bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement of field-dependent surface impedance tensor in amorphous wires with circumferential anisotropy
Authors: Makhnovskiy D. P. , Panina L. V. , Mapps D. J.
Published: July 2002

The surface impedance tensor final_sigmâ is found for 120 μm diameter Co-based amorphous wires as a function of a dc axial magnetic field Hex. Contrary to the usual practice of giant magneto-impedance experiments, the voltage response is measured not only across the wire but also in the external coil (secondary coil), and the wire is subjected to an ac current flowing through it or the coil (primary coil), which allows all the components of the impedance tensor final_sigmazz, final_sigmaφφ and final_sigmazφ=final_sigmaφz to be determined. Under certain conditions, they all are very sensitive to Hex (especially final_sigmaφφ: 100–250%/Oe) but exhibit a different field behavior: the diagonal terms final_sigmazz and final_sigmaφφ are symmetrical with respect to Hex whereas the off-diagonal terms final_sigmazφ=final_sigmaφz are antisymmetrical, and at high frequencies, final_sigmazz has a minimum at Hex=0 where final_sigmaφφ has a maximum. The effect of a moderate dc bias current Ib is demonstrated to be favored for final_sigmazφ and final_sigmaφz, which are nearly zero for Ib=0. These new magneto-impedance characteristics may further expand the application opportunities for the GMI effect.
Magnetotransport and micro-x-ray absorption near-edge structure studies of glass-coated Fe–Ni–Cu microwires
Authors: Wang Kai-Ying , Tang Jinke , Schilling Paul J. , Moelders Nicholas
Published: July 2002

Magnetotransport and micro-x-ray absorption near-edge structure (micro-XANES) studies of glass-coated 20Fe–20Ni–60Cu microwires have been carried out on both as-cast and heat-treated samples. The micro-XANES spectroscopy data were collected at the K edges of Cu, Ni, and Fe with the x-ray microprobe beamline at the Center for Advanced Microstructures and Devices. Comparison of the Fe K-edge spectra from the microwires to standard spectra reveals that the Fe atoms in the as-cast sample are in a face-centered cubic (fcc) configuration and they remain in the fcc phase throughout the annealing processes. Giant magnetoresistance (GMR) has been observed in the microwires and it reaches ∼6.5% at 300 K in a field of 9 T. The MR decreases to ∼2.5% as the annealing temperature increases to 500 °C. The loss of GMR upon annealing is attributed to the growth of Fe–Ni rich magnetic particles. Significant reduction in the number of extremely small particles is obtained after annealing at 500 °C, which shows MR characteristics that are different from the as-cast and 300 °C annealed microwires.
Effects of Ga substitution for Fe on the structure and magnetic properties of Nd8.4Fe87.1−xGaxB4.5 (x=0–2.2) alloys prepared by mechanical alloying
Authors: Cui B. Z. , Sun X. K. , Liu W. , Zhang Z. D. , Geng D. Y. , Zhao X. G. , Liu J. P. , Sellmyer D. J.
Published: July 2002

The effects of a partial substitution of Ga for Fe on the structure and magnetic properties of mechanically alloyed Nd8.4Fe87.1B4.5 were studied. With increasing Ga content, the magnetic properties of the nanocomposite magnets first improve, reach maxima at 1.1 at. % Ga, and then degrade. The addition of Ga helps to control the morphology, especially the grain sizes of both α-Fe and Nd2Fe14B.
Mathematical modeling of thermal runaway in semiconductor laser operation
Authors: Smith W. R.
Published: July 2002

A mathematical model describing the coupling of electrical, optical and thermal effects in semiconductor lasers is introduced. Through a systematic asymptotic expansion, the governing system of differential equations is reduced to a single second-order boundary value problem. This highly nonlinear equation describes the time-independent maximum temperature in the boundary layer adjacent to the mirror facet. The solution of the problem is a multi-valued function of current. The graph of the maximum steady-state temperature as a function of current gives a fold-shaped response curve, which indicates that no bounded steady state exists beyond a critical value of current. For certain device parameters and initial conditions, thermal runaway occurs. A mechanism for the sudden mode of semiconductor laser failure is described in terms of thermal runaway.
High-performance α-Fe/Pr2Fe14B-type nanocomposite magnets produced by hot compaction under high pressure
Authors: Wang Zuocheng , Zhou Shouzeng , Zhang Maocai , Qiao Yi
Published: July 2002

The α-Fe/R2Fe14B-type nanocomposite magnets have been prepared by hot pressing melt spun Pr8Dy1Fe74.5Co10Nb0.5B6 flakes under a conventional pressure P of 125 MPa and high pressures ranging from 1 to 7 GPa. It was found that increasing compaction pressure from 125 MPa to 5 GPa led to marked grain refinement in the magnet and consequently resulted in significant improvement of magnetic properties. When hot pressing under even higher pressure (P>5 GPa), however, the crystallization was constrained and the hot pressed magnets retained a certain amount of amorphous phase besides the Pr2Fe14B-type phase and α-(FeCo) phase, which resulted in the deterioration of the magnetic properties. A remanence of 11.1 kG, coercivity of 10.2 kOe, and maximum energy product of 23.6 MGOe have been achieved in the magnet hot pressed under a pressure of 5 GPa.
The role of capillary condensation of water in the nanoscale friction and wear properties of boron carbide films
Authors: Prioli R. , Reigada D. C. , Freire F. L.
Published: July 2002

