bluuebunny/crossref_metadata_2025 huggingface dataset:
Approximation of isotopic reduced partition function ratios by the WINIMAX polynomial perturbation method (WIMPER)
Authors: Bigeleisen Jacob , Lee Myung W. , Ishida Takanobu , Bigeleisen Paul E.
Published: February 2003

Diagonal H, F, and G matrix approximations to the moments of the eigenvalues are studied for a number of polyatomic molecules. For molecules containing hydrogen atoms the diagonal element approximations to each of the first six moments do not differ from the true values by more than 15%. The results further substantiate the simplified method of analysis of the isotope chemistry of hydrogen in terms of stretching and bending forces. It is shown that the coupling of bending and stretching coordinates makes a significant contribution to the motion of ’’central’’ atoms in a molecule. Corrections to the reduced partition function ratios of isotopic molecules corresponding to uncoupled internal coordinates, the diagonal element approximation, are developed through the use of the WINIMAX polynomials (WIMPER). The magnitude of the corrections are related directly to the off-diagonal F and G matrix elements. Sample calculations show that the diagonal element approximations lead to values of ln (s/s′) f for deuterium substitution within 1% of the exact values at 300°K. It is shown that the diagonal H and diagonal G–diagonal F approximations lead to values of ln(s/s′) f which differ from exact values by 7% and 13%, respectively, at 300 °K for carbon and oxygen isotope substitutions. The second order correction [(h//kT)2]2 leads to approximations of ln(s/s′) f within 3% of the exact values for carbon and oxygen isotope substitutions. Comparisons are made between the WIMPER method and the Singh–Wolfsberg perturbation method. Comparisons are also made between the WIMPER and the direct WINIMAX polynomial expansions of ln(s/s′) f. The areas of physical significance of each of the methods is discussed.
Generalized oscillator strength for the first singlet–singlet transition in p-difluorobenzene and the cross section for energy transfer to acetone
Authors: Klump K. N. , Lassettre E. N.
Published: February 2003

Generalized oscillator strengths have been determined for the first singlet–singlet transition (1B2u←1Ag) in p-difluorobenzene. Using this data and similar data for acetone reported earlier [K. N. Klump and E. N. Lassettre, J. Chem. Phys. 68,886(1978)] the cross section for the resonant energy transfer process C6H4F2(1B2u)+acetone(? 1A1) →C6H4F2(1Ag)+acetone(? 1A2) has been calculated from Huo’s model [J. Chem. Phys. 66, 3572 (1977)]. The calculated cross section is 85 Å2. The disagreement with the experimental value of 56±2Å2 [G. L. Loper and E. K. C. Lee, J. Chem. Phys. 63, 264 (1975)] indicates that the Born approximation does not hold as well as in the case of benzene and acetone.
Matrix isolated alkali halide monomers and clusters
Authors: Martin T. P. , Schaber H.
Published: February 2003

The far infrared absorption spectra of sodium and cesium halide monomers and clusters isolated in solid argon are reported. For low concentrations of alkali halide vapor in argon, four absorption lines were usually seen. The strongest and highest frequency line could be assigned to the monomer. The three remaining lines were due to planar rhombic dimers. For higher concentrations of alkali halide vapor, new absorption lines appeared which seem to be due to trimer and tetramer formation. The equilibrium configurations of alkali halide molecules containing from two to eight atoms were calculated by minimizing the total energy. Both rigid and polarizable ion models were used to describe the two particle interaction. The second derivative of the total energy with respect to the atomic displacements was then calculated and the resulting dynamical matrix diagonalized to give the infrared absorption spectrum.
Observation of the set of the three sublevel phosphorescence spectra.II. Mechanisms of radiative and radiationless transitions of 2,3-dichloroquinoxaline
Authors: Yamauchi Seigo , Azumi Tohru
Published: February 2003

The phosphorescence spectra of 2,3-dichloroquinoxaline in a single crystal of 1,2,4,5-tetramethylbenzene are observed for all the three spin sublevels with the aid of the time-resolved microwave induced delayed phosphorecence. The absolute values of radiative and radiationless decay rate constants are also obtained for the three sublevel phosphorescence. The results are compared with those of quinoxaline reported earlier. The heavy atom enhancement of radiative character is most significant in the Ty spectrum, somewhat less in the Tx spectrum whereas no heavy atom enhancement is obserrved in the Tz spectrum. For the radiationless decay rate constants heavy atom enhancement is observed only in the Ty spectrum. All of the observed features of radiative and radiationless transitions are satisfactorily interpreted.
Ionic solution in a molecular polar solvent
Authors: Ho/ye J. S. , Stell G.
Published: February 2003

We treat here the statistical mechanics of a solution of ions in a polar nonpolarizable solvent. In our model the ions are taken to be charged spheres of equal diameter σ, half of them bearing charge e and the remaining half bearing charge −e. The solvent consists of dipolar spheres of the same diameter σ and the solvent–ion interaction is taken to be a charge–dipole interaction with a hard core, also of diameter σ. Our major interest in the model is its treatment of the molecular solvent on an equal statistical mechanical footing with the ions, rather than as a structureless continuum as in the primitive model. We consider a lowest-order γ-ordered approximation that is exact in the limit of infinite Debye length. Our results yield for the first time the explicit large-r form for the ion–ion, ion–solvent, and solvent–solvent pair distribution functions at arbitrary solvent densities in the limit of vanishing ion charge or ion concentration. A number of new thermodynamic expressions valid in these and related limits are also obtained.
Analysis of some two-phonon infrared absorption bands of polyethylene and the analogous binary combination bands of the triclinic n-paraffins
Authors: Snyder R. G.
Published: February 2003

