bluuebunny/crossref_metadata_2025 huggingface dataset:
An ion beam study of reactive scattering of halide ions by methyl halides
Authors: White N. R. , Scott D. , Huq M. S. , Doverspike L. D. , Champion R. L.
Published: February 2003

Measurements of electron detachment cross sections, reactive charge transfer, and differential elastic and inelastic scattering cross sections for Cl−, Br−, and I− ions on CH3I, CH3Br, and CH3Cl targets are reported. The energy range for the experiments is from 3 to 150 eV. Of the very large number of reaction channels open, it appears that most make significant contributions to the total cross section for sufficiently high projectile energies. The SN2 reaction, which may be dominant at lower energies, was not observed and is probably not significant above the lowest energies in this study. Surviving primary ions exhibit a broad spectrum of inelastic loss, suggesting target breakup.
Approximate calculation of physical properties of enclosed central field quantum systems
Authors: Arteca Gustavo A. , Fernández F. M. , Castro E. A.
Published: February 2003

It is shown that certain formulas satisfied by exact eigenfunctions are very useful to obtain good approximations for the values of physical properties of enclosed central field quantum systems, even though one uses rather poor trial functions. The properties studied in this work (under the assumption of the validity of a very simple model) are the hyperfine splitting and the pressure of the system.
Multiple nucleation pathways near triple points of Ar–Kr mixtures
Authors: Phillips Philip , LaViolette Randall A. , Pratt Lawrence R.
Published: February 2003

Cluster partition functions for droplets of Ar–Kr mixtures are evaluated at 90 and 120 K by the inversely restricted sampling (IRS) algorithm. These quantities and series expansion techniques are used within the droplet model to condensation to predict the condensation curve in the neighborhood of a triple point of the Ar–Kr phase diagrams. Using the same series, the free energy surface for droplet growth is examined for supersaturated states of the mixture. For strongly supersaturated states in the neighborhood of the triple point, we find two saddle-point free energy barriers. It is suggested that these physical circumstances provide a sensitive testing ground for molecular theories of homogeneous nucleation.
Mobilities of poly-L-lysine molecules in low-salt solutions
Authors: Zero Karl , Ware B. R.
Published: February 2003

We have studied the ordinary–extraordinary phase transition of poly-L-lysine solutions using the techniques of quasielastic light scattering (QELS), electrophoretic light scattering (ELS), and fluorescence photobleaching recovery (FPR). Polymer of molecular weight 90 kg/mol was examined in aqueous solutions at a concentration of 1 g polymer/l. The apparent mutual diffusion coefficient, as measured by QELS, was observed to increase sharply by an order of magnitude (from 0.8 to 7×10−7 cm2/s) as the KCl concentration was increased from 2 to 5 mM, in agreement with previous reports by Schurr and co-workers. On the other hand, the tracer diffusion coefficient (measured by FPR) varies only by about 50% (from 4 to 6×10−7 cm2/s) in a broader trend over the range from 0 KCl to 10 mM KCl. The electrophoretic mobility (measured by ELS) mirrors the change in the tracer diffusion coefficient (from 1.9 to 2.6 μm cm/V s). The ELS and FPR data indicate that poly-L-lysine exists primarily as a random coil with little restriction of translational motion due to intermolecular interactions. The QELS data must then be interpreted in terms of fluctuation mobilities whose lifetimes are not characteristic of the motion of individual molecules.
Coexistence and spinodal curves in directionally bonded liquids using the four-cluster approximation
Authors: Bodegom E. , Meijer Paul H. E.
Published: February 2003

We derive the phase diagrams and spinodals of binary liquid systems with anisotropic interactions, such as hydrogen-bonded molecules. The work is based on the four-particle cluster variation method, using a different potential for different contact points. It is shown that the introduction of a cluster larger than previously used by Barker and Fock, leads to a considerable improvement in the shape of the phase diagram and avoids some of the difficulties encountered in their calculation. Phase diagrams are displayed for various choices of the parameters: the number of contact points, the interaction potential, and the order of the approximation.
Configurational statistics of Brownian dumbbells in a quadratic flow
Authors: Keh Huan J. , Anderson John L.
Published: February 2003

The configurational distribution function of dumbbell macromolecules (rigid and linearly elastic) in a quadratic rectilinear flow is obtained. It is shown that, compared to linear flow results, the inhomogeneity of shear rate alters the distribution function in a significant way when the length scale of the inhomogeneity is comparable to the dumbbell size. Based on these configurational distribution functions, the mean slip velocities, and stress contributions of the dumbbells are computed. The analysis shows that the Kramers expression for the stress tensor must be replaced by the Irving–Kirkwood formulism which admits effects of local spatial variation in shear rate or dumbbell concentration on the ensemble averaging of the dumbbell connector force.
X-ray study of the rotator phase of paraffins (IV): C27H56, C28H58, C29H6, C3H62, C32H66, and C34H7
Authors: Doucet J. , Denicolò I. , Craievich A. F. , Germain C.
Published: February 2003

X-ray investigations performed on the rotator phases of n alkanes CnH2n+1 with 26≤n≤34 prove that their structures are different from those exhibited by lower-numbered compounds. Two kinds of phases with monoclinic and triclinic symmetries are encountered both with the molecules tilted with respect to the layer plane. The interlayer stacking modes are compared to those found in thermotropic liquid crystals.
The microwave spectrum of a triplet carbene: HCCN in the X 3Σ− state
Authors: Saito Shuji , Endo Yasuki , Hirota Eizi
Published: February 2003

