bluuebunny/crossref_metadata_2025 huggingface dataset:
Chemiluminescence of CaO from the Ca+N2O and Ca+O3 reactions in solid argon
Authors: Wei Chining-Shiang , Guo Sui-Whei , Lee Yuan-Pern
Published: July 2002

Visible chemiluminescence in the 4300–6600 Å region was observed from the Ca+N2O and Ca+O3 reactions in argon matrix. The broad bands at 6066 and 6246 Å were ascribed to the d 3Δ (or D 1Δ)→a 3Π and the D 1Δ→A′ 1Π transitions of CaO, respectively. A weaker band at 6416 Å (15 586 cm−1) was assigned as emission from the previously unobserved c 3Σ+ state to the a 3Π state of CaO. Emission due to Ca atoms in the 1P(4s4p)→1S(4s2) and the 3P(4s4p)→1S(4s2) transitions was also observed as sharp peaks at 4231 and 6575 Å, respectively. The atomic emitters were excited by energy transfer from CaO*, most probably from the D, d, and c states. The Ca(3P) also receives population via fast relaxation from the 1P state.
Optogalvanic spectroscopy of two-photon dissociation of Tl2 at 532 nm producing large population inversion between Tl(7 2S1/2) and (6 2P3/2) states
Authors: Naqvi A. S.
Published: July 2002

This paper reports the first time achievement of large population inversion between thallium 7 2S1/2 and 6 2P3/2 states through two-photon dissociation of pure Tl2 molecules. The second harmonic of a Nd–YAG laser at 532 nm is used to cause two-photon absorption in Tl2 from ground state Ou− to the repulsive state associated with the atomic 7 2S and 6 2P1/2 states. The relative population of the product atomic species is determined by using a tunable dye laser pumped by the third harmonic of a Nd–YAG laser. The dye laser beam focused for complete spatial and temporal overlap with the photodissociaiton pulse excites the product states to known atomic Rydberg states with subsequent ion detection. Prompt population inversion amounting to 100% is achieved. It is shown that photodissociation transition is mediated through predissociation of the upper bound state to a repulsive state correlated with Tl(7 S1/2) in the region of large internuclear separation.
EXAFS studies of Fe3+ neutralized Nafion: Interpretation of the second peak in the radial structure function
Authors: Pan H. K. , Meagher A. , Pineri M. , Knapp G. S. , Cooper S. L.
Published: July 2002

Fe K-edge extended x-ray absorption fine structure (EXAFS) data are presented for ferric neutralized Nafion membranes treated in various ways. The transmission experiments were performed at room temperature. Some complementary Mössbauer data are also provided. Attention is focused on the second major peak in the EXAFS radial structure functions. In ferric neutralized Nafion, whether dehydrated in air or under reduced pressure, this peak is attributed to the neighboring Fe of an oxo-bridged dimer, Fe–O–Fe. This dimer transforms into a dihydroxo-bridged dimer Fe OHOH Fe upon heating to 150 °C in vacuo. If the heat treatment is done in water then microcrystalline Fe2O3 is formed. Immersion of the ferric neutralized Nafion membrane in KCl, K2CO3, KOH aqueous solutions, or even exposure to NH4OH vapor, results in precipitation of iron inside the membrane. The precipitate has a Fe–O distance of 1.94±0.02 Å and a Fe–Fe distance of 3.11±0.05 Å. Edge-sharing FeO6 octahedra form the basis of the precipitate structure.
EPR studies on an ‘‘unperturbed’’ CrO43− center in K2SO4 crystals
Authors: Sekhar Y. Ravi , Bill H.
Published: July 2002

Electron paramagnetic resonance of CrO2−4 doped K2SO crystals has been studied after x-irradiation at 77 K. The resulting species, a CrO43− center, has the principal g values gx=1.8743, gy=1.8507, and gz=1.9386. The EPR data indicate a small distortion of CrO43− tetrahedra. A temperature variation study of the spectra in the range 1.6–77 K shows that they broaden rapidly above 10 K and no motionally averaged spectrum is observed. The experimental results are discussed in terms of several effects including a possible Jahn–Teller effect.
Solution of the generalized Langevin equation
Authors: Toxvaerd So/ren
Published: July 2002

The generalized Langevin equation is solved numerically by replacing the driving forces by stochastic, Gaussian distributed forces with a Gaussian time correlation. The calculated Brownian trajectories are compared with the corresponding classical mechanical and Monte Carlo trajectories and found to exhibit fractal properties with a dimension equal to two, for time intervals where the stochastic forces are uncorrelated. The classical mechanical trajectories have a lower fractal dimension, whereas the Markovian Monte Carlo trajectories show a fractal dimension equal to 2 even for high resolutions. The result indicates that the long-time memory plays a crucial role for the fractal dimensionality.
Thomas–Fermi limit and leading corrections for atoms and ions
Authors: Gázquez José L. , Ortiz Elba
Published: July 2002