The interpretation of several two-phonon (binary combination) bands in the infrared spectrum of polyethylene is reported. The analysis is based in large part on the infrared spectra of the triclinic n-paraffins, n-CnH2n+2 (n=20, 22, 24, and 26) whose corresponding binary combinations appear with appreciable intensity under a phase difference condition that is analogous to the K=0 condition for the polymer. Several observed two-phonon bands of polyethylene that are uninterpretable in terms of factor-group fundamentals, are found to be associated with critical points that are not at the zone center. A detailed assignment of a band progression of binary states involving the ν3 and ν5 bands leads to new data on the ν5 phonon dispersion curve. It is found that the critical point maximum occurs near 527±5 cm−1, in good agreement with neutron inelastic scattering results, but lower than the value derived from most reported normal vibration calculations.
Crystal field parameters for Eu3+ in KY3F1. II. Intensity parameters
Authors: Porcher P. , Caro P.
Published: February 2003

The relative intensities of the transitions occurring in the fluorescence spectrum of KY3F10: 1% Eu have been measured at 77 and 4 K. By using the computed eigenfunctions for each Stark level, the Judd–Ofelt theory is used for calculations of the electric dipole intensities. The nine phenomenological parameters allowed by the C4v point site symmetry of the lanthanide in KY3F10 are obtained, showing a good simulation of the phenomena. They have been compared in some cases to a priori calculations. The multipolar character of the transition and the importance of the J-mixing effect are underlined.
Crystal field parameters for Eu3+ in KY3F1. III. Radiative and nonradiative transition probabilities
Authors: Porcher P. , Caro P.
Published: February 2003

Processes for radiative and nonradiative decays of the most important emitting levels 5D0, 5D1, and 5D2 of the europium ion doping KY3F10 are investigated. Both magnetic and electric dipole transition probabilities are calculated for the transitions between Stark levels. The ’’forbidden’’ transitions are discussed. From the lifetime measurements versus the concentration of active ions and the temperature variation from liquid helium temperature to 800 K, evidence for multiphonon de-excitation processes is found. The influence of five phonons of 360 cm−1 for 5D1–5D0 de-excitation, six phonons of 420 cm−1 for 5D2–5D1, permits the calculation of the rate constant at 0 K for each level.
Classical differential cross sections for anisotropic potentials
Authors: Budenholzer Frank E. , Gislason Eric A.
Published: February 2003

Classical differential cross sections for a model ion–molecule system are calculated using a recently developed, classical scattering formalism. The deflection angle is computed using classical perturbation scattering theory (CPST). The scattering in the rainbow region is examined in detail; in several cases a second classical rainbow is observed. No rainbow scattering is observed for an ion colliding with a molecule with a moderate dipole moment, but a rainbow pattern is seen with molecules with moderate quadrupole moments. This agrees with the available experimental results. Finally, the topology of classical scattering in the rainbow region is discussed.
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o study of the energies and structures of ketene, oxirene, and ethynol
Authors: Dykstra Clifford E.
Published: February 2003

The structures and relative energies of the lowest closed shell states of ketene, oxirene, and ethynol were studied with ab initio wavefunctions at several levels of theory. Geometries were optimized at the one-configuration self-consistent field (SCF) level of treatment using a double zeta (DZ) basis of 34 contracted Gaussian functions. The optimum structures were then refined with a basis including polarization functions (DZ+P). Correlation effects were included using the method of self-consistent electron pairs (SCEP) which variationally treats singly and doubly substituted configurations and an important subset of the quadruple substitutions. The results show that oxirene is about 80 kcal less stable than ketene, while ethynol is only 35 kcal above ketene.
The inequality formulation of Hund’s rule and a reinterpretation of singlet–triplet energy differences, generalized for molecules at equilibrium geometry
Authors: Colpa J. P. , Brown R. E.
Published: February 2003

An inequality formulation in the open-shell Hartree–Fock approximation for Hund’s multiplicity rule for atoms has been generalized for the energy differences between a singlet and a triplet state of a molecule with different geometries for the two states. A numerical example is given for the lowest 1Πu and 3Πu states of H2. As in neutral atoms the energy difference is dominated by the difference in electron nuclear attraction and the interelectronic repulsion is higher for the triplet state than for the singlet state. The π orbital is more contracted in the triplet state than in the singlet state; the σ orbital gives a minor expansion in going from the 1Πu state to the 3Πu state.
Cluster expansion of the wavefunction. Pseudo-orbital theory based on the SAC expansion and its application to the spin density of open-shell systems
Authors: Nakatsuji H. , Hirao K.
Published: February 2003

