bluuebunny/crossref_metadata_2025 huggingface dataset:
Dependence of dielectric relaxation on ac drive in [Pb(Mg1/3Nb2/3)O3](1−x)–(PbTiO3)x single crystals
Authors: Colla Eugene V. , Furman Eugene L. , Gupta Surya M. , Yushin Nikolai K. , Viehland Dwight
Published: July 2002

Dielectric measurements under different ac drive voltages were performed on single crystals of (PbMg1/3Nb2/3O3)(1−x)(PbTiO3)x [(PMN)(1−x)(PT)x] for 0⩽x⩽0.3. A significant increase in permittivity was observed with increasing ac drive for all compositions. The magnitude of increase depended on temperature and measurement frequency. Alternating current drive effects were found to be a low frequency phenomenon, primarily below 106 Hz. Three distinct ac-drive amplitude ranges are believed to exist in which various types of contributions dominate in the dielectric relaxation: (i) a glassy mechanism is dominant at low drives and gives rise to the typical relaxor ferroelectric characteristics in the linear dielectric response, (ii) a kink or boundary vibration type which is dominant at intermediate drives and gives rise to the nonlinearities in the dielectric response of relaxors under moderate ac drives, and (iii) a domain wall motion type which is dominant at higher drives and results in polarization reversal.
Memory effects of exchange coupling in CoO/Ni81Fe19 bilayers
Authors: Gökemeijer N. J. , Chien C. L.
Published: July 2002

By changing the field-cooling procedure, the value and even the sign of the exchange field HE in CoO/Ni81Fe19 bilayers can be greatly altered. The value of HE can be made to vanish at any temperature below TN. We also show that the magnetization of the Ni81Fe19 is the crucial parameter in the field-cooling procedure. Changing the magnetization during field cooling shows that the exchange bias is an accumulative memory effect, dependent on the thermal and field history of the bilayer. The enhanced coercivity which accompanies the exchange bias, depends only on temperature and not on the field-cooling history.
Enhanced electron attachment to Rydberg states in molecular hydrogen volume discharges
Authors: Pinnaduwage L. A. , Ding W. X. , McCorkle D. L. , Lin S. H. , Mebel A. M. , Garscadden A.
Published: July 2002

We review recent studies on negative ion formation and studies in other areas that are relevant to the role of high-Rydberg states of H2 and H3 in hydrogen negative ion sources. Possible mechanisms for the formation of these excited states are discussed, including the formation of long-lived superexcited (core-excited) Rydberg states. Experimental evidence for negative ion formation via electron attachment to core-excited Rydberg states in a glow discharge apparatus is presented. An expression for the dissociative electron attachment rate constant for Rydberg molecules is derived based on electron capture by a Rydberg molecule due to polarization interaction.
Variable behavior of the current exponent in a microscopic nucleation model for electromigration
Authors: Tammaro M. , Setlik B.
Published: July 2002

A microscopic nucleation model for electromigration on a lattice is proposed. The solution of the master equations for site occupancy produces a variable current exponent with 1<n<2. These results disprove previous claims that n=2 in nucleation models. We show that n=2 is the limiting behavior of all nucleation models and argue that physical regimes exist where the deviation from n=2 is significant.
An rf sustained argon and copper plasma for ionized physical vapor deposition of copper
Authors: Wang W. , Foster J. , Snodgrass T. , Wendt A. E. , Booske J. H.
Published: July 2002

Langmuir probe, optical emission spectroscopy, and biased quartz crystal microbalance measurements were used to investigate an argon and copper plasma used for ionized physical vapor deposition of copper. Copper vapor generated by a magnetron sputter discharge is ionized upon passing through an argon discharge excited by an internal rf induction antenna. Argon plasma characteristics such as electron temperatures Te, plasma densities ne, and plasma and floating potentials Vp and Vf, were studied as a function of argon pressure and rf power. An increase of plasma density versus rf discharge power and argon pressure was observed. The radial profile of plasma density measured by a Langmuir probe reveals a peak ion density at the center of the rf antenna and an increase in the radial ion concentration gradient with argon pressure. The ratios of optical emission intensities from Cu+ ion and Cu neutral lines increase with rf discharge power and argon pressure. The biased quartz crystal microbalance measurements show an increase of both Cu+ ion flux and the ratio of Cu+ ion to Cu neutral fluxes with rf power and argon pressure; however, they also show a decrease of total Cu flux with increasing argon pressure.
Optical properties of epitaxially grown zinc oxide films on sapphire by pulsed laser deposition
Authors: Sun X. W. , Kwok H. S.
Published: July 2002

ZnO thin films were epitaxially grown on c-sapphire substrates by pulsed laser deposition at substrate temperatures of 500–800 °C. The crystal structure of ZnO films follow the epitaxial relationship of (0001)ZnO∥(0001)Al2O3(101̄0)ZnO∥(112̄0)Al2O3. Both room temperature and cryogenic temperature photoluminescence showed a remarkable band-edge transition, and clear excitonic structures could be seen at cryogenic temperature. The optical refractive index was measured in the range of 375–900 nm by varying angle spectroscopic ellipsometry. The simulation was carried out using a Sellmeier equation.
Crystallization behavior of sputter-deposited amorphous Ge2Sb2Te5 thin films
Authors: Jeong Tae H. , Kim Myong R. , Seo Hun , Kim Sang J. , Kim Sang Y.
Published: July 2002

