bluuebunny/crossref_metadata_2025 huggingface dataset:
Porphyrins. XXXII. Absorptions and luminescence of Cr(III) complexes
Authors: Gouterman Martin , Hanson Louise Karle , Khalil Gamal-Eddin , Leenstra Willem R. , Buchler Johann W.
Published: February 2003

Chloro−(tetraphenylporphinato)chromium(III) shows relatively normal visible (Q) bands in the region 610−500 nm and a relatively normal Soret (B) band at 447 nm; in addition, there are prominent bands in the near uv region 400−300 nm (2×104 < ε < 3×104 M−1⋅cm−1) and clear near ir bands in the region 820−610 nm (300 < ε < 2100 M−1⋅cm−1). At 160°K there is one main emission band at ∼815nm (τ < 6 μsec); at 77°K there are emission bands of comparable intensity at both 811 and 847 nm (τ = 44 μsec); at 4°K both emission bands appear with intensity ratio I811/I847 = 0.12. The near ir absorption and emission band at 811 nm is attributed to a ’’trip−quartet’’ excited state formed from the (π,π*), S = 1 porphyrin state coupling to the d3, S = 3/2 electrons of the Cr(III); the 847 nm emission is attributed to the sextet state that arises from the same electron configuration. The prominent near uv bands are attributed to a2u(π), a1u(π) → eg(dπ) charge transfer transitions. Phenol−phenoxo− (octaethylporphinato)chromium(III) shows similar absorption and emission spectra, but the bands are generally blue shifted.
A collision cross section study of the 1 1S → 2 1P and 1 1S → 2 1S transitions in helium at kinetic energies from 200–700 eV. Failure of the Born approximation at large momentum changes
Authors: Dillon Michael A. , Lassettre Edwin N.
Published: February 2003

Differential collision cross sections have been determined for the 1 1S → 2 1S and 1 1S → 2 1P transitions in helium at scattering angles within the range 7.5°−35° and kinetic energies of 200, 300, 400, 500, and 700 eV. Multiple scattering errors have been eliminated by determining the ration of inelastic to elastic cross section as a function of pressure (the dependence is linear) and extrapolating to zero pressure. The error in the absolute cross sections is less than 5% in most instances. Where large corrections for multiple scattering are necessary the error bound is 6%. Generalized oscillator strengths (f) have been calculated from the data for both transitions and compared with the Born approximation calculations (fB) of Kim and Inokuti [Phys. Rev. 175, 176 (1968)]. At a scattering angle of 30° the ratio fB/f (which would be unity if the Born approximation were accurate) decreases from the value 0.86 at 200 eV to 0.16 at 500 eV for the transition 1 1S → 2 1P. At smaller values of the squared momentum change (between 0.5 and 2.5 a.u.) deviations from the Born approximation are still significant (about twice our experimental error) and approach to the Born limit is very slow, imperceptible in fact from 300 to 700 eV. Despite the poor agreement between the differential cross sections measured by us and those calculated from the Born approximation, we have found excellent agreement between the integrated (over angle) cross sections calculated from our measurements and those obtained from the Born approximation (with a small correction for exchange). The reasons for the above observations are discussed, where feasible, and comparisons are made with the experimental the theoretical results of other investigators.
Frictional properties of dilute polymer solutions. I. Rotational friction coefficient
Authors: Felderhof B. U. , Deutch J. M.
Published: February 2003

The theory of Debye and Bueche for the frictional properties of dilute polymer solutions is placed on a microscopic basis. It is shown that the microscopic foundations for the theories of Debye−Bueche and Kirkwood−Riseman are identical, but that the theories differ in their statistical analysis. The Debye−Bueche equations are applied to the rotational friction coefficient of a spherically symmetric polymer with arbitrary radial density distribution. An exact result is derived for a Gaussian distribution of low density. A variational principle of minimum energy dissipation is formulated which is suitable for numerical work.
Perturbation theory for short-range atomic interactions. II. The radius of convergence
Authors: Certain Phillip R. , Merkel Robert R.
Published: February 2003

The perturbation energy series of Byers Brown and Power, applied to the case of one−electron diatomics, is analyzed for various internuclear separations to obtain the radius of convergence. The convergence of the series is limited by a branch point on the real axis for nonphysical values of the perturbation parameter.
Recombination of iodine atoms by flash photolysis over a wide temperature range. VI. I2 in HCl, DCl, HBr, and SO2
Authors: Chang H. W. , Burns George
Published: February 2003

Recombination of iodine atoms in an excess of HCl, DCl, HBr, and SO2 was studied over a wide temperature range using flash photolysis technique. The following data were obtained: logkHClr = 10.287 − 2.798 log(T/300), (300−1000°K); logkDClr = 10.193, (at 300°K); logkHBrr = 10.274 − 2.956 log(T/300), (300−570°K); logkSO2r = 10.272 − 2.574 log(T/300), (300−1070°K). The results suggest that the recombination is dominated, at least at lower temperatures, by the radical−molecule complex mechanism, which requires that the iodine atom forms a weakly bound complex with the third body, i.e., HCl, DCl, HBr, and SO2. In the case of iodine recombination in SO2, simple van der Waals forces appear to contribute to stability of the I−SO2 complex. In the case of iodine recombination in hydrogen halides, the intermediates I−(HX) involved in the reaction appear to be unstable species, in which iodine atoms are interacting with both atoms HX, rather than stable bihalide radicals IHX observed by matrix isolation technique at low temperature. Hydrogen bonding, as well as weak chemical interactions between iodine atom and halogens in hydrogen halides, may contribute to stability of I−(HX) intermediates.
Infrared laser enhanced reactions: Temperature resolution of the chemical dynamics of the O3† + NO reaction systen
Authors: Kurylo Michael J. , Braun Walter , Xuan Chieu Nguyen , Kaldor Andrew
Published: February 2003

