bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonzero temperature variational calculation for the quantum electron gas Slater sum
Authors: Beres R. P. , Pokrant M. A.
Published: February 2003

A nonzero temperature variational principle (NZTVP) for the Slater sum has previously been derived. The Slater sum is written in the form of a Boltzmann factor. The effects of Fermi statistics may be included by Lado’s method. The effective potentials in the Boltzmann factor are approximated by parameterized pair potentials and the parameters are determined by the NZTVP. These effective pair potentials can be used to determine the particle distributions and the thermodynamic properties using the methods developed for classical fluids. This paper presents the results of a sample calculation on the quantum electron gas at three densities which simulates a real calculation on a complicated system in order to see whether calculations using the NZTVP are feasible and to determine what problems may arise in such a calculation. Several simplifying approximations are made so that another calculation which does not restrict the temperature dependence of the parameter may also be done as a test of the results of the sample calculation. These exploratory calculations for the electron gas indicate that calculations using the NZTVP are feasible and that the NZTVP may provide a practical means for computing the properties of matter at all temperatures and densities.
Triplet exciton caging in two dimensions: Magnetic field effects
Authors: Arnold S. , Swenberg C. E. , Pope M.
Published: February 2003

Magnetic field dependence of the room temperature prompt fluorescence under high field conditions from mixed crystals of tetracene containing 2,3- benzocarbazole (BC) as guest molecules is studied both as a function of the mol fraction of BC and the orientation of the external magnetic field. Crystalline mixtures of BC in tetracene were prepared in the mol fraction range 0%–48.1%. For mol fraction x<9% BC, the angular separation between the high field pair state resonances was approximately 52°, whereas for x∼9% this separation dropped to ∼41% and remained at this value for concentrations 9.4%<x<29.0%. This experimental result is interpreted as indicating that the mixed crystal system BC/tetracene undergoes a crystallographic phase transition for x∼9%. An analysis of both the ’’on’’ and ’’off’’ field resonance enhancement in the prompt fluorescence of the host is shown to be consistent with previously reported increase in the fluorescence lifetime with x and existing pair state theory of the magnetic field effects on exciton fission provided the fission branching ratio, ε, is a function of the BC concentration. For x≳9%, ε is a monotonic increasing function of x which is interpreted as an enhancement of the geminate recombination rate of the original triplet exciton fission pair due to presence of exciton reflectors that reduce the size of the domain in which the excitons can freely move.
Self-consistent molecular orbital methods. XVII. Geometries and binding energies of second-row molecules. A comparison of three basis sets
Authors: Collins John B. , von R. Schleyer Paul , Binkley J. Stephen , Pople John A.
Published: February 2003

Three basis sets (minimal s–p, extended s–p, and minimal s–p with d functions on second row atoms) are used to calculate geometries and binding energies of 24 molecules containing second row atoms. d functions are found to be essential in the description of both properties for hypervalent molecules and to be important in the calculations of two-heavy-atom bond lengths even for molecules of normal valence.
1,2,4-Triazole: Vibrational spectra, normal coordinate calculations, and hydrogen bonding
Authors: Bougeard D. , Le Calvé N. , Roch B. Saint , Novak A.
Published: February 2003

The influence of hydrogen bonding on the vibrational spectra of 1,2,4-triazole has been studied using infrared and Raman spectroscopy and normal coordinate calculations. The infrared vapor phase fundamentals have been assigned. The polarized infrared and Raman spectra of triazole single crystals (Pbca,Z=8) have been investigated in the 4000 to 10 cm−1 region. An assignment of the 18 intramolecular fundamentals is given with help of −d1 and −d3 isotopic derivatives and normal coordinate calculations. Correlation field splitting is discussed. Thirty-two of the 45 lattice modes have been assigned to their symmetry species and in terms of rotational, translational, and hydrogen bond vibrations. They are compared to those calculated on the ground of the rigid body and flexible molecule models assuming appropriate intermolecular atom–atom potentials. The variation of the NH stretching and bending force constants upon hydrogen bond formation is correlated with the hydrogen bond force constant (0.35 mdyn/Å), N–N distance and low (1.25) νNH/νND ratio, indicating a positive isotope effect. The structure and breadth of the NH stretching band are discussed; the half-width of the corresponding Raman band decreases from 400 to 10 cm−1 when the temperature is lowered from 300 to 10 K.
Vibrational spectra and normal coordinate analysis of bromochlorofluoromethane
Authors: Diem M. , Burow D. F.
Published: February 2003

Raman spectra (50–4000 cm−1) of the gaseous, liquid, and solid phases along with gas-phase infrared spectra (200–4000 cm−1) of four isotopic species of bromochlorofluoromethane HCBr35ClF, HCBr37ClF, DCBr35ClF, and DCBr37ClF have been observed. Data from all three phases were needed to make a detailed vibrational assignment since Fermi resonance enhancement of several overtone and combination modes results in a rather complex spectrum. Twenty-four distinct gas-phase frequencies were used to calculate a generalized valence force field (GVFF) using structural parameters obtained from gas-phase electron diffraction data.
Thermodynamics of mixtures of nonspherical molecules. III. Fluid phase equilibria and critical loci
Authors: Twu C. H. , Gubbins K. E. , Gray C. G.
Published: February 2003

