bluuebunny/crossref_metadata_2025 huggingface dataset:
Calculations of proton magnetic shielding constants in polyatomic molecules
Authors: Lazzeretti Paolo , Zanasi Riccardo
Published: February 2003

Proton magnetic shielding constants in H2O, NH3, and CH4 molecules have been evaluated within the framework of coupled and uncoupled Hartree–Fock perturbation theories for the Fock–Dirac density matrix, employing several basis sets of gaugeless Gaussian functions. The results yielded by the largest basis sets are in good agreement with experiment, as well as other ab initio calculations. Some quantities, useful in testing the degree of gauge independence of computed screening constants and in evaluating this property for a gauge whatsoever, are also presented. Numerical tests demonstrate the basic soundness of the uncoupled approach to magnetic shielding proposed here, because of its higher accuracy with respect to other uncoupled schemes and its practicality for larger molecules.
Absorption and pitch relationships in dichroic guest–host liquid crystal systems
Authors: Cole H. S. , Aftergut S.
Published: February 2003

The absorption of a dichroic dye dissolved in chiral nematic hosts in the Grandjean texture increased with decreasing pitch of the host. Absorption was higher for hosts of low birefringence and high order parameter. Boundary condition, homeotropic or parallel (homogeneous), had no appreciable effect on absorption, but threshold voltage was higher for the parallel than homeotropic boundary. The results agreed with theoretical plots relating absorption to pitch.
Kinetics of the reactions of O(3P) atoms with the amines CH3NH2, C2H5NH2, (CH3)2NH, and (CH3)3N over the temperature range 298–440 °K
Authors: Atkinson R. , Pitts J. N.
Published: February 2003

Absolute rate constants for the reaction of O(3P) atoms with CH3NH2, C2H5NH2, (CH3)2NH, and (CH3)3N have been determined over the temperature range 298–440 °K using a flash photolysis–NO2 chemiluminescence technique. The Arrhenius expressions obtained were k2[CH3NH2] =9.02×10−12 exp[−(1650±200)/RT] cm3 molecule−1 sec−1, k2[C2H5NH2]=1.13×10−11 exp[−(1275 ±200)/RT] cm3 molecule−1 sec−1, k2[(CH3)2NH]=1.52×10−11 exp[−(550±200)/RT] cm3 molecule−1 sec−1, k2[(CH3)3N] =1.08×10−11 exp[(415±200)/RT] cm3 molecule−1 sec−1, with room temperature rate constants of (5.65±0.57) ×10−13 cm3 molecule−1 sec−1, (1.33±0.14) ×10−12 cm3 molecule−1 sec−1, (6.12±0.62) ×10−12 cm3 molecule−1 sec−1, and (2.20±0.22) ×10−11 cm3 molecule−1 sec−1, for CH3NH2, C2H5NH2, (CH3)2NH, and (CH3)3N, respectively. Possible reaction mechanisms are discussed.
High resolution carbon-13 spin–lattice relaxation study of the nematogen p-methoxybenzylidene–p-?-butylaniline
Authors: Hutton H. , Bock E. , Tomchuk E. , Dong Ronald Y.
Published: February 2003

The 13C relaxation measurements were obtained for all resolved carbon resonances in the nematic and isotropic phases of MBBA. Also obtained were the NOE measurements in the isotropic phase. The study indicates that a flexibility gradient occurs along the n-butyl chain and that the 13C relaxation is governed by the anisotropic rotational diffusion of the entire molecule.
Temperature dependence of carrier generation, and transport in p
a
r
a-terphenyl above and below the 180 °K phase transition
Authors: Burshtein Z. , Williams D. F.
Published: February 2003

Charge carrier drift mobilities have been measured in para-terphenyl single crystals by the transient photoconductivity method. In the a, b, and c′ crystal directions, room temperature electron mobilities were 0.34, 1.2, and 0.25 cm2 V−1  sec−1, respectively, and 0.80 cm2 ⋅V−1 sec−1 in the c′ direction for holes. Electron mobilities in the a, b direction followed a T−0.5 and T−0.7 dependence but for temperatures below the 180 °K phase transition, a T−2.5 dependence is followed. No discontinuity in the carrier mobility temperature dependence were observed. Hole photocurrents in all crystal directions could be measured at low temperatures. Possible mechanisms of carrier transport and carrier generation are discussed.
Crystal structures of Ga(II) and Ga(III)
Authors: Bosio Louis
Published: February 2003

The crystal structures of the high pressure Ga(II) and Ga(III) forms of gallium were studied by single-crystal x-ray diffraction using a diamond anvil device. Ga(II) has a body-centered-cubic structure with a=5.951±0.005 Å and 12 atoms in the centered cell; Ga(III) has a body-centered-tetragonal cell with a=2.813±0.003 Å, c=4.452±0.005 Å, and two atoms (this phase was previously labeled as Ga(II) in works cited in the text).
Mutual neutralization of simple and clustered positive and negative ions
Authors: Smith David , Church Michael J. , Miller Thomas M.
Published: February 2003

