bluuebunny/crossref_metadata_2025 huggingface dataset:
Interface instability in an electric field
Authors: Klinger L. , Levin L.
Published: July 2002

We have studied morphological changes of an interface in a strong electric field which is normal to the initially planar interface. Electromigration along the interface in a two phase metallic system A-B of the immiscible components has been considered. The stresses arising during electromigration of the components were taken into account. A nonlinear equation has been derived for the interface evolution in the electric field, allowing for curvature of the interface. It was shown that the interface diffusion in an electric field leads to the formation of a periodic corrugation on the interface if the components are distinguished by their electric charges. The corrugation increases with time and is transformed into a channel-hillock-like structure. The nonlinear equation was analyzed numerically for the steady-state case. The shape and the growth rate of hillocks on the interface have been calculated.
Characterization of a-C:H:N deposition from CH4/N2 rf plasmas using optical emission spectroscopy
Authors: Clay K. J. , Speakman S. P. , Amaratunga G. A. J. , Silva S. R. P.
Published: July 2002

Optical emission spectra (OES) from CH4/N2 rf plasmas, which are used for the deposition of nitrogen-containing hydrogenated amorphous carbon (a-C:H:N) thin films, have been characterized. Previously unidentified spectral lines have been assigned to atomic N. Further identified species include CH, H, H2, N2, N+2, N, and CN. Variations between spectra from the pure CH4 or N2 plasmas and the mixed CH4/N2 plasma are discussed. The enhancement of excited nitrogen species, with the addition CH4, is attributed to Penning ionization. The observed OES variations of the CH4/N2 plasma with power, pressure, and CH4/N2 ratio are explained in terms of possible reaction mechanisms and their activation, and correlated with preliminary film growth characteristics.
Shallow and deep levels in n-type 4H-SiC
Authors: Evwaraye A. O. , Smith S. R. , Mitchel W. C.
Published: July 2002

The nitrogen levels in 4H-SiC have been determined using thermal admittance spectroscopy. The values of Ec−0.053 eV for nitrogen at the hexagonal site and Ec−0.10 eV for nitrogen at the quasicubic site agree with those reported using other techniques. The deep levels in 4H-SiC were studied using optical admittance spectroscopy. The optical admittance spectrum showed, besides the conductance peak corresponding to band to band transitions, four other conductance peaks. These peaks correspond to photoexcitation of carriers from the defect levels to the conduction band. It is inferred from a comparison with 6H-SiC that the conductance peak b4 is due to excitation of electrons from the vanadium donor at Ec−1.73 eV. The photoconductance build up transients of the Ec−1.73 eV level are described fully by one exponential term. This suggests that only one center contributed to the observed conductance. The decay kinetics of persistent photoconductance due to the Ec−1.73 eV level follow the stretched exponential form. The potential barrier against recapture of photoexcited carriers was determined to be 18 meV for the vanadium donor level in 4H-SiC.
High-energy and high-fluence proton irradiation effects in silicon solar cells
Authors: Yamaguchi Masafumi , Taylor Stephen J. , Yang Ming-Ju , Matsuda Sumio , Kawasaki Osamu , Hisamatsu Tadashi
Published: July 2002

We have examined proton irradiation damage in high-energy (1–10 MeV) and high-fluence (≳1013 cm−2) Si n+-p-p+ structure space solar cells. Radiation testing has revealed an anomalous increase in short-circuit current Isc followed by an abrupt decrease and cell failure, induced by high-fluence proton irradiation. We propose a model to explain these phenomena by expressing the change in carrier concentration p of the base region as a function of the proton fluence in addition to the well-known model where the short-circuit current is decreased by minority-carrier lifetime reduction after irradiation. The reduction in carrier concentration due to majority-carrier trapping by radiation-induced defects has two effects. First, broadening of the depletion layer increases both the generation–recombination current and also the contribution of the photocurrent generated in this region to the total photocurrent. Second, the resistivity of the base layer is increased, resulting in the abrupt decrease in the short circuit current and failure of the solar cells.
Energy distributions of Zr/O/W Schottky electron emission
Authors: Kim H. S. , Yu M. L. , Thomson M. G. R. , Kratschmer E. , Chang T. H. P.
Published: July 2002

The energy distributions of electron emission from a Schottky emitter have been studied at tip temperatures from 1450 to 1800 K and angular current densities from 0.1 to 240 μA/sr. We have observed broadening of the energy distribution, with increase of angular current density and decrease of tip temperature, from 0.4 to 1.32 eV resulting from electron tunneling and electron–electron interaction. Good agreement between the experimental results and predictions from Monte Carlo simulation of the emission process is observed.
Amorphization and defect recombination in ion implanted silicon carbide
Authors: Grimaldi M. G. , Calcagno L. , Musumeci P. , Frangis N. , Van Landuyt J.
Published: July 2002

The damage produced in silicon carbide single crystals by ion implantation was investigated by Rutherford backscattering channeling and transmission electron microscopy techniques. Implantations were performed at liquid nitrogen and at room temperatures with several ions to examine the effect of the ion mass and of the substrate temperature on the damaging process. The damage accumulation is approximately linear with fluence until amorphization occurs when the elastic energy density deposited by the ions overcomes a critical value. The critical energy density for amorphization depends on the substrate temperature and is greatest at 300 K indicating that defects recombination occurs already at room temperature. Formation of extended defects never occurred and point defects and uncollapsed clusters of point defects were found before amorphization even in the case of light ion implantation. The atomic displacement energy has been estimated to be ∼12 eV/atom from the analysis of the damage process in dilute collision cascades.
Optical nonlinearity of Bi2O3 nanoparticles studied by Z-scan technique
Authors: Yu Baolong , Zhu Congshan , Gan Fuxi
Published: July 2002

