
Published: July 2002

A formula for calculating the soft mode frequency and Curie temperature (using ion mass, charge, and the distances between them) was applied to the analysis of the defect structure of nonstoichiometric lithium niobate. Four compositions from near stoichiometric to highly Li2O deficient, including congruent, were studied. The Curie temperatures were calculated for three possible types of defects: (1) oxygen vacancies, (2) niobium vacancies, and (3) lithium vacancies. A comparison between the calculated and experimental values showed that the oxygen and niobium vacancy models did not agree with experimental data. Good quantitative agreement between the calculated and experimental data for the lithium vacancy model allowed us to conclude that this model best describes the defect structure of nonstoichiometric lithium niobate LiNbO3.
Sensitized Ho3+ up-conversion luminescence in Nd3+–Yb3+–Ho3+ co-doped ZrF4-based glass
Authors: Qiu J. , Shojiya M. , Kawamoto Y.
Published: July 2002

As previously reported, a selectively strong green emission due to the Ho3+: (5F4, 5S2)→5I8 transitions is observed in Nd3+–Ho3+ co-doped ZrF4-based fluoride glasses under 800 nm excitation. As an attempt to more enhance Ho3+ up-conversion luminescences in the Nd3+–Ho3+ co-doped ZrF4-based glasses, Yb3+ ions were added to the glasses. As a result it was found that, in 800 nm excitation of 60ZrF4⋅30BaF2⋅(8−x)LaF3⋅1NdF3⋅xYbF3⋅1HoF3 glasses (x=0 to 7), sensitized up-conversion luminescences are observed at around 490 nm (blue), 545 nm (green), and 650 nm (red), which correspond to the Ho3+: F35→5I8, (5F4, 5S2)→5I8, and F55→5I8 transitions, respectively. The intensities of the green and red emissions in a 3 mol % YbF3-containing glass were about 50 times stronger than those in no YbF3-containing glass. This is based on sensitization due to Yb3+ ions. In particular, the green emission is extremely strong so that the Nd3+–Yb3+ –Ho3+ co-doped ZrF4-based glasses have a high possibility of realizing a green up-conversion laser glass. Up-conversion processes for the blue, green, and red emissions were two-photon processes assisted by Nd3+→Yb3+→Ho3+ energy transfer. The up-conversion mechanism in the glasses is discussed.
Generalization of fluctuation induced conductivity in polycrystalline Y1−xCaxBa2Cu3Oy and Bi2Sr2Ca1Cu2O8+δ superconductors
Authors: Ghosh A. K. , Bandyopadhyay S. K. , Basu A. N.
Published: July 2002

We have investigated the fluctuation induced conductivity (FIC) of two high-Tc superconductors (HTSC) namely Bi2Sr2Ca1Cu2O8+δ and Y1−xCaxBa2Cu3Oy. The effective medium theory is used to formulate the Aslamazov–Larkin type equations for these polycrystalline superconductors. For the three dimensional case the temperature exponent remains unaltered since both the exponents for the ab plane and c direction FICs in the crystal are same. But the situation is more complex in the two dimensional case where the counterparts in the crystal have different temperature dependencies. The FIC is evaluated and compared with the theoretically obtained values from the model proposed. We have also evaluated the exponents, the coherence lengths, and the crossover temperature. The analysis based on the present model reveals that these two superconductors are similar concerning their nature of fluctuations. The extent of a particular fluctuation region of course differs from one HTSC to another.
Magnetic properties and underlayer thickness in SmCo/Cr films
Authors: Romero S. A. , Cornejo D. R. , Rhen F. M. , Neiva A. C. , Tabacniks M. H. , Missell F. P.
Published: July 2002