A simple carbene, the HCCN radical, has been identified in the gas phase using a microwave spectroscopic method. The HCCN molecule was generated in a free space absorption cell by the reaction of CH3CN with the microwave discharge products of CF4. Five rotational transitions, each split into three fine structure components, were observed in the region of 110 to 198 GHz. No hyperfine structure was resolved, although some of the observed lines showed broadening. The rotational constant, the centrifugal distortion constant, the spin–spin coupling constant, and the spin-rotation coupling constant were determined with good precision. The observed spectrum is completely consistent with that expected for a linear molecule in a 3Σ state, in agreement with an earlier matrix EPR study of Bernheim et al. [J. Chem. Phys. 43, 196 (1965)].
On the reactive infinite order sudden approximation S matrix
Authors: Baer M. , Kouri D. J. , Jellinek J.
Published: February 2003

We reconsider the S matrix in the l-labeled version of the reactive infinite order sudden (RIOS) approximation. The roles of the γλ, γν, and Bλν are clarified and a general expression obtained for the S matrix which results from averaging over all possible Bλν.
Effect of vibrations on the energy unresolved electron scattering by H2 and D2
Authors: Szalewicz Krzysztof , Kol/os Wl/odzimierz , Monkhorst Hendrik J. , Jackson Ceasar
Published: February 2003

The total (elastic plus inelastic) energy unresolved differential cross section for high energy electron scattering, computed previously [J. Chem. Phys. 77, 1323 (1982)] for H2, has now been obtained for D2. The effects of molecular vibrations are discussed and compared with the data derived from experiment. It is shown that the observed difference between H2 and D2 experimental data was largely an artifact due to an approximate vibrational averaging in the independent atom model employed in the processing of directly measured quantities. However, the effects of vibrational motion of nuclei are large enough to enable an analysis of various approximate molecular potentials.
Electron paramagnetic resonance of Mn2+, VO2+, Cu2+, and x-ray induced centers in Rb2C2O4 ⋅ H2O single crystals
Authors: Jain V. K. , Seth V. P. , Malhotra R. K.
Published: February 2003

The electron paramagnetic resonance spectra of Mn2+, VO2+, Cu2+, and x-ray induced centers in Rb2C2O4 ⋅ H2O single crystals are studied at 9.45 GHz and 290 K. The spectra at 77 K show no appreciable change. The ions appear to enter the lattice at interstitial sites. The doublet separations of forbidden hyperfine transitions (Δm=±1) are studied in Mn2+ doped crystals. The x-ray irradiated single crystals exhibit singlet spectrum and show saturation effect. The spin-Hamiltonian parameters are evaluated at 290 K.
A picosecond CARS study of vibron dynamics in molecular crystals: Temperature dependence of homogeneous and inhomogeneous linewidths
Authors: Schosser Claire L. , Dlott Dana D.
Published: February 2003

An extensive picosecond CARS and spontaneous Raman investigation of temperature dependent dephasing of the totally symmetric vibrons in naphthalene (h8N), perdeuteronaphthalene (d8N), anthracene (h10A), and perdeuteroanthracene (d10A) crystals is presented. The low temperature picosecond decays, ranging from 128 to less than 10 ps, are due to vibrational relaxation by phonon emission. Unlike optical phonons in these crystals, vibron lifetimes of h8N and h10A do not decrease monotonically with frequency, a pattern which is observed in other molecular systems. The low temperature decay rate is proportional to the square of a cubic anharmonic matrix element and a density of difference states function which has a contribution from phonons and lower frequency vibrons. This function is evaluated for each substance and together with measured lifetimes yields matrix elements which are roughly equal for each vibron. We conclude that vibrational relaxation rates are controlled primarily by the density of final states. Relaxation in the deuterated crystals is faster than predicted by this model due to residual isotopic impurities and purely intramolecular vibrational relaxation. Three h8N and one h10A vibrons are studied over an extended temperature range. The homogeneous linewidth increases as Tn with n=2.0–2.4. The observed increase is consistent with the above energy relaxation model up to ∼80 K. From ∼80 K to the melting point, this temperature dependence is consistent with a pure dephasing process involving energy exchange with several thermally populated low frequency vibrons. From ∼250 K to the melting point inhomogeneous broadening is observed. It is suggested that the inhomogeneity is due to slowly (≳10 ps) varying density fluctuations which modulate intermolecular attractive interactions. In this respect, molecular crystals near the melting point resemble some molecular liquids.
A 2H NMR study of molecular motions of nonadecane-d40 in a urea inclusion adducta)
Authors: Casal Hector L. , Cameron David G. , Kelusky Eric C.
Published: February 2003

The 3H NMR spectrum of nonadecane-d40 included in a urea adduct has been recorded at several temperatures in the range 145–273 K. This inclusion adduct is found to undergo a phase change in the range 160–170 K. In the high temperature phase the spectra indicate that the included alkane molecules are freely rotating. The amplitude of rotation is greatly diminished in the low temperature phase; however, the spectra are not those characteristic of the 2H NMR rigid limit. We also present direct evidence that the chain segments next to and including the methyl groups are undergoing motions of greater amplitude than those corresponding to free chain rotation.
Electron diffraction studies of supersonic jets. VI. Microdrops of benzene
Authors: Valente Edward J. , Bartell Lawrence S.
Published: February 2003