Measurements are reported of the rate coefficients, α, for several ion–ion mutual neutralization reactions principally involving NH4+ ions and their ammonia clusters NH4+⋅ (NH3)1,2,3 with several different negative ions. The data were obtained utilizing an ion–ion flowing afterglow plasma combined with Langmuir probe diagnostics. Most of the measurements were obtained at 300 K although the NH4++Cl− reaction has also been studied at 220 and 430 K. Both the absolute magnitude of α and its temperature variation are shown to be in acceptable agreement with theoretical predictions. The α for the cluster ion reactions are very similar to those for the simpler ions, even when both positive and negative ions are large clusters, although an increase in the mean reaction cross section for the cluster ion reactions is discernible. All the α measured to date for both simple and cluster ion reactions, albeit for species of high electron affinity and over the limited temperature range of 180 to 530 K, are within the range (4–10) ×10−8 cm3 s−1, more marked variations occurring with temperature than with ionic mass.
The phonon contribution to the dipolar autocorrelation function of plastic crystals
Authors: Wilde Richard E. , Cohen Simon S.
Published: February 2003

An expression for the phonon contribution to the dipolar-autocorrelation function of plastic crystals has been obtained by considering terms to third order in the dipole-moment expansion. The magnitude of the phonon coupling is directly dependent on the dipole moment induced in the molecules of the crystal due to their translational motions. An example of how the phonon coupling can be estimated is illustrated by considering solid arsine. In the case of highly polarizable molecules the coupling may be significant.
Theory of resonant Raman scattering. III. Toward the total synthesis of molecular spectra and Raman excitation profiles within the generalized vibronic theory and from an exciton-in-molecule viewpoint
Authors: Hong Hwei-Kwan
Published: February 2003

Applications of the Green’s function method to the interpretation of molecular electronic spectra and Raman excitation spectra are further elaborated here. In particular, basic concepts in the generalized vibronic theory of Raman intensity are introduced, as well as the concept of vibrational lattices and the exciton analogy. It is pointed out that the Raman process can be considered from the solid state viewpoint as energy transfer (exciton migration) processes. Therefore, vibronic problems for a polyatomic molecule having M normal modes are analogous to an exciton in M-dimensional space. It is further shown that most problems in one-dimensional space (one mode problems) can be exactly treated with the Green’s function technique in the framework of the generalized vibronic theory. This allows us to handle specifically (1) Franck–Condon effect with both changes in equilibrium positions and force constants upon electronic excitation, (2) nonadiabatic Herzberg–Teller couplings with force constant changes in excited states, and (3) mixed harmonic–quartic oscillators, etc. More complicated vibronic schemes can be dealt with in a similar fashion. For example, vibrational combinations and the Duschinsky effect in electronic and Raman spectroscopy can be treated as pseudo-one-dimensional problems. Green’s functions for multiple mode problems with simple vibronic coupling schemes are given. Some approaches are also provided for the total synthesis of molecular spectra and Raman excitation spectra.
Computer simulations of polyatomic molecules. III. Monte Carlo studies of heteronuclear and homonuclear hard diatomics
Authors: Streett W. B. , Tildesley D. J.
Published: February 2003

Monte Carlo computer simulations have been carried out for fluid systems of 256 and 500 heteronuclear and homonuclear hard diatomic molecules, each consisting of fused hard spheres of diameters σAA and σBB with centers separated by distance L. Nineteen different models have been studied, with elongations L*=L/σAA in the range 0.05⩽L*⩽0.75, and diameter ratios γ=σBB/σAA in the range 0.5⩽γ⩽1.0. The atom–atom distribution functions gαβ(r), for these models are described in detail, and the discontinuities in their first derivatives at distances σαβ+L, predicted by cluster expansion theory, are shown to have a simple explanation in molecular geometry. Pressures have been calculated from a virial-like equation and used to examine trends in pressure with changes in molecular geometry. Structure factors S (K) and direct correlation functions c (r) have been calculated by Fourier transform of the gαβ(r), and the dependence of these functions on molecular geometry has been examined.
Diatomic molecules as perturbed Morse oscillators. IV. Franck–Condon factors for very high J
Authors: Huffaker J. N. , Dwivedi Parmesh H.
Published: February 2003

The perturbed-Morse-oscillator formalism for describing rotating, vibrating diatomic molecules is modified so as to be suitable for very large values of the rotational quantum number J. The modification consists of putting as much as possible of the rotational energy into the unperturbed Hamiltonian, and using the remainder, along with the ’’anharmonic’’ vibrational potential, as perturbations of a Morse–Peckeris oscillator. The accuracy of the resulting vibrational wavefunctions is exhibited by using them to compute Franck–Condon factors for the R branches of the A 1Σu+ −X 1Σg+ band system of Ca2 for J=50 and J=100 and comparing with published RKR results.
Chemical thermodynamic properties of ethylbenzene
Authors: Miller Akeley , Scott Donald W.
Published: February 2003

Thermodynamic functions of ethylbenzene are calculated by standard methods of statistical thermodynamics. These calculations were prompted by new experimental data on ethylbenzene.(AIP)
14N PNQR study of the effect of pressure on the phase transition and molecular reorientation in s-triazine
Authors: Oron Moshe , Zussman Avigdor , Rapoport Eliezer
Published: February 2003

14N pure nuclear quadrupole resonance frequencies and inverse linewidth in s-triazine were measured as functions of hydrostatic pressure to 6.2 kbar along two isotherms at 23.3 and −0.4 °C. The second order phase transition, previously studied at atmospheric pressure, was observed at high pressures. Above the transition pressure pc each resonance line splits into two. The line splittings were fitted to the equation Δν=B (p−pc)β, and the critical exponent β was found to be close to 0.5. The pressure dependence of the transition temperature Tc is described by the (linear) equation Tc(p) =198.8+18.2p, [Tc(°K);p (kbar)]. The present results along isotherms were compared with previous results along an isobar. In the low pressure region the inverse linewidth T2* is dominated by hindered rotations of the s-triazine molecule. The activation volume for this motion is ΔV*=26 cm3/mole.
Reactions of singly and doubly charged argon ions with N2 and O2 in a steady state hollow cathode discharge
Authors: Howorka F.
Published: February 2003

