bluuebunny/crossref_metadata_2025 huggingface dataset:
The nature of the β process for polar solutes in four glassy forming solids
Authors: Ahmed M. S. , Crossley J. , Hossain M. S. , Kashem M. A. , Saleh M. A. , Walker S.
Published: February 2003

Dielectric constant and loss data have been obtained for several rigid dipolar substituted benzenes in up to four glass forming solvents. Measurements were made between 10 and 105 Hz from about 80 K up to room temperature. The broad loss curves were analyzed to give relaxation times which were then used to determine Eyring activation enthalpies ΔHE. For the glassy systems dipole reorientation, at temperatures well below the glass transition, Tg is essentially independent of the solvent and is identified as a β process. The ΔHE values of the β process increase appropriately with molecular size but are largely independent of the glassy medium. The β process for these systems is ascribed to molecular relaxation which may possibly occur in a ‘‘solvent cavity.’’
Free energy of a planar two-phase interface
Authors: Percus J. K.
Published: February 2003

The linear liquid–gas interface of a two-dimensional fluid is modeled by a membrane separating vacuum and incompressible liquid. The density reaction to an arbitrary external potential is written down exactly in the form of the inverse relation. From this, the system free energy is obtained as a functional of the resulting density. It is local, but highly singular, and cannot readily be interpreted as an extension of local thermodynamics.
Orientational ordering of flexible trimers on the square lattice
Authors: Baumgärtner A.
Published: February 2003

It is shown that a system of short flexible chain molecules interacting primarily via a local orientation-dependent potential between different chain segments exhibit a phase transition between a disordered phase and an orientationally ordered phase. The effect of intramolecular and intermolecular interaction on the thermodynamics of small chain molecules each consisting of two flexible jointed segments (trimer) on the square lattice, has been studied using Monte Carlo methods. Systems consisting of up to 4840 chains have been considered. The intramolecular (configurational) energy of one trimer is εB<0 if the two segments are colinear and zero otherwise. The intermolecular energy is εS<0 for two parallel oriented, nearest neighbor segments of two trimers. No evidence for a phase transition is found for vanishing intermolecular interaction (εS=0, εB<0). But continuous phase transitions are found if local orientation-dependent intermolecular interactions are included (εS<0, εB≤0). Various quantities of interest as transition temperature, orientational order parameter and internal energy are discussed.
A unified formulation of the constant temperature molecular dynamics methods
Authors: Nosé Shuichi
Published: February 2003

Three recently proposed constant temperature molecular dynamics methods by: (i) Nosé (Mol. Phys., to be published); (ii) Hoover et al. [Phys. Rev. Lett. 48, 1818 (1982)], and Evans and Morriss [Chem. Phys. 77, 63 (1983)]; and (iii) Haile and Gupta [J. Chem. Phys. 79, 3067 (1983)] are examined analytically via calculating the equilibrium distribution functions and comparing them with that of the canonical ensemble. Except for effects due to momentum and angular momentum conservation, method (1) yields the rigorous canonical distribution in both momentum and coordinate space. Method (2) can be made rigorous in coordinate space, and can be derived from method (1) by imposing a specific constraint. Method (3) is not rigorous and gives a deviation of order N−1/2 from the canonical distribution (N the number of particles). The results for the constant temperature–constant pressure ensemble are similar to the canonical ensemble case.
Calculations on the electric conductivity of a lyotropic mesophase with perforated lamellae
Authors: Photinos P. J. , Saupe A.
Published: February 2003

We calculate the electric conductivity of a system consisting of parallel layers of insulator, embedded into a uniform conductor. Each layer is perforated by holes, which are arranged on a two-dimensional hexagonal lattice. We investigate various stacking patterns of the layers, in volume fractions of 40% to 50%. The results are used to interpret conductivity measurements on the lamellar lyotropic mesophase. We find that on the basis of conductivity anisotropy, a layered structure of disk-shaped aggregates, and a structure of perforated layers are not readily distinguishable. The analysis of experimental data indicates that the amphiphile layers in a lamellar phase can be perforated to about 20% of the area. Also, the interpretation of the conductivity data permits evaluation of the periodicity of the layered structure.
Selective Λ-doublet population of OH in inelastic collisions with H2: A possible pump mechanism for the 2Π1/2 astronomical OH maser
Authors: Andresen P. , Häusler D. , Lülf H. W.
Published: February 2003

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Singlet–triplet perturbations in pyrimidine. Magnetic field effects on collision-induced intersystem crossing
Authors: Matsumoto Yoshiyasu , Pratt David W.
Published: February 2003

We report the discovery of isolated singlet–triplet level anticrossings in the rotationally resolved laser-induced fluorescence spectrum of pyrimidine, and the study of magnetic field effects on the decay properties of the molecule in the vicinity of these avoided crossings, both in the absence and presence of collisions. Analysis of the data shows that the cross section for intersystem crossing induced by low-energy collisions with helium is proportional to the average triplet character of the prepared state, in agreement with the ‘‘too-many level’’, small molecule theory of Freed.
Observation of the ν3 fundamental band of HCO+
Authors: Foster S. C. , McKellar A. R. W. , Sears Trevor J.
Published: February 2003

The ν3 (C–O stretching) band of HCO+ has been detected using a tunable diode laser spectrometer and a liquid nitrogen cooled hollow cathode absorption cell. Eight lines in the R branch, ranging from R(6) to R(17) were measured with an accuracy of 0.001 cm−1. Combining these data in a least-squares fit with the previously measured v3=1, J=1←0 microwave transition, the following parameters were determined: ν0=2183.9496 cm−1, B=44 299.904(10) MHz, and D=82.96(13) kHz.
Improved and extended high-pressure PVT measurements for argon
Authors: Morris Edwin C.
Published: February 2003

