bluuebunny/crossref_metadata_2025 huggingface dataset:
A kinetic model for clustering of water on hydrated protons in a supersonic free jet expansion
Authors: Searcy J. Q.
Published: February 2003

A simple model with three variable parameters is developed for calculating observed clustering data from a free jet expansion. One parameter, σR1, can be associated with a reactive collision diameter for a monomer associating with a cluster to form a larger, excited cluster. A second parameter, σR3, can be associated with a reactive collision diameter for energy transfer from the excited cluster to the surrounding gas. The third parameter, K′, is a constant in a unimolecular decay rate expression for the excited cluster. The reactive cross sections that give the best fit of experimental data are given by (π/4) σ2R1 =9.90×10−16 cm2 and (π/4) σ2R3=5.22 ×10−17 cm2. No value could be assigned to K′. Apparently the excited clusters are stabilized by energy exchange collisions in this particular flow field before there is appreciable unimolecular decay. Good fits of clustering data are obtained even though these same values are used for clusters varying in size from 6 to 26 water molecules. Since the clustering environment is radically different than in near-equilibrium experiments, it is not clear that these numbers can be used with other types of experimental data.
Bonded and nonbonded interactions in saturated alcohols, ethers, thiols, thioethers, and amines
Authors: Kao James W. H. , Chung-Phillips Alice
Published: February 2003

The enthalpies of formation (ΔH°f) of alcohols, ethers, thiols, thioethers, and amines are expressed as linear combinations of structure parameters. The linear coefficients are determined from experimental ΔH°f using the least squares criterion. For these calculations eight structure parameters are used for thiols and thioethers, nine for amines, and ten for alcohols and ethers. The mean and root-mean-square deviations between calculated and experimental values are, respectively, ±0.17 and ±0.25 kcal mole−1 for 30 gaseous alcohols and ethers, ±0.17 and ±0.25 kcal mole−1 for 32 liquid and ±0.19 and ±0.27 kcal mole−1 for 31 gaseous thiols and thioethers, and ±0.12 and ±0.23 kcal mole−1 for 18 liquid amines. Liquid phase data of 28 alcohols and ethers and gas phase data of 13 amines are also treated with this model but the results are not satisfactory. The linear coefficients may be used for predicting unknown ΔH°f and for estimating enthalpy differences between certain conformers. Those corresponding to the various 1,4 nonbonded interactions indicate that the gauche CCCC, CCOC, CCNC, CCOH, and CCNH interactions are repulsive, but the gauche CCCO and CCCN, interactions are attractive. The only 1,5 interactions considered are the gauche–gauche prime CCOCC and CCSCC interactions and they are found highly repulsive. The relative stability of conformers predicted on the basis of these nonbonded interactions agrees quite well with results from experiments and molecular orbital calculations.
Vibrational energy exchange in CO–CO collisions at low temperature
Authors: Gower M. C. , Srinivasan G. , Billman K. W.
Published: February 2003

The V–V exchange rates for CO in the ground electronic state have been measured at a temperature of 75 °K. The rates determined were for the processes CO(v=n)+CO(v=0) →CO(v=n−1)+CO(v=1)−ΔE, with 2?n⩽8. The values obtained are close to published 100 °K data, thus indicating an insensitivity to temperature. It appears that multirotational quantum jumps may be important in explaining the measured rates.
Molecular hard cores of normal fluids
Authors: Bienkowski Paul R. , Chao Kwang-Chu
Published: February 2003

Molecular hard cores of normal fluids have been deduced from analysis of isothermal compressiblity data of dense fluids in light of Gibbons’s hard core equation of state. The hard cores thus obtained are found to follow the principle of corresponding states in the extended three parameter form. The shapes and sizes of the hard cores of some common molecules calculated from the general equations are shown graphically.
Experimental study of the C(3P)+N(4S)+M→CN(B 2Σ+)+M recombination
Authors: Washida N. , Kley D. , Becker K. H. , Groth W.
Published: February 2003

The kinetics of the CN(B 2Σ+→X 2Π) violet band emission produced by the reaction of active nitrogen with cynogen (C2N2) have been studied as a function of the number densities of atomic carbon, atomic nitrogen, and total pressure. CN(B 2Σ+) in vibrational states v?7 is produced by energy transfer from a stabilized metastable CN(4Σ+) precursor CN(4Σ+)+M→CN(B 2Σ+, v?7)+M. CN(4Σ+) is formed in a three body recombination process from N and C atoms C(3P)+N(4S)+M→CN(4Σ+)+M. There is a collisional equilibrium between CN(B 2Σ+) and CN(A 2Π) at energies around 40 000 cm−1. CN(B 2Σ+) in vibrational levels v<7 is populated by energy transfer from CN(A 2Π): CN(A 2Π, v?19) +M→CN(B 2Σ+, v<7)+M. Written as a single step, the CN(B 2Σ+) formation is adequately described by C(3P)+N(4S) +M→CN(B 2Σ+)+M (1). The rate coefficient of Reaction (1) was measured absolutely and is k1= (9.4±2.5) ×10−33 cm6 molecule−2⋅sec−1 (M=argon).
Double absorption photofragment spectroscopy: A new tool for probing unimolecular processes
Authors: Sander R. K. , Wilson K. R.
Published: February 2003

