bluuebunny/crossref_metadata_2025 huggingface dataset:
The bond length and electronic structure of V2
Authors: Langridge-Smith P. R. R. , Morse M. D. , Hansen G. P. , Smalley R. E. , Merer A. J.
Published: February 2003

We report of the first gas-phase electronic spectrum of V2. The dimer was generated in an expansion cooled molecular beam by laser vaporization of vanadium metal. Using the technique of resonant two photon ionization spectroscopy a strong band system with origin near 7000 Å was observed. Rotationally resolved spectra of this band system conclusively demonstrate that the transition is of the type 3Πu(a)←3Σ−g(a), and we assign the 3Σ−g state as the ground electronic state of V2. The short ground state bond length re″=1.77 Å and high vibrational frequency ωe″=535 cm−1 are indicative of extensive 3d-orbital participation in the bonding of this molecule. The unusual case (a) coupling and large second-order spin-orbit splitting observed in the 3Σ−g state implies that the ground state electronic configuration contains a half-filled π(3d) or δ (3d) orbital. The most plausible valence configuration consistent with the data is σg(3d)2πu(3d)+σg(4s)2δg(3d)2. In the excited state there is strong evidence of predissociation occuring for levels higher than 14 872 cm−1 above the ground state, thus providing an upper bound of 1.85 eV for the dissociation energy of V2.
Model calculations of intramolecular interference effects in Rayleigh scattering from solutions of macromolecules
Authors: McClain W. M. , Schauerte J. A. , Harris Robert A.
Published: February 2003

We present an algorithm for seven of the ten observables that govern polarized elastic light scattering from dilute solutions of rigid anisotropic structures of arbitrary size and shape. The algorithm implements the theory of optical interference of electric dipole radiation scattered from different subunits of the structure with exact analytic averaging over all orientations of the whole structure. The phases of the incident and scattered waves are included in the average, and results are exact at all scattering angles. Model calculations show that the polarization observables may carry structural information for scatterers smaller than the optical resolving limit.
Light induced bistability in S2O6F2⇄2 SO3F: Theory and experiment
Authors: Zimmermann E. C. , Ross John
Published: February 2003

Bistability is predicted and experimentally verified in an illuminated, gas phase reaction mixture of peroxydisulfuryl difluoride and fluorosulfate free radicals. The steady state solutions of simple dynamical equations, based on the assumption of spatial uniformity, are in good agreement with hysteretic effects observed in bulk absorption measurements: Sharp transitions between stable branches of different absorption occur as the laser power used to irradiate the system is slowly varied. The appearance of a ‘‘flame’’, previously reported by others and noted here, is explained by an extension of the model which enables calculation of spatial inhomogeneities. The results of the extended model, when bulk averaged, are similar to those for the uniform system; however, large scale structure and steep gradients, corresponding to a region of intense fluorescence, or flame, are predicted in the case of high absorption. Experimental measurements of absorption profiles confirm these predictions. A critique is made of prior work on bistability in the N2O4/NO2 system.
Absolute production rates and efficiencies of NO in electron-irradiated N2/O2 mixtures
Authors: Green B. D. , Caledonia G. E. , Blumberg William A. M. , Cook F. H.
Published: February 2003

The production of vibrationally excited NO in electron irradiated N2-trace O2 mixtures has been measured using a calibrated interferometer-based detection system. The time variation of the NO vibrational populations deduced from the fluorescence was used to determine excitation rates into vibrational levels 1–7. One vibrationally excited NO molecule is created per electron–ion pair. The N(2D)+O2 rection has been identified as the formation mechanism. Metastable N-atom formation mechanisms in these mixtures are discussed. Approximately 2+0.8%−0.5% of the deposited beam energy appears as NO vibrational energy.
Quantum manifestations of classical resonance zones
Authors: De Leon N. , Davis M. J. , Heller E. J.
Published: February 2003