We have studied the pseudo-orbital theory, which is based on the symmetry-adapted-cluster (SAC) expansion of an exact wavefunction proposed previously, in comparison with the conventional open-shell orbital theories, and applied it to the calculations of the spin densities of the first-row atoms, Li(2S), Li(2P), Be+(2S), B2+(2S), B(2P), C(3P), N(4S), O(3P), and F(2P). We have started from the RHF reference wavefunction and considered mainly the spin-polarization excitation operator and its self-consistency effect. This pseudo-orbital theory corresponds to an extension of the UHF and spin-extended HF (SEHF) theories, and yet it is free from the theoretical defects found previously for these theories. The relative magnitudes of the calculated spin densities are predicted to be in the order of the UHF or SEHF, present, and first-order (FO) CI in the decreasing order. This sequence has been confirmed in the calculated spin densities for the first-row atoms. For the three-electron atoms the present theory gives an excellent agreement with experiment, and for boron through fluorine the present results are fairly better than those of the UHF and SEHF theories and reasonable within the orbital theoretic approach. The calculated energies are also satisfactory when the angular correlation is included within the pseudo-orbital theory.
Resonant and inelastic transitions in collision between HCl molecules
Authors: Bhattacharyya S. S. , Saha Samir
Published: February 2003

Transition probabilities and cross sections for rotational transitions from the initial state j01=0, j02=1 have been calculated for an initial relative kinetic energy E0=0.025 eV. The semiclassical exponential approximation has been used. All possible single-molecule and simultaneous transitions as well as intermultiplet transitions have been included with all energetically accessible states. These transitions are caused by the anisotropic short-range, dispersive, inductive, and electrostatic interactions. The influence of pairs of resonating channels on the various inelastic transitions has been discussed. The limitations of the first order perturbation theory calculations are clearly brought out. The importance of various factors determining the relative magnitudes of the cross sections are shown.
Evidence against a strong critical anomaly in the depolarized Rayleigh line of nitrobenzene:n-hexane
Authors: Phillies George D. J. , Chappell Philip J. , Kivelson Daniel
Published: February 2003

The critical behavior of the depolarized Rayleigh linewidth Γ of the liquid system nitrobenzene:n-hexane is in dispute. Atakhodzhaev et al. [Zh. Eksp. Teor. Fiz. Pis. Red. 17, 95 (1973)] reported a 20-fold change in Γ on approaching Tc, while Beysens et al. [Opt. Commun. 15, 436 (1975)] found only a weak anomaly. We report a further study of Γ for 20?T−Tc ?−17 K. We find at most a weak anomaly in Γ, in accord with the results of Beysens et al. Multiple scattering apparently is not responsible for the discrepancy.
Intrachain dynamics and forward depolarized scattering
Authors: Norisuye Takashi , Yu Hyuk
Published: February 2003

The quasielastically scattered spectrum from a dilute polymer solution in the forward depolarized configuration is shown to be generally a superposition of uniformly weighted multiple Lorentzians. This is true irrespective of the dynamic models provided that one samples only the long wavelength modes of the intrachain dynamics, there exists a normal coordinate system for the chain configuration, and the polymer chain is long enough that its mean square end-to-end distance is proportional to the number of chain elements.
Laser magnetic resonance spectroscopy of the ν3 fundamental band of HO2 at 9.1 μm
Authors: Johns J. W. C. , McKellar A. R. W. , Riggin M.
Published: February 2003

The ν3 fundamental band (0–0 stretch; ν0=1097.626 cm−1) of the hydroperoxyl radical HO2 has been studied using the technique of laser magnetic resonance, in which molecular transitions are tuned into resonance with fixed laser lines by means of the Zeeman effect. The HO2 was produced by reacting O atoms with methyl or allyl alcohol in a flow system incorporating an absorption cell inside the CO2 laser cavity. Over 200 resonances involving different MJ components of a-type HO2 transitions with 1⩽N⩽7 and 0⩽Ka⩽4 were assigned, and from an analysis of the spectra the band origin and excited state rotational, centrifugal distortion, and spin–rotation interaction parameters were determined. The possible application of these results to the remote spectroscopic detection of HO2 is discussed.
Effects of H and D order on the thermal conductivity of ice phases
Authors: Ross R. G. , Andersson P. , Bäckström G.
Published: February 2003

Thermal conductivities (λ) of nine ice phases were determined by the transient hot wire method in a temperature range of 100–300 K and under pressures up to 2.2 GPa. Pure D2O as well as D2O–H2O mixtures were found to have values of λ close to those of pure H2O, except in phase VIII. For this phase λ was found to be about 9% lower in the case of pure D2O as well as in 1:1 and 1:3 mixtures of D2O:H2O. The hydrogen ordered phases systematically exhibit higher λ than the disordered ones, and in particular the presence of ice VI′ could be detected by a change of slope at low T in the curve of ln λ versus ln T for phase VI. If the results are fitted to the function λ=CT−n, the values of n fall into two groups. One group with n<1 contains only paraelectric phases, whereas the other group, having higher values of n, contains antiferroelectric phases and phase Ih. The latter phase is exceptional in that λ has a negative pressure coefficient. The results are interpreted in terms of dynamic decoupling of the hydrogen atoms from the lattice and in terms of induced oxygen disorder.
A search for the HF2 and HClF neutral free radicals isolated in argon matrices
Authors: Ault Bruce S.
Published: February 2003