The rate constant for the decay of vibrationally excited ozone, O3†, in the O3† + NO reaction system has been measured from 153 to 373 K. Vibrationally excited O3 was produced in the asymmetric stretch normal mode by absorption of square wave modulated emission from a CO2 laser tuned to the P (30) 9.6 μm transition. Under appropriate experimental conditions, a rapid V→V coupling process involving all three normal modes of O3 is believed to set up a Boltzmann population distribution among them. Reaction or relaxation of O3+ out of this subset of normal modes is observed to proceed through a weighted average of rate constants. From the effects of temperature and buffer gas pressures an assessment can be made as to the predominant loss mechanism for the various modes. While there are three separate convolution schemes which appear to fit our data, we are persuaded to emphasize one whereby all three modes contribute via a reaction channel described by kD = (2.0×10−11) exp(−1525/T) cm3 molecule−1⋅sec−1 while ν2 alone is active in a V→T relaxation process given by kA = (1.0×10−13) exp(−39.2/T cm3 molecule−1⋅sec−1. A comparison of the Arrhenius parameters for the reaction channels of O3† with parameters for the corresponding processes involving thermal O3 yields specific information about the effect of vibrational energy on the reaction dynamics.
A time-dependent variation–perturbation method for the calculation of transition properties and its relation to the random phase approximation
Authors: Huo Winifred M.
Published: February 2003

A time−dependent variation−perturbation (VP) formulation is presented for the simultaneous calculation of the ground state correlation function and the excited state wave function of an atomic or molecular system. The wave functions are chosen for the optimal calculation of transition properties with a relatively small number of molecular integrals. The VP equations so obtained for a singlet system are identical with the equations in the random phase approximation (RPA). On the other hand, if the ground state is a singlet and the excited state a triplet, the VP and RPA equations are different. The latter assumes a ground state correlation function with an open shell component, which is a poor approximation for a closed shell ground state. The orthonormalization condition in the VP scheme is different from that in the RPA. The consequence of this difference is discussed. It is also pointed out that the RPA excitation energy actually contains part of the ground state correlation energy; hence the RPA excitation energy is always lower than the corresponding values in the Tamm−Dancoff approximation and the self−consistent RPA. A variational formulation of the time−dependent problem gives a set of equations which resembles the higher RPA equations.
Calculation of the lattice modes of the isotopic CO2 molecules and their reduced partition function ratios
Authors: Lee Myung W.
Published: February 2003

The lattice dynamics of a unit cell with four CO2 molecules have been calculated from the Raman, infrared, and inelastic neutron scattering spectra. The vibrational frequencies of the isotopic 12C16O2, 13C16O2, 12C18O2, and 12C16O18O are calculated in a pseudoharmonic oscillator approximation. From these vibrational frequencies and the gas phase frequencies, the reduced partition ratios of the various isotopic CO2 molecules are calculated in good agreement with experiment. The lattice dynamics calculation are also compared with calculation from a simple cell model. It is shown that the simple cell model is an excellent approximation for the calculation of the reduced partition function ratios as a consequence of the large separation between the eigenvalues of the internal and external modes, the magnitude of the external eigenvalues relative to kT, and the symmetries of the lattice and the molecule.
Phase transition in metal hexammine complexes. II. The EPR spectra of Ni(NO3)26NH3 and Ni++ doped Zn(NO3)26NH3 and Cd(NO3)26NH3
Authors: Ochi J. A. , Sano W. , Isotani S. , Hennies C. E.
Published: February 2003

The cooperative effect of the ammonias in X(NO3)26NH3 (X = Ni,Zn,Cd) was investigated by EPR. In Ni(NO3)26NH3, the linewidth of the single EPR absorption line undergoes a sudden broadening at Tc = 243 K. This effect is probably a result of a cooperative freezing of the degrees of freedom of rotation of the ammonias, giving the appearance of a crystal field on the Ni++ ions. The crystal field does not split the single line, probably because strong exchange effects assemble the lines. The crystal field parameter D = 0.425 cm−1 was evaluated by applying the Anderson and Weiss theory for the exchange narrowing of the linewidth. Small quantities of the Ni++ ions in Zn and Cd salts do not exhibit strong exchange effects. So the single EPR absorption line above Tc goes to a split line below Tc. For Ni: Zn(NO3)26NH3, Tc = 231 K and D = 0.606 cm−1. For Ni: Cd(NO3)26NH3, Tc = 198 K and D ? 0.3 cm−1. Hysteresis in Tc was also observed for all salts.
Molecular theory of dielectric relaxation
Authors: Sullivan D. E. , Deutch J. M.
Published: February 2003