Recently reported measurements of the density of argon have been repeated using improved experimental techniques. The overall uncertainty in the results has been reduced from 0.05% to 0.03%, and the pressure range extended from 480 to 620 MPa.
Electron spin relaxation in myoglobin below 1 K: Positive identification of a phonon bottleneck
Authors: Muench P. J. , Askew T. R. , Colvin J. T. , Stapleton H. J.
Published: February 2003

Published relaxation data from ten samples of paramagnetic proteins are compared to illustrate the uncertainty which existed in identifying the anomalous low temperature relaxation mechanism in frozen solutions of proteins. Relaxation involving localized two level tunneling states or a phonon-limited direct process can explain the T2 temperature dependence of the relaxation rate that is observed in some proteins at temperatures above 1 K. Relaxation data on myoglobin at a microwave frequency of 16.545 GHz and in the temperature range between 0.4 and 1.2 K are presented. These data exhibit a coth2(ℏω/2kBT) dependence upon temperature and identify the relaxation process as phonon limited.
The polar tensors, effective charges, and infrared intensities of silane
Authors: Ferreira M. M. C. , Bassi A. B. M. S.
Published: February 2003

Symmetry conditions of a partially deuterated silane SiD3H have been used to separate overlapped bands and to determine the correct signs of the dipole moment derivatives for SiH4, SiD4, and SiD3H. These conditions were also able to predict the intensity of the A3′ band of SiD3H. Polar tensor isotopic invariance and the F and G sum rules also played a fundamental role in this work. Separation ratios, polar tensors, and effective charges of the silanes are reported.
Radiative and collisional properties of CsH (A 1Σ+) state. I. Ratios of transition probabilities and radiative lifetimes
Authors: Ferray M. , Visticot J. P. , Telle H. , Sayer B.
Published: February 2003

CsH produced by an electrical discharge in a Cs+H2 mixture is photoexcited to a A 1∑+ (v′, J′) state, either by a cw laser or a pulsed laser. We measure the ratios ρP,R of transition probabilities corresponding to the fluorescence lines A 1∑+ (v′=6, 1≤J′≤18)→X1∑+ (v″=1 and 2, J″) for the P and R branches. ρP,R is observed to vary slowly with J′. We determine also the radiative lifetime τr of A 1∑+ (v′=6, 7, 12, 15, 19) states; τr exhibits a strong decrease (90 to 35 ns) when v′ varies from 7 to 12. These experimental findings will be compared in the following paper (part II) with theoretical predictions.
Deactivation of higher vibration levels of HF by F atoms
Authors: Bott Jerry F.
Published: February 2003

The vibrational relaxation times of HF(v=3) and HF(v=1) in the presence of F atoms have been measured in a slow-flow fluorescence cell at room temperature. The HF is pumped to vibrational levels v=1, 2, and 3 by a pulsed HF laser. It was found that F atoms deactivate HF(v=3) faster than they do HF(v=1) by a factor of 20.8±2.0. This dependence of the deactivation rate coefficient on v agrees with that determined for the exothermic V–V or V-R,T deactivation of HF(v=1 to 3) by N2, O2, D2, and HCl.
Molecular dynamical studies of the dissociation of a diatomic molecular crystal. II. Equilibrium kinetics
Authors: Trevino S. F. , Tsai D. H.
Published: February 2003

The properties of a molecular dynamical model undergoing equilibrium chemical reactions are reported. It is shown that the kinetics of the modeled reaction is consistent with established thermodynamic considerations. Further, at constant pressure, the relation between the Arrhenius energy of reaction ΔE, the potential energy change upon dissociation Δε, and the work done due to the volume change PΔV, viz, −ΔE=−(Δε+PΔV), is satisfied.
A classical trajectory study of the reaction Be+HF(v,J)→BeF(v′J′)+H in three dimensions
Authors: Chapman Sally
Published: February 2003

The reaction Be+HF(v,J)→BeF(v′,J′)+H in three dimensions is studied using the quasiclassical trajectory method. The surface was recently calculated using ab initio techniques. The surface has a high barrier and a noncollinear transition state. The angular dependence of the surface is weak over a fairly wide range of angles. There is a deep potential well representing the stable molecule HBeF. We have explored the effects of reagent translation, vibration, and rotation on the reaction. The surface exhibits a strong preference for product translation, particularly near threshold. Collisions which pass near the deep potential well make a significant contribution to the reaction only when the HF molecule is internally excited. The dynamics of these collisions are in sharp contrast to the more direct ones. These results are related to recent theoretical work on the LiFH system and to experimental work on alkaline earth-hydrogen halide reactions.
The resonance Raman spectrum of phenoxyl radical
Authors: Tripathi G. N. R. , Schuler Robert H.
Published: February 2003