A systematic search has been carried out for the neutral species FHF and FHCl isolated in argon matrices at 14 K. The synthesis of these species was attempted by the photolysis of F2 in the presence of either HF or HCl, either concurrent with, or after deposition into the low temperature matrix. Infrared spectra of these samples gave no evidence for the formation of a species of the form FHF, while evidence was demonstrated for a weakly bound complex between HF and Cl, but with a very different spectrum and structure than the corresponding FHCl− anion. These results are in general agreement with the detailed theoretical calculations that have been performed on these systems.
The Jahn–Teller effect in the lowest charge transfer state of UF6
Authors: Bernstein E. R. , Meredith G. R. , Webb J. D.
Published: February 2003

The neat and mixed crystal emission spectra of UF6 are analyzed. It is found that long uniquantal progressions appear in the ν5 (t2g) vibration built on the origin, ν6 (t2u), and n1ν1 (a1g). The maximum intensity in all these series falls around n5=2. These data are interpreted as being indicative of a substantial molecular Jahn–Teller interaction in the excited charge transfer state. Using a modified Franck–Condon analysis for a three-dimensional vibrational space, it is found that D5 ∼2.02 and that the excited state is distorted along each of three ν5 coordinates by roughly 0.09 Å.
Absolute rate of the reaction of bromine atoms with ozone from 200 to 360 K
Authors: Michael J. V. , Lee J. H. , Payne W. A. , Stief L. J.
Published: February 2003

The rate constant for the reaction Br+O3→BrO+O2 has been measured from 200 to 360 K by the technique of flash photolysis coupled to time resolved detection of bromine atoms by resonance fluorescence (FP–RF). Br atoms were produced by the flash photolysis of CH3Br at λ≳165 nm. [03] was monitored continuously under reaction conditions by absorption at 253.7 nm. At each of five temperatures the results were independent of substantial variations in [O3], total pressure (Ar), and limited variations in flash intensity (i.e., initial [Br]). The measured rate constants obey the Arrhenius expression, k = (7.74±0.50) ×10−12 exp(−603±16/T) cm3 molecule−1 s−1, where the error quoted is two standard deviations. The results are compared with previous determinations which have employed the discharge flow–mass spectrometric technique.
Critical behavior of electrical resistivity in polar + nonpolar binary liquid systems
Authors: Ramakrishnan J. , Nagarajan N. , Kumar Anil , Gopal E. S. R. , Chandrasekhar P. , Ananthakrishna G.
Published: February 2003

The behavior of electrical resistivity in the critical region of three polar + nonpolar binary liquid systems CS2 +(CH3CO)2O, C6H12+(CH3CO)2O, and n-C7H16+(CH3CO)2O is studied. For the mixtures with critical composition, the two phase region shows a conductivity behavior with σ1−σ2∼ (−ε)β with β?0.35. In the one phase region dR/dT has a singularity ε−b with b?0.35. A possible theory of the impurity conduction is given, which broadly explains these results. The possibility of dR/dT being positive or negative is also discussed.
Anomalous effects in photoelectron spectroscopy. The third electronic state of HCN+
Authors: Hansoul J. P. , Galloy C. , Lorquet J. C.
Published: February 2003

The potential energy surface of the ? 2Σ+ state of HCN+ has been calculated ab initio. Due to an avoided crossing process, it exhibits two minima, separated by a saddle point. This entails the following surprising conclusions concerning the corresponding photoelectron band. The band exhibits a vibrational structure in an energy range where the potential energy surface is dissociative. The observed vibrational structure should be assigned to a composite motion involving the in-phase stretching of CH and CN bonds. Since the electronic transition moment strongly depends upon the nuclear geometry, the observed vibrational intensities cannot be related to Franck–Condon factors. The band cannot be described in terms of simple electron removal from a particular molecular orbital. Finally, the dissociation mechanism of HCN+ in photoionization experiments is also briefly discussed. Production of H+ fragments is shown to take place via several conical intersections between potential energy surfaces. Some general comments are added concerning the validity of the orbital picture in photoelectron spectroscopy.
Infrared reflection of NaHF2 and KHF2 and the dipole derivatives of the HF−2 ion
Authors: Cooke Rodney D. , Pastorek Christine , Carlson Robert E. , Decius J. C.
Published: February 2003

The infrared reflection spectra of crystals of NaHF2 and of KHF2 have been studied in the internal mode region, i.e., from about 1100 to 1700 cm−1. Observations were made at oblique incidence angles in the range up to 45°, and both TE and TM polarized reflectivity were recorded for KHF2. Infrared transmission experiments were performed with polycrystalline mulls of both the sodium and potassium salts, in each case over a wide range of hydrogen–deuterium ratios. This spectral information was used to deduce ∂μ/∂Q2 in NaHF2 and both ∂μ/∂Q2 and ∂μ/∂Q3 in KHF2. For each of these anisotropic crystals, an appropriate dipole lattice sum was used to evaluate the effective internal field. It is demonstrated that the transition strength—a dielectric parameter, used to fit the observed reflection, and from which the dipole derivatives were calculated—is rather insensitive to the proportion of quadratic and quartic terms in the potential function. The values found for the dipole derivatives were analyzed in the context of the equilibrium charge–charge flux model of the molecular dipole moment. It is also shown that the dipole–dipole intermolecular coupling model is inadequate to account for the frequency ν2 in an isotopically isolated sample.
Rotated Morse curve–spline potential function for A+BC reaction dynamics: Application to (Cl, HBr), (F,H2), and (H+,H2)
Authors: Wright James S. , Gray Stephen K.
Published: February 2003

