bluuebunny/crossref_metadata_2025 huggingface dataset:
Cross sections for photodetachment of electrons from negative ions near threshold
Authors: Reed Kenneth J. , Zimmerman Albert H. , Andersen Hans C. , Brauman John I.
Published: February 2003

Threshold laws for the cross section for photodetachment of electrons from polyatomic negative ions are derived. The general form for the limiting behavior at threshold is derived using group theory and symmetry considerations. In addition, for energies within a few eV of threshold, a formulation using the Born approximation with partial orthogonalization is developed. The energy dependence of the photodetachment cross section for molecular negative ions is computed and shown to agree well with experimentally determined cross sections.
Cross sections for producing CO+2 (A 2Π–X 2Π) and CO+2 (B 2Σ–X 2Π) fluorescence by photoionization of CO2
Authors: Gentieu E. P. , Mentall J. E.
Published: February 2003

Cross sections have been measured with 0.5 Å resolution between 584 Å and respective thresholds for CO+2 A and B excited states as they fluoresce to the CO+2 ground state. Beutler–Fano resonance profiles are observed in both cross sections converging to the CO+2 C 2Σ ion limit at 640 Å. Characteristics of the initial and final states involved in the ionization process are discussed in detail. The ratio of the absolute magnitudes of the A:B fluorescence cross sections reported in this work is in excellent agreement with results of previous workers near 584 Å, pointing up a fundamental discrepancy between initial and final state measurements of CO+2 A and B excited state populations.
Laser magnetic resonance spectrum of HCO on the D2O 108 μ laser line
Authors: Cook Joel M. , Evenson K. M. , Howard Carleton J. , Curl R. F.
Published: February 2003

An LMR spectrum on the 108 μ D2O laser line has been observed and assigned to the 82,6←71,7 rotational transition of HCO. The spectra of both π (parallel) and σ (perpendicular) polarizations have been fitted by the least squares method, giving excellent agreement between the calculated and observed magnetic fields. The rotational frequency obtained from the fittings has been used to determine the A rotational constant. In addition, the electron spin-rotational splitting of each rotational level and the Fermi constant have been determined.
Linewidths in the vibrational spectra of polymers in dilute solution
Authors: Chang E. L. , Mazo R. M.
Published: February 2003

Damping of the vibrations of polymer chains due to friction with solvent, including hydrodynamic interaction between chain elements, is considered. The linear chain model is treated in detail. It is concluded that the damping constant is independent of the degree of polymerization. This conclusion is based on use of the high frequency hydrodynamic interaction. Only backbone vibrations of the chain are treated; localized group vibrations are not within the purview of the model.
Removal of the discontinuities of the trial function in the computation of the electronic structure of molecules by the intersecting spheres model: Evaluation of the equilibrium geometry of N2 and H2O
Authors: Antoci S. , Mihich L.
Published: February 2003

The trial function in the intersecting spheres model displays discontinuities at the plane boundaries between neighboring spheres; such discontinuities can be removed through the use of spline functions. In this way the intersecting spheres model allows us to perform fully ab initio computations of the electronic structure of molecules.
Theoretical determination of the electronic structure and the spatial arrangement of ferrous iron in deoxygenated sperm whale myoglobin and human hemoglobin from Mössbauer experiments
Authors: Eicher H. , Bade D. , Parak F.
Published: February 2003

The electronic term scheme of ferrous iron in sperm whale myoglobin (Mb) and human hemoglobin (HbA) is evaluated by a Hamiltonian which involves the Coulomb repulsion of the 3d electrons, their interaction with the tetragonally arranged ligands, a small rhombic perturbation of the C4v point symmetry, and spin–orbit coupling. The temperature dependence of the quadrupole splitting in both compounds was measured and the adjustable parameters of the theory were determined by a least squares fit to the experimental results. It was found that the five lowest singlets mainly descend from the rhombic-split 5E term. The low-lying 1A1, 5B2, and 3E levels contribute to the low-energy spectrum too. The least squares fit yields a different term scheme for Mb and HbA. In both compounds, however, the spin–orbit coupling constant λ and the covalency factor α2 of the quadrupole interaction prove to be λ?69 cm−1 and α2?0.89. The calculated electric field gradient agrees with the single crystal experiment on Mb at 77 K. Owing to these results, we discuss within the crystalline field theory the structural properties of the iron cation and its neighboring ligands: In Mb the iron is found to lie 0.4±0.1 Å above the heme plane, while in HbA the out of plane distance is about 0.1 Å greater. The bond length with the imidazole nitrogen is smallest in HbA, indicating a strong interaction with the proximal histidin; calculations give 2.00±0.03 and 2.12±0.05 Å for HbA and Mb, respectively.
Relative yield and rate constants for deactivation of electronically excited iodine atoms produced by methyl iodide photolysis
Authors: Palmer R. E. , Padrick T. D.
Published: February 2003

We have determined the fraction of iodine atoms produced in the electronically excited 2P1/2 state by broadband flash photolysis of methyl iodide. Rate constants for the deactivation of electronically excited iodine by methyl iodide and by methyl radicals have also been evaluated. In addition, an upper limit for the rate constant for the reaction of electronically excited iodine with methyl iodide to produce molecular iodine and methyl radicals has been determined. These measurements have been made by monitoring the gain of an iodine laser amplifier as a function of time.
The effect of the nonlinear adsorptions of benzene and water vapors on molecular transport distribution functions
Authors: Yin Khin M.
Published: February 2003