A new formal relation is obtained for the frequency−dependent dielectric constant of a polar material in terms of a dipole ’’memory function,’’ i.e., a modified correlation function with projected propagator. In contrast to conventional expressions for the dielectric constant in terms of dipole−dipole correlation functions, the memory function expression does not depend on sample shape. The memory function may thus be calculated either by considering the total dipole moment of a molecular sample in vacuum or the dipole moment of a smaller region embedded in the sample. It is shown that this formalism agrees with usual expressions for the dielectric constant and particular relations between the dielectric constant and dipole correlation functions of embedded regions.
Properties of solutions to the Yvon–Born–Green equation for the square-well fluid
Authors: Lincoln William W. , 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, for a temperature−density range bounded by 0 < ϑ < 0.55 and 0 < λ0 < 13. Solutions of the YBG equation throughout this range were found to be unique, and upon examination of the resulting thermodynamic properties, a gas−liquid coexistence region was located and evidence for a high−density, low temperature fluid transition was found. Relative to the range of ϑ − λ0 considered, the down−range properties of the pair−correlation function were used as a guide in carrying through a formal analysis of the YBG equation using basic theorems on the existence and uniqueness of solutions of nonlinear integral equations. Lower bounds on the limit of stability of the pure gas and liquid phase were identified and correlated with the thermodynamic behavior determined via numerical solution of the YBG equation.
Electric field gradient tensors in CuBr2
Authors: Morgen P. , Filho W. Wolney
Published: February 2003

The NQR spectrum of polycrystalline CuBr2 has been obtained at 20°C. Absorption was detected for 65Cu (28.18±0.01 MHz), 63Cu (30.37±0.01 MHz), 81Br (70.10±0.05 MHz), and 79Br (83.98±0.05 MHz). The halogen resonances are discussed in the light of the theory of Townes and Dailey without neglecting the contribution of ionic charges and antishielding. In this approach the electric field gradient tensor is seen to depend on the ionicity of the Cu−Br bonds and on the value of the antishielding factor of the Br− ions. The relation between the two parameters for the cases, where the experimental frequencies are reproduced, is derived. With this relation the antishielding factor calculated by Sternheimer would indicate an ionicity of 90%, while in the purely ionic limit an antishielding factor of 93.8 must be assumed. The presence of finite amounts of covalency in the bonds is evidenced by the copper resonances. Thus, the observed coupling strength for the copper ions is only about 50% of the value observed for a number of octahedrally coordinated Cu2+ compounds.
Study of the structure of molecular complexes. X. Dipole moment surface for the lithium–fluoride–water complex
Authors: Chan E. R. , Clementi E. , Kress J. W. , Lie G. C.
Published: February 2003

The dipole moment for 252 configurations of the Li−F−H2O complex has been computed using Hartree−Fock wavefunctions. The computed values are fitted with an analytical expression. The results clearly indicate that the water dipole moment, having the expected form of point charge−induced dipole moment, acts only as a relatively small perturbance on the dipole moment of the LiF ion pair.
Electron propagator calculations of the photoelectron spectrum for open shell molecules with applications to the oxygen molecule
Authors: Purvis George D. , Öhrn Yngve
Published: February 2003

An approximate self−energy for the electron propagator is derived using the Grand Canonical averaging procedure for open shell molecules. The connection between the overlap amplitudes and the photoionization cross section is discussed. A direct pole and residue search is employed to solve the Dyson equation with a second order self−energy for the oxygen molecule, yielding a theoretical photoelectron spectrum in good agreement with experimental x−ray induced spectra.
Production efficiencies of electronically excited states of barium monohalides
Authors: Bradford R. S. , Jones C. R. , Southall L. A. , Broida H. P.
Published: February 2003

Diatomic barium monohalides (BaX) have been produced in the gas−phase reaction of Ba, entrained in a flowing inert carrier gas, with F2, SF6, Cl2, Br2, and I2. Bright green chemiluminescent flames were observed at pressures from 0.1−10 torr. Emission spectra were obtained from 300−1200 nm. Near infrared emission from BaBr and BaI has been ascribed to transitions to the ground state from previously unobserved electronic states analogous to A and B states in BaF and BaCl. In BaBr, electronic state energies for B 2Σ+, A 2Π3/2, and A 2Π1/2 have been found to be 11 325, 10 604, and 9980 cm−1, respectively; in BaI, corresponding values are 10 417, 9921, and 9268 cm−1. BaX molecules are produced with moderate efficiency into lower lying electronic states (80%−98% in A and B) with an absolute efficiency of approximately 1 photon for each 100 reacted barium atoms. There is a small dependence on pressure with maximum yields occuring near 4 torr. Spectra of C 2Π−X 2Σ+ transitions show Boltzmann distributions in both vibration and rotation corresponding to population temperatures of approximately 3000 K for BaF and 2000 K for BaBr, BaCl, and BaI. Electronic population temperatures show trends similar to those observed in vibration and rotation.
Electron paramagnetic resonance investigation of NO3, NO32−, and O2− in irradiated NH4NO3
Authors: Jarke Frank H. , Ashford Nicholas A.
Published: February 2003

