bluuebunny/crossref_metadata_2025 huggingface dataset:
Generalized van der Waals theory of the isotropic–nematic phase transition
Authors: Gelbart William M. , Baron Barbara A.
Published: February 2003

We extend the generalized van der Waals (GVDW) theory to fluids of anisotropic molecules interacting through angle-dependent potentials which can be broken up into strong short-ranged repulsions and weak long-ranged attractions. This approach provides a description of the gas–liquid phase transition in which corresponding states behavior breaks down with increasing anisotropy of the molecular (hard rod) shape. Allowing for the orientational order characterizing nematic phases of liquid crystals, we propose a GVDW form for the Helmholtz free energy A as a functional of the orientational distribution f (Ω). The molecular shape repulsions 𝓋rep are treated within the approximation of scaled particle theory. The long-range attractions 𝓋attr enter directly through a mean field average which explicitly excludes the relative positions denied to a pair of molecules because of their anisotropic cores. For the special case in which 𝓋rep→𝓋hardsphere and 𝓋attr → 𝓋dispersion ? −Ciso/r6ij - Can cos2ϑij/r6ij (here ϑij is the angle between the long axes of molecules i and j and rij is the distance between their centers), minimization of A[f (Ω)] leads to an equation for f (Ω) which is identical in form to that of the Maier–Saupe (MS) theory of isotropic–nematic phase transitions. Instead of a phenomenological parameter specifying the strength of the mean field potential, we obtain an explicit expression involving Can, the density ρ, temperature T, and molecular size σ. We show further that for nonspherical molecular shapes the effective one-body potential still assumes (to a very good approximation) the familiar form Aoρ+A2ρη P2  (cosϑ), where Ao and A2 are independent of T and ρ, and η is the usual order parameter. However, as a result of the angle dependence in the core-excluded volume which defines the effective attraction as a mean field average over 𝓋attr, the dominant contribution to A2 is determined almost exclusively by the −Ciso/r6ij term rather than by the much smaller anisotropic term −Can cos2ϑij/r6ij. Similarly, the free energy A[f (Ω)] includes a coupling between contributions from the hard rod (length L and diameter σ, say) repulsions and from the dispersional attractions. This result (with no adjustable parameters) is argued to comprise a consistent molecular theory of the Can, Ciso, ρ, T, and shape (L,σ) dependence of the orientational phase transition and order in simple liquid crystals.
Electron-gas model for open shell–closed shell interactions. I. Application to the emission spectra of the diatomic noble-gas halides
Authors: Clugston Michael J. , Gordon Roy G.
Published: February 2003

We extend the electron-gas method of calculation of interatomic forces to the case of one open shell atom interacting with one closed shell atom. The simple case chosen for investigation is the class of noble-gas monohalide excimers. We concentrate on their spectra as numerous data have recently become available. We describe how to extend the electron-gas model to deal with one open shell atom, emphasizing how to set up the two molecular symmetry states. The two main sets of transitions in the noble-gas monohalide spectra are interpreted. For the short-wavelength set, we show that the near degeneracy of the Σ and Π excited ionic states found in a recent configuration interaction study of KrF also takes place in the other members of the molecular class. For the long-wavelength set, we give a simple explanation for the relative pressure sensitivity. Our conclusion is that the electron-gas model can be extended to the calculation of the Π state, but not the Σ state, resulting from the interaction between an atom with one open p shell and a closed shell atom.
Proton magnetic shielding in crystals of α-oxalic acid dihydrate. Comparison with deuteron shielding
Authors: Sagnowski S. , Aravamudhan S. , Haeberlen U.
Published: February 2003

The nuclear magnetic shielding tensors of the carboxyl protons in α-oxalic acid dihydrate, (COOH)2⋅2H2O, have been determined by solid state high resolution multiple pulse techniques. The results, σAv=−11.3±1 ppm relative to liquid H2O, Δσ=29.65±0.2 ppm, and η=0.10±0.02, are compared with deuterium data obtained recently by Vega, Shattuk, and Pines with a novel technique. Our data completely remove the discrepancy between the shielding anisotropies of the protons and the corresponding deuterons reported by Vega et al. on the basis of older multiple pulse proton data of Yeung.
Single-photon-absorption reaction chemistry in the solid state
Authors: Catalano Edward , Barletta Robert E.
Published: February 2003

A description of the technique of single-photon-absorption reaction chemistry in the solid state and preliminary results on four sets of reactants is presented.(AIP)
Radiative lifetime and quenching cross section of the B 1πu state of K2 by time correlated single photon counting using a mode-locked He–Ne laser
Authors: Lemont S. , Giniger R. , Flynn G. W.
Published: February 2003

The radiative lifetime of the B 1πu state of K2* has been measured using the time correlated single photon counting technique with a mode-locked He–Ne laser excitation source. This method provides a direct means of observing the fluorescence decay while previous measurements of the K2 lifetime have involved indirect methods. The radiative lifetime and the quenching cross section for K2*–K collisions were found to be 12.2±0.2 nsec and 162±7 Å2, respectively. The photon counting technique is discussed extensively, especially in light of the use of a mode-locked laser as the excitation light source in place of the usual flashlamp. Possible quenching mechanisms for the K2 B 1πu state are also considered.
The molecular dipole model in monoclinic systems: A study of the sulfate internal optic modes in Li2SO4⋅H2O crystals
Authors: Wu Guo-Jen , Frech Roger
Published: February 2003

