bluuebunny/crossref_metadata_2025 huggingface dataset:
Origin of a magnetic easy axis in pipeline steel
Authors: Clapham L. , Heald C. , Krause T. , Atherton D. L. , Clark P.
Published: July 2002

Oil and gas pipelines are generally magnetically anisotropic, with a magnetic easy axis in the pipe axial direction. This is of interest because magnetic flux leakage tools are commonly used for the detection and sizing of defects. In the present study we investigate the origin of this magnetic easy axis, using an angular magnetic Barkhausen noise technique to characterize the magnetic anisotropy. The texture, microstructure, and residual stress are examined as possible causes of the easy axis, using x-ray pole figure analysis and microstructural examination along with high and low temperature annealing treatments. Our results indicate that plastic deformation and residual stress are responsible for the magnetic easy axis, since an elimination of the residual stresses through low temperature “stress relief” heat treatment produces a magnetically isotropic structure without altering the texture or microstructure. X-ray pole figure analysis supports the conclusion that magnetic anisotropy is not related to texture in these materials. We conclude that the axial magnetic easy axis is due to a compressive residual hoop stress resulting from the cold bending and cold expansion of the pipe during processing.
Electromagnetic direct and inverse problems for a surface-breaking crack in a conductor at a high frequency
Authors: Popov Mikhail , He Sailing
Published: July 2002

The electromagnetic direct and inverse problems for a surface-breaking crack in a conductor are considered at a high frequency. The direct problem, which is transferred to a two-dimensional potential problem in the unfolded configuration, is solved by a boundary integral equation method. The boundary integral equation (a well-posed Fredholm integral equation of the second kind) for the normal derivative of the potential on the interior boundary of the crack is derived. The potential distribution at the open mouth of the crack is used to determine the shape of the crack. The inverse problem is solved by a genetic algorithm combined with a conjugate gradient method.
Solid-solubility limits of Be in molecular beam epitaxy grown AlxGa1−xAs layers and short-period superlattices
Authors: Gaymann A. , Maier M. , Köhler K.
Published: July 2002

The redistribution of Be in highly doped AlxGa1−xAs layers and AlxGa1−xAs/AlAs(GaAs) short period superlattices (SPSL) during molecular beam epitaxy was investigated by secondary ion mass spectrometry (SIMS) depth profiling. Be outdiffuses significantly from these layers and, additionally, segregates in growth direction. Conversely, Be is depleted and incorporated only up to a solid-solubility limit depending on the Al content. SPSLs with shorter period are disordered and show the solid-solubility limits of homogeneous AlxGa1−xAs layers with the same average composition. Be solid-solubility limits in AlxGa1−xAs layers covering the whole range from GaAs to AlAs are derived from the SIMS depth profiles.
Ab initio simulation of defects in energetic materials: Hydrostatic compression of cyclotrimethylene trinitramine
Authors: Kuklja Maija M. , Kunz A. Barry
Published: July 2002

An isotropic compression of both the perfect solid cyclotrimethylene trinitramine (C3H6N6O6), also known as RDX, and of the solid containing vacancies is simulated using the ab initio Hartree–Fock method combined with two different crystal models: a periodic (band structure) and a molecular cluster. We show that an external pressure causes a significant decrease of the optical gap for both the perfect material and the crystal with vacancies. The solid RDX is found to be highly compressible; a pure crystal could be compressed to 57% of its equilibrium volume, whereas crystals containing vacancies are even more compressible. The critical pressure necessary for the insulator–metal transition is also predicted. It is shown that the voids present in a real RDX solid lower the metallization pressure by about 30%. Theoretical results are in close agreement with the experimental data on solid and porous RDX. The influence of defects present in the crystal and the relation to control of the sensitivity to detonation are discussed in detail.
S and Si ion implantation in GaSb grown on GaAs
Authors: Rao Mulpuri V. , Berry Alok K. , Do Thang Q. , Ridgway M. C. , Chi P. H. , Waterman J.
Published: July 2002

Single and multiple energy S and Si ion implantations were performed at room temperature (RT) and 200 °C into GaSb epitaxial layers grown on semi-insulating GaAs substrates. The implanted material was annealed with a Si3N4 cap at 400–600 °C for 5 min. Secondary ion mass spectrometry measurements indicated the thermal stability of the Si and S implants even for 600 °C annealing. Appreciable donor electrical activations of Si and S were obtained only for 200 °C implantation for annealing temperatures ⩾500 °C. For S implantation the calculated substitutional activation is ∼50%. Rutherford backscattering/channeling measurements indicated a lattice quality that is close to the virgin level for samples annealed at 600 °C. N-type conduction was not observed in GaSb bulk material implanted at either RT or 200 °C.
Spin-polarized tunneling in a half-metallic ferromagnet
Authors: Tanaka Clifford T. , Nowak Janusz , Moodera Jagadeesh S.
Published: July 2002