The influence of capillary condensation of water at the atomic force microscope tip and boron carbide film surface on the friction forces and wear of the films is studied by force microscopy. It is shown that friction and wear are strongly dependent on the humidity. At low humidity no wear at the film surface is verified, while at high humidities constant wear rates are measured. The results show that the wear mechanisms of amorphous boron carbide films are highly dependent on the environmental humidity. A wear model based on a film surface interaction with the air moisture induced by the dissipated energy at the microscope tip–surface interface while in relative motion is presented.
Inclusion and assessment of Renner–Teller coupling in transition state theory for Π states: Application to O(3P)+H2
Authors: Wagner Albert F. , Bowman Joel M. , Harding Lawrence B.
Published: July 2002

The inclusion of the Renner–Teller effect in the treatment of linear electronically degenerate transition states in the context of transition state theory is indicated. The magnitude of the effect is shown to be significant for the O(3P)+H2→OH+H reaction at room temperature. Various approximations to the Renner–Teller effect which employ decoupled potential energy surfaces are evaluated over a range of values likely to be encountered in transition states.
Energy levels of lanthanide ions in the cubic Cs2NaLnCl6 and Cs2NaYCl6:Ln3+ (doped) systems
Authors: Richardson F. S. , Reid Michael F. , Dallara John J. , Smith Robert D.
Published: July 2002

The best available energy level data for the cubic Cs2NaLnCl6 and Cs2NaYCl6:Ln3+ (doped) systems are collected together and summarized. These data are analyzed in terms of a parametric model which assumes an octahedral (Oh) site symmetry for the Ln3+ ions and a 4 f N ‘‘free-ion’’ Hamiltonian similar to that used previously in energy level analyses of the LaCl3:Ln3+ systems. The empirical energy level data are fit to the parametrized 4 f N Hamiltonian, and the resulting ‘‘best-fit’’ parameter sets and calculated energy levels are presented and discussed.
Pressure broadening of rotational bands. I. A statistical theory
Authors: Rosenkranz P. W.
Published: July 2002

Absorption of electromagnetic waves by rotational transitions of molecules is formulated for the case in which the wave frequency is displaced from resonance by an amount large compared to the reciprocal duration of a typical binary collision, and also large compared to the differences between frequencies of the strong resonances of the gas. In this far-wing limit, Fano’s relaxation operator is reduced to a scalar parameter which depends on the frequency displacement. This relaxation parameter is not symmetric with respect to reflection about resonance, but becomes symmetric when multiplied by the factor exp(ℏωd/2kT) where ωd is the frequency displacement. The theory applies to dipolar molecules of any shape, in collisions with either dipolar or quadrupolar molecules.
Electron capture phosphoranyl radicals in x-irradiated diphosphine disulfides. A single crystal ESR and a
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Authors: Janssen René A. J. , Sonnemans Max H. W. , Buck Henk M.
Published: July 2002

An electron spin resonance (ESR) study of phosphorus centered electron capture radicals trapped in single crystals of substituted diphosphine disulfides [R2P(S)P(S)R2, R=Me, Et, Ph] irradiated with x rays at 77 K is reported. The principal values and direction cosines of the hyperfine and g tensors were determined. It is shown that in all three compounds a three-electron P–P bond radical is formed. The unpaired electron is found to be symmetrically distributed over the two phosphorus atoms. The direction of the phosphorus hyperfine coupling makes an angle of 20°–30° with the P–P bond. The alkyl substituted diphosphine disulfides reveal furthermore a radical in which the unpaired electron is asymmetrically distributed over the two phosphorus atoms. The electronic configuration of these radicals is best described as trigonal bipyramidal (TBP) with a central phosphorus atom and the unpaired electron in an equatorial position (TBP-e). The second phosphorus atom and a sulfur substituent are located on the apical sites of the TBP. Irradiation yields for R=Et, Ph also a three-electron P–S bond radical and a number of dissociation products. Ab initio quantum chemical calculations of the isotropic and anisotropic hyperfine couplings for the three-electron P–P bond radicals are in good agreement with the experimental values and support the assignment.
Magnetic field modulated infrared laser spectroscopy of the chloronium ClH+2 ion ν2 band
Authors: Kawaguchi Kentarou , Hirota Eizi
Published: July 2002

The chloronium ion (ClH+2) has been detected in the gas phase by infrared diode laser spectroscopy with magnetic field modulation. The ion was generated by a hollow-cathode discharge in an H2, He, and HCl mixture. One hundred and forty eight lines were observed between 1040–1330 cm−1, of which 99 and 25 were assigned to the ν2 fundamental bands of 35ClH+2 and 37ClH+2, respectively. The observed spectra were analyzed by using Watson’s A-reduced Hamiltonian to determine the band origins [ν2(35ClH+2) =1184.1256(2) cm−1, ν2(37ClH+2) =1183.2165(13) cm−1] and rotational and centrifugal distortion constants. The r0 structure was calculated for 35ClH+2 to be r0(H–Cl)=1.3135(87) Å and θ0(HClH)=94.3(12)° with standard errors in parentheses.
The rotational spectrum and molecular geometry of an antihydrogen-bonded dimer of sulfur dioxide and hydrogen cyanide
Authors: Goodwin Elizabeth J. , Legon A. C.
Published: July 2002