The influence of strong dipolar and quadrupolar forces on the fluid phase diagram of binary mixtures is investigated, using the Padé approximant to the free energy proposed by Stell, Rasaiah, and Narang for pure fluids. This approximant is known to give good results for such model fluids in the liquid phase, and is shown here to give quite good results for gases also. Using a Lennard-Jones mixture as the reference system, a study is made of the effect of dipolar and quadrupolar forces on (a) the vapor–liquid equilibrium surfaces, (b) the vapor–liquid critical loci, (c) the azeotropic lines, (d) the liquid–liquid equilibrium curves, and (e) the liquid–liquid critical loci. The model systems exhibit almost all of the major classes of binary fluid phase diagrams that have been observed experimentally.
A p
r
i
o
r
i calculation of the vibron band and of the vibron–phonon and two-phonon combination bands in the Raman spectrum of solid α-N2
Authors: Zumofen Gert , Dressler Kurt
Published: February 2003

The observed shift, splitting, and intensity ratio of the vibrational Raman band in solid α-nitrogen as reported by Anderson et al. have been reproduced in a calculation which is based on the quadrupole moments and polarizabilities of N2 in the gas phase. Furthermore, the intensities and shapes of the weak phonon sideband in the N2 stretching region, as well as of the two-phonon sideband in the lattice region, have been calculated by treating these as combination bands which derive their intensities from electrical anharmonicities, i.e., from quadratic dependences of the polarizability on internal and external nuclear displacements. The contributions from mechanical anharmonicities, i.e., from vibron–phonon coupling through third and higher order terms in the vibrational potential, are found to be negligible in these band intensities. The computations of the lattice dynamics are based on a Lennard-Jones plus quadrupole–quadrupole potential.
ESR linewidths in solution. VIII. Two-component diamagnetic solvents
Authors: Kowert Bruce , Kivelson Daniel
Published: February 2003

The ESR spectra of dilute vanadyl acetylacetonate were studied in several two-component solvents as a function of component concentration. A modified Debye equation gives reasonable results for the rotational correlation time τϑ for a number of solvent systems; this equation has the form τϑ= (4π/3) r⊥3ηκ⊥/kT, where k is the Boltzmann constant, r⊥3 a volume which is constant for vanadyl acetylacetonate in all the systems, η is the solvent viscosity, and κ⊥ is a dimensionless coupling parameter. κ⊥ is κ⊥=κ⊥(A)xA +κ⊥(B)xB where κ⊥(A) and κ⊥(B) are the coupling parameters and xA and xB the mole fractions of solvent components A and B, respectively. In ’’ideal’’ cases, these two equations hold quite well. κ⊥ is independent of temperature, pressure, and viscosity (η), and the excess viscosity (η−η(A)xA−η(B) xB) is small. η(A) and η(B) are the viscosities of pure A and B, respectively. For butanol systems at temperatures decreasing towards room temperature the systems are nonideal and the equations above are not too useful.
Transition moments in infrared-active fundamentals of spherical-top molecules
Authors: Fox Kenneth , Person Willis B.
Published: February 2003

The theoretical expression which relates the total strength of a vibration–rotation band of a tetrahedral or octahedral molecule to an individual line strength and to an appropriate transition electric-dipole moment is re-examined. Values of these transition moments are inferred from experimental band and line strengths. For the few molecules where direct comparisons can be made, the agreement between the values of transition moments obtained by the two methods is satisfactory. Remarkable systematics in the results are found for a variety of spherical-top molecules, which suggests that transition electric-dipole moments may be transferable from molecule to molecule for similar vibrations.
Conformational analysis and rotational barriers for different classes of molecules in terms of many atom exchange interactions
Authors: Lombardi E. , Tarantini G. , Pirola L. , Torsellini P.
Published: February 2003

Conformational analysis of simple molecules containing lone pair electrons such as H2O2, NH2OH, NH2NH2, is carried out as an extension of a recently proposed model where the barrier to internal rotation is assumed to arise only as a result of exchange interactions, calculated to first order using perturbation theory, among nonbonded atoms. Lone pair electrons associated to atoms located on the axis of rotation are effectively included in the model as additional nonbonded atoms. It is shown that exchange interactions among the nonbonded atoms of the extended cluster can be taken alone as responsible for the observed conformations and barriers. The model is then applied to molecules as propane and its derivatives, dimethylamine, dimethyl ether, with two consecutive axes of internal rotation and to the conformational analysis of polyoxymethylene. The results are in good agreement with experiments. The possibility of application to molecules of biological interest is discussed.
On the best average trajectory in the vicinity of energetic threshold
Authors: Billing Gert Due
Published: February 2003