Methods for the fitting of potential energy functions to discrete potential energy data for triatomic molecules are discussed. Several new criteria for successful fitting functions are proposed, and existing criteria are reviewed. Different semiempirical and empirical fitting functions are analyzed in terms of these criteria. The rotated Morse curve–spline (RMCS) method is developed for general triatomic molecules, and applied to the systems Cl+HBr, F+H2, and H++H2. The detailed surface features are well reproduced for Cl+HBr and F+H2, particularly near the minimum energy path. Overall standard deviations between reference functions and RMCS functions are 0.98 and 0.35 kcal/mole for Cl+HBr and F+H2, respectively. Averaged classical trajectories on the F+H2 surface show good agreement between LEPS and RMCS surfaces over the range 0–10 eV, although there is no point-for-point matching of individual trajectories. Relative CPU times for trajectories on the RMCS and LEPS surfaces are 3.3:1. A H+3 potential surface is obtained by the combined use of CNDO/2 energy data and the RMCS fitting function. The resulting surface is scaled to ab initio data by adjustment of the spline functions. It gives a realistic representation of the ab initio surface, except that nonphysical behavior occurs in the outer contours (this was also true for Cl+HBr). The problem of discontinuities arising from the bond angle–bond distance coordinate system is discussed. Possible extensions and limitations of the RMCS method are considered. When Morse curves can describe the asymptotic dissociation channels for A+BC reactions, the RMCS method provides an accurate interpolating function for triatomic potential energy data. Its suitability for classical and quantal studies of reaction dynamics for A+BC systems depends on the magnitude of the discontinuity problem. Computational procedures to study this problem in advance of and during classical trajectories, are given and discussed in detail for F+H2.
The disrete excitation of nonequivalent CH oscillators—a local mode analysis of the high energy overtone spectra of the alkanes
Authors: Greenlay Wayne R. A. , Henry Bryan R.
Published: February 2003

The overtone spectra of the normal alkanes, propane to n-heptane, and several branched alkanes are measured in the liquid or gas phase from 6000 to 15 000 Å. Each overtone band is principally composed of peaks corresponding to the different CH oscillator types in the molecule CH3, CH2, or CH. The localized character of the overtone bands increases with increasing energy. The relative intensity of the CH2 peak to the CH3 peak correlates with the number of CH2 hydrogens in the molecule. Combination bands involving CH local modes and a lower frequency normal mode are identified. The relative intensities of the components of these bands parallel that of the pure CH-stretching overtone bands. Combination bands between different CH-stretching local modes are also observed and tentatively assigned. Deconvolution of the main overtone bands for ΔvCH=3, 4, and 5 of the normal alkane spectra gives further information about individual local mode peaks, particularly with regard to band shape and bandwidth. The local mode frequencies ω1 and diagonal local mode anharmonicity constants X11 for the molecules are obtained from a local mode analysis of both the observed spectral data and deconvolution results. These two parameters form consistent sets for a given CH oscillator type.
Dynamic Padé approximants in the theory of periodic and chaotic chemical center waves
Authors: Ortoleva P.
Published: February 2003

A Padé approximant scheme is introduced to analyze chemical composition center waves. The theory shows how the outer plane wavelike nature of these phenomena, such as spiral or circular waves, couples to the core dynamics. Since the latter may have an autonomous frequency different from that of the plane wave outer behavior, it is shown that all the richness of phenomena found for forced nonlinear oscillations are in principle possible. These include periodic and aperiodic centers, multiple centers compatible with a given outer wavelength, and centers which oscillate at a fraction of the outer plane wave frequency, e.g., subharmonic resonance. The theory provides a simple self-consistent approximate stability analysis for the center waves.
A laboratory study of the reactions of N+, N2+, N3+, N4+, O+, O2+, and NO+ ions with several molecules at 300 K
Authors: Smith D. , Adams N. G. , Miller T. M.
Published: February 2003

A study has been made of the rate coefficients and product ion distributions for the reactions at 300 K of the ions N+, N2+, N3+, N4+, O+, O2+, and NO+ with CH3NH2, NH3, H2S, CH3OH, H2CO, COS, O2, H2O, CH4, CO2, CO, H2, and N2 molecules listed in increasing order of their ionization energies. These measurements are intended as a contribution to stratospheric chemistry. In the binary reactions of the ions of large recombination energy with molecules of low ionization energy, multiple ion products generally result and the rate coefficients are close to gas kinetic. Conversely, the low recombination energy ions NO+ and O2+ generally undergo ternary association reactions with the large ionization energy molecules. The reactions of N2+ and N4+ are very similar, the most common mechanism apparently being direct charge transfer usually followed by fragmentation, the nitrogen–nitrogen bonds in the reacting ions remaining intact. The N+ and N3+ reactions differ from the N2+ and N4+ reactions in that they show a greater propensity to form N–X bonds, X=O, C, S, H, etc. The O+ and O2+ reactions generally proceed via direct charge transfer where energetically possible.
Production of uranium and thorium ions by charge transfer from 0+, N+, N+2, and CO+2
Authors: Rutherford J. A. , Vroom D. A.
Published: February 2003

