bluuebunny/crossref_metadata_2025 huggingface dataset:
Instability and bifurcation analysis for an exothermic surface reaction with diffusive heat and mass transfer
Authors: Pearlstein Arne J.
Published: July 2002

The steady distributions of temperature and concentration in a reactive fluid adjacent to a catalytic wall are investigated. It is shown that for the mass action kinetics considered, the number of steady solutions is either one or three. Several simple sufficient criteria, as well as a slightly more complicated necessary and sufficient criterion, are given for the uniqueness of the steady solution. If the Lewis number Le (ratio of molecular and thermal diffusivities) is at least one, then a steady state that is unique is always stable with respect to small disturbances. For Le≥1, the high and low temperature steady states are stable when the number of steady states is three. When Le<1, even a unique steady state can be unstable, and when there are three steady states, one, two, or three of them can be unstable. The Le≥1 and Le<1 cases are further distinguished by the fact that oscillatory instability can occur for Le<1 but not for Le≥1.
Charge exchange of N2+2 with Ar
Authors: Schulz P. A.
Published: July 2002

The charge exchange of N2+2 is distinguished from charge exchange of N+ in the measurement of the differential cross section. N2+2 charge exchanges with argon to produce predominantly Ar+ in its ground state and N+2 in its C 2Σ+u and D 2Π states. A vibrational progression is observed corresponding to the vibrational frequency in the C state. Also observed is the charge exchange of N+2(A 2Πu). A simple model is used to explain the data.
Absorber speed dependence of the coherence relaxation rate of the J=0–1 transition of N2O
Authors: Fraser Gerald T. , Coy Stephen L.
Published: July 2002

The coherence relaxation rate (T2) of the J=0 –1 transition of 15N2O has been examined using microwave transient techniques. The transient decays were analyzed by a model which considers the absorber speed dependence of the collisional relaxation process [S. L. Coy, J. Chem. Phys. 73, 5531 (1980)]. The decays were also analyzed by the standard speed independent model. In the speed dependent analysis, a linear absorber speed dependence of the form k(v)=k0+k1(v−v̄), where v̄ is the Boltzmann average speed, was assumed. Values of k0=0.0300(1) μs−1 mTorr−1 and k1v̄=0.0099(4) μs−1 mTorr−1 were determined. In the speed independent analysis a relaxation rate, k=0.029 22(6) μs−1 mTorr−1, was found. This corresponds to a pressure broadening parameter 1/2πT2 of 4.65(1) MHz/Torr. A zero pressure transition frequency and pressure dependent frequency shift parameter were also determined. They are 24 274.7865(1) MHz and +30(4) kHz/Torr, respectively. The experimental uncertainties reported above are two standard errors from linear least squares fits of k, k0, k1v̄, and transition frequency, vs sample pressure. The most significant systematic error is a 1% uncertainty in the pressure calibration which contributes an additional 1% error to the slopes determined from the linear least squares fits.
Absolute partial and total electron impact ionization cross sections for CF4 from threshold up to 180 eV
Authors: Stephan K. , Deutsch H. , Märk T. D.
Published: July 2002

Electron impact ionization of carbon tetrafluoride was studied as a function of electron energy from threshold up to 180 eV. A double focusing mass spectrometer system with an improved electron impact ion source was used, alleviating the problems of ion extraction from the source and the transmission of the extracted ions through the mass spectrometer system. Absolute partial ionization cross section functions for the production of CF+3, CF+2, CF+, C+, F+, CF2+3, and CF2+2 in CF4 have been determined. In addition, the total (and the counting) ionization cross section function of CF4 has been determined (summation method) and is compared with calculations based on classical and semiclassical binary encounter approximations. Using nth root extrapolation ionization energies of the following doubly charged fragment ions have been derived: AE (CF2+3) =41.8±0.3 eV, AE (CF2+2) =42.9±0.3 eV, and AE (CF2+)=52.1±0.5 eV. In accordance with previous results no stable CF+4 parent ion has been detected, however, a metastable dissociation process CF+*4→ CF+3+F has been observed.
A thermodynamic ‘‘paradox’’ associated with nonclassical critical phenomena in mixtures
Authors: Wheeler John C. , Morrison Graham , Chang R. F.
Published: July 2002

Nonclassical critical behavior gives rise to an apparent thermodynamic paradox near the critical point in many if not most two-component mixtures. This paradox occurs at both gas–liquid and liquid–liquid critical points. It is resolved by observing that in these mixtures the thermal expansion coefficient is negative sufficiently close to the critical point and diverges to −∞ there. As a consequence, the thermodynamic state is not uniquely determined by the variables P, v, x in the vicinity of the critical point. While for gas–liquid critical points the region in which this occurs is unobservably small, the predicted effect should be observable for liquid–liquid critical points.
Structural anomalies in formate chemisorption on Ni(100)
Authors: Upton Thomas H.
Published: July 2002

Optimization of the geometric structure for a formate moiety (HCOO) chemisorbed in a symmetric bidentate configuration at a bridge site on a model Ni(001) surface reveals exceedingly long nickel–oxygen bonds. We report the results of generalized valence bond and configuration interaction calculations in which the formate oxygen atoms are 1.45 Å above the Ni (cluster) surface. This is well outside the ‘‘normal’’ range for nickel–oxygen bond lengths on surfaces or in bulk compounds. We find an adsorption enthalpy for the formate of 492 kJ/mol and a nickel–oxygen stretch frequency of 335 cm−1. The configuration found in these calculations for Ni(100) is very similar to that recently reported for formate chemisorption on a Cu(100) surface. The bonding is interpreted in terms of a charge-transfer chemisorption interaction that is offset by nonbonding repulsive interactions between adsorbate orbitals and the surface.
Mean-square optical anisotropies and molar Kerr constants of cyclic and linear dimethyl siloxanes
Authors: Mumby Stephen J. , Beevers Martin S.
Published: July 2002