In a previous paper [Billing, Chem. Phys. Let. 30, 391, (1975)] it was shown that the transition probabilities obtained from the ’’classical trajectory’’ equations are improved significantly by introducing an average potential <ψ‖V‖ψ≳ (where ψ is the total wavefunction and V the interaction potential) and an average velocity. With this in mind we recalculated the low temperature values of the He–H2 relaxation time obtained in Billing [Chem. Phys. 9, 359, (1975)] using the arithmetic mean velocity in the energy range 10.786, 1.30 eV]. The relaxation time obtained at 100 K improved by the factor of two, but is still off by a factor of 15 from the experimental value. So far no rigorous derivation of the classical trajectory equations including the average potential velocity exists, but the resulkts shown indicate that suich a derivation should also deal with the prediction of the best average velocity to be used.
Concerning the effect of an applied electric field on the dissociation of ion pairs into free ions
Authors: Freeman G. R.
Published: February 2003

Comments are made on the validity of assumptions made in a recent paper [A. Mozumder, J. Chem. Phys., 60, 4305 (1974)] concerning Onsager’s theory of the effect of an applied field on the dissociation of ion pairs into free ions. (AIP)
Self-consistent field with pseudowavefunctions
Authors: Szasz Levente
Published: February 2003

A computational method is given in which the energy of an atom is computed by using pseudowavefunctions only. The method centers on a model energy expression EM which is similar to the Hartree–Fock energy expression, but contains only pseudowavefunctions. A theorem is proved according to which the Hartree–Fock orbitals can be transformed by a linear transformation into a set of uniquely defined pseudowavefunctions which have the property that when substituted into EM this quantity will closely approximate the Hartree–Fock energy EF. The new method is then formulated by identifying the total energy of an atom with the minimum of EM. Application of the energy minimum principle leads to a set of equations for the pseudowavefunctions which are similar to but simpler than the Hartree–Fock equations. These equations contain pseudopotentials for which explicit expressions are derived. The possibility of replacing these pseudopotentials by simpler model potentials is discussed and the criteria for the selection of the model potential are outlined.
Vibrational relaxation times of F2 with Ar and He and H2 and D2 with He
Authors: Cady W. A. , Diebold Alain C.
Published: February 2003

The vibrational relaxation times for F2 in Ar and He and for H2 and D2 in He are calculated and compared with the data from shock tube experiments. The near-adiabatic approximation developed previously [J. Chem. Phys. 61, 1214 (1974)] was employed to calculate the relaxation times. A comparison of these results with those from the SSH theory, which contain a ’’static’’ approximation, shows the current theory to be as accurate or more accurate than the SSH theory except for the systems dominated by impulsive collisions. Calculations treating transitions in addition to the 1 to 0 transition were also performed and discussed.
Vibrational studies of one-dimensional conductors: K2Pt(CN)4Br0.3⋅3H2O and K2Pt(CN)4Cl0.3⋅3H2O
Authors: Ferraro J. R. , Basile L. J. , Williams Jack M.
Published: February 2003

Raman and infrared spectra of K2Pt(CN)4⋅Br0.3⋅3H2O, KCP (Br); K2Pt(CN)4⋅Cl0.3⋅3H2O, KCP(Cl), and K2Pt(CN)4⋅3H2O were obtained at ambient and nonambient conditions of temperature and pressure. The systems KCP(Br) and KCP(Cl) represent very interesting and unusual materials for ir–Raman spectroscopic studies because while neutron and x-ray diffraction measurements confirm the assignment of a noncentrosymmetric space group, the observable effects of the unit cell asymmetry are so slight, with respect to the vibrations of the Pt(CN)2−4 moiety, that the group may be considered as being present in a symmetric D2h environment (vide infra).
Total inelastic cross sections for potassium ion–atom collisions: Oscillations in the velocity dependence and correlation with molecular structure
Authors: Aquilanti Vincenzo , Casavecchia Piergiorgio
Published: February 2003

Electronic excitation leading to light emission in the wavelength range 350–800 nm has been studied by a crossed ion–atom beam technique for (K+, K) collisions, and the results are interpreted in terms of properties of the potential energy curves for the molecular ion K+2. The investigated velocity range is (1.3–12) ×106 cm s−1. The total cross section for the process K+(3p6 1S0) +K(4s 2S1/2) →K+(3p6 1S0) +K(4p 2P3/2,1/2) increases from threshold up to ∼10−15 cm2 at a velocity of ∼4.5×106 cm s−1, and shows an oscillatory structure. The magnitude and over-all velocity dependence are attributed to a Σ–Π curve crossing, and the oscillations to an interference effect, which is treated as an inelastic ’’glory’’ phenomenon. Cross sections for production of each of the fine structure components of K(4p), 2P3/2, and 2P1/2, have also been measured. Their ratio, which in the investigated velocity range is different from the statistical value, shows additional oscillations, which are discussed in terms of long range interference between alternate semiclassical paths.
Gas phase EPR linewidths and intermolecular potentials. IV. Experimental results for the temperature dependence of the F–He system
Authors: Fisanick-Englot G. J. , Miller Terry A.
Published: February 2003