The third-order optical nonlinear susceptibilities of Bi2O3 nanoparticles coated by a layer of stearic acid in toluene sol and bare in hydrosol were measured by the Z-scan technique. The nonlinear susceptibility of the coated Bi2O3 nanoparticles was found to be two orders of magnitude greater than that of the bulk Bi2O3. For the coated Bi2O3 nanoparticles, the figures of merit related to linear loss and two-photon absorption were determined.
Effects of anomalous permittivity on the microwave heating of zinc oxide
Authors: Martin L. P. , Dadon D. , Rosen M. , Gershon D. , Rybakov K. I. , Birman A. , Calame J. P. , Levush B. , Carmel Y. , Hutcheon R.
Published: July 2002

Highly nonuniform heating has been observed in zinc oxide (ZnO) powder compacts exposed to 2.45 GHz microwaves in oxygen deficient atmospheres such as pure nitrogen or argon. This phenomenon manifests as a localized zone of rapid heating which propagates outward from the sample core, and is documented by real-time surface and core temperature measurements performed during the microwave exposure. Measurements of the complex permittivity, ε″, during heating of identical ZnO samples in a conventional furnace and in a nitrogen atmosphere, demonstrated that ε″ experiences at least one significant maximum between 200 and 500 °C. Mass spectrometry results indicate that the peaks in ε″ correlate well with the rate of desorption of chemisorbed water from the surface of the ZnO powder. It was also noted that the nonuniform heating does not manifest when the microwave exposure is performed in air. Similarly, the anomalous peaks in ε″ are almost completely suppressed during heating in air. It is well known that oxygen adsorbs strongly to the surface of ZnO in the temperature range from room temperature to 300 °C, and that this adsorption results in a drastic decrease in the electrical conductivity and, thus, in ε″. It is proposed, therefore, that the effect of water desorption upon the complex permittivity may be, in effect, counterbalanced by the adsorption oxygen from the atmosphere. The effect of this behavior may be significant during microwave processing, where nonuniform power absorption can result in extremely localized heating.
X-ray sensitivity of a-Se for x-ray imaging with electrostatic readout
Authors: Kasap S. O. , Aiyah V. , Polischuk B. , Baillie A.
Published: July 2002

Currently there is much interest in stabilized amorphous selenium (a-Se alloyed with 0.2–0.3% As and doped with 5–20 ppm Cl) as an x-ray photoconductor in various x-ray imaging systems. Although optical photogeneration in a-Se is well characterized, experiments on its x-ray sensitivity have been limited and, further, have resulted in widely differing conclusions especially for the electron hole pair (EHP) creation energy EEHP. In this work, we first introduce absolute and incremental sensitivity (S and s, respectively) concepts within the electrostatic readout mode of operation and then establish models linking these two sensitivities to the energy EEHP absorbed to create a free EHP. X-ray irradiation induced xerographic discharge experiments have been carried out on vacuum deposited stabilized a-Se x-ray photoconductor films to examine how the sensitivity depends on the charge carrier schubwegs (μτF where μ is the drift mobility, τ is the lifetime, and F is the electric field), the mean photon energy, and the detector thickness L. Experiments indicate that unless both electron and hole schubwegs are much longer than the photoconductor thickness, the sensitivity is reduced. The efficiency of EHP generation per absorbed x-ray photon, i.e., the apparent energy absorbed per free electron hole pair EEHP has been determined as a function of electric field and mean photon energy. EEHP decreases with increasing applied field and energy. By examining EEHP vs 1/F and extrapolating to 1/F=0, a saturation value EEHP0 for EEHP is predicted and estimated (4 to 6 eV) which, within experimental errors, is close to the theoretical expectation of ∼2.2Eg and substantially smaller than previously reported values for EEHP0 (18–29 eV).
Thermal-response time of superconducting transition-edge microcalorimeters
Authors: Irwin K. D. , Hilton G. C. , Wollman D. A. , Martinis John M.
Published: July 2002

We investigate limits on the thermal-response time of superconducting transition-edge microcalorimeters. For operation at 0.1 K, we show that the lower limit on the response time of a superconducting transition-edge microcalorimeter is of order 1 μs due to the heat diffusion time, electrical instabilities, the amplifier noise, and the critical current of the superconducting film. The response time is not limited by self-heating effects and is independent of the intended photon energy. However, design constraints associated with the inductance of the bias circuit make it difficult to achieve the fastest response times for devices with heat capacities high enough for x-ray and gamma-ray detection.
A study of substitutional nitrogen impurities in chemical vapor deposited diamond
Authors: Sitch P. K. , Jungnickel G. , Kaukonen M. , Porezag D. , Frauenheim Th. , Pederson M. R. , Jackson K. A.
Published: July 2002