Mean-square optical anisotropies, 〈γ2〉0, and mean molar Kerr constants, 〈mK〉0, have been calculated for cyclic dimethyl siloxane oligomers having from n=8 to n=20 skeletal bonds. Stringent criteria were used to select ‘‘cyclic’’ conformations of the equivalent open chains based on the relative orientations and rotational states of the terminal bonds. The values obtained for the cyclic oligomers are compared with values calculated by implicit matrix methods for open chain dimethyl siloxanes of equivalent length. Substantial differences between calculated values for cyclic and linear oligomers are observed, and are discussed in terms of the restricted conformational population of the cyclic species. Calculated mean molar Kerr constants are also compared with experimental values measured for cyclic and linear oligomers in the undiluted state at 298 K. It is found that although there is good correlation between the trends calculated and those observed experimentally, discrepancy exists between the absolute values.
Angular correlation and diamagnetic susceptibilities studied by high field NMR. II. Halobenzenes, pyridine, and aniline
Authors: van Zijl P. C. M. , MacLean C. , Bothner-By A. A.
Published: July 2002

The NMR analog of the Cotton–Mouton effect has been employed to determine the anisotropy and asymmetry in the magnetic susceptibility and the angular correlation factors g2 of pyridine, aniline, fluorobenzene, chlorobenzene, and bromobenzene. Results are discussed in comparison with data obtained by microwave spectroscopy, the Cotton–Mouton effect, and Rayleigh scattering.
Non-Newtonian flow of a model liquid between concentric cylinders
Authors: Rainwater J. C. , Hanley H. J. M. , Paszkiewicz T. , Petru Z.
Published: July 2002

Computer simulations of fluids out of equilibrium indicate that even the simplest fluid is in principle non-Newtonian. In particular, the simulations can provide explicitly the pressure tensor as a function of shear rate at a given temperature and density. In this paper the steady state flow of a model soft sphere liquid between rotating vertical concentric cylinders is discussed from a microscopic standpoint, given the coefficients that characterize the pressure tensor. The equations of motion are solved numerically. It is found that the normal pressure differences lead to an enhanced depression of the free surface at the inner cylinder, in contrast to a climbing (Weissenberg effect) which is usually regarded as the consequence of such differences. Reasons for the behavior observed for the soft sphere system are discussed. A consequence of the analysis is that a unique and self-consistent solution of the equations of motion is obtained only if the effects of finite compressibility are included.
Conversions over low barriers. VI. The c
i
s–t
r
a
n
s isomerization of monothioformic acid
Authors: Lazaar K. I. , Bauer S. H.
Published: July 2002

The dynamics of the trans⇄cis isomerization of monothioformic acid (rotation about the HC–SH bond) is a case study for an intramolecular conversion over a significant barrier [microwave spectra indicates Vmax(θ) =3500 cm−1], where the density of vibrational states at the potential maximum is very low, ≈2.5/cm−1. We recorded the NMR spectra of dilute solutions of the acid in CD2Cl2, and of the gas at low pressures, over the temperature range 300–170 K, to determine the exchange relaxation time and to deduce the corresponding kinetic parameters. In spite of difficulties with the preparation and maintainence of the pure material, (i) correct assignment of the NMR resonances were made; (ii) the magnitude of E0sol for isomerization in solution, from k∞u, was found to be compatible with the barrier derived from microwave spectra; (iii) in the gas phase this intramolecular conversion is in the second order regime, but (iv) only an upper limit could be set on the magnitude of E0gas for isomerization, which is at least 2 kcal/mol lower than E0sol.
Vibrational excitation of CH3F by collisions with ‘‘hot’’ H and D atoms
Authors: McGee T. H. , Weston R. E. , Flynn G. W.
Published: July 2002

Hot hydrogen atoms have been produced by photolysis of HBr, HI, and H2S at 193 nm, and HI at 248 nm; D atoms were produced by photolysis of D2S at 193 nm. Vibrational excitation of CH3F in collisions with these hot atoms has been observed by detection of infrared emission at 3.3 μm (ν1, ν4) and 9.6 μm (ν3). Relative probabilities for excitation of these modes have been determined, and the ν3/ν1, ν4 ratio is much larger than is predicted by simple impulsive models. The ‘‘isotope effect,’’ i.e., relative excitation by H atoms compared to D atoms, is 1.4 for both vibrational modes.
Isotope effects in condensed phases, the benzene example. Influence of anharmonicity; harmonic and anharmonic potential surfaces and their isotope independence. Molar volume effects in isotopic benzenes
Authors: Van Hook W. Alexander
Published: July 2002