Direct measurement of the conduction electron spin polarization (P) in epitaxial NiMnSb was performed to test the prediction of half metallicity in this material. Spin-polarized tunneling in NiMnSb/Al2O3/Al junctions showed P of 28%, contrary to the predicted value of 100%. Magnetoresistance measurements in NiMnSb/Al2O3/Ni80Fe20 junctions concurred with this result. The discrepancy between theory and experiment is discussed. Also, the latter junctions show four nonvolatile remanent states due to the NiMnSb magnetocrystalline anisotropy, which has potential as four-level logic elements.
Texture of Al thin films deposited by magnetron sputtering onto epitaxial W(001)
Authors: Madsen Lynnette D. , Svedberg Erik B. , Bergstrom Daniel B. , Petrov Ivan , Greene Joseph E.
Published: July 2002

Highly textured epitaxial metallizations will be required for the next generation of devices with the main driving force being a reduction in electromigration. Herein a model system of 190 nm of Al on a 140 nm layer of W grown on MgO 〈00l〉 substrates was studied. The W layer was 〈00l〉 oriented and rotated 45° with respect to the MgO substrate to minimize the misfit; the remaining strain was accommodated by dislocations, evident in transmission electron microscopy images. From high-resolution x-ray diffraction (XRD) measurements, the out-of-plane lattice parameter was determined to be 3.175 Å, and the in-plane parameter was 3.153 Å, i.e., the W film sustained a strain resulting in a tetragonal distortion of the lattice. XRD pole figures showed that the Al had four fold symmetry and two dominant orientations, 〈016〉 and 〈3 9 11〉, which were twinned with multiple placements on the epitaxial W layer. The driving force for the tilted 〈001〉 and 〈011〉 orientations of Al on W is due to strain minimization through lattice matching. These results show that 〈00l〉 Al deposited at ambient conditions onto W is difficult to achieve and implies that electromigration difficulties are inherent.
Characterization of ejected particles during laser cleaning
Authors: Lu Y. F. , Zheng Y. W. , Song W. D.
Published: July 2002

Laser cleaning is a prospective cleaning method that can be widely used in microelectronics fabrication, archive restoration, and optical apparatus cleaning. Removal of particles from a solid substrate is an important aspect of laser cleaning. Although many studies have been carried out on this subject, few of them are objected to the characterization of the ejected particles in laser cleaning. In this article, a method was developed to “capture” the particles ejected from the substrate after laser irradiation. Detection of both angular distribution and ejection energies was achieved with this method. It was found that the angular distribution of the ejected particles fitted to a Gaussian curve when the laser irradiated normally to the substrate. The distribution curve for the particles ejected from a rough surface has a wider full width at half maximum than that from a smooth substrate. It was also found that the particle ejection energy increased obviously with laser fluence, therefore the laser cleaning efficiency was promoted sharply as laser fluence increased.
Slip sliding away: A novel head–disk interface and its tribology (invited)
Authors: Gui Jing , Tang Huan , Wang Li-Ping , Rauch Gary C. , Boutaghou Zine , Hanchi Jorge , Pitchford Thomas , Segar Peter
Published: July 2002

The continuous reduction of head–disk spacing has made the use of supersmooth media a necessity in gaining ultrahigh magnetic recording areal density. To overcome the stiction barrier associated with supersmooth disks without compromising the head flyability requirement, texture features can be transferred from the disk surface to the slider surface, creating a new type of head–disk interface, the padded slider interface. The tribology of a padded slider interface is in many ways different from that of the traditional head–disk interface with texture on the disk only. In this article, various unique tribological aspects of the padded slider interface are discussed in detail. Both theoretical modeling results and experimental data are presented to elucidate the stiction, friction, and wear behaviors of this novel head–disk interface. It is shown that the padded slider technology offers a viable alternative to the ramp load technology as a head–disk interface solution for the ultrahigh areal density.
Photoacoustic study of the effect of aluminum content on the thermal and thermomechanical properties of AlyGa1−yAs on GaAs in the range (0⩽y≲1)
Authors: Pichardo J. L. , Alvarado-Gil J. J. , Cruz A. , Mendoza J. G. , Torres G.
Published: July 2002

The two layer system AlyGa1−yAs on GaAs is studied in the range (0⩽y≲1). The open photoacoustic cell technique is used to obtain thermal diffusivity, thermal conductivity, heat capacity, and the thermal expansion coefficient. It is shown that the thermal properties of the individual layers can be determined using the low modulation frequency regime models without actually separating the two layer system. The possibilities and advantages of the approach for the thermal characterization of layered semiconductor systems are discussed.
Mechanical and electrical responses of piezoelectric solids to conical indentation
Authors: Sridhar S. , Giannakopoulos A. E. , Suresh S.
Published: July 2002

Mechanical and electrical responses of piezoelectric materials subjected to shallow cone indentation were evaluated. Theoretical analysis based on normal indentation of a transversely isotropic, piezoelectric solid by a conducting steel cone with zero potential bias was compared to experimental results on lead zirconate (PZT-4) and barium titanate ceramics. The effect of the cone angle on the linear elastic assumptions and the depoling of the material below the indenter tip was investigated. The load–displacement and the time–current relations were proven to be specific to the piezoelectric properties of the material. Therefore, instrumented cone indentation can be a viable test in assessing the mechanical and electrical responses of piezoelectric materials in small volumes.
Analysis of the temperature dependent thermal conductivity of silicon carbide for high temperature applications
Authors: Joshi R. P. , Neudeck P. G. , Fazi C.
Published: July 2002