Cross sections for charge transfer from O+, N+, N+2, and CO+2 to uranium and thorium have been determined in the ion energy range from 1 to 500 eV. The experiments were performed using a crossed ion modulated–neutral beam apparatus. All cross sections are found to increase with decreasing ion energy down to the lowest energies studied. The size and shape of the cross-section curves indicates that accidental resonance may be occurring. The uranium and thorium beams were formed by heating the pure metals in specially constructed furnaces. A detailed description of the techniques used to form the beams is given.
Absolute rate of the reaction of hydrogen atoms with ozone from 219–360 K
Authors: Lee J. H. , Michael J. V. , Payne W. A. , Stief L. J.
Published: February 2003

Absolute rate constants for the reaction of atomic hydrogen with ozone were obtained over the temperature range 219–360 K by the flash photolysis–resonance fluorescence (FP–RF) technique. The results can be expressed in Arrhenius form by k = (1.33±0.32) ×10−10 exp(−449±58/T) cm3 molecule−1 s−1 (two standard deviations). The present work is compared to two previous determinations and is theoretically discussed.
Conformational and thermodynamical prediction of the 1,5-methano(10) -annulene, 1,6-methano(10) -annulene and some benzo derivatives
Authors: Espinosa-Müller A. , Meezes F. C.
Published: February 2003

A minimization scheme developed by R. H. Boyd has been applied in order to obtain the geometry of minimum deformation energy, and to evaluate the following thermodynamic functions: entropy, Gibbs free energy, enthalpy, and constant pressure heat capacity, in gaseous states (low pressure) for a series of four methylene-brigded annulenes: 1,5-methano(10) -annulene (I), 1,6-methano(10) -annulene (II), 3,4-benzo-1,6-methano(10) -annulene (III), and 2,3-benzo-1,6-methano(10) -annulene (IV). The relative stability of the first two bridged annulenes is analyzed in terms of the calculated strain energy. Also, we report here the product of inertial moments and vibrational frequencies.
A molecular beam investigation of He, CO, and O2 scattering from Pd(111)
Authors: Engel T.
Published: February 2003

Adsorption of CO and O2 on Pd(111) has been studied with the molecular beam technique. Absolute sticking coefficients for CO and O2 have been determined as a function of coverage, angle of incidence, and substrate temperature. Angular distributions of the scattered molecules show that the probability for capture in the precursor state is unity for CO at low coverages and for all substrate temperatures investigated and 0.5 for O2 at low coverages independent of the substrate temperature. Modulated beam experiments show that Edes. for CO is 32 kcal/mole at low coverages. Phase sensitive detection and waveform analysis show that at high CO coverages on the impurity free surface and over a wide range of coverages on a carbon contaminated surface, three distinct CO species are present on the surface.
Information extent and information distance
Authors: Ruch Ernst , Lesche Bernhard
Published: February 2003

Information extent and information distance are introduced to describe the quantitative aspect of statistical information and gain of information without using prejudices that apply only under special conditions. It is shown that these concepts are closely related to mixing character and mixing distance as defined and discussed in previous papers (Refs. 1, 2, and 5). Several examples are treated to show the use of these concepts and their superiority in some respects over statistical entropy concepts of the conventional type. In a second part graphical illustrations are given to develop an intuitive understanding of partially ordered properties.
Dynamical treatment of unimolecular decomposition reactions. II. Short-range interfragment coupling and incomplete randomization
Authors: Kay Kenneth G.
Published: February 2003

We investigate the dynamics of an isolated polyatomic molecule undergoing unimolecular dissociation. The intramolecular vibrational energy transfer step is treated by applying a theory of vibrational relaxation previously developed for permanently bound molecules. In contrast to an earlier study, the decomposition step is treated here subject to the assumption that, when sufficient energy is concentrated in the reaction coordinate, transitions between internal states of the molecule occur only while the incipient fragments are near their minimum classically allowed separation. Under such circumstances, it is generally not possible to divide configuration space cleanly into a randomized and a nonrandomized region, as required by the RRKM theory of unimolecular reactions. In spite of this, it is shown that, under certain conditions, the RRKM specific rate constant expression may remain valid. More generally, it is shown that the accurate specific rate constant assumes a form predicted by the transition state treatment of unimolecular reactions. The explicit rate expression derived here reveals factors which determine the accuracy of the transition state approximation for unimolecular reactions—the basic, simplifying assumption that a transition state exists which coincides with a ’’configuration of no return’’ for both decomposition and association reactions. Estimates based on our rate formulas suggest that the transition state approximation may often be sufficiently accurate to justify use of the RRKM rate expression. As part of our analysis of randomization and decomposition dynamics, we examine the time-dependent behavior of a molecule which has fragment–fragment interactions as described above but which is constrained to remain bound by the presence of an artificial ’’wall’’ which prevents the fragments from separating completely. It is found that statistical equilibrium may be achieved in this system—a significant result because the system violates some rather stringent assumptions previously invoked to prove randomization in bound molecules. This motivates a future search for a more general theory of intramolecular relaxation.
Phase transitions in the chemisorbed layer W(110) p (2×1)–O as a function of coverage. I. Experimental
Authors: Wang G.-C. , Lu T.-M. , Lagally M. G.
Published: February 2003

