bluuebunny/crossref_metadata_2025 huggingface dataset:
Radio frequency sputter deposition and properties of calcium fluoride thin films
Authors: Maréchal N. , Quesnel E. , Juliet P. , Pauleau Y.
Published: July 2002

Calcium fluoride thin films have been deposited on various substrates by radio frequency sputtering of a hot-pressed CaF2 target in a pure argon plasma using load-locked sputtering equipment. The sputtering power was less than 100 W to avoid crack formation and fracture of the target. As a result, the maximum deposition rate of films was only 2.1 nm/min. The films analyzed by Rutherford backscattering spectroscopy (RBS) were found to be fluorine deficient; depending on the deposition conditions, the content of oxygen atoms incorporated in the films can be as low as the RBS detection limit of 2 at. %. The crystallographic structure of the deposited material was identified to be a CaF2 fluorite phase by x-ray diffraction technique; this phase was preferentially oriented in the (111) direction for films of 0.4–0.5 μm in thickness. This preferred orientation vanished with increasing film thickness. Films of 2.5 μm in thickness were slightly (110) oriented. The sputter-deposited CaFx films (with x≤1.85) exhibited compressive residual stresses. The apparent Knoop hardness of the films deposited on silicon substrates (harder than CaF2) and measured under a load of 10 and 20 g was about 3500 MPa, i.e., twice the hardness of the bulk CaF2 compound. The surface and cross-section morphology of CaFx films was examined by scanning electron microscopy. These films were resistant to oxidation in air at 500 °C. Tribological characteristics of CaFx films sputter-deposited on cobalt-based alloy disks were determined by pin-on-disk tests. The friction coefficient and wear resistance of these films sliding against Cr–C coated pin specimens in air at 500 °C were found to be encouraging. These films are potential candidates to serve as solid lubricant films for precision components (ball bearings) operating at high temperatures in oxidizing ambient.
High gain and wide dynamic range punchthrough heterojunction phototransistors
Authors: Wang Y. , Yang E. S. , Wang W. I.
Published: July 2002

In this article, we propose and demonstrate a novel punchthrough heterojunction phototransistor (HPT). The base of the transistor is lightly doped and completely depleted under the operating condition. The collector bias current can be applied without the base terminal. The transistors exhibit optical conversion gain as high as 1240 at an incident optical power as low as 0.5 μW, and the gain changes less than 15% over a 20 dB range of incident optical power. The transient measurements showed that the transistor has a high response speed than that of conventional two or three terminal HPTs. This represents the best performance of HPTs with similar dimensions. The results of simulation showed that the punchthrough HPTs have much lower noise characteristics than conventional HPTs. The principle reported here can be applied to HPTs made from other material systems, such as AlGaSb/GaSb and InP/InGaAs, for long wavelength optical communications.
Electron-beam generation during the hollow cathode phase of pseudospark discharges
Authors: Pitchford L. C.
Published: July 2002

From calculations using a two-dimensional, hybrid fluid-particle model, the mechanism for the production of the high-current electron beam observed during the hollow cathode phase of pseudospark discharges is identified. The predicted beam properties are in good qualitative agreement with experiments; the onset of the beam current is abrupt and the current in the beam is some 10%–30% of the discharge current. Two distinct components appear in the energy spectrum of the beam. A sizable fraction of the current is carried by electrons with an energy corresponding to the voltage applied to the discharge. A distinct low-energy beam is also predicted with a spread in energy of several 100s of eV.
Compositional control of ferroelectric fatigue in perovskite ferroelectric ceramics and thin films
Authors: Chen Jie , Harmer Martin P. , Smyth Donald M.
Published: July 2002

The mechanism and control of ferroelectric polarization fatigue (loss of polarization with cycling) in donor- and acceptor-doped BaTiO3 ceramics and Pb(Zr1−xTix)O3 (PZT) thin films has been investigated. Experimental results clearly demonstrate that fatigue behavior is related to the defects within the materials. Donor-doped BaTiO3 ceramics showed significantly improved fatigue characteristics when compared with acceptor-doped materials. A similar but reduced effect has been observed in donor-doped PZT thin films. The electric-field-assisted migration of charged species within ferroelectric materials may be responsible for the degradation/fatigue behavior. Results support the expectation that oxygen vacancies play an important role in fatigue that occurs as a result of polarization switching.
Optical properties of cubic and hexagonal CdSe
Authors: Ninomiya Susumu , Adachi Sadao
Published: July 2002

The complex dielectric function, ε(E)=ε1(E)+iε2(E), of hexagonal CdSe has been measured by spectroscopic ellipsometry in the photon-energy range between 1.2 and 5.3 eV at room temperature. The measured spectroscopic-ellipsometry data are analyzed on the basis of a simplified model of the interband transitions. The model is based on the Kramers–Kronig transformation and includes the E0 (E0α ; α=A,B,C), E1 (E1α ; α=A,B,C), and E0′ gaps as the main dispersion mechanisms. The recent SE data of cubic, zinc-blende-type CdSe have also been analyzed with the same model by considering the critical points for the cubic phase (i.e., E0, E0+Δ0, E1, E1+Δ1, and E2). Results are in satisfactory agreement with the experimental data over the entire range of photon energies. To facilitate design of various optoelectronic devices, dielectric-function-related optical constants, such as the complex refractive index, absorption coefficient, and normal-incidence reflectivity, of these crystals are also presented.
Effect of intermixing on the magnetic properties of Co50Fe50/Ni80Fe20 multilayers
Authors: Rengarajan S. , Yun E. J. , Kang W. S. , Walser R. M.
Published: July 2002

