bluuebunny/crossref_metadata_2025 huggingface dataset:
Accurate determination of the temperature and frequency dependent smectic C biaxial permittivity tensor
Authors: Brown Carl V. , Jones J. Cliff
Published: July 2002

The temperature and frequency dependent smectic C bend (B1) and splay (B2) elastic constants and permittivities ε1, ε2, and ε3 have been measured directly for a commercial smectic C liquid crystal mixture. This has been achieved using continuum modelling to fit the ac field dependence of the permittivity in the C2U chevron geometry. The fitting procedure also makes use of measured values for the homeotropic permittivity, the layer tilt angle, and smectic cone angle. This approach removes many of the assumptions which were inherent in previous measurements giving improved accuracy in the results.
Light-induced director-controlled microassembly of dye molecules from a liquid crystal matrix
Authors: Voloschenko D. , Lavrentovich O. D.
Published: July 2002

We report on a light-induced phenomenon in dye-doped liquid crystals (LCs) with the distinctive features of molecular transport and assembly at micron scales. Under single-beam laser irradiation, the dye molecules phase separate from the LC host and assemble onto the cell substrate. Although the intensity of incident light is uniform within the irradiated area, the density of the adsorbed dye is modulated in accord with the director modulation of the LC. The dye molecules form a surface imprint that portrays orientational distortions of the LC host.
Hydriding kinetics of Pd capped YHx switchable mirrors
Authors: van der Molen S. J. , Kerssemakers J. W. J. , Rector J. H. , Koeman N. J. , Dam B. , Griessen R.
Published: July 2002

The kinetics of the metal–insulator transition in polycrystalline, Pd-capped YHx switchable mirrors upon hydrogenation is investigated. Using the accompanying optical transition, we study switching of matrix-like samples with many (∼200) combinations of Pd and Y layer thicknesses. We find that: (i) With increasing Y thickness dY, the switching time τ increases for any constant Pd thickness dPd. (ii) With increasing dPd, there are three regimes. In regime I, it is impossible to switch a device. This can mainly be related to Pd–Y compound formation consuming all Pd within the UHV system, followed by surface oxidation in air. In regimes II and III switching is possible, but only in regime III does Pd form a closed cap layer. The Pd thickness needed for a closed cap layer depends on dY. (iii) An oxide buffer layer hinders Pd–Y interdiffusion, so that a thinner Pd cap layer is needed for switching than in the case without buffer layer. This is interesting for potential applications since it yields a higher optical transmission in the open state of the device.
Evolution of Ge/Si(100) islands: Island size and temperature dependence
Authors: Chaparro S. A. , Zhang Y. , Drucker Jeff , Chandrasekhar D. , Smith David J.
Published: July 2002

Atomic force microscope (AFM) imaging and cross-sectional analysis were used to document the shape evolution of Ge/Si(100) islands, grown by molecular beam epitaxy, as a function of growth conditions. Growth temperatures of 450, 550, 600, and 650 °C with Ge coverages between 3.5 and 14.0 monolayers (ML) were investigated for a deposition rate of 1.4 ML/min. Low coverages produced small hut clusters which then evolved into dome clusters at higher coverages. These dome clusters eventually dislocated after further growth. Higher growth temperatures activated additional pathways for the Ge islands to relieve their strain such as Ge/Si intermixing and the formation of trenches around the islands. Our detailed AFM cross-sectional analysis indicated that dome clusters form several crystal facets in addition to those previously reported.
Change of microscopic tunneling on a macroscopic time scale: Current surge model
Authors: Huang Danhong , Singh Anjali , Cardimona D. A.
Published: July 2002

The current surge model is proposed to calculate the tunneling current in multiple quantum well structures in the presence of a slow time-dependent electric field. The microscopic origin of a zero-bias residual current observed and reported previously [A. Singh and D. A. Cardimona, Opt. Eng. (Bellingham) 38, 1424 (1999)] is explored. The mystery of the observation of a microscopic tunneling change on a macroscopic time scale is uncovered, and an involvement of a very slow physical process is shown. Some new features, such as a current “ripple,” a current instability, a current hysteresis, and a current “arch,” are predicted and confirmed experimentally.
Experimental validation of theoretical models for the frequency response of atomic force microscope cantilever beams immersed in fluids
Authors: Chon James W. M. , Mulvaney Paul , Sader John E.
Published: July 2002

Detailed measurements of the frequency responses of a series of rectangular atomic force microscope (AFM) cantilever beams, immersed in a range of fluids, have been performed to test the validity and accuracy of the recent theoretical model of Sader [J. Appl. Phys. 84, 64 (1998)]. This theoretical model gives the frequency response of a cantilever beam, that is immersed in a viscous fluid and excited by an arbitrary driving force. Very good agreement between experimental measurements and theoretical calculations is found for all fluids considered. Furthermore, a critical assessment of the well-known inviscid model is presented, which demonstrates that this model is not applicable to AFM cantilever beams in general.
Magneto-optical behavior in Co/Pt ultrathin film multilayers
Authors: Fry Richard A. , Bennett Lawrence H. , Della Torre Edward , Farrow Robin F. C.
Published: July 2002