The behavior of substitutional N impurities in chemical vapor deposited diamond is examined theoretically in order to explain recent channeling experiments indicating a dominant onsite incorporation of N. The calculations are based on a combination of density-functional methods at various levels of approximation applied to supercell and cluster models. Neutral charge N impurities in the presence of highly defective carbon regions, such as dangling bonds, strained bonds, and partially developed π bonds are studied. We find a perfectly general argument concerning the position of the substitutional N atom in relation to the position of the Fermi level, Ef: if Ef lies above the A1 level associated with the onsite substitutional N atom, off-site motion in the 〈111〉 direction is observed. Conversely, when Ef falls below A1, N doping charge is transferred to the available deeper lying states. Suitable receptor states include surface dangling bonds, surface reconstruction π bonds, and bulk defects states such as grain boundaries and vacancies.
Solid state C13 and H1 nuclear magnetic resonance investigations of hydrogenated amorphous carbon
Authors: Donnet C. , Fontaine J. , Lefèbvre F. , Grill A. , Patel V. , Jahnes C.
Published: July 2002

Various hydrogenated amorphous carbon films have been analyzed by C13 and H1 nuclear magnetic resonance (NMR) spectroscopies. The films have been deposited from acetylene or cyclohexane by dc plasma enhanced chemical vapor deposition, at various dc biases and gas pressures. The total hydrogen content has been measured by forward recoil elastic scattering (FRES). 13C NMR investigations have been performed in various configurations: high power decoupled to determine the sp2:sp3 carbon ratio, cross polarized at magic angle contact spinning with different contact times to provide information on carbon atoms directly bound to hydrogen, and with dipolar dephasing to study the quaternary carbon atoms. By performing the 13C and 1H NMR measurements on the same samples, it was possible to resolve for the first time the seven different forms of unprotonated and protonated CHx carbon, for both sp2(x=0,1,2) and sp3(x=0,1,2,3) carbon hybridizations, as well as the ratio between bound and unbound hydrogen. The results are discussed and compared with previously published FRES and Fourier transform infrared (FTIR) data. It is shown that NMR and FTIR combined with FRES do not agree systematically on the ratio of bound/unbound hydrogen.
Induced magnetic anisotropy in Co–Mn–Si–B amorphous microwires
Authors: Zhukov A. , Blanco J. M. , González J. , Garcia Prieto M. J. , Pina E. , Vázquez M.
Published: July 2002

The effect of both thermal treatments as well as chemical etching treatments on the magnetic behavior has been investigated in nearly-zero magnetostriction glass-coated amorphous (Co1−xMnx)75Si10B15 (x=0.08, 0.09, and 0.10) microwires. Such a small change in x results in rather strong changes in the hysteresis loop parameters including coercitivity, Hc, and initial magnetic permeability, μ15. This effect was ascribed to the change of sign of the magnetostriction constant with a proper combination of the coercivity and relatively high initial permeability for the as-cast x=0.09 sample. Thermal treatment (temperatures 100–200 °C for 0.5–2 h) as well as chemical etching in 20% diluted fluoridric acid with duration from 0.5 up to 50 min modify this magnetic parameters owing to the internal stresses relaxation process. In particular, annealing under applied magnetic field (field annealing) can improve more significantly these magnetic parameters: increasing both Hc and μ15. Such phenomenology can be interpreted considering the noticeable longitudinal magnetic anisotropy induced by the combined effects of the magnetic field and strong internal stresses arising from the coating during the thermal treatment. The reduction of the glass coating thickness by chemical etching leads to a decrease of the internal stresses from a coating and, consequently, to a decrease of the transverse magnetoelastic anisotropy. Such decrease of anisotropy plays a role similar to that induced by field annealing on the hysteretic behavior.
Spin valve sensors with synthetic free and pinned layers
Authors: Veloso Anabela , Freitas Paulo P.
Published: July 2002

Unshielded spin-valve sensors (6×2 μm2) with both synthetic ferrimagnet free (SF) and synthetic antiferromagnet pinned (SAF) layers are demonstrated, exhibiting high sensitivities, good thermal stability and reduced free layer effective magnetic thickness (teff). The free layer teff can be reduced to ∼10 Å while maintaining its physical thickness near 60 Å, without significant magnetoresistive signal loss (MR∼6%). After current biasing, the sensors show a linearized response, demonstrating a coherent switching of the synthetic free layer. Sensor sensitivity (S) increases with decreasing free layer effective magnetic thickness (S≈0.5%/Oe at teff=10 Å) as expected from the decrease of the free layer demagnetizing field. The coupling field (Hf) between the free and pinned layers increases with decreasing teff (Hf∝Ms2/(teffMefff)). The sensors are thermally stable upon anneals up to 300 °C.
Role of strain in magnetotransport properties of Pr0.67Sr0.33MnO3 thin films
Authors: Wang H. S. , Wertz E. , Hu Y. F. , Li Qi , Schlom D. G.
Published: July 2002