The ground-state rotational spectra of the four isotopic species (32SO2,HC14N), (32SO2,DC14N), (32SO2,HC15N), and (34SO2,HC14N) of a weakly bound dimer of sulfur dioxide and hydrogen cyanide have been observed by means of pulsed-nozzle, Fourier-transform microwave spectroscopy. The 14N-nuclear quadrupole coupling constants determined for the isotopomer (32SO2,HC14N) of this asymmetric rotor molecule are χaa =−2.876(3) MHz and χbb =2.025(4) MHz while the rotational and centrifugal distortion constants are (A0−ΔK) =8633.849(1) MHz, B0=1848.890(2) MHz, C0=1615.863(2) MHz, ΔJ =9.56(1) kHz, ΔJK =114.84(7) kHz, δJ =0.838(6) kHz, and δK =89(1) kHz. The spectroscopic constants have been interpreted in terms of a nonplanar, antihydrogen-bonded geometry of Cs symmetry with S, H, C, and N nuclei lying in the symmetry plane. The HCN molecule is roughly perpendicular to the plane of the SO2 molecule with N lying between the SO2 plane and the H atom. The distance between the SO2 and HCN centers of mass is rc.m. =3.6542(1) Å. The angle between the C2 axis of the SO2 subunit and the NCH axis is 86° and the two axes intersect at a distance of 1.38 Å above the S nucleus.
Electronic absorption of Frenkel excitons in topologically disordered systems
Authors: Schweizer Kenneth S.
Published: July 2002

A self-consistent effective medium theory of the electronic absorption spectra of tightly bound dipolar excitons in simple fluids is developed within the adiabatic picture. The theoretical approach is based on the isomorphism between the path-integral formulation of quantum theory and classical statistical mechanics and is an extension of previous work [D. Chandler, K. S. Schweizer, and P. G. Wolynes, Phys. Rev. Lett. 49, 1100 (1982)]. The consequences of fluid structural disorder on resonant excitation transfer and the statistical fluctuations of single molecule energy levels are simultaneously treated. Detailed numerical calculations are performed to establish the dependence of the absorption spectrum on fluid density, short range order, and the relative magnitude of the resonant transfer vs the single site disorder. The density dependence of the spectral features are found to be a sensitive function of fluid structure and the relative strength of the localizing vs the delocalizing interactions. By comparing the liquid state results with the corresponding crystalline solid behavior, the consequences of topological disorder on the exciton spectrum are identified. The relevance of the theoretical predictions to spectroscopic probes of exciton delocalization in molecular liquids and glasses is discussed.
Theory of the molecular origins of the entropic portion of the Flory χ parameter for polymer blends
Authors: Freed Karl F. , Pesci Adriana I.
Published: July 2002

The lattice model of self- and mutually avoiding polymers is used to compute corrections to the Flory–Huggins entropy of mixing for binary blends of polymers in which the monomers have specific geometric structures and therefore occupy several lattice sites. Ten examples provide an entropic χ of close to the magnitude observed experimentally, thereby giving the first molecular basis for understanding the previously enigmatic and ad hoc entropic χ parameter.
Gas phase kinetics of the reactions of NaO with H2, D2, H2O, and D2O
Authors: Ager Joel W. , Howard Carleton J.
Published: July 2002

The gas phase reactions of the NaO radical with H2, D2, H2O, and D2O were studied in a flow tube reactor at room temperature. The reaction of NaO with H2 has two exothermic product channels, NaOH+H and Na+H2O. Both channels were observed and the Na formation channel produces some Na in the 32P state. The rate constants for the abstraction channel for H2 and D2 reactants are (2.6±1.0)×10−11 and (1.1×0.4)×10−11 cm3 molecule−1 s−1 at 296±2 K. The reaction of NaO with H2O was shown to be second order and the products are assumed to be NaOH and OH. The rate constants for H2O and D2O reactants are (2.2±0.4)×10−10 and (1.2±0.2)×10−10 cm3 molecule−1 s−1 at 298±1 K. The measured NaO+H2O rate constant is compared to the predicted collision rate constant from a model based on the large attractive dipole–dipole force between NaO and H2O. The role of these reactions in mesospheric Na chemistry is briefly discussed.
Concentration depolarization of fluorescence in the presence of molecular rotation
Authors: Fredrickson Glenn H.
Published: July 2002

A theory is presented to describe the observables in fluorescence depolarization experiments when both rotational depolarization and concentration depolarization mechanisms are active. The latter mechanism arises from electronic excitation migration among chromophores and becomes more important as their concentration is increased. We treat the simplest case of randomly distributed chromophores, with fixed centers of mass, undergoing independent rotational Brownian motion. Our formalism reduces to existing theories if either depolarization mechanism is turned off, but makes new predictions in the situation where both are operative. Because electronic excitation transfer and molecular orientation are dynamically correlated, we find that previous ad hoc expressions that neglected such correlations are inappropriate. Approximate expressions for the fluorescence anisotropy are provided that can be tested by time-resolved or photostationary experiments on dye molecules in solution. Extensions of the theory to cases of correlated chromophore positions are straightforward and will allow interpretation of experiments on labeled macromolecular systems.
Self-consistent integral equation applied to the highly charged primitive model
Authors: Belloni Luc
Published: July 2002