We give an asymptotic solution of the general dispersion equation for molecular transport through porous adsorbing solids, taking into account the nonlinearity of the adsorption isotherm. The solution indicates that the nonlinearity causes retardation or advancement of the adsorption front depending on the curvature of the isotherm. This effect is shown quantitatively for nonlinear adsorptions of benzene vapor on silica gel and water vapor on styrene–divinylbenzene copolymer. The model shows that the parameters that characterize the adsorption isotherm can be deduced from the experimentally measurable molecular transport distribution functions. Application of the model for adsorption of gases and vapors at low temperatures is suggested.
Dynamical treatment of unimolecular decomposition reactions. The RRKM formula
Authors: Kay Kenneth G.
Published: February 2003

To explore the range of validity of the RRKM theory of unimolecular reactions, we present a completely dynamical derivation of the RRKM expression for the decomposition rate of isolated molecules. This derivation avoids the usual equilibrium statistical assumptions and expresses the conditions for validity of the RRKM theory in terms of fundamental, static, molecular properties. To carry out this derivation we apply a treatment of energy transfer and decomposition which combines the Wigner–Eisenbud R-matrix approach to scattering with a technique we previously developed for studying the internal dynamics of nonreactive molecules. We obtain a molecular dissociation rate which agrees with the predictions of microcanonical transition state theory by introducing conditions which ensure statistical equilibration of all states describing the molecular fragments in close proximity of each other. We verify that, under the conditions of our derivation, the distribution of product states is statistical, i.e., in agreement with the implications of simple phase space theory, when the RRKM unimolecular rate formula is valid. Our treatment relies on a number of assumptions which may be regarded as sufficient (although not necessary) conditions for RRKM behavior. These include assumptions concerning the relative magnitudes of various interaction matrix elements (and thus the relative rates of intramolecular relaxation, passage along the reaction coordinate, and decomposition), the properties of intramolecular potential energy surfaces, and the energy dependence of certain quantities. One assumption belonging to the last category may not be entirely valid in all relevant cases but can probably be relaxed considerably. Other assumptions seem plausible but ultimately remain somewhat uncertain due to our ignorance about detailed features of potential energy surfaces for polyatomic molecules.
EPR of Gd3+ in NdCl3⋅6H2O
Authors: Misra Sushil K. , Sharp G. R.
Published: February 2003

The electron paramagnetic resonance spectrum of Gd3+ in NdCl3⋅6H2O has been studied with an X-band spectrometer at 295, 78.9, 3.98, and 1.65 °K, and with a K-band spectrometer at 295 and 77 °K. The low temperature data were used to determine the signs of parameters. In particular, the following values for the g tensor and the zero-field splitting parameter b02 were obtained from X-band data: at 295 °K, gzz=1.993±0.005, gxx=1.992±0.005, gyy=1.992±0.005, b02=1.864±0.015 GHz, b22=−1.053±0.015 GHz; at 78.9 °K, gzz=1.997±0.005, gxx=1.991±0.005, b02=1.953±0.015 GHz, b22= −1.366±0.015 GHz; at 3.98 °K, gzz=2.03±0.05, gxx=1.99±0.05, b02=2.02±0.15 GHz, b22=−1.32±0.15 GHz; at 1.65 °K, gzz=2.08±0.05, gxx=1.97±0.05, b02=2.06±0.15 GHz, b22=−1.29±0.15 GHz. (The K-band values were found to be consistent with the X-band values.) The individual values of all the other parameters were also determined. From the angular dependence of the data in the ZX plane the angle χ0, which the Z axis makes with the a vector of the unit cell, was determined to be 58.50±0.25°.
Chain statistics of polyelectrolytes
Authors: Iwasa Kunihiko
Published: February 2003

The characteristic function of the polyelectrolyte chains is given, extending the mode expansion method applied in polyelectrolytes. The mean square end-to-end distance and the expansion factor 〈α2〉 are discussed. Although the dependence of the expansion factor on the salt concentration Cs and on the molecular weight M of the polymer is expressed as 〈α2〉−1 proportional to M1/2/Cs in the excess salt limit, it is suggested that under finite salt conditions the Cs dependence is modified to be of the Cs−1+ε type, with ε≳0. This behavior indicates that the counterion distribution around a polyion reduces the polymer expansion. This tendency agrees with experiments.
Raman spectra of single- and polycrystalline Cs2[ZnCl4] complex
Authors: Wong P. T. T.
Published: February 2003

The polarized Raman spectra of the single-crystalline Cs2[ZnCl4] in the frequency region Δν=0–350 cm−1 at room temperature, and the Raman spectra of the polycrystalline Cs2[ZnCl4] in the same frequency region at 293, 80, and 36 K have been measured. A complete analysis of the spectra in both the metal–ligand optical modes and the lattice optical modes regions has been made; and the distortion of the [ZnCl4]2− ions in the crystal has been discussed on the basis of the present results.
Comments on the behavior of the Yvon–Born–Green equation for the square-well fluid near the critical point
Authors: Co King U. , Kozak John J. , Luks Kraemer D.
Published: February 2003

The Yvon–Born–Green equation under the superposition approximation was solved numerically for a system of molecules interacting via the square-well potential with σ2/σ1=1.85 in the vicinity of the gas–liquid critical point region. The critical point is located, and the critical exponents are determined. A reciprocal temperature perturbation treatment in the context of the Yvon–Born–Green theory is used in conjunction with previously developed stability criteria to locate the gas–liquid and solid–liquid coexistence regions. The methods employed offer encouragement for the use of perturbation approximations in delineating coexistence regions.
Infrared spectra of solid HCl and HBr. The overtone region
Authors: Avrillier S. , Mitra S. S. , Vu H.
Published: February 2003