Previous experimental work on gas phase EPR pressure broadening of F atom lines by He has been extended to investigate temperature effects in the range ∼150–350 °K. The qualitative features of the previous room temperature work persist with unequal line broadening cross sections for the three 2P3/2 Zeeman transitions and with the 2P1/2 Zeeman transition cross section remaining a factor of 50–100 smaller. Considerable variation in the cross sections as a function of temperature is evident, with neither the 2P1/2 nor 2P3/2 cross sections exhibiting a simple temperature dependence. Comparisons with available theoretical predictions indicate that the previous treatments are not adequate to describe the present experimental results.
Calculations on the permanganate ion in the ground and excited states
Authors: Hsu Hsiang-lin , Peterson Carl , Pitzer Russell M.
Published: February 2003

A b initio self-consistent-field calculations have been carried out on the permanganate ion using large Gaussian basis sets. The ground state as well as a number of interesting excited states have been treated. Calculations for the neutral and double negative species derived from the permanganate ion are also given.
Cross sections for rotational energy transfer: An information-theoretic synthesis
Authors: Procaccia I. , Levine R. D.
Published: February 2003

State-to-state cross sections are readily computed by combining the capabilities of the classical trajectories method with an information-theoretic (’’surprisal’’) synthesis. The method is illustrated by an application to rotational energy transfer in several systems, (Ar+N2, Li++H2, Li++D2, H+CO). It is shown that by using classical trajectories to compute a single moment of the distribution of final rotational energy (for a given initial rotational energy) and then maximizing the entropy of the distribution subject to the given value of the moment one can obtain a good prediction of the distribution. Invoking microscopic reversibility, the entire matrix of state-to-state, σ (j→j′), rotational energy transfer cross sections (at the given total energy) is then determined. More averaged quantities, such as the cross sections for inelastic collisions σ (j), are thereby easily obtained. When a realistic potential energy surface is not available or when one requires a simple but reliable prediction, the synthesis can be based on invoking a sum rule. Here the moment of the distribution is not computed via classical trajectories but is expressed as a simple function of the initial conditions. It is shown that available cross sections often satisfy the simplest possible sum rule and hence a synthesis can be readily carried out where the only inputs are the total energy and the rotational constant of the diatomic molecule. The distribution of final rotational states predicted in this way is independent of the nature of the collision partner. The method is illustrated by applications to H+CO, Ar+N2, Li++H2, He+HD, H+H2, and H+D2. Results for the distribution of final rotational state and the dependence of the cross section on the initial rotational state and on the collision energy are in very good accord with classical trajectory and with quantal close-coupling computations.
A successful integral equation theory of simple quantum liquids
Authors: Weres Oleh
Published: February 2003

It is demonstrated that the equilibrium two particle density matrix of a simple quantum liquid may profitably be re-expressed in terms of a Taylor expansion about the diagonal. The resulting arrays of Taylor coefficients are shown to have an intuitive physical significance. A set of three coupled nonlinear integrodifferential equations relating the three most important coefficients is derived. Approximating certain functions in terms of the straightforwardly calculable dilute gas two particle density matrix Taylor coefficients closes these equations, and they are solved numerically for normal liquid helium-4 and liquid para-hydrogen. Satisfactory results are obtained in both cases. In the case of liquid para-hydrogen a simpler related theory is shown to also yield satisfactory results.
Self-diffusion through a liquid–vapor interface
Authors: Mulholland G. W.
Published: February 2003

A diffusion equation containing information about the structure of the liquid–vapor interface is constructed to describe self-diffusion in a one component, two phase fluid. Analysis of the diffusion equation in the limit of vanishing interface thickness leads to interfacial matching conditions, one of which was previously assumed ad hoc. From a study of the asymptotic long time behavior of the equation, it is found that an ordinary self-diffusion experiment can not be expected to provide information about the interface structure. A numerical study of the diffusion equation shows that the reduced time to diffuse through the interface is independent of the ratio of the vapor and liquid densities, which is the one free parameter in the diffusion equation. An experimental test of our prediction that it takes the same time to diffuse from the liquid to the vapor as vice versa is suggested.
Density response functions for molten salts
Authors: Kerr William C.
Published: February 2003

A theory of the density response functions of molten salts is given. The results are obtained by an extension of a theory given by Hubbard and Beeby for a single component liquid. The theory is based on the physical idea that the high frequency motions of the particles are similar to the motions in a disordered solid. The technique used is to consider the linear response of the system to an external disturbance and approximate the exact equation of motion by factoring the higher-order correlation functions which appear therein. The result of Hubbard and Beeby is rederived here by a method which is shorter than the original method, and it is also applied to a charged system rather than to a neutral system. Then this method is generalized to the two-component case. The expressions for the density response functions for the individual species are obtained in a mean-field form appropriately generalized to two components. The screened response functions are determined by the self-motion functions for the two different species, and the polarization potentials are determined by the potential energy part of the fourth moments of the exact density response functions. The moment relations obeyed by the approximate results for the density response functions and the transformation to response functions for the total number or mass density and charge density are given. Finally, approximate dispersion relations for density fluctuations are obtained. These have a branch of ’’acoustical’’ character with frequency tending linearly to zero for long wavelengths, and a branch of ’’optical’’ character with a finite frequency in the long wavelength limit. The factors entering into the limiting optical mode frequency are analyzed, and their magnitudes are estimated.
On the ultraviolet spectrum of t
r
a
n
s-1,3-butadiene
Authors: McDiarmid Ruth
Published: February 2003