We apply the mixed integral equation (ZH) proposed recently by Zerah and Hansen to highly asymmetrical polyelectrolytes within the primitive model. The closure interpolates continuously between the Percus–Yevick and the hypernetted chain closures for charges of same sign and between the HNC and mean spherical approximations for charges of opposite sign. The thermodynamic consistency between the virial and compressibility equations of state is obtained by varying the single parameter in the switching functions. In weakly screened and highly charged suspensions, the new ZH equation improves appreciably the HNC results. The structure factors are essentially modified in the infinite wavelength limit. The two-phase region in the phase diagram is shifted towards the low temperature or the high charge region which allows us to study more asymmetrical systems than with the HNC equation. Since no simulation data are available, the ZH equation is useful to quantify the accuracy of the HNC closure for such systems.
Free radical chain reactions of chlorine with cyclobutane in the solid state: Evidence for radical migration via H atom transfer
Authors: Sedlacek Arthur J. , Wight Charles A.
Published: July 2002

The laser-initiated reaction of chlorine with cyclobutane, deposited as a thin amorphous film at 77 K, is found to produce exclusively chlorocyclobutane and HC1 via a free radical chain mechanism. The reactions are initiated by pulsed laser photolysis of molecular chlorine at 308 nm. Product formation is monitored by Fourier transform infrared absorption spectroscopy. The quantum yield for reaction increases from 1.1±0.3 in an equimolar mixture to a yield of 7.4±0.8 for mixtures rich in cyclobutane. These results are consistent with a reaction mechanism which involves the transfer of a hydrogen atom from cyclobutane to cyclobutyl radical, permitting the radical site to propagate freely in the solid.
Experimental assignments of the 3p Rydberg states of acetone
Authors: McDiarmid Ruth , Sabljić A.
Published: July 2002

A polarization-selected two- and three-photon resonant multiphoton ionization investigation of the 3p Rydberg region of the spectra of acetone and acetone-d6 has been conducted to resolve the controversy concerning 3p Rydberg assignments. Here the 3p Rydberg transitions of acetone are unambiguously assigned. The assigned spectra are then used to assess excited state energy calculations of acetone and to evaluate empirical electron energy loss spectroscopy rules. The vibrational substructures of the Rydberg ←n transitions are used to deduce structural and dynamical information about the Rydberg states of acetone and suggest a possible misassignment of the ground state vibrational spectrum.
Sub-Doppler direct infrared laser absorption spectroscopy in fast ion beams: The fluorine hyperfine structure of HF+
Authors: Coe J. V. , Owrutsky J. C. , Keim E. R. , Agman N. V. , Hovde D. C. , Saykally R. J.
Published: July 2002

We report the development of a new general technique for measuring vibration–rotation spectra of molecular ions with sub-Doppler resolution and with accurate determination of the mass and number density of the carriers of all spectral features. With this method, called direct laser absorption spectroscopy in fast ion beams (DLASFIB), we have carried out the first observation of direct absorption of photons by ions in a fast ion beam. Hyperfine-resolved vibration–rotation transitions of HF+ have been measured, and along with optical combination differences and laser magnetic resonance data, have been analyzed to yield the fluorine hyperfine parameters a, b, c and d for both v=0 and v=1 in the X 2Π state. Comparisons with many-body perturbation theory results are presented.
Highly correlated systems. Ionization energies of first row transition metals Sc–Zn
Authors: Raghavachari Krishnan , Trucks Gary W.
Published: July 2002

The low-lying ionization potentials of the first row transition metal atoms Sc–Zn are calculated using fourth-order Mo/ller–Plesset perturbation theory (MP4) and quadratic configuration interaction (QCI) techniques with large spd and spdf basis sets. Two ionic states have been considered for each atom yielding a total of 20 different ionization processes which we have included in this study. For Sc+–Cu+, the ionic states considered have dns1 and dn+1 orbital occupations and for Zn+, the d10s1 and d9s2 states were studied. The MP4 method accurately reproduces the ionization potentials of Sc–Fe, but is found to be inadequate for Co–Zn. In contrast, the QCI technique performs uniformly for all ionization energies with a mean deviation from experiment of only 0.13 eV (with the spdf basis set) after inclusion of relativistic corrections.
Energy transfer in silver-doped rare gas matrices
Authors: Hebert T. , Wiggenhauser H. , Kolb D. M.
Published: July 2002

The energy transfer between Ag atoms and dimers isolated in rare gas matrices has been studied by time-resolved fluorescence spectroscopy. In the presence of dimers, excitation of the atomic 5s→5p transitions produces not only the emission bands of the Ag atom, but also those of the Ag dimer. This process has been investigated and the mechanism identified as a Förster–Dexter-type energy transfer from silver atoms (donor) to dimers (acceptor). Rise and decay curves of the emissions from atoms and dimers have been analyzed in a concentration dependence study and a critical radius for the energy transfer of 39±5 Å was derived.
Statistical structure of soluble conjugated polymers. I. Conformation and electronic properties
Authors: Viallat Annie , Rossi Giuseppe
Published: July 2002

Recently we proposed a new approach to deal jointly with the presence of conformational disorder and the requirements imposed by the electronic structure of conjugated polymers in solution. The description of the system is based on the one-dimensional tight binding Hamiltonian modified in such a way to account for dependence of the electronic hopping constants on the conformational degrees of freedom. We are able to treat numerically the full statistical problem resulting from such a microscopic description. Effects arising from steric repulsion and other possible sidegroup interactions can be dealt with, using an appropriate effective potential. In this paper we present a number of new results, that we have obtained using this approach. In particular, we analyze in detail the dependence of both conjugation and persistence length on temperature and on the strength of the steric potential. These and the corresponding results for the structure of the energy spectrum suggest the existence of joint electronic and conformational changes in qualitative agreement with the thermochromic effects experimentally observed.
Overtone-excited HN3(X̃ 1A′): Anharmonic resonance, homogeneous linewidths, and dissociation rates
Authors: Foy B. R. , Casassa M. P. , Stephenson J. C. , King D. S.
Published: July 2002

