bluuebunny/crossref_metadata_2025 huggingface dataset:
Deuteron quadrupole coupling in hydrogen-bonded systems. III. Pyridinium salts
Authors: Goldstein Neil , Ragle J. L.
Published: February 2003

Deuterium quadrupole coupling parameters have been measured for the perchlorate, iodide, bromide, chloride, and nitrate salts of pyridinium ion at 77 K. Coupling constants range from 187.2 kHz (η=0.129) for the ClO−4 salt to 128.9 kHz (η=0.185) for the NO−3 salt, and correlate well with what is known about the crystal structure and hydrogen bonding tendencies for the series. A theoretical discussion of the data is given in terms of approximate Hartree–Fock calculations at the STO-3G level, in terms of which a major component of the alteration in field gradient arises from changes in the orbital population on N and D.
Unimolecular processes in vibrationally highly excited cycloheptatrienes. I. Thermal isomerization in shock waves
Authors: Astholz D. C. , Troe J. , Wieters W.
Published: February 2003

The thermal unimolecular isomerization reactions of cycloheptatriene, and of methyl- , ethyl- , and isopropyl-cycloheptatriene, have been studied over the temperature range 900–1400 K in shock waves. Even for these large molecules, falloff behavior has to be taken into account mainly due to weak collision effects. The limiting high pressure rate constants k∞ and the specific rate constants k (E) are derived for use in later photoactivation experiments.
Collisionless nonradiative decay rates of single rotational levels of S1 formaldehyde
Authors: Weisshaar James C. , Moore C. Bradley
Published: February 2003

Fluorescence lifetimes of single rotational levels of the lowest vibrational level of the first excited singlet state of H2CO and D2CO have been measured under collision-free conditions following excitation by a pulsed dye laser. For H2CO, the lifetimes range from 66 ns to 4.2 μs with a median of about 160 ns. Individual lifetimes show no systematic variation with J′, K′, or Erot. K-doublet levels split by as little as 8×10−4cm−1 in S1 are observed to have different lifetimes. The H2CO results are interpreted in terms of a sequential coupling model (S1→S0→continuum) in which the final states are those of the H2+CO dissociation continuum. For D2CO, the lifetimes vary between 5.5 and 8.1 μs and are nearly radiative lifetimes.
Crystal structures of A2FeCl5⋅H2O (A=Rb+, Cs+) and field dependent superconducting susceptometer measurements
Authors: O’Connor Charles J. , Deaver Bascom S. , Sinn Ekk
Published: February 2003

Crystal structures and low field principal crystal axis magnetic measurements are reported for the antiferromagnetic iron (III) salts A2FeCl5⋅H2O; A=Rb+, Cs+. The two compounds undergo a zero field paramagnetic to antiferromagnetic phase transition at 6.5 K (A=Cs+) and 10.05 K (A=Rb+). The spin flop magnetic phase is observed in high field measurements. The magnetic field-temperature phase diagrams are reported. All magnetic measurements were recorded on a superconducting susceptometer. The bicritical points are 6.25 K, 14.5 kOe (A=Cs+), 9.75 K, 17.8 KOe (A=Rb+). Crystal data for Cs2FeCl5⋅H2O: space group Cmcm, Z=4, a=7.442(3) Å, b=17.307(7) Å, c=8.077(7) Å, V=1040 Å3, R=5.2%, 561 reflections. Crystal data for Rb2FeCl5⋅H2O, space group Pnma, Z=4, a=13.825(4) Å, b=9.918(3) Å, c=7.100(2) Å, V=4.9 Å3, 5.7%, 712 reflections.
Electron paramagnetic resonance and phase transition in (NH4)2SeO4:Mn2+ single crystals
Authors: Jain Vimal Kumar , Venkateswarlu Putcha
Published: February 2003

The electron paramagnetic resonance of Mn2+ in ammonium selenate has been studied at X-band and at various temperatures between 77 and 395 K. The spectrum consists of a set of 30 lines corresponding to single type of Mn2+ environment. The principal y axis is along the b axis and the principal z axis makes an angle 54°±2° with the c axis in the ac plane. Around 390 K the spectrum shows an anomalous change in line shapes followed by a complete collapse of the spectrum which is found to be reversible. This has been discussed in terms of a possible first-order phase transition. The spectra at 77 K and at 300 K (including fine structure ΔM=±1 and Δm=0 hyperfine structure) are satisfactorily described by a spin Hamiltonian appropriate for Mn2+ in a rhombic crystal field. The parameters are determined at 77 and 300 K. Forbidden hyperfine transitions Δm=±1 and Δm=±2 have been observed. From the unequal separations between Δm=±1 doublets in M=+1/2→−1/2 transition, the values of the axial and rhombic parts of the quadrupole coupling constants are determined. The observed doublet separations of forbidden hyperfine transitions in different electronic transitions are in fairly good agreement with those calculated. The observed angular variation of the intensities of the allowed and forbidden hyperfine transitions is found to be in good agreement with theory.
Vibrational spectroscopy of matrix isolated diatomics: The infrared line shape
Authors: Korzeniewski Gregory , Metiu Horia
Published: February 2003