The ESR spectrum of NH4NO3 after γ irradiation at 77°K is reported. Resonances assigned to NO32−, NO3, and O2− were observed. No NH4 related radicals were found. (AIP)
Dynamics of chain molecules. II. Spectral distribution of the light scattered from flexible macromolecules
Authors: Perico Angelo , Piaggio Paolo , Cuniberti Carla
Published: February 2003

The separate contributions to the spectral density of the light scattered from a flexible macromolecule in solution have been calculated on the basis of the bead and spring model with hydrodynamic interaction. The interaction enhances the effect of the intramolecular motions with respect to the translational diffusion. A hypothetical homodyne spectrum has been calculated for a chain length of 1500 Å, and the resulting linewidth and height are larger than in the free-draining case.
Rotational transitions in molecule–solid scattering
Authors: Wolken George
Published: February 2003

A quantum mechanical calculation of the collision of H2 with the (001) face of LiF has been carried out and compared with recent experimental data in which rotational transitions were resolved. A simple model potential accounts quantitatively for many of the observed effects. Refined potential parameters are extracted and the effects of some of these parameters on rotational transitions are discussed.
Solid SiH4: Structure and orientational order
Authors: Sears W. M. , Morrison J. A.
Published: February 2003

A powder diffraction study of the two solid phases of SiH4 indicates the structures to be body-centered tetragonal, which suggests that orientational ordering of the molecules is extensive in both phases. The lattice spacings at T = 77 °K of Phase I [63.45 °K<T (melting) = 88.5 °K] are a = 12.5 Å and c = 14.2 Å. General arguments based on the structural data and published optical data lead to the conclusion that the most probable space groups for Phase II (T<63.45 °K) are I4̄, I4, or I4/m. The suggested structures of both phases are consistent with their optical birefringences, which were also measured.
Studies of inelastic molecular collisions using impact parameter methods. II. Exact and high-energy approximate calculations
Authors: Fisanick-Englot Georgia , Rabitz Herschel
Published: February 2003

Exact quantum mechanical impact parameter methods have recently been extended to include inelastic collisions through the use of an effective Hamiltonian. In this paper the nature of the impact parameter formulation will be explored through specific examples. First we consider the case of a calculation for He–para-He2 where the exact impact parameter amplitudes have been computed as a Neumann series of transition matrix elements obtained from an effective Hamiltonian close coupling calculation. The purpose of this calculation is to provide a standard of functional behavior for comparison with amplitudes computed by approximate methods. Such approximate calculations were performed for Ar–TlF scattering using both exponential and fractional high-energy forms for the amplitudes. The two approximations are compared in detail, and it is found that the cross sections resulting from the exponential approximation show reasonable agreement with other calculations for the system using conventional (noneffective) close coupling and various sudden approximations. Ranges of validity for the approximations are discussed and general characteristics of the impact parameter amplitudes and the related cross section integrands are summarized. Comparison is also made with model system calculations discussed in a previous publication. A general procedure is presented for smoothing the highly oscillatory cross section integrands that commonly arise in exponential impact parameter treatments.
Fourier transform NMR in liquids at high pressure. III. Spin–lattice relaxation in toluene-d8
Authors: Wilbur D. J. , Jonas J.
Published: February 2003

The spin–lattice relaxation time (T1) of both ring and methyl deuterons have been measured in toluene-d8 by the Fourier transform inversion recovery method. Measurements were made over the temperature range −35 to 200 °C, and over the pressure range 1 bar to 4 kbar. Viscosities and densities were also measured over most of this range. The T1 data are analyzed in terms of correlation times for isotropic overall molecular reorientation and internal rotation of the methyl group. Using pressure as a variable enables us to separate effects of density and temperature on overall and internal motion of the toluene molecule. Activation energies and activation volumes are reported. The experimental data are interpreted in terms of various theoretical models for the motional dynamics. The rotational diffusion model and the Enskog theory are used to discuss overall motions. For the case of internal motion of the methyl groups, the Langevin model is also considered. Large angle reorientation appears to be important in the overall motion of the toluene molecule at temperatures above 50 °C, whereas the motion of the methyl group is characterized by large angle steps even at lower temperatures. At high temperatures, the motion of the methyl group approaches that of a free rotor.
Theory of vibrational relaxation in the inverting molecules, NH3 and ND3
Authors: Starr Duane F. , Decius J. C.
Published: February 2003

The exceptionally rapid vibrational energy relaxation in ammonia has led to the speculation that the inversion motion is responsible for the higher rate of collisional energy exchange than in related molecules. Exact numerical solution of the scattering equations has been carried out utilizing an atom–atom exponential repulsive potential for one dimensional collisions, with emphasis upon the comparison of models whose internal energy is described by (a) an ordinary harmonic oscillator (noninverting model) and (b) the states that accurately describe the vibrational motion for double minimum potential (inverting model). Although the transition probabilities for the 0+, 0− and 1+, 1− states are high at thermal energies, such transitions do not make a large contribution to the relaxation time found from acoustic experiments. The speed of the observed processes is attributed principally to (a) the ’’acceleration’’ effect which is a consequence of the intermolecular potential of polar molecules and (b) the high impact velocities of rotating molecules with low moments of inertia.
Ferroelectricity in Tl2MoO4 and Tl2WO4
Authors: Sleight A. W. , Bierlein J. D. , Bierstedt P. E.
Published: February 2003