The temperature dependent thermal conductivity of silicon carbide has been calculated taking into account the various phonon scattering mechanisms. The results compared very well with available experimental data. The inclusion of four-phonon processes is shown to be necessary for obtaining a good match. Several important phonon scattering parameters have been extracted in this study. Dislocations are shown to have a strong effect at 300 K, but not as much at the higher temperatures.
Nucleation rate of solidification probed by x-ray absorption temperature scans in undercooled liquid metals
Authors: De Panfilis Simone , Filipponi Adriano
Published: July 2002

A new method to measure the nucleation rate of solidification I(T) in undercooled liquids is described. The method is based on the phase sensitivity of the x-ray absorption coefficient above core–electron absorption edges. By tuning the photon energy to a high-contrast value, the liquid and solid sample fractions can be accurately measured as a function of temperature and time, during the crystallization phase of suitable sample temperature scans. The method can be applied to emulsions of liquid droplets or powder mixtures at high temperatures. The samples are composed of a large number (104–109) of independent micrometric droplets which guarantee excellent statistics. A theoretical treatment of the nucleation statistics, suitable for the interpretation of the experimental data, is developed. The method is applied to the crystallization of undercooled liquid palladium droplets dispersed in sintered Al2O3 powder. The Pd nucleation rate I(T) has been determined in the interval 1560 K<T<1610 K covering a range of 6 orders of magnitude. The data indicate the occurrence of a time independent volume nucleation phenomenon. The results of this fundamental measurement are discussed in light of much interest from both thermodynamical and metallurgical points of view. A fit of our data with a model from the classical theory of nucleation gives a value of the exponential factor compatible with known thermodynamic quantities such as the solid–liquid interfacial energy σ and a preexponential factor Kv on the order of 3.4×1031 m−3 s−1, suggesting the occurrence of a heterogeneous nucleation process.
Local asymptotic gradient corrections to the energy functional of an electron gas
Authors: Pearson Erik W. , Gordon Roy G.
Published: July 2002

Gradient corrections to the energy functional for an electron gas are reconsidered. By interpreting these gradient corrections as terms in an asymptotic series, locally at each point in space, an asymptotic summation procedure is suggested. The new local asymptotic summation improves upon the conventional use of the gradient corrections in several respects: (1) The divergence of higher-order terms is eliminated. (2) The errors in kinetic energy calculated for atoms are reduced by a factor of 3 to 10. (3) The interaction energies of some diatomic systems are qualitatively improved.
Reactions of iron clusters with hydrogen. II. Composition of the fully hydrogenated products
Authors: Parks E. K. , Liu K. , Richtsmeier S. C. , Pobo L. G. , Riley S. J.
Published: July 2002

Reactions of iron clusters with an excess of hydrogen are found to yield fully hydrogenated products FenHm whose compositions remain fixed over a wide range of hydrogen pressures. For n=6 to 131, the observed m values are always even, have narrow ranges, and for many clusters are unique. Up to n=30, nearly stoichiometric 1:1 ratios of m to n are found. Above 30, cluster hydride compositions are consistent with a monolayer of chemisorbed hydrogen on the cluster surfaces. At sufficiently high hydrogen pressures additional hydrogens bind to the clusters, most likely as a second, physisorbed layer. The experimental results are discussed in terms of cluster structure and the relation to bulk iron behavior.
Experimental and a
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Authors: Lehmann Kevin K. , Ross Stephen C. , Lohr Lawrence L.
Published: July 2002

The emission properties of HCP excited to the A, B, and d electronic states have been studied. Lifetimes and quenching rates have been measured. By spectrally resolving the emission spectrum, the energy of 94 vibrational levels of the ground electronic state have been measured to an accuracy of ≈5 cm−1. These energy levels were fit to experimental accuracy by a rigid bender Hamiltonian thereby determining the bending potential over a range of bending angle from 0 to 100° (0–17 500 cm−1). An ab initio bending potential has been computed for HCP and found to be in excellent agreement with the experimentally fitted one over the range that the experimental data span. This potential predicts that HPC has an energy maximum with respect to the bending coordinate. The bending potential decreases monotonically by about 30 000 cm−1 in going from HPC to HCP.
Ultrahigh resolution spectroscopy of C2−: The A 2Πu state characterized by deperturbation methods
Authors: Mead Roy D. , Hefter U. , Schulz P. A. , Lineberger W. C.
Published: July 2002

Spectra of the C2− B 2Σu+–X 2Σg+ transition have been obtained using a merged laser-ion beam spectrometer with sub-Doppler resolution. Strong perturbations of the B state by the previously unobserved A 2Πu state are analyzed to obtain the characteristics of the new state. The A state spectroscopic constants in wave numbers (with estimated standard errors) are found to be Te=4060±180, ωe=1656±20, ωexe=10.80±0.50, Be=1.630±0.005, and A=−24±1. All nine perturbations observed in this work and the five observed in the work of Herzberg and Lagerqvist are explained by the interaction of the A 2Πu state with the B state. These results, as well as the perturbation matrix elements, are compared with theoretical estimates and results for isoelectronic 13-electron species.
Adsorption geometry of hydrogen on Fe(110)
Authors: Moritz W. , Imbihl R. , Behm R. J. , Ertl G. , Matsushima T.
Published: July 2002