Magneto-optical Kerr effect measurements performed on (0.3 nm Co/x Pt)15 multilayers with x=0.3–2.0 nm showed all samples to exhibit strong perpendicular magnetic anisotropy. Recent work has shown that for compositions near (0.3 nm Co/1.2 nm Pt)15, both polar Kerr rotation and ellipticity exhibit bimodal reversals in which the major hysteresis loops have two distinct field-dependent steps due to the contributions of two magnetic phases. To better understand this bimodal magnetization phenomenon, Kerr rotation and ellipticity were determined as a function of radial position from the center to the edge of this disk sample. These measurements exhibit a continuous variation from two-step (bimodal) behavior in the center to single-step (unimodal) behavior at the edge. Since these films were deposited via MBE with a heated substrate, it is believed that this phenomenon is related to thermally induced variations in interface effects such as alloying, abruptness, and morphology.
Effect of Co and Zr on magnetic properties of nanophase PrFeB alloys
Authors: Harland C. L. , Davies H. A.
Published: July 2002

Nanostructured ribbons in the compositional series Pr12(Fe100−xCox)82B6, x=0–30, having mean crystallite sizes dg in the range of 35–45 nm, were produced by overquenching and annealing. These had excellent second quadrant loop shapes and magnetic property combinations (iHc=390–785 kA/m, (BH)max=100–220 kJ m−3), equivalent to those obtained previously by direct quenching to the nanocrystalline state. In particular, these excellent combinations of properties were obtained without additions of a grain refining agent, such as Zr, which has been reported to be a requirement for obtaining good loop shapes and properties for overquenched and annealed ternary melt spun PrFeB alloys. An additional advantage of the Co substitution for Fe is enhanced Curie temperature. It was found, however, that for nanocomposite substoichiometric alloy ribbons (compositions in the series Pr10(Fe100−xCox)84B6, x=0–40) the second quadrant hysteresis loop shapes for overquenched and annealed ribbons were generally poor and, even though Jr was still substantially enhanced (in the range 0.95–1.05 T), the intrinsic coercivities Hci were disappointingly low (275–450 kA/m). Substitution of 1 at. % Zr for Fe in the Pr10Fe84B6 alloy, nevertheless, resulted in an improved loop shape and Hci (525–550 kA/m), with (BH)max increased to 130–140 kJ m−3 (compared with 80–110 kJ m−3 for the Zr-free alloys).
Synthesis of gadolinium oxide magnetoliposomes for magnetic resonance imaging
Authors: Roberts Danielle , Zhu Weibe L. , Frommen Christoph M. , Rosenzweig Zeev
Published: July 2002

A method for the synthesis of gadolinium oxide magnetoliposomes, i.e., nanosized gadolinium oxide magnetic particles coated by a phospholipid membrane, is presented. Magnetoliposomes were prepared by reacting lauric acid coated gadolinium oxide nanoparticles with dimyristoylphosphatidylcholine liposomes prepared using a direct injection method. The gadolinium oxide magnetoliposomes were characterized using transmission electron microscopy imaging, x-ray diffraction, and fluorescence. The magnetic properties of the magnetoliposomes were investigated with a superconducting quantum interference device magnetometer and nuclear magnetic resonance (NMR) spectrometry. Our results indicate that the magnetoliposomes contain approximately spherical nanoparticles averaging 20 nm in diameter. The occurrence of a phospholipid bilayer surrounding the magnetic particles is confirmed both by transmission electron micrographs of samples negatively stained with uranyl acetate and by digital fluorescence imaging microscopy measurements of magnetoliposomes labeled with fluorescein. The particles are paramagnetic at room temperature. NMR measurements show that the ratio between the relaxivities of the particles depends largely on their preparation.
Passivation mechanism analysis of sulfur-passivated InGaP surfaces using x-ray photoelectron spectroscopy
Authors: Tsai Chang-Da , Lee Ching-Ting
Published: July 2002

We present the passivation mechanism and the chemistry of an (NH4)2Sx treated (100) InGaP surface using x-ray photoelectron spectroscopy. The native oxide on the as-etched InGaP surface could be further removed by a suitable (NH4)2Sx-treatment process. The measured x-ray photoelectron spectra revealed that the S atoms, in addition to bonding to the elemental sulfur, also bonded with the In and Ga atoms on the (NH4)2Sx-treated InGaP surface. However, the S atoms occupied the phosphorous-related vacancies instead of bonding with the P atoms. A proposed structural model of a sulfur-passivated surface is then presented.
Structure, function, and use of the magnetic sense in animals (invited)
Authors: Walker Michael M. , Diebel Carol E. , Green Colin R.
Published: July 2002

The hypothesis that animals navigate magnetically is attractive because the earth’s magnetic field provides consistent information about position and direction in all environments through which animals travel. However, the hypothesis has been difficult to test because (i) the structure and function of the sense could not readily be analyzed in the laboratory and (ii) the effects of experimental treatments on behavior in the laboratory and field could not be reliably predicted. Our research is focused on the structure and function of the sense in rainbow trout and on applying the understanding gained in the laboratory to studies of navigation by homing pigeons. We have found iron-rich crystals (most likely single-domain magnetite) in candidate magnetoreceptor cells located within a discrete layer of sensory tissue in the nose of rainbow trout. The candidate receptor cells are closely associated with a branch of the trigeminal nerve that responds to changes in intensity but not direction of magnetic fields. In parallel work, we have developed a model of magnetic position determination in which pigeons derive magnetic analogues of geographic latitude and longitude from (i) the total intensity and (ii) the direction of the intensity slope of the earth’s magnetic field. Taken together with our other results, the model gives us confidence that a coherent understanding of the structure, function, and use of the magnetic sense in animals is now developing.
Magnon-assisted inelastic excitation spectra of a ferromagnetic tunnel junction
Authors: Ando Y. , Murai J. , Kubota H. , Miyazaki T.
Published: July 2002