The vibrational line shape of a diatomic molecule imbedded in a solid matrix is computed with the aid of a one dimensional model. It is assumed that the broadening mechanism is the coupling between the vibrations of the system. As a zero order approximation, we use the harmonic vibrations of the lattice with the diatomic impurity. The harmonic modes are coupled by cubic and quartic force constants. All the necessary force constants are computed from semiquantitative interatomic potentials. The width obtained in this calculation is generally too small and nearly temperature independent. The lattice induced shift (total shift minus the shift caused by the anharmonicity of the diatomic) is toward the blue instead of the red, and is temperature dependent.
The 1H–19F coupled nuclear magnetic relaxation in (NH4)2TiF6
Authors: Watton A. , Koster E. , Sandhu H. S. , Petch H. E.
Published: February 2003

Nuclear magnetic resonance absorption line shapes and spin lattice relaxation times have been measured for the proton and fluorine spin systems in (NH4)2TiF6. At liquid helium temperature the ammonium ions are found to exhibit effects of quantum mechanical tunneling between equilibrium orientations. Over the temperature range 40<T<250 K the spin lattice relaxation of both the proton and fluorine spin systems is controlled by the stochastic reorientation of the ammonium ions. The TiF6= ions remain essentially immobile over this region but their thermal reorientations are found to become effective in the relaxation mechanism above 250 K. The results are discussed in terms of nuclear dipole–dipole cross coupling between the 1H and 19F spin systems.
Amorphous state studies with the Gaussian core model
Authors: Stillinger F. H. , Weber T. A.
Published: February 2003

Properties of the supercooled amorphous state have been studied for the Gaussian core model at reduced density ρ*=1. By monitoring the self-diffusion constant and mean energy during a slow cooling to absolute zero, a glass transition was observed separating supercooled fluid from a rigid amorphous packing. At this glass transition the Wendt–Abraham criterion for the pair correlation function (g(2)min/g(2)max=0.14) is satisfied. However, former suggestions that second-peak splitting in g(2) universally signifies the glassy state for monatomic substances are not supported.
The Xα valence bond theory of weak electronic coupling. Application to the low-lying states of Mo2Cl84−
Authors: Noodleman Louis , Norman Joe G.
Published: February 2003

We show that valence bond (VB) concepts can be introduced into Xα theory. The resulting Xα–VB model yields energy states which either are pure multiplets or can be combined by straightforward projection to give pure multiplets. The new theory should be more computationally efficient than Hartree–Fock-based CI models. A preliminary study of the δ→δ* transition in Mo2Cl84− yields an excitation energy closer to experiment than previous theoretical values, including those obtained to date from GVB–CI calculations.
Electron scavenging in glasses
Authors: Shlesinger Michael F.
Published: February 2003

Electron scavenging reactions in amorphous media are analyzed using a random walk hopping model, which can yield a time dependent rate constant. The underlying waiting time distribution for the time between hops is shown to be able to lead to an exponential of a fractional power of time decay of the reactants, as opposed to simple exponential decay. This type of kinetic behavior has been found by Hamill and Funabashi when replotting electron scavenging data in an appropriate manner. A different waiting time distribution which has been used for the analysis of charge transfer in amorphous xerographic films is shown to lead to an algebraic decay law, as has been observed in various phosphors.
PMR spectroscopy of the tunneling ammonium ion: Generalized line shape theory and some experimental examples
Authors: Lalowicz Z. T. , McDowell C. A. , Raghunathan P.
Published: February 2003

The site symmetry of the NH4+ ion is low in many crystal structures of ammonium compounds. This renders inadequate the commonly considered theoretical models of the tunnel-split ground librational state of NH4+, which are based on a perfectly tetrahedral crystal field due to assumption of equivalent threefold, and neglect of twofold, axes of symmetry. A general treatment of the NMR line shape theory is therefore considered and Hamiltonian matrix elements for tunneling are explicitly calculated in terms of the seven overlap-integral parameters related to the four C3– and three C2–symmetry axes of a tetrahedron. The model allows us to discuss all possible ground torsional level structure of the tunneling ammonium ion. The proton NMR line shapes are calculated for all representative cases, and the results are used in comparison with experimental data to elucidate the ground torsional level structures of NH4VO3, (NH4)2ZrF6, (HN4)2TiF6, (NH4)2TeO4, and (NH4) IO4.
Monte Carlo quasiclassical trajectory study of the collision-induced dissociation of hydrogen by neon
Authors: Lehr Timothy , Birks John W.
Published: February 2003

Cross sections for dissociation from particular vibration–rotation states of H2 on collision with Ne were calculated using the quasiclassical trajectory (QCT) method. An accurate ab initio potential energy surface was used for the Ne–H2 interaction. Two different forms were used for the H2 potential: the ab initio results of Kolos and Wolniewicz and a simple Morse function. Cross sections were similar for both cases and indicated that dissociation occurs from all v states. Both vibrational and rotational energy were shown to enhance the cross sections, and the probability of dissociation was shown to be strongly dependent on the orientation of the collision.
Study of the Ar–N2 interaction. II. Modification of the electron gas model potential at intermediate and large distances
Authors: Lee Sangyoub , Kim Yung Sik
Published: February 2003

The failure of the electron gas model calculation of the Ar–N2 interaction potential at large intermolecular separations [J. Chem. Phys. 68, 5001 (1978)] is remedied by using a model developed earlier by one of the present authors and Gordon. The results give predictions of the Ar–N2 interaction potential for the entire portion of physically interesting region of the potential surface. The spherically averaged potential is compared with the available experimental potentials and shows the improved agreement.
Mott–Hubbard condition for electron localization in the Hartree–Fock band theory
Authors: Whangbo Myung-Hwan
Published: February 2003