The ultraviolet absorption spectra of trans-1,3-butadiene, 1,1,4,4-trans-1,3-butadiene-d4, and trans-1,3-butadiene-d6 between 2300 and 1350 Å have been recorded and analyzed. Four Rydberg series were identified (δ=0.087, 0.21, 0.42, and 0.67). A transition-by-transition comparison of this analysis with those in the literature shows that several of the previously assigned transitions are misidentified. A continuum originating around 1450 Å is reported.
Mode coupling description of dynamics in dilute polymer solutions
Authors: Kapral Raymond , Ng Dominic , Whittington Stuart G.
Published: February 2003

A calculation of the dynamic structure factor for a dilute polymer solution which explicitly takes into account the coupling to the viscous modes of the fluid is presented. The techniques of mode–mode coupling are used to extract this contribution from the Liouville equation for the relevant polymer variable. In the small wave vector regime it is shown that this contribution yields exactly the hydrodynamic contribution to the diffusion coefficient which appears in other theories when the interactions between the monomers via the solvent are calculated by using the Oseen tensor. For large wave vectors the long range correlations in the polymer chain give rise to a nonlocal memory kernel similar to that of the anomalous part of the mutual diffusion coefficient near the binary mixture critical point. The form of this wave vector dependence is examined for several models of the polymer chain.
EPR study of NO2 and NO3 produced in urea nitrate by uv irradiation, x-irradiation, and constituent tritium atom decay
Authors: Wood David E. , Lozos George P.
Published: February 2003

NO2 and NO3 radicals have been produced by uv irradiation, x-irradiation, and constituent tritium atom decay in single crystals of urea nitrate. Two types of NO3 and five types of NO2 were produced, and which of these were observed depended on the type of irradiation and the temperature. At 77 °K two types (two different orientations) of NO2 were formed from tritium decay while only one of these was produced by x-irradiation. The various forms of NO2 were found to irreversibly interconvert from one to another as the temperature increased. Different mechanisms for defect formation have been proposed to account for the different types of radicals formed by x-irradiation and tritium decay at room temperature and 77 °K. The NO3 species which resulted from any of the methods of irradiation at room temperature showed no 14N hfs but did exhibit a significant interaction with five neighboring hydrogen bonded protons. Deuterium substitution caused the proton hfs to collapse into one line, thus simplifying the analysis. This NO3 was the same one reported formed by γ-irradiation. A different type of NO3 without proton hfs was formed when either x-irradiation or tritium substitution was used at 77 °K. These two types did not interconvert. From evaluation of the spin-Hamiltonian parameters the electronic ground states for these NO3 radicals have been elucidated. The low temperature form is found to be a π radical and the high temperature form is a σ radical.
Pattern formation in reacting and diffusing systems
Authors: Fife Paul C.
Published: February 2003

A mechanism is described, whereby stable sharply differentiated (dissipative) structures can evolve naturally within a mixture of reacting and diffusing substances. Our model has two reacting components, with one diffusion coefficient much smaller than the other. Unlike patterned states obtained by small amplitude analysis near uniform states, our structures have large amplitude and serve to divide the reactor into subregions, each corresponding to a distinct phase for the system. The evolution of the structured stationary state from an arbitrary initial distribution occurs in two stages. The first involves differentiation into subregions, and the second involves the migration of the boundaries of the subregions into a stable final configuration. A singular perturbation analysis and the theory of motion of wavefronts is used to deduce these qualitative properties.
A 3Σ+u molecules in the N2 afterglow
Authors: Shemansky D. E.
Published: February 2003

Excitation of N2 by electrons with energy below the ionization threshold produces a strong afterglow in the B 3Πg–A 3Σ+u system. The precursor is identified as the A 3Σ+u state in the v≳6 vibrational levels. The peak electron cross section of the A 3Σ+u–X 1Σ+g system is estimated at about three times the B 3Πg–X 1Σ+g transition. Deactivation rates of the A 3Σ+u v≳6 levels by X 1Σ+g molecules vary over the 8E−12 cm3sec−1–5E−11 cm3sec−1 range. There is some evidence that the higher levels may relax at rates comparable to that of electronic deactivation. Deactivation of B 3Πg by X 1Σ+g molecules has a strong dependence on the vibrational level of the B 3Πg state. The rate coefficients for this process vary between 1E−11 cm3sec−1 and 1E-10 cm3sec−1, with no measureable contribution by vibrational relaxation. The results suggest that production rates of the B 3Πg state in the Lewis–Rayleigh (L–R) afterglow are much more uniformly distributed over the vibrational levels than has been previously assumed. We also suggest it is unlikely that significant amounts of energy pass through the 5Σ+g state in the L–R afterglow. About 25% of the energy of atomic nitrogen recombination enters the B 3Πg state in the L–R afterglow, according to the present results. The factors controlling the A 3Σ+u v=0,1 population in the L–R afterglow appear to be much less well defined than has been suggested in previous literature.
Low temperature specific heat and phonon anomalies in transition metal compounds
Authors: Roedhammer P. , Weber W. , Gmelin E. , Rieder K. H.
Published: February 2003