From an analysis of the low-energy electron diffraction (LEED) intensities we have determined the adsorption geometry of the two ordered H adlayers formed at T<270 K on Fe (110): a (2×1) and a (3×1) structure, with ideal coverages of θ= 1/2 and θ= (2)/(3) . Calculations were performed for different adsorption sites and structural models, taking the Fe–H bond length and the first Fe–Fe interlayer spacing as variable parameters. An R factor analysis was used for quantitative comparison with the experimental data. In both structures the H atoms are adsorbed on highly coordinated (i.e., quasithreefold) sites: The R factors of only the superlattice beams (RZanazzi–Jona=0.26, RPendry=0.55 in the (2×1) and RZJ=0.4, RP=0.58 in the (3×1) structure) are significantly lower than those from models with a long bridge adsorption site (RZJ=0.37, RP=0.66 and RZJ=0.6, RP=0.74). The on top site and the short bridge site can clearly be ruled out. For both structures the minima occur at the same Fe–H interlayer spacing of 0.9±0.1 Å, equivalent to an Fe–H distance of 1.75±0.05 Å or rH=0.47±0.05 Å. From the R factor minimum of all beams

(RZJ=0.23, RP=0.46) the first Fe–Fe interlayer spacing is found to be equal to its bulk value, like on the clean surface. In the (2×1) structure the only possible arrangement of the Had atoms consists of dense packed rows in [001] direction which are separated by a row of unoccupied sites, respectively, due to a delocalization of the H atoms over two neighboring threefold sites, short-range fluctuations can be envisaged. Their influence upon I/V curves and relative intensities of different superlattice beams was analyzed. As a result this effect could be excluded, large domains are required, in which only one type of threefold sites is occupied. For the (3×1) structure a model is favored in which the lateral distribution of the adatoms differs from a previous suggestion. It is shown that this model is more plausible in view of the H–H interactions. The higher density of threefold sites also has implications for the discussion of the 2D phase diagram of H/Fe (110), especially on the requirement of trio interactions.
Irreducible tensors and selection rules for three-frequency absorption
Authors: Andrews David L. , Wilkes Philip J.
Published: July 2002

In this paper rate equations and selection rules are derived for the most general case of three-photon absorption, where the photons all have different frequencies and directions of propagation. The rate equations are re-expressed in terms of irreducible tensor products, and the relationship between these and the more familiar isotropic molecular parameters is given in full for the first time. The selection rules are examined in detail and a classification scheme for the allowed transitions is given together with a method of experimentally distinguishing between the classes. It is shown that once assignment of a transition to one of the 11 possible classes has been carried out, the symmetry of the excited state can in many cases be unambiguously identified. The connection with the classification schemes previously developed for single and double-beam three-photon absorption is also discussed, and finally, a special experimental arrangement is proposed which should give rise to a spectrum in which the only transitions to occur are those forbidden by both single and double-beam excitation.
Coupled cluster calculations with numerical orbitals for excited states of polar anions
Authors: Adamowicz Ludwik , Bartlett Rodney J.
Published: July 2002

A recently proposed technique, which combines numerical orbitals for diatomic systems with coupled cluster methods, has been applied to study excited states of negative ions of polar molecules. Computational strategy is presented and calculations of ground and excited state electron affinities are reported for the LiH and BeO molecules. LiH and BeO are found to have excited state electron affinities of 0.0028 and 0.0234 eV. Both excited state affinities should be amendable to experimental determination.
Low temperature tunneling dynamics in condensed media
Authors: Parris Paul E. , Silbey Robert
Published: July 2002

There has been considerable interest recently in the low temperature dynamics of condensed phase tunneling phenomena. In this paper we consider the interplay between quasiparticle transport and vibrational relaxation; the former taking place via tunneling in a double well potential, and the latter occurring due to interactions of the tunneling system with a harmonic bath. Taking the system–bath interactions to be linear in the bath coordinates, and explicitly allowing for a vibrationally excited well, we present a unified treatment of the weak and strong coupling regimes and obtain reduced equations of motion for the tunneling particle position operator. Solutions are obtained for several important limiting cases. In particular, we find that at sufficiently low temperatures, the dynamical behavior strongly resembles that of multisite spin jump model.
The photoelectron spectroscopy of HO−2
Authors: Oakes John M. , Harding Lawrence B. , Ellison G. Barney
Published: July 2002

The photoelectron spectroscopy of HO−2 and DO−2 has been studied. We find the electron affinities (E.A.) of the corresponding radicals to be E.A.(HO2)=1.078±0.017 eV and E.A.(DO2)=1.089±0.017 eV; the gas phase acidity of hydrogen peroxide is ΔH○acid (HO2–H) =376.4 ±0.6 kcal/mol. The electron spectra show only one active mode which we assign as the HO–O stretch (ω3). By modeling the Franck–Condon factors we deduce a molecular structure for the HO−2 ion: r0(HO–O)=1.50 Å, r0(H–OO)=0.97 Å, and ∠(H–O–O)=104°. A parallel set of GVB/PP, GVB-CI, and POL-CI ab initio electronic structure computations has been carried out for this ion.
Theoretical calculation of ozone vibrational infrared intensities
Authors: Adler-Golden Steven M. , Langhoff Stephen R. , Bauschlicher Charles W. , Carney Grady D.
Published: July 2002