Ion–molecule reactions between the ions Ar+, Ar++, and N2 molecules are studied in the negative glow region of a cylindrical steady-state hollow cathode discharge at pressures between 0.25 and 0.5 torr and at [N2]/[Ar] density ratios of 0.001, 0.002, 0.005, and 0.01. The following rate constants are obtained: for Ar+++N2→N2++Ar+, k=2×10−10 cm3 s−1; Ar+++N2→N++N+Ar, k =1×10−10 cm3 s−1; Ar++N2→N2++Ar, k =0.2–1.5×10−11 cm3 s−1. A value of k=4×10−13 cm3 s−1 for the partial charge exchange Ar+++Ar→2 Ar+ is consistent with the measurements. For comparison the rates of Ar+++O2 (k=1.7×10−9 cm3 s−1) and of Ar++O2 (k=5.8×10−11 cm3 s−1) are determined. The error limits of the k values are ±45%. Dissociative volume recombination of Ar2+ with slow plasma electrons (α=3.1±1.1×10−7 cm3 s−1 at an electron temperature of 1200 K) is the dominant loss process for these ions. Wherever possible, the results are compared with previous ones.
Continued fraction formalism and its application to lattice dynamics
Authors: Jhon Myung S. , Dahler John S.
Published: February 2003

The continued fraction formalism of Mori is derived systematically and its application to nonequilibrium statistical mechanics is carefully studied. We choose applications from lattice dynamics and give general solutions for semi-infinite and infinite harmonic oscillator chains in terms of finite continued fractions. Special cases of these general solutions are considered in detail.
Density difference representation of electron correlation
Authors: Gupta Abha , Boyd Russell J.
Published: February 2003

The density difference ΔD (r1), defined as the difference between the exact radial density function Dex(r1), or our best approximation to it, and the best uncorrelated, that is, the Hartree–Fock, radial density function DHF(r1), is reported for the He, Li, and Be isoelectronic sequences and to a lesser accuracy for the carbon atom. By means of an analysis of the Be density difference in terms of K and L shell contributions some insight is gained for the evolution of the density difference through the He, Li, and Be sequence. In general the effect of electron correlation on the radial density function D (r1) and, of course, on the related density distribution ρ (r1) diminishes as the charge cloud becomes less diffuse. The effect of electron correlation on one-electron distributions is also demonstrated by comparison of 〈rn1〉 expectation values. Analytical Hartree–Fock wavefunctions are reported for H− (energy =−0.4879 2973 a.u.) and Li− (energy =−7.428 2317 a.u.).
Interactions of ion beams with surfaces. Reactions of nitrogen with silicon and its oxides
Authors: Taylor J. Ashley , Lancaster Gerald M. , Ignatiev A. , Rabalais J. Wayne
Published: February 2003

Ion beam studies of chemical reactions between nitrogen and surfaces of silicon and its oxides are reported. A spectrometer system designed for these studies which combines the techniques of x-ray and uv photoelectron spectroscopy, Auger electron spectroscopy, secondary ion mass spectroscopy, low energy electron diffraction, and ion bombardment is described. This work employs XPS and UPS to examine the products induced by 500 eV N+2 beams on targets of elemental Si, SiO, and SiO2. The N+2 ions undergo charge exchange and dissociation at the surface of the target to form hot N atoms. Reaction with Si, produces nitrides which are similar to those of the type Si3N4. Reaction with SiO and SiO2 forms nitrides, with no evidence of nitrate or nitrite formation. The chemical nature of the reaction is suggested by identification of the reaction products through XPS and UPS and energy level shifts. The thickness of the silicon nitride layer on Si(111) formed by 500 eV N+2 bombardment has been determined to be ∼19 Å thick by using the film/bulk Si XPS intensity ratio. Estimates obtained by depth-concentration profiling with 1 keV Ar+ and by using LSS projected ion range calculations agree with this approximate thickness.
Nuclear spin–lattice relaxation in periodically irradiated systems
Authors: Vega A. J. , Vaughan R. W.
Published: February 2003

A master equation for nuclear magnetic relaxation under conditions of periodic and cyclic rf irradiation is derived based on the stochastic Liouville equation. Conditions for the validity of the equation, involving both use of the motional narrowing approximation and the Magnus expansion, are discussed with particular attention given to the simultaneous presence of fluctuating and nonfluctuating interactions. The expressions derived are applied to several irradiation schemes: the Carr–Purcell sequence, where the echo decay time under translational diffusion is calculated; the cw and pulsed versions of spin locking in solids, with special emphasis on the origin and role of spin diffusion and on the exact relationship between the second moment and the prefactor in the T1ρ expression; and, finally, the four- and eight-pulse sequences used for suppression of homonuclear dipolar interactions, where it is shown that the x, y, and z axes of the interaction frame are ’’principal axes’’ of relaxation. Physical interpretation of the calculated results is presented. The relaxation times of cyclohexane under the eight-pulse cycle are experimentally determined and shown to confirm the theory.
Spectra and structure of chiral molecules. III.a) Vibrational assignment and normal coordinate analysis of 1-bromo-1-chloro-1-fluoroethane and 1-bromo-1-chloro-1-fluoroethane?2-d1
Authors: Hudgens B. A. , Diem M. , Burow D. F.
Published: February 2003