The relative stabilities of the delocalized and the localized states of a half-filled band were examined in terms of the Hartree–Fock band theory. The electron localization condition thus obtained is very similar to that derived by Hubbard considering the dynamics of electrons in a narrow band. The relative stability problem of the delocalized vs the localized state in a crystalline chain was also compared with that of the low-spin vs the high-spin state in a weakly interacting metal dimer complex.
Nematic–isotropic phase equilibria in binary mixtures containing rigid rod solutes
Authors: Martire D. E. , Dowell F.
Published: February 2003

It is argued that any realistic model of a liquid crystal system will have to take molecular flexibility into consideration. In binary mixtures, the nematic-isotropic phase equilbria can be described by a lattice model which include molecular flexibility. (AIP)
Gas-to-liquid shifts in NMR and the validity of the second virial coefficient of chemical shielding
Authors: Jameson Cynthia J. , Jameson A. Keith , Parker Harriet
Published: February 2003

NMR spectra are reported for low density gas samples as well as gas-to-liquid shifts for 19F nuclei in several systems. The density dependence of NMR chemical shifts thus obtained gives values of B-A as a measure of higher order terms. (AIP)
Longitudinal diffusion coefficients and the generalized Einstein relation for Cl− ions in Ne, Ar, Kr, and Xe
Authors: Eisele F. L. , Thackston M. G. , Pope W. M. , Ellis H. W. , McDaniel E. W.
Published: February 2003

Longitudinal diffusion coefficients have been measured for Cl− ions drifting in Ne, Ar, Kr and Xe gases under the influence of a uniform constant electric field. The measurements were made at gas temperature 300 °K and pressure below o.5 Torr. (AIP)
Detailed balance in quasiclassical trajectory calculations of thermal rate constants for chemical reactions
Authors: Gray Joni C. , Garrett Bruce C. , Truhlar Donald G.
Published: February 2003

Thermal rate constants for chemical reactions from quasiclassical trajectory calculations are discussed. For H2 + I ? H + HI trajectories for reverse reaction have been calculated giving a complete standard QCT rate constant for it. (AIP)
The radial distribution function of hard rods
Authors: Pickard D. K. , Tory E. M.
Published: February 2003

The radial distribution function of hard rods is discussed and several point not explained in previous paper 1,2 are explained. (AIP)
Temperature dependent mobilities: O2+ and NO+ in N2
Authors: Eisele F. L. , Ellis H. W. , McDaniel E. W.
Published: February 2003

The zero field mobilities of O2+ and NO+ in N are reported over a temperature range of 305°-640°K. The measurements were performed in the pressure range 30–42 Torr. (AIP)
Isothermal diffusion and quasielastic light-scattering of macromolecular solutes at finite concentration
Authors: Wills Peter R.
Published: February 2003

The Brownian motion of a group of hydrodynamically interacting particles suspended in a low-Reynolds-number fluid is described by a generalization of the Einstein diffusion relation. The time integral of each velocity self- or cross-correlation function is found to be equal to one of the mobility tensors governing steady movement of the particles through the fluid. Using the approach of nonequilibrium thermodynamics and linear response theory, this result allows formulation of an expression for the concentration dependence of the Fick’s law diffusion coefficient for mutual binary diffusion in terms of the mobility tensors. The relationship between the concentration dependence of the solute chemical potential, sedimentation rate and mutual diffusion coefficient D is proved. A distinction is made between D and the mean self-diffusion coefficient Ds for solute molecules, thus allowing interpretation of results obtained using different experimental techniques. An argument is presented which indicates that quasielastic light scattering is suitable for the determination of Ds and higher moments of the self diffusion coefficient distribution for a monodisperse solution of macromolecules. The normalized second moment of this distribution is calculated for a solution of spherical macromolecules in terms of the solute volume fraction. It is concluded that quasielastic light scattering may be used to determine the value of a dimensionless parameter which is dependent solely on the shape of the hydrodynamic particle corresponding to a solvated macromolecule.
Fluorescence and thermoluminescence of N2O, CO, and CO2 in an argon matrix at low temperature
Authors: Fournier J. , Deson J. , Vermeil C. , Pimentel G. C.
Published: February 2003

The photolysis of N2O in an argon matrix at 7 °K by 8.4 eV energy photons leads to the emission of the Vegard–Kaplan system of N2 (A 3Σ+→X 1Σ+g) and to the atomic emissions O(1S) →O(1D) and N(2D) →N(4S). The thermoluminescence which accompanies an increase of the matrix temperature is due to O2 (A 3Σ+u→X 3Σ−g). In the presence of CO, the atomic oxygen emission disappears and the CO (a 3Π→X 1Σ) Cameron System is detected. The thermoluminescence in this case is a continuum (3800–5000 Å) apparently due to CO2 (3B2→X 1Σ+g) showing that ground state oxygen atoms, O(3P), react with CO at cryogenic temperatures: CO (X 1Σ+)+O(3P) →CO2(3B2), CO2(3B2) →CO2(X 1Σ+g)+hν.
13C/12C and 18O/16O liquid–vapor isotopic fractionation factors in CO as a function of temperature
Authors: Götz Dieter , Lee Myung W. , Bigeleisen Jacob
Published: February 2003