The Raman spectra of phenoxyl, phenoxyl-d5, and phenoxyl-2,4,6-d3 radicals produced pulse radiolytically in aqueous solutions and observed by time resolved resonance Raman methods are reported. Excitation was mainly in the narrow and moderately intense (0,0) phenoxyl absorption band at 400 nm. These Raman spectra are superimposed on a broad fluorescence in the region of 410–440 nm which is also ascribed to the phenoxyl radical. A very intense Raman band, which is assigned to a mode principally involving the CO stretch (Wilson 7a), is observed at 1505 cm−1. In the fully and partially deuterated radicals this band is at 1489 and 1487 cm−1, respectively, indicating that in the latter instance the vibration is shifted toward a lower frequency, probably by Fermi resonance with an underlying weak vibration. In the protonated radical moderately intense bands are also observed at 990 and 528 cm−1. The higher of these, by virtue of its considerably lower frequency in the deuterated radicals, is assigned to a CH bending mode (Wilson 18a). The other is affected very little by the substitution and is assigned to a CCC bending mode (Wilson 6a). Weak bands at 1157, 1056, and 865 cm−1 in phenoxyl, phenoxyl-2,4,6-d3, and phenoxyl-d5 radicals are assigned to a second CH bending mode (Wilson 9a). Two weak bands are also reported at 1398 and 1331 cm−1 and are attributed to the nontotally symmetric 19b and 14 modes. No Raman band is, however, observed in the 1550–1650 cm−1 region where the Wilson 8a ring stretching motion prominent in semiquinone radicals is expected. The excitation profile of the 1505 cm−1 emission largely follows the narrow absorption spectrum of the radical in the region of 400 nm. Below 390 nm, where the electronic excitation in resonance is to an upper vibrational level, resonance enhancement of the Raman signals is reduced by more than an order of magnitude. The decay of the radical at 10−4 M, as monitored by its Raman signal, corresponds to its loss mainly in second order processes having a rate constant of (2.6±0.3)×109 M−1 s−1, in agreement with measurements by absorption methods at a 50-fold lower concentration.
Molecular multiphoton excitation by phase coherent pulse pairs
Authors: Besnainou S. , Diels J.-C. , Stone J.
Published: February 2003

We theoretically analyze the absorption of media excited by a pair of coherent pulses, as a function of their relative phase, in conditions of multiphoton resonance and negligible dephasing. It is shown that, the periodicity of transmission vs delay and phase, is characteristic of the nature and order of the process. Also a multilevel system may evolve from pure multiphoton resonance to cascade resonance under specific conditions depending on the detunings of the levels involved.
Analysis of Raman excitation profiles for c
i
s-(CH)x and-(CD)x polymers
Authors: Siebrand Willem , Zgierski Marek Z.
Published: February 2003

Excitation profiles of several Raman bands in cis-polyacetylene and its deuterium analog, observed by Lichtmann, Imhoff, Sarhangi, and Fitchen (preceding paper) are compared with the predictions of model calculations. The Raman bands correspond to progressions of totally symmetric carbon–carbon stretching vibrations observed with incident light in the region of the first strong absorption band. The analysis of the profiles of the fundamentals, first overtones and binary combinations, in terms of a model of displaced harmonic oscillators, yields values for the displacement of the modes and thus of the change in carbon–carbon equilibrium distances between the excited state and the ground state of the polyene. The values obtained are comparable to but slightly smaller than those obtained in an earlier analysis of β-carotene, a substituted polyene molecule. The analysis also yields preliminary conclusions about the role of (i) homogeneous and inhomogeneous broadening mechanisms, (ii) factor-group splitting, and (iii) normal-coordinate rotation, in shaping the profiles.
Localization of exciton states in structurally disordered materials
Authors: Blumen A. , Lemaistre J. P. , Mathlouthi I.
Published: February 2003

A comparative study of the influence of the structural disorder on the localization properties of exciton wave functions is presented. The localization behavior of exciton states is analyzed by means of the inverse participation ratio (IPR) and of related quantities. We have performed numerical, configurationally averaged calculations using short-ranged (exchange, octupolar) and long-ranged (dipolar) types of interactions and also including the diagonal disorder. We provide the density of states and the energy distribution of the IPR considering various trap concentrations and interactions which are typical for aromatic crystals like naphthalene.
Nucleation at macroscopic steps
Authors: Mederos L. , Quintana A. , Navascués G.
Published: February 2003

The classical heterogeneous nucleation theory is used to study nucleation at macroscopic steps. Line tension effects are included to give a correct description of the phenomena within the known limits of the theory. A criterion of preferential nucleation at steps (decoration criterion) is given. The expected behavior of the system for low macroscopic contact angles is found.
Objections to a proposal on the rate of entropy production in systems far from equilibrium
Authors: Andresen Bjarne , Zimmermann E. C. , Ross John
Published: February 2003

Theoretical and experimental counter examples are given to a postulate by Sawada that nonequilibrium systems evolve in time along a path of maximum rate of entropy production.
Reply to the Comment by Andresen, Zimmerman, and Ross
Authors: Sawada Yasuji
Published: February 2003

The postulate of the maximum entropy production which the author presented is discussed here in relation to the notion of ‘‘ideal nonequilibrium system.’’
Computation of action integrals for nonseparable classical dynamical systems
Authors: Nesbet R. K.
Published: February 2003

It is shown that classical action integrals J can be expressed as local functions in phase space, computed from a classical trajectory if the frequency constants ω are known. The mean square deviation of components of J from constant values given by this formalism defines a variational objective function whose absolute minimum determines ω and J. Illustrative calculations are presented for a two-dimensional double-well model problem, equivalent to imposing specular reflection at a barrier cutting across a single-well potential. Solutions obtained over a range of values of initial trajectory parameters determine regular solutions of the classical dynamical problem. The proposed formalism essentially constructs a solution of the Hamilton–Jacobi equation from the phase integral computed along a trajectory.
Nonradiative transitions in collisionless perdeuterobenzene
Authors: Knee Joseph L. , Otis Charles E. , Johnson Philip
Published: February 2003