The infrared spectra (3100–200 cm−1) of the gaseous, neat solid and argon matrix, and the Raman spectra of the liquid and solid states are reported for 1-bromo-1-chloro-1-fluoroethane and for the three conformations of 1-bromo-1-chloro-1-fluoroethane-2-d1. Forty-seven distinct frequencies were used to calculate a generalized valence force field (GVFF) and a Urey–Bradley (UBFF) force field.
Spin relaxation and molecular motion in solid trimethylphosphate
Authors: Burnett L. J. , Knowles R. M. , Rottler C. L. , Smiley S. A.
Published: February 2003

Proton and phosphorous spin–lattice relaxation times (T1) were measured in solid trimethylphosphate (CH3O)3PO between 77 and 305 K. In the orientationally disordered ’’plastic’’ phase the observation of a T1 minimum allowed the rate and activation energy (8.0 kcal/mole) for translational diffusion to be determined. In the low temperature ’’hard’’ crystalline phase relaxation effects due to both methyl (1.3 kcal/mole) and methoxy (5.0 kcal/mole) reorientation were observed. In the liquid and supercooled liquid phases proton and phosphorous relaxation data resulted in determination of the rate and apparent activation energy (2.64 kcal/mole) for translational diffusion, along with estimates of the glass transition temperature (70 K) and the ratio of the rms jump distance to molecular diameter (0.63).
Theory of liquid surface tension–bulk modulus relation
Authors: Bhatia A. B. , March N. H.
Published: February 2003

Adopting a gradient expansion of the local free energy of the inhomogeneous liquid–vapor interface, a formula for the liquid surface tension σ is derived from which the Cahn–Hilliard relation between σ and bulk modulus is shown to follow as the lowest order approximation.
Pressure-broadened linewidths of HNO3
Authors: Tejwani G. D. T. , Yeung Edward S.
Published: February 2003

N2 and O2 broadened linewidths of NHO3 at 300°K have been computed for about 1500 transitions of a- and c-type and for about 2500 transitions of b-type. (AIP)
Dissociative attachment and vibrational excitation of F2 by slow electrons
Authors: Hall R. J.
Published: February 2003

Self-consistent dissociative attachment and vibrational excitation cross sections for F2 have been calculated using Herzenberg’s theory of resonant electron scattering. It has been found that the observed electron–F2 attachment data can be explained by a low energy shape resonance. Potential parameters for the 2Σ+u negative ion were varied in order to fit predicted attachment cross sections to measured rate constant data. The best fit was obtained for a negative ion curve which crosses the F2 ground state in the vicinity of the equilibrium internuclear separation, in good agreement with an ab initio calculation for this state. The associated total vibrational cross section has a peak of about 2.0×10−16 cm2 at an incident electron energy of 0.45 eV. A strong dependence of attachment rate on F2 vibrational state is predicted.
Absolute rate of the reaction of atomic hydrogen with ethylene from 198 to 320 K at high pressure
Authors: Lee J. H. , Michael J. V. , Payne W. A. , Stief L. J.
Published: February 2003

The rate constant for the H+C2H4 reaction has been measured as a function of temperature. Experiments were performed with high pressures of Ar heat bath gas at seven temperatures from 198 to 320 K with the flash photolysis–resonance fluorescence (FP–RF) technique. Pressures were chosen so as to isolate the addition rate constant k1. The results are well represented by the Arrhenius expression k1= (3.67±0.66) ×10−11 exp(−1040±42/T) cm3 molecule−1 s−1 (quoted errors are two standard deviations). The results are compared with other studies and are theoretically discussed.
Characterization of eigenvalues by use of orthonormality-constrained variation methods
Authors: Morikawa Tetsuo , I’Haya Y.J.
Published: February 2003

Various kinds of methods are presented for performing variational characterization of eigenvalues of a self-adjoint operator F bounded from below. The methods are derived from construction of ket vectors under the orthogonality and normality constraints through the process of variation. Application of the methods to the expectation value of the operator leads to variational equations, in each of these equations an extremum condition yielding an eigenvalue equation.
Infrared and Raman spectra of hydrogen bonded liquids. Effect of electrical anharmonicity on profiles of hydrogen-stretching bands
Authors: Marechal E. , Bratos S.
Published: February 2003

The effect of electrical anharmonicity on the shape of ir and Raman hydrogen-stretching bands is examined in the case of liquids containing weak hydrogen bonds. This mechanism is shown to have a weak spectral activity and to shift the bands slightly from the positions they would have in the absence of this process. Its efficiency may roughly be estimated by comparing the profiles in ir and Raman spectra.
Inclusion of some higher order effects with the self-consistent electron pairs method for correlated molecular wavefunctions
Authors: Dykstra Clifford E.
Published: February 2003

An efficient technique is presented for the inclusion of some higher order effects in correlated wavefunctions. Diagonal quadruple substitutions (e.g., i2j2→ a2b2) are variationally added to a self-consistent electron pairs (SCEP) wavefunction that includes single and double substitutions relative to a closed shell reference configuration. Through the use of approximate natural orbitals, these configurations include those which are important for the size-consistency correction at the limit of separated systems. While the number of the diagonal quadruple configurations is of the order of the number of doubles, the computational effort for including the quadruples is less than for the doubles, because of the nature of the required matrix elements and the flexibility of SCEP. Variationally additive quadruple corrections to SCEP pair energies are easily found with this new procedure.
Rapid accurate calculation of JWKB phase shifts
Authors: Cohen James S.
Published: February 2003