The specific heat of the following pairs of transition metal compounds (TMC’s) has been measured in the temperature range 1.8<T<100 K : TiN/TiC, NbC/ZrC, and TaC/HfC. Theoretical results for the lattice specific heat, obtained from phonon densities calculated on the basis of appropriate shell models agree well with the experimental data. Values for Debye temperatures and Sommerfeld coefficients are given and compared with previously published data. We show that from the difference in the lattice specific heat between the superconducting and the normal compound of each pair, it is possible to determine the average frequency of the lowest-lying phonon anomalies occurring in the superconducting TMC. For TiN, NbC, and TaC, this frequency was determined to lie at 5.8, 4.8, and 3.1 THz, respectively. These values compare favorably with results from neutron scattering experiments. Thus, specific heat measurements can serve as a valuable tool to determine the mean frequency of the phonon anomalies in the superconducting compounds of this class of materials.
HF infrared chemiluminescence: Energy disposal and the role of the radical fragment in the abstraction of hydrogen from polyatomic molecules by F atoms
Authors: Bogan D. J. , Setser D. W.
Published: February 2003

HF infrared chemiluminescence has been utilized to study the energy disposal for the abstraction of hydrogen by fluorine atoms from polyatomic molecules which yield radical fragments with large stabilization energies. The prototype systems selected for study, methyl benzenes, phenol, and acetonitrile, are cases which yield resonance stabilized radicals as products. Comparison is made to the energy disposal from the reaction of F with the primary C–H bonds of aliphatic hydrocarbons, which have smaller radical stabilization energies. In general the radical stabilization energy, which is associated with major changes in geometry of the radical relative to the parent molecules, was not available to the HF product. The reactions of F + benzene and ethylene also were studied to provide reference data for different types of C–H bonds. The HF vibrational energy distributions have been interpreted using an extension of the information theory which previously has been applied to three body reactions. Vibrational surprisal analyses are developed and discussed for three models of the reference (prior) product distributions: (i) the polyatomic fragment product was treated as an atom, i.e., the three body case, (ii) the rotations of the radical fragment were added to the three body model, (iii) a complete model including all vibrational and rotational modes of the polyatomic radical fragment. For (iii) with the use of the full thermochemical exoergicity linear surprisal plots were found and these plots were used to assign relative populations to HF (v=0). The information-theoretic parameters from the three reference models are compared for a series of F+HR reactions in which R increases in complexity from Cl to CH2C6H5. For reactions with large product stabilization energies, calculations for (i) and (ii) were done with a reduced ’’effective available’’ energy corresponding to the assumption that the energy available to HF was less than the full exoergicity. Some insight is gained into the role of the R fragment in the energy disposal.
A many-body approach to the vibrational structure in molecular electronic spectra. I. Theory
Authors: Cederbaum L. S. , Domcke W.
Published: February 2003

A general Hamiltonian which describes the coupled motion of electrons and nuclei in a molecule is derived. In the one-particle approximation an exact solution for the vibrational intensity distribution of arbitrary electronic transitions (except those between linear and bent configurations) is given. Only the data of the initial electronic state are required and only linear equations have to be solved in the calculation. A formalism to include many-body effects, in which also only properties of the initial electronic state are needed, is presented. To calculate the gross features of the spectral distribution, especially of importance for the interpretation of low resolution spectra, a moment expansion is derived. The influence of the coupling constants appearing in the Hamiltonian on the vibrational spectrum is briefly discussed.
The interdependence and optimization of Gaussian function representations for the fluorine atom
Authors: Kari Roy E. , Mezey Paul G. , Csizmadia Imre G.
Published: February 2003

Three different series of Gaussian basis sets of different dimensions were optimized for the fluorine atom. The basis sets were chosen from a series of function spaces of increasing dimension and each set was subsequently optimized by the method of conjugate gradients. The interdependence of optimum s and p type basis functions and the effects of cross representations were analyzed. The relation between the optima of different basis spaces and the predictability of optimum basis sets were investigated. The ratios of the orbital exponents showed systematic variations when the dimension of the basis space was extended.
Effect of temperature and quencher concentration on vibrational relaxation in condensed media
Authors: Lin S. H. , Lin H. P. , Knittel D.
Published: February 2003

In this paper, we are concerned with the temperature effect on vibrational relaxation and the vibrational energy transfer from the vibrationally excited donor to the acceptor. For the temperature effect, we present numerical results to show the temperature dependence of the rate constant of vibrational relaxation and to discuss the validity of the rate constants obtained from the use of the weak coupling approximation and the strong coupling approximation. It is shown that although the temperature effect is extremely large over the temperature range T=0 to T=ϑE, the Einstein temperature of the medium, for the temperature range T=0 to T=0.3 ϑE, the rate constant varies slowly with temperature. For the vibrational energy transfer, we derive the master equation to describe the time dependent behavior of the excited donor, and the expression for the rate constant of vibrational energy transfer. The master equation is solved to study the temporal behavior of the excited donor as a function of the acceptor concentration. Numerical results are presented to demonstrate the theoretical results.
Semiclassical eigenvalues for nonseparable systems: Nonperturbative solution of the Hamilton–Jacobi equation in action-angle variables
Authors: Chapman Sally , Garrett Bruce C. , Miller William H.
Published: February 2003