The thermal disordering of oxygen chemisorbed on W(110) has been investigated as a function of coverage below half-monolayer coverage by measuring the angular distribution of intensity in the LEED superlattice reflections as a function of temperature. The transition temperature is a function of coverage, being much lower at low coverage. A partial phase diagram is constructed for this overlayer, and the low- and high-coverage limits are interpreted, respectively, in terms of an island–dissolution and an order–disorder transition.
Activated complex in the catalytic oxidation of CO over NiO
Authors: Kobal I. , Senega?nik M. , Barli? B.
Published: February 2003

(CO2)‡ and (CO3)‡ structures were proposed as the activated complex of the rate determining and isotope fractionation governing step in the catalytic oxidation of CO over NiO catalyst. Following Bigeleisen’s formalism, the theoretical rate constant ratios for 13C and 14C substitution were calculated and compared with the experimental values. Computational analysis confirmed the (CO2)‡ activated complex and pointed out that the (CO3)‡ activated complex could not explain the experimetal results.
Absolute rate and temperature dependence of the reaction between chlorine (2P) atoms and methane
Authors: Keyser L. F.
Published: February 2003

The absolute rate of reaction Cl(2P)+CH4→HCl+CH3 was determined between 220 and 423 K using the discharge flow–resonance fluorescence technique. Evidence of curvature is observed in the Arrhenius plot. The data are best fitted by the following Arrhenius expressions: The present results are compared with earlier absolute rate measurements.
Charge distributions and chemical effects. XVII. Valence region energies and electronegativity of atoms and charged species
Authors: Fliszár Sándor
Published: February 2003

The derivative (∂EN/∂N)z of the N valence-electron energry in atoms and charged species can be expressed in a simple manner using the Politzer–Parr partitioning into core and valence regions. A finite difference approximation is presented for (∂EN/∂N)z, thus avoiding the strategy of expressing EN in terms of an approximation leading to (∂EN/∂N)z as a function of this approximation.
Configuration interaction studies of the HeH+ molecular ion. IV. The triplet sigma, pi, and delta states
Authors: Green T. A. , Michels H. H. , Browne J. C.
Published: February 2003

The method of superposition of configurations was applied to the triplet sigma, pi, and delta states of HeH+ which correlate to the separated atom states of principal quantum number less than or equal to 3. The calculations were done for internuclear separations, 0⩽R⩽65.5 a.u., on a mesh adequate for interpolation. Similar calculations on the singlet states have already been reported. The present calculations complete the accurate evaluation of the potential energy curves for this system which are required for low- and intermediate-energy collision studies. In addition to the energy eigenvalues and eigenfunctions, dipole, gradient, and radial coupling matrix elements were calculated for the sigma and pi states. Primarily, this paper presents information on the eigenvalues. The accuracy of the triplet-state calculations is comparable to that obtained for the singlet states. The similarities and differences in the pattern of avoided crossings for the triplet and singlet states are exhibited. These are mainly determined by the differences in the triplet and singlet energy-level schemes of the united and separated atoms.
Perturbed angular correlation studies of chemically generated electric field gradients in Hf compounds
Authors: Ball Jane W. , Kaplan Morton
Published: February 2003

The time-differential-perturbed-angular-correlation (TDPAC) technique has been applied to the study of electric quadrupole interactions in HfO2, HfO(H2PO4) ⋅H2O, and HfP2O7. Strong perturbations were seen in each case, with time evolution characteristic of the static electric field gradients generated by the electron distributions in chemical bonds. Analysis of the experimental data within a theoretical framework has permitted derivation of the electric field gradient parameters associated with the Hf–O bonds in the oxide and the two phosphate compounds. A simple model calculation involving orbital populations appears capable of reproducing the experimental results and provides insight into the bonding features of these relatively complex, but structurally known, systems. The TDPAC signature of each compound appears sufficiently unique so as to offer the possibility of its adaptation as a microscopic probe of more complicated or unknown crystalline and liquid systems.
Photoelectron spectra of Ar2 and Kr2 and dissociation energies of the rare gas dimer ions
Authors: Dehmer P. M. , Dehmer J. L.
Published: February 2003

The photoelectron spectra of Ar2 and Kr2 were determined at 584 Å with resolutions of 18 and 21 meV, respectively, using an apparatus which combines a supersonic molecular beam source with a hemispherical photoelectron spectrometer. Dissociation energies were determined for the weakly bound B 2Π(3/2)g and C 2Π(1/2)u excited states of the dimer ions, and were found to average approximately 30% greater than those predicted by recent theoretical calculations. Optimum potentials were determined for the Ar+2 and Kr+2 ground states by fitting calculated Franck–Condon distributions to the observed photoelectron peaks. As in the case of Xe+2, the optimum Rg+2 potentials retain the shapes determined by theoretical calculations, but they are modified by scaling their dissociation energies to the experimentally determined values. The Morse function is shown to be an inadequate potential form for the Rg+2 ground states, since it is too attractive at large internuclear distance.
The covalent and ionic states of the rare gas monofluorides
Authors: Dunning Thom H. , Hay P. Jeffrey
Published: February 2003