Large clusters of benzene were generated in supersonic flow through a Laval nozzle with carrier gases He, Ne, or Ar. Benzene concentration and carrier pressures were varied over wide ranges. Diffraction patterns recorded from s=0.8 to 12 Å−1 revealed strong intermolecular interference features indicative of supercooled bulk liquid at 100–200 K. Computer simulations of nucleation and cluster growth were carried out and agreed with the following observations. An initial benzene partial pressure of ∼0.1 bar expanded through a nozzle of our design requires a carrier pressure of 2–3 bar to give reasonably complete condensation. Cluster diameter and fraction condensed decrease if (1) benzene mole fraction is reduced at constant carrier pressure, (2) carrier pressure is reduced at constant benzene partial pressure, and (3) carrier molecular weight is decreased. The simulations identify the factors accounting for the trends. Cluster intensities are compared with those calculated for small clusters proposed by Williams, small crystal fragments, and bulk fluid according to the statistical mechanical algorithm RISM. None of the models reproduce observed results within experimental error but comparisons provide information about size, temperature, and structure. Intrinsic deficiencies of RISM are indicated by the present experiments.
Semiclassical vibrational eigenvalues of a three-dimensional Hamiltonian
Authors: Duchovic Ronald J. , Swamy Kandadai N. , Hase William L.
Published: February 2003

A fully coupled realistic mode three-dimensional Hamiltonian representing the H–C–C→H+C=C dissociation is investigated semiclassically. This system has been found to exhibit quasiperiodic behavior above dissociation. Semiclassical vibrational eigenvalues and frequencies have been computed using the Sorbie–Handy method of semiclassical quantization.The semiclassical spectrum has also been computed from the autocorrelation function of the dynamical variable via a Fourier transform.
Two-dimensional hindered internal rotations in activated complexes of the form XH2
Authors: Pacey Philip D. , Wagner Brian D.
Published: February 2003

For reactions of the form X+H2→XH+H, where X represents, H, O, F, or Cl and H represents light hydrogen or deuterium, the usual degenerate, harmonic, bending vibrations of activated complex theory have been replaced by a two-dimensional internal rotation hindered by a sinusoidal potential function. This replacement has the following effects: (a) The zero point energy of the complex is lowered, reducing the Arrhenius activation energy and increasing the reaction rate constant by 45% or less close to room temperature. (b) At very high temperatures, the Arrhenius plot is less strongly curved and the rate constant becomes less than that for the harmonic oscillator case. Provided the best available parameters and tunneling factors are used, the results are in agreement with experiment. At temperatures above about 1000 K, the results of the hindered rotor model approach those of a modified version of simple collision theory. For some reactions, the heat capacity of activation, a measure of Arrhenius plot curvature, exhibits two maxima as a function of temperature; the higher temperature maximum is caused by the hindered internal rotation, the low temperature one by tunneling.
Phase transitions in a lattice gas model for coadsorption
Authors: Huckaby Dale A. , Kowalski Jacek M.
Published: February 2003

A lattice gas with two different sizes of interacting particles on a square lattice is studied. Two large particles are excluded from occupying a pair of nearest neighbor sites and repel if they occupy a pair of next nearest neighbor sites. A small particle on a lattice site repels both large particles and small particles which occupy nearest neighbor sites. Using reflection positivity and the Peierls argument, as many as five different types of ordered phases are proved to exist at sufficiently low temperatures. The model may be useful for the description of competitive adsorption from a gaseous mixture containing adatoms of different radii.
The coordination environment of Eu(III) ions in hydrated A and Y zeolites as determined by luminescence lifetime and EXAFS measurements
Authors: Suib S. L. , Zerger R. P. , Stucky G. D. , Morrison T. I. , Shenoy G. K.
Published: February 2003

Luminescence lifetimes and EXAFS show that the average coordination of Eu3+ in the large-pore zeolite Y is identical to that in aqueous solution, while the coordination number in the small-pore zeolite A is lower, probably because of steric constraints.
Fourier transform pure nuclear quadrupole resonance by pulsed field cycling
Authors: Bielecki A. , Murdoch J. B. , Weitekamp D. P. , Zax D. B. , Zilm K. W. , Zimmermann H. , Pines A.
Published: February 2003

We report the observation of Fourier transform pure NQR by pulsed field cycling. For deuterium, well resolved spectra are obtained with high sensitivity showing the low frequency ν0 lines and allowing assignments of quadrupole couplings and asymmetry parameters to inequivalent deuterons. The technique is ideally applicable to nuclei with low quadrupolar frequencies (e.g., 2D, 7Li, 11B, 27Al, 23Na, 14N) and makes possible high resolution structure determination in polycrystalline or disordered materials.
Uniform chemical shift scaling: Application to 2D resolved NMR spectra of rotating powdered samples
Authors: Aue W. P. , Ruben D. J. , Griffin R. G.
Published: February 2003