We have studied the strain effects on the structural and magnetotransport properties of Pr0.67Sr0.33MnO3 (PSMO) thin films. The PSMO films were epitaxially grown on LaAlO3 (001), SrTiO3 (001), and NdGaO3 (110) substrates that induce biaxial compressive, tensile, and almost no strain in the films, respectively. The film thickness t, varied between 4–400 nm, was used as another controlling parameter of strain for each type of film. There exist two distinct thickness ranges with different thickness dependence of the magnetotransport properties. For t<20 nm, the zero-field resistance peak temperature (Tp) and the high-field magnetoresistance (HFMR) properties are critically dependent on the thickness and the substrate. For t>20 nm, the Tp and the HFMR ratio show weak t dependence. The results show evidence for the effects of the Jahn–Teller type distortion as well as disorders on the resistive transition temperature and the HFMR.
Asymmetric torsion stress giant magnetoimpedance in nearly zero magnetostrictive amorphous wires
Authors: Blanco J. M. , Zhukov A. , Gonzalez J.
Published: July 2002

The effect of the torsion strain on the electrical impedance in (Co0.95Fe0.05)72.5Si12.5B15 amorphous wire has been investigated. The torsion stress impedance ratio, defined as (ΔZ/Z)ξ=[(Z(ξ)−Z(ξmax)]/Z(ξmax), has an asymmetric character with a broad maximum at around 20 rad/m. After Joule heating without torsion (ΔZ/Z)ξ has a tendency to achieve finally a sharp and nearly symmetric shape. Maximum changes of (ΔZ/Z)ξ, of 270% is obtained after optimal conditions of current annealing. Besides, GMI ratio, (ΔZ/Z)H=[(Z(H)−Z(Hmax)]/Z(Hmax) can be significantly improved too reaching 350% in the particular case of current annealing at 550 mA for 0.5 min. The asymmetry of the (ΔZ/Z)ξ in the as-cast state and after torsion annealing could be ascribed to the spontaneous or induced helical anisotropy.
Influence of gas mixture ratio on the luminous efficiency in surface discharge alternating current plasma display panels
Authors: Ahn J. C. , Kim T. Y. , Ko J. J. , Seo Y. , Cho G. S. , Choi E. H.
Published: July 2002

The improvement of luminosity and luminous efficiency is the one of the most important tasks in alternating current plasma display panels. The influence of gas mixture ratio, Ne–Xe and He–Ne(27%)–Xe(3%), on luminosity and luminous efficiency in surface discharge alternating-current plasma display panels has been investigated. It is found that discharge power does not strongly depend on the gas mixtures, while the luminosity for three species mixture gas, He–Ne(27%)–Xe(3%), is shown to be much higher than those for the two species mixture gas of Ne–Xe. The three species mixture gas, He–Ne(27%)–Xe(3%), can be suggested as the one of the candidates for achieving high luminosity and luminous efficiency for color plasma display panels, based on the experimental observations of luminosity, discharge power, response time, and efficiency.
Effect of growth temperature on the electric properties of In0.12Ga0.88As/GaAs p-i-n multiple-quantum-well diodes
Authors: Chen J. F. , Wang P. Y. , Wang J. S. , Wong H. Z.
Published: July 2002

The electric properties of In0.12Ga0.88As/GaAs p-i-n multiple-quantum-well (MQW) diodes, with the MQW layer grown at different temperatures by molecular beam epitaxy, have been investigated. Temperature-dependent current–voltage studies reveal a trap-filled limit current at a low temperature and a generation-recombination current via deep levels at high temperature for a 300 °C-grown sample. Frequency-dependent capacitance and deep-level transient spectroscopy reveal one majority trap at 0.73 eV and two minority traps at 0.71 and 0.43 eV. The 0.73 eV trap is also detected in 550 °C-grown samples, suggesting that it is a common defect in relaxed InGaAs/GaAs MQWs and probably originates from the defect states related to the strain relaxation. The 0.71 eV trap is believed to be the dominating deep level that governs the current conduction due to the activation energy observed in the current–voltage characteristics.
Secondary electron emission characteristics of single-crystal and polycrystalline diamond
Authors: Yater J. E. , Shih A.
Published: July 2002

Secondary electron emission spectroscopy (SEES) is used to examine the transport and emission of low-energy electrons in diamond. In particular, SEES measurements from single-crystal (100) and (111) diamond and polycrystalline chemical vapor deposited (CVD) diamond are compared in order to examine the effect of crystallographic orientation on the emission characteristics. Crystal orientation is found to influence the surface properties of the samples but not the low-energy transport properties. Specifically, very high yields are obtained from negative-electron-affinity (NEA) surfaces of all three samples, indicating that low-energy electrons are transported and emitted very efficiently regardless of crystal orientation. However, the energy distributions measured from adsorbate-covered C(111) surfaces are broader and shifted lower in energy than those measured from corresponding C(100) surfaces. In fact, the energy distributions measured from polycrystalline CVD diamond surfaces appear to be a superposition of the energy distributions measured from the (100) and (111) crystal faces. For all three samples, a broader, lower-energy distribution is measured from cesiated NEA surfaces than from hydrogenated NEA surfaces. This indicates that the electron emission process differs at the two types of surfaces. The emission characteristics observed for the different crystal orientations and adsorbate coverages can be understood by considering the role of surface structure in the emission process.
Bonding and hardness in nonhydrogenated carbon films with moderate sp3 content
Authors: Gago R. , Jiménez I. , Albella J. M. , Climent-Font A. , Cáceres D. , Vergara I. , Banks J. C. , Doyle B. L. , Terminello L. J.
Published: July 2002