Application of the harmonic oscillator cell model (HOCM) for condensed phase isotope effects to the benzene/deuterobenzene system using temperature dependent force fields, which yield good calculated values for vapor pressure isotope effects (VPIE) and reasonable agreement with spectroscopically observed frequency shifts on condensation, gives calculated values of isotope effect on energies of vaporization which are not in agreement with experiment. Pseudoharmonic corrections are inadequate to restore agreement with experiment. The VPIE in benzene is dominated by the contributions of the CH/CD stretching vibrations and consistency is achieved by expanding the model to include anharmonic corrections for these frequencies. In this fashion a combination of thermodynamic measurements of free energies (by vapor pressure) and energies (from the temperature coefficient of the vapor pressure or from calorimetry) can be employed to yield shifts in anharmonic vibrational constants on condensation. In the second part of the paper the molar volume isotope effect (MVIE) and the effects of volume, pressure, and temperature on the internal energy and its isotope effects (as monitored by the expansivity, compressibility, heat capacity, and their derivatives) are interpreted. The low-lying lattice modes make important contributions to these properties.
Vacuum ultraviolet photoabsorption study of SO
Authors: Nee J. B. , Lee L. C.
Published: July 2002

The photoabsorption of SO was investigated by producing the radicals from a pulsed discharge of a trace of SO2 in Ar. The photoabsorption cross section of SO in the 115–135 nm region was measured. A repulsive potential curve is constructed from the observed photoabsorption spectrum.
Optical dephasing of the electronic transitions of delocalized molecular dimer states
Authors: Lee H. W. H. , Fayer M. D.
Published: July 2002

We present a theoretical study of the optical dephasing of the electronic transitions of delocalized molecular dimer states. The dephasing arises from the dynamical interactions of the dimer states with the phonon bath of the host crystal. We introduce a general dephasing model for dimers with strong intermolecular interactions and analyze the consequences of the model with optical Redfield theory. A quantum mechanical microscopic model is developed in which the interaction Hamiltonian of the dimer states and the phonon bath states is clearly delineated. We treat both linear and quadratic excitation-phonon coupling in which the two molecules comprising the dimer may be coupled differently to the phonons. Couplings to acoustic, optical, and pseudolocal phonons (librations) are considered explicitly. The temperature dependence of the dephasing rates are obtained for scattering between delocalized dimer states, scattering to dimer librations and phonon-induced pure dephasing. These results are used to analyze the photon echo studies on pentacene dimers and tetracene dimers in p-terphenyl and on naphthalene dimers in perdeuteronaphthalene. In the pentacene dimers and tetracene dimers, we conclude that the dephasing results from scattering to the dimer librations via a one-phonon process. In addition, a quantitative value for the libron–phonon coupling is determined. In the naphthalene dimer system, we find scattering between delocalized dimer states, induced by linear excitation-phonon coupling, to be the predominant dephasing mechanism. A quantitative value is determined for the phonon coupling matrix element responsible for scattering between the delocalized states.
1H NMR structural rigid lattice and dynamical study of the defect pyrochlores H2Ta2O6, HTaWO6, and their hydrates
Authors: Durand-Le Floch M. , Pannetier J. , Doremieux-Morin C. , Arribart H.
Published: July 2002

The defect pyrochlores H2Ta2O6, HTaWO6, and their hydrates have been studied by proton nuclear magnetic resonance. Rigid lattice spectra of H2Ta2O6 and HTaWO6 exhibit a large Lorentzian contribution which indicates a statistical distribution of the protons over the 48 f  sites of the structure and implies the pairing of protons bonded to the same oxygen [r(H–H)=1.5 Å]. Long range diffusive motion is deduced from the relaxation rates; in all four compounds the diffusion mechanism can be explained by using two independent correlation times τ1 and τ2. The activation energy corresponding to the jump of a proton to the nearest neighbor vacant position is the same (E1=0.15 eV) for three of the pyrochlores. The low temperature spin lattice relaxation of the anhydrous compounds H2Ta2O6 and HTaWO6 is governed by the existence of unequal potential wells.
Comment on electron mobility in the liquid isomeric pentanes
Authors: Stephens J. A.
Published: July 2002

We comment on the large differences in electron mobilities that often exist between structural isomers of nonpolar organic liquids. Focusing on the isomers of liquid pentane as an example, we discuss the relevance and correlation of the first peak observed in the x-ray structure factor S(k) with the observed differences in electron mobility.
Discrete variational Xα studies of core excitation, photoemission, and inverse photoemission for CO and NiCO clusters
Authors: Kojima Isao , Srivastava A. K. , Miyazaki Eizo , Adachi Hirohiko
Published: July 2002

The discrete variational (DV)-Xα method has been applied to CO and NiCO clusters to investigate core excitation, photoemission, and inverse photoemission spectroscopy. The transition state calculations demonstrate several important features of these excitation processes. A significant difference in the appearance of the screening effect due to the core hole is observed between photoemission and core excitation processes. The core excitation shows a greater difference in shifts between the C 1s and the O 1s electron excitations and also a stronger dependence on the bond lengths for Ni–C and C–O than those found for the ionization process. The intensity of the core excitation is briefly described in terms of the calculated dipole transition probability.
Spectral line shape in collision-induced absorption: An improved constant acceleration approximation
Authors: Borysow Jacek , Moraldi Massimo , Frommhold Lothar , Poll J. D.
Published: July 2002

A simple formalism based on path integrals is applied for calculating two-particle intracollisional correlation functions in the semiclassical limit. The parameter-free spectral line model obtained in this way is compared with an exact quantum line shape calculation for collision-induced absorption of the Ne–Ar system at 300 K. Close agreement between model and exact line shape is obtained.
Dependence of the chemiluminescence cross section on spin-orbit state for the reaction of Ca(3P0J) with alkyl bromides
Authors: Furio Nick , Campbell Mark L. , Dagdigian Paul J.
Published: July 2002