The rates and quantum yields of internal conversion and intersystem crossing have been measured for several vibronic levels of perdeuterobenzene. The molecule existed in the cold and collisionless environment of a supersonic expansion, while the excited state dynamics were measured by time-resolved multiphoton ionization mass spectrometry. It is found that the onset of rapid internal conversion is lowered by deuteration to ∼2400 cm−1 as compared to 3300 cm−1 for benzene-h6.
Hybrid quantum scattering algorithms for long-range potentials
Authors: Alexander Millard H.
Published: February 2003

We investigate hybrid scattering codes which combine the log-derivative integrator (LOGD) of Johnson at short range with one of the three following integrators at moderate to long range: the multichannel WKB algorithm of Johnson, the variable-interval-variable-step (VIVS) algorithm of Parker, Schmalz, and Light, or a new algorithm (AIRY) based on the linear reference potential algorithm of Gordon modified to propagate the log-derivative matrix directly without determination of perturbation corrections. In calculations of S matrices for the collision of two HF molecules, where the off-diagonal coupling decreases only as R−3, the AIRY integrator is found to be 3 times faster than the WKB integrator, at least 5 times faster than the VIVS integrator, and 19 times faster than direct application of the LOGD integrator in the long-range region.
Observation of high vibrational excitation in HCN molecules produced from 193 nm photolysis of 1,3,5-triazine
Authors: Goates S. R. , Chu J. O. , Flynn G. W.
Published: February 2003

Infrared emission from the ν2 bending mode and ν3 C–H stretching mode of HCN have been observed following 193 nm pulsed excimer laser photolysis of 1,3,5-triazine. Using a simple harmonic oscillator analysis, the number of ν2 bending quanta produced in HCN from photolysis of sym-triazine was found to be 70 times larger than the number of ν3 C–H stretching quanta. The combination of a high density of bending vibrational states in HCN and favorable geometry changes which occur in going from 1,3,5-triazine to three HCN molecules, appear to give an unusually pure distribution which maximizes vibrational energy in the HCN bending mode.
The ‘‘most likely’’ potential energy curve for the lowest 3Σ+u state of Li2
Authors: Konowalow Daniel D. , Regan Rebecca M. , Rosenkrantz Marcy E.
Published: February 2003

A most likely potential curve for the 1 3Σ+u state of Li2 with constants De=340.6±15 cm−1, ωe=65.6±0.9 cm−1, ωexe=3.16±0.04 cm−1, Re=7.77±0.22a0 is obtained by a judicious combination of three disparate ab initio computations.
Studies of solvated ion motion: Molecular dynamics results for dilute aqueous solutions of alkali and halide ions
Authors: Nguyen H. L. , Adelman Steven A.
Published: February 2003

A comparative molecular dynamics study of the motion of the alkali cations Li+, Na+, and K+ and the halide anions F− and Cl− in the Stillinger–Rahman revised central field model for water is presented.
Synchrotron radiation study of vibrationally resolved partial photoionization cross sections of CO2 between 64 and 80 nm
Authors: Hubin-Franskin M.-J. , Delwiche J. , Morin P. , Adam M. Y. , Nenner I. , Roy P.
Published: February 2003

Vibrationally resolved partial photoionization cross sections of CO2 have been measured for the X̃ 2Πg ionic state for wavelengths between 64 and 80 nm and for Ã 2Πu and B̃ 2Σ+u from 64 to about 2 nm below the respective thresholds. The autoionization of the Rydberg series converging to Ã and those converging to B̃ is discussed in terms of population of two vibrational levels of the ground electronic state of the ion. The Rydberg series with limit at C̃ 2Σ+g (000) are shown to decay mainly to B̃ 2Σ+u in its (000) and (100) vibrational levels and to a lesser manner to X̃ 2Πg. They do not populate the Ã 2Πu state.
Single-crystal EPR study of bromobis (diethyldithiocarbamato) iron (III) diluted in the lattice of bromobis (diethyldithiocarbamato) arsenic (III)
Authors: Decurtins S. , Wickman H. H.
Published: February 2003

The single-crystal EPR spectra of Fe(S2CNEt2)2Br diluted in As(S2CNEt2)2Br have been recorded at liquid–helium temperature. They display a typical rhombic pattern with g′X =3.07(2), gY =5.08(2), g′Z =1.95(2) in agreement with a ground Kramers doblet, ‖±〉≂‖S=3/2, MS =±1/2〉. The signals exhibit ligand hyperfine structure from the79,81 Br ions. Due to the overlapping of four, symmetry related signals, only a partial analysis of this hyperfine tensor was achieved. It was found that the ligand A-tensor anisotropy is not pronounced and that the magnitudes of the three principal components lie in the range 80–100×10−4 cm−1.
High resolution infrared spectroscopy of the ν1 (NH stretch) and ν2 (CH stretch) bands of HCNH+
Authors: Altman Robert S. , Crofton Mark W. , Oka Takeshi
Published: February 2003

The infrared spectrum of the ν1-band (NH stretch) and the ν2-band (CH stretch) of protonated hydrogen cyanide HCNH+ has been observed based on recent theoretical predictions. This is the first observed spectrum of this ion in any wavelength region. From a least-squares analysis of the rovibrational bands, the following spectroscopic constants have been obtained (in cm−1): ν1 (NH stretch) ν2 (CH stretch) ν0 3482.844 1(10) 3187.863 8(4) B1    1.228 633(39)    1.228 494(19) B0    1.236 024(37)    1.236 067(18) D1    1.574(43)×10−6    1.610(22)×10−6 D0    1.596(42)×10−6    1.620(21)×10−6 It is hoped that the rotational constants determined in this paper will lead to the identification of the microwave spectrum of this molecule in interstellar space.
The rotationally resolved Ã 1A″←X̃ 1A′ spectrum of expansion cooled NCNO: Vibrational fundamentals, rotational constants, and perturbations
Authors: Noble M. , Nadler I. , Reisler H. , Wittig C.
Published: February 2003