An ab initio dipole moment function for ozone has been computed using the CASSCF (complete active space self-consistent field) method, and forms the basis for a calculation of ozone infrared band intensities. Vibrational wave functions were generated using the variational method with potential energy surfaces derived from experimental force constants. Computed values of the permanent dipole moment, dipole moment derivatives, and infrared band strengths are all found to be in remarkably good agreement with experiment. Intensities are predicted for hot bands for which experimental values are unavailable, and implications for atmospheric ozone spectroscopy are discussed. As the dipole moment matrix element signs are now established for nearly all of the observed bands, further refinement of the dipole moment function is possible.
Mechanisms of dissociative single and double photoionization of NO: Analytical photoion spectroscopy
Authors: Masuoka Toshio
Published: July 2002

Photoion branching ratios for NO have been measured by photon and electron impact in the region between 20 and 80 eV. These branching ratios were differentiated with respect to the photon energy. This provides a new analytical photoion spectroscopy, which clearly indicates predissociation of the electronic states of NO+ and the σ-type shape resonances into the fragment ions N+ and O+. Predissociation of the NO2+ states and the Rydberg states converging on them has also been observed.
A laser–ion beam study of the photodissociation dynamics of the (CO2)+3 cluster
Authors: Kim Hyun-Sook , Jarrold Martin F. , Bowers Michael T.
Published: July 2002

The results of a study of the visible wavelength photodissociation dynamics of the (CO2)+3 cluster ion are presented. The experiments were performed using a crossed laser–ion beam apparatus. The photodissociation spectrum, product branching ratios, product angular distributions, and product energy distributions are reported. The main product from visible wavelength photodissociation of (CO2)+3 is CO+2. The results suggest photodissociation of (CO2)+3 is a two-step process: photon absorption by a (CO2)+2 chromophore, instantaneous ejection of a CO2 monomer unit derived from the chromophore, and formation of a transient dimer species; this is followed by dissociation of the transient dimer species which may survive for several rotational periods.
Photofragmentation of mass-resolved carbon cluster ions: Observation of a ‘‘magic’’ neutral fragment
Authors: Geusic M. E. , McIlrath T. J. , Jarrold M. F. , Bloomfield L. A. , Freeman R. R. , Brown W. L.
Published: July 2002

Mass-resolved carbon cluster ions from C+3 to C+20 have been fragmented with 248 nm light. For any initial cluster C+n, the dominant fragment is C+n−3. The fragmentation is primarily linear in laser intensity for n>5, but there are indications that C+3 and C+5 fragment only with the absorption of two or more 248 nm photons. The photodissociation cross sections change dramatically with cluster size.
Non-Markovian activated rate processes: Comparison of current theories with numerical simulation data
Authors: Straub John E. , Borkovec Michal , Berne Bruce J.
Published: July 2002

We calculate the barrier crossing rate constants for a Brownian particle in a double well potential experiencing a non-Markovian friction kernel using a full stochastic simulation. We compare the simulation results with recently proposed interpolation formulas which are based on the Grote–Hynes theory and the energy diffusion mechanism. We find that such formulas can fail by orders of magnitude in a physically interesting regime. Slow activation in an effective dynamic double well potential is probably responsible for the deviations observed.
Time- and frequency-resolved light scattering by isolated molecules. I. Nonperturbative treatment of a single-doorway model
Authors: Wang S. L. , Lim E. C. , Siebrand W.
Published: July 2002

Light scattering by isolated small and intermediate-size molecules is studied theoretically. A quantum-mechanical model is developed that treats the molecule and the radiation field on equal terms and allows a nonperturbative solution for arbitrary wave forms and field strengths. The molecular states are described in terms of a basis set of zeroth-order states chosen so as to minimize the number of excited molecular states in the laser frequency range that combines radiatively with the initial (ground) state, i.e., that act as doorways for the field. Detailed calculations are restricted to the case of a single doorway. Closed-form expressions are obtained for the frequency-resolved scattering rates and time-resolved scattering spectra, including fluorescence and Rayleigh–Raman components, for a variety of conditions, including δ(t) excitation, excitation by a Lorentzian light pulse, by a coherent train of pulses and stationary excitation. The case of nonlinear scattering by very intense pulses is also investigated. Model calculations are reported for systems of 1–3 molecular levels. It is found that in general phase relations are maintained so that the coherence survives as long as the signal.
Studies of reactions of importance in the stratosphere. VI. Temperature dependence of the reactions O+NO2→NO+O2 and O+ClO→Cl+O2
Authors: Ongstad Andrew P. , Birks John W.
Published: July 2002