High-resolution spectra have been obtained for the predissociative N–H stretching overtone levels 5ν1 (15 120 cm−1) and 6ν1 (17 670 cm−1) of HN3, cooled in a free-jet expansion. The spectral bandwidth (Doppler limited at 0.007 cm−1 full width at half-maximum (FWHM) is sufficiently narrow to reveal the homogeneous linewidths of individual rovibrational transitions in the 6ν1 band, for which we previously measured the dissociation lifetime in time-domain experiments. Two distinct manifestations of vibrational coupling characterize the spectra: (a) anharmonic mixing of the N–H stretch with other vibrational motions to give a complex spectrum of vibrational eigenstates and (b) homogeneous widths of the resultant states determined by the dissociation lifetime. The results are discussed with reference to previous studies of overtone spectroscopy and intramolecular mixing. Time-domain measurements of dissociation rates are reported for four vibrational levels with zero-order labels 5ν1 +νx. Over the range 15 100 cm−1–17 700 cm−1, the dissociation rate increases monotonically with vibrational energy, with no apparent ‘‘mode-specific’’ variations. This is interpreted in terms of the highly mixed character of any particular vibrational eigenstate that is excited.
An a
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o intermolecular potential for the carbon monoxide dimer (CO)2
Authors: van der Pol A. , van der Avoird A. , Wormer P. E. S.
Published: July 2002

We have constructed an analytical potential energy surface for CO–CO by means of ab initio calculations for the electrostatic and first-order exchange interactions and by the use of accurate dispersion coefficients recently calculated in our group. Parameter-free damping functions account for second-order exchange and penetration effects. The anisotropy of this potential is represented by an expansion in spherical harmonics for the molecules A and B, up to LA, LB=5 inclusive. The second virial coefficients calculated with this potential, including quantum corrections, lie within the experimental error bars over a wide temperature range.
Mobility of helium ions in neon: Comparison of theory and experiment
Authors: Johnsen R. , Tosh R. , Viehland L. A.
Published: July 2002

The mobility of helium ions in neon gas has been computed using an interaction potential inferred from spectroscopic data and the results are compared to values obtained by a drift-tube experiment. The computed values accurately reproduce the dependence of the mobility on E/n0 at two temperatures, 77 and 300 K, thus supporting the potentials inferred from molecular spectroscopy.
Vector correlations in the photofragmentation of HN3
Authors: Gericke K.-H. , Theinl R. , Comes F. J.
Published: July 2002

Hydrazoic acid was excited to its lowest electronic excited state Ã 1A″ and the fragments were analyzed by high resolution Doppler spectroscopy. The NH fragment is rotationally cold, while N2 is strongly internally excited fint(N2)=0.48. The Λ doublets are populated statistically. The alignment of NH rotation vs the transition dipole moment of the parent is low (βμJ≤0.14). The vector correlation between the translational (vNH) and rotational (JNH) motion of the NH fragments is positive and increases with increasing JNH, indicating a preferential parallel alignment of vNH and JNH(βvJ≂0.40). The observed correlation between the transition dipole moment of the parent and the NH(1Δ) recoil velocity is negative at low NH rotations [βμv(JNH=2)=−0.4] and increases to positive values with increasing JNH. The HN3 distorts from a nonplanar configuration after excitation of a linear-bent electronic transition in the NNN framework, resulting in a strong N2 rotation and relatively weak NH rotation. The upper potential surface must be dependent on the torsional angle of the NN–NH system.
The infrared spectrum of cyclotetraoxygen, O4: A theoretical investigation employing the single and double excitation coupled cluster method
Authors: Dunn Kevin M. , Scuseria Gustavo E. , Schaefer Henry F.
Published: July 2002

Cyclotetraoxygen is hypothesized to be a substance of high energy density. In order to aid in the identification of this molecule, its structure and infrared spectrum have been predicted using the coupled cluster single and double excitation (CCSD) method. At this level of theory, the bond length is predicted to be 1.473 Å and the O–O–O–O torsional angle is 27°. The heat of formation is estimated to be 83 kcal/mol; thus O4 is predicted on a per atom basis to lie 21 kcal/mol above two separated O2 molecules. The full quartic force field was determined from theory and an anharmonic prediction of the infrared-active fundamentals places them close to 815 and 724 cm−1.
The A 1Σ+u state of the potassium dimer
Authors: Lyyra A. M. , Luh W. T. , Li L. , Wang H. , Stwalley W. C.
Published: July 2002

Using resolved fluorescence from a 1Σ+g Rydberg state to the A 1Σ+u state of K2, we have observed the lowest vibrational levels (v′≤12) of the A 1Σ+u state. Levels v′=21 to 62 (well beyond the previously known v′=12 to 18 region) were observed using the polarization optical–optical double resonance technique. Molecular constants and a Rydberg–Klein–Rees (RKR) potential energy curve based on these constants are given. Franck–Condon factors for the K2, A 1∑+u–X 1∑+g system based on this new A 1Σ+u potential are also given.
Molecular dynamics simulation of diffusion of simple gas molecules in a short chain polymer
Authors: Takeuchi Hisao , Okazaki Keiji
Published: July 2002