With a generalization of a procedure suggested by Pack, the JWKB phase shift has been evaluated using a number of Gauss–Mehler quadrature formulas. Extraordinary accuracy is obtained with convenient low-order formulas employing the weight function (1−x)1/2 or (1−x)−1/2. The method has been tested with Lennard-Jones (6–12), exponential, exp-6, exp-5, exp-4, exp-3, Morse, and Yukawa pontentials with excellent results in all cases. A change of variable is shown to make the method efficient even for high energies and small impact parameters. The present method is superior in speed and accuracy to the method proposed by Kennedy and Smith, which was formerly considered the best for numerical evaluation of JWKB phase shifts.
CO2 photolysis revisited
Authors: Slanger T. G. , Black G.
Published: February 2003

Measurements have been made of the absolute oxygen atom yields from CO2 photolysis at 1470 and 1302–1306 Å; in both cases the yield is unity. The CO generation rates have also been measured in these two wavelength regions, and they are proportional to the oxygen atom generation rates; by implication the CO quantum yields are thus also unity. The CO deficiencies observed in earlier work on this subject are probably caused by heterogeneous processes.
Zero field optically detected magnetic resonance (ODMR) of dihydroporphin
Authors: van der Bent Sievert J. , Schaafsma Tjeerd J.
Published: February 2003

Zero field ODMR results are presented for the lowest triplet state T0 of dihydroporphin free base (chlorin), representing the basic skeleton for many photosynthetic pigments. ISC rates are found to be kx=435±35 sec−1, px=62±13%; ky =173±11 sec−1, py=36±8%; kz=23.4±0.7 sec−1, pz=2±0.5%; where kx, ky, kz and px, py, pz represent total decay rate constants and relative populating rates of spin levels ‖x〉, ‖y〉 and ‖z〉 of T0. Upon prolonged irradiation with 400 nm light, the central protons of dihydroporphin free base are rearranged, as is evident from the appearance of new zero field transitions; no shift in fluorescence spectra is observed during this phototransformation. ZFS shifts due to the effect of different crystal field orientation and H–H axis orientation have been obtained separately. The results are consistent with a model based on four possible orientations of a C2v dihydroporphin molecule in a crystal site with twofold or lower symmetry.
Near ir spectra of some hexahydrated 3d metal fluorosilicates
Authors: De Debabala , Datta Sunil K.
Published: February 2003

The near ir spectra (from 1 to 2.5 μ) of MSiF6̇6H20 (M=Mn, Fe, Co, Ni, and Zn) which belongs to the rhombohedral system, space group R3̄ or R3̄m, Z=1, exhibit two broad and intense bands at 1.45 and 1.93 μ, besides several smaller ones, at the room temperature. At 80 K the resolution improves for all the bands, whereas the 1.45 μ band is resolved into three sharp peaks lying at about 1.42, 1.46, and 1.48 μ. On deuteration all of them decrease in intensity and two new bands appear at 1.64 and 2 μ, the latter showing resolution into sharp peaks at 1.965, 1.98, and 2.02 μ at 80 K. Many of these bands have been assigned as different combinations and overtones of H2O or D2O vibrations with the value of the fundamental frequencies ν1, ν2, ν3, of H20 as 3480, 1604, 3520 and of D2O as 2553, 1174, 2623 (in cm−1), in close agreement with those available from ir and Raman data. The 1.45 and 2 μ bands, which are shown to belong to the same assignment, have very similar temperature dependence, especially in the Co salt. Some results on polarized spectra have been presented.
Further studies on noise and fluctuations in kinetic systems
Authors: Chen Yi-der
Published: February 2003

Recently, we have presented a general formalism for the calculation of the noise power spectrum matrix of a set of macroscopic quantities in chemical reaction systems. In general, the noise power spectrum matrix G(ω) can be expressed in closed form in terms of the irreducible relaxation matrix M and the Fokker–Planck moment matrix B of the system. The relaxation matrix is called irreducible because all the system’s conservation relations are used to eliminate the dependent variables. In this paper we illustrate (with an example) that G(ω) can also be calculated using the reducible relaxation matrix by considering only some (or none) of the conservation relations. In addition, we show that, for systems at equilibrium and for linear systems at nonequilibrium steady state, the full Fokker–Planck moment matrix of the system can be expressed as a product of the full relaxation matrix of the system and a diagonal matrix whose diagonal elements are the equilibrium (or steady-state) concentrations of the species in the system. Thus, for these systems the calculation of G(ω) does not require the matrix B. With this relation it is easy to show that the ’’auto’’ noise power spectrum of any equilibrium system can be expressed as a sum of simple positive Lorentzians.
Förster migration of electronic excitation between randomly distributed molecules
Authors: Haan Steven W. , Zwanzig Robert
Published: February 2003

This article deals with the diffusivelike migration of electronic excitation between molecules that are randomly distributed in space. A generalized (non-Markoffian) diffusion equation is obtained, and the generalized diffusion coefficient is expanded to second order in molecular concentration. At early times the mean squared displacement of excitation increases as the 5/6th power of time, instead of the usual first power of time. We conjecture that, at long times, the migration may be described by a conventional (Markoffian) diffusion equation.
Optimized Rouse–Zimm theory for stiff polymers
Authors: Bixon Mordechai , Zwanzig Robert
Published: February 2003