Ferroelectric behavior has been discovered in Tl2MoO4, Tl2WO4, and their solid solutions. The Curie temperature is 38 °C for the molybdate and 11 °C for the tungstate. Transitions at intermediate temperature are observed in the solid solution. This is a first-order transition of the type 6/mmmFmm2(s); thus, it is fully ferroelectric and partially ferroelastic. Other transitions in Tl2MoO4 and Tl2WO4 at higher temperatures were also examined.
On infinite order sudden approximations for an arbitrary potential energy
Authors: Hunter L. W.
Published: February 2003

The infinite order sudden approximation (IOSA) for rotational excitation, introduced by Pack and co-workers, is extended to arbitrary potential energies in an independent derivation of results also obtained by Secrest by another route. Close connections are established with the collision theory of Curtiss and co-workers. In addition, an IOSA for general rotating and vibrating molecules is presented.
An exploratory study of reactant vibrational effects in CH3 + H2 and its isotopic variants
Authors: Chapman Sally , Bunker Don L.
Published: February 2003

An empirically calibrated potential energy surface, obtained previously [J. Chem. Phys. 61, 21 (1974)], was used in a trajectory study of the effect of reactant energy partitioning on cross section for CH3 + H2 → CH4 + H. Artificial variations in the barrier position, and artificial and naturally occuring changes in the H masses, were also made. For natural CH3 + H2, H2 vibration enhances and CH3 out-of-plane bending depresses the cross section, at constant (25 kcal) translational energy of approach. The isotope substitution and barrier position effects are more complex and not predictable from triatomic A + BC generalizations. The relationship of these results to experimental ones is discussed.
Absolute unimolecular decay rates of energy selected C4H6+ metastable ions
Authors: Werner Arthur S. , Baer Tomas
Published: February 2003

The rate constants for the unimolecular decomposition of energy selected C4H6+ ions have been measured using the recently developed technique of zero kinetic energy (ZKE) photoelectron–photoion coincidence spectroscopy in conjunction with time-of-flight (TOF) mass spectrometry. The C4H6+ ions were prepared from 1,3-butadiene, 1,2-butadiene, cyclobutene, 2-butyne, and 1-butyne by photoionization. Absolute rate constants for unimolecular decay were determined from the TOF distributions of mass selected C3H3+ daughter ions. For parent metastables prepared with energies between 0.05 and 0.5 eV above the appearance potentials for the lowest fragmentation process, decomposition to C3H3+ and CH3, total unimolecular decay lifetimes were in the range 0.5–4 μsec. The experimental TOF decay curves were found to be exponential, with rate constants increasing monotonically with increasing excitation energy. These measurements also indicate that parent C4H6+ metastables rearrange to a common structure regardless of the neutral precursor. A comparison of the experimental rate constants with the predictions of the quasiequilibrium theory (QET) reveals a discrepancy of over two orders of magnitude near the dissociation threshold. The evidence suggests that the large experimentally determined threshold rate of 105 sec−1 is not associated with a kinetic shift, which was found to be negligible. In addition to the lifetime studies, ZKE photoelectron spectra, photoelectron–photoion coincidence spectra, and breakdown curves obtained directly from the coincidence measurements are presented for all five C4H6 isomers.
Criterion for excess electron localization in liquid alkanes
Authors: Fueki Kenji , Kevan Larry
Published: February 2003

A criterion for electron localization in alkanes, base on transient fluctuations in the liquid leading to cavities of molecular size, is developed and shown to correctly predict localization, and conversely quasifree behavior, for those alkanes including methane for which the conduction electron energy has been measured.
Electric field deflection of molecules with large amplitude motions
Authors: Bernstein Lawrence S. , Pitzer Kenneth S.
Published: February 2003

The Stark effect is derived for several plausible models for molecules such as XeF6 and IF7 which are believed to show pseudorotation or other large-amplitude internal motion. These results are related to the behavior of these molecules in electric-field-deflection molecular beam experiments. The absence of focusing of XeF6 is compatible with an effective dipole moment of distorted XeF6 of up to 1.2 D for free pseudorotation but only 0.2 D if pseudorotation is restricted by a potential barrier with C4v orientation 770 cm−1 above C3v. Additional examples are treated for XeF6 as well as models appropriate to the MX7 molecules.
Liquid–vapor asymmetries in pure fluids
Authors: Mulholland George W. , Zollweg John A. , Sengers J. M. H. Levelt
Published: February 2003

The coexistence curve and the compressibility-like quantity (∂ρ/∂μ)T at the coexistence curve are calculated for Mermin’s decorated lattice gas. We tabulate the Padé approximants to the Ising model properties which were used in these calculations. It is shown that for the proper choice of the adjustable parameter in the model the asymmetries in the coexistence curve and in (∂ρ/∂μ)T over a large temperature range, Tc/2 < T < Tc, are qualitatively similar to those in real fluids. Also, similar correlations between the overall coexistence curve asymmetry and the amplitudes of divergences for the critical region occur in the model and in real fluids. Based on our model calculations, several predictions for fluid behavior near the critical point are listed: (1) The asymptotic asymmetry in the coexistence curve and in (∂ρ/∂μ)T at the coexistence curve should be most observable for fluids with large overall asymmetry such as SF6, NH3, and H2O; (2) the range of the asymptotic asymmetry for (∂ρ/∂μ)T is expected to extend somewhat beyond t = 1 − (T/Tc) = 0.1 for such fluids; (3) in order to obtain an estimate for the compressibility exponent γ′ that is within a few percent of the true asymptotic value, experiments will have to be confined to the range t < 10−2. Recent experimental data for (∂ρ/∂μ)T at the coexistence curve are found to be consistent with the model for t ≳ 10−2, but are inconclusive closer to the critical point.
19F relaxation in paramagnetic O2AsF6
Authors: Rigny P. , Falconer W. E.
Published: February 2003

