bluuebunny/crossref_metadata_2025 huggingface dataset:
Carbon-13 NMR of liquid crystal solutions with magic-angle spinning. Hindered rotation in 6-(dialkylamino)fulvenes
Authors: Afzal Jalees , Fung B. M.
Published: July 2002

With magic-angle spinning, the carbon-13 NMR spectra of nematic liquid crystals having positive anisotropy of the magnetic susceptibility show sharp peaks. Solutes which have carbon-13 peaks in the ‘‘window’’ regions of the solvent spectra can be conveniently studied. The hindered rotation of the dialkylamino group in 6-(dimethylamino)fulvene, 6-(diethylamino)fulvene, and 6-(N-piperidyl)fulvene in a liquid crystal solvent (E8) has been investigated by carbon-13 NMR with magic-angle spinning. The results are compared with those obtained in isotropic solutions of the same compounds in CDCl3 and CDCl3+E8. The free energy of activation (ΔG≠) of each compound in the three solvents is the same within experimental error.
Excitation functions for the chemiluminescent reactions of the ground state IVA atoms with F2
Authors: Rosano W. J. , Parson J. M.
Published: July 2002

Time-of-flight studies for the chemiluminescent reactions of Sn, Ge, and Si with F2 have been conducted to determine the excitation functions, σ(ET), for the formation of the A 2Σ+ state of the IVA monofluorides. In all cases σ increases with increasing collision energy. This may be the result of a barrier in the entrance channel of the potential energy surface (PES) and/or the velocity dependence of a nonadiabatic transition from reactant to product PES. σ(ET) has also been analyzed as a product of a statistical factor and a dynamical factor. The latter was found to decrease with increasing collision energy suggesting an attractive interaction between reagents. This result may be explained qualitatively in terms of the relative amount of mixing of covalent and ionic surfaces.
Uniform semiclassical self-consistent field and adiabatic calculations of complex energy eigenvalues for nonseparable systems
Authors: Farrelly David
Published: July 2002

The semiclassical self-consistent field (SCF) procedure is extended to the calculation of complex energy eigenvalues corresponding to metastable resonant states. By use of the complex Hellmann–Feynman theorem a fully uniform SCF procedure is developed which is equally suitable for states lying below or above any barriers in the potential, and, unlike the primitive semiclassical SCF originally developed by Ratner, Gerber, and co-workers, undergoes a smooth transition through the barrier top region. A detailed numerical investigation is presented for a coupled double barrier problem supporting no bound states. Comparison is made with uniform semiclassical calculations based on an adiabatic separation of the wave function as well as with accurate quantum complex coordinate SCF calculations, and quantum CI results available in the literature. Our calculations allow a comparison of the usefulness of the SCF and adiabatic approximations in predicting mode specificity.
Reflexive pair production probability study for equal spheres in a saturated random distribution in three dimensions
Authors: Hanson Howard G.
Published: July 2002

A study of reflexive pair formation probability for a three-dimensional random distribution of spheres. The numerical density (0.704 sphere centers/unit volume) of the distributions studied is approximately the previously known saturation density of 0.709. A probability of 0.517±0.004 was found for the spheres. This can be compared to the known value of 0.5926 for the Poisson distribution points. Apparently the probability of formation of reflexive pairs is inhibited when the entities being studied possess a finite impenetrable volume. Coordination numbers for reflexive and nonreflexive pair members are found to be essentially the same.
Neutron scattering from charge stabilized suspensions undergoing shear
Authors: Ackerson Bruce J. , Hayter John B. , Clark Noel A. , Cotter Lise
Published: July 2002

Small angle neutron scattering(SANS) is used to investigate the local order in aqueous, charge stabilized suspensions of 103 Å diameter polystyrene latexspheres at ∼14 wt. % solids. These samples which evidence a close packed close packed structure in equilibrium are observed to undergo a transition to an amorphous order when sustaining a sufficiently large and steady shear. The shear melting phenomonology is different from that observed by light scattering for very dilute (0.16 wt. % solids) suspensions of polystyrene particles which form bcc lattices in equilibrium. Furthermore, neutron scattering reveals more detail than light scattering for concentrated suspensions and indicates a larger degree of three-dimensional ordering under steady shear flow than is implied by sliding layer models for similar systems.
Zeeman, hyperfine, and magnetic dipolar effects on the EPR spectrum of a cubic copper tetramer
Authors: Rubins R. S. , Black T. D. , Barak J.
Published: July 2002

The Zeeman interaction for an isolated multiplet of a cubic Cu2+ tetramer, characterized by the total spin quantum number S, is shown to be essentially isotropic, with g=(g∥+2g⊥)/3, where g∥ and g⊥ are single Cu2+ ion components. Matrix elements are given for the off-diagonal part of the Zeeman interaction within the three S=1 multiplets. The hyperfine structure for each isotope in a [100] direction is shown to be a 13 line structure. The intracluster magnetic dipole–dipole (MDD) interaction has nonzero matrix elements within the three S=1 multiplets, which are degenerate for an isotropic exchange interaction with equal coupling constants. With the MDD interaction treated as a perturbation of the Zeeman interaction, it is found that the single line at g=2.10, expected on the basis of the Zeeman interaction alone, is split into four lines separated from the former line by ±h0 and ±2h0, where h0≂480 G. The MDD coupling constant, calculated assuming point dipoles, is an order of magnitude too small to explain the zero-field splitting of the S=2 multiplet measured by EPR in Cu4OCl6(TPPO)4.
Generalized Langer corrections
Authors: Collins Michael A.
Published: July 2002