The dependence of the chemiluminescence cross section on initial spin-orbit level has been measured for the reaction of metastable Ca(4s4p 3P0J) with Cl2, CH3Br, CH2Br2, CH2=CHCH2Br, and C6H5CH2Br, using an optical pumping state selection technique. Optical pumping depletion was also used to determine what fraction of the chemiluminescence was due to the second metastable Ca(4s3d 1D) level present in the incident atomic beam. Chemiluminescence cross sections for both the 3P0 and 1D reactions were separately determined; total attenuation and intramultiplet mixing cross sections were also estimated. The Ca(3P0) spin-orbit dependence for the chemiluminescence channel in all the reactions studied was substantial and of similar magnitude, with an order of reactivity J=2>J=1>J=0. This spin-orbit selectivity is related to differences in the evolution of the asymptotic spin-orbit states onto the various electrostatic surfaces at smaller separations. It is remarkable that this selectivity persists with polyatomic reagents.
Far-infrared spectra of methane in condensed rare gas matrices under high pressures
Authors: Nanba T. , Obriot J. , Fondére F. , Marteau Ph.
Published: July 2002

Far-infrared absorption spectra of methane molecule diluted in condensed argon, krypton, and xenon matrices have been obtained at 4.2 K under high pressures. It was found that in the CH4–Ar system the vibrational absorption band grew and its peak position showed a blue shift as the applied pressure increased. On the other hand, the rotational absorption bands in the CH4–Xe and the CH4–Kr systems showed a red shift. The different responses of the rotational absorption band and the vibrational band to the applied pressure are discussed in relation to the induction mechanism of the dipole moment in the molecule.
Theoretical studies of vibrationally assisted reactions of the O3 ⋅ NO van der Waals complex
Authors: Arnold Craig , Gettys Nancy S. , Thompson Donald L. , Raff Lionel M.
Published: July 2002

The exchange reaction and dissociation dynamics of two O3 ⋅ NO van der Waals complexes upon vibrational excitation has been determined at two different internal energies from the results of quasiclassical trajectories. The dynamics for such complexes is found to resemble that for chemical reactions occurring under matrix isolation conditions and to be significantly different from the O3+NO bimolecular collision dynamics. Mode specificity is found for reaction, vibrational predissociation, and intermode energy transfer. Structure specificity is also observed for the van der Waals complexes. In most cases, the asymmetric stretching mode of O3 is found to be the most effective in promoting reaction. For predissociation and intermode energy transfer, the O3 bending mode is usually the most effective. We find that a five-step mechanism consisting of two non-RRKM reactions, a non-RRKM energy transfer step, and two RRKM steps is required to explain the overall reaction. Excitation of the hindered rotational of NO about the O3 symmetry axis is found to significantly influence the dynamics in that partitioning of less than 2% of the energy into such motion dramatically increases the predissociation rate and, by inference, the intermode energy transfer rate. Excitation of the NO vibrational mode is found to be much less effective in promoting reaction or vibrational predissociation on this potential-energy surface.
Hartree–Fock approximation for D-dimensional two-electron atoms
Authors: Loeser J. G. , Herschbach D. R.
Published: July 2002

The Hartree–Fock method for two-electron atoms is generalized to spaces of arbitrary dimensionality. The problem is exactly soluble in two limiting cases, D→1 and D→∞, for any value of the nuclear charge Z. Numerical calculations of the ground-state energy are reported for a wide range of D and for Z=1 to 6, with an accuracy typically better than one part in 1010. Together with previous variational calculations employing the Pekeris method, these results permit the correlation energy to be evaluated with an accuracy typically better than one part in 106. The correlation energy is found to be largely independent of Z for any D. For a given Z, the correlation energy decreases smoothly as D increases; to a good approximation it is simply linear in 1/D. However, the correlation energy remains appreciable even in the limit D→∞; as a fraction of the total energy, for Z=2 the correlation energy varies from 2.28% at D=1 to 1.45% at D=3 to 0.99% as D→∞.
Coexistence curve of acetonitrile and cyclohexane liquid system
Authors: Vani V. , Guha S. , Gopal E. S. R.
Published: July 2002

The paper reports a detailed determination of the coexistence curve for the binary liquid system acetonitrile+cyclohexane, which have very closely matched densities and the data points get affected by gravity only for t=(Tc−T)/ Tc≂10−6. About 100 samples were measured over the range 10−6<t<5×10−1. The order parameter exponent is found to be 0.322±0.004 and the value of Δ is nearly 0.50±0.05 as expected theoretically. There is also a definite anomaly in the diameter which is carefully examined with reference to the predictions of the various theories. The data, when expressed in terms of the volume fraction as the composition variable and analyzed in the asymptotic region, give an exponent of 0.9 for the diameter in keeping with the (1−α) exponent theoretically predicted, the value of α being 0.11 in binary liquids. The extrapolation properties of the Wegner expansion procedure is somewhat inferior to the extrapolation obtained using a renormalization group (RG) crossover expansion. The use of the new variables proposed by Johnston et al. and Vnuk in making the coexistence curve appear symmetric in the new variables is pointed out.
Studies on chemical effects on x-ray intensity ratios of Kβ/Kα in nuclear decay of technetium nuclides 99mTc, 97mTc, and 95mTc
Authors: Yamoto I. , Kaji H. , Yoshihara K.
Published: July 2002