The carbon and oxygen isotope fractionation factors in the liquid–vapor equilibrium (LVIFF) of carbon monoxide have been determined over the temperature range 81–125° K. The results are in excellent agreement with previous isotopic vapor pressure data which cover the range 68–82° K. The results are analyzed in terms of an anharmonic cell model by the FG matrix method. The anharmonicity is approximated by the use of a temperature dependent F matrix. Analysis of the LVIFF and VPIE data by this method gives good agreement with experiment and the previous theoretical calculation by Gordon of the mean square force, the mean square torque, and the translation rotation coupling at 77.5° K. The present calculations of the mean square force and experimented data on the mean energy in liquid CO are used to derive a set of intermolecular potential parameters for CO using the WCA perturbation theory. If a m-6 potential is assumed, the best fit of all the data is obtained for m=14, ε/k=109° K, and σ=3.858Å. The latter are consistent with potential parameters derived from second virial coefficients and gas viscosity.
Analysis of torsional spectra of molecules with two C3v internal rotors. XIV. c
i
s- and t
r
a
n
s-2-butene
Authors: Durig J. R. , Hudson S. D. , Natter W. J.
Published: February 2003

The low frequency Raman and far infrared spectra of gaseous cis- and trans-2-butene have been recorded from 70–450 cm−1. Considerable torsional data have been obtained for the trans-2-butene molecule with the Au torsional fundamental being observed at 163.6 cm−1 in the infrared spectrum. The overtone of the Bg fundamental was observed in the Raman spectrum at 432 cm−1. The torsional data have been used to determine the potential function based on a semirigid model and the effective barrier was found to be 713 cm−1 (2.04 kcal/mole). The cosine–cosine coupling term, V33, was found to be zero whereas the sine–sine coupling term, V33', has a value of −48 cm−1 (−0.14 kcal/mole) and V60=V06=−4 cm−1 (−0.01 kcal/mole). For the cis-2-butene molecule only the B2 torsional mode was observed at 101.0 cm−1 which gives an effective barrier of 250 cm−1 (0.87 kcal/mole). These data are compared to the corresponding quantities obtained earlier by other methods.
Perturbation theory for the free energy of hard quadrupolar diatomics
Authors: Martina Esteban , Stell G. , Deutch J. M.
Published: February 2003

The perturbation theory for the free energy of hard spheres with an embedded quadrupole is extended to molecules with a nonspherical core. Our system consists of quadrupolar hard homonuclear diatomics. The free energy is calculated to order Q6 in the quadrupole strength. Several expressions for hard diatomics are examined to obtain the free energy of the unperturbed system. The term of order Q2 is calculated using recent Monte Carlo results for the reference system and it is found that it gives an important contribution to the total free energy. The Padé approximant (PA) formed for the quadrupolar hard spheres with the terms of order Q4 and Q6 is evaluated with an effective diameter calculated analytically using the isothermal compressibility of the hard diatomics. It is concluded that it is necessary to take into account the nonsphericity of the molecules in the calculation of the free energy in order to obtain reliable results for real systems.
Are the homogeneous linewidths of spin resonance (ODMR) and optical transitions related?
Authors: Zewail Ahmed H.
Published: February 2003

This paper examines two problems that are central to the question raised in the title. First, we describe formally the origin of homogeneous broadening of ODMR and optical transitions in the presence and absence of spin-orbital couplings between the lowest triplet and other triplet or singlet states. Second, we deduce expressions that relate the spin-to-optical widths to the rates of orbital and spin scatterings in the singlet and triplet states. In the absence spin-orbital coupling, it is found that the optical width caused by pure dephasing has a much different origin than the ODMR width. In the presence of spin-orbital coupling, on the other hand, the two widths are related and depend on the phonon-induced scattering cross section of the ground electronic state and the other spin sublevel that is involved in the ODMR transition and is not perturbed by spin-orbital interactions. Throughout the paper the coupling of only one triplet sublevel to the singlet was chosen to sufficiently represent the effect of spin-orbital coupling on ODMR and optical widths. Finally, using these theoretical findings we compare the results with some available data on aromatics (ππ*states) and azines and carbonyls (nπ*states) and also conjecture on possible future experiments.
Collision-induced line shapes calculated with a hard sphere model
Authors: Strauss Herbert L. , Weiss Shmuel
Published: February 2003

The induced-dipole autocorrelation functions for an exponential dipole and for a quadrupole-induced dipole are computed for a hard sphere potential. The first function is compared to collision-induced absorption results for rare gas mixtures. The shortcomings of the hard sphere model for the description of these short-ranged phenomena are considered. The second function is used to fit the experimental H2–Xe rotational spectrum. Good agreement between the experimental and the calculated spectrum is obtained. It is concluded that the hard sphere model provides a good description of the longer ranged quadrupole-induced spectra.
Angular and rotational energy moments of cross sections in atom–diatom scattering
Authors: Wood Robert R. , Curtiss C. F.
Published: February 2003

The evaluation of the various averages of the rotationally inelastic cross sections which appear in the kinetic theory developments of Wang Chang, Uhlenbeck, and deBoer is discussed and carried out numerically. The generalized phase shift (GPS) approach is used to obtain the rotational energy moments of the degeneracy averaged cross sections. These quantities are developed as power series in the ’’strength’’ of the anisotropic portion of the interaction potential, and the first few terms are evaluated in the classical limit. A nonspherical model potential for the interaction of an atom and a homonuclear diatomic molecule is introduced, and the ’’cross sections’’ are numerically evaluated for five sets of model parameters. One choice of the parameters represents the Ar–N2 system, and the other choices are used to examine the effects on the scattering results of varying specific molecular quantities.
Stochastic reduction for molecular multiphoton processes
Authors: Mukamel Shaul
Published: February 2003