A direct method for obtaining the correct classical limit quantization of general angular momenta is presented. The generalized Langer corrections so obtained are shown to arise from a stationary phase approximation to the coordinate transformation matrix elements. A couple of illustrative examples and results are provided.
Molecular properties of CN− ions in alkali cyanide crystals
Authors: Fowler P. W. , Klein Michael L.
Published: July 2002

A b initio coupled Hartree–Fock calculations of the multipole moments and dipole polarizability of CN− in the high temperature solid phase of NaCN are reported and compared with results for the free anion. The size, polarizability, and anisotropy of the electron cloud are all considerably reduced in the crystal. The variation of these properties with the orientation of CN− in the cation cage is also investigated and found to be small. These findings give a microscopic basis for models derived from molecular dynamics simulations of alkali cyanide crystals.
Comment on ‘‘Probability oscillations in single pass curve crossings: Semiclassical predictions of nonmonotonic dependence on crossing velocity’’
Authors: Miller William H.
Published: July 2002

The electronically non−adiabatic transition probability for a two−stat curve crossing shows oscillations as a function of translational energy. It is contended that these oscillations arise from semiclassical interference between trajectories that hop from one curve to the other at different times. (AIP)
Synergistic methane formation kinetics for hydrogen impact on carbon
Authors: Haasz A. A. , Davis J. W.
Published: July 2002

A physical/chemical model is presented for the reaction kinetics for methane formation from carbon, due to bombardment by energetic (∼100’s eV) H+ ions and thermal (∼1 eV) H0 atoms. While the model was developed for H+ and H0, it can be readily applied to nonhydrogenic energetic particles (ions or atoms, e.g., Ar+, He+, He) in combination with thermal (∼1 eV) hydrogen (again ions or atoms) impacting on carbon. Both collisional (in the case of the energetic particles) and chemical reaction processes are included. Special cases of sub-eV H0 alone, energetic H+ alone, and combined H0 plus H+ were considered and fitted to experimental data. Generally good agreement was found between theoretical predictions and experimental results over the experimental flux and H+ energy ranges studied (H0 flux: 6×1014–7×1015 H0/cm2 s, H+ flux: 6×1012–5×1015 H+/cm2 s, H+ energy: 300 eV/H+ and 1 keV/H+).
Restricted primitive model for electrical double layers: Modified HNC theory of density profiles and Monte Carlo study of differential capacitance
Authors: Ballone P. , Pastore G. , Tosi M. P.
Published: July 2002

Interfacial properties of an ionic fluid next to a uniformly charged planar wall are studied in the restricted primitive model by both theoretical and Monte Carlo methods. The system is a 1:1 fluid of equisized charged hard spheres in a state appropriate to 1 M aqueous electrolyte solutions. The interfacial density profiles of counterions and coions are evaluated by extending the hypernetted chain approximation (HNC) to include the leading bridge diagrams for the wall–ion correlations. The theoretical results compare well with those of grand canonical Monte Carlo computations of Torrie and Valleau over the whole range of surface charge density considered by these authors, thus resolving the earlier disagreement between statistical mechanical theories and simulation data at large charge densities. In view of the importance of the model as a testing ground for theories of the diffuse layer, the Monte Carlo calculations are tested by considering alternative choices for the basic simulation cell and are extended so as to allow an evaluation of the differential capacitance of the model interface by two independent methods. These involve numerical differentiation of the mean potential drop as a function of the surface charge density or alternatively an appropriate use of a fluctuation theory formula for the capacitance. The results of these two Monte Carlo approaches consistently indicate an initially smooth increase of the diffuse layer capacitance followed by structure at large charge densities, this behavior being connected with layering of counterions as already revealed in the density profiles reported by Torrie and Valleau.
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o SCF-CI study of anharmonic vibrational effects in the HCO–HOC radical pair
Authors: Pauzat F. , Chekir S. , Ellinger Y.
Published: July 2002

We report the results of a series of SCF-CI calculations on the ground state of the HCO–HOC radical pair. The calculations have focused on an accurate determination of the geometries, force constants, and anharmonic effects; special attention has been devoted to the role of these effects on IR frequencies and rotational constants. The calculations have been performed using a localized representation for the wave function and include the effects of electron correlation by a first order treatment of the most active electrons plus all single and double excitations from bonding to antibonding orbitals. The zero point energy corrections have been determined by an explicit treatment of the vibrational functions including harmonic and anharmonic contributions. Harmonic force constants are in good agreement with experiment. Strong anharmonic corrections are found in these molecules (for example, 189 cm−1 for C–H stretching in HCO; 291 cm−1 for O–H stretching in HOC). The calculations demonstrate that a vibrational treatment beyond the harmonic level is necessary for quantitative correlations with experimental values. This appears particularly important to obtain accurate rotational constants as illustrated by the corrections of 17.45, −0.23, and −0.30 GHz found for HCO.
The domain Green’s function method
Authors: Moskowitz Jules W. , Schmidt K. E.
Published: July 2002