A new technique, double absorption photofragment spectroscopy (DAPS), has been developed for observing the time evolution of unimolecular processes. Molecules isolated from collisions in a molecular beam are prepared in a well defined intermediate state by an initiation laser pulse. As this state evolves in time, it is monitored by a second probe laser pulse which, after a chosen delay, photodissociates the molecules. Information about the state to which the molecules have evolved by the time of the probe pulse can be extracted from the mass, density, and angular and energy distributions of the recoiling fragments. Simple models are explored for the functional form of the angular distributions which can be expected in DAPS experiments. This new technique is demonstrated on the I2 B state (used as a real rather than virtual intermediate) whose lifetime is found to vary between 770 and 1600 nsec, depending upon the set of B state v′J′ levels prepared by the initiation laser pulse. Future DAPS experiments should be able to observe unimolecular electronic and nuclear change over a time span from 10−12 to 10−3 sec, probing such processes as rearrangement, dissociation, internal vibrational transfer, intersystem crossing, and internal conversion.
Photofragment spectrum of bromine
Authors: Oldman R. J. , Sander R. K. , Wilson K. R.
Published: February 2003

We have measured the photofragment spectrum of Br2 at visible and near uv wavelengths using the absorption of a single photon to probe ungerade states and double photon absorption to probe gerade states invisible in the ordinary absorption spectrum. In these experiments, a pulsed polarized laser beam intersects a molecular beam in a vacuum chamber, and Br atom photodissociation fragments are detected with a mass spectrometer (i) as a function of the angle between the light polarization direction and the recoil path of the detected atoms, measuring the Ω quantum number of the excited molecular state, and (ii) as a function of time after the laser pulse, measuring the atomic states with which the molecular state correlates. The experimental results at 18 780, 21 450, and 28 810 cm−1 are in accord with the predictions of Mulliken that the visible absorption spectrum of Br2 is due to transitions to the A 1u(3Π), B 0+u(3Π), and 1u(1Π) states. We have also found double photon dissociation processes involving absorption of one 18 780 and one 9390 cm−1 photon, as well as such processes involving two 18 780 cm−1 photons. Calculations, following Mulliken, of the approximate vertical energies of the newly observed final dissociative states in these double photon processes are in accord with our experimental observations.
The effect of reagent energy on chemical reaction rates: An information theoretic analysis
Authors: Levine R. D. , Manz J.
Published: February 2003

The effect of changing reagent vibrational and rotational energy on the reaction rate has been analyzed for over 20 chemical reactions. In most cases the selectivity in energy requirements could be characterized by a single (’’consumption potential’’) parameter, even when the reactivity varied by many orders of magnitude. The reactions analyzed covered atom–diatom and diatom–diatom collisions and included both simple rearrangement (’’exchange’’) reactions as well as collision induced dissociation (CID) and quenching of electronically excited states. The results were derived both from experiments and classical trajectory computations and include the variation in reactivity at both a given total collision energy and at a given translational (and rotational) temperature. In all cases the analysis was based on evaluating the surprisal of the energy consumption, i.e., the observed (or computed) reaction rate constant was compared to the rate expected on prior grounds when all states (at a given total energy) react with the same rate. The excess internal energy of the reactants is not necessarily available for reaction. Hence the consumption potential is not invariably of a definite sign. For highly endoergic processes, both experiments and trajectory computations show that often the increase in reaction rate due to reagent vibrational energy is over and above that expected on purely prior grounds (i.e., that expected due to the increase in the total available energy). The enhancement of an endoergic reaction rate by reagent vibrational energy is particularly significant for the lower vibrational states. As the excitation energy of the reagent approaches the endoergicity of the reaction, the enhancement of the rate is considerably lower. For exoergic, thermoneutral, and mildly endoergic reactions the increase in rate due to reactant vibrational energy is usually less than expected on prior (i.e., statistical) grounds. The effect of reagent rotational energy appears to be strongly correlated with the ’’steric requirements’’ of the reaction. For reactions with a preferential direction of attack (a ’’cone of acceptance’’) the enhancement in the reaction rate is often less than expected on prior grounds. The surprisal analysis demonstrates different rotational energy consumption for even and odd J states in the F+H2 (v,J) reaction. Several examples of reactions with two (or more) different types of products have been analyzed. It is found that often the different reaction paths have qualitatively different energy requirements. In particular, reagent vibrational energy tends to strongly favor the most endoergic path, over and above the energetic effect expected on prior grounds. Selective excitation of reagents does provide a discriminatory (i.e., nonstatistical) method for changing the branching ratio.
Electronic energy transfer from metastable argon atoms to krypton atoms
Authors: Piper L. G. , Setser D. W. , Clyne M. A. A.
Published: February 2003

The interaction between metastable argon atoms (3P2,0) and krypton atoms has been studied at room temperature using the flowing afterglow technique. Measurements of the emission intensities from the excited-krypton levels show that only Kr(5p[3/2]2) and Kr(5p[3/2]1) are primary products from Ar* (3P2). The pressure dependence of the emission intensities from other 5p krypton states shows that these are produced by collisional cascade from the 5p[3/2]2,1 levels; some rate constants for these cascade processes are reported. Absorption measurements using the 123.6 nm resonance transition of Kr demonstrate that the emitting Kr(5s 3P1) state carries no excess translational energy; therefore, it must be produced only via radiative cascade from Kr(5p) levels. Thus Ar (3P2) excitation rate constants of 5.6 and 0.65×10−12 cm3  molecule −1⋅sec−1 are established for excitation to Kr(5p[3/2]2) and Kr(5p[3/2]1), respectively. The Ar(3P2)+Kr reaction can serve as a reference for obtaining rate constants for excitation of other species via comparison of relative emission intensities and the technique is even useful in the vacuum ultraviolet if the appropriate branching ratio for the radiative cascade from 5p[3/2]2,1 to Kr(5s 3P1), established here as 0.301±0.006, is utilized. The emission from Kr* also can serve as a reference for determining absolute emission rates if [Ar(3P2)], [Kr], and the emission intensities are simultaneously measured. Quenching of Ar(3P0) by Kr gave an Ar–Kr* excimer emission, which peaks around 756.5 nm, emission from Kr(5p[1/2]0) at 758.7 nm, and formation of lower Kr(5p) states or deactivation to lower Ar(4s) states. Based upon the pressure dependence of these emission intensities, the excimer is formed by a fast three-body reaction with Ar* (3P0), and Kr(5p[1/2]0) is excited both by a slow two-body reaction with Ar(3P0) (∼3.4×10−15 cm3 molecule−1⋅ sec−1) and by bimolecular quenching of the excimer.
Direct measurement of spontaneous predissociation using coaxial laser–molecular beams
Authors: Lum R. M. , McAfee K. B.
Published: February 2003