The sulfate ion internal optic modes in monoclinic lithium sulfate monohydrate (Li2SO4⋅H2O) are studied using a molecular dipole model. The polarization direction of the B symmetry phonons lies in the a–c plane, however a specific direction is not given by symmetry considerations. An analysis of polarized near-normal incidence infrared reflection data complemented by Raman data is sufficient to uniquely determine the polarization of the phonon. In these systems a given A symmetry phonon is ’’paired’’ with a particular B symmetry phonon, that is, both phonons are correlated through the same site group internal mode which in this crystal must be of A symmetry. The correct ’’pairing’’ assignments for several internal modes can be found from an application of the molecular dipole theory to the experimental data.
Intermolecular vibrations of a crystalline molecular complex
Authors: Chen Freeman P. , Prasad Paras N.
Published: February 2003

The nature of intermolecular vibrations in a crystalline molecular complex is examined using a simple model of a crystal with one molecule of a 1:1 complex per unit cell. It is shown that two limiting descriptions can result depending on the coupling strength between molecular components in the complex: (i) The giant molecule limit describes the case of a strong coupling where the intermolecular motions are the motions of the complex as one entity. (ii) The sublattice limit is applicable in the weak coupling case. Here, the motion can be characterized as an intermolecular motion belonging to the sublattice of the individual components. The phonon bands of naphthalene:octafluoronaphthalene crystals are examined within the framework of the simple model. The results predict phonons to be close to the sublattice limit. Our experimental study of Raman phonon spectra using isotopic mixed crystals of the complex is in agreement with the theoretical predictions and show that the low frequency phonons belong predominantly to the C10F8 sublattice, while the high frequency phonons characterize the C10H8 sublattice motions.
Vibrational spectra and intermolecular forces of mercuric halide complexes of 1,4-dioxane and tetrahydrofuran
Authors: Kobayashi Masamichi , Kohyama Masaki , Tadokoro Hiroyuki
Published: February 2003

Infrared and Raman spectra of mercuric chloride complexes of 1,4-dioxane and 1,4-dioxane-d8 and mercuric bromide complexes of tetrahydrofuran and tetrahydrofuran-d8 were measured in a solid state at various temperatures. Optically active Raman and far-infrared bands due to the lattice vibrations and to the stretching and bending modes of the mercuric halide molecules were detected. On the basis of the observed frequency data the intra- and intermolecular force constants were evaluated by the least-squares fitting of the normal frequencies. A strong interacting force was estimated for the nearest Hg⋅⋅⋅O pairs compared with the ordinary van der Waals-type intermolecular interactions. The frequencies of the Hg–X (X is Cl or Br) stretching vibrations in the complexes are lower than those of the corresponding crystalline mercuric halides, and they increase with rising temperature. These facts suggest a coordination character of the nearest Hg⋅⋅⋅O bonding in the complexes.
Orthogonality constraints in electron scattering by open-shell targets: Comments on a paper by Riley and Truhlar
Authors: Rescigno T. N.
Published: February 2003

S-wave e−-H scattering in the static-exchange approximation as considered by Riley and Truhlar is commented upon. The orthogonality constraints are discussed.(AIP)
Comparisons of Monte Carlo and RISM calculations of pair correlation functions
Authors: Chandler David , Hsu C. S. , Streett William B.
Published: February 2003

The solutions of the RISM integral equation are compared with Monte Carlo results for three hard-core diatomics, including both homonuclear and heteronuclear models. The comparison shows that the RISM theory is qualitatively reliable and that the sizes of quantitative errors in the theory are predicted accurately by internal consistency tests.
Infrared studies of new one-dimensional tetracyanoplatinates containing the bifluoride ion
Authors: Basile L. J. , Ferraro J. R. , Williams Jack M.
Published: February 2003

New one-dimensional tetracyanoplatinate (TCP) conductors of the type M2[Pt(CN)4](FHF)0.39–0.40 where M=Rb+, Cs+, and K2[Pt(CN)4](FHF)0.30⋅xH2O, were studied by infrared techniques and the fluorine in these compounds was shown to be present as the bifluoride ion (FHF)−. The asymmetric hydrogen–fluorine stretching vibration was found to be cation dependent, and occurs at higher frequency for the K+ salts, as compared to the Rb+ and Cs+ salts. In contrast to the anion and cation-deficient one-dimensional TCP conductors, only one intense CN stretching mode at ∼2150 cm−1 was observed. The rubidium and cesium salts are lustrous, metallic-gold in color, and appear to be optically different from the reddish-bronze prototype materials, K2[Pt(CN)4]X0.3 ⋅3H2O where X=Br or Cl.
A basic thermodynamic method of determining absolute temperatures by utilizing the maximum entropy of electronic multiplets near liquid helium temperatures
Authors: Giauque W. F. , Fisher R. A.
Published: February 2003