Infrared spectra of solid HCl and HBr have been investigated in the second and third overtone regions at several temperatures and pressures. The overtone spectra have been interpreted in terms of long-wavelength one-phonon ’’proper harmonics,’’ i.e., excitation to higher vibrational levels in the same molecules, and two-phonon excitations involving molecules in neighboring sites with different phases. In hydrogen bonded solids like HCl and HBr the two spectra are well separated. Additional spectral structure is explained as combinations with lattice modes. The effect of phase transformation from the disordered orthorhombic to the ordered orthorhombic phase of solid HBr on its spectrum has also been investigated.
Electron paramagnetic resonance spectrum and spin relaxation for Ti(H2O)63+ in aqueous solution and in frozen glass
Authors: Wilson Richard C. , Myers Rollie J.
Published: February 2003

The EPR spectra of acidified solutions of Ti(III) were observed both above and below the freezing point. In the frozen glass, an axial spin Hamiltonian was found for the Ti(H2O)63+ complex with g∥=1.988±0.002 and g⊥=1.892±0.002. When warmed above −50 °C, broadening occurs which follows 1/T2=2.90×1014 e−ΔE/kT sec−1, where ΔE=1850±50 cm−1. This same broadening is followed for both the solution and frozen glass. The g values are explained assuming that the 1850 cm−1 value is the first excited orbital state energy, and the relaxation is explained as a resonant Raman process.
Two relaxation time model for orientational motion of molecules in a gas reservoir
Authors: Malo Otieno
Published: February 2003

A classical treatment of a two relaxation time model is presented, and it is shown from a solution of the general initial value problem that the Debye relaxation rate is equal to the rate of angular momentum relaxation.
Direct determination of the rotational barrier in ethane using perturbation theory
Authors: Nee Tsun-Shi , Parr Robert G. , Bartlett Rodney J.
Published: February 2003

A transition state perturbation method is derived which permits the direct determination of the energy barrier to hindered rotation in ethane as opposed to the usual subtraction of two large numbers. The method is implemented by using the coupled Hartree–Fock perturbation theory (CPHF). The value obtained for the rotational barrier of ethane through third order is 2.60 kcal/mole, while the infinite-order result gives 3.17 kcal/mole. A generalized derivation of the energy formulas in CPHF is given which provides expressions for the energy to any order. This also establishes the CPHF analog of the theorem in ordinary perturbation theory that the energy correct to order 2n+1 can be obtained from perturbed orbitals of order n.
Chemiluminescence studies. V. Production of electronically excited alkali atoms in reactions of Li, Na, and Cs with NF3 and of Cs with F2
Authors: Luria M. , Eckstrom D. J. , Edelstein S. A. , Perry B. E. , Benson S. W.
Published: February 2003

The magnitudes and distributions of alkali atom emission have been measured for Li+NF3, Na+NF3, Cs+NF3, and Cs+F2 flames. The production of excited states in all cases follows a gi(Eion−Ei)3 dependence, where gi and Ei are the degeneracy and energy of the state, and Eion is the ionization energy of the alkali atom. This suggests that the primary excitation mechanism involves electron–ion recombination. Examination of other data shows that this energy dependence of excitation may hold in a variety of reaction systems and experimental conditions. Measured photon yields for atomic emission ranged from 0.2% to 3%; considerably higher values would be expected in the absence of radiation trapping. There is no evidence for substantial population inversions between any of the atomic states.
Statistical mechanics of polar systems. II
Authors: Ho/ye J. S. , Stell G.
Published: February 2003

The results of Part I of this work [J. Chem. Phys. 61, 562 (1974)] are extended. A new formally exact expression for the dielectric constant ε of a polar nonpolarizable fluid is derived. It involves a ’’core parameter’’ Θ that depends upon the way one decomposes the pair potential into a reference and perturbing term, reducing to our earlier expression given in Part I for Θ=0 and to the expression proposed by Nienhuis and Deutch for Θ=1. It is shown how the Clausius–Mosotti, Onsager, and Wertheim expressions for ε can all three be regarded as mean-field approximations, each associated with a different value of Θ. Each expression becomes exact for a somewhat different model of a continuum fluid. A mean-field theory is given for phase transitions that involve dipolar ordering; the theory becomes exact (as do the other mean-field results of the paper) for a model in which the Kac inverse-range parameter γ goes to zero.
Potential energy curves from the electron gas model. II. The ion–rare gas interactions
Authors: Gianturco F. A.
Published: February 2003

The electron gas model already proposed in the literature for computing potential energy curves between neutral, spherical systems is here extended to nonneutral interactions by employing a physically intuitive and numerically simple form of the induction forces within the overlap region. Calculations are performed for Li+, Na+, and K+ interacting with heavy rare gases, from neon through xenon, and the results are compared with the available molecular beam data. The relevant potential parameters are also presented for all the examined cases.
The spectrum of Nd3+:LaCl3
Authors: Crosswhite H. M. , Crosswhite Hannah , Kaseta F. W. , Sarup R.
Published: February 2003