The Z−1 perturbation expansion is used to derive a formula of the energy of an ion in the limit of large nuclear charge Z and number of electrons N of the form E=∑nZ(7−n)/3 fn(q) where q=N/Z and fn(q)=q(1−n)/3[b0n+b 1nq+b2nq2] with b1n≂[(7−n)/3]Cn−2b0n and b2n≂b0n −[(4−n)/3]Cn. The constants b0n correspond to the asymptotic expansion of the zero-order perturbation coefficient ε0(N) and the constants Cn correspond to the neutral atom binding energy E=∑nCnZ(7−n)/3. The first function f0(q), which corresponds to the Thomas-Fermi limit, is then used to obtain approximate analytical expressions for the first derivative at the origin S(q) and the radius of the ion, χ0(q), of the Thomas-Fermi screening function. The expressions for f0(q), S(q), and χ0(q) provide an excellent representation of the numerical solutions. The function f1(q) is used to show that the value of the coefficient of the leading correction to the Thomas-Fermi energy C1 is 1/2. Finally, it is shown that the description of the ratio of the total energy and the nuclear-electron attraction energy is greatly improved over the Thomas-Fermi values by including the leading corrections.
Valence orbital momentum distributions in s-triazine
Authors: Hiser S. , Chornay D. J. , Coplan M. A. , Moore J. H. , Tossell J. A.
Published: July 2002

Valence orbital momentum distributions, ρ(q), are obtained for s-triazine, C3H3N3, from (e,2e) spectroscopy and split valence SCFMO calculations. The separation energy spectrum simulated using the calculated ρ(q) and ionization energies from Green’s function calculations is in good agreement with experiment from 9–20 eV, but at higher energy the simulated spectrum has higher intensity than experiment, indicating that the Green’s function calculation underestimates the complexity of the inner valence region. The calculated momentum distributions have their maxima at higher values than observed experimentally for the outermost valence orbitals, but the difference in momentum distributions between the antisymmetric 4e′ N2p lone pair orbital of s-triazine and the N2p lone pair orbital of ammonia is reproduced well by the calculations. Similarly, calculated and experimental differences of Fourier transformed ρ(q) for the 4e′ and 3a′1 N2p lone pair orbitals of triazine are in agreement and can be interpreted using plots of 4e′ and 3a1 orbital averages and differences from the SCFMO calculations. The Fourier transformed momentum distribution of the 4e′ orbital clearly shows the presence of both nearest-neighbor C and second-nearest-neighbor N interactions.
Semiclassical vibrational wave functions for electronically excited DCN: A highly quantum mechanical system
Authors: Ozment Judy , Chuljian David T. , Simons Jack
Published: July 2002

A semiclassical spectral quantization (SQ) technique quite similar to that developed by De Leon and Heller is applied to a rigorous test case—the highly quantum mechanical vibrational motion of DCN on the excited C 1A′ potential energy surface. By using essentially arbitrary classical trajectories, SQ energies and wave functions are obtained for the 12 bound vibrational states of this system. These results turn out to be in remarkably good agreement with those also obtained in this lab from a totally quantal treatment of the same 12 vibrational states. Observations are made concerning the limitations of the SQ method and its performance when state degeneracies are present.
Radiative and nonradiative decay rates of molecules adsorbed on clusters of small dielectric particles
Authors: Liver Naomi , Nitzan Abraham , Freed Karl F.
Published: July 2002

In this work we shall develop a formalism which extends previous calculations of lifetimes of excited molecular states near planar dielectric surfaces and near single small dielectric particles to situations where the molecule is adsorbed on a cluster made of several dielectric spherical grains. The nonradiative relaxation rate is seen to be relatively weakly dependent on the cluster structure and, if the molecule is adsorbed on one of the grains, is well approximated by the single grain result. The radiative decay rate and hence the quantum yield are much more sensitive to the geometry of the cluster which determines the position (in frequency) of the dielectric resonances of the cluster. The formalism developed here may be used to evaluate local field intensities at different positions within a cluster of dielectric grains under the influence of an external incident field.
Two and three particle distribution functions of one-dimensional lattice fluids
Authors: Johnson Elijah
Published: July 2002

A pair of exact equations are used to determine the two and three particle distribution functions of lattice fluids. The particles of a lattice fluid exclude one or more lattice sites and their positions are sites on a lattice. Accurate particle distribution functions are obtained. The major obstacle to obtaining accurate results seems to be computer round-off error. A procedure is given for circumventing this round-off problem.
Cold jet infrared absorption spectroscopy: Fermi resonance in the ν5/2ν8 bands of PF5
Authors: Yamamoto Satoshi , Takami Michio , Kuchitsu Kozo
Published: July 2002

The infrared vibration-rotation spectrum of PF5 in the 1025 cm−1 region is measured in a supersonic free jet. The spectrum in this region consists of the ν5 (E′) and 2ν8 (E′) bands which are strongly coupled by a Fermi interaction. The unperturbed molecular constants for both states and the interaction term are determined by including this interaction explicitly: ν0(ν5)=1025.7371(4) cm−1, ν0(2ν8)=1024.4483(4) cm−1, ζ5=0.603 36(15), ζ8=0.406 80(8), and W588=2.087 99(12) cm−1. The influence of a higher-order Coriolis interaction is identified as the origin of extraordinary A1A2 splittings in the K=1(+l) levels.
Two particles on a sphere
Authors: Dawson K. A.
Published: July 2002

It is shown that zero angular momentum states of previously studied model systems are integrable. Thus, two electrons moving on a surface may be studied by first removing the surface symmetries. This yields an ordinary differential equation in the internal coordinate angle.
A reduction scheme for explosive chemical kinetics
Authors: Fernández Ariel
Published: July 2002