The effect of interface mixing on the magnetic properties of multilayer thin films consisting of N Co50Fe50/Ni80Fe20 bilayers was investigated for N=1–25. A fixed 50 Å Ni80Fe20 thickness was used in all the multilayers, but the Co50Fe50 layer thickness was varied as 1260 Å/N. With increasing N, the saturation magnetorestriction (λs) of the multilayer films increased from 40 to 58 ppm, but both the anisotropy field (Hk) and the coercivity (Hc) decreased, from 86 and 30 Oe, to 20 and 2.5 Oe, respectively. λs varied linearly with the 1/modulation thickness (D), confirming that the increase was due to interface mixing in the bilayers. From x-ray data, and the variations of the saturation magnetization (4πMs) and resistivity (ρ) of the multilayers, it was determined that the interfacial layer resulted from the diffusion of Fe from the NiFe layers into the CoFe layers. The sensitivity (dλ/dH), and the permeability of the multilayers also increased with N. The utility of this combination of properties for magnetostrictive sensor applications has been demonstrated in a related work.
Red luminescence in phosphorous-doped chemically vapor deposited diamond
Authors: te Nijenhuis J. , Olsthoorn S. M. , van Enckevort W. J. P. , Giling L. J.
Published: July 2002

Luminescence studies have been performed on phosphorous-doped diamond films deposited by hot-filament chemical vapor deposition. A broad luminescence band, centered around 1.9 eV is revealed, in the cathodo luminescence spectra of homoepitaxial and polycrystalline films, whereas the blue band-A luminescence, which is characteristic for undoped diamond, is quenched in the presence of phosphorus. The 1.9 eV luminescence band could not be excited by Ar+ laser light (2.54 eV) and did not show a zero-phonon line in the spectrum. Therefore, we suggest that this band is a red form of band-A luminescence, related to electron-hole recombination at substitutional phosphorous and a phosphorus-vacancy complex.
High-rate deposition of YBa2Cu3O7−x films by hot cluster epitaxy
Authors: Takamura Yuzuru , Yamaguchi Norio , Terashima Kazuo , Yoshida Toyonobu
Published: July 2002

The growth rate and crystallinity of YBa2Cu3O7−x (YBCO) films were investigated in connection with the cluster size and the growth mode in order to clarify the high-rate deposition of high-quality epitaxial films from clusters in the plasma flash evaporation method. The films were deposited from clusters that were not accelerated by bias voltage but were self-activated in a thermal plasma. With increasing cluster size, the growth rate increased drastically at the point of the growth mode transition from spiral to two-dimensional cluster nucleus growth. After the transition, the film was still well epitaxial and have the minimum value of the full width at half maximum of the (005) x-ray rocking curve (FWHMrc). A 1-μm-thick, nonspiral growth, monolayer smooth epitaxial YBCO film was successfully deposited at a growth rate of 16 nm/s. FWHMrc for the films was less than 0.14°. It was revealed experimentally that the deposition from “hot” clusters with large sticking probability onto a high-temperature substrate is highly effective for the deposition of high-quality films at a high rate.
1/f
noise in magnetite films
Authors: Raquet B. , Coey J. M. D. , Lind D. M. , von Molnár S. , Anane A. , Koch R. H.
Published: July 2002

The low frequency electrical noise has been studied in Fe3O4 films as a function of temperature and in a frequency range from 0.1 to 100 Hz. At room temperature, Hooge’s coefficient is around 80, at least three orders of magnitude greater than normally found for metals. A huge increase of the normalized noise is observed at the Verwey transition, whose kinetics cannot be described in the framework of the Dutta model. We show that the 1/f noise scales with the inverse of the number of carriers involved in the hopping conduction. The high noise level is analyzed in terms of slow electronic exchanges between a critical conduction network and traps or excitations widely distributed in energy near the Fermi level.
Oxygen diffusion in ultrafine grained monoclinic ZrO2
Authors: Brossmann Ulrich , Würschum Roland , Södervall Ulf , Schaefer Hans-Eckhardt
Published: July 2002

The diffusion of oxygen in ultrafine grained, undoped monoclinic ZrO2 was studied using O18 as tracer and secondary ion mass spectroscopy profiling. Samples with a relative mass density of 97%–99% and average crystallite sizes of 80 or 300 nm were prepared from Zr by sputtering, inert-gas-condensation, oxidation, in situ consolidation of nanocrystalline (n-)ZrO2 powder and subsequent pressureless sintering at 950 or 1050 °C in vacuum. Volume and interface diffusivities were directly determined from the O18 diffusion profiles in n-ZrO2 in the type B and type A regime of interface diffusion. The diffusion of O18 in interfaces in undoped n-ZrO2 is 103–104 times faster than in the bulk of the crystallites throughout the temperature range of 450 to 950 °C studied. These diffusivities are independent of the crystallite size in the range of 70–300 nm. The activation energies QV=2.29 eV and QB=1.95 eV for the volume (QV) and interface diffusion (QB) are considerably higher than the diffusion activation energies found in the fast ion conductors Ca- or Y-stabilized ZrO2. Based on the present data on oxygen diffusion in ZrO2, the cation and anion diffusivities of other binary oxides are discussed.
Experiments on space-charge waves in electron beams propagating through a resistive-wall channel
Authors: Suk H. , Wang J. G. , Reiser M. , Zou Y.
Published: July 2002