The laser excitation spectrum (Ã 1A″←X̃ 1A′, T′0 =11 339 cm−1) of expansion cooled nitrosyl cyanide (NCNO) has been observed at rotational resolution using the technique of two-photon photodissociation followed by laser induced fluorescence detection of the CN fragment. All six excited state fundamental vibrations are assigned. The Ã 1A″ state geometry is derived from the measured rotational structure, and corresponds to a planar, trans conformation. Most of the bands are strongly perturbed by ground state levels, and there is strong evidence suggesting that Ã 1A″/X̃ 1A′ mixing leads to the vibrational predissociation.
Nonequilibrium chemical instabilities in continuous flow stirred tank reactors: The effect of stirring
Authors: Horsthemke W. , Hannon L.
Published: February 2003

We present a stochastic model for stirred chemical reactors. In the limiting case of practical interest, i.e., fast stirring, we solve for the characteristic function in steady state and derive expressions for the stationary moments through a perturbation expansion. Moments are explicitly calculated for a generic model of bistable behavior. We find that stirring decreases the area of the bistable region essentially by changing the point of transition from the high reaction rate state to the low reaction rate state. This is in remarkable agreement with the experimental findings of Roux, et al. Our results indicate that stirring should not be considered simply as an ‘‘enhanced diffusion’’ process and that nucleation plays only a minor role in transitions between multiple steady states in a continuous flow stirred tank reactor (CSTR).
Temperature-dependent ESR hyperfine constants for nitroxides and orientational correlation time determination
Authors: Lee Sook , Ames D. P.
Published: February 2003

A direct experimental observation of temperature-dependent electron-spin-resonance (ESR) hyperfine coupling constants has been made for TANOL nitroxide radicals in a polycarbonate polymer. This observation is based on the accurate rigid-limit spin Hamiltonian constants obtained from the ESR powder spectra of deuterated TANOL enriched with 15N isotope (I=1/2) (TANOLD–15N) at 77 K, and the isotropic hyperfine splitting factor obtained from the completely motionally shifted 15N ESR hyperfine lines at temperatures >445 K. From this reduction in the isotropic hyperfine splitting, it is determined that the hyperfine splitting factor ‖Az/gzμB‖ is reduced by 0.16 and 0.11 mT for TANOL–15N and TANOL–14N, respectively, when the temperature is increased from 77 to 445 K. The latter value for normal TANOL–14N is consistent with the temperature-dependent hyperfine splitting factor ‖Az/gzμB‖ deduced recently from the application of a new theoretical and experimental approach for studying slow-tumbling nitroxide radicals. It is shown that the temperature-dependent nitroxide hyperfine coupling constants will directly affect the measurements of slow orientational motional correlation times by conventional methods.
Jahn-Teller EPR spectra of Cu2+ in MgSiF6⋅6H2O
Authors: Rubins R. S. , Tello Lucio N. , De D. K. , Black T. D.
Published: February 2003

The 34 GHz EPR spectrum of Cu2+ in MgSiF6⋅6H2O showed a ‘‘static’’ Jahn–Teller effect at 4.2 K with two inequivalent Jahn–Teller sites per unit cell. The six axially symmetric sets of Cu2+ lines had their z axes parallel to the three tetragonal axes of two cubes, which were rotated by approximately 40° with respect to each other about a common [111] axis, which is the crystal c axis. The measured spin-Hamiltonian parameters at 4.2 K for each set of lines were g∥=2.47±0.01, g⊥=2.10±0.01, and ‖A∥‖=(110±3)×10−4 cm−1. There was a gradual decrease in the anisotropy of the spectrum on warming the crystal, with a single, nearly isotropic line being observed above 220 K. At 270 K the spectrum had axial symmetry about the c axis with g′∥ =2.23±0.01 and g⊥ =2.25±0.01. The temperature evolution of the spectrum was interpreted in terms of a Boltzmann distribution over inequivalent distorted Jahn–Teller configurations, with one potential well lowered by an amount Δ≊105 cm−1 below the other two.
Effect of deuterium substitution on the triplet state dipole moments of substituted benzaldehydes
Authors: Winkler I. C. , Hanson D. M.
Published: February 2003

The laser induced phosphorescence excitation spectra of 2, 4, 5-trimethylbenzaldehyde -1h1 and-1d1, and 2, 5-dimethylbenzaldehyde -1h1 and -1d1 isolated in durene single crystals have been obtained, and the dipole moment difference vectors associated with the bands identified, from their intensities and positions in the spectra, as the T1 and T2 origins have been measured. Since the data indicate that the T2 origin is mixed with T1 vibronic levels, the symbol T2 is used only as a convenient label. The T2 band of TMB shifts 26 cm−1 to the red upon deuteration, and the corresponding band in DMB shifts 36 cm−1 to the blue. The magnitude of the dipole moment difference vector found for this band is +1.2 D in TMB and −2.0 D in DMB. The spectral shifts upon deuteration and these dipole moment difference values both are consistent with the T2 band in TMB being predominantly ππ* in character and the T2 band in DMB being predominantly nπ*. Upon deuteration, the dipole moment difference vector decreases for TMB and appears to increase for DMB. These variations may indicate that the coupling of the T2 origin with the T1 vibronic levels decreases in TMB and increases in DMB upon substitution of deuterium for the aldehydic hydrogen. No deuterium isotope effect on the dipole moment difference vectors for the T1 origin bands of these molecules was observed.
The dependence of the velocity autocorrelation function on the intermolecular potential and on the memory function
Authors: Toxvaerd So/ren
Published: February 2003