The role of combined fluctuations in the input and output flows is analyzed on a model for explosive kinetics presenting reactive intermediates. In the deterministic limit the system is an extension of other kinetic schemes extensively studied which have third-order autocatalytic elementary steps. However, it represents a more realistic situation since it does not involve such low-probability third-order collision processes. It has an additional fast-relaxing degree of freedom: the concentration of an intermediate species. The multiplicative noise in the evolution of the fast variable corresponds to the fluctuations in the input flow. Only if these fluctuations are neglected in the adiabatic elimination limit, our model yields the results as obtained with previous models. However, applying the center manifold techniques, we show that these fluctuations are crucial since they can reinforce or inhibit the explosive tunneling effect caused by the noise in the output flow rate.
EPR measurement of Cu2+–Fe2+ exchange in FeSiF6 ⋅ 6H2O at 4.2 K
Authors: Rubins R. S. , De D. K.
Published: July 2002

Six inequivalent Cu2+ EPR spectra were observed at 4.2 K in single crystals of FeSiF6 ⋅ 6H20. The estimated parameters gz∼2.38 and θ∼40°, where θ is the angle between the ionic z axis and the c axis, differ from those measured in crystals of similar structure. These differences have been explained in terms of an isotropic Cu2+–Fe2+ exchange Hamiltonian JS1 ⋅ S2, with J=+(0.030±0.003) cm−1, which gives a contribution gex∼−5.05 J sin2 θ, where θ is the angle between the external magnetic field and the z axis. Perpendicular to the c axis, an independent estimate of +0.034 cm−1 for J was made from the low-field displacement of a satellite spectrum.
Angular correlation and diamagnetic susceptibilities studied by high field NMR
Authors: van Zijl P. C. M. , MacLean C. , Bothner-By A. A.
Published: July 2002

High field high resolution NMR has been used to determine the components of the magnetic susceptibility tensor and the angular correlation factors g2 for nitrobenzene, toluene, and benzene. For nitrobenzene an accurate concentration dependence study of the effective susceptibility anisotropy and asymmetry has been made. The angular correlation around the three principal molecular axes has been studied. For all compounds the g2 factor along the main molecular axis is in agreement with that found by the Cotton–Mouton effect and depolarized Rayleigh scattering studies. For nitrobenzene and toluene g2 cannot be interpreted in terms of simple monomer–dimer equilibria. For benzene a dimer model is shown to describe the liquid behavior satisfactorily.
On the behavior of Nd3+ in the single crystals of hydrated selenate and sulfate: Magnetic measurements and a crystal field investigation
Authors: Neogy D. , Nandi J.
Published: July 2002

The magnetic measurements on the single crystals of octahydrated neodymium selenate have been carried out in the temperature range 90–300 K. The results are very close to that reported for the isomorphous Nd-sulfate. The observed magnetic moment shows a nearly smooth linear variation with temperature and a least squares fit of the Curie–Weiss law to the observed χ̄ yielded −44.5 K as the paramagnetic Curie temperature. An analysis of the results has been made with a crystal field (CF) of D4 symmetry and an excellent simulation of the observed magnetic behavior of Nd3+ ion in selenate and sulfate was obtained. The CF splitting of the ground and excited multiplets of the ground term and the corresponding eigenfunctions were obtained by the simultaneous diagonalization of the CF and spin orbit interaction matrix in the complete 52-dimensional basis of the 4I term; intermediate coupling was also taken care of to a large extent. The g factors and the anomaly in the electronic heat capacity have been determined. The effect of the proposed CF on the nuclear level splitting and its consequences have been worked out and discussed.
Self-avoiding lattice walk confined between two planes interacting with them
Authors: Ishinabe Takao
Published: July 2002

Self-avoiding walk on a lattice is investigated as a model of dilute polymer solutions in a narrow slit between plates. The limiting free energy is estimated as a function of interplate distance and temperature from the exact enumeration for up to 20 steps on the tetrahedral lattice. If the walk interacts with both plates with an attractive potential, there is a threshold value which coincides with the adsorption temperature Tc for the single plane. As the interplate distance decreases, a repulsive force between plates increases for T>Tc while an attractive force increases for T<Tc. If the walk interacts with only one of the plates, there is always a repulsive force. The results are in qualitative agreement with the theory of DiMarzio and Rubin for the random walk model. Analogical relationship between the constrained walk and the critical behavior of magnetic film is also discussed.
Magnetic and electronic properties of HOPG/MoF6 graphite intercalation compounds: An ESR study
Authors: Vaknin D. , Davidov D. , Selig H. , Yeshurun Y.
Published: July 2002

The diamagnetic MoF6 molecule intercalates into graphite (HOPG) according to the reaction nC+MoF6 → C+xn(MoF−6)x (MoF6)1−x. Combination of ESR and magnetic susceptibility measurements indicate that the charge transfer is incomplete with x∼0.2. The system HOPG/MoF6 is unique in that ESR resonances due to both the 4d1 intercalant species (MoF−6) and conduction carriers can be detected. The ESR spectra of stage III exhibit an additional resonance which is probably associated with graphite interior layers.
Associative electron detachment: O−+H→OH+e−
Authors: Acharya Prabhat K. , Kendall Rick A. , Simons Jack
Published: July 2002