Chemical shift scaling experiments which reduce the number and intensity of sidebands in high field magic angle sample spinning (MASS) spectra are described. They rely on the fact that the scaling pulse sequence reduces the effective size of the shift anisotropy, so that moderate speed spinning results in spectra devoid of sidebands of appreciable amplitude. In principle, the technique allows a realization of the full signal strength available in high field MASS spectra. In addition a number of other interesting features have emerged from the experiments. First, the problem of decoupling in the presence of a scaling sequence has been solved by arranging the rf field strengths to mismatch the Hartmann–Hahn condition. Second, it is shown that the scaling cycles used in the experiments are uniform over a wide frequency range with respect to both the scaling factor and the linewidth. Third, under certain conditions, the frequencies for scaling and spinning are about the same size and consequently new features appear in the spectra which we refer to as combination sidebands, combination artifacts, and zero and νr artifacts. We believe these features will be shared by many multiple pulse/MASS spectra. Finally, we demonstrate that a complete separation of centerbands and sidebands in high field MASS spectra is possible by incorporating chemical shift scaling into a two-dimensional experiment.
A theoretical study of resonance Raman scattering from molecules. IV. Anharmonic effect
Authors: Kono H. , Ho Z. Z. , Lin S. H.
Published: February 2003

We have investigated the effects of anharmonicity on the resonance Raman scattering by using the Morse potential for molecular vibration. The Raman intensity is computed by the conventional sum-over-states method. Several conditions in which the anharmonicity effects are manifest have been derived from our numerical calculations. We have also compared our exact results with those of the short time approximation, which is the most tractable approximation in the time-dependent theory of Raman scattering proposed by Heller et al.
Generalized theoretical treatment of axially symmetric ESR hyperfine centers under slow orientational diffusion motion
Authors: Lee Sook , Ames D. P.
Published: February 2003

The ensemble averaging based on the solution of an isotropic diffusion equation is employed to calculate analytical expressions for angular-dependent motional linewidths and shifts in spectral line position of axially symmetric ESR hyperfine centers, which undergo isotropic, small-angle, slow-orientational diffusion motion. The resultant theoretical expressions are consistent with those calculated by other investigators using different theoretical approaches. As a result of the present calculations, the entire ESR hyperfine spectra associated with slow-tumbling motions can be calculated in polycrystalline/amorphous substances. A detailed investigation of these theoretical spectra supports the validity of the previously proposed simple method of determining experimental rotational correlation times from the behavior of the parallel-edge lines of nitroxide ESR powder spectra.
On NO2 excited state lifetimes and g factors in the 593 nm band
Authors: Bylicki F. , Weber H. G. , Zscheeg H. , Arnold M.
Published: February 2003

The investigations reported here aim to resolve a controversy concerning the excited state lifetimes and g factors. The results reveal a correlation between the values of lifetime and g-factor measurements, which was not seen before.
Unimolecular dissociation rates of the chlorobenzene cation prepared by multiphoton ionization
Authors: Durant J. L. , Rider D. M. , Anderson S. L. , Proch F. D. , Zare R. N.
Published: February 2003

We have studied the unimolecular reaction C6H5Cl+→C6H+5 +Cl as a function of internal energy by using multiphoton ionization (MPI) on a supersonically cooled jet of chlorobenzene to prepare the excited parent ion. Specific rate constants k(E) were obtained from an analysis of the distorted C6H+5 peak shape in a time-of-flight (TOF) mass spectrometer. Decomposition in the wavelength region 265–270 nm is shown to proceed by two-photon ionization followed by one-photon absorption. The internal energy of the excited parent ion is characterized in a separate experiment which measures the photoelectron kinetic energy distribution, obtained over the same wavelength range. The specific rate constants found in this study compare well with previous literature values, thus validating this MPI TOF technique for the determination of ion decomposition rates.
Vibrational energy disposal in polyatomic ion–molecule reactions: SF−6+H, D→SF−5+HF(v), DF(v)
Authors: Hamilton Charles E. , Bierbaum Veronica M. , Leone Stephen R.
Published: February 2003

Vibrational state distributions of the SF−6+H, D→SF−5 +HF(v=0–12), DF(v=0–17) ion–molecule reactions are investigated with the flowing afterglow-infrared chemiluminescence technique. The nascent distribution for the hydrogen reaction is (0.00)v=1 : (0.17)v=2 : (0.30)v=3 : (0.24)v=4 : (0.13)v=5 : (0.11)v=6 : (0.05)v=7 and for the deuterium reaction is (0.00)v=1 : (0.06)v=2 : (0.11)v=3 : (0.14)v=4 : (0.14)v=5 : (0.23)v=6 : (0.15)v=7 : (0.08)v=8 : (0.09)v=9. The fractions of the available energy deposited into the HF and DF vibrations are 0.37 and 0.38, respectively. The distributions do not show the characteristics of a statistical distribution that might be expected if a long-lived complex occurs in the reaction. Instead, the distributions show a moderate amount of vibrational excitation due to an initial attractive energy release. Since the fraction of the available energy deposited into the diatomic vibration correlates well with the attractive energy release in L+HH′ systems, a larger fraction of the energy is released as repulsion during S–F bond scission, favoring product translation and SF−5 vibration. The collisions are most likely direct with negligible effects due to secondary encounters.
Theory of chemical reactions of vibronically excited H2(B 1Σ+u). II. Noble gas dihydrides
Authors: Nicolaides C. A. , Zdetsis Aristides
Published: February 2003