SmCo films with a Cr underlayer have been investigated as potentially attractive candidates for high density recording media. Magnetron sputtering was used here to produce Cr/SmCox/Cr films on Si (100) substrates. The magnetic films were deposited at a substrate temperature of 350 °C and an Ar pressure of 5 mTorr. Cr underlayers were deposited both at 25 °C and at 350 °C and exhibited different textures. Layer thicknesses were evaluated using Rutherford backscattering spectroscopy, while SmCo grain size and underlayer texture were determined from a Rietveld analysis of x-ray diffraction data. The magnetic properties were measured with a vibrating sample magnetometer. The resulting films had in-plane coercivities in the range 4–8 kOe. For a SmCo layer with a thickness of 200 nm, the Cr underlayer thickness was varied in the range 75–300 nm to study its effect on intergranular coupling in the films. For thermally demagnetized samples, both magnetizing and demagnetizing remanence curves were measured and used to evaluate switching field distributions and δm plots. The underlayer texture was found to affect magnetic properties more strongly than underlayer thickness.
Reduction mechanisms for defect densities in GaN using one- or two-step epitaxial lateral overgrowth methods
Authors: Vennéguès P. , Beaumont B. , Bousquet V. , Vaille M. , Gibart P.
Published: July 2002

A transmission electron microscopy study of the reduction mechanisms for defect densities in epitaxial lateral overgrown (ELO) GaN films is presented. In the standard one step ELO, the propagation of defects under the mask is blocked, whereas the defects in the window regions thread up to the surface. We propose an alternative two step ELO method. In a first step, dislocations close to the edge of the (0001) top facet bend at 90°, thereby producing a drastic reduction in the density of defects above the window. After the coalescence, induced by lateral growth in a second step, dislocations are mainly observed in the coalescence boundaries. The density of defects is decreased to 2×10−7 cm−2 over the entire surface and areas nearly 5 μm wide with 5×106 cm−2 dislocations between the center of the windows and the coalescence boundaries are obtained.
Preparation and thermoelectric properties of A8IIB16IIIB30IV clathrate compounds
Authors: Kuznetsov V. L. , Kuznetsova L. A. , Kaliazin A. E. , Rowe D. M.
Published: July 2002

Polycrystalline samples of clathrate compounds Ba8Ga16Si30, Ba8Ga16Ge30, Ba8Ga16Sn30, and Sr8Ga16Ge30 were prepared by direct melting and characterized using X-ray powder diffraction and differential thermal analysis. The Ge- and Si-based clathrates melt congruently, whereas Ba8Ga16Sn30 melts incongruently. At room temperature the Ge- and Si-based clathrates possess a moderate negative Seebeck coefficient and a high electron concentration in the range of 7×1020–9×1020 cm−3 while Ba8Ga16Sn30 exhibits substantially lower electron concentration of 2.2×1019 cm−3. The Seebeck coefficient and electrical resistivity were measured over the range 100–870 K. The temperature dependence of transport properties of the clathrates is typical for heavily doped semiconductors. The transport properties were analyzed using a standard semiconductor transport model. There is a good agreement between the assumed model and experimental temperature dependence of the Seebeck coefficient in the extrinsic conductivity range for all studied clathrates apart from Ba8Ga16Ge30. The calculated effective masses of the clathrates range from 0.9 to 3 of the free electron mass. The estimated ZT values are 0.7 for Ba8Ga16Ge30 at 700 K and 0.87 for Ba8Ga16Si30 at 870 K. The potential for thermoelectric applications of these materials is assessed.
Porous media gas diffusivity from a free path distribution model
Authors: Tokunaga Tetsu K.
Published: July 2002

A model for the effective diffusivity in porous media is developed from simple considerations. This derivation is analogous to those used in force path models for electron and phonon diffusion in solids. (AIP).
Rotational-level-dependent quenching of A 2Σ+ OH and OD
Authors: Copeland R. A. , Dyer Mark J. , Crosley David R.
Published: July 2002