Localized longitudinal space-charge waves in a transport channel with a resistive wall are investigated experimentally. The space-charge waves with various pulse widths are generated in electron beams with energies of 3.0–8.5 keV and currents of 30–135 mA, and they are transported through a resistive channel with a resistance of 5–10 kΩ and a length of 1 m. The resistive channel, made of a glass tube with a resistive material coating, is surrounded by a long solenoid which provides the focusing for the beam. The localized space-charge waves are measured with current monitors and energy analyzers at the entrance and exit of the resistive channel, and the wave-amplitude growth and damping rates are determined. The measured results are compared with the theory in the long-wavelength range. For localized waves with short widths, where the long-wavelength approximation is not valid any more, dispersion and distributed capacitance effects on the growth/damping rates are discussed. In addition, preliminary results on the energy width measurements of space-charge waves are presented.
Defects in ZnSe/ZnTe multiple quantum well-based pseudo-ohmic contacts to p-ZnSe
Authors: Tomiya S. , Kijima S. , Okuyama H. , Tsukamoto H. , Hino T. , Taniguchi S. , Noguchi H. , Kato E. , Ishibashi A.
Published: July 2002

The microstructure of ZnSe/ZnTe multiple quantum well-based pseudo-ohmic contacts to p-ZnSe was investigated using transmission electron microscopy and high-resolution transmission electron microscopy. In the case of samples consisting of five ZnSe/ZnTe multiple quantum wells, both pure edge Lomer dislocations and 60° dislocations were identified at the interface between the ZnSe/ZnTe multiple quantum wells and the ZnTe overlayer, along with partial dislocations bounding stacking faults. The dominant dislocations at the interface are Lomer dislocations. In the case of samples grown under group II-rich conditions, the interface exhibits corrugations. At the top and bottom of the corrugations, the Lomer dislocations are dominant and in the slope of the corrugations, 60° dislocations are dominant. In the case of samples grown using migration-enhanced epitaxy, V-shaped defects consisting of three dislocations associated with two stacking faults are formed. The total Burgers vector of the V-shaped defects is a〈100〉. The increasing total thickness and the number of ZnSe/ZnTe multiple quantum wells leads tend to make the dominant defects dissociated 60° dislocations.
Hetero- and homoepitaxial Nd3+-doped LaF3 thin films grown by molecular beam epitaxy: A spectroscopic study
Authors: Lahoz F. , Daran E. , Zhang X. , Muñoz-Yagüe A. , Cases R. , Alcalá R.
Published: July 2002

Hetero- and homoepitaxial Nd3+-doped LaF3 thin films have been grown by molecular beam epitaxy. Two different orientations of CaF2 substrates, (111) and (110), have been used for the heteroepitaxial structures. High-resolution emission and excitation spectra as well as the decay time of the emission have been measured. The spectroscopic measurements demonstrate that one Nd3+ site is present in the LaF3 layers grown on CaF2(111) substrates but two slightly different Nd3+ centers are resolved in the films on CaF2(110) substrates. One Nd3+ site has been found in the homoepitaxial sample. Slight differences are observed between the centers found in the LaF3 layers and the one observed in the Nd3+-doped LaF3 bulk crystal. For the homoepitaxial layer, the linewidths are similar to those of the bulk crystals, whereas for the heteroepitaxial layers, a broadening is observed.
Evolution of defect complexes in silicon single crystals with heavy fluence 10 MeV proton irradiation
Authors: Khan Aurangzeb , Yamaguchi Masafumi , Hisamatsu Tadashi , Matsuda Sumio
Published: July 2002

We have investigated the defect structure of 10 MeV proton irradiated Czochralski-grown Si single crystals and space solar cells with boron-doped p-Si base layer using deep level transient spectroscopy measurements to characterize both vacancy interstitials and their complex-type defects and to monitor their evolution upon annealing at temperatures ⩽500 °C. We have observed quite different annealing behavior of the deep levels for conduction-type converted samples (n-type) irradiated at 1×1014 p/cm2 as compared to an intermediate dose of 3×1013 p/cm2. The observed concentrations of the minority carrier level at EC−0.20 eV and the new electron level at EC−0.71 eV that can be seen in type converted samples, have been found to be enough to account for the carrier removal effects. The present study also throws light on the fact that heavy proton irradiation not only changes the structure of the device (from p to n type) but also makes the defect structure complex as compared to the simple defect structure in low dose samples. Isochronal thermal annealing after heavy irradiation provides interesting insight into defect interactions. In particular, the new observed prominent electron level (EC−0.71 eV) in type converted cells exhibits a mutual thermal transformation with hole level (EV+0.36 eV) upon annealing.
NO thermally desorbed from a saturation coverage on Pt(111): Internal state distributions
Authors: King David S. , Mantell David A. , Cavanagh R. R.
Published: July 2002