An implementation of the domain Green’s function Monte Carlo algorithm is described. Unlike the short time approximation, the method is variational and exact within the limits of the fixed node approximation. The systems investigated include LiH, Be in the ground and first excited states, a study of the C2v insertion pathway of Be into H2, and H2O. We predict a barrier of 0.190 hartree for the insertion reaction with a statistical accuracy of ±2%. Better than 90% of the correlation energy is recovered in each case, which makes these some of the most accurate computations to date.
Complex dynamic NMR spectra in the fast exchange limit
Authors: Gamliel D. , Luz Z. , Vega S.
Published: July 2002

Symmetries in Liouville space are used to construct analytical relaxation equations for the density matrix of chemically exchanging systems in the fast exchange limit. The method is particularly useful when a symmetry group for the exchange superoperator can be found. The method is applied to the proton NMR of molecules which are dissolved in liquid crystals, and which undergo isomerization reactions, such as ring inversion and bond shift rearrangement. The results are compared with exact calculations in order to check the range of validity of the fast exchange approximation. For completeness the slow exchange approximation equations are also compared with the exact procedure. It is found that the approximate equation in both limits can faithfully reproduce the NMR spectra over most of the dynamic range and often without a gap. The approximate equations significantly shorten the computation time, making it possible to simulate dynamic line shape for as big a molecule as cyclohexane.
Primary reactions in x-irradiated inositol studied by ESR and pulse radiolysis
Authors: Nilsson Gösta , Westerling Jan , Lund Anders , Lindgren Mikael
Published: July 2002

Primary reactions in x-irradiated single crystals of the two crystal modifications of myo-inositol have been investigated by ESR and pulse radiolysis. Two primary radicals have been determined by ESR at 77 K, the alkoxy radical and the hydroxyalkyl radical, and trapped electrons have been observed at 4.2 K in the hydrated crystal of myo-inositol. The kinetics of the trapped electron have been determined with pulse radiolysis. The alkoxy radical is bleachable with visible light, but the signal in anhydrous myo-inositol recovers when the light is switched off.
Evolution of the magnetic properties during the electrochemical doping and undoping of polyacetylene (CH)x with lithium
Authors: El-Khodary A. , Bernier P.
Published: July 2002

Electrochemical cells employing cis-rich polyacetylene as one electrode have been built in order to follow continuously in situ the evolution of its ESR characteristics during successive doping and undoping cycles. Very significant and almost reversible variations of the linewidth ΔHpp and asymmetry ratio A/B have been observed on either thin or thick samples. These variations are directly correlated with (i) the doping induced isomerization, (ii) the conductivity increase of the (CH)x film upon doping, and (iii) the sudden transition for the electronic spins system from a Curie-type behavior (degenerate semiconductor) to a Pauli-type one (metal).
Quantum reactive scattering for A+BCD→AB+CD reactions: Coupled channel distorted wave theory
Authors: Natanson Grigory A. , Schatz George C.
Published: July 2002

This paper presents a general theoretical formalism for describing quantum reactive scattering in four atom reactions of the type A+BCD→AB+CD based on the coupled channel distorted wave method (CCDW). In this formalism, we have derived expressions for the Schrödinger equation in body-fixed axis (bfa) coordinate systems associated with the A+BCD and AB+CD arrangement channels, and we have reduced the CCDW scattering matrix to a six dimensional internal coordinate quadrature in terms of the bfa wave functions. Explicit expressions for the nonreactive (coupled channel) centrifugal and potential coupling matrices are derived for both A+BCD and AB+CD using the bfa frame isomorphic Hamiltonian. The interconnection between A+BCD and AB+CD coordinate systems is explored in detail, and it is found that the resulting CCDW scattering matrix is similar in form to that derived earlier for atom–diatom collisions, with the main conceptual difference associated with use of an Eckart frame to define the vibrational wave functions for BCD. The relation between the Eckart and scattering frames is discussed in depth. Also, the imposition of parity decoupling in the CCDW calculation is developed for both the CC Hamiltonian matrices and the distorted wave overlap.
Influence of rotational states on molecular multiphoton excitation
Authors: Chang Johnny , Wyatt Robert E.
Published: July 2002

The role of rotational states in IR multiphoton excitation (MPE) is examined for a model spherical top molecule. While the common belief that inclusion of rotational states can enhance MPE by overcoming anharmonic detunings and increasing the number of pathways for excitation is correct for some systems, it is shown that the simultaneous increase in deexcitation pathways can lead to significant rotational heating of lower vibrational bands, and therefore, to a deenhancement of MPE for other systems. The problem of diagonalizing prohibitively large matrices is circumvented by using an artificial intelligence tree pruning algorithm to select the most important basis states for the multiphoton dynamics.
Classical dynamics study of intramolecular energy flow in benzene
Authors: Bintz Karen L. , Thompson Donald L. , Brady John W.
Published: July 2002

The flow of energy from highly excited CH overtones in benzene is investigated by quasiclassical trajectory methods for atomic motions restricted to the molecular plane. Various initial conditions and potential-energy surfaces (harmonic and anharmonic) are examined. The results are in accord with the rapid energy transfer rates observed in experiments. Comparisons are made with other theoretical studies.
Orientational correlations in fluids of hard convex bodies. I. General theory
Authors: She Richard S. C. , Evans Glenn T.
Published: July 2002