A basic absolute method of measuring thermodynamic temperatures in the liquid helium range is proposed. It is based on the principle that the entropy, S, of strongly quantized isolated electronic multiplets with n members must be in R lnn. At such temperatures nearly the entire entropy (above nuclear entropy) of many magnetic substances is due to the related electronic multiplets. In high magnetic fields the electronic entropy can be reduced essentially to zero in a limiting exponential manner. In its simplest terms, the calorimetric heat q required during isothermal demagnetization must obey Kelvin’s definition, which in this case is T=q/ΔS=q/R lnn. An equivalent method involving intersecting isentropes and isoerstedic heat capacity curves is discussed. Several preliminary experimental results indicate that the helium vapor pressure scale T58 is low by 0.24%.
Extension of Koopmans’ theorem. IV. Ionization potentials from correlated wavefunctions for molecular fluorine
Authors: Ellenbogen James C. , Day Orville W. , Smith Darwin W. , Morrison Robert C.
Published: February 2003

Uncorrelated (Hartree–Fock) ab initio calculations have proven unable to predict the energy ordering of 3σg and 1πu molecular orbitals as observed in electron spectroscopy experiments on fluorine (F2). The correct ordering was obtained, however, by applying an extension of Koopmans’ theorem [J. Chem. Phys. 62, 113 (1975)] to an MC–SCF correlated wavefunction for F2 which contained only one configuration beyond the one used in Hartree–Fock. With the addition of still further configurations to the wavefunction for the neutral molecule, the ordering of the extended Koopmans’ valence orbital energies was maintained and the correspondence improved between those and the experimental values.
Excited state spectroscopy, subpicosecond predissociation, and solvation of diatomic XeO in solid rare gas hosts
Authors: Goodman Julie , Tully J. C. , Bondybey V. E. , Brus L. E.
Published: February 2003

Time, wavelength, and polarization resolved fluorescence techniques are used to investigate the photophysics of weakly bound diatomic XeO following frequency doubled dye laser excitation near 2200 Å. Photoselection experiments prove that the ’’Green bands’’ contain both E 1Σ→C 1ΣΠ and E 1Σ→B 1Σ fluorescences. The B 1Σ state, which correlates with O(1D)+Xe, is at least three vibrational quanta deeper than previously thought. An isotopic reversal of linewidths between Xe16O and Xe18O in the E 1Σ→B 1Σ spectra is interpreted in terms of subpicosecond homogeneous predissociation of B 1Σ. Solid phase homogeneous predissociation theory is discussed, and the isotopic sensitivity of the appropriate Franck–Condon factors is numerically investigated. The C 1Π and B 1Σ potentials are unambiguously deeper and more harmonic in solid Ar than in vacuum. This change in potential curve shape appears to reflect increased admixture of charge transfer (Xe+O−) components in the solid phase electronic wavefunctions.
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o investigation of geometry changes during inversion of NH3, NH2F, NHF2, NF3 and PH3, PH2F, PHF2, PF3
Authors: Schmiedekamp Ann , Skaarup Steen , Pulay Péter , Boggs James E.
Published: February 2003

A b initio SCF molecular orbital calculations with complete geometry optimization have been performed using consistent basis sets for each member of the series NH3, NH2F, NHF2, NF3 and PH3, PH2F, PHF2, PF3. The calculations were repeated using the same basis sets with the molecules constrained to the planar configuration which is the transition state for inversion, all other geometrical parameters being energy optimized. In the planar configuration, shorter N–F, N–H, and P–H bonds resulted as well as a change from the nearly equal pyramidal bond angles to highly asymmetric planar bond angles with &HXH≳ &HXF≳ &FXF. Omission of polarization functions from the basis set produced longer P–N and N–F bonds. Regardless of the inclusion or exclusion of d atomic orbitals in the basis set, substitution of fluorine for hydrogen yields regular increases in inversion barrier heights. Exclusion of d atomic orbitals results in inversion barriers that are smaller by nearly the same amount for all molecules in a series. The increase in barrier with substitution of fluorine for hydrogen is correlated with an increase of s character in the lone pair. Dipole moment and components, bond moments, and lone pair moments are discussed. Effects of varying the d orbital exponent coefficient on the central atom were examined.
The invalidity of rank-one separable approximations to off-shell t matrix elements for atom–atom interactions
Authors: Wong J. K. C. , Brumer P. , Shapiro M.
Published: February 2003

One term separable approximations to off-shell two body t matrix elements for atom–atom interactions are considered. A ratio method, which simultaneously tests all one term approximations, is applied to two typical molecular potentials, those of HF and He2. The results show that a rank-one separable approximation is unreliable. The Kowalski–Noyes and pole dominance approximations are considered as explicit examples.
Kinetic theory of rough sphere fluids: Density fluctuations
Authors: Mehaffey J. R. , Desai Rashmi C.
Published: February 2003

We construct a kinetic equation for the phase space density correlation function (PSDCF) in a rough sphere fluid (at the generalized Enskog level) which is valid for arbitrary wave number and frequency. The kinetic equation yields the first two kinetic sum rules exactly; also the density–density correlation function implied by it has the correct first four sum rules exactly. We use the short time behavior of PSDCF and the generalized hydrodynamics approach to construct approximate interpolation formulas for a variety of space–time dependent correlation functions related to conserved variables in the fluid. The coupling of modes described by these correlation functions to the test particle density fluctuations is responsible for the known asymptotic power law decay in the linear and angular velocity correlation functions. As a by-product of this study, we also obtain expressions for various (bare) transport coefficients in a dense rough sphere fluid.
A flash photolysis, infrared chemiluminescence study of the rate constant for the recombination reaction F+O2+M
Authors: Chen Hao-Lin , Trainor Daniel W. , Center R. E. , Fyfe W. I.
Published: February 2003