A b initio configuration interaction calculations have been carried out on the electronic states of RgF (Rg=Ne, Ar, Kr, and Xe) arising from the covalent, Rg(1S)+F(2P), and ionic, Rg+(2P)+F−(1S), separated atom limits. Spin–orbit effects, which have been shown to be important in the ionic states, are included in a semiempirical manner. The potential curves of all of the covalent states were found to be repulsive while those of the ionic states were strongly bound (De∼5–6 eV). Only those transitions for which ΔΩ=0 were found to be strong. The intense laser emissions at 193 nm in ArF, 248 nm in KrF, and 352 nm in XeF all arise from a transition between the lowest ionic Ω=1/2 state and the lowest covalent Ω=1/2 state (III1/2–X1/2 in our notation). The calculated lifetimes of the III1/2–X1/2 laser transition varies from 2.6 nsec (NeF) to 12 nsec (XeF).
Information theory analysis of angular momentum disposal in chemical reactions
Authors: Case D. A. , Herschbach D. R.
Published: February 2003

Several vector properties of single collisions between gas-phase molecules can now be measured by molecular beam techniques. Such properties consist of correlations among the directions of relative velocity vectors and angular momentum vectors of the reactant and product molecules. Here we examine the utility of information theory for analysis of these multidimensional correlations. Vector distributions generated from impulsive and statistical models for an A+BC→AB+C atom exchange reaction are analyzed, with particular emphasis on the triple angle correlation relating the initial and final velocity vectors to the product rotational momentum. For the impulsive model, the entropy deficiencies for the angular degrees of freedom are found to be larger than those for the energy distributions and the largest contribution comes from the triple angle correlation. However, it is argued that information theory cannot assess the comparative value of different kinds of data. Also discussed are the effects of approximate dynamical constraints on multidimensional entropy deficiencies.
Vibrational spectra of liquid crystals. XI. Correlation functions obtained from 4-octyloxy, 4′-cyanobiphenyl CN stretching in smectic, nematic, isotropic, and solution phases
Authors: Bulkin Bernard J. , Brezinsky Kenneth
Published: February 2003

Correlation functions from the Raman scattering associated with the C=N stretching mode of the mesogen 4-octyloxy, 4′-cyanobiphenyl (80 CB) in aligned smectic, aligned nematic, and isotropic phases have been calculated. These functions, calculated from IVH spectra, show that the band contour is dominated by vibrational relaxation processes. Correlation functions calculated from spectra of the C=N band of the mesogen dissolved in benzene, carbon tetrachloride, chloroform, and methyl thiocyanate suggest that ’’vibrational dephasing’’ is primarily responsible for the bandwidth and shape in liquid crystal phases, in the isotropic phase, and in solutions. The solution studies indicate the presence of a wide distribution of local environments in the mesomorphic and isotropic phases of the mesogen as well as in CHCl3 and CH3SCN. These environments average out to a more narrow distribution, i.e., there is ’’motional narrowing,’’ in C6H6 and CCl4. The results lead to an understanding of why motional narrowing occurs for certain bands and certain solvents.
Effect of coherent transients on time-resolved four-level double resonance signals
Authors: Morita Norio , Kano Satoru , Shimizu Tadao
Published: February 2003

It is found that the time-resolved four-level double resonance signal critically depends on the frequency and the intensity of the probe field. The observed results are well explained by the theoretical analysis in which the coherent transient interaction between the molecules and the coherent probe radiation is taken into consideration. An importance of this effect in determination of collisional relaxation constants is demonstrated both experimentally and theoretically.
Spontaneous pattern formation in precipitating systems
Authors: Feinn D. , Ortoleva P. , Scalf W. , Schmidt S. , Wolff M.
Published: February 2003

A series of new experiments on patterned precipitation are presented which clearly show the symmetry breaking aspects of these systems. In one configuration a system consisting of lead iodide, water, and agar is shown to display a symmetry breaking instability in precipitation from a supersaturated gel solution prepared by supercooling. In another configuration the system is maintained out of equilibrium for long times by feeding a gel layer from opposite sides with lead nitrate and potassium iodide (or similar pairs of precipitating reactants). In this latter experiment the precipitation in the plane of the reaction gel layer is shown to form spontaneous patterning, breaking the symmetry imposed by the uniform baths of lead nitrate and potassium iodide. The patterns range from mosaic to speckled in nature depending on the concentrations of the reactants and the thickness of the gel layers. A new mechanism for patterned precipitation is proposed which is based on a consideration of the surface tension of the precipitate particles. Feedback in this view is provided because the larger particles may grow at the expense of smaller ones. The mechanism is not inconsistent with the Ostwald–Prager theory of Liesengang rings but unlike the latter can explain pattern formation in the absence of imposed gradients. The present theory may also be used to explain secondary bands in the classic Liesengang experiments as well as several types of spontaneous center patterns and a divergence in the coherence or pattern length found in the experiments presented here.
The Compton profile of urea
Authors: Reed W. A. , Snyder L. C. , Guggenheim H. J. , Weber T. A. , Wasserman Z. R.
Published: February 2003

The Compton profile of single crystal urea is measured and compared to theoretical calculations of profiles of the molecule, two dimer configurations, and an estimate of the crystal. The major features of the measured anisotropy are predicted by the theoretical calculations for an isolated urea molecule. The effect of the hydrogen bonding between molecules is small.