We examine the concept of nodal breakup of wave functions as a criterion for quantum mechanical ergodicity. We find that complex nodal structure of wave functions is not sufficient to determine quantum mechanical ergodicity. The influence of classical resonances [which manifest themselves as classical resonance zones (CRZ)] may also be responsible for the seeming complexity of nodal structure. We quantify this by reexamining one of the two systems studied by Stratt, Handy, and Miller [J. Chem. Phys. 71, 3311 (1974)] from both a quantum mechanical and classical point of view. We conclude that quasiperiodic classical motion can account for highly distorted quantum eigenstates. One should always keep this in mind when addressing questions regarding quantum mechanical ergodicity.
Integrated infrared intensities and the atomic polar tensors in SO2
Authors: Kim Kwan , King W. T.
Published: February 2003

The integrated infrared intensities of the fundamental modes in SO2 have been measured by the Wilson–Wells–Penner–Weber method. The results are 20.60±0.07 km/mol for ν1 at 1151 cm−1, 25.33±0.04 km/mol for ν2 at 518 cm−1, and 192.13±0.14 km/mol for ν3 at 1362 cm−1. The polar tensors derived from the observed intensities are in essential agreement with those obtained from an ab initio calculation. The effective atomic charge for the oxygen atoms in SO2 is shown to have a magnitude close to that found in other oxygen-containing molecules.
Electronic structure and spectra of the lowest five 1Σ+ and 3Σ+ states, and lowest three 1Π, 3Π, 1Δ, and 3Δ states of NaK
Authors: Stevens Walter J. , Konowalow Daniel D. , Ratcliff Lyn B.
Published: February 2003

We calculate the potential energy curves for all molecular states of NaK which may be obtained from the interactions Na(3s)+K(4s), Na(3s)+K(4p), Na(3p)+K(4s), Na(3s)+K(5s), Na(3s)+K(3d), and for the Δ states corresponding to the interactions Na(3d)+K(4s) and Na(4p)+K(4p) by full-valence configuration interaction computations which utilize effective core potentials to describe the core electrons, the core-valence orthogonality constraints, and the core-valence correlation (CVC) energy. The differences between our computed curves and those deduced from experimental spectra are generally small and can be accounted for by: (1) the modest size of the basis set, which is insufficiently diffuse to describe Na− and K− resonances associated with excited charge transfer interactions and related molecular Rydberg character, and (2) the approximate way in which the CVC interaction is included.
High pressure phase transitions and hydrogen-bond symmetry in ice polymorphs
Authors: Schweizer Kenneth S. , Stillinger Frank H.
Published: February 2003

A recently developed ground state variational wave function theory of hydrogen-bonded crystals is employed to investigate the high pressure behavior of several ice polymorphs. The theory accurately accounts for short and long range proton correlations and quantum mechanical tunneling, all of which are important in the high pressure regime. Attention is focused on pressure induced order–disorder phase transitions and hydrogen-bond symmetrization for the ices VII, VIII, and Ic. We find a strongly first order transition (driven by proton tunneling) at approximately 330 kbar from the antiferroelectrically ordered ice VIII phase to a highly ionized form of proton disordered ice VII. Despite the large degree of ionization, significant short range proton order remains at the transition as indicated by the bimodality of the proton charge density along the bond. The crossover to a unimodal situation, i.e., hydrogen-bond symmetrization, is predicted to occur at 450 kbar. The effects of deuterium isotope substitution on these phenomena are also studied. Calculations for the lower density cubic form of ice indicate that it may be possible to observe symmetrical hydrogen bonds in this material at a significantly lower pressure than those required for the dense ices.
An isotopic disorder as a possible cause of the intrinsic electronic localization in some materials with narrow electronic bands
Authors: Berezin A. A.
Published: February 2003

A new hypothesis relating the possibility of the existence of the intrinsic centers of electron (hole) localization in some crystalline materials with very narrow electronic (hole) bands is put forward. For the sake of definiteness, the discussion is based on the case of beta-rhombohedral boron—the material with relatively large isotopic mass difference. We consider the supposition that the centers of intrinsic localization could result from Anderson electronic localization (AEL) induced by a random distribution of boron isotopes 10B and 11B among the regular sites of a complex unit cell. An estimation of the width of the narrow band required for the AEL is made in the frameworks of the one-oscillator electron-vibrational model. The obtained upper estimate for the width J≂0.3 meV seems to be a realistic one to provide the necessary order-of-magnitude for the AEL or, at least, to be comparable in strength with other possible sources of localization. This can be especially plausible in view of the possibility of the polaron effect in beta-boron, which strongly reduces the width of narrow electronic bands.
Calorimetric, spectroscopic, and structural studies of anhydrous zinc bromide
Authors: White M. A. , Chieh C. , Anderson A. , Staveley L. A. K.
Published: February 2003