Earlier, the authors proposed a method for constructing an optimized Rouse–Zimm theory for an arbitrary polymer system. This method is applied here to a model of a stiff polymer. The spectrum of relaxation times and the intrinsic viscosity obtained from the optimized Rouse–Zimm theory are discussed. Comparison with known results for the limiting cases of a flexible chain, a rigid ring, and a rigid rod, shows that this procedure gives a quite good approximation at low frequencies. Its basic disadvantage is its inability to account for the observed high frequency behavior of the intrinsic viscosity of stiff polymers.
Kinetic theory of chemical reactions in dense fluids
Authors: Kapral Raymond
Published: February 2003

A kinetic theory description of a dense chemically reacting fluid is presented. The kinetic equation takes into account the effects of correlated binary collision events and involves the ’’repeated ring’’ operator. Expressions for the rate coefficient are derived from the kinetic equation by means of projection operator techniques. The resulting expressions for the rate coefficient incorporate several features. They show how the dynamics of the reactive collision events lead to a coupling to the diffusive modes of the fluid. The rate coefficient expression is valid in the limit where binary dynamics dominate as well as in the diffusion controlled limit where collective solvent effects play an important role. The frequency and time dependence of the rate kernels and rate coeffiicients are also calculated. The manner in which the kinetic theory can be used to take into account the contributions to the rate coefficient which are higher order in the concentration is described and the relation to other theories is discussed.
The electronic structure and coordination geometry of the oxyheme complex in myoglobin
Authors: Churg Antonie K. , Makinen Marvin W.
Published: February 2003

The polarized single crystal electronic absorption spectra of oxymyoglobin, carboxymyoglobin, and cyanomyoglobin are presented. Four regions of the oxymyoglobin spectrum in which charge-transfer states are detected are defined as probes of oxyheme orbital structure and coordination geometry. The z-polarized absorption intensities near 15 500, 21 000, and 31 500 cm−1 is assigned to porphyrin–iron charge-transfer transitions, while the broad x,y-polarized band at 10 800 cm−1 is assigned to porphyrin(π) →O2(πg) promotions. Correlation of the observed transition energies and polarizations of the charge-transfer states with predictions based on extended Hückel studies of oxyheme complexes [Kirchner and Loew, J. Am. Chem. Soc. 99, 4639 (1977)] indicates that the electronic absorption spectrum of sperm whale oxymyoglobin is diagnostic of a bent (Fe–O–O) oxyheme coordination geometry with totally spin paired, ground state, electronic configurations of the iron and the dioxygen ligand.
The four-center charge distribution of the carbonate ion and the lattice energies of calcite and aragonite
Authors: Yuen Pui Sum , Lister M. W. , Nyburg S. C.
Published: February 2003

The charge distribution of carbonate ions in calcite and in aragonite have been calculated on the principle that the crystal experiences no net expansive or compressive force and no ion within it experiences a net force or couple. The charges q (in ‖e‖) for calcite are q (C) =+0.95±0.26, q (O) =−0.98±0.09 and for aragonite q (C) =+0.89, q (O1) =−0.94, and q (O2) =−0.98 (errors unknown). These estimates differ widely from those of other authors (not using the zero-force principle) but are in close accord with the best ab initio MO calculations currently available.
Light scattering from systems undergoing spinodal decomposition
Authors: Metiu Horia
Published: February 2003

It is suggested that double light scattering from systems undergoing spinodal decomposition may be detectable and should be included in the theory, when comparison to experiment is made. For systems having spherical molecules the existence of detectable double scattering suggests that the use of polarized light scattering will permit one to obtain more information concerning the dynamics of the decomposition. The depolarized light scattering, detected at 90°, contains no single scattering contribution and it is therefore a direct measure of the time evolution of the four point concentration correlation function. The polarized light scattering measures the combined effect of single and double scattering and it is therefore a measure of the time evolution of the two- and four-point correlation functions.
Single site multiphonon and energy transfer relaxation phenomena in BaF2:Er3+
Authors: Miller Marvin P. , Wright John C.
Published: February 2003

The individual charge compensated sites of Er3+ have been identified in BaF2 crystals containing 0.01–1.0 mole % Er3+ using a narrow bandwidth tunable dye laser and it was found that the sites are not adequately described by the model of single Er3+–F−I pairs. The sites have been classified as single Er3+–F−I pair sites or cluster sites on the basis of the concentration dependence, the number of transitions exhibited, and the existence of efficient nonresonant energy transfer between ions within a site. The energy transfer rates have been found for the cluster sites for two different transfer processes and are compared with previous results reported for similar processes in CaF2:Er3+. The appearance of cluster sites at concentrations as low as 0.01 mole %Er3+ is in agreement with previous work in CaF2 and SrF2 and exemplifies the need for consideration of clustering to describe the defect site equilibria in crystals of this type.
Theory of small-signal ac response of solids and liquids with recombining mobile charge
Authors: Macdonald J. Ross , Franceschetti Donald R.
Published: February 2003

An exact, small-signal theory of the impedance of an electrode/material/electrode system under quite general conditions is presented. The system, assumed flat band, consists of a slab of material between two identical plane-parallel electrodes. The material may be a nondegenerate electronic semiconductor or an ionic conductor. Solid ionic conductors considered are Schottky and Frenkel defect materials, possibly containing neutral defect pairs and/or aliovalent impurities, and fast ion conductors such as Na-β-alumina. Liquid ionic conductors treated include unsupported strong, weak or potential electrolytes, and possibly fused salts and oxides. Both intrinsic and extrinsic conduction conditions are included, with a single species of negative mobile charge of arbitrary valence and mobility and a single species of positive mobile charge of arbitrary valence and mobility assumed present. Intrinsic and extrinsic equilibrium and dynamic generation and recombination processes are taken into account. The boundary conditions employed permit the charge carriers to react directly at the electrode, to be adsorbed without reaction, or to react after the formation of an adsorbed intermediate. The general solution and various simplified special cases are discussed in detail. The general solution in the form presented here is sufficiently simple that it can be used without approximation as the fitting function in a newly developed weighted nonlinear least squares fitting procedure which treats simultaneously the real and imaginary parts of a complex function such as impedance. A discussion is presented showing how fitting parameter estimates thus derived from experimental data may be used to obtain a set of basic physical parameters characterizing the experimental electrode/material system. Finally, the physical interpretation and significance of many of the basic parameters is discussed in detail.
Dielectric dispersion and dielectric friction in electrolyte solutions. II
Authors: Hubbard J. B.
Published: February 2003