The properties of the fluorine spins of O2AsF6 are dominated by the coupling with dioxygenyl unpaired electrons. Study of the longitudinal relaxation shows that a phase transition takes place at 255±3°K. Motions of the dioxygenyl cations are shown to take place and are discussed.
Electronic spectroscopy of propadiene (allene) by electron impact
Authors: Mosher Oren A. , Flicker Wayne M. , Kuppermann Aron
Published: February 2003

The electron impact excitation of propadiene (allene) has been studied experimentally at impact energies of 20, 40, and 60 eV and scattering angles from 6° to 80°. Two transitions with maxima at 4.28 and 4.89 eV are identified as singlet → triplet excitations. The magnitude of the splitting between these transitions is a measure of the interaction between the two perpendicular π molecular orbitals. The significance of these triplet excited states in the interpretation of previous electronic energy transfer experiments and sensitized photochemical studies is briefly discussed. Two very weak transitions are observed between 5.0 and 6.5 eV. The angular dependence of the corresponding cross sections indicates that these are spin-allowed but symmetry-forbidden transitions. A weak singlet → singlet transition is seen with a maximum at 6.74 eV. This feature has also been optically observed and attributed to an electric dipole-allowed ? 1A1→1 1E transition. This assignment is discussed in the light of the present results. The first strong transition, the ? 1A1→1 1B2 π→π* transition, appears with a maximum at 7.24 eV. Higher energy-loss features between 7.95 and 10 eV probably involve excitations to Rydberg states. A previously unreported transition to a superexcited state is observed with a maximum at 11.25 eV.
Molecular complexes of I2: A Mössbauer effect study
Authors: Bukshpan S. , Goldstein C. , Sonnino T. , May L. , Pasternak M.
Published: February 2003

Molecular complexes of I2 with different organic solvents, in frozen solution, were studied using Mössbauer effect in 129I. The results obtained for the hyperfine constants indicate that the complexes may be divided into three main groups with increasing bond strength, according to the organic solvent used: (a) π solvents, (b) alcohols and ether, and (c) amines. The same classification was also used in the analyasis of various properties of the I2 complexes. Some general conclusions on the electronic nature of the I2-complexant bond are derived and discussed.
Classical theory of atomic collisions. II. Low energy scattering
Authors: Gryziński Michal/
Published: February 2003

It is shown that large number of low energy scattering phenomena can be consistently explained if time-varying electric multipoles are ascribed to the atomic shell. Scattering experiments allow for estimation of the atomic multipoles and their absolute values and frequencies. It is discovered that atoms of the same chemical properties have multipoles of the same order and only their absolute values and their frequencies are different. From the whole analysis the conclusion is drawn that unperturbed atomic field must be
Multicentric expansions for isosceles H++3 configurations
Authors: Hernández Juan Antonio , Carbó Ramón
Published: February 2003

Three, four, and ten center expansions, using Gaussian orbitals have been used to study the isosceles energy maps of the H++3 ion. Results confirm the inexistence of a bound state, and are compared with those of other authors.
Effects of collisions on the magnetic quenching of positronium
Authors: Mills Allen P.
Published: February 2003

A theory describing the behavior of positronium (Ps) in a magnetic field is presented including the effects of Ps collisions. Such effects are especially important if the Ps is formed in a medium containing unpaired electrons since Ps collisions with these electrons can provide an effective spin exchange mechanism. By integrating the equation of motion for the density matrix, it is shown that the dependence of any annihilation yield N on the static magnetic field x is of the form N (x) = (g + hx + kx2) / (l + mx2) for a wide class of quenching mechanisms with time independent reaction rates. The coefficients g, h, k, l, and m are evaluated explicitly for the 2γ and 3γ yields from Ps formed from polarized positrons assuming that the quenching mechanisms can be characterized by an effective spin exchange rate and an effective pickoff rate. The yields are also evaluated assuming zero positron polarization in formulas which include the interesting Ps chemical reaction rates directly. The formulas derived can be used to obtain information about such chemical reaction rates from magnetic quenching data. Diagrams which permit the quick estimation of magnetic quenching effects as a function of the quenching rates are presented. The formalism has also been extended to include an rf magnetic field and is used to compute the shift of the measured Ps hyperfine structure interval as a function of quenching and microwave power. The shifts are found to be extremely small as expected.
Inversion of orbiting scattering from elastic collisions of reactive molecules
Authors: Anderson R. W. , Herschbach D. R.
Published: February 2003