It is shown how the Hamilton–Jacobi equation for a multidimensional nonseparable system can be efficiently solved directly in action-angle variables. This allows one to construct the total (classical) Hamiltonian as a function of the ’’good’’ action-angle variables which are the complete set of constants of the motion of the system; requiring the action variables to be integers then provides the semiclassical eigenvalues. Numerical results are presented for a two-dimensional potential well, and one sees that the semiclassical eigenvalues are in good agreement with the exact quantum mechanical values even for the case of large nonseparable coupling.
Stochastic theory of molecular collisions
Authors: Augustin Stuart , Rabitz Herschel
Published: February 2003

This paper investigates treating inelastic collisions as a stochastic process for the diffusion of probability between quantum states. It is shown that when the interaction is strong, the time-dependent Schrödinger equation may be approximated by a Pauli master equation. Under certain circumstances this can be further approximated by a Fokker–Planck partial differential equation. These equations can be used to model various collision phenomena. Inelastic and dissociative collision processes are discussed, and it is indicated how the theory may be extended to systems with several degrees of freedom. The theory extends smoothly between (a) the perturbation and (b) statistical limits, although it should be better near limit (b). Nevertheless, comparisons even near limit (a) with a similar exact quantum mechanical collinear vibrational problem show that the diffusion model gives qualitative agreement and predicts certain trends correctly. For example, the usual sudden and adiabatic regimes are readily achieved, total probability is conserved, and multiquanta transitions are easily handled. It is hoped that the conceptual simplicity of the model will be useful in analyzing and perhaps correlating the behavior of large theoretical and experimental collision problems.
Note on a bubble model for excess electrons in liquid hydrocarbons
Authors: Hammer H. , Schoepe W. , Weber D.
Published: February 2003

It is shown that a simple bubble model for electrons in liquid hexane that has been suggested recently does not satisfy the necessary stability criteria if the macroscopic properties of liquid hexane are used. Stability can be achieved, however, by assuming a higher dielectric constant ε than the macroscopic value. For hexane and some other hydrocarbons we have varied ε until the stable bubble model yields the experimentally determined activation energies of the mobility. The resulting values of ε turn out to be nearly the same for all liquids investigated.
A comprehensive parametrization and computational program for the analysis of MCD and absorption data
Authors: Caldwell D. J. , Eyring H.
Published: February 2003

A tractable method is presented for separating vibronic and static effects, which does not require extensive vibrational analysis of each molecule. The parameters so obtained can be expressed in terms of explicit matrix elements for a computation using only equilibrium position geometry and orbitals. The behavior of these vibronic quantities can provide an extremely sensitive means for studying the effects of substitution on the transition density matrix. This paper has been designed with sufficient flexibility in both the phenomenological and computational formalism to indicate how results of one procedure suggest appropriate modifications in the other. Benzene and its derivatives provide a fitting example for summarizing the procedure.
Ion cyclotron resonance studies of endothermic reactions of UF−6 generated by surface ionization
Authors: Beauchamp J. L.
Published: February 2003

Ion cyclotron resonance spectroscopy is used to investigate endothermic reactions of UF−6. At temperatures below those required for electron emission, UF−6 is formed by negative surface ionization on a directly heated rhenium filament. The reactant ion is accelerated to a known energy and traverses the source region of the ICR cell in the direction of the magnetic field. Slow products from reactive and nonreactive encounters are confined in the trapping well and detected in the usual manner. The formation of UF−7, UF−6, UF−5, and UF−3 from the reaction of UF−6 with UF6 is studied over the energy range 0–40 eV in the center of mass. The cross section for UF−7 formation exhibits a threshold at a relative kinetic energy of 2.7 eV. Reaction of UF−6 with BF3 leads to formation of BF−4 with a threshold of 1.6 eV in the center of mass. From these thresholds, E.A.(UF6) =4.9±0.5 eV is deduced and the bond dissociation energies D (UF5–F−) =108±6 kcal/mole and D (UF6–F−) =46±10 kcal/mole are calculated. The endothermic reaction of UF−6 with SF6 to produce SF−6 is also investigated, the cross section for the process exhibiting a gradual onset at ∼7.5 eV, considerably above the thermodynamic threshold. The experimental techniques for using ion cyclotron resonance spectroscopy to study endothermic reactions of positive and negative ions formed by surface ionization are considered in detail.
Paramagnetic relaxation reagents as a probe for translational motion of liquids
Authors: Hexem John G. , Edlund Ulf , Levy George C.
Published: February 2003