The crystallization behavior of amorphous Ge2Sb2Te5 thin films were investigated by using differential scanning calorimetry, x-ray diffraction, and optical reflectivity measurements. The analysis of in situ ellipsometry isotherm Ge2Sb2Te5 films based on the Johnson–Mehl–Avrami equation revealed that the crystallization process near 150 °C was a two-step process. In this alloy thin film, the kinetic exponents in the Johnson–Mehl–Avrami equation were about 4.4 for the first stage and 1.1 for the second stage. A kinetic model based on the cascaded crystallization is proposed. The proposed model showed good agreement with the experimental results obtained by transmission electron microscopy and x-ray diffraction.
Crystallographically engineered BaTiO3 single crystals for high-performance piezoelectrics
Authors: Park Seung-Eek , Wada Satoshi , Cross L. E. , Shrout Thomas R.
Published: July 2002

Dielectric and piezoelectric properties of BaTiO3 single crystals polarized along the 〈001〉 crystallographic axes were investigated as a function of temperature and dc bias. Electromechanical coupling (k33)∼85% and piezoelectric coefficients (d33)∼500 pC/N, better or comparable to those of lead-based Pb(Zr, Ti)O3 (PZT), were found from 〈001〉-oriented orthorhombic crystals at 0 °C, as a result of crystallographic engineering. A rhombohedral BaTiO3 crystal polarized along 〈001〉 also exhibited enhanced piezoelectric performance, i.e., k33∼79% and d33∼400 pC/N at −90 °C, superior to PZTs at the same temperature. It was found that the crystal structure determined the (in)stability of the engineered domain state in BaTiO3 single crystals. Rhombohedral (3m) crystals at −100 °C exhibited a stable domain configuration, whereas depoling occurred in crystals in the adjacent orthorhombic phase upon removal of the E field.
Analysis of average thermal stresses in passivated metal interconnects
Authors: Wikström A. , Gudmundson P. , Suresh S.
Published: July 2002

Volume-averaged thermal stresses in passivated metal interconnects on Si substrates are derived for situations where the thickness to width ratio of the interconnect lines is “small” or “large.” The analysis provides different components of volume-averaged stresses for the most general case of thermal and elastic anisotropy in the passivation layer, the interconnect line, and the substrate. It is shown that the theoretical predictions, particularly those for the hydrostatic stresses, are in agreement with detailed finite element calculations for a wide range of line and passivation geometries of practical interest. The theoretical predictions of average hydrostatic stresses are also found to be in reasonable agreement with available experimental results for thermal stresses derived from x-ray diffraction measurements on passivated Cu lines. The present theoretical results are shown to be far more accurate than prior stress analyses for periodic passivated lines based on Eshelby’s theory of inclusions.
50 T pulsed magnetic fields in microcoils
Authors: Mackay K. , Bonfim M. , Givord D. , Fontaine A.
Published: July 2002

In this article we present a system based on microcoils capable of generating pulsed magnetic fields up to 50 T. We discuss the current generation and the measurement techniques currently used. We have measured the Faraday rotation for different paramagnetic materials, glass, gadolinium gallium garnet, Pr gallium garnet, and Si and have used the results to calibrate the magnetic field achieved. We discuss the relative merits of such a system compared to conventional ones, as well as the field characteristics obtained.
Dielectric properties and field-induced phase switching of lead zirconate titanate stannate antiferroelectric thick films on silicon substrates
Authors: Xu Baomin , Ye Yaohong , Cross L. Eric
Published: July 2002

Thick (∼5 μm) films of antiferroelectric compositions in the lead zirconate titanate stannate family of solid state solutions have been fabricated by sol–gel methods on platinum-buffered silicon substrates. Dielectric properties, electric field induced ferroelectric polarization, and associated elastic strain and the temperature dependence of the dielectric response have been explored as a function of composition. Films with high tin content are shown to undergo a diffuse antiferroelectric–paraelectric phase transition with temperature, probably because of compositional inhomogeneity associated with the high tin content. This type of film also demonstrates a diffuse field-induced antiferroelectric–ferroelectric phase switching under high electric field with the appearance of “slim loop” double hysteresis, which can be attributed to the compositional heterogeneity and the high level of tensile stress in the film because of the thermal mismatch between the film and substrate. On the other hand, the film with high zirconium and low tin content demonstrates a typical antiferroelectric–paraelectric phase change with a sharp peak in dielectric constant on increasing temperature and a typical antiferroelectric–ferroelectric phase switching under high electric field with the appearance of “square loop” double hysteresis. Corresponding to the characteristic of polarization-field hysteresis the film with high tin content possesses a gradual increase of phase switching elastic strain with applied field, reaching a maximum strain level ∼0.38% and suitable for analogue actuation. The film with high zirconium content possesses a sharp jump of phase switching strain with applied field and reaches a maximum strain level ∼0.48%, attractive for high strain digital applications.
Energy influx from an rf plasma to a substrate during plasma processing
Authors: Kersten H. , Stoffels E. , Stoffels W. W. , Otte M. , Csambal C. , Deutsch H. , Hippler R.
Published: July 2002

The energy influx delivered by an rf plasma to a metal substrate has been studied by a calorimetric method with a thermal probe. By changing the substrate voltage, the influence of the kinetic energy of the charge carriers to the thermal power could be determined. The measured energy influx for an argon plasma can be explained mainly by ions, electrons, and their recombination. In the case of an oxygen plasma, where the energy influx is under comparable conditions about 50% higher, also other transfer mechanisms such as surface-aided atom association and relaxation of rovibrational states have to be taken into consideration.
Micromagnetic simulation of thermal effect in longitudinal thin film disk media
Authors: Peng Qingzhi
Published: July 2002