The reactions of two free radicals, nitrogen dioxide and chlorine monoxide, with oxygen atoms, O+NO2→NO+O2 and O+ClO→Cl+O2, have been investigated in a fast flow system over the temperature range 220–387 K, at a total pressure of 2.3 Torr. Reaction rates were measured under psuedo-first-order conditions with either ClO or NO2 in large excess and the decay of oxygen atoms monitored by adding nitric oxide at the end of the flow tube and detecting the O+NO+M chemiluminescence. The reactions are of interest, because they are thought to be the rate determining steps in two catalytic chains which destroy odd oxygen (O and O3) in the stratosphere. Measurements of the rate constants over the stated temperature interval yielded the Arrhenius expressions kO+NO2=(6.58±0.52)×10−12 exp[(142±23)/T] and kO+ClO=(2.61±0.60)×10−11 exp[(97±64)/T]. Thus, both reactions display small negative activation energies. In addition, experiments showed no pressure effect on the reaction rate in the range 2.3–5.3 Torr and at a temperature of 230 K. Addition of a partial pressure of 1 Torr of oxygen also had no effect on the rate constant at 230 K, suggesting that ClO ⋅ O2 formation is of little or no significance under stratospheric conditions.
New elastic electron scattering factors for the elements for incident energies of 10, 40, 60, and 90 keV
Authors: Ross A. W. , Fink M.
Published: July 2002

An improved set of scattering factors for all neutral elements has bee completed for inclusion in the new edition of the  International Tables of X−ray Crystallography . These calculations are compared with the former electron scattering factors and the deviations between the two are discussed.(AIP)
Electronic states and nature of bonding of the molecule PdGe by all electron a
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Authors: Shim Irene , Kingcade Joseph E. , Gingerich Karl A.
Published: July 2002

In the present work we present all-electron ab initio Hartree–Fock (HF) and configuration interaction (CI) calculations of six electronic states of the PdGe molecule. The molecule is predicted to have a 3Π ground state and two low-lying excited states 3Σ− and 1Σ+. The electronic structure of the PdGe molecule has been rationalized in a simple molecular orbital picture. As part of the PdGe molecule the Pd atom essentially retains its (4d)10 ground term configuration. The chemical bond in the PdGe molecule has been interpreted in terms of donation and backdonation of charge. The bond is polar with a small charge transfer from the Pd to the Ge atom. The dissociation energy of the PdGe molecule has been determined from the mass spectrometric equilibrium data combined with the theoretical results as D00 =252.0±10.5 kJ mol−1.
The full CCSDT model for molecular electronic structure
Authors: Noga Jozef , Bartlett Rodney J.
Published: July 2002

The full coupled-cluster model (CCSDT) single, double, and triple excitation method defined by the wave function exp(T1+T2+T3)‖Φ0〉 is formulated and computationally implemented for the first time. Explicit computational equations are presented. The method is applied to numerous examples including BH, FH, C2H2, CO, Ne, F−, and H2O to assess its applicability to the correlation problem. Results from CCSDT agree with full CI, to an average error of less than 1 kcal/mol even for difficult bond breaking examples. A series of results for various approximate, but computationally more efficient versions of the full CCSDT model are also studied and shown to give results in excellent agreement with CCSDT. Additional comparisons with fifth-order MBPT are reported.
The dissociation of dense liquid nitrogen
Authors: Ross Marvin
Published: July 2002

A theoretical model has been derived to examine the dissociation of shock-compressed liquid nitrogen. An important feature of the model is the introduction of a binding energy for the atomic state which leads to a volume-dependent dissociation energy. Calculated shock pressures and shock temperatures are in agreement with experiment. The model correctly predicts the highly unusual feature of a reflected shock point lying above the principal Hugoniot and of a decrease in the temperature of a reflected shock. The shock cooling is shown to be the consequence of the volume dependent dissociation energy. The overall transition is from a molecular to a metallic liquid.
Photoelectron dynamics of the valence shells of benzene as a function of photon energy
Authors: Carlson Thomas A. , Gerard Patrick , Krause Manfred O. , Grimm Frederick A. , Pullen B. P.
Published: July 2002

Angle-resolved photoelectron spectroscopy was carried out on the first nine valence orbitals of benzene using synchrotron radiation as a photon source. From these data cross sections σ and angular distribution parameters β were obtained over a photon energy of 10 to 34 eV. The experimental results are compared with calculations for the same parameters, based on the multiple scattering Xα method. Considering the complexity of the molecule, the comparison is gratifying. A number of shape resonances, which are predicted, have been identified experimentally. The orbital assignments for the photoelectron spectrum of benzene have been reexamined in view of the cross sections and angular distributions and have for the most part been verified.
Square well orthobaric densities via spinodal decomposition
Authors: Chapela Gustavo A. , Martínez-Casas Sergio E. , Varea Carmen
Published: July 2002

Orthobaric densities of a square well fluid are obtained by means of a molecular dynamics simulation. Hard wall boundary conditions in one direction and periodic boundary conditions in the other two, added to a mean density close to the critical have two effects: a fast spinodal decomposition that condenses a slab of liquid at the center of the box plus a better sampling of the whole orthobaric curve. Results for the liquid densities compare well with previously obtained data while vapor densities are more sensitive to capillary effects. These finite size effects are analyzed within a lattice gas model, especially those produced by changes in box size and mean density. The method is well suited to predict orthobaric and miscibility curves for binary mixtures once the orthobaric curves of each molecular species have been properly adjusted to experiment.
Analysis of covalent bonding by nonergodic Thomas–Fermi theory
Authors: Nordholm Sture
Published: July 2002