Chemical effects of characteristic x-ray intensity ratios of Kβ/Kα were investigated for the second d-group element Tc in the decay processes 99mTc → 99Tc (two-step isomeric transition), 97mTc → 97Tc (one-step isomeric transition), and 95mTc → 95Mo (electron capture). The objective nuclides were produced by (n, γ)β−, (d, xn), and (α, xn) reactions and were chemically separated from the target materials. The x rays were measured with a pure germanium detector, and the Kβ x rays were analyzed into the two components K′β1 and Kβ2 using a computer program. The chemical effect of the intensity ratio was more pronounced for K′β2/Kα than for Kβ1/Kα, as expected. The effect was larger in KTcO4 than in the other species of Tc2S7, K2TcCl6, and Tc metal. The effect in the two-step isomeric transition in 99mTc was found to be larger than in the one-step isomeric transition in 97mTc. The effect in the electron capture in 95mTc (Mo x rays) was similar to that in 97mTc although the ratio Kβ/Kα was smaller in 95mTc than in 97mTc, reflecting the change of nuclear charge. The tendency of the observed chemical effect was explained by taking into account the interfering factors which involve Pauling’s ionicity in chemical bonding between Tc and its counter atom.
Branching ratio for Penning and associative ionization in collisions Ne*(3P2,0) with Kr at thermal energies
Authors: Brunetti B. , Vecchiocattivi F. , Volpi G. G.
Published: July 2002

The Penning and associative cross sections for the collisions between metastable neon atoms and krypton have been measured as a function of the collision energy in the range 0.025–0.35 eV. The measurements have been performed in a crossed beam experiment, using the time-of-flight technique for the velocity selection and a quadrupole mass spectrometer for the mass analysis of the product ions. The data have been analyzed using a classical model and assuming semiempirical potential energy curves for the three possible states of the NeKr+ ion. The analysis indicates that in the high energy limit the ionization leads to the production of the NeKr+ ion in its ground state.
Electron mobility in supercritical butanes as a function of density and temperature
Authors: Itoh Kengo , Nakagawa Kazumichi , Nishikawa Masaru
Published: July 2002

The mobility of excess electrons in supercritical n- and isobutane was measured as a function of density at several temperatures. The density-normalized mobility μN in both isomers goes through a minimum at a density below the respective critical density and the mobility is quite temperature dependent in this region, then goes through a maximum above the critical density where it is rather insensitive to temperature. The minimum in isobutane is not reproduced by the Cohen–Lekner equation with the structure factor S(K) estimated from the velocity of sound, while it is well accounted for by the model in n-butane. This and other characteristics in the mobility behavior for n-butane are typically those of nonspherical hydrocarbons such as ethane and propane, but are intermediate between spherical and nonspherical hydrocarbons for isobutane.
Photoelectron study of the valence level cross sections of XeF2 from 21 to 50 eV photon energy
Authors: Yates B. W. , Tan K. H. , Bancroft G. M. , Coatsworth L. L. , Tse J. S. , Schrobilgen G. J.
Published: July 2002

Using monochromatized synchrotron radiation, the gas phase photoelectron spectra of the first five valence orbitals of XeF2 have been obtained from 21–50 eV photon energy. From these spectra, the branching ratios and partial cross sections have been calculated. Theoretical branching ratios and partial cross sections have been obtained from MS-Xα calculations and compared with experiment. The generally good agreement between experiment and theory confirms the DV-Xα and MS-Xα orbital assignments: 5πu <10σg <3πg <4πu <6σu, in order of increasing binding energy. Somewhat surprisingly because of the lack of antibonding virtual orbitals above threshold and the long Xe–F bond length (2.00 Å), a number of strong resonances are predicted and observed. For example, resonances are predicted at 4.8 (πg channel), 11.6 (πu, δu channels), and 14.3 eV (σu channel) kinetic energies. The observed resonance positions are usually in reasonable agreement with these. The nature of these resonances are discussed, and compared to similar features in O2, F2, and CO2.
Phase transitions in M(ClO4)2 ⋅ 6H2O (M=Mg, Zn). Investigations by adiabatic calorimetry and infrared spectroscopy
Authors: White Mary Anne , Falk Michael
Published: July 2002

Infrared spectra of Mg(ClO4)2 ⋅ 6H2O at 10 K indicate a complex structure containing at least 16 distinct types of water protons. At 107.9 K this structure transforms sharply to a much simpler disordered form (ΔStrans=7.754 J K−1 mol−1), and at 273.2 K it transforms more gradually to a form in which all water protons are equivalent (ΔStrans=8.25 J K−1 mol−1). There is also a phase transition at 325.36 K (ΔStrans=21.9 J K−1 mol−1). Infrared spectra of Zn(ClO4)2 ⋅ 6H2O at 10 K indicate a disordered structure, with a continuum of proton environments, probably related to the structure of the Mg compound above 107.9 K. The phase transitions in the Zn compound at 70.1, 209.8, 290.5, and 349.0 K (ΔStrans=3.70, 0.08, 9.50, and 21.6 J K−1 mol−1, respectively) occur without a major change in the infrared spectrum. The measured entropy changes in the Mg, Zn, and Cd compounds appear to be related to the number of distinguishable types of water protons that already exist at 10 K.
On water’s ability to dissociate ion pairs
Authors: Goldman Saul , Backx Peter
Published: July 2002