Micromagnetic simulation is utilized to study thermal effects in longitudinal thin film media. A Monte Carlo method is adopted to compute the thermally assisted switching process in a large time scale (from seconds to years). Throughout the study, the ratio of film thickness to grain size is fixed to be 1.67 as grain size varies between 8 and 15 nm. The effective media bulk coercivity is simulated by assuming an applied field duration of 1 s. The thermal decay of both the bulk remanent magnetization and recorded transitions is studied. The thermal effects are compared for media with different intergranular exchange couplings. Media with larger exchange coupling is less susceptible to thermal effect.
Non-Arrhenius temperature dependence of magnetic after effect
Authors: Swartzendruber Lydon J. , Rugkwamsook Pattana , Bennett Lawrence H. , Della Torre Edward
Published: July 2002

The rate of magnetization change (magnetic after effect) which occurs after the magnetic field applied to a magnetic material is switched suddenly to a new value, is generally assumed to increase when the temperature is increased. Deviation from this temperature behavior, which has been observed in several cases, has been ascribed to changes in pertinent magnetic properties, such as magnetocrystalline anisotropy, activation volume, coercive field, etc., with temperature. We report here on the magnetic after effect temperature dependence of a magnetic recording material with magnetic properties that change very little with temperature over a wide temperature range but which, nevertheless, deviates considerably from a monotonic increase in decay rate with increasing temperature. We postulate that this behavior is explained by replacing the Maxwell–Boltzmann statistics by the Bose–Einstein statistics appropriate for the magnon energy distribution.
Effect of lamination period and deposition angle on FeAlN–Al2O3 multilayers
Authors: McNeill K. A. , Bell A. M. , O’Kane W. J. , McLaughlin T. K. , Maass W.
Published: July 2002

Magnetic properties and microstructure have been investigated as a function of lamination period in 1-μm-thick FeAlN/Al2O3 multilayers. It has been seen that coercivity is weakly dependent on FeAlN thickness in the lamination, consistent with a change in morphology. Within this thickness range, Hk is reduced and dispersion increases accordingly. As the deposition angle is increased from 0° to 40°, there is little change in coercivity, while Hk increases, from 4 to 8 Oe. Saturation magnetostriction was found to exhibit a strong linear dependence (from +1×10−6 to −5×10−6) over the thickness range studied, with weak dependence on the deposition angle. High frequency permeability degraded with increasing FeAlN thickness, which is believed to reflect the reduced anisotropy in these samples. Microstructural analysis via x-ray diffraction and transmission electron microscopy has indicated that the films contain predominately α-Fe, and that Al2O3 spacer layers of 20 Å are sufficient to interrupt columnar grain growth. Furthermore, columnar grains formed in samples with thick FeAlN layers do not exhibit canting when grown on tilted substrates.
Sputter deposition of 45% Ni-55% Fe for high moment top pole applications
Authors: Rook K. , Gupta S. , Hieronymi R.
Published: July 2002

We present the properties of dc magnetron sputter-deposited 45% Ni-55% Fe. Optimal magnetic properties obtained in 1.4 μm films were: 4πMs∼16.4 kG, Hce∼3.7 Oe, Hch∼0.8 Oe, Hk∼9.6 Oe, at a deposition rate of 29 Å/s. A linear designed experiment revealed that: higher gas pressure significantly lowers both Hce and Hk; higher (rf) substrate bias increases α90 and Hch, while decreasing Hk; and higher substrate magnetic field worsens Hch and α90. Augmentation of the experimental matrix and analysis of higher order terms implied that a combination of intermediate pressure and small bias provides optimal magnetic properties. From x-ray diffraction, the 45–55 NiFe is fcc, with lattice parameter 3.578–3.584 Å, in fair agreement with the bulk value of ∼3.585 Å. Grain sizes in the range 150–180 Å were estimated. Analysis of the (111): (200) peak intensity ratio showed that higher substrate bias or lower pressure both significantly enhance the (111) texture. Correlation was observed between the magnetics and the crystalline texture; less [111] texture gives lower coercivity. Data on 2000 Å films for seedlayer applications are also presented.
Soft magnetic properties of [FeTaN/N-rich FeTaN]n multilayer films
Authors: Kim Choong-sik , Shin Dong-hoon , Jeong Jong-han , Ahn Dong-hoon , Nam Seoung-eui , Kim Hyoung-june
Published: July 2002

We investigated the soft magnetic properties of FeTaN multilayers which consist of Fe78.8Ta8.5N12.7 films with a periodic insertion of thin (∼100 Å) N-rich FeTaN layers. The FeTaN multilayer leads to an enhancement of magnetic anisotropy compared to the single layer. This enhancement can be explained in terms of increased tensile stresses in the multilayer after heat treatments. The degree of anisotropy is found to depend on the stacking number of [FeTaN/N-rich FeTaN]n layers. The stress-induced anisotropy ultimately improves the high-frequency characteristics through restricting ferromagnetic resonance. The optimum multilayer exhibits the following good soft magnetic properties: permeability of 2500, coercivity of 0.1 Oe, magnetic anisotropy field of 6 Oe, and stable frequency response up to 300 MHz.
Electron correlation effects at the Gd(0001) surface
Authors: Shick A. B. , Pickett W. E. , Fadley C. S.
Published: July 2002