A new molecular dynamics algorithm enables us to calculate the coefficients in the Mori expansion of the velocity autocorrelation function φ(t) accurately. These coefficients are used to analyze the influence of the attractive forces and to find memory functions for φ. The results for a Lennard-Jones system show that the attractive forces contribute to φ with a Gaussian-like term and with the maximum at a time which corresponds to about five mean collision times. The effect on φ of increasing the density can be separated into a short-time effect, caused by a short-time memory, and an effect of the same form and location as the effect from the attractive forces. The short-time memory can be described by a simple Gaussian and with a relaxation time in good agreement with the mean collision time obtained from perturbation theories.
Ultrasound measurements of polystyrene solutions in the binodal, theta, and good-solvent regions
Authors: Fenner D. B.
Published: February 2003

The absorption spectra and velocity of longitudinal ultrasound are reported for solutions of monodisperse polystyrene, with molecular weights of 9×104 and 6×105, in cyclohexane. The solutions were studied in the homogeneous region above the miscibility coexistence curve (binodal), including the critical-point, theta, and good-solvent conditions, with concentrations in the dilute, semidilute, and concentrated regions. Samples near the critcal point for optical opalescence did not show an ultasound-absorption anomaly, as expected in binary mixtures. The excess ultrasound-absorption spectra, from 1 to 90 MHz were always found to be satisfactorily fit with the Kawasaki-type two-mode multiple relaxation, allowing three adjustable parameters. The fit characteristic-frequency parameter was found to follow scaling relationships: increasing with the temperature interval above the binodal and decreasing with the solution concentration. Thus, we infer that this excess absorption was caused by polymer-segregation fluctuations, rather than critical fluctuations. Slowing down of chain dynamics occurred near the binodal in concentrated solutions, and, for the higher molecular-weight sample, in the good-solvent concentrated region. The latter effect was due to chain entanglement. The characteristic frequency was independent of molecular weight in the Daoud–Jannink scheme of corresponding states. Under theta conditions, this frequency had a weak local minimum at the critical concentration of corresponding states, and showed a sharp decrease in the concentrated region. Various empirical exponents for the scaling of this frequency are determined from the data. The overall temperature, concentration, and molecular-weight dependencies of the measured characteristic frequencies are more consistent with the polymer-chain dynamics of self-diffusion or reptation, than that of mass diffusion. The low-frequency sound velocity showed no anomalies, was always slightly larger than that for the pure solvent, and increased with concentration.
Semiflexible self-avoiding polymers
Authors: Petschek R. G.
Published: February 2003

A simple two dimensional model for semiflexible polymers is discussed. A rigorous lower bound is obtained for the free energy and a rigorous upper bound is obtained for the energy. These bounds evaluated for infinite systems are shown to be in serious conflict with results obtained by a Monte Carlo simulation of a finite system.
Effects of inhomogeneity in the infrared photodissociation spectra of (C2H4)2, C2H4⋅HF, and C2H4⋅HCl
Authors: Casassa M. P. , Western Colin M. , Janda Kenneth C.
Published: February 2003

A general multilevel band shape model for describing predissociation spectra is presented. The model is applicable to systems of noninteracting molecules for which the predissociation rate is greater than the Rabi frequency. The multilevel band shape formula is used to analyze previously reported results for (C2H4)2, C2H4⋅HF, and C2H4⋅HCl. Using a reasonable set of molecular constants two apparently contradictory band shape measurements of the (C2H4)2 ν7 vibrational predissociation transition are shown to be in agreement. The anomalous width of the high power pulsed band shape measurement is attributed to orientational inhomogeneity of a hybrid band. Band shape simulations of C2H4⋅HF and C2H4⋅HCl indicate that the former spectrum is affected by rotational substructure while the latter is not. Other sources of inhomogeneity, including Fermi resonance, are discussed in terms of their effects on band shapes and intensities.
Spin-correlated crystal field parameters for lanthanide ions substituted into LaCl3
Authors: Crosswhite H. M. , Newman D. J.
Published: February 2003

Fits to spectral energy levels for Ho3+:LaCl3 and Gd3+:LaCl3 establish, for the first time, empirical values of the lanthanide spin-correlated crystal field parameters. The fitted rank 6 parameters are consistent with the superposition model analysis and provide the solution to a long standing problem in the fitted values of the crystal field parameters for trivalent lanthanide ions in LaCl3.
Theoretical study of H2O–HF and H2O–HCl: Comparison with experiment
Authors: Szcześniak M. M. , Scheiner Steve , Bouteiller Y.
Published: February 2003

The H bonds in H2O–HF and H2O–HCl are studied and compared using ab initio molecular orbital methods and the results compared to experimental data. Basis sets used are: (i) triple valence 6-311G** and (ii) double ζ with two sets of polarization functions. Electron correlation, included via second- and third-order Mo/ller–Plesset perturbation theory, is found to have profound effects on both systems, particularly H2O–HCl. Both H bonds are strengthened substantially with a concomitant reduction in length. H-bond energies and geometries calculated at correlated levels are in excellent accord with available experimental information. In both systems, all levels of theory indicate the equilibrium geometry contains a pyramidal arrangement about the oxygen atom. However, the difference in energy between this structure and a C2v planar arrangement is found to be small enough that consideration of probability amplitudes in the ground vibrational level leads to nearly equal likelihood of observing either geometry. Agreement between experimental vibrational frequencies in H2O–HF and those calculated at correlated levels and involving quadratic, cubic, and quartic force constants is quite good. An explanation is offered for the increase in HX bond length which occurs at SCF and correlated levels upon H-bond formation based upon nearly linear relationships between this length on one hand and subunit dipole moment and polarizability on the other. The dispersion energy is found to be a very sensitive, almost exactly linear function of the increase of H–X bond length. This energy contributes substantially to the weakening of the HX bond upon complexation.
Short time relaxation processes in liquids from viscosity and light scattering studies in molten KCl 2BiCl3
Authors: Torell L. M. , Ziegler D. C. , Angell C. A.
Published: February 2003