Rate constants kQ for collisional quenching of A 2Σ+, v′=0, OH and OD have been measured for specific rotational levels N′ of the radical and a wide variety of collision partners. Through measurements of the time-dependent laser-induced fluorescence in a low pressure discharge flow at room temperature, we observe a decrease in kQ with increasing rotational quantum number for most quenchers. The internal levels of the collision pairs appear unimportant from experiments involving deuterium substitution. A comparison of rotationless rates for different colliders [kQ(N=0)] with calculations based on collision complex formation indicate that attractive forces play a role in the quenching process.
A singular perturbation approach to non-Markovian escape rate problems with state dependent friction
Authors: Dygas M. M. , Matkowsky B. J. , Schuss Z.
Published: July 2002

We employ singular perturbation methods to examine the generalized Langevin equation (GLE) with state and memory dependent friction. The GLE describes the dynamics of a Brownian particle in an arbitrary potential field, acted on by a fluctuating force characterizing the collisions between the particle and the thermal bath. We consider the situation in which the collisions are not restricted to occur instantaneously and additionally these interactions are nonlinear. This leads to a non-Markovian description of the dynamics of the particle. We compute explicit analytical expressions for the mean exit time τ of the Brownian particle from the potential well in which it is confined. Thus we derive expressions for the activation rate κ from the well, which is inversely proportional to τ. In the examples considered, the effect of the state dependent friction is to increase τ and thus to lower κ.
Two-color threshold photoionization of jet-cooled aniline: Vibrationally selective autoionization
Authors: Hager James , Smith Mark A. , Wallace Stephen C.
Published: July 2002

We report the results of investigations of the autoionization of jet-cooled aniline-h7 and -d7. Using two-color photoionization techniques, we have observed autoionization of high lying Rydberg levels containing quanta of the nontotally symmetric vibrational modes 10b, I, and 15. No autoionization is observed for Rydberg states containing only totally symmetric vibrational excitation. In all of the spectra in which autoionization is found, the mechanism is vibrational–electronic coupling (vibrational autoionization) with a characteristic Δv=−1 propensity. An upper limit for the autoionization rate of 2×1012 s−1 has been determined from linewidth measurements. Possible explanations are presented and discussed for the activity of only nontotally symmetric vibrational modes in the autoionization process.
Studies of the reaction of O+2 with deuterated methanes
Authors: Barlow S. E. , Van Doren J. M. , DePuy C. H. , Bierbaum V. M. , Dotan I. , Ferguson E. E. , Adams N. G. , Smith D. , Rowe B. R. , Marquette J. B. , Dupeyrat G. , Durup-Ferguson M.
Published: July 2002

In the gas phase O+2 reacts with methane at 300 K to produce a hydrogen atom and the CH3O+2 ion. The structure of this ion has recently been determined to be H2COOH+, methylene hydroperoxide ion. The reaction rate coefficients and product distributions have now been measured at 300 K for the CHnD4−n isotopes. The reaction shows both inter- and intramolecular isotope effects, e.g., CH2D2 reacts more slowly than methane and more rapidly than CD4, but loses hydrogen or deuterium with equal probability. The ion readily transfers HO+ to alkenes, CS2, and many other neutral molecules. The reaction with CS2 has been used to investigate the isotopic distribution within mixed isotope product ions. In addition, the reaction rate coefficients for both CH4 and CD4 have been measured as functions of temperature between 20 and 500 K; in both cases a clear minimum is observed in the reaction rate coefficient near room temperature. A mechanism for the reaction is proposed which allows us to model the temperature dependence of the reaction rate coefficient over the entire range for which there are data.
Brillouin scattering studies of isotopic effects in solid ammonia
Authors: Kiefte H. , Penney R. , Breckon S. W. , Clouter M. J.
Published: July 2002

The technique of high resolution Brillouin spectroscopy has been used to determine the adiabatic elastic constants and the elasto-optic coupling (Pockels) coefficient ratios of oriented single crystals of (solid I) ND3 at temperatures near the gas–liquid–solid triple point. The values of the elastic constants at 196.0 K are C11=83.3, C12=44.0, and C44=49.6 (in units of kbar) with an estimated absolute uncertainty of ±2%. The values of the elasto-optic coefficient ratios are P12/P11=0.90 and P44/P11=0.16 for ND3 and 0.89 and 0.16 for NH3, respectively. Other than that expected from the mass ratio, no significant isotopic differences are evident.
State-to-state N2(A 3∑+u) energy pooling reactions. II. The formation and quenching of N2(B 3Πg, v′=1–12)
Authors: Piper Lawrence G.
Published: July 2002