We have performed full-potential linearized augmented plane wave calculations of the Gd(0001) surface using the local density approximation (LDA) together with the Hubbard U (LDA+U) total energy functional. The use of LDA+U instead of LDA total energy calculations leads to a ferromagnetic ground state for both bulk Gd and the Gd surface, in agreement with experimental observation. The calculated downward shift of 4f eigenvalues for the Gd surface is in agreement with experimentally observed binding energies. Surface strain relaxation leads to a 90% enhancement of the interlayer surface-to-bulk effective exchange coupling. Application of a Landau–Ginzburg-type theory yields a 30% enhancement of the Curie temperature at the surface, in very good agreement with the experiment.
Interface stress in Au/Ni multilayers
Authors: Schweitz K. O. , Bo/ttiger J. , Chevallier J. , Feidenhans’l R. , Nielsen M. M. , Rasmussen F. B.
Published: July 2002

The effect of intermixing on the apparent interface stress is studied in 〈111〉-textured dc-magnetron sputtered Au/Ni multilayers by use of two methods commonly used for determining interface stress. The method using profilometry and in-plane x-ray diffraction does not take intermixing into account and yields an apparent interface stress of −8.46±0.99 J m−2. However, observed discrepancies between model calculations and measured high-angle x-ray diffractograms indicate intermixing, and by use of the profilometry and sin2 ψ method the real interface stress value of −2.69±0.43 J m−2 is found. This method also reveals a significant and systematic change of the stress-free lattice parameter of both constituents as a function of modulation period which is shown to account for the difference between the two findings. The method using in-plane diffraction is thus shown to be inapplicable to interface stress determinations in systems exhibiting a modulation period-dependent stress-free lattice parameter. Finally, a deviation of the interface stress in the Au/Ni sample with the smallest modulation period as compared to specimens with larger bilayer lengths is observed to be concurrent with a significant decrease in the interface roughness measured by x-ray reflectivity, which suggests that the deviation is of geometrical origin.
Electrical and photovoltaic properties of Cu-doped p-GaSe/n-InSe heterojunction
Authors: Shigetomi S. , Ikari T.
Published: July 2002

GaSe(Cu)/InSe heterojunctions have been formed by bringing the cleavage surface of undoped n-InSe and Cu-doped p-GaSe into direct contact. Transport and phototransport properties are studied by the measurements of capacitance–voltage, current–voltage, and the spectral response of short-circuit current. Moreover, the efficiency parameters under illumination are estimated by using the open-circuit voltage and short-circuit current. These characteristics of GaSe(Cu)/InSe heterojunctions are compared with those of GaSe(Un)/InSe heterojunctions fabricated by undoped p-GaSe and n-InSe. The series resistance of GaSe(Cu)/InSe heterojunctions is found, the value of which is about 103 times lower than the corresponding value of GaSe(Un)/InSe heterojunctions. A short-circuit current density of 9.0 mA/cm2 and an open-circuit voltage of 0.42 V on GaSe(Cu)/InSe heterojunctions are obtained under illumination of 120 mW/cm2 of a halogen lamp. The short-circuit current of GaSe(Cu)/InSe heterojunctions is about one order of magnitude lager than that of GaSe(Un)/InSe heterojunctions. These experimental results indicate that the low-resistivity of Cu-doped GaSe is effective for the electrical and photovoltaic properties of GaSe/InSe heterojunctions.
Brownian dynamics of polymer chains in membranes
Authors: Muthukumar M.
Published: July 2002

We present a theory of the Brownian dynamics of polymer chains embedded in two-dimensional hydrodynamic membranes. The excluded volume effect and both the intra- and interchain hydrodynamic interactions are included. We show that, in the dilute solution limit, the mean square displacement of a labeled monomer obeys the familiar diffusive law just as the center of mass of the whole chain. Furthermore, our calculations show that the interchain hydrodynamic interaction does not induce the screening of the hydrodynamic interaction to the leading order in concentration and that the mode dependence of the concentration-dependent relaxation times arises solely from the excluded volume effect. However, at very high concentrations, the square of the hydrodynamic screening length decays exponentially with concentration in membranes in contrast to an algebraic decay under isotropic conditions. In the screened regime, the effective viscosity of the membrane depends exponentially on the concentration demonstrating the absence of free-draining behavior at any concentration. These predicted dynamical properties of chains trapped in a membrane are in distinct contrast with the laws in three dimensions.
Diffusion in potentials. A method for solving the Smoluchowski equation
Authors: Benesi Alan J.
Published: July 2002

A method for solving the Smoluchowski equation for arbitrary potential energy surfaces in any number of dimensions is presented. The method is based on the postulate that the time evolution of the probability density at sufficiently short times and for sufficiently small space increments is separable into a diffusion component and a deterministic component. The method is verified by observing that a known analytic solution to the Smoluchowski equation for the case of a particle diffusing isotropically in a parabolic one-dimensional potential can be duplicated to any desired degree of accuracy by choosing small enough space and time increments. The method is also applied to the case of a single particle diffusing isotropically in a periodic potential, and to the case of two-dimensional conformational diffusion about the glycosidic torsion angles of maltose oligomers within the torsion angle conformational potential.
Vertical and adiabatic ionization energies of NH−4 isomers via electron propagator theory and many body perturbation theory calculations with large basis sets
Authors: Ortiz J. V.
Published: July 2002