Radiationless transitions of isolated molecules have been observed using a laser–molecular beam technique. Transitions of the laser excited beam molecules leading to predissociation and other unimolecular processes are detected as a modulation of the molecular beam. Within the 0.05 Å Doppler width of the 5145 Å line of an argon-ion laser, spontaneous predissociation of the B 3Π+0u state of bromine has been directly observed at wavelengths which produce selective excitation of the individual Br2 isotopic species. The peaks in the modulated mass signal amplitude corresponding to the predissociation process have a width of approximately 500 MHz. Our measurements indicate that predissociation is the predominant decay mechanism of the Br2 B 3Π state at 5145 Å.
Comments on high frequency molecular reorientation in liquids
Authors: Kivelson Daniel
Published: February 2003

The analysis of high frequency molecular reorientation in liquids is complicated by the presence of intermolecular effects. The interpretation of the data in terms of angular velocity correlation functions and dynamic molecular models is often subject to uncertainties due to slight differences in the definitions of relevant correlation functions and times. A brief discussion of some of these problems is presented.
Model potential for solid xenon
Authors: Agarwal T. N. , Gupta R. K.
Published: February 2003

Phonon dispersion curves in the three principal symmetry directions have been computed using a central force rigid atom model. Our theoretical results using modified (exp. 6) Buckingham potential are found to compare well with the recent neutron scattering data.
Dependence of the pure quadrupole resonance frequency on volume and temperature
Authors: Stahl E. A.
Published: February 2003

Bayer showed that the pure quadrupole resonance frequency was a function of temperature. Kushida, Benedek, and Bloembergen showed that the quadrupole resonance should be a function of both temperature and volume. Many others have shown the volume or pressure dependence either theoretically in terms of partial derivatives or experimentally. It is the purpose of this paper to show the volume dependence in a compact form by considering the Debye influence on pure quadrupole resonance.
Stimulated surface ionization: A new effect
Authors: Helbing R. K. B. , Hsieh Y. F. , Pol V.
Published: February 2003

Stimulated surface ionization of alkali atoms adsorbed on a tungsten surface by a molecular beam is reported. The effect is demonstrated for sodium and rubidium beams ionizing adsorbed potassium. (AIP)
Non-Maxwellian H and F velocity distributions in an H2–F2 reaction
Authors: Riley Merle E. , Matzen M. Keith
Published: February 2003

Time-dependent and quasi-steady-state solutions for the velocity distribution functions of H and F atoms in a burning, homogeneous, and isotropic H2–F2 gas mixture show an increase in the rates of the H+F2 and F+H2 chemical reactions that approaches a factor of two larger than the thermalized rates. The mixture is ignited by dissociating a small fraction of the F2 molecules. Approximations to the time-dependent Boltzmann equation for the system reduce it to a form which is linear in the H and F distribution functions. This limits the solutions to early time behavior and small initial F-atom concentrations, but it allows a general picture of the burn rates to be developed from a moderate number of calculations.
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o study of the zero-field splitting parameters of 3B1u benzene
Authors: Langhoff Stephen R. , Davidson Ernest R. , Kern C. William
Published: February 2003

The electron spin dipole–dipole contribution to the zero-field splitting of benzene in its lowest triplet state (3B1u in D6h symmetry) is determined theoretically by ab initio methods. Two hexagonal conformations are considered, distinguished by having carbon–carbon bond lengths appropriate to the 1A1g ground state (1.395 Å) and to the 3B1u state (1.427 Å). In addition, two distorted forms of D2h symmetry are treated, one having a compressed or ’’quinoidal’’ structure and the other having an elongated structure. All calculations are carried out with a double-zeta basis of contracted Gaussian-lobe functions. The correct microscopic spin-dipole Hamiltonian is used and all integrals are evaluated accurately. The hexagonal conformation with 1.427 Å bond lengths gives results in best agreement with experiment. A large configuration-interaction wavefunction leads to D=0.1676 cm−1, to be compared with D=0.1580 cm−1 obtained from the electron resonance spectra of benzene in a C6D6 host crystal. Both the quinoid and elongated forms corresponding to 2% – 3% distortions give zero-field-splitting patterns that are qualitatively different from experiment. Hence, these small deviations from purely hexagonal symmetry are found to have extremely large effects on the unpaired electron spin density in the molecule. The experimentally determined conformation appears to be slightly elongated, but the distortions must be much smaller than those considered in the present study and certainly less than 1%. Electron correlation is also found to have a very large effect on the splitting parameters. For example, the Hartree–Fock value of D for the hexagonal (1.427 Å) form is 0.1087 cm−1 as compared to 0.1676 cm−1 obtained from a large configuration-interaction wavefunction. An extensive analysis of the effect of electron correlation on the splitting parameter shows that excitations of the types σ2→π2 and π2→σ2 have little effect on D. By contrast, excitations of the types π2→π′2 (or π→π′) and σπ→σ′π′ increase D, whereas those of the type σ2→σ′2 (or σ→σ′) decrease D. The sum of the absolute values of the configuration-interaction effects for these classes of excitations is greater than the value of D itself. The implications of these results on previous semiempirical calculations are discussed.
Magnetothermodynamics of single crystal CuSO4⋅5H2O. VI. Properties below 0.5 °K by heat introduction with constant fields to 33 kG along the γ axis. The initial T5 dependence of entropy and heat capacity for dipole–dipole magnetic interactions
Authors: Fisher R. A. , Brodale G. E. , Hornung E. W. , Giauque W. F.
Published: February 2003