Brillouin scattering, viscosity, and calorimetric studies of the congruently melting glass-forming compound KCl⋅2BiCl3 have been made to clarify and extend a pattern of relaxation behavior seen over wide temperature ranges in a recent study of the simple ionic liquid system KNO3 +Ca(NO3)2. Viscosity data for the two systems covering nearly four orders of magnitude superimpose when scaled by the glass temperature Tg ratio, and comparable parallels are seen in the hypersonic absorption behavior. Sound velocity and absorption data are consistent with single relaxation time kinetics at high temperature in accordance with findings for the KNO3 +Ca(NO3)2 system. In each system high temperature Arrhenius behavior breaks down when τ>10−11s. It is shown in the present system that this is forced by the approach of the system to a thermodynamic limit where liquid and crystalline entropy contents would equalize: the limit falls only 26 K below Tg. A relationship to the behavior of argon is considered.
The homogeneous nucleation of nonane
Authors: Adams G. W. , Schmitt J. L. , Zalabsky R. A.
Published: February 2003

The homogeneous nucleation rate of n-nonane has been measured as a function of temperature and supersaturation ratio in a precision fast-expansion chamber. The measured nucleation rate ranges from 102 to 105 drops/cm3 over the temperature range 215–270 K. The results have been compared to the classical theory and to the classical theory with the RKC replacement factor. The RKC theory functional form is the basis for an empirical rate equation to fit the data. A full listing of the thermodynamic constants used for the reduction of the data is given.
Bulk properties of two-phase disordered media. I. Cluster expansion for the effective dielectric constant of dispersions of penetrable spheres
Authors: Torquato S.
Published: February 2003

We derive a cluster expansion for the effective dielectric constant ε* of a dispersion of equal-sized spheres distributed with arbitrary degree of impenetrability. The degree of impenetrability is characterized by some parameter λ whose value varies between zero (in the case of randomly centered spheres, i.e., fully penetrable spheres) and unity (in the instance of totally impenetrable spheres). This generalizes the results of Felderhof, Ford, and Cohen who obtain a cluster expansion for ε* for the specific case of a dispersion of totally impenetrable spheres, i.e., the instance λ=1. We describe the physical significance of the contributions to the average polarization of the two-phase system which arise from inclusion-overlap effects. Using these results, we obtain a density expansion for ε*, which is exact through second order in the number density ρ, and give the physical interpretations of all of the cluster integrals that arise here. The use of a certain family of equilibrium sphere distributions is suggested in order to systematically study the effects of details of the microstructure on ε* through second order in ρ. We show, furthermore, that the second-order term can be written as a sum of the contribution from a reference system of totally impenetrable spheres and an excess contribution, which only involves effects due to overlap of pairs of inclusions. We also obtain an expansion for ε* which is exact through second order in φ2, where φ2 is the sphere volume fraction. We evaluate, for concreteness, some of the integrals that arise in this study, for arbitrary λ, in the permeable-sphere model and in the penetrable concentric-shell model introduced in this study.
Inherent structures and distribution functions for liquids that freeze into bcc crystals
Authors: Weber Thomas A. , Stillinger Frank H.
Published: February 2003

The collection of potential energy minima in a condensed phase determines its inherent packing structures. We have examined these inherent structures for a simple model substance which (like Na) freezes into a bcc crystal. Molecular dynamics trajectories at several different temperatures were periodically sampled, and each of the configurations was ‘‘quenched’’ by a steepest-descent construction into a nearby potential energy minimum. The resulting collections of inherent structures possess quenched pair correlation functions that are nearly independent of the initial-state temperature, provided the latter correspond to fluid states. Attempts to reconstitute the equilibrium pair correlation functions by thermally broadening the quenched versions, using harmonic Einstein or Debye vibrational approximations, were clear failures. Evidently the true broadening phenomenon entails substantial anharmonicity.
Point defects in bcc crystals: Structures, transition kinetics, and melting implications
Authors: Stillinger Frank H. , Weber Thomas A.
Published: February 2003

Structures corresponding to various potential energy minima have been examined for a classical model whose pair interactions produce a body-centered-cubic crystalline ground state. The method used is molecular dynamics computer simulation for 128 particles with frequent steepest-descent mapping onto nearby minima. The elementary structural excitation out of the crystalline absolute minimum is creation of a vacancy, split-interstitial defect pair. This excitation process upon repetition shows a defect softening of the medium. Transition states (saddle points) have been located for some pairs of neighboring minima, and vibrational modes have been calculated for minima and for transition states. A simple melting theory based on these observations is proposed which satisfactorily describes the model’s first-order melting behavior.
Frequency and intensity patterns of LAM progression in weakly coupled chains: The case of stearic acid and stearyl alcohol
Authors: Minoni Giuseppe , Zerbi Giuseppe , Rabolt J. F.
Published: February 2003