Internal state distributions were obtained by laser-excited fluorescence for NO desorbing from a saturation coverage on Pt(111). Two major peaks are observed in the thermal desorption spectrum at crystal temperatures Ts=200 and 340 K. The rotational state distributions for each desorption feature were Boltzmann, described by the rotational temperatures TR=195±10 K and 305±10 K, respectively. The two spin-orbit states were occupied to a degree consistent with equilibration between spin and rotational degrees of freedom, both Lambda doublet species were statistically populated and no molecular alignment was observed.
Generalized Tamm–Dancoff approximation (GTDA) and random-phase approximation (GRPA) calculations on LiH, Be, and Li2
Authors: Sangfelt E. , Chowdhury R. Roy , Weiner B. , Öhrn Y.
Published: July 2002

The theory for polarization propagator calculations based on a correlated reference state in the form of a spin singlet restricted antisymmetrized geminal power (AGP) is discussed. Particular attention is paid to the calculation of excitation energies and transition moments for the molecules over all internuclear separations, i.e., not just single point vertical energies, which is done in standard RPA or in higher-order treatments using perturbation methods. Simple applications to the Be atom, and the LiH and Li2 molecules are used to illustrate how this simple correlated state, which is completely determined by the choice of orbital basis (i.e., no choice of configurations is necessary), can cure the notorious consistency problems and the instabilities against the breaking of symmetry, which plague standard RPA based on a Hartree–Fock (HF) single determinantal reference state.
PbS in polymers. From molecules to bulk solids
Authors: Wang Y. , Suna A. , Mahler W. , Kasowski R.
Published: July 2002

The transition of PbS from molecular to bulk form has been observed in polymer films. As the particle size decreases the band gap shifts to the blue and eventually approaches the transition energy of the first allowed excited state, X→A, of a PbS molecule. Discrete absorption bands also appear. The electron-hole-in-a-box model with effective mass approximation cannot explain the observed size dependence. We have developed two theoretical models, both including the effect of band nonparabolicity, that successfully explain the observed size dependence down to about 25 Å.
A phase space analysis of the collinear I+HI reaction
Authors: Skodje Rex T. , Davis Michael J.
Published: July 2002

The collinear I+HI reaction is studied using an approach based on the concepts of nonlinear dynamics. Three closed regions in phase space are constructed by connecting the dynamical manifolds emanating from physically important periodic orbits. It is shown that many features of the reaction dynamics can be understood with reference to these regions. The oscillating reaction probability in this system is shown to stem from the geometrical pattern of overlap of heteroclinic oscillations of an interaction region. The process of complex formation is quantitatively described in terms of passage into a well defined complex region of phase space. The phase space representation predicts that the complex formation probability oscillates with energy and suggests that the complex lifetime might oscillate as well. We have carried out simulations which confirm both of these effects. The vibrational adiabatic approximation for the reaction is assessed relative to the exact classical dynamics.
Measurements of the forces between a metal surface and mica across liquids
Authors: Parker John L. , Christenson Hugo K.
Published: July 2002

Results of direct measurements of the interactions between a silver surface and a mica surface across liquids are presented. In the nonpolar liquid octamethylcyclotetrasiloxane oscillatory solvation forces are found, the range of which is shorter than between two mica surfaces. In aqueous electrolyte solutions long-range double-layer forces are present. At smaller separations the interaction is attractive in conductivity water but steeply repulsive in 10−4 M sodium carbonate.
The Dufour and Soret coefficients of isotopic mixtures from equilibrium molecular dynamics calculations
Authors: Vogelsang R. , Hoheisel C.
Published: July 2002

The Soret and Dufour cross correlations functions (CCFs), as determined by equilibrium molecular dynamics calculations, are compared for soft sphere and Lennard-Jones (LJ) mixtures. The isotopic mixture was investigated to avoid separate determination of the partial enthalpy of the system. For the LJ system, potential truncation and particle number effects on the thermal diffusion coefficient were additionally studied. Main findings are: (i) Soret and Dufour CCFs are numerically equivalent, but the fluctuations in the latter are more pronounced, particularly for the LJ mixture. (ii) Using a symmetrized form of the tensor occurring in the heat current, differences between the two CCFs are noticeable, when the Stoermer–Verlet integration algorithm is applied. (iii) The particle number dependence of the CCF, and hence of the thermal diffusion coefficient, is very small.
Determination of the bound and quasibound states of Ar–HCl van der Waals complex: Discrete variable representation method
Authors: Choi Seung E. , Light J. C.
Published: July 2002

The ArHCl (HCl; ν=0) van der Waals (vdW) molecule has a large number of bound and rotationally predissociative (resonance) states for total angular momentum in the range of 0≤J≤60. Using the Jacobi coordinates and the total angular momentum representation in the body-fixed reference frame, the Hamiltonian is evaluated in the discrete variable representation (DVR) of the stretch and bend internal vibrational basis and a basis of parity adapted rotation functions. The facile and effective application of the DVR is greatly enhanced by an appropriate choice of the basis set. The sequential diagonalization and truncation of the Hamiltonian permit accurate and efficient determination of eigenstates. Using Hutson’s H6 potential energy surface, the energies and wave functions of all bound and resonance states are computed for selected J’s up to J=60. A novel flux projection technique facilitates semiquantitative evaluation of the lifetimes of all states and, in particular, the simple identification of the resonance states in the ℒ 2 eigenvector basis.
Cluster ions and multiply charged ions formed in frozen CO2 molecules under heavy ion impact
Authors: Tawara H. , Tonuma T. , Kumagai H. , Matsuo T. , Shibata H.
Published: July 2002