Infrared chemiluminescence was observed from vibrationally excited HF when mixtures of HF/O2/F2 were flash photolyzed over the wavelength region 2500–3500 Å. We have interpreted this excitation as being the result of termolecular recombination reactions involving F atoms. A simple kinetic model was derived to explain these observations and a comparison of model predictions to the observed time resolved chemiluminescence signals showed reasonable agreement. Based on this model, third order kinetic rate constants for the reaction F+O2+M→FO2+M at 298 °K were determined and are 5.0±0.6, 15±3, 8.4±0.9, 5.4±0.6, and 2500±500×10−33 cm6/molecule2 sec for M=F2, O2, Ar, He, and HF, respectively. The rate constant for M=DF was found to be about two times slower than that of HF.
Nuclear modulation of the electron spin echo envelope in glassy materials
Authors: Mims W. B. , Peisach J. , Davis J. L.
Published: February 2003

We have made experimental studies and computer simulations of the nuclear modulation effect in electron spin echoes for a number of frozen solutions in which paramagnetic ions from the 3d and 4f groups are coordinated with hydrogen- and deuterium-containing ligands. The aim was to discover to what extent electron spin echo measurements can be used to characterize the nuclear environment and to find the radial distances of the nearer nuclei for paramagnetic complexes embedded in glassy matrices. The method is easiest to apply for protons and yields information regarding the radial distribution of the nuclei as well as the distances of the nearest neighbors. According to the theory of the modulation effect, the same results and the same modulation depths are to be expected for all I=1/2 nuclei. The method can also be used where the coordinating nuclei are deuterons, but errors introduced by the quadrupolar interaction lead to uncertainties ?12% in the measurement of the radial distance. Deuteron interactions are easily seen against a proton background since the period is quite distinct, and the modulation pattern is deeper. It is shown that in the limiting case of small quadrupolar interactions the modulation depth parameter is proportional to I (I+1).
A theoretical study of the depolarized Rayleigh scattering time-correlation function of a dilute polymer solution
Authors: Lin Y.-H. , Wang C. H.
Published: February 2003

The depolarized Rayleigh spectra of macromolecules in solution have been studied theoretically. From a general expression of the time-correlation function, two correlation functions respectively related to molecular rotation and local segmental motion are obtained in a natural way. Previous work has not demonstrated this connection. The result of the present analysis is valuable in the interpretation of the experimentally observed spectra of polymers in solution.
A theoretical analysis of mercury molecules
Authors: Smith Earl W. , Drullinger R. E. , Hessel M. M. , Cooper J.
Published: February 2003

A theoretical analysis of experimental observations on bound–free electronic transitions in mercury molecules is presented. Potential curves and A values are derived for these transitions and the possibility of emission from a mercury trimer is also discussed briefly. These data are then used to produce a simple model for an optically pumped 335 nm mercury laser. A table is given which predicts the small signal gain at 335 nm as a function of temperature, density, and the excimer density in the vapor.
Internal energy of CH+ produced by the C+(H2,H)CH+ reaction
Authors: Jones C. A. , Wendell K. L. , Koski W. S.
Published: February 2003

The energy spectrum of CH+ from the reaction C+(H2,H)CH+ was measured with a tandem mass spectrometer with improved energy resolution as a function of projectile kinetic energy. These spectra indicate the presence of the three lowest electronic states of CH+ and product vibrational excitation was observed in favorable cases. The internal energy distribution in the CH+ product was in satisfactory agreement with published results of phase space theory calculations for this reaction.
High resolution 14N NMR in solids: Single crystals of ammonium hydrogen oxalate hemihydrate
Authors: Wolff E. K. , Griffin R. G. , Watson C.
Published: February 2003

Proton decoupled 14N NMR spectra of ammonium hydrogen oxalate (AHOX) single crystals have been employed to determine the electric field gradient tensor at the 14N sites. Despite the 99.4% abundance of 14N, the spectral lines are narrow (∼300 Hz) and show two inequivalent 14NH4+ ions with (e2qQ/h) =130 kHz, η=0.39 and (e2qQ/h) =350 kHz, η=0.27. On the basis of motional averaging arguments, we surmise that these nonvanishing field gradients arise primarily because of lattice distortions of the N–H distances. The results indicate that it should be relatively easy to obtain 14N spectra in single crystals, and thus to examine a number of phenomena which have heretofore been inaccessible.
Successive perturbation of phonon bands in an organic alloy
Authors: Bellows James C. , Prasad Paras N.
Published: February 2003

Successive perturbation of the phonon bands is presented as a method to determine the criteria for formation of localized phonons in an organic alloy. The method is applied to the p-dihalobenzenes. The host is p-C6H4Cl2. The other isomorphic p-dihalobenzenes are used as guests with increasing perturbation in the order p-C6H4ClBr, p-C6H4Br2, p-C6H4ClI, and p-C6H4BrI. It is found that the phonon bands remain amalgamated throughout the entire range of concentration for all perturbation strengths. The absence of localized phonons indicates either a large bandwidth (k dispersion) for the various phonon subbands (branches), or a significant coupling among the various phonon subbands. Several theoretical approaches for mixed crystals are summarized with a comparison of their relative merits. The highest frequency phonon exhibits the behavior predicted by the virtual crystal approximation. However, the more sophisticated average T-matrix approximation is required to adequately explain the line position and linewidths of the lowest frequency phonon.
Quantum mechanical close coupling approach to molecular collisions. Averaged definite parity j
z-conserving coupled states approximation
Authors: Shimoni Yair , Kouri Donald J.
Published: February 2003