The recently discovered chemically bound excited state of HeH2 from large MRD–CI calculations can be explained in terms of the highly ionic character of the H2B 1Σ+u excited state at R=4.0 a.u. and the resulting binding between it and the polarized He via electrostatic and overlap (with the diffuse ‘‘H −’’ center) effects. Based on this explanation, a theory for chemical reactions of normally nonreactive systems is proposed. These reactions may occur when one of the reacting molecules is excited vibronically to a state of maximum ionicity. The H2B 1∑+u state presents a convenient test case. State-specific, open shell SCF calculations have been carried out for the He+H*2, Ne+H*2, and Ar+H*2 systems. The results for He+H*2 agree with the MRD–CI calculations and demonstrate that electron correlation is reasonably constant over the potential energy surface. NeH*2 and ArH*2 molecules are predicted for the first time to bind chemically in an excited 1A′ state at geometries similar to HeH*2 and close to a crossing with the unbound ground state into which they can decay radiatively or via nonadiabatic coupling. Our results suggest that, in accordance with the previous observations by Fink et al. [J. Chem. Phys. 56, 900 (1972)] of quenching of fluorescence of the H2B 1Σ+u→X 1Σ+g transition in a He+H*2 system, quenching should be observed in Ne+H*2 and Ar+H*2 systems, but this time for higher vibrational numbers.
Hard sphere sequential impulse model for endoergic reactions: Angular and velocity distributions of the products
Authors: Safron S. A. , Coppenger G. W. , Smith V. F.
Published: February 2003

Formalism is developed for the scattering of the products of the reaction A+BC → AB+C, produced by the sequence of collisions: A strikes B which then strikes C. The collisions are treated as purely hard sphere impulses except that energy may be absorbed in either collision. Analytical expressions are given for the differential cross sections of the scattered C, which may be converted easily to those of the scattered AB product. Angular-velocity contour diagrams for C are presented which illustrate the features of the model. Formulas for the total reactive cross sections are also given. Finally, comparisons are made between this model and other widely used models.
An improved basis set for H2O. Test on Rydberg excited states
Authors: Durand Gérard , Volatron François
Published: February 2003

The optimization of a basis set for H2O is presented via the test and improvement of two standard and two adapted to H2O basis sets. The valence+polarization part is optimized at the SCF level; then, a new method for determining Rydberg orbital exponents is presented at the SCF+CI level. The achieved basis is tested on the calculation of the electronic spectrum of H2O. Results are compared to theoretical and experimental data.
Local-density Hartree–Fock theory of electronic states of molecules with self-interaction correction
Authors: Pederson Mark R. , Heaton Richard A. , Lin Chun C.
Published: February 2003

A scheme for incorporating the self-interaction correction (SIC) to the local density approximation of the Hartree–Fock theory of electronic structure of molecules is presented. This method is applied to the N2 molecule and the resulting orbital energies and total energy are in good agreement with the Hartree–Fock values.
Symmetry-specific densities of vibrational levels for polyatomic molecules
Authors: Sinha Amitabha , Kinsey James L.
Published: February 2003

An extension of the algorithm of Stein and Rabinovitch for exact counting of the densities and sums of vibrational states of polyatomic molecules is described. The new algorithm permits the same sort of efficient computation of exact counts by symmetry species. Several examples for molecules belonging to various point groups are given, and a high-energy limiting distribution among various symmetry species is conjectured.
A comparative theoretical analysis of the photochemistry of the methyl radical and related systems
Authors: Yu H. T. , Sevin A. , Kassab E. , Evleth E. M.
Published: February 2003

Portions of the ground and 3s,3px,y,z Rydberg excited state C–H and C–H2 fragmentation pathways are computed for the CH3 radical at the 4-31G+Rydberg+CI level. It is concluded that the B̃ state photochemistry of CH3 should give principally C–H bond rupture in analogy with the N–H bond rupture occurring in the Ã state of NH3. Higher energy photolyses should give a mixture of products whose mechanisms of formation will be difficult to establish. Rydberg extended molecular orbital and state correlation diagrams are developed for these transformations. The theoretical relationship between the behavior of the B̃ state C–H bond rupture surface in CH3 and the fragmentation occurring in the Ã states of NH3, H2O, and CH4 is established. Finally, an analysis is made as to the probable adiabatic photochemistry of the 3s Rydberg states of alkyl radicals.
Monte Carlo simulation of diffusion controlled colloid growth rates in two and three dimensions
Authors: Meakin Paul , Deutch J. M.
Published: February 2003

The kinetics of diffusion limited aggregation have been explored in two and three dimensions using Monte Carlo simulations. If the initial particle concentration is very low, our model is equivalent to the Witten–Sander model of diffusion limited aggregation, and the resulting cluster has a Hausdorff dimensionality (D) substantially lower than the ordinary Euclidean dimensionality (d) (D≂5d/6). Under these conditions, the radius of gyration (Rg) of the clusters grows according to the rate law Rg(t)∼t[1/(2+D−d)] obtained previously. If the initial particle concentration is large, the radius of gyration increases linearly with time, and the clusters are uniform (D=d) on all but very short length scales. In general, the clusters grow like Witten–Sander clusters during the early stages of growth. As the clusters grow larger and larger, they become less and less dense until their density approaches that of the surrounding medium. At this stage of growth, there is a crossover from a growth exponent of 1/(2+D−d) for Rg to linear growth, and the dependence of the radius of gyration on cluster size crosses over from Rg∼Nβ (β≂6/5d) to Rg∼Nβ′ (β′=1/d). The structure of the cluster is Witten–Sander-like on short length scales but uniform on long length scales. At no stage does the cluster growth follow the classical t1/2 behavior. In two dimensional space, the rate of increase of mass is given by N(t)∼t for diffusion limited aggregation in the early stages of growth.
Cold jet infrared absorption spectroscopy: The ν3 band of PF5
Authors: Takami Michio , Kuze Hiroaki
Published: February 2003