Diffusion of simple gas molecules in polymers has been studied using molecular dynamics (MD) computer simulations. Two model systems were used, each composed of short chain molecules having twenty segments and small molecules interacting with the segments. One of the systems is a mimic of polymethylene with oxygen as a solute. For the other system, potential parameters are identical except that the torsional potential of the chains is set to zero to alter the chain flexibility. Self-diffusion coefficients of the small molecules and relaxation times of internal rotations of the chains were calculated at around the density of amorphous polyethylene and at temperatures of 250 to 300 K above glass transitions of the systems. Behavior of the models obtained within the time scale of usual MD simulations agrees well with experimental observations for amorphous polymers above glass transition temperatures. The free volume theory of simple liquids is found to be applicable to the diffusion as well as the relaxation. The diffusion strongly depends on amount of free volume so that thermal expansion of the system considerably affects the diffusion. Both dynamic properties show Arrhenius-type temperature dependence regardless of the chain flexibility. Although the amount of free volume is a dominant factor for the diffusion, the distribution and dynamics of free volume, which depend on the chain flexibility, clearly affect the diffusion for a fixed amount of free volume; the high chain flexibility causes large diffusion coefficients and a low apparent activation energy of diffusion.
Bidimensional tunneling dynamics of malonaldehyde and hydrogenoxalate anion. A comparative study
Authors: Bosch Enric , Moreno Miquel , Lluch José M. , Bertrán Juan
Published: July 2002

One-dimensional and bidimensional tunneling splittings have been calculated in malonaldehyde (MA) and hydrogenoxalate anion (HX) systems. Two different monodimensional paths have been considered: the intrinsic reaction path (IRP) and the linear reaction path (LRP). A bidimensional model that includes the coupling between the proton transfer motion and the vibration of the heavy atoms is then used. We find that with the bidimensional model the splittings are 2 orders of magnitude greater than the monodimensional ones, and close to the previous experimental and theoretical values for the MA when zero point energy is introduced. At all levels of calculation we obtain that the splitting is greater in the MA than in the HX. This fact is attributed to the different size of the rings through which the proton transfer occurs.
Self-diffusion of Brownian particles with hydrodynamic interaction and square step or well potential
Authors: Cichocki B. , Felderhof B. U.
Published: July 2002

We study self-diffusion in a dilute system of Brownian particles with hydrodynamic interaction, hard core repulsion, and an additional square step or well potential. In our model the hard core radius coincides with the hydrodynamic one. The short-time and long-time self-diffusion coefficients are calculated exactly to first order in the volume fraction. They show a wide variety of behavior upon variation of the potential.
Simulation of polymer chains in elongational flow. Steady-state properties and chain fracture
Authors: Cascales J. J. Lopez , de la Torre J. Garcia
Published: July 2002

The behavior of polymer chains in steady, uniaxial elongational flows is studied using the Brownian dynamics simulation technique. Two different types of chain models are considered. One is the bead-and-spring Rouse chain and the other is a chain with breakable connectors that obey a Morse potential. The dynamics of Rouse chains and Morse chains is simulated both without and with hydrodynamic interaction (HI) between chain elements. From the simulated trajectories, steady-state properties such as chain dimensions and elongational viscosities are calculated. When HI is accounted for by using the Rotne–Prager–Yamakawa tensor, the calculated dimensions and viscosities are appreciably lower than when it is neglected. Carrying out simulations with varying elongational rate, it is possible to observe stretching and finally the fracture of the polymer chains. The critical elongational rate, corresponding to infinite elongation in the case of Rouse chains, and the fracture of the Morse chains has been characterized as a function of chain length. When the short length of the simulated chains is accounted for adequately, we find that the elongational rate needed for fracture ε̇f scales with molecular weight M as ε̇f∝M−2. This result, which had already been predicted rigorously without HI, holds in practice as well when hydrodynamic interaction is considered.
Numerical solution of the time-dependent Schrödinger equation in spherical coordinates by Fourier-transform methods
Authors: Dateo Christopher E. , Metiu Horia
Published: July 2002

We present a numerical Fourier-transform method for solving the time-dependent Schrödinger equation in spherical coordinates. The time propagation is accomplished with a novel implementation of a Crank–Nicholson scheme combined with a Feit–Fleck split-operator method. Calculations on model problems give results that are in excellent agreement with those obtained by other methods. The procedure avoids the numerical instabilities associated with the division by the sine of the polar angle. The propagator is unitary, does not require the calculation of any matrix elements, and uses no matrix multiplications other than fast Fourier transforms.
Fourier transform infrared spectroscopy of the BH3 ν3 band
Authors: Kawaguchi Kentarou
Published: July 2002

The gas-phase infrared spectrum of the BH3 ν3 band was observed in the 2600 cm−1 region by a Fourier transform infrared spectrometer. The borane molecule was produced with a discharge in a B2H6 and He mixture. The spectral pattern was consistent with the D3h structure of borane. The observed spectrum between 2450–2730 cm−1 was analyzed to determine band origins, rotational constants, and centrifugal distortion constants. The ν3 band origins of 11BH3 and 10BH3 were 2601.5743(15) and 2615.7935(21) cm−1, respectively, with one standard deviation in parentheses. The B–H bond length was determined for the first time experimentally to be r0=1.190 01(1) Å from the ground state rotational constant B0.
Vibrational relaxation of ozone in O3–O2 and O3–N2 gas mixtures from infrared double-resonance measurements in the 200–300 K temperature range
Authors: Ménard J. , Doyennette L. , Ménard-Bourcin F.
Published: July 2002