Inelastic-electron-tunneling spectroscopy (IETS) has been applied to investigate the spin dependent tunneling process for a Ta(50 Å)/Fe20Ni80(60 Å)/IrMn(300 Å)/Co(60 Å)/Al(13 Å)-oxide/ Co(40 Å)/Fe20Ni80(200 Å) spin-valve-type tunnel junction. IET spectra for both parallel and antiparallel magnetization configurations of ferromagnetic electrodes showed a strong peak at 12 mV. The subtraction spectrum defined by the difference between the spectra of both the configurations was calculated. Spin-independent inelastic excitation processes are not affected by an external magnetic field, and thus, the subtraction spectrum indicates the inelastic modes induced only by the magnetic origin. It showed a strong peak at 12 and 20 mV for the positively and negatively biased direction of the bottom electrode, respectively. The tendency of the tunneling magnetoresistance ratio to decrease with bias voltage agreed with the shape of the subtraction spectrum. By assuming the surface magnon excitation, we obtained the distributions of the correlation length and the Curie temperature for both ferromagnetic electrode surfaces faced on the insulator.
Dielectric properties of polystyrene–aluminum-nitride composites
Authors: Yu Suzhu , Hing Peter , Hu Xiao
Published: July 2002

For polymer composites to be used in electronic packaging, they must have a good combination of thermal and dielectric properties. A composite of aluminum-nitride (AlN) particles dispersed around polystyrene matrix particles has been synthesized in this study. The purpose of using this microstructure is to improve the thermal properties of the polymer at the low-filler content with a minimal increase in the dielectric constant of the polymer composite. The dielectric relaxation behavior of polystyrene–AlN composites has been investigated with broadband dielectric relaxation spectroscopy. The experimental results indicate that the dielectric property of polystyrene–AlN composites is a function of polystyrene particle size, AlN filler concentration, temperature, and frequency under this dispersion state. The dependence of Maxwell–Wagner–Sillars or interfacial polarization of polystyrene–AlN composites on AlN volume fraction has also been studied. The Davidson–Cole equation is used to fit the experimental Cole–Cole plot.
Thermal expansion coefficients of low-k dielectric films from Fourier analysis of x-ray reflectivity
Authors: Bouldin C. E. , Wallace W. E. , Lynn G. W. , Roth S. C. , Wu W. L.
Published: July 2002

We determine the thermal expansion coefficient of a fluorinated poly(arylene ether) low-k dielectric film using Fourier analysis of x-ray reflectivity data. The approach is similar to that used in Fourier analysis of x-ray absorption fine structure. The analysis compares two similar samples, or the same sample as an external parameter is varied, and determines the change in film thickness. The analysis process is very accurate and depends on no assumed model. We determine a thermal expansion coefficient of 55±9×10−6 K−1 using this approach.
Alloying of Co ultrathin films on Pt(111) with Ag buffer layers
Authors: Shern C. S. , Su C. W. , Wu Y. E. , Fu T. Y.
Published: July 2002

The structure at the interfaces of Co/Ag/Pt(111) was studied by low-energy electron diffraction, ultraviolet photoelectron spectroscopy, Auger electron spectroscopy, and depth profiling. An atomic exchange occurs between Co and Ag before the formation of a Co–Pt alloy. Ag atoms start moving to the top at 425 K when the coverage of Co is one monolayer. The temperature of the complete exchange between Ag atoms and Co atoms is dependent on the thickness of the Ag buffer layer. The Co–Pt alloy develops after the atomic exchange is complete. The especially small surface free energy of Ag and large strain energy in this system are proposed as the driving force for the exchange.
Effect of the band-tail states on the exciton peaks in GaN epilayers grown on sapphire substrates
Authors: Kang T. W. , Yuldashev Sh. U. , Bolotin I. L. , Park S. H. , Kim D. Y. , Won S. H. , Jung K. S. , Kim T. W.
Published: July 2002

Photoluminescence (PL) measurements have been carried out to investigate the effects of the band-tail states on the exciton lines in unintentionally doped and Mg-doped GaN epilayers grown on sapphire substrates by using plasma-assisted molecular beam epitaxy. The results of the PL spectra for the Mg-doped epilayers show that the peak positions of the bound exciton lines shift to higher energy with increasing temperature within the low-temperature region. The radiative recombinations of the carriers are related to the band-tail states, and the temperature-dependent blueshifts are analyzed by using a Gaussian distribution of charged impurities. The calculated thermal activation energies of the band-edge emission lines show that those lines in Mg-doped GaN epilayers are related to ionized donor bound exciton recombinations. These results indicate that the positions and the intensities of the exciton peaks observed in Mg-doped GaN films are significantly affected by the concentration of the magnesium dopant.
Laser–plasma interactions in 532 nm ablation of Si
Authors: Han Gyoowan , Murray P. Terrence
Published: July 2002