A new jz-conserving approximation for treating molecular collisions, the averaged definite parity jz (ADPjz) approximation, is presented in detail and applications to the He+H2 system and the Walker–Light model reported. The ADPjz method is a new general method for introducing the effects of different turning points into jzCCS-type approximations. This is achieved by means of a relabeling of the T matrices by an orbital angular momentum type index L in place of total J. The ‖TLλ(j ‖j0) ‖2 are then averaged in such a way as to incorporate the effects of various orbital angular momenta l̄ in the range ‖J-jm‖⩽l̄⩽J+jM, where jM=max(j,j0). In addition, parity effects are introduced by the restriction that only l̄ of a given parity enters the average to produce a given ‖TpJλ(j ‖j0) ‖2. The numerical results for He+H2 indicate that the ADPjz method gives quantitative agreement with close coupling (CC) for elastic scattering partial cross sections at the definite parity level, in dramatic contrast to the standard jzCCS method. In addition, the ADPjz method corrects for the too rapid falloff with J of the standard jzCCS method. Results for inelastic transitions are also improved compared to the standard jzCCS but not as much as for elastic transitions. Again, the too rapid falloff with J encountered in standard jzCCS is corrected to a significant extent. Integral cross sections are also consistently improved in the ADPjz method and it is shown that for those cases where jzCCS integral cross sections are good, the ADPjz and jzCCS results will be close. However, even in those cases, we find the ADPjz method to be consistently closer to CC than jzCCS. It is also found that the ADPjz’s inclusion of the effects of different turning points by averaging over l̄ values extends the range of validity of jz—conserving methods to much larger J values than previously expected. Finally, the method requires the solution of no more coupled equations than the standard jzCCS method so that the computational effort is essentially the same in the two approaches. This fact combined with the greater accuracy of the ADPjz (especially for elastic transitions and also for inelastic ones) makes the ADPjz the method to use in neutral atom–molecule collisions.
Infrared absorption spectra of gaseous HD. III. Collision-induced fundamental band of HD in HD–Kr and HD–Xe mixtures at room temperature
Authors: Prasad R. D. G. , Reddy S. Paddi
Published: February 2003

Spectra of the enhancement of absorption of the collision-induced infrared fundamental band of HD in HD–Kr and HD–Xe mixtures have been investigated at room temperature for different base densities of HD and a number of total gas densities up to 140 amagat. These spectra exhibit a narrow line at the R1(1) position, in addition to the well known collision-induced O, Q, and S branches. The occurrence of this new feature in the enhancement spectra, as explained recently by us elsewhere, is due to a phenomenon termed ’’intracollisional interference.’’ The binary absorption coefficients of the band derived from the measured integrated absorption coefficients are (6.3±0.1) ×10−35 and (10.1±0.3) ×10−35 cm6 s−1 for HD–Kr and HD–Xe, respectively. The characteristic half-width parameters, δd and δc of the overlap-induced transitions, and δq (and δq′) of the quadrupole-induced transitions are derived for both HD–Kr and HD–Xe from an analysis of the profiles of the enhancement of absorption. As described in Papers I and II of this series for other binary mixtures of HD, the variation of δc, the half-width of the intercollisional dip of the Q branch, with the density of the perturbing gas Kr as well as Xe has been investigated. In both HD–Kr and HD–Xe, the quadrupole-induced transitions show negative wavenumber shifts from the corresponding calculated positions for the free HD molecule. In particular, for HD–Xe, these (∼7 cm−1 at a density of 70 amagat of Xe) are found to vary linearly with the partial density of Xe.
Mixing character and its application to irreversible processes in macroscopic systems
Authors: Mead C. Alden
Published: February 2003

The ’’Principle of Increasing Mixing Character,’’ first described by Ruch and the present author, is presented in a somewhat modified formulation. The theory is then developed in such a way as to make it applicable to irreversible processes in nonideal systems. In systems described by a large number of parameters for which the ’’exponential assumption’’ (essentially the same as the well-known local-equilibrium assumption) can be made, the entire content is contained in a single, intuitively appealing expression. The result is applied to a simple example: diffusion in the Landau model of a nonideal solution.
Cooperative pseudo-Jahn–Teller model for the sequence of ferroelastic transitions in barium sodium niobate
Authors: Paquet Daniel
Published: February 2003

A microscopic model is described which accounts for the sequence of two ferroelastic phase transitions in Ba2NaNb5O15. It is shown that the phase transformations, which involve opposite symmetry changes on raising the temperature, can be explained on the basis of a cooperative pseudo-Jahn–Teller mechanism. In the model, it is assumed that two d electrons localized on each niobium ion interact with their oxygen-octahedral environment. The electronic energy levels, which are degenerate in a cubic symmetry, are partially split by the spontaneous polarization of this ferroelectric crystal. At each niobium site, the resulting ground and excited electronic levels are both coupled to the surrounding octahedra distortion by a pseudo-Jahn–Teller interaction. Finally, the various octahedra distortions couple cooperatively to determine the ferroelastic spontaneous strain of the crystal. In the framework of this theory, the lowest transition at 110 K has its origin in the temperature dependence of the populations of the electronic excited states. At higher temperature (T≳573 K), the increase of the entropy restabilizes the undistorted paraelastic phase. Possible experimental tests of the model are discussed. In particular, it is predicted that observable changes, of definite sign, must occur in the transitions temperatures, either by application of a static electric field along [001] or by substitution of the niobium ions by Jahn–Teller inactive ions.
Intersystem crossing rates and saturation parameters in the triplet state for rhodamine, fluorescein, and acridine dyes
Authors: Yamashita Mikio , Kuniyasu Akio , Kashiwagi Hiroshi
Published: February 2003