We have carried out a series of Monte Carlo simulations on oppositely charged contact and near-contact ion pairs immersed in two different models for the solvent. One model (ST2 water) was relatively water-like; the other model, in effect a polar solvent, had the same dipole moment as ST2 water, but it did not have the ST2-model’s point-charge distribution. We found that the water-like solvent exerted a stronger and qualitatively different dissociating force on the contact ion pair, relative to the behavior of polar solvent. By examining the solvation shells around a contact or near-contact ion pair, we were able to identify one structural characteristic that contributes to the greater dissociating force with the ST2 model.
Projection operator analysis of macrotransport processes
Authors: Pagitsas Michael , Nadim Ali , Brenner Howard
Published: July 2002

Projection operator methods are used to derive a systematic perturbation expansion scheme that provides formal justification for the otherwise ad hoc arguments underlying generalized Taylor dispersion theory. The latter theory is a coarse-graining procedure for eliminating the internal variables from multidimensional phase-space convective-diffusion-type transport equations in the limit of long times. In addition to rationalizing the otherwise ad hoc Lagrangian moment scheme, projection operator methods are further used to: (i) investigate nonasymptotic effects arising from ‘‘memory’’ of the initial conditions; (ii) establish the existence of higher-order contributions beyond second-order terms in the physical-space spatial gradients. Explicit criteria are developed for when these third- and higher-order non-Gaussian terms may be neglected in the coarse-grained, Taylor dispersion or macrotransport equation.
Isotope separation and yield calculations for vibrationally enhanced oxidation of nitrogen
Authors: McLaughlin David F. , Christiansen Walter H.
Published: July 2002

The separation of nitrogen isotopes by low temperature reaction of vibrationally excited nitrogen gas with oxygen has been studied, in which the formation of 15NO is theoretically favored. The potential yield and isotope separation coefficient β for this process were examined using a numerical simulation of the kinetic processes, which incorporated a steady-state isothermal model of the 14N2 and 14N15N vibrational distribution functions coupled with a non-steady-state kinetic model of the chemical system including N*2, O*2, N, O, and their reaction products. In the absence of O2, the vibrationally enhanced rate coefficient for the reaction N*2+O → NO+N was observed to be inversely proportional to the concentration of O atoms, due to VT loading of the N*2 distribution function. O2 was also found to greatly reduce the rate coefficient due to efficient depletion of the highly excited species via the reaction N2(v)+O2(0) → N2(v−1) +O2(1). Computed reaction yield increases dramatically if both the O*2 and N*2 vibrational temperatures are elevated, but only at the expense of greatly reduced β. The effective separative work for this process was estimated.
Many-body calculations of hyperfine constants in diatomic molecules. I. The ground state of 16OH
Authors: Kristiansen P. , Veseth L.
Published: July 2002

Magnetic hyperfine parameters (Frosch and Foley parameters) have been computed for the 2Π ground state of 16OH by use of many-body perturbation theory. The computations are complete to third order in the many-body expansion, and they were repeated for a series of internuclear distances around re to reproduce the vibrational dependencies of the parameters. Parameters were computed for the three lowest vibrational levels, and were found to be within 1%–2% of the very accurate experimental values. The rather strong vibrational dependencies of the parameters were reproduced with accuracies of 80%–90%. Finally, centrifugal distortion corrections to the magnetic hyperfine parameters were also computed, and for the one parameter (dD) observed, the error was about 5%. The vibrational wave functions needed were obtained from a published accurate CI potential curve. Thus, no empirical results are incorporated in the present ab initio calculations.
Radio frequency–infrared double-resonance and multiphoton Lamb dips in a dressed molecule
Authors: Arimondo E. , Sasso A. , Allegrini M.
Published: July 2002

The molecular response in laser intracavity experiments where molecules interact simultaneously with a radio frequency field and an infrared one is analyzed theoretically. In the quantized field approach the molecule–radio frequency interaction is treated exactly. For the laser intensity the density matrix equations are solved either in a third-order perturbation treatment or through a matrix continued fraction analysis. The resonance line shapes in different two-level and three-level configurations are analyzed numerically.
The rotational spectrum, geometry, and intermolecular force constant of the heterodimer of hydrogen cyanide and fluoroform
Authors: Goodwin Elizabeth J. , Legon A. C.
Published: July 2002

The ground-state rotational spectra of six isotopic species of an intermolecular complex formed by hydrogen cyanide and fluoroform have been measured using the pulsed-nozzle, Fourier-transform microwave technique. The rotational constant B0, the centrifugal distortion constants DJ,DJK,HJ, HJK, and HKJ, and, where appropriate, the nuclear quadrupole coupling constants χ(14N) and χ(D) have been determined for each of the species HC14N⋅⋅⋅HCF3, DC14N⋅⋅⋅HCF3, HC14⋅⋅⋅DCF3, HC15N⋅⋅⋅HCF3, HC15N⋅⋅⋅DCF3, and DC15N⋅⋅⋅HCF3. For HC14N⋅⋅⋅HCF3 the values are as follows: B0=1151.2991(4) MHz, DJ=1.91(1) kHz, DJK=282.75(6) kHz, HJ=−0.1(1) Hz, HJK=44.3(5) Hz, HKJ=53(5) Hz, and χ(14N)=3.948(8) MHz. The form of the spectra and magnitudes of the rotational constants allow the conclusion that the geometry of the complex is of C3V symmetry with the nuclei in the order HCN⋅⋅⋅HCF3 and lead to r(N⋅⋅⋅C)=3.489(2) Å for HCF3 species but 3.483(1) Å for DCF3 species. The intermolecular stretching force constant determined from DJ is kσ=3.52 N m−1.
Calculation of rovibrational spectra of water by means of particles-on-concentric-spheres models. II. Excited states of stretching vibrations
Authors: Natanson Grigory A. , Ezra Gregory S. , Delgado-Barrio Gerardo , Berry R. Stephen
Published: July 2002