The magnetic and thermodynamic properties of a 3.700 cm diam spherical single crystal of CuSO4⋅5H2O have been investigated from 0.02 to 0.5 °K with various constant fields from 0 to 33 000 G along the γ axis. Heat introduction was used at fields of 0, 1000, 3000, 5000, 10 000, 20 000, 30 000, and 33 000 G, in combination with previously measured entropy references, to determine the thermodynamic temperature, enthalpy, internal energy, and heat capacity in the several fields. Measurements of magnetic moment along isentropes were correlated with temperature in the region below 0.5 °K. It has been known for some time that CuSO4⋅5H2O consists of two relatively independent magnetic systems, each of one-half mole. The linear chain system loses its entropy in a Schottky curve with a maximum near 1.5 °K. We have found that the lower system has a lambda transition with a heat capacity maximum at 0.027 °K. At temperatures below this maximum the enthalpy change (H–H0 °K) is proportional to S6/5. A consequence of this is that the heat capacity, C=5S, will vary as T5. This is the fourth substance we have found to have this type of behavior, which we associate with essentially complete dipole–dipole interactions. We have also confirmed the measurements of van Tol and Poulis who showed that the linear chain system has a transition near 0.1 °K, due to interchain condensation. The magnetothermodynamic properties have been tabulated over the general range 0.02 – 0.5 °K. Methods of correcting ’’adiabatics’’ to isentropes have been discussed.
Dielectric relaxation in 1,1,1-trichloroethane/cyclohexane solutions
Authors: Finsy R. , Van Loon R.
Published: February 2003

Complex dielectric permittivity measurements on liquid 1,1,1-trichloroethane and cyclohexane and several mixtures of these liquids are reported at centimeter and millimeter wavelengths and 25 °C. It appeared possible to discriminate, with the reported permittivity measurements and the correlation functions given by Nee and Zwanzig and by Lobo et al., the models of dielectric relaxation proposed by Glarum and Cole and by Nee and Zwanzig in favor of the latter. The theories of dielectric relaxation in polar liquids of Nee and Zwanzig and of Lobo et al. are extended to solutions and in particular to binary mixtures of polar and nonpolar liquids.
Theoretical study of H+2 spectroscopic properties. IV. Adiabatic effects for the 2pπu and 3dσg electronic states
Authors: Bishop David M. , Shih Shing-Kuo , Beckel Charles L. , Wu Fun-Min , Peek James M.
Published: February 2003

Adiabatic corrections to the potential curves for the 2pπu and 3dσg states of H+2 are reported. The effects of these corrections in altering spectroscopic propterties of H+2 are described in some detail; likely consequences are indicated in two astrophysical applications.
Band structure of the one-dimensional metallic (SN)x crystal
Authors: Zunger A.
Published: February 2003

A self-consistent LCAO tight binding calculation of the band structure of the one-dimensional (SN)x crystal is performed. Convergence of the band structure as a function of the interaction range and the number of translational irreducible representations that are allowed to interact (via the Hartree–Fock elements) as well as the SCF iteration cycle convergence are examined. The crystal is shown to possess a partially occupied valence band in accord with its experimentally established metallic behavior. Bond alternancy is investigated by examining the stability of the total energy with respect to conformational changes. The alternant structure is shown to be more stable than the equal-bond structure. Analysis of the charge distribution in the system reveals a low ionicity of the S+δN−δ bond (δ=0.18e) and points to the possibitlity of formation of cross bonds between nonbonded S–S pairs. The possibility of the occurrence of a Peierls instability was investigated by searching for a superlattice of model conformational distortions in the (SN)x chain that will both produce a gap around the Fermi level and lower the total Hartree–Fock crystal energy. Such an instability was not found, and the crystal was shown to behave like a metal for all the conformations studied.
Symmetry, nodal surfaces, and energy ordering of molecular orbitals
Authors: Wilson E. Bright
Published: February 2003

The relationship between geometrical symmetry and the nodal surfaces of molecular orbitals is explored and the minimum number of nodal regions for MO’s of each species for the common point groups is tabulated. Some old theorems relating the number and nature of the nodal regions of a wavefunction to the rank order of the eigenvalue are reviewed and reproven. Finally, the application of these results to a partial ordering of molecular orbital energies is illustrated. Although some approximations are involved, the conclusions do not depend on nearest neighbor or similar restrictions.
Polarized crystal spectra of CrF2 from 6000 to 38 OOO cm−1
Authors: Lim Poh Eng , Stout J. W.
Published: February 2003