Amorphous carbon films with an sp3 content up to 25% and a negligible amount of hydrogen have been grown by evaporation of graphite with concurrent Ar+ ion bombardment. The sp3 content is maximized for Ar+ energies between 200 and 300 eV following a subplantation mechanism. Higher ion energies deteriorate the film due to sputtering and heating processes. The hardness of the films increases in the optimal assisting range from 8 to 18 GPa, and is explained by crosslinking of graphitic planes through sp3 connecting sites.
Diffusion barrier properties of single- and multilayered quasi-amorphous tantalum nitride thin films against copper penetration
Authors: Chen G. S. , Chen S. T.
Published: July 2002

Tantalum-related thin films containing different amounts of nitrogen are sputter deposited at different argon-to-nitrogen flow rate ratios on (100) silicon substrates. Using x-ray diffractometry, transmission electron microscopy, composition and resistivity analyses, and bending-beam stress measurement technique, this work examines the impact of varying the nitrogen flow rate, particularly on the crystal structure, composition, resistivity, and residual intrinsic stress of the deposited Ta2N thin films. With an adequate amount of controlled, reactive nitrogen in the sputtering gas, thin films of the tantalum nitride of nominal formula Ta2N are predominantly amorphous and can exist over a range of nitrogen concentrations slightly deviated from stoichiometry. The single-layered quasi-amorphous Ta2N (a-Ta2N) thin films yield intrinsic compressive stresses in the range 3–5 GPa. In addition, the use of the 40-nm-thick a-Ta2N thin films with different nitrogen atomic concentrations (33% and 36%) and layering designs as diffusion barriers between silicon and copper are also evaluated. When subjected to high-temperature annealing, the single-layered a-Ta2N barrier layers degrade primarily by an amorphous-to-crystalline transition of the barrier layers. Crystallization of the single-layered stoichiometric a-Ta2N (Ta67N33) diffusion barriers occurs at temperatures as low as 450 °C. Doing so allows copper to preferentially penetrate through the grain boundaries or thermal-induced microcracks of the crystallized barriers and react with silicon, sequentially forming {111}-facetted pyramidal Cu3Si precipitates and TaSi2 Overdoping nitrogen into the amorphous matrix can dramatically increase the crystallization temperature to 600 °C. This temperature increase slows down the inward diffusion of copper and delays the formation of both silicides. The nitrogen overdoped Ta2N (Ta64N36) diffusion barriers can thus be significantly enhanced so as to yield a failure temperature 100 °C greater than that of the Ta67N33 diffusion barriers. Moreover, multilayered films, formed by alternately stacking the Ta67N33 and Ta64N36 layers with an optimized bilayer thickness (λ) of 10 nm, can dramatically reduce the intrinsic compressive stress to only 0.7 GPa and undergo high-temperature annealing without crystallization. Therefore, the Ta67N33/Ta64N36 multilayered films exhibit a much better barrier performance than the highly crystallization-resistant Ta64N36 single-layered films.
Origin of MeV ion irradiation-induced stress changes in SiO2
Authors: Brongersma M. L. , Snoeks E. , van Dillen T. , Polman A.
Published: July 2002

The 4 MeV Xe ion irradiation of a thin thermally grown SiO2 film on a Si substrate leads to four different effects in which each manifests itself by a characteristic change in the mechanical stress state of the film: densification, ascribed to a beam-induced structural change in the silica network; stress relaxation by radiation-enhanced plastic flow; anisotropic expansion and stress generation; and transient stress relaxation ascribed to the annealing of point defects. Using sensitive wafer-curvature measurements, in situ measurements of the in-plane mechanical stress were made during and after ion irradiation at various temperatures in the range from 95 to 575 K, in order to study the magnitude of these effects, the mechanism behind them, as well as their interplay. It is found that the structural transformation leads to a state with an equilibrium density that is 1.7%–3.2% higher than the initial state, depending on the irradiation temperature. Due to the constraint imposed by the substrate, this transformation causes a tensile in-plane stress in the oxide film. This stress is relaxed by plastic flow, leading to densification of the film. The anisotropic strain-generation rate decreases linearly with temperature from (2.5±0.4)×10−17 cm2/ion at 95 K to (−0.9±0.7)×10−17 cm2/ion at 575 K. The spectrum of irradiation-induced point defects, measured from the stress change after the ion beam was switched off, peaks below 0.23 eV and extends up to 0.80 eV. All four irradiation-induced effects can be described using a thermal spike model.
Infrared diode laser study of the hydrogen bifluoride anion: FHF− and FDF−
Authors: Kawaguchi Kentarou , Hirota Eizi
Published: July 2002

The ν3 vibration–rotation transition of the hydrogen bifluoride anion in the X̃ 1Σ+g state has been detected for the first time by infrared diode laser spectroscopy using the magnetic field modulation technique. The anion was generated by a hollow cathode discharge in a mixture of H2 and a fluorine-contained molecule such as CF4, C2F4, and CHF3. The carbon compound was indispensable to produce the anion. Identification of the species was based on the spectral pattern showing intensity alternation, the magnitude of the rotational constant (B″=0.334 181 cm−1), and the ion drift velocity sign determined by the velocity modulation method. The observed ν3 frequencies of FHF−(1848.699 cm−1) and FDF−(1397.236 cm−1) are much different from the reported values in the solid phase. By contrast, the ν1 and ν2 frequencies estimated from the centrifugal distortion constant and the perturbation of Coriolis interaction between the ν1+ν2 and ν3 states are in good agreement with solid state measurements.
Multidimensional Franck–Condon integrals and Duschinsky mixing effects
Authors: Kupka H. , Cribb P. H.
Published: July 2002