We have studied the state-to-state excitation of N2(B 3Πg, v′=1–11) in energy pooling reactions between N2(A 3∑+u, v′=0,1) molecules and subsequent quenching in collisions with molecular nitrogen. Excitation of vibrational levels 10, 2, and 3 appears to be much stronger than excitation of the other vibrational levels. In addition, we failed to observe any emission from v′=12 even though it is energetically accessible. The excitation rate coefficients are quite large, 7.7×10−11 cm3 molecule−1 s−1 for the pooling of two N2(A, v′=0) molecules, and roughly a factor of three larger for energy pooling events involving N2(A, v′=1). The effective rate coefficients for electronic quenching of N2(B) by N2 are also quite large, ≈3×10−11 cm3 molecule−1 s−1. Comparison of our quenching results with the laser-excited, real-time quenching studies of Rotem and Rosenwaks indicates agreement only within factors of 2–3.
Infrared spectroscopy of the cluster ions H+3⋅(H2)n
Authors: Okumura M. , Yeh L. I. , Lee Y. T.
Published: July 2002

The vibrational spectra of the clusters H+3(H2)n were observed near 4000 cm−1 by vibrational predissociation spectroscopy. Spectra of mass-selected clusters were obtained by trapping the ions in a radio frequency ion trap, exciting vibrational transitions of the cluster ions to predissociating levels, and detecting the fragment ions with a mass spectrometer. Low resolution bands of the solvent H2 stretches were observed for the clusters of one to six H2 coordinated to an H+3 ion. The red shift of these vibrations relative to the monomer H2 frequency supported the model of H+9 as an H+3 with a complete inner solvation shell of three H2, one bound to each corner of the ion. Two additional bands of H+5 were observed, one assigned as the H+3 symmetric stretch, and the other as a combination or overtone band. High-resolution scans (0.5 and 0.08 cm−1) of H+n, n=5, 7, and 9 yielded no observable rotational structure, a result of either spectral congestion or rapid cluster dissociation. The band contour of the H+5 band changed upon cooling the internal degrees of freedom, but the peaks remained featureless. The observed frequencies of H+7 and H+9 agreed well with ab initio predictions, but those of H+5 did not. This deviation is discussed in terms of the large expected anharmonicity of the proton bound dimer H+5.
Melting and phase space transitions in small clusters: Spectral characteristics, dimensions, and K entropy
Authors: Beck Thomas L. , Leitner David M. , Berry R. Stephen
Published: July 2002

The Grassberger–Procaccia method has been employed to study the transitions which occur as a classical Ar3 cluster, modeled by pairwise Lennard-Jones potentials, passes from a rigid, solid-like form to a nonrigid, liquid-like form with increasing energy. Power spectra and lower bounds on the fractal dimensions and K entropies are presented at several energies along the caloric curve for the Ar3 cluster. In addition, the full spectrum of Liapunov exponents has been computed at these same energies to get an accurate value of the K entropy. Chaotic behavior, though relatively small, is observed even at low energies where the power spectrum displays largely normal-mode structure. The degree of chaotic behavior increases with energy at energies where some degree of regularity is observed in the spectrum. However, at energies that just allow the system to pass into and across saddle regions separating local potential minima, the phase space appears to be separable into a region within the equilateral triangle potential well where the behavior is highly chaotic, and a region of lower dimensions and less chaos around the saddle of the linear configuration. Dimensions from approximately three to eight are observed. A clear separability of time scales for establishment of different extents of ergodicity permits the determination of fractal dimensions of the manifold on which the phase points moves, for time scales of physical, i.e., observable significance. We believe this to be the first evaluation of the dimensionality of the space on which the phase point moves, for a Hamiltonian system displaying this range of dimensions.
A new method for carrying out free energy perturbation calculations: Dynamically modified windows
Authors: Pearlman David A. , Kollman Peter A.
Published: July 2002