Single-crystal Si was ablated by pulsed 532 nm laser radiation, and the volume of removed material and the time-resolved current of ejected ions were measured. These data were used to determine the ion fraction of ejected material. The ion fractions provide direct evidence that the break point is due to the laser-plasma interaction. This is confirmed by the speed distributions of the positive ions and from the integrated intensities of the low-speed ion component.
Optical properties of fluorinated diamond-like carbon films produced by pulsed glow discharge plasma immersion ion processing
Authors: Hakovirta M. , He X. M. , Nastasi M.
Published: July 2002

Pulsed glow discharge plasma from a 1 Pa gas mixture of acetylene (C2H2) and hexafluoroethane (C2F6) was used to produce fluorinated diamond-like carbon (F-DLC) films on glass and polymethyl-methacrylate (PMMA) substrate. The composition of the F-DLC coatings was measured by using Rutherford backscattering spectroscopy and elastic recoil detection analysis techniques. The transmittance, absorption coefficient, and optical band gap of 100 nm thick F-DLC coatings was measured by using an ultraviolet/visible spectrometer. The friction and wear properties were measured with a conventional pin-on-disk device. In addition, contact angle measurements were taken in order to determine the nonwetting properties of the coatings. The results showed an increase in nonwetting properties, transmittance, and optical band gap with increasing fluorine content in the coatings. The increased fluorine contents suppressed the incorporation of hydrogen and increased the optical band gap energy, which is quite different from the general DLC films whose optical properties are highly improved with increasing amount of hydrogen incorporated in the films. Furthermore, the F-DLC coatings on PMMA and glass substrates proved to have low friction and wear and similar nonwetting properties such as Teflon®.
N2+ Meinel band quenching
Authors: Piper Lawrence G. , Green B. David , Blumberg William A. M. , Wolnik Stanley J.
Published: July 2002

We have measured the rate coefficients for quenching the A 2Πu state of N2+ by air to be (7.0±0.4), (7.5±1.0), and (7.0±1.0)×10−10 cm3 molecule−1 s−1 for vibrational levels 2–4, respectively. Rate coefficients for quenching vibrational level 2 by molecular nitrogen and oxygen are (7.5±0.8) and (6.2±0.6)×10−10 cm3 molecule−1 s−1, respectively. Our results show that Meinel-band quenching becomes significant at altitudes below100 km.
Dynamics of intramolecular vibrational-energy redistribution (IVR). II. Excess energy dependence
Authors: Felker Peter M. , Zewail Ahmed H.
Published: July 2002

The results of picosecond-resolved measurements of intramolecular vibrational-energy redistribution (IVR) in jet-cooled anthracene at different excess energies are presented. From these results, the nature of IVR as a function of vibrational energy, the relevant time scales for the process, and the details of pertinent vibrational couplings are determined.
A renormalized kinetic theory of dilute molecular gases: Chattering
Authors: Cole R. G. , Evans D. R. , Hoffman D. K.
Published: July 2002

The phenomenon of molecular chattering (a collision between two rigid ovaloids involving two or more impulsive hits) is investigated with emphasis on the importance of these collisions on kinetic theory calculations for dilute molecular systems. To facilitate this analysis, the pseudo-Liouville formalism, commonly employed in the kinetic theory of rigid spheres, is generalized to a form appropriate to rigid ovaloids. From this formalism, we derive a chattering expansion of the bracket integrals, the terms of which successively incorporate higher order chattering sequences. Finally, the results of scattering trajectories between a rigid sphere and a rigid ellipsoid are reported. It is demonstrated that the neglect of chattering sequences leads to large errors when calculating the bracket integrals for realistic model parameters; the errors ranging from ∼1% to as high as ∼200%.
The thermodynamical stability of the heterogeneous system with a spherical interface
Authors: Yang Arthur Jing-Min
Published: July 2002

A one component system, a liquid drop in equilibrium with its vapor, is studied in the discussion of the stability of a heterogeneous system with a spherical interface. Due to the complexity of the second order functional differentiation of the thermodynamical potential function, a simpler treatment with the use of surface thermodynamics is provided instead. The Gibbs free energy and the Grand potential are found to be at a saddle point if respective natural thermodynamical variables are held constant. The Helmholtz free energy for this system at constant T, V, and N is either at a saddle point or a minimum depending on the relative ratio of the two phases. This is in agreement with a previous result. The method developed here can be used in a homogeneous nucleation to study the initial fluctuation which results in the formation of the critical nucleus. It can also be generalized to study the stability of the multicomponent system such as an emulsion. The use of the surface thermodynamics to study a kinetic process is discussed briefly.
The elastic constants of condensed matter: A direct-correlation function approach
Authors: Lipkin Michael D. , Rice Stuart A. , Mohanty Udayan
Published: July 2002