The saturation of the population in the triplet state T1 using a laser excitation ESR technique was observed directly at 77 K. The saturation parameters Isat and the singlet-to-triplet intersystem crossing rate kST were determined for acridine red (AO), rhodamine S (RS), rhodamine B (RB), rhodamine 6G (R6G), fluorescein disodium salt (FDS), 2′,7′-dichlorofluorescein (DF), eosin Y (EY), eosin B (EB), acridine orange (AR), and acridine yellow (AY) in rigid ethanolic solutions. On the basis of the results, the intersystem crossing mechanisims of rhodamine (type R), fluorescein (type F), and acridine (type A) types of the ten dyes are discussed.
The high temperature oxidation of tetrafluoroethylene
Authors: Keating Eugene L. , Matula Richard A.
Published: February 2003

The high temperature oxidation of tetrafluoroethylene (C2F4) in excess argon has been studied behind reflected shock waves. Kinetic data were obtained for three fuel/oxidant ratios and all experiments were carried out in the temperature and total pressure ranges 1670–2500 °K and 4.0–6.9 atm, respectively. The rate of formation of COF2 was monitored in emission at 5.5 μ while the rate of reaction of CF2 was monitored simultaneously in absorption at 2804 Å. The Arrhenius rate constant for the elementary reaction CF2+O2 5→COF2+O is given by the expression k5= (2.01±0.30) ×1013 exp(−26 500±600/RT) cm3/mole⋅sec. Based on both the simultaneous experimental measurements and analytical mechanistic considerations, it was shown that the initial rate of COF2 formation could be utilized to evaluate k5 under all experimental conditions. However, owing to the presence of competing reactions, the initial rate of reaction of CF2 could not be utilized in all cases to evaluate the elementary rate constant k5.
Theoretical study of electron–helium resonance scattering below the first excitation threshold
Authors: Hata Jiro
Published: February 2003

The Breit–Wigner formula is derived by using the many-body perturbation theory, and the derived formula is applied to the investigation of the p-wave resonance scattering below the first excitation threshold of electron–helium collision. Two p-wave resonances below the first excitation threshold are obtained in the present calculation including the correlation effects between the projectile electron and the target electrons. It is also found that the resonance positions are 19.67 and 19.73 eV. These values are in good agreement with the experiment of Golden et al.
A quickly converging method for computing electrostatic crystal field parameters
Authors: Faucher M. , Caro P.
Published: February 2003

Ewald’s method is utilized to obtain quickly convergent second and fourth order crystal field parameters in the point charge hypothesis. The procedure consists first in expressing crystal field parameters following Racah’s formalism Bkq as functions of partial derivatives of 1/R in Cartesian coordinates, then in differentiating correspondingly Ewald’s expression of 1/R. Electrostatic parameters of Nd2O3 are computed by this method. Perfect convergence is met by performing the summation within a 12 Å sphere in the direct space and a 8 Å−1 sphere in the reciprocal space.
The momentum distribution of a low-density quantum system
Authors: Berman D. H. , Bruch L. W.
Published: February 2003

A density expansion for the one-particle momentum distribution of a quantum gas is developed formally, and the binary, two-particle cluster term is evaluated analytically for a one-dimensional gas of hard rods and numerically for a three-dimensional Lennard-Jones gas with 4He parameters. The two-particle cluster is divided into a direct Boltzmann gas term and an exchange quantum statistics term. The competing effects of quantum statistics and of the intermolecular forces in determining the form of the non-Maxwellian distribution are displayed for the Lennard-Jones gas.
Magnetic exchange interactions between hydrated complexes of iron group ions: Mn2+, Co2+, and Ni2+ in La2Zn3(NO3)12⋅24H2O
Authors: Culvahouse J. W. , Francis C. L.
Published: February 2003

The double nitrate crystals La2Zn3(NO3)12⋅24H2O have two sites, X and Y, into which the divalent iron group ions may be substituted. Significant exchange interactions exist for nearest neighbor X–X pairs and X–Y pairs. For Mn, Co, and Ni, there are six possible X–X pairs and nine possible X–Y pairs since the latter do not possess inversion symmetry. Electron paramagnetic resonance of pairs is used to study Mn(X) –Co(X), Ni(X) –Co(X), Ni(X) –Mn(Y), Co(X) –Mn(Y), and Mn(X) –Co(Y) interactions. These new results combined with earlier results for nine of the other ten possible combinations are used to analyze the interaction in terms of exchange between orbitals. For the X–Y interactions, it appears that only the eg orbitals are significantly involved, and a model is developed for a semiquantitative explanation of that interaction in terms of the covalency of the complex and the two hydrogen bonds between the water molecules of the X and Y complexes and a noncomplexed water molecule which is a receptor for the bonds. The degree of covalency required in the hydrogen bonds appears to be in accord with current descriptions. For X–X pairs it is clear that t2g orbitals are involved in the exchange process, and an attempt is made to use the data for X–X pairs to deduce the individual orbital contributions, assuming that the orbital exchange parameters do not vary with the number of d electrons. It is concluded that the strain induced by Mn ions in the lattice affects the orbital exchange parameters for the X–X pairs as it does for the X–Y pairs, and this prevents a definitive determination. Values are deduced for three of the seven orbital exchange constants which are valid if none of the other four are exceptionally large.
Semiclassical theory of molecular collisions in a laser field
Authors: Yuan Jian-Min , Laing John R. , George Thomas F.
Published: February 2003