A variety of cluster ions, positive or negative, as well as multiply charged atomic ions have been observed from the frozen CO2 targets under (MeV/amu) energetic, highly charged projectile ion impact. Their spectra are found to be quite different from those produced in the cooled expanding CO2 gas targets.
Molecular dynamics studies of the liquid–vapor interface of water
Authors: Townsend R. Michael , Rice Stuart A.
Published: July 2002

We report the results of molecular dynamics simulations of the liquid–vapor interface of water. Two different water–water interactions were studied. The density profile of the interfacial transition region between liquid and vapor is monotone with a 10%–90% width of 3.45 Å, in good agreement with the value determined from x-ray reflectance measurements, namely 3.30 Å. The water molecules in the interface tend to lie with the HOH bisector in the plane of the surface and one OH bond pointing out of the surface. As the density falls in the transition region the tetrahedral structure of the bulk liquid breaks up and there is a tendency towards dimerization. The diffusion constant in the interfacial region is 58% larger than that in the bulk region.
Quantum resonance dynamics for the I+HI reaction in three dimensions: An adiabatic treatment using Jacobi coordinates
Authors: Grayce Beverly B. , Skodje Rex T.
Published: July 2002

The quantum mechanical resonance states for the I+HI chemical reaction on the Manz–Römelt LEPS (London–Erying–Polanyi–Sato) surface ‘‘A’’ are calculated in three dimensions for the case of total angular momentum equal to zero. The problem is simplified to a two degree of freedom system through the adiabatic elimination of the fastest time scale motion. The adiabatic reduction is carried out in Jacobi coordinates, which allows the correct identification of the fast motion in all dynamically relevant regions. The resonance energies and wave functions are obtained using a stabilization technique on the adiabatically reduced system. A total of 68 resonance states were located for the J=0 dynamics. A number of bend excited resonances that have not been previous calculated are identified. Some considerations from classical mechanics are shown to be useful in understanding the quantum dynamics.
A coupled chemical burster: The chlorine dioxide–iodide reaction in two flow reactors
Authors: Dolnik Milos , Epstein Irving R.
Published: July 2002

The dynamical behavior of the chlorine dioxide–iodide reaction has been studied in a system consisting of two continuous flow stirred tank reactors (CSTRs). The reactors are coupled by computer monitoring of the electrochemical potential in each reactor, which is then used to control the input into the other reactor. Two forms of coupling are employed: reciprocally triggered, exponentially decreasing stimulation, and alternating mass exchange. The reaction, which exhibits oscillatory and excitable behavior in a single CSTR, displays neuronlike bursting behavior with both forms of coupling. Reciprocal stimulation yields bursting in both reactors, while with alternating mass exchange, bursting is observed in one reactor and complex oscillation in the other. A simple model of the reaction gives good agreement between the experimental observations and numerical simulations.
On the contribution of multiplicative noise terms to the Langevin equation for rotational relaxation
Authors: Coffey W. T.
Published: July 2002

The Langevin equation for the rotational Brownian motion of two and three dimensional rotators has multiplicative noise terms if it is transformed into an equation for the dipole moment. It has been heuristically argued following Langevin that the average of the multiplicative noise terms vanishes for the inertial response. This enables one to write down the hierarchy of differential-difference equations governing the time behavior of the averages of the solution (usually generated by separating the variables in the Fokker–Planck equation) directly from the transformed Langevin equation. It is shown by interpreting the transformed Langevin equation as a Stratonovich equation how the assumption that the average of the multiplicative noise terms vanishes is justified.
Alignment of Hg–Ar van der Waals molecule photofragments following photodissociation
Authors: Quayle C. J. K. , Bell I. M. , Takács E. , Chen X. , Burnett K. , Segal D. M.
Published: July 2002

We have measured the alignment of the photofragments produced by photodissociation of Hg–Ar van der Waals molecules using light around 253.7 nm. The molecules are produced in the X0+ ground state in a supersonic expansion. Tunable laser light close to the Hg 6 1S0–6 3P1 transition is then used to excite the molecule to the B1± state, in which it dissociates. The alignment of the resulting Hg(3P1) asymptotic atoms is probed by tuning a delayed second laser to the 435.8 nm Hg 6 3P1–7 3S1 atomic transition and monitoring the subsequent fluorescence on the 7 3S1–6 3P2 transition around 546.1 nm. These measurements have been made, we believe for the first time, as a function of dissociating energy from very close to threshold to higher energies. The alignment shows a significant rise with increasing photon energy in line with a semiclassical picture of orbital locking at lower energies and rapid decoupling at higher energies. The results exhibit good agreement with the results of a fully quantum mechanical theory of the system.
Free doubly negative tetrahalides
Authors: Weikert H.-G. , Cederbaum L. S.
Published: July 2002