New measurements of the optical spectrum of trivalent neodymium in a hexagonal lanthanum chloride host have been made at 4 and 77 K at wavelengths down to 2500 Å (40 000 cm−1). Crystal quantum number assignments for new energy levels were derived from these results and for most of the earlier levels not already classified. A least-squares fit with a mean error of 8.1 cm−1 was carried out for 101 levels, using 20 adjustable parameters in a Hamiltonian containing operators for two- and three-particle electrostatic interactions; spin–orbit, spin–other-orbit and spin–spin magnetic interactions; two-particle pseudomagnetic configuration-mixing interactions; and single-particle crystal-field interactions.
Infrared spectra of the M+HBr2− and the M+HClBr− ion pairs and their deuterium analogs isolated in argon matrices at 15 K
Authors: Ault Bruce S. , Andrews Lester
Published: February 2003

The matrix reactions of alkali bromide salts and HBr and DBr have been investigated in argon matrices. In each case, an intense band was observed from the reaction product, which was assigned to the antisymmetric stretch, ν3, of the HBr2− and DBr2− anions in the M+HBr2− and M+DBr2− ion pairs. Comparisons to literature spectra, in conjunction with quartic deuterium shifts, indicate that the anion has a linear, centrosymmetric geometry in the isolated ion pair. A less intense band was also observed which has been assigned to the combination band ν1+ν3 for the HBr2− anion. Reaction of alkali bromide salts with HCl and alkali chloride salts with HBr have been shown to yield identical absorptions for the M+HClBr− ion pairs. The spectra suggest that both type I and type II HClBr− anions exist, and the resulting form may be determined by cation position.
The magnetic structure of the rare earth tetraborides ErB4 and DyB4
Authors: Schäfer W. , Will G. , Buschow K. H. J.
Published: February 2003

Neutron diffraction studies were performed on the compounds ErB4 and DyB4 at 4.2 and 300 K. Both compounds are antiferromagnetically ordered at 4.2 K. A determination of the magnetic structure type is given for ErB4 and DyB4. The magnetic intensities were studied as a function of temperature, leading to Néel temperatures TN=16.3 K (ErB4) and TN=21 K (DyB4). The relative stability of several simple magnetic structures, appropriate to RB4, are discussed in terms of the RKKY model, together with previously reported results of magnetization and NMR studies on these compounds.
van der Waals interactions of carbon monoxide
Authors: Parker Gregory A. , Pack Russell T
Published: February 2003

Padé approximant methods and experimental frequency dependent polarizabilities are used to calculate reliable van der Waals C6 coefficients for the interaction of CO with He, Ne, Ar, Kr, Xe, H, Li, Na, K, Rb, Cs, H2, N2, O2, CO2, and CO. Error bounds are given for the C6 coefficients and for the CO oscillator strength sums.
Molecular oxygen enhanced fluorescence of organic molecules in polymer matrices: A singlet oxygen feedback mechanism
Authors: Kenner Rex D. , Khan Ahsan U.
Published: February 2003

The surprising observation that molecular oxygen, the universal quencher of luminescence of organic molecules, can reverse its role and act as an enhancer of fluorescence in polymer matrices was originally recorded by Geacintov et al. Bolton, Kenner, and Khan investigated the phenomenon of oxygen enhanced fluoresence of organic molecules in polymer matrices and attributed the enhancement to a singlet oxygen feedback mechanism in which there is a repopulation of the singlet manifold of the organic molecule via energy transfer from singlet molecular oxygen. In this paper we examine the preliminary results and consider the relative merits of the singlet oxygen feedback mechanism and a trivial mechanism involving ground state depletion of the organic fluorescer in the evacuated samples. Optical cross beam experiments, designed to compare the extent of ground state depopulation of the organic molecule with the amount of enhancement observed, indicate that the depopulation mechanism can account for only a small part of the fluorescence enhancement. Furthermore, we report the discovery of oxygen stimulated fluorescence in similar systems under conditions in which the singlet manifold of the organic molecule can be populated only by energy transfer from singlet oxygen. Oxygen stimulated fluorescence is a direct and unequivocal confirmation of the basic idea of the singlet oxygen feedback mechanism and raises a fundamental question about the mechanism of oxygen quenching of the excited singlet states of organic molecules.
Effect of vibrations on the internal rotation barrier in ethane
Authors: Kirtman Bernard , Palke William E. , Ewig Carl S.
Published: February 2003

The effect on the ethane internal rotation barrier of (1) zero-point vibrational averaging and (2) distortions in equilibrium geometry during internal rotation is determined. We find that the (previously ignored) averaging term is 374±90 cal/mole which is substantially larger, and opposite in sign, to the distortion correction of −161±20 cal/mole. The isotope shift, other experimental tests, and some important implications of our results are discussed.
Light scattering by impurities at a liquid interface
Authors: Titulaer U. M. , Deutch J. M.
Published: February 2003

We discuss the scattering of light by anisotropic impurities located at a fluid interface. Our aim is to determine which type of information about the arrangement and motion of the impurities can be obtained from precise light scattering experiments. Since the paper is intended to be exploratory rather than exhaustive, we concentrate our attention on the simplest nontrivial case; a number of possible complications are discussed in a more qualitative way.
Detailed quantum transition state theory
Authors: Light J. C. , Altenberger-Siczek A.
Published: February 2003