The coupled oscillator model previously described for the interpretation of longitudinal acoustical modes (LAM) of weakly coupled chain molecules has been extended in order to account for the observed frequency and intensity patterns in stearic acid and stearyl alcohol. The model and its general validity are discussed and a specific comparison is made with data obtained from Raman measurements on hexatriacontane (n-C36H74), stearic acid (n-C17H35–COOH), and stearyl alcohol (n-C18H37OH).
Solvent dependence of the deuteron quadrupole coupling constant of CDCl3 determined by 2H spin-lattice relaxation and Raman line shape studies
Authors: Gregory R. V. , Asdjodi M. R. , Spencer H. G. , Beyerlein A. L. , Savitsky G. B.
Published: February 2003

The deuterium quadrupole coupling constant in deuterochloroform was determined in an inert solvent (C6H12) and in polar solvents (neat liquid, C6H6, and THF), by combining NMR relaxation data with Raman line shape analysis of the ν1 band (2253 cm−1) of deuterochloroform. The QD value in a 20 mol % solution of CDCl3 in C6H12 was found to be (186±4 kHz), significantly higher than any previously reported value obtained in liquid crystals. A substantial lowering of the quadrupole coupling constant was observed in polar solvents.
Predissociative lifetimes of single rovibronic levels of the B 3Π0+(v′=3) state of ICl
Authors: Suzuki Takanori , Kasuya Takahiro
Published: February 2003

Laser induced fluorescence (LIF) spectra of the 3–0 band of the B 3Π0+–X 1Σ+ system of I 35Cl and I 37Cl were observed for the first time by the use of a narrow bandwidth pulsed dye laser. Fluorescence lifetimes of the order of 1 ns have been successfully obtained for the rotational levels in v′=3 of the B 3Π0+ state. These measurements have shown that a new model of curve crossing has to be considered for the predissociation other than the well known B–Y crossing or the recently predicted heterogeneous crossing. On the assumption of Voigt profile for each hyperfine component, the line profile was analyzed to give the quadrupole interaction ‖ΔeQq‖ of approximately 0.065 cm−1 for v′=3. The asymmetry of the type of Fano’s profile was recognized on the observed line profile.
The ν1 band of the DO2 radical by difference frequency laser and diode laser spectroscopy: The equilibrium structure of the hydroperoxyl radical
Authors: Lubic Karen G. , Amano Takayoshi , Uehara Hiromichi , Kawaguchi Kentarou , Hirota Eizi
Published: February 2003

The ν1 band of the DO2 radical was observed by difference frequency laser and diode laser spectroscopy with Zeeman modulation. The radical was produced by a 60 Hz discharge in a mixture of CH3OD or CD3OD and O2 flowing through a multiple reflection absorption cell. The observed lines, 181 in total, of six subbands with Ka =2←3, 1←2, 0←1, 1←0, 2←1, and 3←2 were included in a least-squares analysis to determine molecular parameters for the upper state: A=326 929.3(92), B=31 683.68(36), C=28 779.96(46), εaa=−26 039(38), εbb=−381.7(29), εcc =4.3(22), in MHz, and the band origin ν0=2 549.224 65(36) cm−1, where the values in parentheses denote one standard error in units of the last quoted decimal place. By combining this data with that for the ν2 and ν3 bands of DO2 by Uehara et al. and that for HO2 already available in the literature, the equilibrium structure of the hydroperoxyl radical was calculated with estimated uncertainties: re (O–H)=0.9707±0.0020 Å, re(O–O)=1.330 54 ±0.000 85 Å, and θe (HOO)=104.29±0.31 Å.
Spin-Hamiltonian input parameters in the EPR analysis of liquid phase copper complexes
Authors: Basosi Riccardo , Antholine William E. , Froncisz Wojciech , Hyde James S.
Published: February 2003

Electron paramagnetic resonance (EPR) spectra of the copper complex of bis(dimethyldithiocarbamato) copper (II) [65Cu(dtc)2] in the liquid phase (o-toluidine at 20 °C, η≂4.39 cP) have been obtained at five widely varying microwave frequencies: 1.19, 2.51, 3.44, 9.12, and 34.78 GHz. A two-stage procedure has been developed for analysis of these spectra under assumptions of coincident axial magnetic tensors and isotropic rotational diffusion. In the first stage, linewidths, and dynamic second-order frequency shifts are extracted from the spectra. Algebraic manipulation of motional narrowing linewidth and shift equations yields preliminary values for the magnetic parameters and the rotational correlation time together with error estimates. In the second stage, full spectral simulations are made in which goodness-of-fit is optimized over the multidimensional space that is determined by these preliminary values and errors. The first stage is only possible by inclusion of dynamic second-order shifts; a unique solution cannot be obtained from linewidth data alone. Using this procedure, effective rigid-limit parameters have been obtained from liquid-phase data alone. The assumption that these parameters are the same in frozen solution and in the liquid phase is avoided. The high quality of the simulations over a wide range of microwave frequencies gives confidence in the theoretical formulation.
The gamut of alkoxy radicals
Authors: Box Harold C. , Budzinski Edwin E. , Freund Harold G.
Published: February 2003

It is shown that various radicals exhibiting diverse ESR and ENDOR spectral characteristics are nonetheless a closely related family of alkoxy radicals. The relationship is established by correlating the g tensor with crystal structure and by relating dihedral angles inferred from proton hyperfine couplings to dihedral angles inferred from the g tensor and crystal structure. The analysis also serves to demonstrate that an ESR absorption observed in x-irradiated single crystals of uridine 5′-monophosphate is due to an alkoxy radical.