It is shown that adiabatic separation of high-frequency stretching modes from bending and overall rotational motions in triatomic molecules XY2 leads naturally to the particles-on-a-sphere (POS) model treated previously [J. Chem. Phys. 81, 3400 (1984)]. Solution of the rovibrational problem using a further approximation in which stretching motions are treated as uncoupled modes is then investigated in detail. It is shown that, for states with a significantly larger number of quanta in one bond than the other, the POS model in this approximation yields energy levels that are essentially identical with those for the particles-on-concentric-spheres (POCS) model, where the latter is obtained using a different decoupling of the basic set of differential equations.
Free-proton range ENDOR study of the inhomogeneous radical distribution in irradiated succinic acid
Authors: Miyagawa Ichiro , Swann Jack T.
Published: July 2002

Free-proton range ENDOR signals were studied in the case of an x-ray irradiated polycrystalline sample of succinic acid. The line shape of the out-of-phase signal, which is believed to arise from the radical cluster with the lowest radical density, was found to be strongly deformed under an intense microwave field.
Average wave function method for multiple scattering
Authors: Singh Harjinder , Dacol Dalcio K. , Rabitz Herschel
Published: July 2002

A general approximation scheme applicable to scattering of atoms and molecules off multicenter targets, is proposed. The total potential is replaced by a sum of nonlocal, separable interactions. Each term in the sum projects the wave function onto a weighted average in the vicinity of a given scattering center. The resultant solution is an infinite order approximation to the true solution, and choosing the weighting function as the zeroth order solution guarantees agreement with Born approximation to second order. In addition, the approximation also becomes increasingly more accurate in the low energy long wavelength limit. A nonlinear, nonperturbative iterative scheme for the wave function is proposed. An extension of the scheme to multichannel scattering suitable for treating inelastic scattering is also presented.
Propensities for rotational energy transfer in the B 3Πg (v=3) state of nitrogen
Authors: Katayama D. H.
Published: July 2002

A two laser, double resonance technique is used to determine propensities for collision induced rotational energy transfer by Ar atoms in the N2 B 3Πg (v=3) state. The results show that e/f parity conservation is much more rigorous for the low J values of the 3Π0 component as compared to the 3Π1 or 3Π2 rotational manifolds. These results are in good agreement with the prediction of Alexander and Pouilly [J. Chem. Phys. 79, 1545 (1983)]. The breakdown of e/f parity conservation for the relatively low value of J=7 in the Ω=0 manifold and collisional transfers from this level to the Ω=1 and 2 spin components are discussed in terms of the effects of spin uncoupling from the internuclear axis. There does not appear to be any obvious propensity for J specificity in the collision induced transfer between any two of the three rotational manifolds separated by the spin-orbit interaction.
Kinematic model of oscillating thermal decomposition reactions of solids
Authors: Strobel Hans
Published: July 2002

A simultaneous system of first-order partial differential equations is proposed for a quasi-isothermal description of experimental results on the decomposition of gypsum obtained by Heide et al. (1980). The equations resemble Volterra–Lotka-type models of population dynamics. They are shown to contain parameter regions leading to self-oscillations and oscillating spatial structures which may explain the observed oscillations of overall decomposition rate. The properties of slightly modified equations with respect to front propagation and creation are inspected for the possibility of providing a second, alternative explanation of experimental facts, basically taking into account the external heating of the sample.
Analytic expressions for the equilibrium distributions of isomer groups in homologous series
Authors: Alberty Robert A. , Oppenheim Irwin
Published: July 2002

The equilibrium distribution of isomer groups in a homologous series is a function of the ratio of the number of hydrogen atoms to the number of carbon atoms at a given temperature and is independent of the pressure, except for homologous series with the molecular formulas CnH2n. Alternatively, the distribution can be taken to be a function of the equilibrium partial pressure of ethylene for all homologous series. An advantage of using the partial pressure of ethylene as the independent variable is that the equilibrium distribution can be expressed as an analytic function of the carbon number at higher carbon numbers. The distribution functions have been derived for the alkanes and alkenes for the case that the standard Gibbs energies of formation of the isomer groups are a linear function of carbon number. Experimentally it is found that a linear relation is approached as the carbon number is increased. Expressions are derived for the mean carbon number and the carbon number contributing the largest weight fraction for given conditions, without calculating the whole distribution using a general equilibrium program.
A stochastic dynamics approach to quantum Boltzmann statistical mechanics
Authors: Lamm Gene
Published: July 2002

An alternative to the standard Metropolis-based approach for evaluating quantum statistical mechanical ensemble averages is introduced. The method is based on the close similarity between the Bloch equation and the Smoluchowski diffusion equation. Stochastic ‘‘Bloch trajectories’’ are constructed from local solutions to the Bloch equation for the density matrix and used to evaluate Boltzmann ensemble averages and partition functions. This is analogous to the use of Brownian dynamics trajectories in diffusion calculations. Sample results for selected one-dimensional systems are presented and the extension to multidimensional problems is discussed.
The effect of pressure on electronic excitations in TCNQ and its complexes
Authors: Tkacz M. , Jurgensen C. W. , Drickamer H. G.
Published: July 2002