We describe a simple, systematic and physically transparent method for calculating the elastic constants of condensed matter. This approach is equally useful when applied to such diverse materials as alkali halides and nematic and smectic A liquid crystals, as we report in this paper. Our analysis involves regarding the periodic density of the ordered phase to be representable as a small perturbation to the uniform density distribution of the corresponding fluid phase. We implement this idea by making use of recent work on the density wave theory of freezing and the statistical mechanics of nonuniform systems. The theory makes the role of the structure of the medium explicit while leaving the role of the intermolecular potential implicit. We find, for example, that the elastic constants of an alkali halide crystal can be expressed in terms of the curvatures of the Fourier transforms of the charge–charge and number–number direct correlation functions of the corresponding fluid evaluated at the reciprocal lattice vectors of the Bravais lattice. We also report molecular representations for the Frank elastic constants of a nematic liquid crystal and a smectic A liquid crystal.
Structural properties of solutions of spherical micelles: Effect of finite size of small ions
Authors: Nägele G. , Klein R. , Medina-Noyola M.
Published: July 2002

The effects of a finite size of the small ions (counterions and ions of 1–1 electrolytes added to the supporting solution) on the structural properties of solutions of spherical micelles are studied. This study is based on the primitive model of the solution and the mean spherical approximation. The structure obtained using the primitive model is contrasted with that of the one component macrofluid model for these systems. At high ionic strengths, interesting effects are observed in the short-range correlations between the macroparticles, directly related with the finite size of the small ions. Similar effects are also observed on the structure of the electrical double layer around each macroparticle. The consequences of these results for the interpretation of recent neutron scattering experiments from ionic micellar systems are briefly discussed. It is found that for small amounts of added salt a given experimental structure factor can be reproduced with a smaller effective charge on a micelle in the present model than in the previously used one-component macrofluid model. For large amounts of added salt, however, the finite size of the small ions may give rise to the opposite trend.
Scattering theory of photodissociation in strong laser fields: Photon induced predissociation in IBr
Authors: Shapiro M. , Bony H.
Published: July 2002

Theory of photodissociation in a strong laser field is developed in a systematic way. Photon absorption and molecular dissociation are treated uniformally using time-dependent quantum scattering theory. The connection between the physically realizable situation, in which a multimode coherent pulse dissociates a molecule and the computationally convenient occupation number representation, is made in a very explicit way. The coherence (or incoherence) of the radiation field and that of the wave packet of scattering states generated by it are shown to be linked together in a very straightforward fashion. The analytic properties of the scattering wave functions and the existence and duration of off-the-energy-shell transients are shown to be connected. The theory is applied to studying the laser assisted dissociation, in a two color experiment, of IBr. The role of two-photon absorption vs photon catalyzed dissociation is investigated.
On the adiabatic invariance method of calculating semiclassical eigenvalues
Authors: Johnson B. R.
Published: July 2002

In this paper we analyze and further develop the adiabatic invariance method for computing semiclassical eigenvalues. This method, which was recently introduced by Solov’ev, is basically an application of the Ehrenfest adiabatic hypothesis. The eigenvalues are determined from a classical calculation of the energy as the time dependent Hamiltonian H(t)=H0+s(t)H1 is switched adiabatically from the separable reference Hamiltonian H0 to the system Hamiltonian H0+H1. A systematic study is carried out to determine the best form for the switching function, s(t), to maximize the rate of convergence of the energy to its adiabatic limit. Five switching functions, including the linear function used by Solov’ev, are defined and tested on three different systems. The linear function is found to have a very slow convergence rate compared to the others. The classical energy is shown to be a periodic function of the angle coordinates of H0. The coefficients of the Fourier series representation of this function are then shown to be related to the classical energy expectation value and classical energy uncertainty which we define and compare to their quantum mechanical counterparts. Four example problems are analyzed and solved in the course of this investigation. They are: (i) the forced harmonic oscillator, (ii) the harmonic oscillator with time dependent frequency, (iii) the nondegenerate, and (iv) the degenerate two dimensional coupled oscillator problems. In the degenerate oscillator case, we have discovered a correction, similar to the Langer correction in WKB theory, which significantly increases the accuracy of the semiclassical eigenvalues. States that are a linear combination of several energy eigenstates are discussed and it is demonstrated that the energy expectation value of these nonstationary states can also be computed semiclassically.
The oxygen–water reaction on Pd(100): Observation of a precursor complex
Authors: Nyberg C. , Uvdal P.
Published: July 2002

The oxygen–water reaction on Pd(100) has been studied by electron energy loss spectroscopy (EELS) and low energy electron diffraction. A precursor complex for hydroxyl formation was identified in the ν(O–H) loss region of the vibrational spectrum when water was adsorbed on the Pd(100) p(2×2)O surface. The observations indicate that the precursor complex contains several water molecules. The formation of the water–oxygen precursor complex and thus the reaction is inhibited by the presence of hydrogen on the surface. The reaction is demonstrated to start at the defects of the p(2×2)O structure. Water does not adsorb on the perfect p(2×2)O structure above 140 K, at low pressures.
The importance of diffuse f functions for transition metals
Authors: Langhoff Stephen R. , Bauschlicher Charles W.
Published: July 2002

The importance of a diffuse f function for determining the dissociation energy (De) of Cu2 and the 3F–3D energy separation in Ni atom is investigated. It is found that the diffuse f contributes at most 0.05 eV to the De of Cu2 when added to a basis containing tight f functions and a flexibly contracted d basis. The diffuse f function is found to decrease the 3F–3D separation in Ni, but by substantially less than the tight f functions.
Vibrational relaxation of N2 by collision with He atoms
Authors: Banks A. J. , Clary D. C. , Werner H.-J.
Published: July 2002