The ν3 band of PF5 is studied at a low rotational and vibrational temperature using a tunable diode laser. Infrared absorption in a pulsed supersonic free jet of PF5, pure or seeded in Ar, is observed by phase-sensitive detection synchronized with the pulse frequency. Well resolved K structure is observed for the ν3 fundamental and the ν3+ν7←ν7 hot band. Seven molecular constants are determined for the ν3 fundamental and five constants for the hot band. The splitting due to Berry pseudorotation is not found in these two bands under the present resolution of 10 MHz.
Time resolved 360 to 600 nm luminescence produced by the pulse radiolysis of water vapor
Authors: Freeman C. G. , Quickenden T. I. , Sangster D. F.
Published: February 2003

Water vapor was excited by a 0.53 MeV pulsed electron beam and examined for luminescence emission in the 360–600 nm region. Spectral lines arising from the Balmer transitions H(n=4→n=2), H(n=5→n=2), and H(n=6→n=2) of the hydrogen atom and the transitions 3p4D05/2→3s4P3/2, 3p4D07/2→ 3s4P5/2, and 3p4D01/2→3s4P1/2 of the O+ ion were observed. The Balmer lines were found to be quenched by water vapor with rate constants at 293 K of (9.77±0.26)×10−9, (10.67±0.45)×10−9, and (12.3±2.3)×10−9 cm3 molecule−1 s−1, respectively. The excited O+ ions were also quenched strongly by water vapor but line intensities were insufficient for rate measurements to be carried out.
Structure and internal rotation of H2O–CO2, HDO–CO2, and D2O–CO2 van der Waals complexes
Authors: Peterson K. I. , Klemperer W.
Published: February 2003

The radio frequency and microwave spectra of H2O–CO2, HDO–CO2, and D2O–CO2 have been measured by molecular beam electric resonance spectroscopy. Rotational constants, deuterium quadrupole coupling constants, and dipole moments are reported.
A recoupled states approximation for molecular scattering theory
Authors: McLenithan Kelly , Secrest Don
Published: February 2003

Recent work is reviewed on the theory of angular momentum decoupling approximations in inelastic molecular scattering theory. It is shown that the failure of these approximations is more extensive than previously believed, particularly for completely state-selected m transitions. A systematic study of a number of centrifugal sudden approximations is made and discussed. A new method is presented for improving these approximations, which recouples the approximate solutions. Completely state-selected differential and integral cross sections are computed under the centrifugal decoupling approximation and its correction for Ne+HD scattering at a total energy of 31.5 meV. Five choices of the partial wave decoupling parameter (including the three known ones) are studied. The numerical results show that with the partial wave parameter chosen to be the arithmetic mean of the initial and final l-quantum numbers, the coupled states approximation gives excellent results for orientation averaged transitions, but fails dramatically for state-selected m transitions. The corrected centrifugal decoupling approximation gives significantly more accurate inelastic differential and integral state-selected cross sections than its uncorrected counterpart. The extension of this correction technique to the energy sudden and the infinite order sudden approximations is discussed.
Analytic solution of relaxation in a system with exponential transition probabilities. III. Macroscopic disequilibrium
Authors: Forst Wendell
Published: February 2003

The exponential transition probability, in the version that permits an analytical solution of the relaxation problem, is used to compute a number of macroscopic (bulk) observables for a model system based on multiphoton excitation of SF6 coupled to a rare-gas heat bath. Two extreme cases are considered: Initial excitation as a delta function, or as a Poisson distribution. It turns out that regardless of initial conditions, all macroscopic observables are functions of time, including the relaxation time, so that the system does not undergo simple exponential decay. This is because the first moment of the exponential transition probability does not satisfy the linear ‘‘sum rule.’’ The exponential transition probability causes the overall (or bulk) average of energy transferred (〈〈ΔE〉〉) to be constrained to a maximum which is independent of the nature and level of initial excitation, thus producing a bottleneck in the macroscopic relaxation process when excitation is sufficiently high. The consequence is that the initially more highly excited system takes longer to reach steady state, with a relaxation time that is initially nearly proportional to initial excitation and which decreases as the system approaches steady state. It is only in the immediate vicinity of steady state that simple exponential relaxation takes place, with the shortest relaxation time. Several consequences of this, particularly the population distribution as a function of time, are illustrated and discussed.
Electronic spectra of the mixed complexes of s-tetrazine with He and Ar
Authors: Haynam Christopher A. , Brumbaugh Donald V. , Levy Donald H.
Published: February 2003