For elastic scattering produced by an attractive, spherically symmetric potential, the classical deflection angle is a single-valued function for impact parameters equal to or greater than that which gives orbiting at the centrifugal barrier. If reaction removes contributions from smaller impact parameters, the potential in this outer region can be derived from the elastic scattering by a simple inversion procedure. This is obtained by slightly modifying the Firsov method, so as to integrate from small angles inwards. Close to the centrifugal barrier, the scattering is determined primarily by the first and second derivatives of the potential, and an approximate inversion method for obtaining the derivatives is also given. These procedures are applied to data for nonreactive scattering of K atoms from Cl2, Br2, I2, and ICl. The results indicate the orbiting model is a useful approximation for such reactions in the thermal range, although a less simple treatment is necessary at higher energies. The potential parameters obtained are discussed in terms of the ’’electron-jump’’ mechanism for these reactions and are related to other models and experiments.
Multiple-crossing electron-jump model for reactions of metal atoms with diatomic halogen molecules
Authors: Gislason Eric A. , Sachs Judith Grobe
Published: February 2003

The reaction of metal atoms with diatomic halogen molecules via an electron jump is examined using the multiple-crossing formalism of Bauer and Fisher. This permits the calculation of reactive cross sections from known properties of the atom and the molecule. Application of the model to reactions of I2 yields cross sections in good agreement with available experimental data. The model predicts that vibrational excitation via curve crossings should become an important process at energies above thermal, and cross section estimates are obtained. In addition, the relationship between the location of the electron jump and the dynamics of the reaction is examined. Finally, the model presented here is compared with other electron-jump models.
Beam maser measurements of hyperfine structure in 14N2O
Authors: Casleton K. H. , Kukolich S. G.
Published: February 2003

High resolution measurements of hyperfine structure on the J = 1 → 0 rotational transitions in 14N2O were made with a molecular beam maser spectrometer. All seven hyperfine components arising from quadrupole interactions by two nitrogen nuclei were completely resolved. This permitted determination of both quadrupole coupling constants as well as the two spin–rotation constants. Values of the best least-squares fit parameters are (eqQ)end = −776.7±1.0 kHz, (eqQ)central = −269.4±1.8 kHz, Mend = −2.35±0.20 kHz, and Mcentral = −2.90±0.26 kHz. The spin–rotation constants are used to calculate magnetic shielding parameters for the two nitrogen nuclear sites. The 1 → 0 rotational transition frequency is 25 123 246 kHz.
Determination of the antishielding factor of the chlorine ion in CoCl2⋅2H2O
Authors: Liu Y. S.
Published: February 2003

The electric field gradient (EFG) tensor at the Cl site in a single crystal of CoCl2⋅2H2O has been calculated using the point-charge point-dipole model plus the contribution from covalent bonds with the two nearest neighboring Cu ions. The value of ionicity i and that of the antishielding factor γ∞ were varied to obtain the EFG tensor. It has been found that the best values for i and γ∞ which give rise to EFG tensors compatible to experimental results of NQR absorption and NMR measurement are i = 0.911 and γ∞ = −31.0, obtained by using a simple point-charge model, without including the contribution of induced dipole moments.
Molecular beam study of the mechanism of catalyzed hydrogen–deuterium exchange on platinum single crystal surfaces
Authors: Bernasek S. L. , Somorjai G. A.
Published: February 2003

The hydrogen–deuterium exchange reaction was studied by molecular beam scattering on low and high Miller index crystal faces of platinum in the surface temperature range of 300–1300°K. Under the condition of the experiments which put strict limitation on the residence time of the detected molecules, the reaction product, HD, was readily detectable from the high Miller index, stepped surfaces (integrated reaction probability, defined as total desorbed HD flux divided by D2 flux, is ∼10−1) while HD formation was below the limit of detectability on the Pt(111) low Miller index surface (reaction probability <10−5). Atomic steps at the platinum surface must play a controlling role in dissociating the diatomic molecules. The exchange reaction is first-order in D2 beam pressure and half-order in H2 background pressure. This observation indicates that an atom–molecule reaction or possibly an atom–atom reaction is the rate-limiting step in the exchange. The absence of beam kinetic energy dependence of the rate indicates that the molecular adsorption does not require activation energy. The surface is able to store a sufficiently large concentration of atoms which react with the molecules by a two-branch mechanism. The rate constants for this two-branch mechanism were determined under conditions of constant H atom coverage, reducing the bimolecular reaction to a pseudo-first-order reaction. At lower temperatures (<600°K) the rate constant for the exchange is k1 = (2±1) ×105 exp(−4.5±0.5 kcal/RT) sec−1. The rate determining step appears to be the diffusion of the D2 molecule on the surface to a step site where HD is formed via a three-center (atom–molecule) reaction, or via a two-center (atom–atom) reaction subsequent to D2 dissociation at the step. At higher temperatures (≳600°K) the reaction between an adsorbed H atom and an incident D2 gas molecule competes with the low temperature branch. The rate constant for this branch is k2 = (1±2) ×102 exp(−0.6±0.3 kcal/RT) sec−1. The catalyst action of the platinum surface for the exchange reaction is due to its ability to adsorb and dissociate hydrogen molecules with low activation energy and to store atomic hydrogen on the surface thereby converting the gas phase molecule–molecule reaction to an atom–molecule or atom–atom reaction of low activation energy.
Hindered asymmetric top states for chemical reactions
Authors: Harms Susan H. , Wyatt Robert E.
Published: February 2003