Reduced equations of motion (REM) are derived for molecular multiphoton processes by constructing a small set of relevant operators corresponding to populations and coherences of the molecule ’’dressed’’ by the radiation field and adopting the Mori projection operator technique. The resulting REM contain time-dependent dephasing operators which are intimately related to our reduction scheme and whose definition and evaluation do not rely on any perturbative expansion in intramolecular interactions. These operators are expressed in terms of intramolecular dipole correlation functions which are the key quantities in the present formulation and are then evaluated using a simple model. In the limit where our physical system may be separated into a ’’system’’ and a ’’bath’’ with a weak interaction, we recover the ordinary expressions of line broadening. The present formulation provides us with a hiearchy of REM which interpolate smoothly all the way from the coherent to the incoherent driving limits and are in good quantitative agreement with the existing experimental data.
Molecular dynamics studies of Lennard-Jones liquid mixtures. V. Local composition in several kinds of equimolar mixtures with different combining rule
Authors: Nakanishi Koichiro , Toukubo Kazuo
Published: February 2003

Local fractions of two components around each kind of central particle have been calculated for the purpose of obtaining detailed structural information on binary model mixtures from molecular dynamics data for three equimolar mixtures of Lennard-Jones fluids with different combining rules for the energy parameter. It was found that well-defined local fractions can be determined in each case and that they are different from the overall mole fractions in nonideal mixtures. The validity of empirical formulations for local compositions has been tested against the present results and it was suggested that the Boltzmann factor relation between local and overall compositions may be inadequate to describe relative configuration of two components around a central particle.
Effects of fluctuation on dissipative structures. I. Effects of fluctuation on chemical oscillation systems
Authors: Iwamoto Kazutoshi , Senō Manabu
Published: February 2003

A simple chemical system showing oscillatory behavior is proposed and analyzed in detail especially concerning the effect of fluctuations. The chemical system under influence of fluctuation shows occasionally induced pulselike oscillations. This behavior is similar to that of the Belousov–Zhabotinskii reaction near the terminus of oscillations. The effects of fluctuation on the Brusselator and the Oregonator models were also examined. Through these investigations, we conclude that chaotic oscillation arises from the finite deviation induced by a large fluctuation near the critical transition state.
High pressure syntheses of cubic BN using water, urea, and boric acid catalysts
Authors: Kobayashi Toshio
Published: February 2003

Cubic BN is successfully synthesized under high pressure by using three nonmetal catalysts, i.e. water, urea, and boric acid, and the mechanism is explained. The synthesis is accomplished with the aid of ammonium borate flux which is produced by the reaction between starting hexagonal BN and catalysts at pressures above 4.3–4.8 GPa and temperatures above 800–1000°C. The resulting cubic BN is a white powder, with grains having a tetrahedral crystal morphology and a 0.1–0.3 μm size. At pressures below 4.2 GPa cubic BN is found to reconvert to hexagonal BN in the presence of ammonium borate, showing that hexagonal BN is stable in this region. The lower pressure limit in the synthesis of cubic BN is surmised to be based on the cubic–hexagonal BN equilibrium line in the ammonium borate coexisting systems.
A search for multicritical points in liquid mixtures: The shield region and the three-state Potts point
Authors: Das B. K. , Griffiths Robert B.
Published: February 2003

In a study of the mixture, ethylene glycol + water + lauryl alcohol + nitromethane + nitroethane we have observed a region of four coexisting liquid phases between 37 and 42 °C. Analysis of the compositions of the coexisting phases provides fairly convincing evidence that they belong to a ’’shield’’ region noted previously in theoretical studies of a three-component model and a van der Waals model of binary mixtures. Such a region could be the starting point in a search for the multicritical point D2. On the other hand its presence is good evidence for the absence in this mixture of the fluid analog of the three-state Potts model multicritical point C2. This is in agreement with studies in systems with symmetry-breaking phase transitions and lends support to the hypothesis that C2 does not occur in three-dimensional systems.
Temperature effect on the collision-induced emission of S(1S) with He, Ar, N2, H2, Kr, and Xe
Authors: Black G. , Sharpless R. L.
Published: February 2003

Collision-induced emission from S(1S) has been studied in collisions with He, Ar, N2, H2, Kr, and Xe at 232 and 425°K. The S(1S) was made by OCS photodissociation at 157.7 nm. These results, in conjunction with an earlier study at 296°K, show the importance of bound molecules in the collision-induced emission with Xe—a dissociation energy of ?0.06 eV is estimated for the 2 1Σ+ potential of XeS [the potential arising from Xe+S(1S)]. Evidence for collision-induced emission from bound molecules is found for all except He. In contrast to the other gases, collision-induced emission is a minor process in the removal of S(1S) by hydrogen, accounting for only 0.36% of quenching collisions at 232°K decreasing to 0.12% at 425°K.
Structure and dynamics of the biphenyl ring torsion in solid neon and argon
Authors: Baca A. , Rossetti R. , Brus L. E.
Published: February 2003