A new methodology for carrying out free energy perturbation calculations is derived and implemented in an extensively modified version of the amber 3.0 program (U. C. Singh, P. K. Weiner, J. W. Caldwell, and P. A. Kollman, Department of Pharmaceutical Chemistry, University of California, San Francisco, 1986). In this method, the local slope of the accumulated free energy vs λ curve is used to choose the width, δλ of the next window. This method has been evaluated on several test cases: the free energies of solvation for the noble gases neon, krypton, and xenon; and the free energy of solvation of methane. In each case, good agreement between theory and experiment is obtained in a reasonably short amount of simulation time using the new methodology. In contrast, when these test cases are run using a standard fixed δλ methodology for the same lengths of time, much worse results are obtained. This attests to the ability of the new method to reduce the radius of convergence for these types of free energy calculations.
Vibrational relaxation of H2O from 295 to 1020 K
Authors: Zittel P. F. , Masturzo D. E.
Published: July 2002

A laser induced fluorescence/heated flow tube method has been used to measure rate constants for vibrational relaxation of the coupled ν1 and ν3 stretching levels, the 2ν2 bending overtone level, and the ν2 bending level of H2O by H2O and rare gases from 295 to 1020 K. The probability per collision for self-relaxation of the coupled stretching levels decreased monotonically from 0.146 at 295 K to 0.055 at 1020 K. The probability for self-relaxation of the 2ν2 level decreased monotonically from 0.59 at 295 K to 0.32 at 947 K. The probability for self-relaxation of the ν2 level decreased with temperature from 0.26 at 295 K to 0.16 at 947 K. At all temperatures the dominant path for H2O self-relaxation was relaxation of the stretching levels to the bending overtone level followed by V→T,R relaxation of a single bending quantum to the bending fundamental level. Relaxation of all levels by He and Ar was approximately two orders of magnitude less efficient than H2O self-relaxation. The rate constant for relaxation of the H2O stretching levels by oxygen atoms was measured at 315 and 520 K, and was a factor of 5–10 smaller than the H2O self-relaxation rate constant.
The growth modes of vapor-deposited bismuth on a Cu(110) surface
Authors: Clendening W. D. , Campbell Charles T.
Published: July 2002

The interactions of vapor-deposited Bi with Cu(110) have been studied with x-ray photoelectron spectroscopy (XPS), thermal desorption mass spectroscopy (TDS), and low-energy electron diffraction (LEED). There is a net repulsive Bi–Bi interaction in the adlayer such that the heat of adsorption decreases from ∼69 to 51 kcal/mol as θBi increases from ∼0.3 to ∼0.7. At low coverages (i.e., long Bi–Bi distances), this repulsive interaction is relatively isotropic, with the Bi forming a quasihexagonal c(2×2) adlayer. At high coverages (i.e., short Bi–Bi distances), the effective Bi–Bi interaction potential is very anisotropic, wherein there are Bi–Bi repulsions parallel to the troughs of the Cu(110) surface, but Bi–Bi attractions across these troughs. This leads to p(n×1) overlayer structures up to completion of the monolayer. Systematic trends in the structural growth modes for metal adlayers showing net repulsive lateral interactions on one-dimensionally corrugated substrates are analyzed. It is found that three types of growth modes can be predicted depending upon a size parameter (the ratio of the metallic diameter of the adatom to the trough–trough separation of the substrate) and the nature of the substrate (fcc vs bcc).
Statistical-mechanical theory of a new analytical equation of state
Authors: Song Yuhua , Mason E. A.
Published: July 2002