The relaxation of ozone laser excited to ν1 and ν3 vibrational states has been studied by means of a time-resolved infrared double-resonance technique using two CO2 lasers. Measurements have been performed in O3–M mixtures (M=O2 and N2) over the 200–300 K temperature range, as found in the upper atmosphere. Rate coefficients for the V–T,R deexcitation of the ν2 and (ν1,ν3) states and for the (ν1,ν3)→ν2 intermode transfer have been deduced from the measurements. They show a positive temperature dependence consistent with that predicted by the Landau–Teller law, which is based upon short-range repulsive interactions. These results should be useful to model more precisely the distribution of the population of ozone upon its various vibrational states in the upper atmosphere.
A molecular dynamics simulation study on nematic–isotropic phase transition of rod-like molecules in NpT ensemble
Authors: Lee S. H. , Kim H. S. , Pak H.
Published: July 2002

The nematic–isotropic (N–I) phase transition of rod-like molecules is investigated by molecular dynamics (MD) simulation. This study is focused on three different aspects. First, a more realistic model than a hard core system is employed. Second, the MD simulation enables us to calculate time-dependent functions. Last, an isothermal–isobaric (NpT) ensemble is used to consider the change of volume at the N–I transition. The results of MD simulation suggest that the interatomic potential plays an important role in the N–I phase transition of rod-like molecules. The MD simulation of the N–I phase transition of rod-like molecules can predict the ‘‘weak first orderedness’’ accurately. Both translational and rotational diffusion coefficients of rod-like molecules in the system with a smaller value of σ are larger than those with a larger value of σ, and the calculated rotational reorientation time of rod-like molecules in isotropic phases lies in the 10−11–10−10 s range.
Dielectric study of the structure of hyperquenched glassy water and its crystallized forms
Authors: Johari G. P. , Hallbrucker Andreas , Mayer Erwin
Published: July 2002

The dielectric permittivity and loss tangent of hyperquenched glassy water (HGW) have been measured for fixed frequencies of 1 and 10 kHz from 80 K to its crystallization temperature and corresponding measurements have been made of the crystallized forms. The effect of thermal cycling has been investigated. Except for a shoulder at T<Tg for 10 kHz measurement, there are no features in the loss tangent until the beginning of crystallization when the increase in tan δ of the water above Tg is dominated by the decrease on the formation of cubic ice and a peak appears. The dielectric loss of HGW at 80 K is ≊5 times more than that of cubic ice formed on crystallization by heating to 195 K and 2.8 times higher at Tg. The dielectric loss of the cubic ice formed on crystallization tends towards a plateau value prior to rapidly increasing with increasing temperature, which is evidence for a low temperature relaxation which vanishes on conversion to hexagonal ice. The plateaulike region indicates the presence of topologically disordered structure of intergranular water. The loss tangent lacks any feature that can be attributed to the presence of ‘‘dangling’’ OH groups. A comparison of these results with those of in vacuo sintered vapor-deposited amorphous solid water (ASW) [Johari et al., J. Chem. Phys. 95, 2955 (1991)] shows that the presence of ≊5% cubic ice in HGW causes its crystallization to occur more rapidly, leading to larger grains or less intergranular space and a lower value of dielectric loss of the cubic ice formed by heating it, than that formed by heating ASW. Implications of these results for the heat evolved in the phase transformation process have been discussed.
Absolute partial cross sections for the parent ionization of the CFx (x=1–3) free radicals by electron impact
Authors: Tarnovsky V. , Becker K.
Published: July 2002

We report absolute partial electron-impact ionization cross sections from threshold to 200 eV for the formation of the parent CFx+ ions from the CFx free radicals (x=1–3). Fast (3–3.5 kV) beams of CF3, CF2, and CF radicals were prepared by near-resonant charge transfer of CF3+, CF2+, and CF+ with triethylamine (TEA), Xe, and NF3. The CF and CF3 neutral beams were found to contain primarily ground state neutral radicals with some evidence for the presence of a small amount of vibrational excitation (0.5 eV or less). Contributions to the CF2+ ion signal obtained at electron energies several electron volts below the 11.4 eV threshold for the ionization of ground state CF2 radicals indicate the presence of CF2 metastables (presumably in the 3B1 state) in the CF2 neutral beam. The level of CF2 metastable contamination was found to depend critically on the exact experimental conditions. At 70 eV, the absolute parent ionization cross sections are 0.38±0.07 Å2 (CF3→CF3+), 1.03±0.16 Å2 (CF2→CF2+), and 1.25±0.19 Å2 (CF→CF+), respectively.
Calculations of the relative free energies of aqueous solvation of several fluorocarbons: A test of the bond potential of mean force correction
Authors: Gough Craig A. , Pearlman David A. , Kollman Peter
Published: July 2002