Use of the paramagnetic relaxation reagent tris- (acetylacetonato) chromium(III) [Cr(acac)3] proves capable of probing the solution structure of organic liquids. The theory describing the relaxation effects of this chelate is presented, wherein it is assumed that the nuclear spin–lattice relaxation mechanism is due to the modulation of the electron–nuclear dipole–dipole interaction. The electronic relaxation time Te1 (or τs) and the mutual translational motion of the paramagnetic species and the solvent molecules, in our case carbon tetrachloride, are considered the only causes of this modulation. Since for the paramagnetic molecules it is possible to calculate the electronic relaxation times, such systems are well defined for the description of translational diffusion. It is found that a jump diffusion model for translational motion explains the experimental data with jump distances comparable to the diameter of the solvent molecules.
Atomic and molecular calculations with the model potential method. IV
Authors: Bonifacic V. , Huzinaga S.
Published: February 2003

Results of model potential calculations are reported for the ground state of transition metal elements, Sc through Zn. We also present satisfactory results of calculations on both the ground and selected excited states of Fe, Fe++, Ni, and Ni++ with the model potential method. The model potentials were constructed to replace the 1s2 2s2 2p6 3s2 3p6 core, and also the 1s2 2s2 2p6 3s2 core, thus including 3p electrons as the ’’valence’’ electrons.
Magnetic linear dichroism in S4 symmetry: Experimental evidence for complex coefficients in wavefunctions for Nd3+ in SrMoO4
Authors: Moreau N. , Boccara A. C.
Published: February 2003

A detailed calculation of magnetic linear dichroism (MLD) is performed in the case of S4 symmetry for ions with an odd number of active electrons, using perturbation theory. First it is shown that MLD anisotropy provides reliable assignments of the symmetry of the excited states. Then, in case of sinusoidal variation of the MLD amplitude, it is demonstrated that the complex part of the wavefunctions coefficients introduces phase differences between the signals associated to different transitions. This shows unambiguously that the use of six parameters to account for S4 symmetry instead of five in the simplifying assumption of D2d symmetry does not reduce simply to the adjunction of one more adjustable parameter but is of prime physical importance. Experiments confirm the theoretical predictions. In addition, for the example of Nd3+ in SrMoO4, the evaluation of MLD amplitude using the wavefunctions calculated by Karayianis from experimental absorption data are shown to be in quantitative agreement with experiment.
Energy bands in reactive collisions. I. H+H2 on the collinear SSMK surface
Authors: Wright James S. , Tan K. G. , Laidler Keith J.
Published: February 2003

The collision of H+H2 on the collinear Shavitt, Stevens, Minn, and Karplus (SSMK) surface is studied for incident energies of 0–2 eV. By assuming a fixed zero-point energy for H2, and systematically varying the incident energy of the atom and the vibrational phase angle of the molecule, the trajectories divide into a continuous series of reactive (R) and unreactive (U) bands. At the R–U boundary, trajectory times and product vibrational energies reach a maximum. From the bands, the reaction probability may be obtained with a high degree of precision. Band continuity is shown clearly by means of linear and polar plots. The reaction probabilities obtained from the band plots are discussed and compared to other quasiclassical and quantum-mechanical studies.
Self-consistent-field wavefunctions using a symmetry-restricted annihilation of single-excitations procedure
Authors: Yarkony David R. , Schaefer Henry F. , Bender Charles F.
Published: February 2003

The form of Brillouin’s theorem appropriate for restricted Hartree–Fock calculations is used to determine rigorous self-consistent-field wavefunctions for the 3A1, 3E, 3A2, 1A2, and 1E states of the ... 2e3 3e manifold of CH3CO+, a manifold for which traditional Fock operator techniques are unsuitable. The results of an approximate method previously reported are compared with these exact results are found to be in quite good agreement.
The linear electric field effect for low spin ferric heme compounds
Authors: Mims W. B. , Peisach J.
Published: February 2003

We have measured the linear electric field induced g shifts for a series of low spin ferric heme compounds. Measurements were made by the electron spin echo method on noncrystalline samples at liquid helium temperatures. Some of these samples consisted of proteins and protein derivatives; others were made from heme reacted with small ligand molecules. Analytic expressions relating the observed changes in spin echo amplitude to certain of the g2-shift coefficients Bij have been derived for the case where the Zeeman field H0 is set at either end of the electron paramagnetic resonance (EPR) spectrum. Elsewhere in the spectrum the observations can be described in terms of a shift parameter σ which is obtained by noting the time integral of the applied field E required to cause a 50% reduction in the echo amplitude. This shift parameter is approximately equal to the mean value of ‖δg/g‖ +E. In order to illustrate qualitatively the way in which σ depends on the values of the Bij and on the H0 setting, numerical computations of the mean value of ‖δg/g‖ have been made for a typical set of g values and for unit values of the Bij. Computations have been made for the two cases of E∥H0 and E⊥H0. A simplified crystal field model has been used to interpret the data. The results suggest the presence of extensive back bonding between the Fe3+ ion and the axial ligands of the heme complex, this back bonding being several times larger for an RS− ligand than for ligands involving nitrogen. The results also suggest that myoglobin hydroxide is characterized by a crystal field which is exceptionally low for the S′= (1/2) ferric heme group of compounds.