A semiclassical theory of radiative transitions for molecular collisions in an intense laser field is developed. The resulting expressions for the S-matrix involve electronic-field potential surfaces which are constructed from field-free adiabatic surfaces and transition moments. The electronic-field surfaces, which exhibit avoided crossings due to radiative coupling, are analytically continued to complex intersection points (branch points). Transitions between two such surfaces are effected by classical trajectories propagating on one surface and passing smoothly to the other surface through a branch point. Each of the two surfaces is seen to be a branch of a quadruple-valued potential function, and the branch-point structure of this function is related to field-free nonadiabatic coupling and dynamic radiative coupling. Interference effects between these two kinds of couplings are discussed and illustrated for model potentials. Numerical results show enhanced transition probabilities for the collinear process Br(2P3/2)+H2(v=0)+h/ω →Br(2P1/2)+H2(v=0). Experimental investigations of laser enhanced energy transfer, interference effects, and the possible formation of long-lived complexes, which are not necessarily present in the field-free case, are suggested for a molecular beam experiment carried out in the presence of a laser beam.
The theoretical determination of the B 1Πu potential energy curve for Li2
Authors: Kahn Luis R. , Dunning Thom H. , Winter Nicholas W. , Goddard William A.
Published: February 2003

The Li2 B 1Πu potential energy curve has been calculated with a multiconfiguration SCF (MCSCF) wavefunction. Several different types of wavefunctions and basis sets have been examined and their accuracy determined. The most accurate wavefunction used predicts a binding energy of 0.3015 eV (0.08 eV above the experimental value of 0.385 eV) and a potential barrier of 0.0724 eV with its maximum at 10.6 bohr. It is argued that the theoretical value of the barrier is an upper bound to the experimental value. The long range behavior of the potential energy is found to match smoothly onto the form predicted from dispersion forces.
Mechanism of chemiluminescent chain reactions in Mg catalyzed N2O–CO flames
Authors: Benard D. J. , Slafer W. D.
Published: February 2003

The concentrations of several species present in a low pressure Mg–N2O–CO flame were monitored by a combination of spectroscopic and modulated CO2 laser induced fluorescence measurements. The concentrations of Mg(3P) atoms, CO2, and MgO in the 3Δ and B 1Σ states were measured as functions of the partial pressures of N2O, CO, CO2, N2, and Ar. The results obtained support a multistep chain reaction mechanism which incorporates excited atomic and molecular intermediates. Quenching and pressure dependence show that the continuum emitter is not the molecular intermediate as previously supposed and that collisionally induced intersystem crossing in MgO accounts for the apparent violations of spin conservation that occur when Mg(3P) atoms are generated. The strong chemiluminescent MgO bands appear to result from the reaction Mg(3P)+N2O→MgO*+N2 and these are discussed in terms of a correlation diagram. The excitation of the MgO B 1Σ state is shown to be simultaneous with the 3Δ state and to occur by a nonadiabatic surface crossing mechanism.
Approximation methods for certain total charge transfer probabilities in collision processes
Authors: Shipsey Edward J.
Published: February 2003

Approximation methods are presented for certain charge exchange total probabilities and the sums of charge transfer probabilities appearing in rate constants. New approximations for the two and three state total probabilities are given. An iteration procedure is also demonstrated.
Infrared band intensities and polar tensors: Properties of effective charge ξα and its relation to the sign of dipole moment derivatives
Authors: Prasad P. L. , Singh Surjit
Published: February 2003

Polar tensors for a variety of molecules, XY4, XY3, CH3X, and C2F6, are evaluated from infrared band intensities. The effective charges ξα for all atoms are reported. It is found that the difference in the ξα2 value of the central atom and the sum of the ξα2 values the terminal atoms is maximal for the preferred sign combination of dipole moment derivatives δμg/δQi in all the molecules considered. Making use of this property, we find it possible to determine unequivocally the sign of dipole moment derivatives with respect to normal coordinates δμg/δQi. It is noted that the ξα and ξαβ values follow the trend in electronegativity of atoms and ionic character of bonds, respectively, in a series of molecules.
Molecule–molecule collisions: Semiclassical calculations of rotational excitation for the H2–CO2 system using the exponential approximation
Authors: Guha Evelyn , Saha Samir
Published: February 2003

The formalism of the semiclassical exponential approximation is developed for rotational transitions in collisions between two linear molecules. The H2 and CO2 molecules are treated as rigid rotors and their interaction potential includes anisotropic contributions from the short-range repulsive forces as well as from the long-range dispersion, induction, and electrostatic interactions. The calculations are made over a wide range of energy below the rotational threshold of H2 and for several initial rotational states of both molecules. Neglecting the small probability of de-excitation of H2 when possible, the rotational inelasticity in CO2 is found to depend significantly on the rotational state of its collision partner. The effect of varying the repulsive anisotropy parameter on the inelastic cross sections is also investigated.
Proton spin–lattice relaxation in selectively deuterated p,p′-azoxyanisoles
Authors: Hayamizu Kikuko , Yamamoto Osamu
Published: February 2003