Electron propagator theory (EPT) is applied to calculating vertical ionization energies of NH−4 in two stable geometries. The first is a complex of a hydride coordinated to one of the hydrogens of the solvent ammonia molecule, while the other isomer has tetrahedral symmetry. Both structures have been optimized using second order many body perturbation theory with a 6-311++G(d) basis. The former structure is predicted to be 0.42 eV more stable than the latter. A series of third order EPT quasiparticle calculations is carried out to demonstrate basis set saturation. The best EPT calculations give vertical ionization energies of 1.20 eV for the solvated hydride structure and 0.42 eV for the tetrahedral isomer. Both of these calculations are within 0.1 eV of anion photoelectron spectroscopy peaks and suggest that the tetrahedral isomer is responsible for one of these features. After removing an electron from the solvate structure, the complex settles into a shallow van der Waals minimum with a hydrogen atom weakly bound to the ammonia molecule. Calculations on the tetrahedral neutral indicate that there is little relaxation energy along the neutral final state potential energy surface, explaining the sharpness of the observed photoelectron peak.
Charge transfer and structured vibrational distributions in H++CH4 low-energy collisions
Authors: Chiu Ying-Nan , Friedrich Bretislav , Maring Wolfram , Niedner Gereon , Noll Martin , Toennies J. Peter
Published: July 2002

Inelastic and charge transfer collisions of protons with methane molecules have been investigated in a perpendicular-plane crossed beam experiment via the detection of the scattered protons and H atoms, respectively. Time-of-flight analysis of the protons and H atoms at scattering angles 0°≤θ≤10° and collision energies 10≤E≤30 eV provided information on internal energy distributions of the CH4 and CH+4 products. Excitation of the n(ν1 ,ν3) +m (ν2 ,ν4) type vibrations, with n,m=0, 1, 2,⋅⋅⋅was found to be the most probable assignment of the observed structured energy distributions of CH4 (1 A1 ) at θ≤4°. At θ>4°, the energy transfer increases steeply up to the dissociation limit while the vibrational structure was no longer resolved. In the case of charge transfer, the observed narrow internal energy distributions corresponding to a most probable average internal energy of CH+4 of about 0.95 eV was centered at the recombination energy of the proton indicative of quasiresonant charge transfer. In addition, fragmentation of CH+4 formed in charge transfer collisions of H+ with CH4 was investigated in an independent experiment using mass spectrometric analysis to identify the individual fragment species. The relative intensities of the parent and fragment ions (i.e., of CH+4, CH+3, and CH+2) were found to be in good agreement with the known values of the appearance potentials of the fragment ions and the distribution of the CH+4 internal energy as obtained from the differential cross sections. A mechanism is proposed to explain the experimental results based on vibronic symmetry correlation theory. This mechanism deals with vibronic interactions in the compound quasimolecule CH+5 and explains the origin of the unexpected excitation of infrared inactive modes [e.g., ν2 (E)] of the tetrahedral methane. The effects of Jahn-Teller distortions of the CH+4 charge transfer product are also discussed.
Desorption kinetics of hydrogen and deuterium from Si(111) 7×7 studied using laser-induced thermal desorption
Authors: Koehler B. G. , Mak C. H. , Arthur D. A. , Coon P. A. , George S. M.
Published: July 2002

The desorption of hydrogen and deuterium from Si(111) 7×7 was studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) mass spectrometry. Isothermal LITD measurements enabled the surface coverage of hydrogen and deuterium to be monitored as a function of time. These isothermal results were used to obtain accurate desorption kinetics of hydrogen and deuterium from the high-temperature β1 state on Si(111) 7×7. The desorption of hydrogen displayed second-order kinetics with an activation barrier of 61±4 kcal/mol and a preexponential factor of 1.2×101±1.3 cm2/s. Likewise, the desorption kinetics of deuterium displayed second-order kinetics with an activation barrier of 59±3 kcal/mol and a preexponential factor of 2.8×100±1.0 cm2/s. These desorption activation barriers yield upper limits of 82.6 and 81.6 kcal/mol for the Si–H and Si–D chemical bond energies, respectively, on Si(111) 7×7. TPD results obtained as a function of hydrogen coverage were consistent with second-order desorption kinetics. The TPD experiments were also used to measure hydrogen coverages and to calibrate the LITD signals. In addition, LITD techniques were used to study the surface diffusion of hydrogen on Si(111) 7×7. No evidence of significant hydrogen surface mobility (D≤10−9 cm2/s) was found for surface temperatures as high as 740 K.
High resolution frequency selective photochemistry of phycobilisomes at cryogenic temperatures
Authors: Köhler W. , Friedrich J. , Fischer R. , Scheer H.
Published: July 2002

Highly frequency selective photochemistry at cryogenic temperatures is used to gain information on the excited state dynamics of large biomolecular aggregates, the phycobilisomes from the blue–green alga (cyanobacterium) Masticogladus laminosus. In particular, we show that in spite of the well organized structure of these aggregates disorder on a microscopic level dominates the optical spectra. The hole burning reaction in the resonantly excited chromophores is most probably due to a conformational change in the neighborhood of the chromophore. From the widths of the holes energy transfer times between different pigments on the order of 16 ps are determined. These transfer times are independent of the excitation energy.
Vibrational predissociation of He–I2*(v)–Ne: An approximate quantal study
Authors: Villarreal Pablo , Varade Andrés , Delgado-Barrio Gerardo
Published: July 2002