Associative Electron Detachment processes are important experimental events that can readily be modeled using modern theoretical methods. Experimental methods to date have only allowed one to obtained the relative vibrational distribution of the neutral product molecules. Using a non-Born-Oppenheimer, nonadiabatic, viewpoint that utilizes a fully ab initio approach, we are able to obtain absolute rates (∼104 s for the O−+H system) for transitions from an initial state specified by collision energy and impact parameter, to specific vibrational and rotational states of the neutral OH and a detached electron. The fact that these rates are slow for the O−+H system is due to the large electron affinity of OH (1.8 eV). These rates have an obtuse propensity favoring vibrationally and rotationally hot products. This propensity arises from contributions that are independent and dependent of the angular momentum of the system, an aspect that is of substantial experimental interest. A detailed study of O−+H→OH(V’,J’)+e− is given and generalizations for detachment processes in other systems (F−+H, LiH, BH−) are made.
Surface-induced chemical oscillations and their influence on space- and time-periodic patterns
Authors: Riecke Hermann , Kramer Lorenz
Published: July 2002

Within the framework of a reaction-diffusion model we study the influence of active or passive surfaces (e.g., membranes) on extended chemical systems. The surface is described by boundary conditions. Using linear stability analysis we find time-independent boundary conditions which induce oscillatory instabilities of the homogeneous state leading to boundary oscillations. Two cases are of special interest:(i) autocatalysis takes place only in the bulk, whereas coupling occurs only at the surface (or vice versa); (ii) all reactions take place at the surface but bulk diffusion is still essential for the oscillations. Phase diffusion theory and numerical simulation show that for oscillating and excitable (or bistable) bulk systems these boundary conditions can act as a pacemaker leading to phase-locked and quasiperiodic oscillations. The stability of the arising phase waves is discussed. For systems with stationary spatial bulk patterns boundary oscillations can constitute an efficient wavelength selection mechanism.
Photocurrent thresholds and angular momenta for electrons solvated in some glassy solids
Authors: Baird J. K. , Lee Li Kao , Meehan E. J.
Published: July 2002

An equation relating the photocurrent just above threshold to the binding energy of the solvated electron is compared to experimental data. (AIP)
EPR study of VO2+ ions doped in single crystals of NH4I and NH4Br
Authors: Chand Prem , Upreti G. C.
Published: July 2002

The results of the EPR study of VO2+ doped in NH4I and NH4Br single crystals in the temperature range 430–77 K are reported. The observed spectra suggest a liquid-like behavior of these crystals for VO2+. A single eight line isotropic spectrum was observed in the temperature ranges ∼373 to ∼245 K in NH4I and ∼430 to ∼233 K in NH4Br. No spectrum was observed in the temperature ranges ∼245 to ∼173 K and ∼ 233 to ∼223 K in NH4I and NH4Br crystals, respectively. A spectrum with two sets of eight isotropic lines was observed from ∼173 K for NH4I and ∼223 K for NH4Br to 77 K. The observed mI dependence and temperature variation of linewidths are discussed. The anomalies in temperature dependence of linewidth confirmed the structural phase transitions in the host lattices.
Symmetry breaking in molecular calculations and the reliable prediction of equilibrium geometries. The formyloxyl radical as an example
Authors: McLean A. D. , Lengsfield B. H. , Pacansky J. , Ellinger Y.
Published: July 2002

A systematic approach to symmetry breaking in molecular calculations, based on MCSCF and multireference CI (MRCI) wave functions, is presented. A series of MCSCF expansions is generated by successively incorporating resonance effects and size effects into the wave functions. The character of the potential surface obtained at each level is analyzed. As an example, the potential energy curves of the ground state (σ) and the first excited state (π) of the formyloxyl radical (HCO2) are characterized. The σ and π equilibrium structures are shown to be symmetric, with an adiabatic σ−π excitation energy of 9.2 kcal/mol. Unlike earlier theoretical studies, our MCSCF model produces a qualitatively correct potential surface. Therefore, we are able to extract reliable vibrational frequencies from the MRCI potential surface.
Theory of multiphoton excitation of methane in intense laser fields. I. A purely vibrational treatment
Authors: Chang Johnny , Ding Shi-Liang , Wyatt Robert E.
Published: July 2002

The quantum dynamics of methane in an intense laser field is investigated through the time dependence of band transition probabilities, photon absorption, and their long-time-averaged analogs. A local mode Hamiltonian is used to describe the vibrational spectrum, and the stretching terms are compared with the well-known Hecht Hamiltonian for tetrahedral molecules. In this first study, the molecule is constrained to pure vibrational motion which, as the results demonstrate, is inadequate for multiphoton excitation into the higher stretching bands (>3 quanta) even at a field intensity of 1 GW/cm2.
Least squares fits of one-dimensional tunneling expressions to the results of three-dimensional quantum scattering calculations for H+H2
Authors: Furue H. , Pacey P. D.
Published: July 2002

Expressions have been fitted by least squares methods to a curved Arrhenius plot obtained from three-dimensional quantum scattering calculations for the reaction of hydrogen atoms with parahydrogen. The fitted expressions were based on transition state theory with tunneling in one dimension through parabolic and Eckart activation barriers. The preexponential factor, the effective barrier height, and the curvature of the barrier top were treated as variable parameters. One expression fitted the quantum rates within 3%. The values of the barrier parameters did not depend strongly on the expression or temperature range chosen, but the expressions for parabolic barriers did not converge at the lowest temperature. The effective barrier height deduced was equal to the threshold energy from quantum or quasiclassical scattering calculations, but was only about three-quarters of the height of the three-dimensional barrier used in the scattering calculations, in contradiction of the principle of vibrational adiabaticity. The effective barrier height and preexponential factor agreed with versions of transition state theory in which either the symmetric stretching or bending vibrations in the complex were treated classically. The curvature of the top of the effective one-dimensional barrier was compared to the curvature of the original three-dimensional barrier.
Kinetics of the H2–NF2 system
Authors: Koffend J. B. , Gardner C. E. , Heidner R. F.
Published: July 2002