Polarized absorption spectra of single crystal CrF2 from 6000 to 38 000 cm−1, at temperatures of 293, 80, 21 and 5.5 °K, are reported. In the (010) plane the principal axes of refractive index are near the [100] and (001) crystallographic directions. Spectra are presented with light propagated along [100], [010], (001), [101], and (101), each with two polarizations. Four narrow lines, of full width at half-height 4 cm−1 and identified from their polarization properties as magnetic dipole transitions, were found at 10 394, 16 986, 18 122, and 20 270 cm−1. Oscillator strengths of the lines and bands are calculated. The observed spectra are interpreted as electronic transitions of a Cr2+ ion in approximate D4h symmetry from a ground 5B1g level to excited quintette and triplet levels. Ligand field parameters are B=895 cm−1, C=3152 cm−1, Dq=1039.4 cm−1, Ds=−371 cm−1, and Dt=320 cm−1. CrF2 was found to be transparent between 1323 and 5000 cm−1.
Generalized Langevin equation approach for atom/solid-surface scattering: Inelastic studies
Authors: Doll J. D. , Myers L. E. , Adelman S. A.
Published: February 2003

The generalized Langevin equation approach for gas/solid-surface scattering is applied to the study of gas translational to surface vibrational energy transfer. A local Brownian approximation to the exact dynamics is used for these studies. Finite temperature lattice studies are facilitated by directly solving the appropriate Fokker–Planck equation.
Repulsive potentials for Na+–R and Al+–R (R= rare gas atoms) derived from beam experiments
Authors: Kita S. , Noda K. , Inouye H.
Published: February 2003

Repulsive potentials for Na+–R and Al+–R (R=rare gas atoms) are derived from integral scattering experiments using the projectile ions in the energy range 0.5–4 keV. The potentials are well represented by an exponential formula, V (R) =A exp(−αR). The numerical values of the parameters, A (eV) and α (Å−1), are Na+–He, 1200, 4.92; Na+–Ne, 5350, 5.12; Na+–Ar, 11 340, 4.68; Na+–Kr, 9600, 4.33; Al+–He, 325, 3.50; Al+–Ne, 1220, 3.93; and Al+–Ar, 3170, 3.86. The statistical computation published by Kim and Gordon [J. Chem. Phys. 60, 4323 (1974)] for Na+–R is in good agreement with the experiments, and the values for Al+–He and Al+–Ar computed by a similar statistical method are also in good agreement with the experimental results, while Al+–Ne resulted in a significant discrepancy.
Theory of two maxima in the temperature dependence of the spin–lattice relaxation rate of rotating three-spin systems in solids
Authors: Hubbard Paul S. , Johnson Charles S.
Published: February 2003

The spin–lattice relaxation of a system of three identical spin-1/2 nuclei at the corners of an equilateral triangle is calculated on the assumptions that the relaxation is due to the dipole–dipole interactions between the nuclei, and that the motion of the triangle is a hindered rotation about an axis perpendicular to the plane of the triangle, the hindered rotation being described by a ’’stochastic six-well model.’’ In this model there are six equilibrium orientations about the fixed axis, separated alternately by angles Δ and (2π/3)−Δ. The system rotates by jumping randomly from one of these orientations to an adjacent orientation. There are in general four different jump probabilities per unit time: ν′1 for a jump of +Δ, ν′−1 for a jump of −Δ, ν1 for a jump of (2π/3)−Δ, and ν−1 for a jump of −(2π/3)+Δ. This model is plausible for some cases in which the potential for the rotational motion includes a term of period π/3 in addition to a term of period 2π/3. If cross correlations of the three different dipole–dipole interactions are omitted in the calculation, the relaxation is a simple exponential decay with relaxation time T1. Expressions for 1/T1, and for the average of 1/T1 over all orientations of the axis of hindered rotation, are obtained for the general stochastic six-well model. For the special case in which ν1=ν−1≪ν′1 =ν′−1, it is shown that 1/T1 and (1/T1)av can have two distinct maxima as a function of temperature. The nonexponential relaxation which results when cross correlations of the different dipole–dipole interactions are included in the calculation is also considered, and it is shown for the case ν1=ν−1≪ν′1=ν′−1 that there can be two distinct maxima in the relaxation rate as a function of temperature, and that the form of the nonexponential relaxation in the temperature ranges of the two maxima can be obtained from a previous calculation.
Raman study of the high pressure phase transitions and the volume dependent lattice anharmonicity of NH4Br
Authors: Ebisuzaki Y.
Published: February 2003

Raman spectra of NH4Br are reported for temperatures between 195 and 407 K and pressures between 1 bar and 42 kbar. This domain includes the stability fields of the disordered NH4Br II, the parallel ordered NH4Br IV, and the high pressure phase of NH4Br V and the II–V and V–IV transitions. A detailed phase diagram including the high pressure transtions determined in this work is also presented. The Grüneisen constants for the lattice phonons at 296 K are 3.2 and 1.2 [ν5(TO)], 0.72 and 0.46 (2ν6), and 3.3 and 0.75 (ν7) in NH4Br II and NH4Br V, respectively. NH4Br V is interpreted as a mixed phase and its spectrum as the superposition of the spectra of two other phases, III (antiparallel arrangement of NH4+ ions) and IV (parallel arrangement). The most salient features of the results for NH4Br V, namely the small volume-dependent anharmonicity and the anomalous and strongly temperature-dependent intensities of the Raman bands, can be explained in terms of a mixed phase. The intensity of one group of peaks (antiparallel) decreases, while that of the other (parallel) increases as the temperature is lowered in the field of NH4Br V. The five lattice phonons of NH4Br V which become Raman inactive in the phase IV belong to cluster or domains in which NH4+ ions are aligned as in NH4Br III.
Oscillator strength sums S (−3) to S (2) and C6 for the atoms He to Kr
Authors: Cummings F. E.
Published: February 2003