We present an analytical equation of state based on statistical-mechanical perturbation theory for hard spheres, using the Weeks–Chandler–Andersen decomposition of the potential and the Carnahan–Starling formula for the pair distribution function at contact, g(d+), but with a different algorithm for calculating the effective hard-sphere diameter. The second virial coefficient is calculated exactly. Two temperature-dependent quantities in addition to the second virial coefficient arise, an effective hard-sphere diameter or van der Waals covolume, and a scaling factor for g(d+). Both can be calculated by simple quadrature from the intermolecular potential. If the potential is not known, they can be determined from the experimental second virial coefficient because they are insensitive to the shape of the potential. Two scaling constants suffice for this purpose, the Boyle temperature and the Boyle volume. These could also be determined from analysis of a number of properties other than the second virial coefficient. Thus the second virial coefficient serves to predict the entire equation of state in terms of two scaling parameters, and hence a number of other thermodynamic properties including the Helmholtz free energy, the internal energy, the vapor pressure curve and the orthobaric liquid and vapor densities, and the Joule–Thomson inversion curve, among others. Since it is effectively a two-parameter equation, the equation of state implies a principle of corresponding states. Agreement with computer-simulated results for a Lennard-Jones (12,6) fluid, and with experimental p–v–T data on the noble gases (except He) is quite good, extending up to the limit of available data, which is ten times the critical density for the (12,6) fluid and about three times the critical density for the noble gases.

As expected for a mean-field theory, the prediction of the critical constants is only fair, and of the critical exponents is incorrect. Limited testing on the polyatomic gases CH4, N2, and CO2 suggests that the results for spherical molecules (CH4) may be as good as for the noble gases, nearly as good for slightly nonspherical molecules (N2), but poor at high densities for nonspherical molecules (CO2). In all cases, however, the results are accurate up to the critical density. Except for the eight-parameter empirical Benedict–Webb–Rubin equation, this appears to be the most accurate analytical equation of state proposed to date.
Where does overtone intensity come from?
Authors: Lehmann Kevin K. , Smith Alice M.
Published: July 2002

The factors that influence overtone intensity are examined using realistic potential and dipole moment functions that model the acetylenic stretching vibration. It is found that mechanical anharmonicity is more important than electrical anharmonicity, especially at higher overtone levels but a quantitative treatment requires both contributions which are not simply additive. It is found numerically that the inner wall of the potential dominates in determining overtone intensity. This can be justified by heuristic arguments and by the properties of the sensitivity functional of the transition dipole matrix elements to changes in V(r). It is suggested that absolute overtone intensities could be useful in determining the inner walls of hydrogen stretching vibrations.
Spectra of dangling OH bonds in amorphous ice: Assignment to 2- and 3-coordinated surface molecules
Authors: Buch V. , Devlin J. P.
Published: July 2002

Interpretation is provided for the recently measured stretch spectra of the dangling OH and OD bonds in amorphous ice deposits. The spectral bands are doublets; the higher and the lower frequency feature of each doublet is assigned respectively to 2- and 3-coordinated water molecules on the surface. The assignment is based on combined computational and experimental evidence. Annealing of 2-coordinated surface molecules is demonstrated in a simulated amorphous H2O cluster.
Interpolated variational transition-state theory: Practical methods for estimating variational transition-state properties and tunneling contributions to chemical reaction rates from electronic structure calculations
Authors: Gonzalez-Lafont Angels , Truong Thanh N. , Truhlar Donald G.
Published: July 2002