The reaction of hydrogen atoms with NF2 was studied in a flow tube using pulsed KrF laser initiation of H2/NF2/Ar mixtures at 440 K. The quantum yield for NF(a) production from the 249 nm photolysis of NF2 was determined to be 0.10±0.05. Several important reactions in this system were investigated, and rate coefficients were determined at 440 K. A value of (1.03±0.20)×10−30 cm6/molecule2 s was obtained for the three-body recombination of F atoms with NF2 and Ar. Rate constants for the relaxation of HF(v=2) and HF(v=3) by NF2 are (9.7±1.0)×10−14 and (2.5±0.5)×10−13 cm3/molecule s, respectively. Study of the quenching of NF(a) by NF2 yields a rate coefficient of (2.7±1.0)×10−16 cm3/molecule s. Evidence is presented that indicates the disproportionation of NF(a) is three orders of magnitude slower than that of ground state NF. Modeling results are presented which agree well with experimental data.
Recombination of radicals in solution. A theoretical study on the influence of two- and many-site hopping on the microwave field effect
Authors: Lersch W. , Michel-Beyerle M. E. , Knapp E. W.
Published: July 2002

Unpaired electron spins on radicals in solution can hop between several molecules of the same species. Two types of hopping process are considered: (1) intermolecular hopping, where the unpaired spin can jump to many molecules and (2) intramolecular hopping, where it jumps between the two equivalent sites of a connected dimer. In both cases the hopping process leads to characteristic changes of the magnetic field dependence of the triplet recombination yield observed by reaction yield detected magnetic resonance (RYDMR) experiments. This allows one to discriminate between many- and two-site hopping and to determine the value of the hopping rate.
On the measurement of molecular anisotropies using laser techniques. III. Detection of the higher multipoles
Authors: Bain A. J. , McCaffery A. J.
Published: July 2002

In this paper we discuss the problem of measuring the higher moments (K≥2) of a generalized anisotropic distribution of molecular rotors. Two photon absorption techniques may be used to obtain these quantities and the appropriate expressions for linear and circular dichroism are derived.
A charge flow model for the high derivatives of the molecular dipole moment. II. Application to oxygen containing triatomics
Authors: Hylden Jeffrey L. , Overend John
Published: July 2002

The charge flow model which was developed in the previous paper has been applied to the series of oxygen containing triatomics SO2, O3, CO2, and OCS. The error in the charge flow parameters (as determined from least squares fitting) are found to depend only slightly on the error in the measurement of the fundamental intensities, and most of the error comes from the uncertainty in the potential. Since the harmonic potentials are more well known than the higher order terms, the linear charge flow parameters are more certain. It is found that the linear charge flow parameters do correspond to chemical intuition but in an quantitative rather than qualitative manner. The results of Gussoni for carbon–hydrogen bonds in various hydrocarbons indicates that the hybridization of the bond is important in determining its characteristics . Our results for the series CO2, OCS, and SO2 indicate that hybridization is not as important as the electronegativity of the central atom and of the other atoms in the molecule in determining the charge flow parameters. Because of this sensitivity to other atoms, it is not believed that the charge flow parameters will be very transferable except for molecules that are very similar chemically. It is hoped that this chemical sensitivity will allow the charge flow model to be applied to problems of chemical interest.
High-pressure Raman and x-ray studies of the alkali cyanides up to 27 GPa
Authors: Strössner K. , Hochheimer H. D. , Hönle W. , Werner A.
Published: July 2002

We have investigated the phase diagrams of the alkali cyanides NaCN, KCN, RbCn, and CsCN in the pressure range up to 27 GPa, using the Raman and energy-dispersive x-ray techniques. We completed the information concerning the concept of a general phase diagram of the alkali cyanides with our measurements. In the case of KCN and RbCN we found at room temperature the expected high pressure phase. This phase IV, with a monoclinic structure (space group Cm) can be followed up to about 27 GPa. A different behavior was observed for NaCN. This crystal has in the pressure range between 10–15 GPa a coexistence region between the antiferro-ordered orthorhombic phase B and the ferro-ordered monoclinic phase. However, this monoclinic phase IVa in NaCN has a different structure with space group Pm (C1s). Finally the combination of the x-ray and Raman data allowed the determination of the structure of CsCN at low temperature and at high pressures. The structure is rhombohedral with space group R3m (phase VII).
Adsorbed polymers under flow. A stochastic dynamical system approach
Authors: Armstrong Robert , Jhon Myung S.
Published: July 2002