The existence and properties of the closely related tetrahedral tetracoordinated dianions BeF2−4, MgF2−4, and CaF2−4 in the gas phase are discussed. It is shown that these dianions exist in long-lived intermediate states bound by a barrier to dissociation. To definitely establish whether the added electrons remain bound, the outer-valence ionization potentials have been computed via the Green’s function formalism which takes account of relaxation and electron correlation effects beyond the one-particle approximation. The results show that the potential barrier as well as the binding of the excess electrons of the dianions increase along the sequence BeF2−4, MgF2−4, and CaF2−4. The possibility of dynamic autodetachment is analyzed by computing the potential energy surfaces of the lowest anionic and dianionic states of the tetrafluorides at both the independent-particle and the correlated levels. Strong effects of hole localization as a symmetry breaking phenomenon are observed accompanying the outer-valence ionization of the dinegative system sharing most of the outer-valence electron density on the symmetry equivalent fluorine ligands. Attempts are made to interrelate the nature or character of the chemical bonding with the thermodynamic stability of the dianion which also increases in going from BeF2−4 to MgF2−4 and finally to CaF2−4. In this connection, some observations have been made which do not fit into expectations as taught by chemical experience. Calculations on the tetrachloride dianions BeCl2−4, MgCl2−4, and CaCl2−4 have also been performed. The major findings are briefly discussed.
The recovery of polymer melts after shear and elongational flows
Authors: Borgbjerg Ulrik , de Pablo Juan José , Öttinger Hans Christian
Published: July 2002

This work examines the constrained recoil after steady shear flow and the free recovery after steady elongational flow of monodisperse polymer melts. Calculations are performed in the framework of the Curtiss–Bird theory using both conventional numerical integrations and nonequilibrium Brownian dynamics. The latter approach is shown to have significant advantages, particularly at high shear or elongational rates. The predicted curves of recovery as a function of shear rate exhibit an unusual maximum for which several explanations are proposed.
Macromixing effects on the Kumpinsky–Epstein model of a chlorite-iodide reaction in a stirred tank
Authors: Hsu T. J. , Lee D. J.
Published: July 2002

A numerical investigation of the macromixing effect on the bistable ClO−2+I− reaction based on the global fluid flow field was undertaken in this work. Results obtained in this study indicated that the mixing intensity has a significant effect on the location and transition point of the thermodynamic branch, but has nearly no effect on the flow branch.
Hyperfine-resolved rovibrational spectrum of the X 2Π state of HI+
Authors: Chanda A. , Ho W. C. , Dalby F. W. , Ozier I.
Published: July 2002

A high resolution spectroscopic study of HI+ has been carried out for the first time. The absorption spectrum in the inverted 2Π ground electronic state has been measured with a linewidth of 0.004 cm−1 between 1995 and 2245 cm−1. A total of 117 vibration–rotation transitions were observed with a tunable diode laser spectrometer coupled to an ac glow discharge cell employing velocity modulation. Lines were measured in the vibrational fundamental of the 2Π1/2 spin substate and in the three lowest (v+1←v) bands of the 2Π3/2 spin substate. A good fit to the data was obtained using a standard vibration–rotation, fine structure Hamiltonian. Equilibrium values were determined for 16 molecular parameters including the harmonic vibrational frequency ωe, the rotational constant Be, and the Λ-doubling constants pe and qe. A review of the ground state properties of the hydrogen halide ions HX+ (X=F, Cl, Br, I) shows that the harmonic force constant is, to excellent approximation, a linear function of the internuclear spacing. In ten different vibration–rotation transitions of the two spin substates of HI+, hyperfine splittings were observed. A total of 58 relative splittings of hyperfine components were analyzed to determine the iodine quadrupole coupling constant eQq0 as well as the Frosch–Foley magnetic hyperfine constants a, (b+c), and d. The results have been used to investigate the electronic properties of the ion.
On the singlet–triplet separation in methylene: A critical comparison of single- versus two-determinant (generalized valence bond) coupled cluster theory
Authors: Balková Anna , Bartlett Rodney J.
Published: July 2002

We present a critical comparison of the performance of the single- vs two-determinant-reference coupled cluster method, including its generalized valence bond version, for the classic multireference problem of the singlet–triplet separation in methylene. After demonstrating excellent agreement between the two-determinant coupled cluster method with single and double excitations and the full configuration interaction method for a double zeta polarization (DZP) basis, we adopt an extended atomic natural orbital basis and obtain harmonic frequencies for the two states to give T0=10.30 kcal mol−1 and 8.86 kcal mol−1, respectively, for the single- and two- determinant-reference coupled cluster results, compared to the experimental value, 8.998±0.014 kcal mol−1. Adding triples, the corresponding single-reference value is 9.35 kcal mol−1. We also consider stretched geometries of CH2 as a stringent test of our approach. Comparisons are made between the two-determinant coupled cluster values, including the single and double excitations and their triples excitation corrected coupled cluster counterparts, obtained using various choices of orbitals, including the generalized valence bond ones.
Molecular dynamics simulation of gaseous ion-motion in electrostatic fields
Authors: Koutselos Andreas D.
Published: July 2002

A molecular dynamics (MD) method has been developed for the simulation of the motion of ions in neutral gases under the influence of homogeneous electrostatic fields. The method treats the translational motion of the ions and gas molecules classically and thus requires as input the ion–molecule interaction potentials. The continuous dissipation of a part of the ion-energy to a thermal bath is accomplished through the introduction of ‘‘iconical interactions’’ between ions and images of the neutrals created and stored in the memory of a computer during ion–atom encounters. The steady ion motion is then simulated by usual equilibrium MD methods using periodic boundary conditions. The resulting ion mobilities, effective temperatures, and third other velocity-distribution moments, expressed as skewness parameters, compare well with available results derived from the moment solution of Boltzmann equation and Monte Carlo simulations using the same interaction potentials in the cases of K+ in He and Ar, as well as of Ba+ in Ar. The additional reproduction of experimental data provides an independent test for the accuracy of the employed model potentials. Although the method has been applied to atomic systems it is easily extendable to the more complex molecular systems but at the expense of computer time.
Alternative Hamiltonian for molecular dynamics simulations in the grand canonical ensemble
Authors: Lo Chaomei , Palmer Bruce
Published: July 2002