The detailed quantum transition state theory (DQTST) is presented. It allows quantum effects to be obtained from greatly simplified calculations by applying boundary conditions corresponding to flux in the product direction at the transition state and solving for nonreactive scattering S-matrix elements. These are used to obtain the reaction probability from specific initial states. It is compared to the transition state theory (TST) and the complete quantum calculations for the planar H+H2 reaction. The convergence of DQTST to the exact results is shown to be very good.
Elastic scattering measurements of the Ar2* (3Σ+u) well depth
Authors: Gillen K. T. , Saxon R. P. , Lorents D. C. , Ice G. E. , Olson R. E.
Published: February 2003

Angular distribution measurements of the scattering of Ar(3P2) from Ar(1S0) at center of mass collision energies from 5 to 10 eV yield a prominent rainbow maximum at τ=Eϑ=81±3 eV-deg. Combined with recent high energy ground state scattering data and information derived from the Ar2* emission continua, the result implies that De=0.78±0.04 eV and Re=2.33±0.02 Å in an assumed Morse form for the lowest 1u, 0−u(3Σ+u)Ar2* excimer potential.
Spectroscopic studies of diatomic noble gas halides: Analysis of spontaneous and stimulated emission from XeCl
Authors: Tellinghuisen Joel , Hoffman J. M. , Tisone G. C. , Hays A. K.
Published: February 2003

A XeCl emission band system with peak intensity near 3080 Å is reported and analyzed. The vibrational analysis of this spectrum includes 24 violet-degraded bands assigned to Xe35Cl and Xe37Cl, and yields (for Xe35Cl) ΔTe=32 405 cm−1, ωe′=195.2 cm−1, ωe″=26.3 cm−1, and De″=255 cm−1. The weakly bound 2Σ+ ground state of XeCl resembles a van der Waals state, whereas the excited state is the analog of the CsCl ground state. Approximate potential curves are derived for these XeCl states. Lasing is observed on the 0–1, 0–2, and 0–3 bands of this transition, with a peak output power of 1.5×104 W.
Lifetimes and energy transfer near the dissociation limit in bromine
Authors: McAfee K. B. , Hozack R. S.
Published: February 2003

The fluorescence lifetime and quenching cross sections of several specifically excited vibrational and rotational states in the bromine B 3Πou+ state have been measured near the dissociation limit. The lifetime is found to depend sharply upon J′ and v′ allowing the relationship between predissociation rates and quantum states to be examined in detail. Lifetimes are found to decrease monotonically as the dissociation limit is approached. However, one state within a few wavenumbers of the limit is found to reverse the monotonically decreasing series. Quenching cross sections are found to increase systematically as the limit is approached. Collisional energy transfer between specific quantum states is observed, but does not interfere with lifetime measurements.
Computer simulation of multiple chain systems—equation of state of hard sphere chains
Authors: Curro John G.
Published: February 2003

A Monte Carlo method was used to determine the equation of state of systems of chains with a hard sphere potential. These equation of state results can be applied to the osmotic pressure of polymer solutions where polymer–solvent interactions are neglected, or to the external pressure of a system of neat chains. The calculations were performed on 15 and 20 unit chain systems over a wide range of density, and comparisons are made with analytical theories appropriate to each density.
k=0 vibrational spectrum for solid orthorhombic HCl
Authors: Gillis N. S. , Raich J. C. , Kanney L. B. , Bickermann A.
Published: February 2003

The harmonic k=0 lattice vibration spectrum of orthorhombic HCl has been determined employing a potential model consisting of dipolar, quadrupolar, and exponential-6 atom–atom interactions. The hydrogen bond has been simulated using a model due to Lippincott and Schroeder. By fitting the calculated frequencies to the low temperature vibrational data, the parameters which appear in the potential model have been determined subject to the constraint that the lattice equilibrium conditions are satisfied. Comparison is made with available experimental data.
Anisotropic exchange in linear chain complexes of copper(II)
Authors: McGregor K. T. , Soos Z. G.
Published: February 2003

An improved formula for the second moment of the electron paramagnetic resonance line is found for the case of both anisotropic exchange (a.e.) and magnetic dipole interactions between localized spins. The width Γ of the exchange-narrowed EPR line is then obtained by using the Blume–Hubbard result for spin dynamics and by preferentially weighting the secular a.e. and dipolar terms in one-dimensional systems. The angular and temperature dependence of Γ for copper(II) pyrazine nitrate are reported. The linewidth is adequately described in this linear chain system once the a.e. interaction and the one-dimensional linewidth correction are included. The EPR data confirm the postulated direction for strong exchange and provide a convenient method for evaluating the a.e. term. Previously reported single crystal EPR linewidths data for two other linear chain complexes, copper(II) benzoate trihydrate and copper(II) pyridine chloride, are also analyzed in terms of a.e. along the chain. In all three crystals, a.e. is comparable to the magnetic dipole–dipole interaction.
Dynamics of nuclear Overhauser enhancement in proton decoupled carbon-13 nuclear magnetic resonance
Authors: Opella Stanley J. , Nelson Donald J. , Jardetzky Oleg
Published: February 2003

The contribution of nuclear Overhauser enhancements to signal intensities in proton decoupled 13C nuclear magnetic resonance spectra can be determined by comparing the spectrum obtained with continuous proton irradiation to the spectrum obtained with a gated decoupler technique. The gated decoupler technique utilizes a sequence of acquiring data with the decoupler on followed by a delay time with the decoupler turned off; under appropriate conditions, it gives a fully decoupled spectrum with no nuclear Overhauser enhancements. The mathematical relationship describing the delay requirements for accurate intensity comparisons depends on the ratio of the acquisition and delay times to the longitudinal relaxation time of the carbon, as well as on the magnitude of the nuclear Overhauser enhancement.
The electronic and vibrational spectra of t
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s-[NiCl4(H2O)2]2− anions in crystalline Rb2NiCl4⋅2H2O and the effective symmetry of mixed-ligand nickel(II) complexes
Authors: Shankle G. E. , Bates J. B.
Published: February 2003