Recent experiments have shown that porous filters preadsorbed with polymer molecules exhibit an anomalously high pressure drop at high rates of flow. We have modeled the adsorbed polymers as dynamical systems and have found that the introduction of hydrodynamic interaction between molecules destabilizes at a high applied shear. As a direct result this instability will cause the molecules to unravel and stretch far into the cross section of the pore, and thus by inference, cause the observed anomalously high pressure drop. Although much of this paper is devoted to the stability characteristics of the deterministic system, Brownian motion is also considered, and an account of the statistics of the Brownian system when the deterministic system becomes unstable is given. The examples revealed in this paper are not of sufficient complexity to calculate with any accuracy the magnitude of this anomalous pressure drop. We simply present a procedure by which a large variety of more complex models could be undertaken and their ultimate effect clearly understood.
Concentration dependent translational self-friction coefficient of rod-like macromolecules in dilute suspensions
Authors: Hatziavramidis D. , Muthukumar M.
Published: July 2002

A general theory for the concentration dependent translational self-friction coefficients of rod-like macromolecules in suspensions is presented. Adopting the Kirkwood–Riseman model for the polymer and using a multiple scattering cluster expansion approach we have generated explicit expressions for the friction coefficients at infinite dilution as well as the leading terms of the concentration dependent friction coefficients for both theta solutions and solutions of hard rod-like macromolecules. The leading terms of the concentration dependent friction coefficients in both cases show a weak dependence on the molecular weight of the polymer, a dependence which is absent from the corresponding coefficients for spheres and flexible polymer chains in solution.
Vibronic structure of MCD spectra. I. Non-Condon effects in molecules with nondegenerate electronic states
Authors: Zgierski Marek Z.
Published: July 2002

A model is formulated for calculation of MCD spectra of molecules with nondegenerate electronic states. It takes into account differences between nuclear equilibrium configurations of the excited electronic states that are coupled by the magnetic field. These differences lead to the appearance of new (‘‘non-Condon’’) terms in the expression for the B term describing magnetic optical activity. It is shown that these terms have a strong effect on the vibrational structure of MCD spectra. They cause this structure to differ from the vibrational structure of the corresponding absorption spectra and can lead to sign changes within a single, isolated electronic manifold. It is argued that the experimental MCD spectra of planar aromatic hydrocarbons and their derivatives show clear signs of the presence of such non-Condon effects.
Contribution of dispersion to the properties of H2S--HF and H2S--HCl
Authors: Szczȩśniak M. M. , Scheiner Steve
Published: July 2002

A b initio calculations are carried out using a doubly polarized basis set. Dispersion, evaluated by second-order Mo/ller–Plesset perturbation theory (MP2), is found to have a profound influence on the stabilities and structures of the H-bonded complexes. The contribution of dispersion to the H-bond energies of H2S--HF and H2S--HCl is 44% and 69%, respectively, placing this attractive term second in magnitude only to electrostatics. Reductions of the intermolecular distance of 0.17 and 0.34 Å result from inclusion of correlation effects. Nevertheless, the influence of dispersion upon the angular characteristics of the complexes is rather minor as the relative orientations of the subunits are controlled chiefly by electrostatic factors. The HF--HSH geometry appears to be a true minimum on the potential energy surface but is much less stable than the H2S--HF structure. Comparison of the above systems with previous results for H2O--HF and H2O--HCl reveals a number of regular patterns. Replacement of either first-row atom of H2O--HF with one from the second row equally diminishes the strength of the H bond; a further reduction to roughly half of the ΔE for H2O--HF occurs when both O and F are exchanged. Comparison between the calculated and observed X--Y distances suggests that the relative changes due to substitutions of O and F by S and Cl are predicted very well by MP2, indicating that this approach is capable of accurately reproducing relative (if not absolute) values of R(X--Y) as well as ΔE. The contribution of dispersion to the interaction energy is magnified by each substitution by a second-row atom; these exchanges also produce drastic increases in the correlation-induced contraction of the H bond.
Towards a state-to-state transition state theory
Authors: Kuppermann A. , Levine R. D.
Published: July 2002

We assume that, having arrived at the transition state, the branching into the different product states is independent of the initial quantum states of the reactants. This assumption plus the familiar transition state approximation (that the reaction rate is the rate of the passage across the barrier) yields an expression for the state-to-state cross section in terms of the state-to-all one, as well as microcanonical rate constants. Models, adiabatic correlations, purely statistical considerations, or collinear computations can provide the required input for the theory. Exact quantal computations on the 3D H + H2 reaction are found to satisfy the assumed factorization quite well. Furthemore, reaction probabilities derived from a line-of-centers model, with a barrier height dependent on the approach angle, account for the probabilities derived from the exact quantal computation.
Effect of nonionic solutes on the freezing potential of dilute ionic aqueous solutions
Authors: Rastogi R. P. , Tripathi A. K.
Published: July 2002

Experiments are described which show that sucrose considerably reduces the freezing potential of very dilute solutions of alkali halides, NH4Cl and (NH4)2CO3 while urea enhances the magnitude of such potentials. Results have been explained on the basis of a hypothetical model based on nonequilibrium structure of ice containing hydrogen bonded structure and free water molecules in which foreign ions are incorporated during freezing and the growth of phase occurs by the acquisition of hydronium and hydroxyl ions which form free water molecules in the nonequilibrium ice phase.
Electron spin resonance of the two-dimensional antiferromagnet Cs2MnCl4
Authors: Zaspel C. E.
Published: July 2002

The angular dependence of the line shape of Cs2MnCl4 indicates that this compound is a two-dimensional antiferromagnet. However, the angular dependence of the linewidth is smaller than expected. An intraplane exchange constant of J/kB =12.6 K is obtained.
The dynamic longitudinal charge of the hydrogen atoms in ice X
Authors: Klug D. D. , Whalley Edward
Published: July 2002