Proton spin–lattice relaxation rates 1/T1 are observed for the nematic liquid crystal PAA and its selectively deuterated species PAA-d6 and PAA-d8 in the nematic and in the isotropic states at frequencies of 15 and 55 MHz. The main relaxation process for the ring protons of PAA-d6 is the thermal fluctuation of the order director, and the translational diffusion of the molecules also contributes to the relaxation rate of these protons. On the other hand, the methyl protons of PAA-d8 relax predominantly through the translational diffusion in the nematic and isotropic phases. The thermal fluctuation of the order director makes a small contribution to the spin relaxation of the methyl protons in the nematic state and in the isotropic state near the transition point.
A classical trajectory study of inelastic collisions between highly vibrationally excited KBr and Ar
Authors: Matzen M. Keith , Fisk G. A.
Published: February 2003

We present a study of inelastic collisions between highly vibrationally excited KBr and Ar based on three-dimensional classical trajectory calculations. Calculations are performed for three closely related potential energy surfaces chosen to allow determination of the importance of an attractive well. Comparison with experiment indicates that both depth and shape of an attractive well are important for calculating detailed scattering distributions. Analysis of selected trajectories indicates that double impact collisions play an important role in the overall vibrational deactivation mechanism. Double impact collisions occur frequently for all three surfaces and, unlike the situation in one dimension, transfer energy very efficiently.
On the transport properties of the van der Waals fluid. III. Explicit calculation of the shear viscosity
Authors: Napiorkowski M. , Piasecki J. , Seghers M. , Résibois P.
Published: February 2003

We present numerical data for the effect of the attractive forces on the shear viscosity, as they come out of the van der Waals theory developed by Résibois, Pomeau, and Piasecki.
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o model calculations for benzene
Authors: Fink William H.
Published: February 2003

By using calculational procedures that (a) perform a variational calculation on a portion of molecular electronic structure and (b) project the best representation of a set of orbitals into a basis set reduced in size, a model calculation of benzene can be produced from a four CH unit fragment of the molecule. The two terminal CH units are the fixed part which are defined by projection from the result of the last variational calculation on the middle two CH units. The self-defining cycles are continued until consistency between cycles is reached. All electron repulsion integrals are evaluated exactly for the four CH unit fragment. The full benzene molecular wavefunction may then be constructed by replicating the variational space of the calculation in a ring reproducing the benzene molecular geometry. Several variants of the calculation have been examined and are reported.
Photodissociation of thiophosgene
Authors: Okabe Hideo
Published: February 2003

The photolysis of thiophosgene produces CS(A 1π) below the incident wavelength (1270 Å) from which ΔHf °0(SCCl2) ?7.92±1 kcal mol−1 (33.1±4 kJ mol−1) is derived. The quantum yield for the production of Cl atoms has been determined from the amounts of HCl produced from the photolysis of thiophosgene–heptane mixtures. At 2537 Å the yield is unity over the pressure region 0.4–80 torr (0.05–10 kPa) of thiophosgene, indicating that the primary photochemical process is almost entirely SCCl+Cl. Both at 3660 and 4358 Å the quantum yield of Cl production increases with a decrease in thiophosgene pressure and is 0.90±0.17 and 0.38±0.19, respectively, as the pressure approaches zero. From the pressure dependence of the quantum yield the lifetime of the excited state 1A2 is estimated to be about 55 nsec at 4358 Å and about 6 nsec at 3660 Å. The photolysis data of the 1A2 state of thiophosgene are correlated with the fluorescence lifetime measured previously. D°0(SCCl–Cl) =63.4±0.5 kcal mol−1 (265.3±2 kJ mol−1) and ΔHf °0(SCCl) =43±1 kcal mol−1 (180±4 kJ mol−1) are derived. The photochemistry of thiophosgene is compared with that of phosgene and formaldehyde. A new method of Cl isotopic enrichment is suggested.
Changes in surface morphology and vaporization rate upon vaporization of arsenic (111) cleavage surfaces
Authors: Dowell Michael B. , Hultman Carl A. , O’Neal H. Roger , Rosenblatt Gerd M.
Published: February 2003

The initial vaporization of freshly cleaved As(111) surfaces has been studied by a combination of microbalance weight-loss measurements and continuous microinterferometry. Vaporization is accompanied by formation and growth of triangular pits on the originally flat surface. The slopes (5°–9°) and other characteristics of the pits show that they have a terrace–ledge structure and form at dislocations. Pit sides have constant slope indicative of an average, constant terrace width between ledges. Isolated pits grow outwards at a constant rate while mass loss from a freshly cleaved increases quadratically with time. The mass-loss rate attains steady state, independent of the pit density, when the surface is covered by intersecting pits. The slopes of pits formed on surfaces sublimed into a vacuum are greater than those on surfaces vaporized under saturations nearer equilibrium. The pit growth rates and characteristics, as well as the time-dependent mass-loss rates, are quantitatively consistent with a vaporization mechanism in which the vaporization rate is proportional to the total ledge length on the surface. The results imply that formation of As4 vapor species occurs with equal probability, per unit length of ledge, along any ledge segment.