Raman, far infrared, and electronic absorption spectra have been recorded for the [NiCl4(H2O)2]2− anion in dirubidium tetrachlorodiaquonickelate(II), Rb2NiCl4⋅2H2O. The vibrational spectra are analyzed in terms of an effective anion symmetry sufficiently low to remove all eigenstate degeneracies, while the d-electron ligand-field band shapes do not reflect the low crystallographic site symmetry of the Ni2+ ion. The effect of low symmetry and magnetic ordering on the polarized intensities of some zero-phonon electronic fine-structure lines is discussed, and the energy separations between these zero-phonon origins and vibronic side bands are correlated with the observed anion normal modes of vibration.
The velocity of sound with derived state properties in helium at −175 to 150 °C with pressure to 150 atm
Authors: Gammon B. E.
Published: February 2003

The velocity of sound was determined in helium on 14 equally spaced isotherms from −175 to 150 °C and at pressures from 10 to 150 atm. The results were correlated with an abbreviated perturbation equation of state which employs both the Lennard-Jones (n,6) and the exponential repulsion potentials. Six parameters in the equation of state were adjusted to fit the velocity data within the experimental precision of about 0.001%. The derived results reliably predict literature (P,V,T) values and other velocity-of-sound data for pressures as high as 7000 atm and for temperatures from −223 to 577 °C. A value of the gas constant was derived, R=8314.79±0.35 J K−1⋅kmol−1, which is statistically consistent with the accepted value. The results also support a recent gas thermometry result that indicates the 1968 temperature scale may be in error by 0.012 K at −183 °C.
Relative spectral measurements of the entropy Rayleigh factor: Comparison with present theories
Authors: Beysens D.
Published: February 2003

Numerous theories have been put forward to express the Rayleigh factor of pure fluids. This factor is a complex function of many parameters, and it is not surprising to find that no decisive experimental verification exists. By means of scattered light spectral analysis, it is nevertheless possible to separate the isotropic entropy Rayleigh line from the other contributions. For relative measurements, the entropy line intensity is a function of only a few well known parameters, and it is therefore easy to check the various theories. These theories predict quite different refractive index variations; we have therefore performed the intensity measurements (by means of a 213 MHz free spectral range double-passed Fabry–Perot) on ten liquids whose refractive indices lie between 1.3 and 1.6. The results are in good agreement with the theories proposed by Rocard, by Vuks, and by Yvon, the latter two giving similar formulations, whereas the Einstein or the Fixman formulations show consistent discrepancies.
Measurement of ionic mobilities in dielectric liquids by means of concentric cylindrical electrodes
Authors: Allen Augustine O. , de Haas Matthijs P. , Hummel Andries
Published: February 2003

When a liquid, placed in an electric field between coaxial cylindrical electrodes, is ionized uniformly by a short pulse of x rays, the current resulting from the subsequent motion of the inward-moving ions decays with time differently from that due to the outward-moving ions. This effect provides a convenient method, here described in detail, for assigning mobilities to the positive and negative ions present. Mobilities were determined as a function of temperature for the ions formed in dilute solutions of the electron-trapping molecules SF6, CH3I, CH3Br, CH3Cl, or O2 in cyclohexane or 2,2,4-trimethylpentane. In all these cases the negative ions were more mobile than the positive. Only one major negative ion mobility and one major positive ion mobility was to be seen in the solutions, with the exception of O2 in 2,2,4-trimethylpentane, which gave two negative ions of different mobilities.
Detection of LiH and NaH molecular beams by laser fluorescence and measurement of radiative lifetimes of the A 1Σ+ state
Authors: Dagdigian P. J.
Published: February 2003

We report here detection of supersonic molecular beams of LiH and NaH using the tunable laser fluorescence technique. From measurement of the relative intensities of individual rotational lines in the excitation spectrum, beam rotational temperatures Trot=600±100 °K for LiH and Trot= 230±50 °K for NaH have been obtained. Direct measurement of the fluorescence decay of excited A 1Σ+ state molecules after pulsed laser excitation has allowed determination of the radiative lifetimes of several (v′,J′) levels. We find τ (v′=2,J′=3) =29.4±1.3, τ (v′=5,J′=3) =30.5±1.3, and τ (v′=7,J′=12) =36.9±1.9 nsec for 7LiH and τ (v′=8,J′=3) =22.7±1.6 nsec for 23NaH, where the quoted uncertainties represent three standard deviations. These values agree to within the ∼10% quoted experimental uncertainties with theoretical lifetimes calculated using ab initio electronic transition moments of Docken and Hinze (LiH) and Sachs, Hinze, and Sabelli (NaH).
Electron emission from methane, ammonia, monomethylamine, and dimethylamine by 0.25 to 2.0 MeV protons
Authors: Lynch D. J. , Toburen L. H. , Wilson W. E.
Published: February 2003