Ice X is a phase having centrosymmetric hydrogen bonds that are made by compressing ice VII or VIII to ∼500 kbar. The dynamic longitudinal charge of its O–H stretching oscillation, which is defined as the change of dipole moment with O–H distance, has been determined by extrapolating from the integrated infrared intensities of the O–H stretching vibration of a dilute solution of HDO in D2O ice VII in the range 24–189 kbar. It is about one electronic charge, which corresponds to an integrated infrared absorption intensity that is 45 times greater than the value in the vapor, and is negative.
The 1/Z expansion for heavy positive atomic ions
Authors: Senatore G. , March N. H.
Published: July 2002

The 1/Z expansion of the total energy E(Z,N) is compared with a recent result giving the total energy of heavy positive atomic ions in the weak ionization limit N/Z near to unity. The leading term in the asymptotic limit of the high order coefficients of the 1/Z expansion for large N is thereby determined precisely. The results are finally compared and contrasted with the treatment by Stillinger of the He-like series with N=2.
Comment on ‘‘A proposal for the proper use of pseudopotentials in molecular orbital cluster model studies of chemisorption’’
Authors: Igel-Mann G. , Stoll H.
Published: July 2002

In recent SCF calculations for M5CO clusters (M=Cu, Ni, Al), Bagus et al. found that serious errors arise when pseudopotentials are introduced for the Cu and Ni atoms. Using the Cu pseudopotential and basis set given by Bagus et al., in SCF/CI calculations for Cu2, we find that serious errors arise indeed, but that the bulk of these errors can be eliminated by (i) a proper account of core–core interaction and (ii) enlargement of the basis set.
A comparison of energy estimators used in quantum Monte Carlo calculations
Authors: Doll J. D. , Freeman David L.
Published: July 2002

Path-integral Monte Carlo calculations in quantum statistical mechanics have been performed using either discretized methods or Fourier methods. In each of these methods the internal energy has been calculated using either temperature differentiation or direct operation on the density matrix by the Hamiltonian. It is shown that the variance of the internal energy calculated by operation of the Hamiltonian on the density matrix in the Fourier method is independent of the number of Fourier components included in the expansion of the paths for a number of systems. The variance of the internal energy obtained from the other methods is shown to grow with the size of the expansion used for all systems.
Quantum calculations for diffractive and rotationally inelastic H2/LiF (001) scattering
Authors: Drolshagen Gerhard , Kaufhold Achim , Toennies J. Peter
Published: July 2002

Exact close-coupling calculations for H2 rigid rotor scattering from a LiF (001) surface are presented. Using a standard model potential, rotation-diffraction transition probabilities are reported for a range of collision energies up to 0.7 eV and for various surface corrugations. The calculations are aimed to serve as a reference for approximate approaches. The exact transition probabilities are compared with existing quasiclassical trajectory calculations and with results obtained by the matrix-diagonalization sudden method. In agreement with previous approximate treatments it is found that the coupling between rotations and diffractions is only weak so that both can essentially be treated independently.
Microwave optical double resonance of HNO: Dipole moment of HNO in Ã 1A″(100)
Authors: Takagi Kojiro , Suzuki Tetsuo , Saito Shuji , Hirota Eizi
Published: July 2002

The MODR technique has been used to observe the Stark splittings of some rotational lines for HNO in the Ã 1A″(100) state. The components of the dipole moment of HNO inÃ 1A″(100) have been determined as μa =1.057(6) D and 1.089(6) D for Ka=2 and Ka=3, respectively and μb=1.311 (8) D.
On the effects of adsorbate aggregation on the kinetics of surface reactions
Authors: Silverberg M. , Ben-Shaul A. , Rebentrost F.
Published: July 2002

The effects of lateral interactions between chemisorbed reactants on the mechanisms of surface reactions are studied by Monte Carlo simulations for a model system. The model describes a reaction between two adsorbates, A and B, which upon reaction form a rapidly desorbing product. The surface is described by a square lattice with attractive nearest-neighbor interactions between the A atoms; wAA/kT=−2.5. The B atoms are regarded as ‘‘random walkers,’’ unaffected by lateral interactions. (wAB=wBB=0.) The A atoms are allowed to aggregate before B is adsorbed on the surface. Reaction can occur when the much more mobile B’s reach the boundary of an ‘‘island’’ of A atoms. The island structures depend on the aggregation period allowed for A and on coverage. Most of the simulations model a reaction beginning after A aggregation has reached a long-lived (yet nonequilibrium) state, characterized by finite and ramified islands with strongly indented boundaries. Typical island characteristics, such as their average boundary length, average size, and specific populations of A atoms with all possible numbers of A neighbors, as well as coverages and reaction rates are followed as a function of time during the aggregation and reaction periods. The rates are much lower than those expected from a random distribution of reactants. The kinetic laws observed are nonstandard. Models assuming arrangement of the aggregating species in regularly shaped islands are shown to be oversimplified. Application of the quasichemical approximation to the calculation of reaction rates is shown to be inadequate.
Semiclassical ergodic properties of the Henon–Heiles system
Authors: Ramachandran B. , Kay K. G.
Published: July 2002