A version of scaled particle theory (SPT) is developed for a thermodynamic system comprised of a single hard convex body (HCB) in a fluid of spheres. In two limits, corresponding to an infinitesimally and a macroscopically scaled HCB, the present version of SPT yields the same angle-dependent constraints on the contact radial distribution function (rdf) as those derived by Boublik for the angle-averaged rdf. For noninfinitesimally scaled HCB’s, a new orientation-dependent condition on the contact rdf is determined by means of a differential bulging technique.
Theory of continuum surface Raman scattering from electrons in metals
Authors: Das Sanjukta , Gersten Joel I.
Published: July 2002

A theory for continuum Raman scattering from a bounded free electron gas is developed. The theory is able to account for the observed magnitude of the effect. By suitably generalizing the theory we are also able to account for the observed spectral features. Our theory implies that it is possible to measure densities of states to photon spectroscopic accuracy.
Diatomic–diatomic molecular collision integrals for pressure broadening and Dicke narrowing: A generalization of Hess’s theory
Authors: Monchick L. , Hunter L. W.
Published: July 2002

The line shape theory of Hess draws its attractiveness from the fact that it approaches the correct asymptotic theories at low and high pressures. In this paper, Hess’s theory is generalized slightly to describe overlapping lines of diatomic molecules immersed in a bath of diatomic molecules. A ‘‘spherical’’ approximation is introduced in which the collision integrals are averaged over the propagation direction. In terms of a reduced S matrix evaluated in three commonly used coupling schemes, the pressure broadening cross sections agree with well-known results, and the isotropic Dicke line narrowing cross section is expressed in a form suitable for numerical computation. Both cross sections can be expressed as linear sums of generalized cross sections having the same formal structure.
Studies of depolarized and polarized Rayleigh–Brillouin spectra of a supercooled liquid: Salol
Authors: Wang C. H. , Zhang J.
Published: July 2002

An extensive interferometric study of polarized and depolarized Rayleigh–Brillouin spectra of neat salol was carried out as a function of temperature and scattering angles. Considerable dispersion is observed in the isotropic spectrum, as manifested by the nonlinear temperature dependence in the longitudinal hypersonic frequency and a maximum in the linewidth. The relaxation behavior of the longitudinal hypersonic frequency is shown to be due to the bulk viscosity. Upon decreasing the sample temperature, the shear wave spectrum changes from diffusive to oscillatory behavior. In the oscillatory region, the shear wave frequency decreases linearly with increasing temperature and extrapolates to a vanishingly small value at the temperature at which the longitudinal acoustic wave linewidth is maximum. Curve fitting the VH spectrum to theory shows that the shear frequency is proportional to q; the shear wave linewidth and the rotation–translation coupling parameter to q2. These results are consistent with a linear viscoelastic theory, or the microscopic displacement density theory (taking in the small q limit) previously developed.
Adiabatic, diabatic field-free and dressed molecular bases in a semiclassical treatment of laser induced transitions near pseudocrossings
Authors: Errea L. F. , Méndez L. , Riera A.
Published: July 2002

We study the characteristics of charge exchange processes in ion–atom collisions, induced by a strong laser field whose wavelength is resonant to the splitting at a pseudocrossing of two molecular adiabatic energies, where the transition dipole has a sharp maximum. To calculate the charge exchange cross section we use a semiclassical approach and a molecular expansion for the electronic wave function. Using the formalism of Macías and Riera, and of Ho, Chu, and Laughlin, the properties and practical advantages of field-free and dressed molecular wave functions in that expansion are studied in detail. In practice we have found the former basis to be more advantageous from the computational point of view. A limitation of the Floquet approach of Ho et al. is obtained. As an illustration, we treat O8++H(1s) charge exchange collisions in presence of a laser field whose frequency is quasiresonant with the energy splitting at the pseudocrossing between the 7iσ and the 6hσ energy curves. The importance of the (usually neglected) diagonal ac Stark terms is stressed.
Structure and magnetism of N
N
N′N′-tetramethyl-p-phenylenediamine -pentacyanocyclopentadienide (TMPD-PCCp)
Authors: Radhakrishnan T. P. , Soos Z. G. , Endres H. , Azevedo L. J.
Published: July 2002

TMPD-PCCp crystallizes in the triclinic space group P1̄ with cell parameters a=6.814(2), b=10.595(3), c=14.540(3) Å, α=74.65(2), β=81.93(2), γ=79.04(2)°, V=989.3 Å3, Z=2, and wR=0.064. The complex exhibits novel mixed stacking of TMPD+ ion radicals and diamagnetic PCCp− ions and is weakly antiferromagnetic, with a Weiss constant of 2.6 K. An empirical model for ferromagnetic superexchange is presented for properly oriented ion radicals forming mixed stacks with symmetric diamagnetic spacers.
Improved predictions of equilibrium properties for spin-polarized atomic nitrogen
Authors: Ferrante Robert F. , Stwalley William C.
Published: July 2002