The oscillator strength sums S (2), S (1), and S (−1) calculated using analytical Hartree–Fock wavefunctions and the sums S (−2) and S (−3) from an approximate uncoupled Hartree–Fock calculation are given for the atoms He to Kr. Trends are discussed as well as the accuracy of the Hartree–Fock calculations. An extensive compilation of accurate sums is used to assess the relative usefulness and accuracy of recently proposed bounds and approximations to the spherically averaged dispersion coefficient C6. Values of C6 for the first three rows are presented.
The electrical and magnetic properties of (TTF)(I)0.71
Authors: Somoano R. B. , Gupta A. , Hadek V. , Datta T. , Jones M. , Deck R. , Hermann A. M.
Published: February 2003

The synthesis and transport properties of the salts (TTF) (iodine)x have been investigated. These salts are quasi-one-dimensional compounds containing cation radicals only, in contrast to the cation radical–anion radical systems such as (TTF)(TCNQ). The phase diagram of (TTF)(I)x is discussed, and found to consist of two conducting, stoichiometric phases, x=2.0 and 3.0, and two nonconducting, nonstoichiometric phases, x=0.71 and 2.3. The transport properties of (TTF)(I)0.71 are discussed in detail. This phase is highly conducting, σRT∼350 Ω−1⋅cm−1, undergoes a sharp transition to a less conducting state at ∼210 °K, and exhibits an unusual hysteresis in some of the transport properties upon thermal cycling. The nature of the high temperature ground state is examined.
The fluorescence excitation spectrum of rotationally cooled NO2
Authors: Smalley Richard E. , Wharton Lennard , Levy Donald H.
Published: February 2003

The fluorescence excitation spectrum of NO2 was measured in the region 5708–6708 Å using a tuneable dye laser as an excitation source. The NO2 was cooled to a rotational temperature of ∼3 K by expansion with argon as a carrier gas through a supersonic nozzle. This cooling drastically reduced the rotational structure and thereby permitted a clear separation and analysis of 140 vibronic bands found in this 1000 Å region of the spectrum. The results indicate that most of the fluorescence in this region is due to a 2B2 electronic state, and that this state is so heavily perturbed by high lying levels of the ground electronic state that a substantial fraction of all ground state levels of appropriate vibronic symmetry in this region have appreciable fluorescence intensity.
An approach to rubber elasticity with internal constraints
Authors: Ronca G. , Allegra G.
Published: February 2003

The effect of configurational hindrances in unswollen rubberlike networks is taken into account by assuming the existence of a proper correlation between the macroscopic deformation and the rms fluctuations of the network junctions along the various directions of space. Following James and Guth’s approach, some junctions have been fixed at the surface of a proper parallelepiped in order to avoid collapse of the network, and the above correlation is expressed through the introduction of appropriate δ functions into the configurational partition function. Results identical with those previously arrived at by Flory–Hoeve–Ciferri are obtained by assuming that the rms fluctuation components transform as the macroscopic stretch ratios. This assumption cannot hold under high deformations because it implies that the junction fluctuations increase without limit with the deformation. Therefore an intermediate model is proposed where (i) the average ms fluctuations of the junctions are independent of strain, as in James–Guth theory, and (ii) the ratios between the rms (mean square) fluctuations are equal to those between the corresponding stretch ratios as in Flory–Hoeve–Ciferri theory. The intermediate model can explain, at least to some extent, the so-called Mooney effect. It is also shown that the relation −[∂ln(σ/T)/∂T]L,V=dln〈r2〉0/dT is valid for James–Guth theory only.
Differential charge exchange cross sections in AB+–AB collisions
Authors: McCann K. J. , Flannery M. R. , Hornstein J. V. , Moran T. F.
Published: February 2003

Differential cross sections for vibrational excitation in H+2(X 2Σ+g, ν0′) –H2(X 1Σ+g, ν0″), N+2(X 1Σ+g, ν0′) –N2(X 1Σ+g, ν0″), O+2(X 2Πg, ν0′) –O2(X 3Σ−g, ν0″), CO+(X 2Σ+, ν′0) –CO(X 1Σ+, ν0″), and NO+(X 1Σ+, ν0′) –NO(X 2Πr, ν0″) charge transfer reactions have been determined for incident ions with 0.004 to 2.2 keV kinetic energies. Differential cross sections for excitation of individual charge transfer channels have been computed as a function of scattering angle using a multistate impact parameter treatment of the collision processes. At low kinetic energies, coverged cross sections are obtained with the inclusion of a relatively small number of product vibrational states in the wavefunction expansion of the system while a large number of product states must be included at higher energies. Small angle scattering within several degrees dominates the charge transfer, with the scattering becoming more concentrated in the forward direction at higher kinetic energies.
Tunneling frequencies of the ammonium ion in a tetrahedral crystalline field
Authors: Smith David
Published: February 2003

The tunneling frequencies of the ammonium ion in a tetrahedral crystalline field have been computed as a function of the barrier to rotation and the librational frequency. For NH4I, the tunneling frequency determined by the analysis of the proton magnetic resonance line shapes is in good agreement with the predicted value.
The generalized valence bond description of O2
Authors: Moss B. J. , Bobrowicz F. W. , Goddard W. A.
Published: February 2003

A b initio calculations using the generalized valence bond (GVB) method have been carried out for the lowest triplet and singlet states of O2 at internuclear distances (R) between 2a0 and 6a0. In contrast to other orbital descriptions, GVB leads correctly to ground state oxygen atoms as the bond length is increased to infinity. This proper behavior requires optimization of the spatial orbitals themselves and of the permutational coupling between them as well. Analysis of the results as a function of R is straightforward. Constructing a simple configuration interaction (CI) wavefunction using the GVB orbitals leads to excellent potential curves, accounting for 94% of the bond dissociation energy. The calculated adiabatic separation of the singlet and triplet states is 1.09 eV, which is 0.11 eV above the experimental Te.
Rotationally inelastic diffraction of molecular beams: H2, D2, HD from (001) MgO
Authors: Rowe R. Grant , Ehrlich Gert
Published: February 2003