An alternative to the Hamiltonian of Çağin and Pettitt for performing molecular dynamics simulations in the grand canonical ensemble is presented and used as the basis for a new algorithm. The algorithm is tested on the ideal gas and the truncated and shifted Lennard-Jones fluid. Simulations are used to calculate the vapor–liquid coexistence points for the Lennard-Jones system and are found to be in agreement with previous calculations using Gibbs ensemble calculations and with the Nicolas equation of state. Simulations are also performed on the Lennard-Jones solid.
Limiting values for Mo/ller–Plesset second-order correlation energies of polyatomic systems: A benchmark study on Ne, HF, H2O, N2, and He...He
Authors: Klopper Wim
Published: July 2002

Limiting values for Mo/ller–Plesset second-order (MP2) correlation energies are provided for the ten-electron systems Ne, HF, and H2O, for the N2 molecule, and for the weak He...He interatomic interaction energy. These limiting values were obtained by the MP2-R12 approach. This approach differs from traditional MP2 theory by employing first-order wave functions which explicitly depend on the interelectronic coordinates rij. With the MP2-R12 method, the atomic orbital (AO) basis set limits for the systems under study are reached. The calculations provide insight into AO basis set requirements for methods with linear rij dependence (R12 methods), e.g., for coupled cluster methods, or multireference configuration interaction methods. Moreover, it is expected that the results have the potential to serve as valuable benchmarks for further developments in the field of explicitly correlated wave functions, for example for expansions in terms of Gaussian geminals (Gaussian functions which depend on rij). The present calculations on HF, H2O, and N2 provide the most accurate second-order correlation energies to date for these systems.
Dynamics of a glass-forming system: 11B NMR of B2O3
Authors: Maekawa Hideki , Inagaki Yousuke , Shimokawa Shigezo , Yokokawa Toshio
Published: July 2002

The dynamics of the relaxation processes in a glass-forming system, B2O3, was investigated by means of 11B nuclear magnetic resonance (NMR). Using a homemade high temperature NMR probe, we collected NMR data over a wide temperature range from room temperature to 1200 °C. The NMR data were interpreted in terms of a Fourier transform of the Kohlrausch decay function, f(t)=exp[−(t/τc)−b], where the parameter b varied from 0 to 1. The temperature dependence of τc and b in the decay function was estimated by using both the data from a 11B NMR longitudinal relaxation and a line shape measurement at each temperature. Above 800 °C, the NMR data were well simulated by a single exponential decay of the function (i.e., b=1). Below 800 °C, stretched exponential was introduced to the simulation with the b parameters of 0.6 and 0.8. An Arrhenius plot of τc showed a bend at around 600 °C, which indicates the existence of two distinct reorientational processes crossing each other at that temperature. Below 600 °C, an almost linear dependence of the logarithm of τc vs the inverse of temperature with the activation energy of 40 kJ/mol was observed. This process persists below the glass transition temperature. Above 600 °C, the temperature dependence of τc became non-Arrhenius-like and was identical with that of the previous relaxation measurements. The isotropic chemical shift for the B2O3 melt suggests that the network structure constructed from the BO3 triangle is preserved in the whole temperature range.
A model of crystal polarization in β-poly(vinylidene fluoride)
Authors: Carbeck Jeffrey D. , Lacks Daniel J. , Rutledge Gregory C.
Published: July 2002

A model of the crystal polarization of β-poly(vinylidene fluoride) utilizing an atomic potential energy function based on the shell model of electronic polarization is developed. Lattice constants, crystal polarization, and dielectric constants at finite temperatures are determined through minimization of the Gibbs free energy calculated using consistent quasi-harmonic lattice dynamics. Molecular dynamics is used to include the effects of thermal oscillations of the dipoles. We find that in going from a single chain in vacuum to a chain packed in the crystal the repeat unit dipole increases by approximately 50% or 0.9 debye. Increasing temperature results in a decrease in polarization due to: (i) an increase in the unit cell volume, (ii) a decrease in the local electric field, and (iii) an increase in the magnitude of dipole oscillations. It is found that the dipole oscillation is consistent with the excitation of a single rotational lattice mode.
Vibrational transitions of coupled stretching and bending overtones in chloroform
Authors: Beckmann Kai , Gerhards Markus , Kleist Einhard , Bettermann Hans
Published: July 2002

The intensities and frequencies of Fermi-coupled stretching and bending overtone transitions in CHCl3 were calculated by means of a variational method. Symmetrized two-dimensional ab initio potential and dipole moment surfaces were determined at the MP2 level using the 6-31G** basis set. The Hamiltonian for the CH- stretching motion and the simultaneously excited twofold degenerate CH- bending vibration is expressed most easily in cylindrical coordinates. Absorption intensities up to the Δv=7 CH-stretching overtone above 16 300 cm−1 are calculated and are compared to former experimental values and theoretical results. New quantitative intracavity measurements for the N=6 polyad are presented. Relative errors between absolute experimental intensities and the calculated values are less than 30%.
Induced infrared absorption of oxygen physisorbed on NaCl films
Authors: Dai David J.
Published: July 2002