A reanalysis of several equilibrium properties of electron spin-polarized atomic nitrogen is presented, utilizing parameters from a new and improved theoretical calculation of the 7Σ+u state of molecular nitrogen which represents the interaction potential between two such spin-polarized atoms. The new potential well is significantly more shallow than previously assumed, resulting in substantial changes in the critical and triple point pressures and temperatures as calculated in the quantum theorem of corresponding states framework. The corresponding molar volumes are only slightly affected because the hard sphere radius is unchanged. Bulk samples are not expected to exhibit significant quantum behavior. Earlier conclusions on the relative stability of this material to single spin-flip processes remain unaltered.
The efficiency of photosynthetic molecularly based electronic devices
Authors: Kundu Kalyan , Phillips Philip
Published: July 2002

We present in this paper a detailed treatment of the transport properties of photosynthetic molecularly based electronic devices. A primary problem we address are the conditions that must be obtained for molecular electronic devices to efficiently effect charge separation. We study a general d=1 site-disordered stochastic hopping model consisting of N impurities and a random distribution of fluctuating hopping rates. Each impurity is treated as a symmetric well. A relaxation mechanism is included on the first site to simulate decay of the initially created excitation. Using the formalism recently developed by us [K. Kundu and P. Phillips, Phys. Rev. A (in press)], we derive exact expressions for the quantum yield, transit times, and the moments of the distribution of transit times. We show explicitly that the efficiency of charge separation in molecular electronic devices is determined almost entirely by the efficiency of the first electron transfer step. This result is relevant to recent experimental results on charge separation in the quinone–viologen and chlorophyllic systems. Nanosecond electron transit times are predicted for molecular assemblies consisting of five molecular subunits and an effective activation energy of 0.55 eV. The significance of this result to the feasibility of molecular photodiodes and recent experiments on chlorophyll-like systems is discussed.
Coverage dependent phase transition of pyridine on Ag(110) observed by second harmonic generation
Authors: Heskett D. , Song K. J. , Burns A. , Plummer E. W. , Dai H. L.
Published: July 2002

Second harmonic generation from a Ag(110) surface is shown to be sensitive to submonolayer converages of O2, O, pyridine, and benzene. The second harmonic intensity as a function of exposure indicates that the orientation and bonding of pyridine adsorbed at 110 K changes at about 0.3 L.
A state-to-state study of the electron transfer reactions N+2(X̃,v′=0–2)+Ar(1S)→N2(X,v) +Ar+(2P3/2,1/2)
Authors: Liao C.-L. , Xu R. , Ng C. Y.
Published: July 2002

Relative vibrational-state-selected total cross sections σv′, v′=0–2, for the reaction N+2(X̃,v′=0–2)+Ar(1S0)→N2(X,v +Ar+(2P3/2,1/2) [reaction (1)], over the center-of-mass collisional energy (Ec.m.) range of 1.2–320 eV have been determined using the crossed ion–neutral beam photoionization apparatus. The experimental results at Ec.m.=1.2–40 eV are in agreement with those obtained in previous experimental and theoretical studies, indicating that σ0 is substantially less than σ1 and σ2. As Ec.m. is increased, σ0 becomes comparable to σ1 and σ2 in the Ec.m. range of ∼140–200 eV. At Ec.m.=260 and 320 eV, the cross sections are in the order σ0>σ1>σ2. The fractions of Ar+(2P1/2) resulting from reaction (1), Xv′→1/2, v′=0–2, at Ec.m.=4–320 eV have been measured by the charge exchange method. The measurement shows that the Ar+ product ions are predominantly(≳80%) formed in the 2P3/2 state, an observation qualitatively in accord with the predictions of semiclassical multistate calculations. The predicted values for Xv′→1/2, v′=0–2, at Ec.m.=8, 20, and 40 eV are higher than the experimental values. The values for X0→1/2 at Ec.m.=8–320 eV and X1→1/2 at Ec.m.=4–40 eV are found to increase as Ec.m. is increased, showing the behavior of an endothermic process. The values for X1→1/2 and X2→1/2 remain approximately constant at the the Ec.m. ranges of 40–320 and 8–200 eV, respectively. The measured relative state-to-state cross sections for reaction (1) and the reaction Ar+(2P3/2,1/2)+N2(X,v=0)→Ar(1S0) +N+2(X̃,v′) are consistent from the consideration of microscopic reversibility.
Concentration dependence of diffusion-controlled reactions among static reactive sinks
Authors: Torquato S.
Published: July 2002

We evaluate a rigorous lower bound on the rate constant k associated with diffusion-controlled reactions in a porous medium composed of impenetrable spherical sinks, for a wide range of sink volume fractions. The results are compared to an effective-medium approximation for k. It is found that the lower bound gives a useful estimate of the rate constant for virtually all sink concentrations.
Pair diffusion at close range in liquids
Authors: Evans Glenn T. , Kivelson Daniel
Published: July 2002

We offer a ‘‘molecular’’ derivation of a generalized Smoluchowski equation for the relative diffusional motion of two particles in a liquid. We introduce approximations which make the equation useful for the study of motion at close range and high frequency, the conditions of particular interest in the study of chemical reactions in liquids.
Classical trajectory study of the orientation dependence of the reaction CH3I+K→KI+CH3
Authors: Blais Normand C. , Bernstein Richard B.
Published: July 2002