In the coherent scattering of molecules from a crystal surface, energy of molecular rotation can be transferred to translational motion. This interchange has been studied for thermal beams of H2, D2, and HD diffracted from (001) surfaces of MgO prepared by cleavage in an ultrahigh vacuum. Strong new diffraction peaks are identified for in-plane scattering, beyond those predicted for elastic interactions. The location of these peaks is in agreement with calculations based on the assumption that phonons are not excited during the collision with the lattice. In the experiments, only those rotational transitions are detected for which there are extrema in the variation of the scattering angle with the wavenumber of the incident molecules. Scattering at these critical angles is analyzed and it is shown that for beams formed by effusion, these extrema lead to sharply peaked scattering maxima, which facilitate detection of rotational–translational interchanges. The efficiency of rotational transitions is found to be high, comparable to elastic scattering, and to depend strongly upon the angle between the incident beam and the scattering surface.
Molecular orbital theory of the hydrogen bond. XIV. Disubstituted carbonyl compounds as proton acceptors
Authors: Del Bene Janet E.
Published: February 2003

A b initio SCF calculations with a minimal STO-3G basis set have been performed to determine the ground state equilibrium structures and energies of dimers in the series HOH...OCR2, where R is CH3, NH2, OH, or F. The equilibrium structures of the water–acetone, water–urea, and water–carbonyl fluoride dimers are similar to those of the water–acetaldehyde, water–formamide, and water–formyl fluoride dimers, respectively, in which the substituents are ’’cis’’ to water with respect to the carbonyl CO bond. No equilibrium water–formic acid dimer of this type was found, and no equilibrium water–carbonic acid dimer exists in which water is the proton donor molecule. There is a greater differentiation of dimer stabilities in the series having the disubstituted carbonyl compounds as proton acceptors, which may be related to enhanced pi-donating and sigma-withdrawing effects of the substituents. Thus, the order of increasing dimer stability in the combined series HOH...OCRR′ is F2<H,F<H2<H,CH3< (CH3)2<H,NH2< (NH2) 2. CI calculations show that the dimer singlet n→π* transition energies are well approximated as the sum of the n→π* transition energy in the monomer R2CO and the hydrogen bond energy in the corresponding dimer HOH...OCR2, suggesting that the hydrogen bond is broken in the dimer n→π* state.
A configuration interaction study of the ground state molecular properties of NO2
Authors: Jackels Charles F. , Davidson Ernest R.
Published: February 2003

An ab initio SCF–CI wavefunction for the 2A1 ground state of NO2 has been calculated using a ’’double zeta plus polarization’’ quality basis set. The CI energy, −204.374346 a.u., is the lowest which has been reported for this molecule. The CI wavefunction is discussed and Mulliken population analyses are presented. Several one-electron properties, including the magnetic hyperfine splitting parameters, are reported for both the SCF and CI wavefunction.
Gaussian basis sets suitable for accurate valence-shell calculations using the model potential method
Authors: McWilliams D. , Huzinaga S.
Published: February 2003

Pople’s extended Gaussian-type basis, usually termed 4-31G, was adapted for use with the model potential method. Molecular calculations were performed successfully for N2, H2O, CH4, NH3, HCN, PH3, H2S, and ClF. Substantial savings were achieved in computing cost.
PNO–CI (pair-natural-orbital configuration interaction) and CEPA–PNO (coupled electron pair approximation with pair natural orbitals) calculations of molecular systems. IV. The molecules N2, F2, C2H2, C2H4, and C2H6
Authors: Ahlrichs R. , Lischka H. , Zurawski B. , Kutzelnigg W.
Published: February 2003

SCF, IEPA–PNO, CEPA–PNO, and PNO–CI calculations have been performed for the molecules N2 and F2 at their experimental equilibrium distances with two basis sets, a ’’small’’ basis that contains one d set per atom and a ’’standard’’ basis with two d sets and one f set. Potential curves of these molecules are calculated with the small basis sets. The molecules C2H2 and C2H4 are calculated with the small basis (which contains additionally one p set on H) and with a ’’hydrocarbon’’ basis that is smaller in the s,p part, but includes the same polarization functions. For C2H6 in both its staggered and eclipsed forms only the hydrocarbon basis is used. The computed correlation energies are analyzed in terms of quantities defined in Part I, in particular, in terms of the IEPA pair correlation energies εμIEPA and the error ΔEIEPA of the IEPA energy. A comparison is made between the results in the canonical and the localized representations and a partially localized description in which the σ–π separation is preserved. The IEPA error is rather large for all of these molecules, especially for N2. The IEPA error in the localized representation changes smoothly in the series C2H2 to C2H6; whereas in the canonical representation it varies by almost two orders of magnitude. A new geometry optimization for staggered and eclipsed ethane is carried out, since previous optimizations turned out to be unsatisfactory. Correlation, however, does not affect the geometries significantly; its effect on the rotational barrier is very small. For N2 and F2 in the neighborhood of the equilibrium distances the CEPA potential curves turn out to be very close to the exact ones (and yield good equilibrium distances and force constants); whereas the PNO–CI curves (and, of course, more so the SCF and IEPA curves) are unacceptable. At large internuclear distances, where the weight of the leading determinant in the full wavefunction becomes smaller than ∼0.8 the CEPA curve is unsatisfactory.
Hyperfine and superhyperfine EPR spectra of Tc(IV) and Re(IV) in tin dioxide single crystals
Authors: Pieper H. H. , Schwochau K.
Published: February 2003