The extent of phenyl ring torsional relaxation possible in low temperature frozen solvents has been investigated for biphenyl in solid neon and argon. In neon biphenyl is trapped in a twisted form, while in argon both twisted and planar forms occur. The twisted forms appear to undergo substantial movement in ϑ towards a planar form before excited state luminescence occurs. The planar form in argon undergoes a highly anharmonic, low frequency (∼8 cm−1), large amplitude ground electronic state torsional oscillation. The excited state splitting (571 cm−1) between the two phenyl rings is 177 cm−1 less than the value reported in crystalline biphenyl. The excited state vibronic structure is considerably different from that observed in crystalline biphenyl. The difference between torsional relaxation in soft solids and viscous flowing fluids, as well as the ability of soft solids to store elastic energy of compression, are discussed.
The atom dipole interaction model of Raman optical activity: Reformulation and comparison to the general two-group model
Authors: Prasad P. L. , Nafie Laurence A.
Published: February 2003

The atom dipole interaction (ADI) model of Raman optical activity (ROA) is reformulated and the ADI expressions for the induced polarizabilities are presented in Cartesian tensor notation. These revised expressions are compared with those of the general two-group (GTG) model. It is shown that the ADI model incorporates antisymmetric tensor contributions to the polarizability which is an important source of optical activity not included in the GTG model. The ROA intensity parameters for the HCBrClF and DCBrClF are presented and the importance of the antisymmetric tensor contributions in this molecule is demonstrated.
ArF laser photolysis of OCSe. II. Effect of vibrational excitation on Se(1S) quantum yields
Authors: Bischel W. K. , Bokor J. , Dallarosa J. , Rhodes C. K.
Published: February 2003

The dependence of Se(1S) production on the vibrational excitation of the parent molecule OCSe by a CO2 laser operating at 10.6 μm has been examined for a photolysis wavelength of 193 nm produced by an ArF laser. Measurements of the absorption cross section for several CO2 laser lines as well as a determination of the infrared energy deposition for the P (34) and P (44) transitions have been made. A vibrationally induced enhancement of 25% in the Se(1S) concentration has been observed and is interpreted as resulting from a vibrationally induced increase in the UV absorption coefficient.
Vibrational and rotational relaxation of cyclohexane in the pure state and in solution
Authors: Arndt R. , McClung R. E. D.
Published: February 2003

Correlation functions φ (t) and correlation times for the ν22(Eg) band of C6H12 in the pure liquid, in liquid solutions in CCl4, CHCl3, CH3I, and CS2 and in some of the corresponding solid phases have been obtained from spontaneous Raman scattering. Reorientational correlation times of C6D12 in these media were obtained from 2D spin relaxation measurements. Measurements of the relative intensities of the site group components of ν22 in solid phase II of C6H12 and of the second moment of ν22 in neat C6H12 liquid showed the dominance of the βxx–βyy, βxy contribution to this Raman band. The reorientational contributions to Φ (t) were estimated from the reorientational correlation times of C6H12 by assuming the applicability of the J-diffusion model and the equality of the angular momentum correlation times for C6D12 and C6H12. The vibrational correlation functions obtained showed a slight increase in vibrational relaxation rate with increasing temperature, the absence of large differences in vibrational relaxation in liquid and plastic solid phases, and a decrease in vibrational relaxation rate when C6H12 is dissolved in polar and nonpolar solvents. Variations in the reorientational correlation time of C6H12 with solvent and temperature are interpreted in terms of the anisotropy of the intermolecular potential and the solution viscosities.
Generalized Langevin equation approach to reaction dynamics in liquids
Authors: Kapral Raymond , Shin K. J.
Published: February 2003

A microscopic formulation for the description of reaction dynamics in liquids is presented. The theory is based on a generalized Langevin equation approach and can be carried out at different levels of description. It also properly accounts for the dynamics of recollision events and static structural correlations. Configuration space and phase space versions of the theory are presented and compared. The configuration space theory illustrates how generalized diffusion equations for the description of the chemical relaxation may be derived and shows that the proper treatment of static structural correlations is important. The phase space version of the theory shows how this formulation may be used to derive kinetic equations for the chemical relaxation processes. The phase space expression for the rate kernel is reduced to the configuration space result by projecting onto the diffusive eigenfunction of the independent particle diffusive propagator. This reduction shows interplay between the collision dynamics and static structural correlations. The results are compared with earlier theories and possibilities for future developments are discussed.
Asymptotic expansion of the porous medium, effective diffusion coefficient in the Knudsen number
Authors: Ho Fun Gau , Strieder William
Published: February 2003

A variational principle based on the mean free path kinetic theory of gases is used to derive an expansion of the effective diffusion coefficient in the Knudsen number for a bed of overlapping, solid spheres. The effects of tortuosity are rigorously and properly considered by explicitly including the flux of diffusing material around obstructions in its path in the trial function. A formula for the ratio of Knudsen to bulk tortuosity in terms of the void fraction is derived. For small void fraction the reciprocal additivity of Knudsen and bulk effective diffusivities is obtained in analytical form.
Temperature dependence in the multiphoton dissociation of 32SF6
Authors: Duperrex R. , van den Bergh H.
Published: February 2003