A computational method for development of hindered asymmetric top rotational wavefunctions for atom–diatom chemical reactions is presented. The external rotation of the triatomic system is that of a nonrigid asymmetric top that is in no way hindered. Internal rotation (bending) of the system is described by hindered rotor functions of the internal angle. The internal rotation-bending angle is the natural collision coordinage γ; a rotating-axes system located by Euler angles is used to orient the three particle triangle. The basis states are obtained as linear combinations of hindered symmetric top states by perturbatively adding on Coriolis and asymmetry terms in the Hamiltonian while allowing limited configuration interaction among unperturbed levels. The coupling mixes states of different K (projection of the total angular momentum on the rotating Z axis). A single rotational potential minimum is considered in computing symmetric and asymmetric top energy level correlation diagrams. The usefulness of this rotational basis in close-coupling reactive scattering calculations is discussed.
Hindered asymmetric top wavefunctions for three-dimensional H+H2
Authors: Harms Susan H. , Wyatt Robert E.
Published: February 2003

The perturbation technique for generating hindered asymmetric top rotor states for atom–diatom reactions described previously is applied to the H+H2 exchange reaction. The external rotation of the H3 system is that of an unhindered, nonrigid asymmetric top. Internal rotation (bending) of the system is described by hindered rotor functions of the internal angle. The method consists first of diagonalization of the energy matrix in the hindered symmetric top representation. The bending potential is modeled from the Porter–Karplus surface for H3. A limited number of the eigenfunctions obtained are then used as a basis for forming a matrix representation of the Coriolis–asymmetry coupling operators. A selection rule is derived which aids in the choice of hindered symmetric top basis states. The asymmetric top hindered rotor functions obtained are linear combinations of the unperturbed functions. Symmetry features of the hindered rotor functions are discussed. Numerical results are presented in the form of eation of the Coriolis–asymmetry coupling operators. A selection rule is derived which aids in the choice of hindered symmetric top basis states. The asymmetric top hindered rotor functions obtained are linear combinations of the unperturbed functions. Symmetry features of the hindered rotor functions are discussed. Numerical results are presented in the form of energy correlation diagrams and wavefunction plots.
Hall coefficient for non-Markoffian carrier motions
Authors: Harris S.
Published: February 2003

We first calculate the Ohmic and Hall conductivities at infinite carrier dilution for the case where the charge carrier motion is described by a Gaussian, but not necessarily Markoffian, solvent averaged propagator. We then consider the same calculation when the propagator is given by a hydrodynamical model which generalizes the classical Brownian motion model, and which we show is not Gaussian. In both cases, the Hall coefficient found is unity, the identical value as in the case of classical Brownian motion.
Measurement of Rayleigh ratio for several pure liquids using a laser and monitored photon counting
Authors: Pike E. R. , Pomeroy W. R. M. , Vaughan J. M.
Published: February 2003

The Rayleigh ratios of several liquids have been measured using a He–Ne laser and established techniques of high-speed, monitored photon counting with dead-time corrections and low dark rate. A simple well-defined geometry at 90° scattering angle was used and the following values of R90,u (× 106) at 633 nm and 22°C were obtained: carbon disulfide 42.64, benzene 8.51, toluene 10.38, methanol 1.422, carbon tetrachloride 2.954, and water 0.494. Errors are estimated to be less than 2%. These values are in generally good agreement with other recent measurements at narrow angle at the same wavelength, but after scaling by the accepted formula would seem to be somewhat smaller than quoted values at shorter wavelengths.
The vacuum-ultraviolet photolysis of the difluoroethylenes
Authors: Guillory William A. , Andrews George H.
Published: February 2003

The vacuum-ultraviolet (vuv) photochemistry of the various isomeric difluoroethylenes has been investigated in argon and carbon monoxide matrices at 10 and 24 K. Selective excitation at various frequencies of the broad π*←π absorptions with N and Br resonance line sources results in two major processes and additional minor processes based on infrared intensities. As major processes, all three difluoroethylenes photodissociate into fluoroacetylene (FA) and HF via molecular elimination, and all give rise to to the same free radical C2HF2, presumably via atomic detachment. No acetylene (A) or difluoroacetylene was observed. Isomerization of the three difluoroethylenes into each other was observed as the minor processes. The results of experiments conducted in CO matrices, wherein almost total quenching of the atomic detachment process and normal occurrence of molecular elimination were observed, strongly suggest that the former occurs via intersystem crossing (ISC) and the latter via internal conversion (IC).
A unified approach to the interpretation of displacive and order–disorder systems. I. Thermodynamical aspect
Authors: Aubry S.
Published: February 2003

Some properties of a linear chain of harmonically coupled double wells are studied by the technique of the transfer operator. Exact values for the chain specific heat, the displacement probability density, and the first six even moments of the dynamical displacement correlation function are obtained. The model is shown to be a prototype for displacive as well as for order–disorder transitions depending on the strength of the harmonic coupling. Criteria for the harmonic behavior of the system are derived. By this method, it was not possible to find the exact form of the dynamical correlation functions for very anharmonic cases; this is so in general for order–disorder systems, but also for a small wave vector and temperature region in displacive systems.