The electron paramagnetic resonance spectra of Tc(IV) and Re(IV) in tin dioxide have been observed at X- and Q-band frequencies at liquid helium temperature. Both ions with a spin ground state of S=3/2 occupy lattice sites of D2h symmetry. The hyperfine structures show large anisotropies, especially in the case of Re(IV), which can be explained by the large crystal field parameters. The spectra fit with a conventional spin Hamiltonian for orthorhombic symmetry correspond to the electron transition 1/2↔−1/2. The obtained parameters for the c-axis and the magnetic z-axis coincident are Besides the hyperfine lines, both systems exhibit a partly resolved superhyperfine structure due to an interaction of the unpaired electrons with adjacent tin nuclei.
Molecular polarizabilities and susceptibilities from Frost-model wavefunctions
Authors: Amos A. T. , Yoffe J. A.
Published: February 2003

Average polarizabilities and susceptibilities of a number of molecules are computed from Frost-model wavefunctions using a form of symmetry-adapted double perturbation theory. The anisotropy of α and χ is found for a few molecules using the elliptical Gaussian form of the Frost model. The results obtained are in reasonable agreement with experiment and other calculated values.
Vibrational energy transfer in CO2–CO mixtures from 163 to 406 °K
Authors: Starr D. F. , Hancock J. K.
Published: February 2003

Vibrational energy transfer rates in CO2–CO mixtures have been measured from 163 to 406 °K using the laser excited vibrational fluorescence method. The near resonant V→V transfer step CO(v=1)+CO2(000)ke⇌ke′ CO(v=0)+CO2(001)+ΔE=−206 cm−1 has been measured in the endothermic direction and after detailed balancing [ke′=ke exp(−ΔE/kT)] is in excellent agreement with data taken in the exothermic direction at and above room temperature. V→R,T quenching of the CO2(001) level by CO2 and CO has also been studied. The CO2 self-relaxation rate reverses in temperature dependence between 250–300 °K with rates becoming progressively faster as temperature decreases. The total quenching rate for CO2 deactivation in CO2–CO collisions behaves similarly to the self-relaxation rate. It was not possible in the present analysis to separate and distinguish the pathways for V→R,T deactivation in equilibrated CO2–CO mixtures. The cross relaxation terms (CO+2–CO) and (CO+–CO 2) are both expected to be important at low temperatures.
Vibrational relaxation and photochemistry of HCl(v=1,2) and Br atoms
Authors: Leone Stephen R. , Macdonald R. Glen , Moore C. Bradley
Published: February 2003

A laser-excited fluorescence method is used to measure the total deactivation rates of HCl(v=1) and (v=2) by Br atoms. The rate constant for v=1 is (2.8±0.52) ×10−13 cm3 molecule−1 sec−1 (295 °K) and for v=2 is (1.8±0.33) ×10−12 cm3 molecule−1 sec−1 (294 °K) The deactivation of HCl(v=1) by Br occurs through V→T,R energy transfer, while the deactivation of HCl(v=2) is believed to occur largely by reaction. The rate constant for deactivation of HCl(v=1) by Br2 is (3.26±0.13) ×10−14 cm3 molecule−1 sec−1 (295 °K). The reaction between HCl(v=2) and Br is discussed as a possible means of separating Cl isotopes by vibrational photochemistry.
Molecular photoelectron spectroscopy at 132.3 eV. The second-row hydrides
Authors: Banna M. S. , Shirley D. A.
Published: February 2003

Photoemission spectra of the second-row hydrides CH4, NH3, H2O, and HF, as well as Ne, obtained with ultrasoft (132.3 eV) x rays from the yttrium Mζ line and with soft x rays are compared and discussed. The 2s-derived 2a1 or 2σ orbitals show large relaxation energies, as do the 2s orbitals in the free atoms. The high binding energies of the 2a1 orbitals in CH4 and NH3 indicate that much of the bond energy resides in these orbitals. Bond energies estimated from changes in the average valence-electron binding energies from atoms to hydrides show rough agreement with literature values. Relative molecular orbital peak intensities show dramatic changes from 132.2 eV to 1253.6 eV photon energies, with the atomic cross-section ratio 2p/2s near unity at 132.3 eV and near 0.1 at 1253.6 eV. This difference allows peaks to be assigned to molecular orbitals in some cases by visual inspection, on the basis of atomic orbital composition. Comparison with theoretical intensities based on plane-wave or OPW continum final states shows qualitative agreement at best, and clear disagreement in some cases. Quantitative agreement will require a more sophisticated theory. Values of σ (2p)/σ (2s) for atomic C, N, O, F, and Ne were derived from the spectra at both photon energies.
High resolution proton NMR study of gypsum, CaSO4⋅2H2O
Authors: McKnett C. L. , Dybowski C. R. , Vaughan R. W.
Published: February 2003

Multiple pulse nuclear magnetic resonance techniques have been used to measure the proton chemical shift tensor in gypsum, CaSO4⋅2H2O, and to characterize the proton exchange occurring in this material. The orientation of the room-temperature averaged proton chemical shift tensor is as would be expected from the H2O molecular symmetry alone, and the low temperature results are consistent with a tensor of near axial symmetry along the O–H bond direction (σ∥=+2.1 ppm, σ⊥ (in H2O plane) =−18.4 ppm, and σ⊥ (perpendicular to H2O plane) =−17.6 ppm±1 ppm). In addition, a value of 1.54±0.01 Å was determined for the interproton distance within the H2O molecule in gypsum.