The vibrational predissociation (VP) of the He–I2(B 3Π0+u,v)–Ne complex is studied in the range of initial vibrational excitations 25≤v≤35. The rare gas atoms are restricted to move on a perpendicular plane to the I2 axis. A simple addition of pairwise Morse atom–atom interactions is used to describe the potential energy surface. The breaking up of the I2–Ne bond is found to be the dominant dissociation channel for v≤29. However, due to the anharmonicity of the I2 stretch, the situation for v>29 is just the opposite and the He atom escapes first.
Determination of atomic point charges and point dipoles from the Cartesian derivatives of the molecular dipole moment and second moments, and from energy second derivatives of planar dimers. I. Theory
Authors: Dinur U. , Hagler A. T.
Published: July 2002

The asymptotic pairwise interaction between nonbonded atoms in planar dimers is analyzed. It is pointed out that the second derivative of the energy with respect to the perpendicular coordinates of the two nonbonded atoms enables to extract the parameters of the pairwise electrostatic interaction from the total energy. In the limit of large intermolecular separations the coupling second derivatives of the energy are shown to be determined by the atomic polar and quadrupolar tensors of the isolated monomers. It is further shown that the dipole moment of planar molecules can be expressed in terms of the out of plane Cartesian derivatives of its perpendicular component 〈z〉, and that the molecular quadrupole moments can be expressed in terms of these derivatives and the out of plane Cartesian derivatives of the second moments 〈xz〉 and 〈yz〉. These derivatives form a unique and well defined decomposition of the molecular multipoles into atomic quantities that can be formally interpreted as atomic point charges and point dipoles. These atomic point charges and point dipoles completely determine the asymptotic total and pairwise interactions in planar and rigid dimers.
Photofragmentation of the Ne⋅⋅⋅ICl complex: A three-dimensional quantum mechanical study
Authors: Roncero O. , Beswick J. A. , Halberstadt N. , Villarreal P. , Delgado-Barrio G.
Published: July 2002

Converged three-dimensional quantum mechanical calculations for photofragmentation of the Ne⋅⋅ICl van der Waals molecule in the energy region of the electronically excited B(3∏0+) state of ICl are presented and compared with experiments. Lifetimes and final state distributions for the ICl fragments were determined for vibrational predissociation from the lowest van der Waals level in the B(v′=2) channel. Good agreement between theory and experiment was achieved using a sum of atom–atom pairwise potentials. This potential energy surface predicts the equilibrium geometry of the complex to be bent at 140° with the Ne atom towards the Cl end of ICl. The diabatic vibrational golden rule (DVGR) approximation, as well as the rotational infinite order sudden approximation (RIOSA), have been tested again the full 3D calculations. Analysis of the quasibound wave function reveals that the highly inverted rotational distribution of the ICl fragments observed in the experiment, is not due to zero-point bending motion. It is more likely to be due to a rotational rainbow effect enhanced by the favorable initial geometry of the complex. The effect of the excitation of the bending van der Waals mode in the complex has also been studied. As compared with the lowest level, a longer lifetime and a different rotational distribution of the fragments is predicted. The results presented in this work not only elucidates many dynamical aspects of vibrational predissociation for the Ne⋅⋅⋅ICl complex, but also provide benchmark data for the study of other theoretical methods and approximations.
Photodissociation of F2 and mobility of F atoms in crystalline argon
Authors: Feld J. , Kunttu H. , Apkarian V. A.
Published: July 2002

Experimental studies on the photodissociation of F2 in 12 K crystalline argon solids are reported, and compared with molecular dynamics (MD) simulations of the same system. At excess energies above 2.5 eV,the dissociation probability is near unity, in agreement with theory. At the longest experimentally accessible wavelength for photodissociation, 450 nm, which corresponds to a dissociation excess energy of 1.16 eV, the permanent dissociation probability is reduced to 0.05. This is an order of magnitude smaller than that predicted by the simulations. Possible sources for this discrepancy are discussed. Long range migration of F atoms, predicted by MD simulations, is demonstrated to occur both upon photodissociation of F2, and upon relaxation of argon fluorides. In the latter case, it is shown that with the initial impulse, the F atoms migrate on average a length of ∼70 Å in the lattice. The thermal recombination of F atoms in both solid Ar and Kr are also characterized. The experimental studies are conducted in free standing crystals of argon, by following the photogeneration of F atoms. This is accomplished by monitoring the argon fluoride emissions upon charge transfer excitation over the ArF(B←X) excitation at 193 nm. Two distinct charge transfer states contribute to the observed emission. The first, with an emission maximum near 315 nm and a lifetime of 180(±10) ns, is assigned to Ar2F. The second with a maximum at 355 nm and a lifetime of 700(±50) ns is assigned to Ar6F stabilized by trapping of F atoms in interstitial sites. Pump–probe measurements indicate a strong transient absorption from both of these states, precluding the possibility of observing any stimulated emission over these exciplexic transitions in the solid state.
Ionization potentials and radii of atoms and ions of element 104 (unnilquadium) and of hafnium (2+) derived from multiconfiguration Dirac–Fock calculations
Authors: Johnson Elijah , Fricke B. , Keller O. L. , Nestor C. W. , Tucker T. C.
Published: July 2002