van der Waals complexes of s-tetrazine, H2C2N4, with the rare gases He and Ar have been formed in a supersonic free jet and observed by the technique of laser induced fluorescence. The resolution of rotational structure in the π*←n electronic transition (1B3u←1Ag) of the s-tetrazine chromophore allows the structure of these complexes to be measured. The complex between s-tetrazine and one argon atom is similar to that observed between s-tetrazine and He. The argon lies near the out-of-plane A rotational axis with a ground state separation of 3.44 Å, which decreases to 3.40 Å in the excited state. When the number of complexing atoms is two or less they are found to always bind directly above and below the tetrazine ring. Complexes containing three or more rare gas atoms have been observed and their structures have been measured. In the case of three or more rare gas atoms the out-of-plane preferential binding sites are filled by the first two atoms and additional atoms must find new sites. As long as one or more argon atoms are present, additional rare gas atoms bind to an argon atom which is itself bound to the tetrazine ring at an out-of-plane site. No rare gas atom binding to the periphery of the tetrazine ring has been observed.
A model for the interaction of two electric double layers in two dimensions: The metal electrolyte interface and the Donnan membrane
Authors: Blum L.
Published: February 2003

A model for the interface of two media with different background densities, separated by a charged bilayer, is solved exactly in a two dimensional, one component plasma at reduced temperature 2. When the two media are in direct contact (no gap), then we can think of it as a model of a classical metal and an electrolytic solution or just two metals. The contact potential, or potential of zero charge appears to be a very simple function of the ratio of the background densities. We also find that the potential does not depend on the surface charge, a fact that we explain using a corollary of the perfect screening theorem. A second case of interest is the case of the two media separated by a gap that in our case could be charged: this is a model of a two dimensional membrane, or the polarization layer of the inner Helmholtz region. Very surprisingly, we find that also the surface dipole is completely screened by the charges surrounding the interface, so that the potential drop across the interface only depends on the logarithm of the charge densities, as found by Ballone, Senatore, and Tosi for the most simple case of a discharged contact.
Novel behavior of biased correlated walks in one dimension
Authors: Kang K. , Redner S.
Published: February 2003

The asymptotic properties of a biased correlated walk model in one dimension are treated analytically. In this model, a weight factor p is associated with the walk each time it visits a new site, and different fugacities are assigned for steps to the left and to the right. The range 0≤p≤1 corresponds to attractive correlations, and this case describes aspects of biased diffusion on a lattice with randomly distributed static traps. A generating function for the correlated walk model is defined which equals the configurational averaged survival probability for biased diffusion in the presence of traps. The quantity is found to decay exponentially with N, the number of steps in the walk. We find an interesting transition in the average number of visited sites. For weak bias, this quantity varies as N1/3, suggesting a zero drift velocity; for strong bias it varies linearly with N, indicating a transition to a finite drift velocity.
High overtone C–H and O–H transitions in gaseous methanol: Theory
Authors: Sage Martin L.
Published: February 2003

High vibrational overtones observed in the laser photoacoustic spectrum of gaseous methanol are interpreted in terms of coupled anharmonic C–H bond modes and an anharmonic O–H bond mode coupled to harmonic C–O and <HOC modes. An ab initio SCF calculation is used to estimate the harmonic part of the potential and the dipole moment operator.
Polytypism in n-fatty acids and low-frequency Raman spectra: Stearic acid B form
Authors: Kobayashi Masamichi , Kobayashi Tohru , Itoh Yuzo , Sato Kiyotaka
Published: February 2003

Single crystals of single-layered (mon) and double-layered (orth II) polytypes of stearic acid B form were obtained and their structures were investigated by the x-ray diffraction and vibrational spectroscopic methods. Two polytypes exhibited quite different Raman spectra in the frequency range from 65 to 2 cm−1. The Raman bands appeared as singlets in mon, while they split into doublets with different polarization in orth II through the interlamellar interactions between two successive layers contained in the unit cell. The frequencies of the phonon modes in orth II were found to be lower than the corresponding ones in mon, indicating that orth II (or mon) was the high-temperature (low-temperature) stable form.
A multichannel quantum defect analysis of two-state couplings in diatomic molecules
Authors: Mies Frederick H. , Julienne Paul S.
Published: February 2003

A multichannel quantum defect analysis (MCQDA) for non-Coulomb potentials is applied to two-channel coupling in diatomic molecules. Given the exact 2×2 scattering matrix S(E) obtained at a single energy above the dissociation threshold E>0, we can extract an analytic, energy-insensitive 2×2 matrix Y(E) which can be extrapolated across thresholds and yield a complete description of predissociating molecules. The predicted widths, shifts, and line shapes obtained from MCQDA are in quantitative agreement with the exact numerical results and confirm the remarkable simplicity that can be achieved in describing diatomic systems near dissociation limits. The analysis is equally applicable to either adiabatic avoided crossings or diabatic curve crossings. The validity of MCQDA is independent of coupling strength and can quantitatively describe strongly overlapped predissociating resonance states. The application of MCQDA to the bound state spectrum, when both channels are closed, is discussed.
On the quadrupole moment of the hydrogen molecule in excited states
Authors: Rychlewski Jacek
Published: February 2003