A quantum mechanical study of vibrational relaxation in the collision of N2 with 3He and 4He is reported. An ab initio potential energy surface has been computed using the coupled electron pair approximation with an extended basis set. Cross sections for v=1 → 0 vibrational relaxation have been calculated on this surface by using the centrifugal sudden approximation (CSA). The calculated vibrational relaxation rate coefficients show a very encouraging agreement with those measured in laser fluorescence experiments for the temperature range 82–291 K. Over this temperature range the rate coefficients increase by a factor close to 100. The CSA computations are also used to test the accuracy of the vibrational close coupling, rotational infinite order sudden, and the breathing sphere approximations.
Condensation of a supersaturated vapor. VI. The diffusivity and the size of photonucleating clusters in vapors containing CS2
Authors: Chen Chin-Cheng , Wen F. C. , Katz Joseph L.
Published: July 2002

When a supersaturated vapor containing a low concentration of CS2 is illuminated with ultraviolet light, copious nucleation occurs. This paper presents a determination of the diffusivity of the photonucleator by analyzing the decay in the nucleation rate which occurs in a diffusion cloud chamber after blocking the light. Equations describing the mass transfer of clusters of different sizes were derived and solved to obtain the relationship between their diffusivities and the decay of the rate of nucleation. The resultant diffusivities lead to a lower limit for the radius of the CS2 photonucleators of about 5.4 Å. Such a size is much larger than that of any photoproducts of CS2. We conclude that the CS2 photonucleators are stable (or very long-lived) products resulting from the reaction of photoexcited CS2 with other vapor phase molecules (e.g., nonane). The effectiveness of clusters of these photonucleators in causing nucleation is also examined. We show that a generalization of the Becker–Doering version of nucleation theory is consistent with these results.
Laser photolysis of HNO3 at 222 nm: Direct determination of the primary quantum yield of OH
Authors: Jolly G. S. , Singleton D. L. , McKenney D. J. , Paraskevopoulos G.
Published: July 2002

A method has been developed and used in the direct determination of the primary quantum yield of OH in the laser photolysis of nitric acid at 222 nm. The amount of laser light absorbed was determined with a calibrated disk calorimeter. The amount of OH produced was determined by absorption spectroscopy using the resonance line at 308.3 nm and a published value of its line strength. The primary quantum yield of OH at 296 K was found to be 0.89±0.08 (95% confidence interval) with an uncertainty of +17%, −8% due to potential systematic errors. Within the experimental uncertainty, this result is consistent with a previous determination of a quantum yield of unity for the other product of photolysis, NO2.
Collision induced transitions between 2Π and 2Σ states of diatomic molecules: Quantum theory and collisional propensity rules
Authors: Alexander Millard H. , Corey Gregory C.
Published: July 2002

We develop the exact quantum description, free of any dynamical approximations, of rotationally inelastic collision induced transitions between 2Π and 2Σ electronic states of a diatomic molecule. An explicit connection is made between the matrix elements of the electrostatic coupling, described in an asymptotically exact diabatic basis, and the results of an ab initio calculation of the appropriate atom–molecule adiabatic electronic wave functions of A′ and A″ symmetry. Analysis of the quantum close-coupled equations demonstrates that the use of Franck–Condon approximations in the description of E → E energy transfer is unjustified and, furthermore, that in collisions involving homonuclear diatomic molecules the s/a permutation-inversion symmetry of the molecular wave functions will be rigorously conserved. The extension of the infinite-order sudden approximation to electronically inelastic 2Π → 2Σ processes allows us to predict two new collisional propensity rules: (a) When Δ J=0 the cross sections will become vanishingly small for transitions which conserve the e/f symmetry index of the molecular wave function. (b) In a high-J Hund’s case (b) limit transitions from either the F1 or F2 2Π-state manifolds will populate only one of the Σ-state spin-doublet levels, consistent with a physical model in which the electronic spin S is a spectator so that the relative orientation of N and S is preserved during the collision.
Phase transition and glass transition in rubidium cyanide
Authors: Shimada Tetsuro , Matsuo Takasuke , Suga Hiroshi , Luty Fritz
Published: July 2002

Heat capacity of a pure rubidium cyanide crystal has been measured between 14 and 330 K with an adiabatic calorimeter. A first order phase transition was found at (132.30±0.05) K in agreement with the previous dielectric study. The transition enthalpy and entropy are 1280 J mol−1 and 10.2 J K−1 mol−1, respectively, of which (742±2) J mol−1 and (5.65±0.02) J K−1 mol−1 are the discontinuous parts. Under the assumption that the orientational disorder of the cyanide ion in the cubic phase is eightfold, a residual entropy of 5.43 J K−1 mol−1 was derived from the transition entropy and related to the head-to-tail disorder of the anions. The disorder causes a glass transition at 30 K. The enthalpy relaxation time determined by spectroscopic use of the adiabatic calorimeter agrees very well with the dielectric relaxation time extrapolated to the lower temperature, giving evidence for the frozen disordered state to be a dipolar glass.
A theory of relaxation processes with application to a two-dimensional rotor
Authors: Fulton Robert L.
Published: July 2002