Absolute cross sections, differential in electron energy and emission angle, have been measured for ejection of electrons from methane, ammonia, monomethylamine, and dimethylamine by 0.25 to 2.0 MeV protons. Electron energy distributions are presented for 12 emission angles between 15° and 125°. The results indicate that emission cross sections for electrons ejected with energy greater than about 1 eV show no effects related to molecular structure for this group of molecules, within experimental uncertainty. The double-differential cross sections are found to be scalable in terms of the number of weakly bound electrons per molecule. Total ionozation cross sections are determined by integration of the double-differential cross sections with respect to energy and angle. The results are compared with the results for other low-Z polyatomic molecules, and also in the case of methane, with results from electron impact ionization.
Water and its relation to broken bond defects in fused silica
Authors: Stolen R. H. , Walrafen G. E.
Published: February 2003

We have studied the effects of water and defects on the Raman spectrum of fused silica. Intensities of defect lines at 604 and 490 cm−1 were found to increase with fictive temperature and to decrease with increasing OH content. The results suggest that water in the melt is preferentially trapped at the defect sites. The increased sensitivity obtained by using an optical fiber as the sample enabled us to see for the first time the Si– (OH) stretching vibration at 970 cm−1. The Si–(OH) intensity agrees with that of the 1100 cm−1 dangling oxygen line in a silica fiber weakly doped with K2O. A comparison with other spectral features shows that the Raman cross section for the Si–O stretch is extremely weak. This weak cross section contributes to the weak Raman intensity of the 1100 cm−1 lines in both fused and crystalline SiO2 and also explains why the dangling oxygen stretching vibration is not observed by Raman scattering in neutron damaged silica. Measurements of the much stronger 3690 cm−1 OH stretching vibration in a series of bulk samples of known OH content suggest this line as a useful measure of OH in fibers. In addition we find a water induced low frequency (30 cm−1) band which is weakly Raman and strongly infrared active.
Many-body perturbation theory applied to molecules: Analysis and correlation energy calculation for Li2, N2, and H3
Authors: Freemen David L. , Karplus Martin
Published: February 2003

The correlation problem is analyzed in terms of Goldstone diagrammatic perturbation theory. A hole-line expansion for the correlation energy is defined and used with matrix partitioning techniques to determine the diagrams contributing to various forms of pair theory and to configuration interaction treatments of the usual type. The presence of certain terms in the double excitation configuration interaction formulation that cancel in higher order is demonstrated. The nature of various approximations to the correlation correction is determined. To illustrate the analysis, certain of the approximations are used in correlation energy calculations with multicenter Slater basis sets on Li2, N2, and H3. Comparison with complete configuration interaction calculations are made for Li2 and H3; the diagrammatic calculation, which is much simpler than a full CI treatment, is found to be a good approximation to the latter.
The clustering of O2 and He to Li+
Authors: Colonna-Romano L. M. , Keller G. E.
Published: February 2003

The clustering of O2 and He to Li+ has been studied using a drift tube–mass spectrometer at 319 K. For O2 clustering, the rate coefficients for cluster ion formation and collisional dissociation were measured as a function of E/N. For clustering to He, only an equilibrium constant was determined. The relative importance of the clustering of several atmospheric gases to Li+ in the ionosphere is discussed.
Monte Carlo study of systems of linear oligomers in two-dimensional spaces
Authors: Okamoto Hiroshi
Published: February 2003

The statistical mechanical properties of systems of linear r-mers (r=4, 5, and 6) in a square lattice space and of those in a two-dimensional continuous space were studied by Monte Carlo simulations. In the lattice space systems, the site fractions of the 4-mers ranged from 0.08 to 0.75, those of the 5-mers from 0.075 to 0.800, and those of the 6-mers from 0.12 to 0.667. In the continuous space systems, the concentrations were in more narrow ranges. In the lattice space systems, statistical quantities of finite systems were extrapolated to those of infinite systems. Pressures or osmotic pressures of the lattice space systems were close to the Huggins and the Miller–Guggenheim theoretical ones, while pressures of the continuous space systems, in dilute concentrations ranges, were better described by the Flory theory. The occurrence of each conformation of the r-mers was random in dilute concentration ranges but not in dense ranges. The nonrandomness was much increased in the continuous space systems, especially in the case of the 4-mers. No sign of the cooperative aligning of the r-mers was found in the lattice space systems.
Evidence for trapped-exciton fluorescence in anthracene crystals at room temperature
Authors: Kalinowski J. , Godlewski J. , Chance R. R.
Published: February 2003

The fluorescence variations due to imposed exciton–charge carrier interactions in anthracene are examined as a function of emission wavelength. In the short wavelength region of the emission spectrum, the observed modulation of the fluorescence intensity due to the presence of trapped charge carriers is relatively small and, with the exception of the highly reabsorbed fluorescence at very short wavelengths, is independent of emission wavelength. At longer wavelengths, however, a series of maxima develop along with a tenfold increase in the modulation. These observations are interpreted in terms of emission from trapped singlet excitons. Though the results are consistent with those expected with a discrete exciton trap at 0.43 eV, they may also be interpreted as indicating a series of exciton traps with depths in the range 0.4–1.0 eV. Results are also presented for tetracene-doped anthracene crystals which support the hypothesis that the traps are perturbed molecules of the host crystals, rather than impurities.
Single-rotational-level lifetimes from measurements of linewidths in the B3Π+←X1Σ+ system of ICI
Authors: Olson C. D. , Innes K. K.
Published: February 2003