The relative free energies of aqueous solvation of several fluorinated derivatives of methane were calculated using the free energy perturbation (FEP) method. The calculations in general duplicated the experimental free energies with relatively good accuracy, but the calculation of the bond potential of mean force (bond-PMF) contribution [D. A. Pearlman and P. A. Kollman, J. Chem. Phys. 94, 4532 (1991)] was necessary in order to get the most satisfactory agreement with experiment. In particular, it was necessary to use this contribution to obtain even qualitatively correct results for the relative free energies of hydration of methane and tetrafluoromethane. The reasons for this are discussed in terms of the accurate calculation of the effect of the size of the solute. In addition, it is noted that the bond-PMF contribution is important even for FEP calculations not involving large changes in size, such as the ethanol to ethane perturbation, if the length of a bond to a disappearing atom is changed during the perturbation. The relative free energy of aqueous solvation for ethanol and ethane was calculated to demonstrate that if the bond between the oxygen and the hydroxyl hydrogen being removed is ‘‘shrunken’’ during the perturbation without including the bond-PMF correction, the calculated free energy is too low by ∼3 kcal/mol.
Experimental and theoretical study of the reaction K+HCl
Authors: Helmer M. , Plane J. M. C.
Published: July 2002

The reaction K+HCl→KCl+H was studied by the pulsed photodissociation at 193.3 nm of KCl vapor to produce K atoms in an excess of HCl and He bath gas, followed by time-resolved laser induced fluorescence spectroscopy of atomic K at 766.5 nm [K(4 2P3/2–4 1S1/2)]. The HCl concentration was monitored by absorption spectroscopy at 184.9 nm. This reaction exhibits non-Arrhenius behavior, with the rate coefficient being given by k(252 K<T<780 K)=(1.69±0.52)×10−10 exp[−(15.21±2.00) kJ mol−1/RT]+(1.51±0.12)×10−11 exp[−(4.94±1.72) kJ mol−1/RT] cm3 molecule−1 s−1. The quoted uncertainties are 2σ. This result is in very good accord with several molecular beam studies, whose relative reaction cross sections can now be put onto an absolute basis. Ab initio calculations were then employed to determine the saddle points on the reaction potential energy hypersurface as a function of the K–Cl–H angle. There is a marked steric effect, with the reaction proceeding through a linear transition state or one that is strongly bent (θKClH=49.1°). The reaction is also characterized by a late barrier, in accord with the observed enhancement of the reaction cross section by vibrational excitation of the HCl. Transition state theory calculations with the linear transition state are shown to be in excellent accord with the experimental results, and indicate that the non-Arrhenius behavior of the reaction is caused by a very loose transition state, rather than a significant contribution to the reaction from vibrationally excited HCl at higher temperatures. Finally, the influence of the reverse reaction on the chemistry of meteor-ablated potassium in the upper atmosphere is discussed.
Spectral line shape parameters for HF in a bath of Ar are accurately predicted by a potential inferred from spectra of the van der Waals dimer
Authors: Green Sheldon , Hutson Jeremy
Published: July 2002

An interaction potential for Ar–HF, which was recently determined from extensive spectroscopic data for the van der Waals dimer and which includes dependence on the HF vibrational state, has been used with accurate close-coupling molecular scattering calculations to predict line shape parameters for the pure rotational and the fundamental and first overtone vibrational bands of HF in a bath of Ar. Agreement with experiment is good; in fact, considering inconsistencies among the experimental values, the theoretical values may be the more reliable. This confirms the accuracy of the Ar–HF interaction potential, including the dependence on the HF vibrational level which is sensitively probed by the line shift cross sections.
The existence of structure progressions and wetting transitions in intermediately disordered monolayer alkyl chain assemblies
Authors: Allara David L. , Parikh Atul N. , Judge Elizabeth
Published: July 2002

Partial monolayers of uniform length alkyl chains [CH3(CH2)n–; n=10–21] bonded at constant coverage onto a disordered lattice of silanol groups on amorphous SiO2, exhibit a distinct change in surface wetting behavior for 14≤n≤17 which correlates closely with a significant shift in the average conformational ordering of the chains. These data provide definitive evidence for the existence of a family of surface-constrained, intermediately ordered phases of discrete length, flexible chains and imply that broad classes of these families exist.
Experimental evidence for an isomeric transition of size selected methanol hexamers
Authors: Buck Udo , Ettischer Ingo
Published: July 2002

We have measured the predicted isomeric transition between the two energetically lowest lying isomers of S6 and C2 symmetry of (CH3OH)6. The clusters are size selected by momentum transfer in collisions with He, and identified by their infrared spectra. The transition occurs at a nozzle temperature of 368 K corresponding to a cluster temperature of 23 K.
Pulsed gradient spin echo nuclear magnetic resonance for molecules diffusing between partially reflecting rectangular barriers
Authors: Coy A. , Callaghan P. T.
Published: July 2002

The application of pulsed gradient spin echo nuclear magnetic resonance (NMR) to the case of molecules trapped between two plane parallel boundaries, has been examined theoretically, with computer simulation and by experiment. A new closed-form analytic expression is obtained for the averaged propagator and the echo attenuation when the walls have finite relaxivity and this expression is verified by computer simulations. It is shown that ‘‘diffraction’’ effects are still strongly apparent when wall relaxation is taken into account and that deviations in the barrier spacing parameter obtained from the position of the echo minimum, are weak. In particular we show that for the pulsed gradient spin echo (PGSE) pulse separation time on the order of a2/2D, the deviation is less than 10% provided that the relaxation is not so severe as to reduce the zero gradient signal amplitude below 10% of its unrelaxed value. We further examine the influence of finite gradient pulse and find, as with wall relaxation, that diffusion during the gradient pulse has the effect of shifting to higher q the position of the first minimum in the echo diffraction pattern. The diffusive diffraction effect is demonstrated experimentally using a stack of pentane-filled microcapillaries of wall spacing 100 μm, and fits to the data yield realistic values for the known experimental parameters.