Presented herein is a thorough investigation of the consequences of coupling ion migration to dielectric relaxation in the solvent. A self-consistent continuum model is developed from both local (stress tensor) and global (dissipation function) considerations. The friction coefficient of a moving ion is computed in terms of the dimensionless coupling parameter where e is the ion charge, η is the solvent viscosity, ε0 and ε∞ are the low and high frequency dielectric constants of the solvent, respectively, τD is the Debye dielectric relaxation time of the pure solvent, and R is the ionic radius. For small ions the computed frictional drag is practically independent of ion radius, provided some degree of hydrodynamic slip is allowed at the ion surface. The effect of ion migration on the dielectric relaxation time of the solution is analyzed in detail. It is shown that the Debye–Falkenhagen effect should lead to an increase in the observed relaxation time, while the kinetic ion–solvent coupling introduces one or more relaxation times, all less than the Debye time of the pure solvent.
CO2 laser induced reactions of SF5Cl
Authors: Leary K. M. , Lyman J. L. , Asprey L. B. , Freund S. M.
Published: February 2003

Isotopically selective reactions of SF5Cl have been induced by intense CO2 laser radiation at frequencies near the sulfur–fluorine stretching fequency ν8 at 909 cm−1. The experiments were performed at low pressure (0.25 to 4.0 torr) with neat SF5Cl and several SF5Cl–diluent mixtures. The reaction yield, product distribution, and isotopic selectivity were determined for various reaction conditions. A reaction mechanism was proposed with simple sulfur–chlorine bond cleavage being the primary photolytic reaction.
Singlet–triplet anticrossings between the doubly excited 3 1K state and the g (3d)  3Σ+g state of H2
Authors: Freund Robert S. , Miller Terry A. , Jost R. , Lombardi M.
Published: February 2003

The 3 1K, v=1 level of H2 has recently been interpreted as an adiabatic mixture of vibronic levels of the EF 1Σs+ and the GK 1Σg+ double minimum states, with currently undetermined nonadiabatic corrections. Anticrossing spectra, which are sensitive to these state mixings, have been observed for the 3 1K, v=1, N level and the (3d)  3Σg+, v=1, N level of ortho-H2 for the rotational levels N=1 and 3. Linewidths, which are determined by either the singlet–triplet Fermi-contact hyperfine interaction or the spin–orbit interaction, as well as magnetic g values, help provide a detailed description of the 3 1K state. Measured singlet–triplet zero field separations are combined with previously published values to provide the best available value for the energy T0(a 3Σ) of the lowest triplet level (a 3Σ, v=0, N=0) with respect to the ground state, 95 076.36±0.20 cm−1.
Diffusion of excess electrons in one- and two-component molecular solutions. A theoretical model
Authors: McHale Jeanne , Simons Jack
Published: February 2003

A connection is made between the experimentally observed diffusion coefficient (D) of excess electrons in molecular fluids and the rate at which the excess electrons ’’jump’’ from one solvent molecule to another. A simple one-dimensional nearest-neighbor model is employed to develop a diffusion equation description of the microscopic excess-electron density function. Solution of this diffusion equation yields, in the one-component solution case, an expression for D in terms of the nearest-neighbor distance of the liquid, the solvent anion’s diffusion coefficient, and the intermolecular electron hopping rate. The diffusion equation has also been solved in general for a binary solution as well as in two especially interesting limiting cases: when the rate of hopping from one kind of molecule (A) to the other (B) is much faster (slower) than the A-to-A or B-to-B hopping rates. In each case, it is demonstrated how the solution composition dependence of D can be used to extract the various electron hopping rates.
One-photon and consecutive two-photon excitation of Ca2 fluorescence in solid krypton at 12 K
Authors: Miller John C. , Andrews Lester
Published: February 2003

Laser-excited emission from the Ca2 van der Waals dimer isolated in solid krypton has been studied. Analysis of the strong red emission at 14 000 cm−1 gives ωe″=80 cm−1, ωex″e=0.35 cm−1 for the ground state molecule. Observation of unrelaxed emission up to v′=7 and anti-Stokes emission due to sequential two-photon excitation are discussed in terms of excited and ground state vibrational relaxation rates. Resonance Raman scattering from Ca2 at Δν=80, 158, and 236 cm−1 was also observed.
Kinetic study of H+HF(v=3): Kinetic isotope effect and temperature dependence
Authors: Bott J. F. , Heidner R. F.
Published: February 2003

Rates of HF(v=3) removal by H and D atoms were measured between 200 and 295 K in a laser-induced fluorescence–discharge flow–tube apparatus. The removal rates by H atoms were found to increase from 6.3×1013 cm3/mol s at 295 K to 10×1013 cm3/mol s at 200 K. The removal of HF(v=3) by D atoms is somewhat slower, but the removal rates have a similar negative temperature dependence. There are several mechanisms by which H atoms can remove HF(v=3), i.e., reaction to form H2+F or deactivation to HF(v=0, 1, or 2) with or without exchange of the F atom. The several possibilities are discussed and compared to theoretical calculations.
The effects of band shapes on circular dichroism spectra of chromophore aggregates
Authors: Hemenger R. P.
Published: February 2003