The multiphoton dissociation of 32SF6 at 10.59 μm has been studied as a function of the temperature between 223 and 413 K. The observed decrease of the reaction yield with increasing temperature varies strongly with the laser energy fluence at the applied conditions. Rate constants for the unimolecular decomposition of sulfur hexafluoride induced by the monochromatic infrared radiation are derived at 223, 293, and 343 K.
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o calculations of the collision-induced dipole in He–H2. II. SCF results and comparison with experiment
Authors: Wormer Paul E. S. , Van Dijk Gerard
Published: February 2003

Using the SCF-LACAO method we have calculated dipole moments of the He–H2 ’’supermolecule’’ for a number of geometries pertinent for the interpretation of the collision-induced IR spectrum. From the results parametrized expressions for μ have been obtained, which are compared with expressions used earlier in experimental analyses. Beside the contributions to μ from the isotropic overlap and He induction by the H2-quadrupole, we have found that also the interference of the latter term with the corresponding short range term is important. The zeroth moments of the spectral densities of the rotation–translation band and the enhanced fundamental band have been calculated and are compared with recent experiments.
Monte Carlo studies of lattice-model polymer chains. IV. Equilibrium dimensions and distributions in Rouse coordinates
Authors: Verdier Peter H.
Published: February 2003

Distributions in the squares of the seven lowest Rouse coordinates for simple lattice models of polymer chains of up to 64 beads, with and without excluded volume, have been obtained by dynamical simulation on a digital computer. The expansion of the mean-square values of the coordinates is found to depend primarily, though not entirely, upon the ratio of the number of beads in the chain to the index of the Rouse coordinate. The distribution functions for the squares of the Rouse coordinates for chains both with and without excluded volume constraints are close to Gaussian with the sole exception of the lowest Rouse coordinate for chains with excluded volume, which exhibits a pronounced depletion at small values. This depletion is very similar to that previously observed in distributions in the square of end-to-end length.
Correlation effects in the spectra of polyacenes
Authors: Tavan Paul , Schulten Klaus
Published: February 2003

In order to describe the electron correlation in the excited singlet π,π* states of the polyacenes [C4n+2H2n+4], we have carried out PPP–SCF–CI calculations including all single and double excitations in the CI expansion up to n=5, including all triple excitations up to n=3, and all quadruple excitations up to n=2. Compared to previous CI descriptions which included single excitations only, e.g., the classic work of Pariser [J. Chem. Phys. 24, 250 (1956)], our calculations lead us to predict the following: (1) ’’new’’ excited states entailing the promotion of two electrons from the ground state (some of them predicted previously by other authors), and (2) a partial reordering of those (well-known) excited states already accounted for by a S-CI representation. Single and double excitations in a CI expansion (D-CI) satisfactorily describe the ordering of all excited states up to 7 eV; the effect of higher excitations is to correct the excitation energies overestimated by the D-CI description. Our predicted spectra provide a consistent assignment of all one- and two-photon spectral data but do not yield a quantitative agreement.
Rate constants for the reactions of atomic boron with O2, SO2, CO2, and N2O
Authors: Sridharan U. C. , DiGiuseppe T. G. , McFadden D. L. , Davidovits P.
Published: February 2003

The gas phase reactions of atomic boron with O2, SO2, CO2, and N2O have been studied using a diffusion flame technique. Atomic boron is produced in a microwave discharge of 1% B2H6 in helium and diffuses into a chamber containing the oxidizer also diluted in helium. The temperature was approximately 300 °K. The rate constants were determined from atomic absorption measurements of boron density in the flame. The rate constants for the O2 and SO2 reactions are (9±7) ×10−12 cm3 molecule−1sec−1 and (7±5) ×10−12 cm3 molecule−1sec−1, respectively. An upper limit of 5×10−13 cm3 molecule−1sec−1 is estimated for the rate constants of the CO2 and N2O reactions.
Depolarized light scattering by simple liquids
Authors: Keyes T.
Published: February 2003

A new theory is presented for the scattering of light by simple fluids. The theory naturally leads to the idea that, in multiple scattering by a group of spherical particles, the shape of the region excluded (’’shaped cavity’’) by the group to the rest of the system is of considerable importance. The large existing disagreement between theoretical and experimental scattering intensities for simple liquids is explained using the shaped cavity idea, which has already been used most successfully for scattering from nonspherical molecules.
The role of intermolecular potential well depths in collision-induced state changes
Authors: Lin H.-M. , Seaver Mark , Tang K. Y. , Knight Alan E. W. , Parmenter Charles S.
Published: February 2003

A relationship is developed from two distinct theoretical approaches to correlate the rate constants kM or cross sections σM for a series of added gases M which collisionally induce a state transformation A*→B. The correlation derived from theory is where C is a constant and εA*M is the intermolecular well depth between A* and M. We observe that experimental data can be described by a related correlation where β is a constant and εMM is the well depth between pairs of M molecules. This correlation is shown to be general. It works for electronic state deactivation in atoms, intersystem crossing and internal conversion in S1 polyatomics, rotational and also vibrational relaxation in S1 polyatomics, predissociation in diatomics and polyatomics, and vibrational relaxation in a free radical as well as in a molecular ion. The theory is appropriate only when attractive forces dominate the interaction, and this seems consistent with the experimental data. The correlation thus provides a simple means to distinguish between attractive and repulsive interactions. The correlation also reveals that collision partners do not substantially modify the intrinsic S1-T mixing during collision-induced intersystem crossing.
A method to estimate intermolecular potential well depths for species in both ground and excited electronic states
Authors: Parmenter Charles S. , Seaver Mark
Published: February 2003