Multiconfiguration relativistic Dirac–Fock (MCDF) values have been computed for the first four ionization potentials (IPs) of element 104 (unnilquadium) and of the other group 4 elements (Ti, Zr, and Hf). Factors were calculated that allowed correction of the systematic errors between the MCDF IPs and the experimental IPs. Single ‘‘experimental’’ IPs evaluated in eV (to ±0.1 eV) for element 104 are: [104(0),6.5]; [104(1+),14.8]; [104(2+),23.8]; [104(3+),31.9]. Multiple experimental IPs evaluated in eV for element 104 are: [(0−2+),21.2±0.2]; [(0−3+),45.1±0.2]; [(0−4+),76.8±0.3]. Our MCDF results track 11 of the 12 experimental single IPs studied for group 4 atoms and ions. The exception is Hf(2+). We submit our calculated IP of 22.4±0.2 eV as much more accurate than the value of 23.3 eV derived from experiment.
Analytic evaluation of energy gradients for the singles and doubles coupled cluster method including perturbative triple excitations: Theory and applications to FOOF and Cr2
Authors: Scuseria Gustavo E.
Published: July 2002

The analytic energy gradient for the singles and doubles coupled cluster method including a perturbative correction due to triple excitations [CCSD(T)] is formulated and computationally implemented. Encouraged by the recent success in reproducing the experimental equilibrium structure and vibrational frequencies of ozone, the new CCSD(T) gradient method is tested with two other ‘‘difficult’’ quantum chemistry problems: FOOF and Cr2. With the largest basis set employed in this work [triple zeta plus two sets of polarization functions (TZ2Pf)] at the CCSD(T) level of theory, the predictions for the O–O and O–F bond lengths in FOOF are 1.218 and 1.589 Å, respectively. These figures are in good agreement with the experimental values 1.216 and 1.575 Å. Based on CCSD calculations with even larger basis sets, it is concluded that the error of 0.014 Å in the O–F bond length at the TZ2Pf/CCSD(T) level of theory is due to the remaining basis set deficiency. On the other hand, the CCSD(T) prediction for the equilibrium bond length of Cr2 (1.604 Å), obtained with a large (10s8p3d2f1g) basis set capable of achieving the Hartree–Fock limit, is still 0.075 Å shorter than experiment, clearly indicating the importance of higher than connected triple excitations in a single-reference treatment of this particular problem.
Analytic gradients for coupled-cluster energies that include noniterative connected triple excitations: Application to c
i
s- and t
r
a
n
s-HONO
Authors: Lee Timothy J. , Rendell Alistair P.
Published: July 2002

An efficient formulation of the analytic energy gradient for the single and double excitation coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations, denoted CCSD(T), is presented. The formulation presented here has a smaller computational cost than any previous formulation, and the algebraic manipulations that lead to the additional savings may be applied generally to the analytic gradient of Mo/ller–Plesset perturbation theory energies. The energy contribution from connected triple excitations scales as n3on4v+n4on3v, and the additional work needed for the gradient scales as 2n3on4v+2n4on3v, where no is the number of doubly occupied orbitals and nv is the number of unoccupied orbitals. The new formulation has been implemented in an efficient set of programs that utilize highly vectorized algorithms and has been used to investigate the equilibrium structures, harmonic vibrational frequencies, infrared intensities, and energy separation of cis- and trans-HONO.
Spectroscopic studies of jet-cooled AgAu and Au2
Authors: Bishea Gregory A. , Morse Michael D.
Published: July 2002

Resonant two-photon ionization spectroscopy has been applied to the jet-cooled Au2 and AgAu molecules. Three new band systems of Au2 and two new systems of the poorly characterized AgAu molecule have been observed. Excited state lifetime measurements have been made, and assignments of the excited states are suggested. For Au2 the a 3Σ+1u←X 1Σ+g transition has been detected, and vibrational levels of the a 3Σ+1u state have been observed up to v′=33, which lies only 120 cm−1 below the convergence limit of the system. This allows a precise confirmation of previous high temperature Knudsen effusion measurements of the bond strength of Au2 as D00(Au2)=2.290±0.008 eV. In addition, the excited states of Au2 of 0+u symmetry are shown to have significantly shorter fluorescence lifetimes than the 1u states, and this is explained as resulting from an admixture of Au+Au− ion pair character in these Ω′=0+u states. The ionization potential of Au2 has been bracketed as IP(Au2)=9.20±0.21 eV, which may be combined with the values of D00(Au2) and IP(Au) to provide the dissociation energy of the Au+2 ion as D00(Au+2)=2.32±0.21 eV. Detailed comparisons with theoretical results are made for Au2, and assignments of the A and B states of AgAu to Ω′=0+ and Ω′=1, respectively, are proposed.
An electronic mechanism for electron pairing in antiferromagnetic bridged mixed-valence systems
Authors: Ondrechen Mary Jo , Gozashti Saeed , Wu Xu Monica
Published: July 2002

One- and two-dimensional bridged mixed-valence systems consisting of metal atoms M and polarizable ligands L are discussed. It is argued that when the valence state of the metal atom is changed, the energy gap and the effective resonance integral between M and L orbitals are also changed. The former is due to electron–electron repulsion and the latter to ligand polarization. A Hubbard model Hamiltonian with an occupation-dependent resonance integral is adopted to describe these systems. Approximate solutions are obtained by the Hartree–Fock method. One-dimensional chains and two-dimensional nets are studied, in which metal ions are joined together by bridging ligands. The fully oxidized system is taken to have one electron per metal orbital, with the ligand orbitals vacant. The electron spin vectors are aligned in an antiferromagnetic fashion. Upon the addition of extra electrons, low-lying vacancies can be formed under certain conditions, which can lead to interesting p-type conductive properties in one or two dimensions. In the one-dimensional system, the formation of such a low-lying vacancy requires that the regional orbitals which contain the added electron are delocalized over seven units or more. (If the bridging ligands are monatomic and the metal–ligand bond distance is about 2 Å, then the delocalization length is about 12 Å.)