In many cases, variational transition states for a chemical reaction are significantly displaced from a saddle point because of zero-point and entropic effects that depend on the reaction coordinate. Such displacements are often controlled by the competition between the potential energy along the minimum-energy reaction path and the energy requirements of one or more vibrational modes whose frequencies show a large variation along the reaction path. In calculating reaction rates from potential-energy functions we need to take account of these factors and—especially at lower temperatures—to include tunneling contributions, which also depend on the variation of vibrational frequencies along a reaction path. To include these effects requires more information about the activated complex region of the potential-energy surface than is required for conventional transition-state theory. In the present article we show how the vibrational and entropic effects of variational transition-state theory and the effective potentials and effective masses needed to calculate tunneling probabilities can be estimated with a minimum of electronic structure information, thereby allowing their computation at a higher level of theory than would otherwise be possible. As examples, we consider the reactions OH+H2, CH3+H2, and Cl+CH4 and some of their isotopic analogs. We find for Cl+CH4→HCl+CH3 that the reaction rate is greatly enhanced by tunneling under conditions of interest for atmospheric chemistry.
On the construction of coarse-grained models for linear flexible polymer chains: Distribution functions for groups of consecutive monomers
Authors: Baschnagel J. , Binder K. , Paul W. , Laso M. , Suter U. W. , Batoulis I. , Jilge W. , Bürger T.
Published: July 2002

Coarse-grained models for linear flexible polymers are constructed defining effective segments by taking together n successive chemical monomers of a polymer chain, for n=1,2,3,... . The distribution function Pn(l) for the length l of such effective segments is studied as well as the distribution function Pn(ϑ) of the angle between successive effective segments. If n is large enough, all these distribution functions tend towards universal limiting functions. For small n, information on chemical structure and effective potentials for the various degrees of freedom of the polymer chains is still preserved. Using polyethylene (PE) as one example, it is shown that these distribution functions for small n depend somewhat on the choice of the model for the effective potential (and the degrees of freedom included). Bisphenol-A-polycarbonate (BPA-PC) as a second example, serves to study to which extent these distribution functions Pn(l) and Pn(ϑ) differ for chemically different polymers, such as PE and BPA-PC. Consequences for the molecular modeling of polymeric materials are briefly discussed.
High resolution threshold photoelectron spectroscopy of aniline and aniline van der Waals complexes
Authors: Zhang Xu , Smith Jonathan M. , Knee J. L.
Published: July 2002

Zero electron kinetic energy threshold photoelectron spectroscopy is applied to jet cooled aniline and the van der Waals molecules aniline–Ar, aniline–(Ar)2, and aniline–CH4. The monomer cation spectrum is assigned and more precise values of the vibrational frequencies are determined. The spectra of the cation complexes reveal significant vibronic activity indicative of a significant change in complex geometry upon ionization. The change in complex binding energy upon ionization is obtained from a determination of the complex ionization potentials. For the first time zero electron kinetic energy is used to probe van der Waals complex predissociation on the S1 electronic surface. Both reactants (complex) and products (monomer) of the reaction are observed in the photoelectron spectrum. Details of the mechanism of the zero electron kinetic energy threshold photoionization process and its impact on the observation of van der Waals molecules are discussed.
Toluene–ammonia clusters: Ion fragmentation and chemistry
Authors: Li Shijian , Bernstein Elliot R.
Published: July 2002

The toluene/ammonia cluster system is studied by mass resolved excitation spectroscopy: like the toluene/water system, an extensive ion chemistry is found to exist generating ammonia solvated protons and (solvated) benzyl radicals. Extensive cluster fragmentation (dissociation) is observed even at ‘‘threshold ionization energies.’’ Nozzle delay and deuteration studies are applied to elucidate the ion chemistry and dissociation of this cluster system. Proton transfer/cluster fragmentation reactions in this system are driven by the stability of the benzyl radical and (NH3)mH+ and solvation stabilization of the toluene ion. Cluster fragmentation by loss of a single ammonia molecule is not observed. Fragmentation is so extensive for this system that spectral features could not be identified with parent clusters.
Electrolytes constrained on fractal structures: Debye–Huckel theory
Authors: Lee Song Hi , Hubbard Joseph B.
Published: July 2002

We calculate, via the Debye–Huckel theory, the effect of ionic screening on the thermodynamic properties of an electrolyte confined topologically to an isotropic random fractal structure embedded in three dimensions. It is found that screening effects are generally important, even for low fractal dimensions. In many instances crossover effects occur, whereby with increasing ionic strength, a low dimension fractal is more stabilized by screening than a fractal of higher dimension.