The fundamental induced infrared absorption of O2 physisorbed on annealed NaCl films is observed for the first time. Analysis of spectroscopic and thermodynamic data is consistent with molecules adsorption on defect sites and on Na+, Cl−, and the center of lattice square of smooth NaCl(100) faces of the film crystallites. Isotherms of O2 on three smooth-face surface sites at 58 and 60 K are reported with isosteric heats of adsorption of −9.0±1.0, −7.1±1.0, and −7.1±1.0 kJ/mol, respectively. The infrared bands of molecules adsorbed on NaCl films are close to the gas phase fundamental vibrational frequency.
Coupled Hartree–Fock calculations of molecular magnetic properties annihilating the transverse paramagnetic current density
Authors: Zanasi Riccardo
Published: July 2002

The reliability of the continuous transformations of origin of the current density method, which makes the transverse paramagnetic current vanish (CTOCD-PZ), for the prediction of nearly gauge-origin independent molecular magnetic susceptibility and gauge-origin independent nuclear magnetic shielding, is proved on the basis of a fairly large number of calculations. It is shown that, within the computational scheme provided by the coupled Hartree–Fock perturbation theory (CHF), convergence towards the presumed Hartree–Fock limit, for magnetic susceptibility and proton magnetic shielding, is systematically reached using basis sets which are smaller than those required by conventional common origin and CTOCD-DZ techniques. For second-row nuclear magnetic shieldings a variant of the CTOCD-PZ method, which shifts the origin of the current towards the nearest nucleus for points close to nuclei, as suggested originally by Keith and Bader with the CSDGT method [T. A. Keith and R. F. W. Bader, Chem. Phys. Lett. 210, 223 (1993)], gives likewise good results with affordable basis sets.
Second-order recoupling of chemical-shielding and dipolar-coupling tensors under spin decoupling in solid-state NMR
Authors: Ernst Matthias , Bush Seth , Kolbert Andrew C. , Pines Alexander
Published: July 2002

The source of the residual line broadening in continuous-wave (cw) decoupled spectra under magic-angle sample spinning conditions is reexamined. It is shown that an important contribution to the line broadening comes from a second-order recoupling of the heteronuclear dipolar-coupling tensor and the chemical-shielding tensor of the irradiated spin. Such an interference between the two tensors leads to a sum of a zeroth-rank, a second-rank, and a fourth-rank tensor component in the Hamiltonian. The zeroth-rank and the fourth-rank tensor components are not averaged out under magic-angle sample spinning (MAS) conditions, requiring the use of higher-order averaging such as double rotation (DOR) for obtaining narrow lines. This broadening is distinctly different from off-resonance decoupling effects which transform as a second-rank tensor and can be averaged out by MAS. The properties of this second-order recoupling as a source of structural information are explored, and the conditions for removing the broadening in systems with weak homonuclear dipolar-coupling networks are discussed.
Microwave spectrum and molecular structure of the dihydrophosphoryl radical H2PO in the X̃ 2A′ ground electronic state
Authors: Hirao Tsuyoshi , Saito Shuji , Ozeki Hiroyuki
Published: July 2002

The dihydrophosphoryl radical H2PO in the X̃ 2A′ ground electronic state was studied by microwave spectroscopy using a source-modulated spectrometer combined with a free-space cell. The radical was generated by dc-glow discharge in a mixture of PH3 and CO2 and spectral lines of H2PO were observed in the 140–380 GHz region. The spectral pattern, including hyperfine structure, suggests that the radical has a pyramidal structure with Cs symmetry. About 1000 spectral lines of H2P16O and 250 lines of H2P18O were measured for the fine and hyperfine structures of rotational transitions up to N=10−9. Nineteen molecular constants including the hyperfine coupling constants of the phosphorus and hydrogen nuclei were precisely determined by a least-squares fit of the observed lines. Molecular structural parameters were derived from the rotational constants: r(PO)=1.4875(4) Å, r(PH)=1.4287(14) Å, ∠HPO=115.52(10)°, and ∠HPH=102.56(14)° with three standard deviations in parentheses. The PO bond is intermediate between the normal single and double PO bonds. The spin density of unpaired electrons was ascertained from the hyperfine coupling constants for the P and H atoms and is consistent with the molecular structure determined.
Nonlinear optical and vibrational properties of conjugated polyaromatic molecules
Authors: Rumi Mariacristina , Zerbi Giuseppe , Müllen Klaus , Müller G. , Rehahn Matthias
Published: July 2002

Raman spectra of oligo-p-phenylenes, oligorylenes, and oligoacenes of different chain lengths have been obtained in the solid state and in solution. Among the properties studied, particular attention is devoted to frequency and intensity dispersion of the Raman bands with increasing conjugation length and to the vibrational second order hyperpolarizability γ r. The results obtained are compared with those relative to polyenic systems. The behavior of the various classes of molecules studied is in some cases different both in absolute values and trends. This fact is discussed in order to clarify the influence of the topology of the π-electron system on the properties of conjugated materials and to determine whether the presence of aromatic rings in the main chain can confine π electrons and so reduce delocalization. Oligorylenes turn out to be the compounds with the largest vibrational γ r. The results also indicate that absolute Raman intensity shows strong intensity dispersion with conjugation length and can be used as a powerful tool in characterizing conjugated compounds.