This phenomenon is unlikely to be realizable in one dimension for the present type of system since it only occurs in parts of parameter space with an unrealistically small value for the electron–electron repulsion U. However, in two dimensions, low-lying vacancies are sometimes predicted by the present model for reasonable values for U. Furthermore, the minimum delocalization radius required for the generation of low-lying vacancies is somewhat smaller in two dimensions than in one. The relative stability of one isolated versus two paired excess electrons is also explored, although some crude assumptions about the delocalization lengths of two vs one excess electrons had to be made. When a single excess electron is delocalized over three units or more, the total energy for two paired excess electrons may be lower than that for two isolated excess electrons. Possible superconductivity by such an electronic mechanism is discussed.
Optimization of a multideterminant wave function for quantum Monte Carlo: Li2 (X 1Σ+g)
Authors: Sun Zhiwei , Barnett Robert N. , Lester William A.
Published: July 2002

A wave function constructed as a product of a four-determinant function and a symmetric correlation function is employed in Monte Carlo computations of the ground-state energy of Li2 at Re = 5.05 Bohrs. Wave function parameters are determined by a fixed-sample minimization of deviations of the local energy. Although the variational Monte Carlo energy for this function lies, as expected, below that of a similar wave function constructed with a single determinant, the four-determinant function/correlation function wave function gives no improvement in quantum Monte Carlo energy. However, the unoptimized four-determinant function/correlation function wave function does yield an energy in excellent agreement with the estimated exact result. The poorer energy of the optimized function is caused by degradation of the nodal structure during parameter optimization.
Multireference coupled cluster calculations on CH2+
Authors: Ghose Keya Basu , Pal Sourav
Published: July 2002

In this paper we examine the potential energy surface of CH2+ to reinvestigate the controversy surrounding the stability of this dication using the Fock space version of multireference coupled cluster theory.
Mixed-mode oscillations and incomplete homoclinic scenarios to a saddle focus in the indium/thiocyanate electrochemical oscillator
Authors: Koper Marc T. M. , Gaspard Pierre , Sluyters J. H.
Published: July 2002

The complex mixed-mode and chaotic oscillations in the reduction of indium(III) at a hanging mercury drop electrode in thiocyanate solution are studied in experimental detail in the parameter plane. With the aid of a simple model developed previously, we critically discuss the relation of the experimentally observed periodic–chaotic sequences with those predicted by the Shil’nikov theory of homoclinic orbits. It appears helpful to distinguish between two fundamentally different types of mixed-mode behavior: ‘‘type 1’’ related to the incomplete homoclinic scenarios observed in the present system and its model; ‘‘type 2’’ related more directly to the Shil’nikov behavior and the classical Rössler type of reinjection mechanism. The definitions and properties of both types of mixed-mode oscillation are worked out in some detail.
Multiproperty determination of a new N2–Ar intermolecular interaction potential energy surface
Authors: Beneventi L. , Casavecchia P. , Volpi G. G. , Wong C. C. K. , McCourt F. R. W.
Published: July 2002

A new multiproperty potential energy surface for the N2–Ar intermolecular interaction is reported. The present determination is based upon molecular beam total differential and integral scattering data, taken together with the temperature dependence of the interaction second virial coefficient, transport properties, transport property field effects, and relaxation phenomena, such as pressure broadening of the depolarized Rayleigh line and longitudinal nuclear spin relaxation. The primary fit has been made to the beam scattering and virial data, and refinements to the potential parameters thus determined have been made by employing the data available for the gas phase transport and relaxation phenomena. The potential energy surface employed is an empirical Morse–Morse–spline–van der Waals form, in which the potential parameters depend upon the angle between the N2 figure axis and the line joining the centers of mass of N2 and Ar. No N2 stretching dependence has been included in the present determination. Comparison is made between the present potential energy surface and two other previously published N2–Ar potential energy surfaces. The present potential energy surface provides the best overall agreement for all available gas phase data for N2–Ar mixtures, and can thus be recommended for calculations of all properties of such mixtures that depend upon the intermolecular interaction.
Inability of the hypernetted chain integral equation to exhibit a spinodal line
Authors: Belloni Luc
Published: July 2002

The hypernetted chain (HNC) integral equation is solved for a variety of systems including the simple fluid of hard spheres with attractive Yukawa tail, the symmetrical and highly asymmetrical mixtures of charged particles, and the mixtures of hard spheres with positively nonadditive diameters. All these systems exhibit a liquid–gas or fluid–fluid phase separation. We are concerned with the behavior of the HNC solution near the two-phase region in the phase diagram. In all cases, the HNC equation does not have solutions inside a certain region whose boundary line is not a spinodal line. As the boundary is approached, the isothermal compressibility deduced from the compressibility route does not diverge, but tends to finite values. The isotherms and isochores terminate at the boundary line in square root branch points. This behavior is directly correlated to the existence of multiple HNC solutions. Physical and nonphysical solutions coincide on the boundary line. This universal HNC behavior is compared with that of the mean spherical approximation and Percus–Yevick integral equations.