Lines in the 3−1 band of the B3Π0+←X1Σ+ system of ICl vapor havew been recorded using a Fabry–Perot interferometer spectrometer. A striking increase in linewidths is observed for transitions ending on excited-state rotational levels above J′=37. Even at low J′, the linewidths are approximately twice the expected value. Examination of the probable causes of the broadening in the spectrum points to shortened excited-state lifetimes and yields 3×10−11 sec for the lifetime of I35Cl in the J′=41 level and about 2×10−10 sec for J′ values below 20. These may be compared with the radiative lifetime, 9.8×10−7 sec, obtained from areas of some of the profiles measured here. The observed short lifetimes are discussed in terms of a homogeneous predissociation suggested also by earlier workers. The data require that the outer limb of the potential energy curve of the B state interact with an unstable O+ state. In addition it is suggested that a weaker, heterogeneous predissociation contributes to the widths for levels of low J′.
Uniform reduced interaction dipole for all rare gas pairs
Authors: Bar-Ziv Ezra , Weiss Shmuel
Published: February 2003

A large body of translational absorption spectra of the rare gases was used to evaluate a uniform reduced interaction dipole for all rare gas pairs. This dipole, obtained through the use of realistic intermolecular potentials, was free from artifacts which distorted previous dipoles, obtained from cruder models. Interesting correlations were found between individual dipole strengths and molecular properties. Similarities between the dipole and potential functions were noted.
The Hugoniot and shock initiation threshold of lead azide
Authors: Davies F. W. , Zimmerschied A. B. , Borgardt F. G. , Avrami Louis
Published: February 2003

The Hugoniot of unreacted dextrinated lead azide has been determined using plate impact techniques in a 2.5 in. bore gas gun. The test specimens were compacted to a density of 3.4 gm/cc. Piezoresistive gauges were used to determine both the impact stress and the stress profile propagated through the sample. Each shot provided transmitted wave data for four sample thicknesses. The Hugoniot is linear up to 10 kbar. It can be represented by the equation σ=41.7 Up (σ in kilobars and Up in mm/μsec). The initiation threshold to long duration (3.5 μsec) shocks was 6.0 kbar based on evidence of reaction in the explosive inferred from increases in stress with propagation distance. Changes of wave speed were found to be noticeable only at higher stresses. Above 6 kbar the delay in initiation decreases with increasing stress levels. The initiation sensitivity to short duration (0.1 μsec) shocks was determined by impacting the lead azide with thin flyers accelerated by exploding foils. The impact velocity was determined by observation with a high-speed framing camera and the transmitted stress profiles were measured by x-cut quartz gauges. The short-pulse initiation threshold for polyvinyl lead azide is 4.2 kbar for a density of 3.6 gm/cc, and 2.1 kbar for dextrinated lead azide for a density of 2.9 gm/cc. Comparison of initiation threshold measurements suggests that there is a minimum thickness, or a run-up distance, before detonation occurs which is independent of pulse width with stresses up to 10 kbar. For long pulse (3.5 μsec) experiments with the gas gun, no evidence of detonation was detected for 1 mm thick samples. Detonation occurs with run distances in the range of 1–2 mm for impact stresses of 8.9 kbar. For stresses greater than 6.0 kbar, evidence of detonation was noted after a 2 mm run. In the thin flyer plate experiments at stresses of 8 kbar dextrinated lead azide displayed a 2 μsec initiation delay. Close to the initiation threshold the stress profiles in the explosive show a gradual transition from an unreactive shock to a stable detonation. It is postulated that reaction occuring behind the shock front produces pressure waves which travel through the explosive interacting with the unreacted explosive ahead of the reaction front causing a nonuniform rate of growth of the shock. This behavior is similar to that observed for heterogenous secondary explosives. The wave speed of the initial wave front propagating in the explosive changes abruptly from that appropriate to unreacted material to the detonation velocity.
Static and dynamic structure factors for interdimensional systems. I. Chainlike crystals
Authors: Gavish B. , Imry Y.
Published: February 2003

The displacement–displacement space–time correlation functions and static and dynamic structure factors are calculated for a chainlike harmonic lattice. The continuous transition from 3D to 1D behavior of the various quantities as functions of the interchain coupling is analyzed. Criteria for probing the 1D behavior are obtained along with suggestions and predictions for relevent scattering experiments. Existing experimental results are found to be compatible with the theory.
Discussion of Clarke’s ’’graph theoretic’’ method: Application—the Rayleigh–Bénard instability of a binary mixture
Authors: Maltman Kim , Laidlaw W. G.
Published: February 2003

The ’’graph theoretic’’ method of Clarke is analyzed with a view to application to stability problems other than chemical reactions. The extended version—essentially the extension of the vertex aspects of the method is applied to the analysis of the Rayleigh–Bénard (convective instability) for a binary mixture.
Absolute unimolecular decay rates of energy selected metastable halobenzene ions
Authors: Baer Tomas , Tsai Bilin P. , Smith Deborah , Murray P. Terrence
Published: February 2003

We have measured the absolute rates of X loss from metastable, energy selected, C6H5X+ (X=Cl, Br, I) ions. A photoion–photoelectron coincidence technique was used to collect time of flight distributions of fragment ions formed from precursor ions with selected internal energies. The measured rates, which are between 6×104 and 3×106 sec−1, are compared with predictions of the statistical theory of unimolecular reactions. Over the energy range covered (0.7 to 1.7 eV above the reaction threshold) the agreement between experiment and theory is excellent. This agreement is in distinct contrast to a similar investigation of the rates of C4H6+ fragmentation which, at the reaction threshold, were three orders of magnitude faster than predicted.