It is proposed that covalent bonding can be understood as due to dynamical constraints on electronic motion in atoms which are partially relaxed upon molecule formation. The corresponding delocalization of electronic motion and settling of the electron density stabilizes the molecule. A preliminary implementation of the analysis is based on a form of Thomas–Fermi theory extended so as to account for dynamical constraints leading to a nonergodic population of phase space. Simple estimates for light atoms indicate that nonergodic effects in atoms correspond to large amounts of energy only partially released upon molecule formation due to a variety of antibonding mechanisms.
Differential cross sections for ionization of water vapor by high-velocity bare ions and electrons
Authors: Miller J. H. , Wilson W. E. , Manson S. T. , Rudd M. E.
Published: July 2002

A semiempirical model of single differential cross sections (SDCS) for ionization of water vapor by fast electrons and bare ions is presented. At low secondary-electron energy, the model is based on an asymptotic expansion of the first Born approximation with coefficients, that are independent of projectile properties, evaluated from experimental photoabsorption and proton-impact ionization data. As the secondary-electron energy increases, the model converges to a binary-encounter approximation. Comparisons with measured differential, total, and dissociative cross sections for ionization of water by fast electrons are used to test the model. For primary electrons with energy greater than about 500 eV, agreement with these data is generally within experimental uncertainty; however, some discrepancies of uncertain origin exist.
The triplet 3s,3d complex of HD
Authors: Keiding So/ren R. , Bjerre Nis
Published: July 2002

High-resolution laser photofragment spectroscopy of the 3s,3d←2p c 3Π−u transitions in HD has been performed in a fast mass and state-selected beam. Transition wave numbers and rotational energy levels are given for v=0–3 of the h3∑+g, g 3∑+g, i 3Πg, and j 3Δg states. The data are represented by an effective Hamiltonian which accounts for the rotational breakdown of the Born–Oppenheimer approximation (L-uncoupling). The model parameters indicate a vibrational Born–Oppenheimer breakdown in the i 3Πg state. Good agreement with the most accurate ab initio calculations is obtained.
Thickness of fluid interfaces near the critical point from optical reflectivity measurements
Authors: Beysens D. , Robert M.
Published: July 2002

The optical reflectivity of fluid interfaces near the critical point is analyzed in the light of recently improved estimates of the surface tension and measurements of the bulk correlation length. In contrast to earlier analyses, it becomes possible to discriminate between the two leading phenomenological theories of fluid interfaces, the intrinsic structure theory originated by Maxwell and van der Waals and the capillary wave theory as developed by Buff, Lovett, and Stillinger. For one of the two systems studied, cyclohexane–methanol, capillary wave theory is found to fit the reflectivity data well, while intrinsic structure theory is in strong disagreement with the reflectivity data. A simple combination of the two theories does not significantly improve the performance of capillary wave theory. For the second system studied, sulfur hexafluoride, neither capillary wave theory nor intrinsic structure theory fits the reflectivity data well, while their combination gives partial agreement with experiment.
Nascent rovibrational distributions of HCl+(A 2∑+) and HBr+(A 2∑+) produced from He(2 3S) Penning ionization of HCl and HBr
Authors: Obase Hiroshi , Tsuji Masaharu , Nishimura Yukio
Published: July 2002

He(2 3S) Penning ionization of HCl and HBr leading to HCl+(A) and HBr+(A) has been investigated spectroscopically by using a low-pressure experimental apparatus coupled with a flowing afterglow source. The vibrational distribution of HCl+(A) agrees well with the result obtained by Penning ionization electron spectroscopy (PIES), which shows a Franck–Condon like distribution. In contrast, the vibrational distribution of HBr+(A) is more deexcited than the PIES one shifting to lower vibrational levels relative to Franck–Condon factors for ionization. These findings indicate that the collisional perturbation occurs only at the entrance channel for the He(2 3S)/HCl system, while at both entrance and exit ones for the He(2 3S)/HBr system. The rotational temperature of HCl+(A) decreases from 600±100 K for v′=0 to 300±100 K for v′=5, while that of HBr+(A) is 450±50 K for v′=0 and 400±50 K for v′=1.
Electronic absorption spectroscopy of molecular ions in plasmas by dye laser velocity modulation: The A←X system of N+2
Authors: Radunsky Michael B. , Saykally Richard J.
Published: July 2002

A computer controlled velocity modulation dye laser spectrometer is used to measure the A 2Πu←X 2Σ+g (7,3) band of N+2. The Doppler-limited absorption spectra are fit to a model Hamiltonian to obtain precise parameters for the states. The ion rotational temperature was determined to be 461±11, and the translational temperature was estimated to be near 1200 K.
Photoionization of molecular Rydberg states: H2, C 1Πu and its doubly excited states
Authors: Chupka William A.
Published: July 2002