Using the three-body approximation for the subject reaction (taking CH3 to be a structureless particle), a potential energy surface has been constructed to encompass the key features of the observed reaction dynamics. To accommodate the inference from the existing experimental data on the dependence of the reaction probability upon the ‘‘angle of attack’’ γ the potential energy surface has an orientation-dependent barrier which increases essentially linearly with cos γ from threshold up to a cut-off angle γ∼130° corresponding to a 50° steric cone of nonreaction. Classical trajectory calculations of the reaction cross section σR at collision energies from threshold to 0.14 eV show a strong orientation dependence, with dσR/d cos γ decreasing nearly linearly with cos γ to the energy-dependent cut-off angle predicted from the standard angle-dependent barrier models. The relation between such a calculated orientational opacity function and experimental data on the dependence of the cross section upon the initial angle of attack is evaluated and implications for future experiments employing oriented molecule beams discussed.
Millimeter-wave observations of vibrationally excited SO and CS produced by laser photolysis
Authors: Kolbe W. F. , Leskovar B.
Published: July 2002

The methods of steady-state and pulsed millimeter-wave rotational spectroscopy were used to probe the vibrationally excited products, CS and SO produced in the laser photolysis of CS2 and SO2 at 193 nm. The time dependence of the vibrational populations was measured for CS up to v=8 and for SO up to v=5. For CS, a Boltzmann-like population distribution was found with a vibrational temperature of 5795 K corresponding to a vibrational energy of 9.8 kcal mol−1. In the case of SO, significant excitation was observed only for the states v=0, 1 and 2 with v=2 having the largest initial population. A small percentage of the SO molecules were found in v=5. The precursor gases were diluted in Ar to enhance the ratio of rotational to vibrational relaxation times. Because rotational transitions were probed, it was necessary to wait for rotational equilibrium to be established before making the measurements. As a result, the vibrational populations may have been partially relaxed at the time of measurement. In order to evaluate the extent of relaxation, a discussion of the various relaxation processes is given.
Two-point distribution function for a dispersion of impenetrable spheres in a matrix
Authors: Torquato S.
Published: July 2002

Results are reported for the two-point distribution function G2(r) of two-phase random media composed of equisized impenetrable spheres in a matrix. G2 arises in various expressions for transport and mechanical properties of heterogeneous materials.
ESR studies of O−2 adsorbed on silica gel: Photoformation and rotational dynamics
Authors: Howe Russell F. , Timmer William C.
Published: July 2002

An ESR investigation is reported of oxygen radicals formed by UV radiation of silica gel surfaces in the presence of O2. The species O−2 and O−3 are identified. The O−2 radical shows a temperature dependent signal shape between −196 and 25 °C which is interpreted in terms of Brownian rotational surface diffusion about an axis perpendicular to the O–O bond and to the surface. Correlation times for surface diffusion are estimated from comparison of observed and computer simulated spectra, and an activation energy for surface diffusion of 2.4±0.8 kJ mol−1 calculated. Comparison with previously reported spectra for W/SiO2 and Ti/SiO2 surfaces indicates that the mobile O−2 species formed in all three systems must be located on the SiO2 support, and that the mobility is due to the absence of strong interaction with transition metal sites.
NH+3 as a paramagnetic probe of molecular motion: Application to phase transitions in (NH4)2SO4-type solids
Authors: Rakvin B. , Dalal N. S.
Published: July 2002

Using line broadening effects in the EPR of NH+3 radicals, correlation times (τ) were measured over a wide temperature range around the ferroelectric transition (Tc) of (NH4)2SO4. An anomalous decrease in τ is observed well above Tc, as is a critical slowing down and coexistence of para- and ferroelectric phases about Tc, suggesting a Landau-type mean-field approach as an alternative model for this improper ferroelectric. The good agreement between the τ for the NH+3 fluctuations measured with EPR and that for the NH+4 measured earlier via NMR, infrared, and neutron scattering over a wide temperature range, suggests that the NH+3 radicals can serve as sensitive probes of the low frequency (∼1010 Hz) reorientations of the NH+4 ions in the NH4+-containing lattices.
Reactive collision dynamics by far wing laser scattering: Mg+H2
Authors: Kleiber P. D. , Lyyra A. M. , Sando K. M. , Zafiropulos V. , Stwalley W. C.
Published: July 2002

We have measured the far wing absorption profiles of the MgH2 collision system leading to both the nonreactive formation of Mg* and into two distinct final rotational states of the reaction product MgH (v″=0, J″=6, 23). We have observed qualitatively expected behavior including a pronounced red wing in the reactive absorption profile indicating strong reaction probability on the excited attractive potential surfaces. We have also observed novel aspects of the excited state dynamics including reactive vs nonreactive channel competition effects and a strong far blue wing reactive absorption suggesting significant reaction probability even for trajectories on the repulsive surfaces. We have developed a simple theoretical model to semiquantitatively explain our experimental results. This model uses standard quasistatic theory to estimate the absorption probability as a function of detuning between levels of MgH2 and with assumed nonreactive vs reactive branching ratios, accounts for the subsequent evolution on the excited potential surfaces. This theory correctly predicts the overall shapes of the profiles and in general gives reasonable predictions for the relative magnitudes of the wing intensities.
Semiclassical algebraic description of inelastic collisions
Authors: Benjamin I.
Published: July 2002