The quadrupole moment functions Q(R) for three excited states a 3Σ+g, B 1Σ+u, and B′1Σ+u of the hydrogen molecule are calculated for a wide range of bond lengths, using highly flexible wave functions depending explicitly on the interelectronic distance. The shapes of the quadrupole moment functions are interpreted in terms of simple quantitative considerations regarding the character of the wave functions. It is found that Q(R) for the excited states converge very slowly to the asymptotic values (R→∞). The Born–Oppenheimer values of Q(R) are averaged over nuclear motion to give the quadrupole moments in a number of vibration-rotation states for H2, HD, and D2.
Static disorder in crystals of the anthracene–tetracyanobenzene charge transfer complex
Authors: Boeyens Jan C. A. , Levendis Demetrius C.
Published: February 2003

Crystals of anthracene TCNB are disordered at room temperature and undergo an order–disorder phase transformation on cooling. Calculation of the appropriate maps of potential energy environment as a function of concerted anthracene ring rotation, and subsequent rigid body refinement with published data for the room temperature crystal, showed that anthracene TCNB is statically disordered. Anthracene randomly occupies two positions at ±6° away from the crystallographic mirror plane. The disordered model converged at R=0.058 in the space group C2/m.
Asymptotic behavior of two-electron atomic wave functions
Authors: Patil S. H.
Published: February 2003

The relation between the asymptotic behavior of atomic wave functions and their energies is used to determine model wave functions and their energies, for two-electron atoms H−, He, Li+, and Be++.
Thermodynamics of homonuclear diatomic fluids from the angular median potential
Authors: MacGowan David , Waisman Eduardo M. , Lebowitz Joel L. , Percus Jerome K.
Published: February 2003

The use of the angular median potential as a temperature-independent spherical reference system for approximating molecular fluids is tested for its predictions of thermodynamics. Calculations have been carried out for a wide range of homonuclear diatomics with continuous atom–atom potentials believed to be representative of the full range of simulation data available for such systems. The results for the pressure are surprisingly good both in the detonation regime and around the triple point. In the latter case, however, the internal energies for highly elongated molecules with attractive potential wells are considerably too positive. Comparison with other perturbation theories indicates that the median reference system gives better pressures but poorer energies than RAM, and that in many cases, especially for purely repulsive potentials, it gives results of comparable accuracy to those obtained with nonspherical reference systems.
Molecular dynamics investigation of homogeneous nucleation for inverse power potential liquids and for a modified Lennard-Jones liquid
Authors: Mountain Raymond D. , Brown Alan C.
Published: February 2003

The influence of the harshness of the repulsive part of the pair potential on the onset of the process of homogeneous nucleation of the crystal from the supercooled liquid has been investigated using the computer simulation method of molecular dynamics. An inverse twelfth power and an inverse seventh power system were studied along with a modified Lennard-Jones system. We find that the times required for both the onset and the completion of the homogeneous nucleation process increase significantly with the harshness of the repulsion. Local orientational order in supercooled liquids was also examined. Values of the orientational order parameter Q6 intermediate between those of a normal liquid and of a crystal were found to be associated only with amorphous solid states.
Time correlation functions of the Smoluchowski level of description of solutions and suspensions
Authors: Sung Wokyung , Friedman Harold L.
Published: February 2003

Time-correlation functions at the Smoluchowski level of description, in which the variables are only the coordinates of the solute particles, apparently violate the stationarity and interchange symmetries that are well known characteristics of time correlation functions formulated in the full Hamiltonian level of description. It is shown that the evolution of a dynamical variable A that depends only on the solute coordinates may be described by the random Smoluchowski equation dA(t)/dt=S†A(t)+RA(t), where S† is the adjoint of the operator S that governs the evolution of the distribution function f (q, t) of solute configurations, i.e., df/dt=Sf. The ‘‘random force’’ RA is determined so as to resolve the anomalies mentioned above. The theory of the random force term itself leads to the formulation of a simulation procedure for Smoluchowski-level systems that reduces to the Ermak–McCammon algorithm in the case that A is the set of solute coordinates.
Three-dimensional DIM-3C potential energy surfaces for the reactions H+XY and X+HY (X,Y=F, Cl, Br, I)
Authors: Last I. , Baer M.
Published: February 2003

The modified diatomics-in-molecules method including a three-center term (DIM-3C) was applied to calculate the three-dimensional potential energy surfaces of 17 H+XY and X+HY reactions, where X and Y are halogen atoms.
Analysis of the true parameters in the correlated effective valence shell Hamiltonian of Li2 and comparison with semiempirical parameters
Authors: Takada Toshikazu , Freed Karl F.
Published: February 2003

The correlated ab initio effective valence shell Hamiltonian method is applied to study the theoretical basis of the bond length dependence of semiempirical parameters. The Li2 molecule is chosen because it only has two valence shell electrons, a simplicity which makes it possible to compare directly the semiempirical parameters with the true parameters obtained from the effective valence shell Hamiltonian calculations. Three qualitatively different basis sets are employed. One set has a character similar to Löwdin orthogonalized atomic orbitals and is used to examine the validity of the zero differential overlap approximation. The bond length dependence of the calculated correlated integrals are compared with those assumed by the semiempirical Mulliken and Linderberg approximations for the resonance integral βμν, the Mataga–Nishimoto and Ohno–Klopman schemes for the two-center Coulomb repulsion integrals γμν, and the Mulliken approximation for two-center exchange and hybrid integrals.