The problem of calculating circular dichroism (CD) spectra for aggregates of chromophores with nonoverlapping wavefunctions is formulated in a general way with regard to band shapes. An immediate consequence of this formulation is that a series of exact and approximate results derived previously for aggregate absorption spectra apply to CD as well. It is shown that the contribution to aggregate CD from magnetic transition dipoles of the individual chromophores can always be written as a function of the intrinsic CD of the noninteracting chromophores. It is further shown, making use of a specific example, that there are whole classes of aggregate CD spectra which arise solely as a consequence of finite bandwidths. These spectra are qualitatively different from what is expected in the exciton limit, i.e., the limit of vanishing bandwidth.
Relativistic and nonrelativistic effective core potentials for xenon. Applications to XeF, Xe2, and Xe2+
Authors: Wadt Willard R. , Hay P. Jeffrey , Kahn Luis R.
Published: February 2003

Valence electron calculations on the low-lying electronic states of XeF, Xe2, and Xe2+ are reported using recently developed nonrelativistic and relativistic effective core potentials. The results indicate that apart from spin–orbit coupling relativistic effects are negligible for xenon. Comparison with previous all-electron calculations reveals that the valence electron dissociation and transition energies agree to 0.1 eV or better with all-electron results. The errors in the calculated values for Re are slightly greater (0.05 Å for XeF and 0.1 Å for Xe+2). The valence electron calculations generally appear to underestimate repulsive two-center interactions leading to significant errors at small internuclear separations in the repulsive wall of the potential curves. Comparison with the valence electron results of Lee, Ermler, Pitzer, and Winter shows excellent agreement, indicating that the simple, approximate approach used here for including relativistic effects in the effective core potential is valid.
Determination of the 14N quadrupole tensor of phenazine in its lowest excited triplet state by ODNQR
Authors: Dinse K. P. , Winscom C. J.
Published: February 2003

The first complete determination of the 14N nuclear quadrupole tensor in the photoexcited triplet state of an organic molecule was made feasible by an alternative to ODENDOR. Instead of using a simultaneous desaturation method (cw-ODENDOR), a procedure involving sequential population transfer between specific electron–nuclear spin levels was adopted. This new method is demonstrated for phenazine-d8 in a Shpolskii matrix and yields ‖e2qQ/h‖= (4.36±0.02) MHz; η=+0.21±0.03.
Collective vibrational modes of adsorbed CO
Authors: Mahan G. D. , Lucas A. A.
Published: February 2003

The interpretation, in terms of dipole interactions, of the dependence on coverage of the stretching frequency of CO adsorbed of metal sufaces is re-examined. We take into account substrate image effects and dielectric screening of the adsorbed layer. We also provide a theory of the statistical fluctuations in random, partially filled layers. The dipole interactions provide a frequency shift of about 10 cm−1, which is only about one third of that usually observed.
The relative permittivity of supercooled water
Authors: Hodge I. M. , Angell C. A.
Published: February 2003

Approximate relative permittivity data for water down to −35°C have been derived from measurements of the Maxwell–Wagner interfacial polarization which occurs at the surface of emulsified water droplets. Although the emulsifying agent strongly influences this polarization in a complex way, a novel method of data analysis enables the permittivity and conductivity to be obtained in a simple and direct manner. The derived data are in essential agreement with literature data near 0°C. To within an estimated experimental uncertainty of ±2% the present data conform to a critical exponent expression with the same critical temperature (228 K) found for many other properties of supercooled water, though the divergence is very weak.
A theoretical study of phase transitions in bidimensional monolayers
Authors: Firpo J. L. , Dupin J. J. , Albinet G. , Bois A. , Casalta L. , Baret J. F.
Published: February 2003

We propose a theoretical study of phase transitions in long hydrocarbon chain monolayers, lying on aqueous substrate. We use a model taken from that applied by Di Marzio to liquid crystals: each molecule can occupy several aligned sites on the holder (1 to 5 for us). We also take into account the attractive interactions between the hydrocarbon chains in the framework of a mean field approximation: it gives a first order phase transition in a diagram showing surface pressure variations in terms of the molecular density, noted (π, ρ) diagram. We also introduce an order–disorder transition and we show that it is either of the first order, or of the second order, according to the physical conditions. The (π, ρ) isotherms are in good qualitative agreement with experimental results.
Thermodynamic properties of a quasi-ionic alloy from electromotive force measurements: The Li–Pb system
Authors: Saboungi Marie-Louise , Marr Jane , Blander Milton
Published: February 2003

Electromotive force measurements of the activity of lithium in lithium–lead liquid alloys are reported in the temperature range T=770–932 K. The partial molar excess Gibbs free energies are negative. Variations with composition exhibit an inflection point around an atomic fraction of lead equal to 0.2 which corresponds to the ratio of the valences of Li and Pb. The inflection point, as well as structural data, may be interpreted by assuming an at least partly ionic character of the alloy. A set of partial structure factors defined by Faber and Ziman are calculated as a function of the composition at the long-wavelength limit. Their variations support the hypothesis of a local short-range order around the above-mentioned composition (0.2). Finally, a modeling approach based on an electron transfer from Li to Pb is used to explain some of the characteristics of this class of alloys.