A general method for calculating multidimensional Franck–Condon integrals for polyatomic molecules is given. These integrals are derived by means of a multivariable generating function which incorporates both the transformation of the normal mode coordinates between initial and final electronic states and their frequency changes. The normal mode transformation or mode mixing (Duschinsky effect) scrambles the occupations of the normal modes, leading to unusual distributions, which at certain values of the angles of rotation are confined to some of the modes only or even to a single mode. Mathematically, this selectivity can be described in terms of normal mode displacements generated by the rotation matrix from the potential minima of the ground and excited states. These normal mode displacements, as well as the set of mixed frequency change parameters due to mode mixing, have no counterparts in the parallel mode approximation and are the reason that multidimensional Franck–Condon integrals are of formidable analytical complexity. This complexity is analyzed with particular attention to the symmetry of the integrals with respect to the interchange of quantum occupation numbers. Finally calculation of scattered intensities in the resonance Raman process is made, showing a strong dependence of the corresponding cross section upon the rotation angle.
Adiabatic compressibility and structure of aqueous solutions of ethyl alcohol
Authors: Onori G.
Published: July 2002

The adiabatic compressibility of ethanol–water mixtures has been measured as a function of temperature and alcohol mole fraction; the measurements were carried out at 5 °C intervals over the 10–40 °C range of temperature and over the entire range of cosolvent concentration. The experimental results are compared with other physicochemical measurements available for these mixtures and discussed in terms of possible mechanisms of molecular aggregation in the various regions of ethanol concentration.
Accurate ionization potentials of phenol and phenol-(H2O) from the electric field dependence of the pump–probe photoionization threshold
Authors: Lipert Robert J. , Colson Steven D.
Published: July 2002

The adiabatic ionization potentials of phenol and phenol-(H2O) have been obtained from pump–probe photoionization threshold measurements in a supersonic molecular beam time-of-flight mass spectrometer. The observed photoionization thresholds were found to vary linearly with the square root of the ion-extraction electric field over fields ranging from 27 to 500 V/cm. Extrapolations to zero field yields IPs of 68 623±10 cm−1 for phenol and 64 035±10 cm−1 for phenol-(H2O). Thus, hydrogen bonding to one water molecule lowers the ionization potential of phenol by 4590±15 cm−1. The advantages of this technique of measuring ionization potentials are discussed.
Origin of dipole moment enhancement in the formation of SiF4–NH3 dimer
Authors: Keith T. A. , Bader R. F. W.
Published: July 2002

The dimerization of SiF4 and NH3 is accompanied by a substantial 1.63 a.u. increase in dipole moment. Theoretical analysis of the charge distribution of the SiF4–NH3 system using the theory of atoms in molecules shows that the origin of this increase is primarily the large geometrical change which occurs within the SiF4 molecule when the dimer is formed, rather than the changes in atomic polarization or interatomic or intermolecular electron transfer. The relative displacement of the highly charged fluorine atoms of SiF4 away from NH3 upon dimer formation accounts for nearly 90% of the dipole moment enhancement. This result is in agreement with predictions made recently based on experimental results.
Capillary condensation in the lattice gas model: A Monte Carlo study
Authors: Binder K. , Landau D. P.
Published: July 2002

A three-dimensional lattice gas model with nearest-neighbor attractive interaction confined to a slitlike adsorbing capillary of thickness D is studied by computer simulation, varying the chemical potential μ, temperature, T, as well as the strength of the short-range interaction between the walls of the capillary and the gas. We show that the chemical potential μc(D) at the condensation transition in the capillary is shifted relative to its bulk value μc(∞) according to the Kelvin equation, μc(D)−μc(∞)∝D−1, for large enough D. Deviations are found for small D, however (of the order of 10 lattice spacings), particularly under conditions where for μ=μc(∞) the surfaces of the capillary for D→∞ are wet. For D=16 lattice spacings we also locate a capillary condensation critical point, and study the distortion of the gas–fluid coexistence curve due to the attractive walls. Profiles of density and local energy across the capillary are studied, and data for the temperature dependence of the local density at the wall both in the gas and in the liquid phase are presented. It is shown that these quantities vary very smoothly near the wetting temperature TW of the bulk (D→∞) system.
Ammonia alane
Authors: Marsh Craig M. B. , Hamilton Tracy P. , Xie Yaoming , Schaefer Henry F.
Published: July 2002

A b initio studies employing basis sets approaching triple-ζ plus double-polarization plus f function quality are reported for AlH3NH3, AlH3, and NH3. The self-consistent-field method was employed for all basis sets; the single and double excitation configuration interaction and the comparable coupled-cluster methods were utilized with a double-ζ plus polarization functions basis set. The dissociation energy of AlH3NH3 is predicted to be 26 kcal/mol after correcting for zero-point vibrational energy, in qualitative agreement with the rough estimate of 30 kcal/mol by Haaland for Al–N bond strengths. In keeping with the dative nature of the Al–N bond, the AlH3 and NH3 moieties do not undergo large changes upon the formation of AlH3NH3. The resemblance of AlH3NH3 to the related compound BH3NH3 is quite strong. The equilibrium geometry, harmonic vibrational frequencies, infrared intensities, atomic charges, and dipole moment of ammonia alane are reported.
Confinement effects on dipolar relaxation by translational dynamics of liquids in porous silica glasses
Authors: Korb J.-P. , Xu Shu , Jonas J.
Published: July 2002