An algebraic semiclassical approach to the calculation of vibrational transition probabilities in inelastic collisions between molecules is presented. Translational motion is treated classically, while vibrational motion is described quantum mechanically using the generalized coherent state of a proper Lie algebra. This leads to a set of linear differential equations for the parameters of the coherent state, coupled to the classical Hamilton equations. Use is also made of a time dependent canonical transformation to simplify the algebraic structure. Two examples are treated explicitly: colinear collision of an atom and a diatom and a diatom–diatom collision. Good agreement with the exact quantum results is found.
Cyanide adsorption on silver and gold overlayers on island films as determined by surface enhanced Raman scattering
Authors: Murray Cherry A. , Bodoff Stephanie
Published: July 2002

Cyanide adsorption in air on silver, gold, and overlayers of gold on silver and silver on gold is investigated by surface enhanced Raman scattering, using island films to enhance the CN Raman scattering by ∼4 orders of magnitude. We studied pure metal island films as well as either silver or gold islands coated with the other metal with overlayer thicknesses ranging from 0.2 to 4 monolayers (Ml). We studied the coverage dependence of the cyanide adsorption for CN coverages ranging from 0.01 to ∼1 Ml. The Raman spectra reveal both considerable adsorption similarities and noteworthy differences for CN adsorption on the pure metals as compared to the overlayers. The spectra also reveal the order of adsorption of the CN into various possible sites, including the substrate metal sites. For each substrate system studied, we find two new distinct linear, on-top CN species adsorbed to two different types of surface site. The harmonic force constants and vibrational modes of the chemisorbed CN differ substantially on silver and gold, and are yet again different for the thin overlayer systems, due to the incomplete buildup of the overlayer three dimensional band structure.
Magnetic susceptibility of autoreduced silver zeolite A
Authors: Texter John , Kellerman Richard , Gonsiorowski Thomas
Published: July 2002

Static magnetic susceptibility measurements have been carried out on autoreduced silver zeolite A to investigate the spin state of the principal silver cluster formed. This autoreduction procedure produces the so-called yellow form of silver zeolite A, where the color arises from an optical absorption in the neighborhood of 2.7–2.9 eV (previously assigned to a linear and paramagnetic Ag+23 cluster). Our static susceptibility data show unequivocally that this cluster is not paramagnetic and necessitate a reevaluation of the mechanism of autoreduction in silver zeolite A.
Theory of photoisomerization of impurity molecules in solids
Authors: Lin S. H. , Zhang Xing-Guo , Wu George Y. C.
Published: July 2002

In this paper we have presented the density matrix method to derive the master equations for describing the photoisomerization of molecules in solids that take into account the interplay of optical absorption, vibrational relaxation, and molecular isomerization. We have also calculated the isomerization rate constant induced by anharmonic coupling and Coriolis coupling, which is applicable to the whole temperature range from 0 K to the melting temperature of solids. We have shown that the mode selectivity of the photoisomerization of molecules in solids can be interpreted by using the derived master equations.
On the Compton defect
Authors: Bell F.
Published: July 2002

Simple analytical formulas are given for Compton profiles obtained from an exact first Born approximation for K, L, and M subshells. Compton profile asymmetries and Compton defects are investigated and compared with experimental data.
Theory of chemical exchange in zero field NMR: Two-site flips
Authors: Jonsen P. , Luzar M. , Pines A. , Mehring M.
Published: July 2002

In zero field NMR, the spectra are characterized purely by the quadrupolar or dipolar interactions, which are sensitive to molecular motion. An analytical theory is presented which describes the effect of two-site flips of a deuteron on its zero field NMR spectrum and demonstrates the potential of this novel technique in the investigation of molecular dynamics.
Pressure effects on the far infrared spectra of dimers, trimers, and aggregates of HCl molecules trapped in Ar and Kr matrices at 4.2 K
Authors: Obriot J. , Fondere F. , Marteau Ph.
Published: July 2002

By slowly cooling under pressure diluted solutions of HCl molecules in argon or krypton we have obtained different aggregation states of these molecules: dimers, trimers, and even aggregates and microcrystals. Thick solid samples (up to 14 mm) have been cooled down to 4.2 K and compressed within the range 0–5 kbar using a piston–cylinder device. The new spectroscopic data on dimers and trimers, in connection with some results previously obtained at low pressure in various matrices, lead us to confirm that the band frequencies of dimers and trimers are mainly sensitive to the intermolecular separation and do not appreciably depend on the nature of the rare gas matrix. The HCl aggregates are clearly distinguished from the microcrystals because of the quite distinct behavior observed for their corresponding translational modes. Finally, emphasis is made on the first observation of an unexpected and strong absorption band, centered at 200 cm−1, which disappears irreversibly under the effect of pressure. Its possible origin is discussed and tentatively ascribed to structure defects.
Flow modification by polymers in strong elongational flows
Authors: Rabin Yitzhak , Henyey Frank S. , Creamer Dennis B.
Published: July 2002

Flow modification by dilute solutions of polymers in strong elongational flows is analyzed using a two-state model according to which the polymer contribution to the stress balance in the fluid is negligible as long as the polymers are in their coiled state (or are in the process of stretching) and becomes significant as they approach their steady configuration corresponding to an almost fully stretched state. The flow field is separated accordingly into weakly and strongly modified regions and, using mass and momentum conservation considerations, we obtain the flow profiles in these regions. Our prediction of a dip in the velocity and strain rate profiles agrees with experimental observations.
Rod to coil transition and aggregation in soluble polydiacetylenes: Semidilute regime
Authors: Peiffer D. G. , Chung T. C. , Schulz D. N. , Agarwal P. K. , Garner R. T. , Kim M. W.
Published: July 2002