The relationship lnσM=lnC+[(εA*A*) (εMM)]1/2/kT correlates the cross sections σM for a state change A*→B induced by a series of added M gases with the intermolecular potential well depths for A*...A* pairs and M...M pairs. This correlation is used with literature data concerning A*→B to deduce εA*A* for electronically excited atoms (Na, Ne, Ar, Xe) and electronically excited molecules (I2, SO2, CH3OH, glyoxal, propynal, benzene). The well depths are generally observed to exceed the ground state values by factors of 2–10. Large well depths are also observed for sec-butyl radicals and for the C5H9+ ion with high vibrational excitation. The correlation also provides an alternate means to measure ground state well depths εMM. In cases where secure comparisons are available, the well depths so derived usually lie within 20% of values found from transport measurements or virial coefficients. The correlation seems a useful alternative to empirical estimating procedures when data from conventional methods are not available.
Glass tricritical points
Authors: Dowell F.
Published: February 2003

The theoretical discovery of a symmetrical tricritical point in a binary mixture of polymeric glasses is reported. A mean field simple cubic lattice model for mixtures of semiflexible chain molecules having simple Lennard-Jones interactions has been used to calculate the phase diagram of the system. Effects of pressure on the mixture have also been studied.
Excited states and photochemistry of saturated molecules. VII. Potential energy surfaces in excited singlet states of methane
Authors: Gordon Mark S. , Caldwell James W.
Published: February 2003

Potential energy surfaces are investigated for a number of low-lying excited singlet states of methane, using a split valence plus Rydberg basis set and singly excited CI. Of the six minima found, the lowest two are valence states. The lowest minimum corresponds to the products 1 1B1 CH2+1 1Σg+ H2, in agreement with the observed threshold photochemistry, and is accessible by five separate routes from the lowest vertical state. The second valence minimum is a square planar structure. Of the four Rydberg minima detected, one is a local minimum on the otherwise dissociative B1 surface and two correspond closely to CH4+ structures previously determined. None of the minima have a structure from which the radical products CH3⋅+H⋅ are likely to be obtained.
Electron energy loss spectroscopy of pristine and radiation damaged polyethylene
Authors: Ritsko John J.
Published: February 2003

The spectra of valence and core electronic excitations of polyethylene from 1 to 390 eV were measured with a resolution of 0.11 eV by transmission electron energy loss spectroscopy. Structure at 9.5, 13.2, 14.7, 16.6, 19.6, 21.6 eV and a continuum of core excitations beginning at 287.5 eV was measured and compared with theoretical calculations of molecular excitons and interband transitions. The momentum dependence of the fundamental absorption threshold at 7.2 eV indicates that the absorption edge is due to highly localized excitons. From the energy loss spectra the complex dielectric response function and optical properties were calculated and compared with existing optical data. Radiation induced changes in the electronic structure were observed by their characteristic excitation spectra which were shown to be due to formation of polyenyl chromophores in the polymer backbone.
Knudsen effusion mass spectrometric determination of the dissociation energy of diniobium, Nb2(g), and the heat of sublimation of solid niobium
Authors: Gupta Satish K. , Gingerich Karl A.
Published: February 2003

The dimer of niobium, Nb2(g), has been observed under thermal equilibrium conditions by the high temperature mass spectrometric technique, employing an unconventional design for the Knudsen cell. From the partial pressures of Nb and Nb2 measured in the 2545–2677°K temperature range, the dissociation energy of Nb2(g) has been determined to be D0°=503±10 kJ mole−1 or 120.2±2.4 kcal mole−1 and D°298=511±10 kJ mole−1 or 122.0±2.4 kcal mole−1. By combining D (Nb2) with the heat of vaporization of niobium, the heat of formation of Nb2(g) is derived as ΔH°f,298=916±12 kJ mole−1 or 218.9±2.9 kcal mole−1. Also, the heat of vaporization of Nb has been redetermined as ΔH°v,298=713±7 kJ mole−1 or 170.4±1.6 kcal mole−1 and ΔH°v,0=710±7 kJ mole−1 or 169.7±1.6 kcal mole−1. The dissociation energy of Nb2(g) is in good agreement with the value predicted previously and is consistent with the trends indicated for the dissociation energy of the diatomic molecules of the first and second transition series metals.
On the temperature dependence of the 14N NQR in K-TCNQ
Authors: Murgich Juan
Published: February 2003

The temperature dependence of the 14N nuclear quadrupole resonance (NQR) spectrum of K-TCNQ was measured from 77 to 300 K. The NQR frequency shifts with temperature can be explained by considering the effects of the thermal averaging of the quadrupole parameters and the rearrangement of the TCNQ− and K+ stacks. The 14N NQR data are consistent with the observed Peierls transition found for K-TCNQ.
NMR dynamic frequency shifts and the quadrupolar interaction
Authors: Werbelow Lawrence G.
Published: February 2003

A brief account of the quadrupolar interaction is presented. It is demonstrated that second-order dynamic frequency shifts must be considered for multipolar nuclides relaxed by spacially isotropic quadrupolar interactions. Hence, each single quantum coherence is characterized by a unique precessional frequency. A short discussion of the intensities and evolution of these transverse magnetization composites is also included for completeness. The findings of this work supplement the theory of spin 3/2 quadrupolar labelling and exchange techniques as applied to biomolecular systems.