A general theory of the relaxation of a system in contact with a second system (called the heat bath) which need not be in equilibrium is presented. The theory, which avoids the use of projection operators, is developed in such a fashion that the effects of the interaction between the system and the heat bath can be consistently approximated by an ordering in terms of the degree of the fluctuation in the heat bath variables. This gives a natural classification in terms of differing regimes, the lowest nontrivial of which is the quadratic, or Gaussian, regime. The cubic and higher order fluctuations represent increasingly complex regimes. The exact equations of motion for the average of the single time system variables and for the two-time correlation functions involving the system variables are found in the Gaussian regime. (The general theory is not limited to the two-time correlation functions.) In the classical limit these lead to equations of motion for the three quantities: the average of the single time variables; the two-time, or pair, fluctuations; and the average of the Poisson bracket of a pair of system variables (the

dissipative function) for systems in and not in equilibrium. Some exact consequences of the equations when the heat bath and the system are in thermal equilibrium are found. In particular the exact equilibrium distribution function of the system variables is determined and it is demonstrated that the system variables obey the fluctuation–dissipation theorem. Various approximation schemes are devised. When the bath and the system are in thermal equilibrium the approximate equations of motion obey the fluctuation–dissipation theorem and have as a possible steady state solution the exact equilibrium distribution. The theory is applied to the relaxation of a rigid rotor constrained to rotate in two dimensions. The first corrections to the diffusion constant D due to the nonzero correlation times of the bath variables and a finite moment of inertia of the rotor are found. Connection is made with dielectric friction when the rotor is endowed with a permanent dipole moment which interacts with the bath variables via an electric field. A symmetrized version of the Nee–Zwanzig friction constant gives one contribution to the diffusion constant. In addition, when the moment of inertia is finite and the correlation time of the bath variables is nonzero, there is a contribution which tends to increase the diffusion constant.
Pattern formation in precipitation processes. I. The theory of competitive coarsening
Authors: Venzl Gerd
Published: July 2002

The theory of competitive coarsening is formulated and analyzed for applications to the formation of macroscopic precipitation patterns. Previous work is extended to account for ‘‘local coarsening’’ which results if processes on smaller length scales are properly eliminated. Formally the theory includes the width of the local particle size distribution as a dynamical variable and avoids the assumption of monodispersivity. The linear analysis shows that long-wavelength instabilities of the spatially homogeneous system are suppressed in comparison with the results of previous work. Numerical simulations of the nonlinear model are applied to locally perturbed homogeneous systems and produce one precipitation band surrounded by a growing region essentially void of precipitate. Recurrent banding and satellite induction which result under the assumption of monodispersivity are not explained by this theory. The random patterns found experimentally in supercooled PbI2 solutions containing a gel are interpreted as dissolution patterns driven by exceptionally large particles which act as local perturbations. Using an analogy to percolation problems this point of view can rationalize part of the varying appearance of the patterns as well as the sensitivity to the initial salt concentration. The theory also qualitatively describes the ‘‘focusing effect’’ found in recent experiments on Liesegang band formation.
Electrical properties of ammonia and the structure of the ammonia dimer
Authors: Liu Shi-Yi , Dykstra Clifford E. , Kolenbrander Kirk , Lisy James M.
Published: July 2002

Electrical multipole moments, polarizabilities, and hyperpolarizabilities of ammonia have been calculated with large basis sets through application of derivative Hartree–Fock (DHF) theory. With these properties, the orientational dependence of the electrical interaction between two ammonia molecules has been examined. The electrical interaction model indicates that the orientational angles of the two ammonias with respect to the a-inertial axis are strongly coupled. Internal rotation about the ammonia C3 axes is coupled as well. There are 18 equivalent minimum energy structures for the ammonia dimer and the lowest barrier for inversion is found to be about 250 cm−1. The predicted equilibrium structure differs noticeably from the recently determined experimental structure. Certain of the assumptions used in obtaining the experimental structure, we feel are not justified, in light of the coupling of internal motions apparent in the potential surface.
Radiative lifetimes of the υ′3=0, 1, and 2 levels of CH3S(Ã 2A1)
Authors: Black Graham , Jusinski Leonard E.
Published: July 2002

Radiative lifetimes for CH3S(Ã 2A1) in the υ3 =0, 1, and 2 levels are 760±60, 810±60, and 197±15 ns, respectively. These results are in disagreement with previous work on the υ′3 =0 and 1 levels although in agreement for the υ3 =2 level. Since the previous work was in a beam, it is possible that loss from the viewing region contributed to the shorter lifetimes reported previously for the lower levels. Quenching by (CH3S)2 is fast for all three levels.
MRD-CI potential surfaces using balanced basis sets. IV. The H2 molecule and the H3 surface
Authors: Wright James S. , Kruus Erik
Published: July 2002