A theory of dipolar relaxation by translational diffusion of a nonwetting liquid confined in model porous media is presented. We obtain expressions of the rates of spin-lattice relaxation 1/T1, spin–spin relaxation 1/T2, and spin-lattice relaxation in the rotating frame 1/T1ρ, which depend on the average pore size d. The frequency variations of these rates are intermediate between the two-dimensional and three-dimensional results. At small frequency they vary logarithmically for small d and tend progressively to a constant with increasing d. For small pore sizes we obtain quadratic confinement dependences of these rates (∝1/d2), at variance with the linear (∝1/d) relation coming from the biphasic fast exchange model usually applied for a wetting liquid in porous media. We apply such a theory to the 1H NMR relaxation of methylcyclohexane liquid in sol-gel porous silica glasses with a narrow pore-size distribution. The experiments confirm the theoretical predictions for very weak interacting solvent in porous silica glasses of pore sizes varying in the range of 18.4–87.2 Å and in the bulk. At the limit of small pores, the logarithmic frequency dependencies of 1/T1ρ and 1/T1 observed over several decades of frequency are interpreted with a model of unbounded two-dimensional diffusion in a layered geometry. The leveling off of the 1/T1ρ low-frequency dependence is interpreted in terms of the bounded two-dimensional diffusion due to the finite length L of the pores. An estimate of a finite size of L=100 Å is in excellent agreement with the experimental results of the transmission electron microscopy study of platinium-carbon replicated xerogels.
All the bound vibrational states of H3+: A reappraisal
Authors: Henderson James R. , Tennyson Jonathan , Sutcliffe Brian T.
Published: July 2002

The 3D discrete variable representation (DVR) calculations of Henderson and Tennyson [Chem. Phys. Lett. 173, 133 (1990)] are reanalyzed to find the source of the nonvariational behavior highlighted by Carter and Meyer [J. Chem. Phys. 96, 2424 (1992)]. The discrepancy is found to be caused not by the use of incorrect boundary conditions, but by a failure of the quadrature approximation commonly used in DVR calculations. Corrected DVR calculations show variational but slow convergence. Calculations using the same intermediate vectors as the nonvariational calculations and a corrected final Hamiltonian show greatly enhanced convergence. The vibrational band origins computed with this method are converged to within 2 cm−1 up to 35 000 cm−1. A complete list of these is presented and comparisons made with previous predictions.
Hysteresis models and their application to the spin-state transition in dithiocyanatobis(2,2′-bi-2-thiazoline)iron(II)
Authors: König E. , Kanellakopulos B. , Powietzka B. , Nelson J.
Published: July 2002

The hysteresis at the spin-state transition (T̄c≂180.6 K) in [Fe(bt)2(NCS)2] has been studied by magnetic susceptibility measurements (bt=2,2′-bi-2-thiazoline). Two series of minor hysteresis loops between fixed temperatures TA=177.0, TB=183.0 K and TA=177.7, TB=183.7 K, respectively, have been obtained. Their areas are practically equal over a large range of μ2eff values. The results are consistent with the requirements of the classical Preisach model and some of its generalizations. Right and left boundary minor loops have been obtained for the widths ΔT=6.0 and ΔT=7.2 K. The areas order on two curves with almost equal maximum F/Ftotal values. A phenomenological approach is proposed to explain the results. It is based on the fact that the properties of minor loops are associated with the abrupt vs gradual character and the more or less symmetrical nature of the transition. It is pointed out that according to the Everett model, the transition should proceed by conversion of independent domains of high-spin (HS) and low-spin (LS) molecules.
A computer simulation study of the temperature dependence of the hydrophobic hydration
Authors: Guillot B. , Guissani Y.
Published: July 2002

The test particle method is used to evaluate by molecular dynamics calculations the solubility of rare gases and of methane in water between the freezing point and the critical point. A quantitative agreement is obtained between solubility data and simulation results when the simulated water is modeled by the extended simple point charge model (SPCE). From a thermodynamical point of view, it is shown that the hierarchy of rare gases solubilities in water is governed by the solute–water interaction energy while an entropic term of cavity formation is found to be responsible for the peculiar temperature dependence of the solubility along the coexistence curve, and more precisely, of the solubility minimum exhibited by all the investigated solutes. Near the water critical point, the asymptotic behaviors of the Henry’s constant and of the vapor–liquid partition coefficient, respectively, as deduced from the simulation data follow with a good accuracy the critical laws recently proposed in the literature for these quantities. Moreover, the calculated partial molar volume of the solute shows a steep increase above 473 K and becomes proportional to the isothermal compressibility of the pure solvent in the vicinity of the critical point as it is observed experimentally. From a microscopic point of view, the evaluation of the solute–solvent pair distribution functions permits to establish a relationship between the increase of the solubility with the decrease of the temperature in cold water on the one hand, and the formation of cages of the clathrate-type around the solute on the other hand. Nevertheless, as soon as the boiling point of water is reached the computer simulation shows that the water molecules of the first hydration shell are no longer oriented tangentially to the solute and tend to reorientate towards the bulk. At higher temperatures a deficit of water molecules progressively appears around the solute, a deficit which is directly associated with an increase of the partial molar volume. Although this phenomenon could be related to what is observed in supercritical mixtures it is emphasized that no long range critical fluctuation is present in the simulated sample.
Stabilization energies of Cl−, Br−, and I− ions in water clusters
Authors: Perera Lalith , Berkowitz Max L.
Published: July 2002