Visible absorption, rheological measurements, polarizing light microscopy, and temperature–time dependent quasielastic light scattering was used to examine the dilute–semidilute properties of polydiacetylene solutions, specifically poly(4BCMU). A good (tetrahydrofuran) and poor (toluene) solvent were selected for examining solution properties over a broad range of solvent quality. In this particular instance, poly(4BCMU) dissolved in both the pure solvents and their respective mixtures. Based on the observation that dramatic color changes (yellow–orange–red) can occur with the hydrodynamic radius remaining invariant, it is quite likely that the color changes are due to a local modification in the conjugation length. That is, on a molecular level, a local stiffening of the segments occurs as the magnitude of the red absorption component increases. Interestingly, the stability of the solutions towards aggregation/phase separation also correlates well with the absorbance value of the red peak. Furthermore, careful thermal aging measurements confirm that as the aggregation process proceeds, the absorption spectrum remains unchanged, although the initially relatively broad spectral peaks do become significantly easier to resolve. As a result, an ‘‘intermediate’’ peak (due to an intermediate conjugation length) in the orange region is resolvable. Finally, polarized light microscopy confirms that the aggregates formed from thermally aged solutions containing an orange/red component are highly anisotropic, indicative of high chain alignment. Similar results are obtained from evaporating off the solvent from a dilute yellow solution. The implications of this self-alignment process are discussed.
Similarity transference of molecular parameters. I. The atomic polar tensors of cyanoacetylene
Authors: Neto B. B. , Ramos M. N. , Bruns R. E.
Published: July 2002

A similarity transference procedure for the calculation of molecular parameters is proposed. The theoretical relationships between the direct transference method normally used and the proposed procedure are discussed. The importance of adequately defining the similarity models used in the transference calculations is emphasized. As an example, similarity models are constructed using the experimental and the STO-3G and 4-31G molecular orbital values of the atomic polar tensors of the HCN, C2H2, CH3CN, C4H2, CH3CCH, and C2N2 molecules. Partial least squares calculations based on these similarity models and using the STO-3G and 4-31G values of the atomic polar tensors of HC3N result in estimates of the experimental tensors of cyanoacetylene which have about one-half or less the root mean square error of the molecular orbital values. Also the partial least squares detection of potentially unreliable estimates of polar tensor elements is illustrated.
Localization effects in the Auger spectra of ring nitrogen systems: Pyridine, poly(2-vinyl)pyridine, borazine, and boron nitride
Authors: Rye R. R. , Kelber J. A. , Kellogg G. E. , Nebesny K. W. , Lichtenberger D. L.
Published: July 2002

The N(KVV) Auger spectra of gas phase pyridine (C5H5N) and borazine (B3N3H6), and of solid phase poly(2-vinyl)pyridine (PVP) and hexagonal boron nitride [(BN)x] are reported and analyzed. The data indicate two Auger ‘‘fingerprint’’ types of nitrogen. Ammonia (NH3) is the prototype for the first, where three of the five valence electrons are σ bonding and the other two are the lone pair. This localized electronic structure gives rise to relatively sharp features in the N(KVV) spectrum. Typical of the second fingerprint type is pyridine, where there are two σ bonding electrons, a lone pair of electrons, and one electron contributing to the delocalized π system. Theoretical nitrogen Auger transition energies and intensities are calculated for pyridine to demonstrate the general origin of the overlapping features in the relatively broad N(KVV) spectrum of this molecule. PVP fits into the second fingerprint type while borazine and boron nitride give nitrogen Auger spectra more like ammonia. Approximate calculations using the equivalent core concept are used to clarify the relationship between the ammonia, borazine, and boron nitride spectra. It is shown that in these systems the initial Auger state (core–hole) largely localizes the bonds and lone pair on the nitrogen. The Auger spectra show that it is the σ, π and nonbonding orbital characters that provide the Auger fingerprint.
Coupled Hartree–Fock calculations of 31P NMR shieldings in phosphorous fluorides and PO3−4
Authors: Tossell J. A. , Lazzeretti Paolo
Published: July 2002

A b initio coupled Hartree–Fock perturbation theory (CHFPT) calculations on PF3, PF+4 , PF5, PF−6, and PO3−4 using large polarized Gaussian bases satisfactorily reproduce observed 31P NMR chemical shifts. In PF3 the 31P NMR shielding constant σ increases as the P–F bond distance or the F–P–F angle is decreased. The 31P chemical shift δ of PF+4 is predicted to be ∼−15 ppm (vs 85% H3PO4). Although the average values of the isotropic NMR shieldings are overestimated by the calculations (e.g., σPav calc. 372.5 ppm vs expt. 259±20 ppm in PF3) the anisotropy in σP is well reproduced (calc. 244.7 ppm vs expt. 228±2 ppm). The ∼200 ppm smaller 31P NMR shielding of PF3 compared to PF5 arises partly from a diamagnetic contribution lower by ∼100 ppm and partly from the more negative paramagnetic contribution arising from mixing of the 8a1 P lone pair MO with unoccupied orbitals of e symmetry in PF3.