Explanations are offered for some anomalies seen in photoelectron spectra resulting from single-photon ionization of unperturbed single-configuration Rydberg states of molecules. Some anomalous behavior may be due to occurrence of Cooper minima and shape resonances. Stronger effects can be due to excitation of the ion core producing doubly excited states whose decay characteristics and their effects on photoelectron spectra are discussed. As an example, anomalous photoelectron spectra produced in resonant multiphoton ionization of H2 via its C state are attributed to the excitation and autoionization of the 1Πg(2pσ)(2pπ) state. Formation of excited atomic hydrogen is predicted. The value of such experiments in investigating doubly excited states in general is discussed as well as the problem of preparation of ions in selected states.
The solution of the hypernetted chain and Percus–Yevick approximations for fluids of hard nonspherical particles. Results for hard ellipsoids of revolution
Authors: Perera A. , Kusalik P. G. , Patey G. N.
Published: July 2002

In this paper we describe a general approach which allows the hypernetted chain (HNC) and Percus–Yevick (PY) integral equation theories to be solved numerically for fluids of hard nonspherical particles. Explicit results are given for fluids of hard ellipsoids of revolution and comparisons are made with recent Monte Carlo calculations. It is found that for dense systems of highly anisotropic ellipsoids the HNC and PY closures give significantly different results. The HNC theory is superior predicting the existance of a nematic phase in qualitative agreement with computer simulations. The PY approximation strongly and erroneously suggests that the isotropic phase is stable throughout the liquid regime.
Mass spectrometry of and standard enthalpies of formation of gaseous sodium tetrachloroaluminate and its dimer
Authors: Wahlbeck P. G.
Published: July 2002

Knudsen cell mass spectrometry was performed on vapors from a sample containing NaAlCl4(l) and NaCl(s) at an average temperature of 500 K. The vapor was analyzed using electron bombardment with 50 eV electrons. Several ions were detected which permitted the conclusion that the vapor consisted of AlCl3, Al2Cl6, NaAlCl4, and Na2Al2Cl8. Ion intensities were determined as a function of the temperature of the vapor in the Knudsen cell. Data were treated thermodynamically using the van’t Hoff equation. Statistical mechanical calculations were performed for NaAlCl4(g). The data yielded the following thermodynamic data: standard enthalpy of formation at 500 K of NaAlCl4(g), −997.8±6.0 kJ/mol; standard enthalpy of formation at 298.15 K of NaAlCl4(g), −994.8±8.0 kJ/mol; standard enthalpy of formation at 500 K of Na2Al2Cl8(g), −2174.7±8.0 kJ/mol.
Two-photon photodissociation dynamics of state-selected NO2
Authors: Bigio Laurence , Grant Edward R.
Published: July 2002

Quantum states of NO2 are selected and then photodissociated by resonant two-photon photoexcitation. The total photolysis energy is scanned over a region from 50 cm−1 below to 300 cm−1 above the threshold for production of NO(X̃ 2Π)+O(1D). This channel yielding excited oxygen is observed to dominate the production of vibrational ground state NO. Diatomic product J and Λ doublet state distributions are probed by resonant two-photon ionization. The photodissociation cross section for production of specific NO quantum states is found to be structured in the photolysis wavelength. This structure is assigned to intermediate resonance in the two-photon photolysis. Rotational structure is identified in this pattern, and confirmed by separate optical–UV–double resonance spectroscopy using the same intermediate states in combination with levels of the 3pσ 2Σ+u Rydberg state of NO2. Though photodissociation dynamics are found to be a very sensitive function of photolysis wavelength, different wavelengths that promote different transitions through the same intermediate state yield very similar dynamics. The existence and apparent patterns of photoselection in the product state dynamics are discussed in the light of recent theoretical developments incorporating electronic degrees of freedom in state-to-state photofragmentation.
Systematic features of the energy dependence of radiationless processes in large molecules: The substituted naphthalenes
Authors: Jacobson Benjamin A. , Guest Joyce A. , Novak Frank A. , Rice Stuart A.
Published: July 2002

This work examines, both experimentally and theoretically, nonradiative decay processes in a series of substituted naphthalenes. We report single vibronic level fluorescence lifetimes and fluorescence excitation spectra of jet-cooled 2-chloronaphthalene, 1- and 2-fluoronaphthalene and 1- and 2-methylnaphthalene over an energy range of about 0–4000 cm−1 in S1. While the 00 nonradiative rates of these molecules vary by a factor of 30, the energy dependences of the nonradiative rates are quite similar. At low vibrational energies the nonradiative rates depend sensitively on the level excited, but in general they increase with energy. As energy increases, the nonradiative rates become less sensitive to the level excited and eventually become almost independent of vibrational energy. We can qualitatively predict this behavior using a thermodynamic formalism which treats the density of states as an intramolecular entropy and avoids the calculation of vibrational coupling terms. In addition we report the fluorescence lifetime of the S1 vibrationless level of 1-chloronaphthalene, and single vibronic level fluorescence spectra of levels up to 1396 cm−1 in 2-chloronaphthalene. Most of the fluorescence excitation and fluorescence spectra show substantial enhancement of the origin inensity and vibrational mode mixing in S1 compared to naphthalene. We briefly discuss these substituent effects and make some tentative assignments of S1 vibrational levels. We also discuss substituent effects on the 00 fluorescence lifetimes. In particular, the order of magnitude increase in the decay rate of 1-chloronaphthalene relative to 2-chloronaphthalene cannot be explained using CNDO/S calculations.