This paper investigates the applicability of the concepts of classical ergodicity and the results of semiclassical ergodic theory to quantum mechanical systems that are away from the classical limit and that do not necessarily become ergodic as ℏ → 0. To carry out this study, a new quantity, called the ‘‘constancy’’ FA of classical property A, is introduced. This constancy is defined in terms of the autocorrelation function of A and its statistical equilibrium value. It measures, on an absolute scale, the extent to which the dynamics of A behave statistically. The FA for a set of quadratically integrable properties A can be used to define the ‘‘degree of ergodicity’’ of a classical system with respect to this set. This analysis motivates introduction of the quantum analog of FA as a means of judging the applicability of classical ergodicity concepts to quantum mechanical systems. To illustrate these ideas, calculations for the Henon–Heiles system are carried out which compare the quantum and classical analogs of the constancies and of the ‘‘almost microcanonical’’ autocorrelation functions from which they are formed. The results indicate that the quantum and classical systems exhibit similar forms of partial ergodicity at high energy. This conclusion supports the approach introduced here for identifying the quantum implications of classical ergodicity. As a consequence of the good quantum-classical agreement, the partially ergodic nature of the classical behavior is reflected in the distribution of certain quantum-mechanical matrix elements. At lower energy, the applicability of classical ergodicity concepts to this system is more limited due to differences in the quantum and classical dynamics. The kinds of quantum-classical discrepancies that limit the implications of classical ergodicity for quantum systems are identified.
HeI α photoelectron modulation spectroscopy of H2 and D2
Authors: van der Meer W. J. , van Lonkhuyzen H. , Butselaar R. J. , de Lange C. A.
Published: July 2002

A new method for the determination of photoionization cross sections is presented. The technique is used to obtain cross sections of H2 and D2 relative to those of H and D atoms from accurate HeI α photoelectron intensity measurements. With the virtually exact atomic cross section calculations available for H and D atoms our experiments provide absolute H2 and D2 photoionization cross sections. Ultraviolet photoelectron (UV PE) spectra of vibrationally excited hydrogen (v″=1,2) and deuterium (v″=1,2,3), free from overlapping bands due to vibrational ground state molecules, are presented for the first time. Positions and intensities of all observed features are excellently reproduced using well-known molecular and ionic constants and accurate ab initio potential energy curves. Combination of the cross section determinations of H2 and D2 with intensity measurements of vibrationally excited H2 and D2 reveals some qualitative information on H*2 and D*2 cross sections and  β parameters.
Inelastic electron tunneling spectroscopy of silane coupling agents adsorbed on alumina
Authors: Alexander J. D. , Gent A. N. , Henriksen P. N.
Published: July 2002

Inelastic electron tunneling spectroscopy (IETS) has been used to obtain vibrational spectra of alkoxysilanes adsorbed on aluminum oxide. The spectra reveal that: (1) monoalkoxysilanes are not adsorbed whereas trialkoxysilanes are readily adsorbed, (2) the reaction mechanism of triethoxysilanes and trimethoxysilanes is the same, and (3) the reaction mechanism under anhydrous conditions utilizes surface OH or OD groups present on alumina. These results support the hypothesis that the alkoxy groups exchange with OH groups on the surface to form siloxane bonds, and that these bonds are stabilized with trialkoxysilanes. For monoalkoxysilanes, either the bonds do not form so readily, or they are not stabilized by further condensation of the adsorbed species. Instead, the monoalkoxysilanes appear to catalyze further oxidation of the alumina surface.
Elastic theory of supercoiled DNA
Authors: Tanaka Fumihiko , Takahashi Hitoshi
Published: July 2002

This paper estimates the configurational energies of a closed DNA molecule in highly supercoiled states. Calculation is based on an elastic rod model characterized by the bending and torsional stiffness constants. We show that the global constraint due to topological restriction leads to highly branched metastable structures. We also clarify the conditions for formation of supercoils by linear stability theory and obtain low-frequency vibrational spectra of relatively simple supercoiled structures. It is shown that even a small number of duplex unwindings can lead to the instability of a circle for a wide range of elastic constants.
Chemiluminescent reaction of tellurium with fluorine: The A 2Πi–X 2Πi and B 2Σ+–X2Πi transitions
Authors: Thorpe William G. , Carper W. Robert , Davis S. J.
Published: July 2002

Te reacts with fluorine to give a strong chemiluminescence between 3800 and 8700 Å. The emission spectrum contains both 2Σ+–2Πi and 2Πi–2Πi systems. TeF emission bands are of the Hund’s type (a) without spin forbidden intermultiplet crossing. Spin orbit coupling of the X 2Πi state is −497 cm−1 as determined from the 2Σ+–2Πi transition. Vibrational constants are reported for all states and compare favorably with those obtained from theory.
On the representation of the electric charge distribution in ethane for calculations of the molecular quadrupole moment and intermolecular electrostatic energy
Authors: Hansen F. Y. , Alldredge G. P. , Bruch L. W. , Taub H.
Published: July 2002

It is shown that a simple atom-site monopole model is inferior to one which includes higher-order local multipoles to represent the intramolecular charge distribution of ethane. Unlike the latter model, the local monopole representation predicts the wrong sign for the molecular quadrupole moment and gives a repulsive rather than an attractive electrostatic interaction at typical intermolecular distances. In the local multipole model, the atom-site dipoles give the largest contribution to both the molecular quadrupole moment and the intermolecular interaction.