The electronic spectra of TCNQ and TCNQ complexes has been investigated at high pressure up to 150 kbar. The observed pressure shifts are discussed in terms of intermolecular interactions. The results are also compared with available theoretical calculations.
Dynamics of charge transfer Ar++(3P)+He(1S)→Ar+(2P)+He+(2S) at low collision energies: Comparison of experimental results with quasiclassical calculations of the differential cross sections
Authors: Friedrich B. , Pick Š. , Hládek L. , Herman Z. , Nikitin E. E. , Reznikov A. I. , Umanskii S. Ya.
Published: July 2002

Differential cross sections of the elementary process Ar++(3P)+He(1S)→Ar+(2P)+He+(2S) at collision energies between 0.6 and 1.6 eV were calculated using the quasiclassical treatment. Analytic potentials providing an approximation of the electronic energy of the system were employed to calculate deflection functions. Coupling of the diabatic terms was evaluated using the asymptotic method, and the Landau–Zener model was applied to account for the transition probability. Semiquantitative agreement of the calculated differential cross sections with those obtained in crossed beam experiments was achieved.
ESR line shape studies of N-(4-n butyl benzilidine) 4-amino 2,2,6,6-tetramethyl piperidine 1-oxide (BBTMPO) in toluene
Authors: Hwang J. S. , Pollet P. , Saleem M. M.
Published: July 2002

BBTMPO was synthesized by a condensation of 4-n butyl benzaldehyde and 4-amino 2,2,6,6-tetramethyl piperidine N-oxide (4-tempamine). The shape of the resulting spin probe, estimated using the Dreiding models, is a spherocone of length 10.7 Å capped by hemispheres of radii 6.6 and 4.0 Å, respectively. The magnetic parameters of BBTMPO in toluene at 77 K were determined. Variable temperature ESR line shape studies in the temperature range of 142–211 K revealed that BBTMPO exhibited anisotropic rotational diffusion in toluene along the long axis of the spherocone and in the direction parallel to the N–O bond. Rotational diffusion along the long axis was found to be 3±0.5 and 7±0.5 times faster for frozen and liquid toluene, respectively, than the other two axes. The line shapes have been simulated by using the stochastic Liouville theory of slow-motional effects on ESR spectrum. The slow tumbling spectrum (158 K) in the model-sensitive region showed good agreement for a Brownian model. The plot of τR vs 1/T over the whole temperature range yield a good linear fit with two slopes, with the activation energy for reorientation process of 4.9 kcal/mol in the liquid state, and 9.9 kcal/mol in the solid state. The higher activation energy of rotational reorientation coupled with a lower anisotropy of molecular reorientation (N) in the solid state, indicates that the rotational degrees of freedom is more restricted in the frozen state than in the liquid state. Molecular reorientation for BBTMPO in toluene was analyzed in terms of the hydrodynamic free space model for molecular relaxation in liquids. The stickiness factor S was found to be 0.6 while the anisotropic interaction parameter κ which measures the coupling of rotation to translation was found to be 0.7. For perdeuterated 2,2,6,6-tetramethyl-4-piperidine N-oxide (PD-Tempone) in toluene the stickiness factor was calculated to be 0.3, and κ was 0.4. The doubling in size and weight in going from PD-Tempone to BBTMPO increases the stickiness factor and κ from 0.3 to 0.6, and 0.4 to 0.7, respectively. The similarity in the structure of BBTMPO to that of liquid crystal methoxy benzilidine butyl aniline (MBBA) and its potential use as a spin probe for MBBA is discussed.
Photoinduced reorientation in an electron donor–acceptor complex: An optically detected ENDOR study of triplet traps in anthracene/tetracyanobenzene
Authors: Bos F. C. , Schmidt J.
Published: July 2002

We have performed an optically detected electron-nuclear double resonance (ODENDOR) study of one of the low-lying triplet excitations in the charge transfer complex anthracene/tetracyanobenzene. The study shows that the spectroscopic characteristics associated with this specific excitation are related to an almost complete charge transfer from anthracene to a nearest neighbor tetracyanobenzene molecule. This photoinduced charge transfer results in a reorientation of the (effectively charged) partners of the EDA complex. The possible impact of this result for the other types of low-lying triplet excitations is discussed.
Observation of surface roughening on Ni(115)
Authors: Conrad Edward H. , Aten Ralph M. , Kaufman Donald S. , Allen Lynn R. , Engel Thomas , den Nijs Marcel , Riedel Eberhard K.
Published: July 2002

We present evidence that the equilibrium topography of the Ni(115) surface in the temperature range 100 K<T<1000 K is that expected above its roughening temperature. This conclusion is based on a detailed line shape analysis of the specular diffraction peak at anti-Bragg angles as a function of temperature. The experimental results are combined with a generic phase diagram for stepped (11m) surfaces of face centered cubic (fcc) metals and is estimated that the roughening temperature of Ni(115) is below 50 K, that of Ni(113) approximately 200±50 K, and that of Ni(001) well above 420 K.
Rotating waves in reaction-diffusion systems with folded slow manifolds
Authors: Sultan Rabih , Ortoleva Peter J.
Published: July 2002

A scaling approach to multiple time scale systems with folded slow manifolds is applied to reaction-diffusion systems. Plane periodic wave trains, rotating waves, and static patterns are obtained. The periodic (planar and rotating) waves are shown to arise as a finite amplitude bifurcation at zero frequency.