Molecular dynamics computer simulations were performed on clusters of Cl−(H2O)n (n=2,...,15). From the simulations we calculated the stabilization energies of the anion in the cluster. These energies were compared with the values of stabilization energies obtained from the photodetachment spectra of X−(H2O)n clusters (X=Cl−, Br−, or I−). The comparison confirms the hypothesis that the anion is attached to the water cluster.
Determination of approximate lumping schemes by a singular perturbation method
Authors: Li Genyuan , Tomlin Alison S. , Rabitz Herschel , Tóth János
Published: July 2002

A singular perturbation method is employed for the determination of an approximate nonlinear lumped model for a chemical kinetic system described by a set of first order ordinary differential equations with a group of small positive parameters corresponding to different time scales. New variables, called purely fast variables, are introduced and determined. Substituting their explicit expressions into the original kinetic equation system yields a lumped differential equation system containing the independent variable t. The lumped system can reach any desired accuracy for any initial composition. A further approximation to this lumped system, obtained by omitting transient exponential functions of t, is shown to define the dynamics of the system on a slow invariant manifold. Two simple examples are used to illustrate this approach.
The structure of small nickel clusters. I. Ni3–Ni15
Authors: Parks E. K. , Zhu L. , Ho J. , Riley S. J.
Published: July 2002

The geometrical structure of small nickel clusters is probed via molecular adsorption of nitrogen on their surfaces. Nitrogen uptake patterns can be rationalized with the proposed structures if it is assumed that N2 binds to every exposed nickel atom, that the binding energies decrease with increasing metal—metal coordination, and that atoms that are four or less coordinate can bind two nitrogen molecules. In some cases nitrogen adsorption causes a change in cluster structure, usually to one that can accommodate more nitrogen molecules. Cluster structures are proposed for all clusters (bare and nitrogenated) in the 3–15-atom size range except Ni4 and Ni11. The nitrogen uptake for Ni4 is consistent with virtually any structure, and the data for Ni11 could not be interpreted in terms of a specific structure. In general, nickel cluster structures are different from those found for rare gas clusters as well as those derived from bulk packing. A comparison of the experimental results with existing theoretical calculations is presented.
Numerical Hartree–Fock energies of low-lying excited states of neutral atoms with Z≤18
Authors: Tatewaki Hiroshi , Koga Toshikatsu , Sakai Yoshiko , Thakkar Ajit J.
Published: July 2002

Numerical Hartree–Fock calculations have been performed for low-lying excited states of the neutral atoms from He to Ar. Total energies, orbital energies, and the mean values of r of the outermost orbitals of each symmetry are tabulated as an aid to calibration of algebraic basis sets.
Cr5+ luminescence from chromium-doped SiO2 glass
Authors: Herren Markus , Nishiuchi Hiroki , Morita Makoto
Published: July 2002

The room-temperature photoluminescence spectrum of SiO2 glass: 0.1% Cr prepared by sol–gel method is reported. It consists of a broad band centered at 14 900 cm−1 with a width of 4200 cm−1 and a lifetime in the μs order and is attributed to the d–d transition of Cr5+ ions (3d1 electron configuration). Absorption and emission spectra give no evidence for the presence of Cr3+ or Cr4+ ions.
Relativistic Hartree–Fock by means of stationary direct perturbation theory. I. General theory
Authors: Kutzelnigg Werner , Ottschofski Edgar , Franke Robert
Published: July 2002

Perturbation theory of relativistic effects (if done properly, i.e. avoiding spurious singularities as they arise, e.g. if one starts with the Foldy–Wouthuysen transformation) has many advantages and only few drawbacks compared to a nonperturbative relativistic theory. In this paper relativistic Hartree–Fock theory for closed-shell states is combined with the stationary direct perturbation theory of relativistic effects in powers of the inverse velocity of light c−1 (which in hartree units is equal to the fine structure constant α). After a formulation for an arbitrary electron interaction the explicit expressions for the Dirac–Coulomb Hamiltonian are presented. The first step is always a nonrelativistic Hartree–Fock calculation, followed by the construction of the perturbation adapted zeroth order bispinors in terms of their large and small components φ(0)i and χ(0)i, respectively. The leading relativistic correction to the energy c−2E(2) is obtained as an expectation value involving φ(0)i and χ(0)i. The higher-order corrections c−4E(4) and c−6E(6) require that first the lowest order relativistic corrections φ(2)i and χ(2)i to the Hartree–Fock MOs are constructed by means of a coupled-Hartree–Fock-type approach. A hierarchy of approximations to the full treatment up to O(c−6) is proposed, in three of which the lowest order relativistic correction c−2E(2) to the energy is calculated exactly, while two-electron contributions to c−4E(4) and c−6E(6) are neglected in part or entirely. Finally the use of the Breit interaction and related interactions is discussed.