The utility of midbond functions in molecular calculations was tested in two cases where the correct results are known: the H2 potential curve and the collinear H3 potential surface. For H2, a variety of basis sets both with and without bond functions was compared to the exact nonrelativistic potential curve of Kolos and Wolniewicz [J. Chem. Phys. 43, 2429 (1965)]. It was found that optimally balanced basis sets at two levels of quality were the double zeta single polarization plus sp bond function basis (BF1) and the triple zeta double polarization plus two sets of sp bond function basis (BF2). These gave bond dissociation energies De =4.7341 and 4.7368 eV, respectively (expt. 4.7477 eV). Four basis sets were tested for basis set superposition errors, which were found to be small relative to basis set incompleteness and therefore did not affect any conclusions regarding basis set balance. Basis sets BF1 and BF2 were used to construct potential surfaces for collinear H3, along with the corresponding basis sets DZ*P and TZ*PP which contain no bond functions. Barrier heights of 12.52, 10.37, 10.06, and 9.96 kcal/mol were obtained for basis sets DZ*P, TZ*PP, BF1, and BF2, respectively, compared to an estimated limiting value of 9.60 kcal/mol. Difference maps, force constants, and relative rms deviations show that the bond functions improve the surface shape as well as the barrier height.
Thermal disruption of the inherent structure of simple liquids
Authors: LaViolette Randall A. , Stillinger Frank H.
Published: July 2002

Short range order in liquids formally may be viewed as an inherent structure (amorphous particle packings) that has been smeared by thermally induced vibrational distortions. In order to study these distortions, we have employed molecular-dynamics computer simulation, with steepest-descent quenching to potential energy minima, for a model system resembling liquefied noble gases. A typical multidimensional path connecting the liquid configuration to its quench packing is found to be tortuous and characteristic of substantial anharmonicity. In this respect, thermodynamically stable liquids differ qualitatively from crystals and low- temperature amorphous solids. On the basis of evidence from quench path geometry, and by observing how particle pairs redistribute as a result of the quench, an hierarchical domain clustering picture emerges to characterize vibrational anharmonicity in liquids.
Two-photon circular dichroism in lanthanide (III) complexes
Authors: Sztucki J. , Strȩk W.
Published: July 2002

The theory of two-photon circular dichroism in lanthanide (III) systems was developed within the crystal field model. From the selection rules for the process four different types of transitions were distinguished and their dissymmetry factors were estimated.
Carbon K-shell excitation of gaseous and condensed cyclic hydrocarbons: C3H6, C4H8, C5H8, C5H1, C6H1, C6H12, and C8H8
Authors: Hitchcock A. P. , Newbury D. C. , Ishii I. , Stöhr J. , Horsley J. A. , Redwing R. D. , Johnson A. L. , Sette F.
Published: July 2002

The carbon K-shell excitation spectra of gaseous cyclic hydrocarbons, both saturated (cyclopropane, cyclobutane, cyclopentane, cyclohexane) and unsaturated (cyclopentene, cyclohexene, and cyclooctatetraene), have been recorded by electron energy loss spectroscopy under dipole-dominated conditions. These are compared to the NEXAFS spectra of multilayers and monolayers of C4H8, C5H8, C6H12, and C8H8 on Pt(111). Multiple scattering Xα calculations of the spectra of cyclopropane, cyclobutane, and cyclohexane are also reported. In most cases the gas and solid spectra are essentially the same indicating that intramolecular transitions dominate in the condensed phase. The NEXAFS polarization dependence of the condensed phases has assisted spectral assignments and the determination of the molecular orientation in the monolayer phase. In the saturated species a sharp feature about 3 eV below the carbon 1s ionization threshold is identified as a transition to a state of mixed Rydberg/valence character with the π*(CH2) valence component dominating. Except for cyclopropane the positions of the main σ * resonances correlate with the C–C bond lengths in a manner similar to that reported previously for noncyclic aliphatic molecules. In the spectra of monolayer C6H12, C5H8, and C8H8 spectral broadening and weak additional features are observed which are attributed to molecule–surface interactions.
Non-Markovian theory of activated rate processes. VI. Unimolecular reactions in condensed phases
Authors: Nitzan Abraham
Published: July 2002

The non-Markovian theory of activated rate processes developed by Carmeli and Nitzan is applied to investigate unimolecular reactions in condensed phases with particular emphasis on the molecular size (number of internal degrees of freedom) dependence of the effect of solvent friction on the reaction rate. The model consists of one reaction coordinate coupled to n−1 nonreactive modes. The molecule solvent interaction is treated within the context of the generalized Langevin equation. The reaction dynamics may be roughly described as two consecutive processes: the well (energy diffusion) dynamics where it is assumed that fast intramolecular vibrational relaxation and slower overall molecular energy diffusion dominate the process, and the barrier dynamics where it is assumed that the motion along the reaction coordinate is only weakly coupled to the nonreactive modes. This model leads to a result for the reaction rate which, as in the one-dimensional case, is obtained as the inverse of the sum of two times: the barrier crossing time and the energy diffusion time. The latter is very sensitive to molecular size and becomes extremely short for large molecules. Correspondingly, the Kramers turnover region is predicted to occur for low molecular weight solvent in the high pressure gas phase, as was found in recent experiments. For higher viscosities the rate is dominated by the barrier crossing time with a large (larger for larger molecules) transition state rate plateau and with a falloff for high viscosities. Recent interesting results by Straub et al. which have pointed out the dominance of spatial diffusion in the well for extremely high viscosities (overdamped well motion) are argued to be irrelevant for most molecular situations.