Measurements of the heat capacity of ZnBr2 from T=10 to 300 K, as well as Raman, infrared, and x-ray diffraction investigations reveal some very interesting features of this material. The unit cell (space group I41/acd) is large (Z=32), and composed of supertetrahadral Zn4Br10 subunits. The large unit cell and the large effective mass of the subunits, respectively, give rise to a rich vibrational spectrum and many low frequency modes. These have been used quantitatively to account for the unusually high heat capacity of ZnBr2 at low temperatures.
Velocity correlations in supercooled and nucleated liquid rubidium
Authors: Brown Alan C. , Mountain Raymond D.
Published: February 2003

The momentum transport is studied for supercooled liquid rubidium, via molecular dynamics. The transport is investigated on a microscopic scale by measuring the contribution to the two point velocity correlation function for various separations of the two points. The correlation functions are measured in both the normal and supercooled liquid and the solid phase. It is found that momentum is transported by two different mechanisms; by correlated oscillations associated with the maximum of g(r), and by strongly diffusive modes associated with the minimum of g(r). The amplitude of the diffusive or liquid-like mode increases upon nucleation, and remains large at low temperatures in the solid phase.
Diffusion in a laminar flow: Shear rate dependence of correlation functions and of effective transport coefficients
Authors: Hess Siegfried , Rainwater James C.
Published: February 2003

The diffusion equation for independent Brownian particles suspended in a fluid undergoing plane Couette shear flow is solved in Fourier space by means of the Campbell–Baker–Hausdorff expansion for the product of exponentials of noncommuting operators. Explicit solutions are derived and numerically evaluated for an initial Gaussian distribution with no source and for a continuous stationary source with a Gaussian spatial distribution. For the latter problem, the tensor describing the curvature of the steady-state distribution at the origin is analyzed in some detail and is shown to possess a dependence on shear rate very similar to that of the pressure tensor obtained in computer simulations of simple liquids under shear by Hanley and Evans.
Observation of surface-enhanced Raman scattering for transition-metal hexaammine cations at the outer Helmholtz plane: Implications for enhancement mechanisms at electrochemical interfaces
Authors: Tadayyoni M. A. , Farquharson Stuart , Weaver Michael J.
Published: February 2003

Surface–enhanced Raman scattering (SERS) has been observed at silver electrodes for several transition-metal hexaammine complexes even though these cations cannot bind to the metal surface and are excluded from the electrochemical inner layer by the presence of a monolayer of chloride or bromide anions. The intensity of the SERS metal-ammine stretching vibrations νM−N are comparable to those seen for closely related pentammine complexes that are bound to the surface via specifically adsorbed coordinated ligands. For hexammineruthenium(III), changes in the νM−N frequency with potential indicate the presence of an interfacial Ru(NH3)3+/2+6 couple. The formal potential of this couple obtained from the potential dependence of the SERS intensities is consistent with the Raman scattering ions being located at the outer Helmholtz plane. The implications of these results to current models of SERS in electrochemical environments are noted.
Franck–Condon distributions in the Penning ionization of HCl by metastable helium
Authors: de Vries Mattanjah S. , Tyndall George W. , Martin Richard M.
Published: February 2003

Rotational and vibrational constants of HCl+(A) are determined following the collision of HCl and He (25) at 40 meV. (AIP)
Limitations of the rotating wave approximation in the theory of molecular collisions in a laser field
Authors: Last I.
Published: February 2003

The radiative transition in a molecular collision system is usually described as a motion on so-called electronic-field potential surfaces, obtained by application of the rotating wave approximation (RWA). The RWA is shown to be valid if the time of interaction in the resonance region is larger than a third of the light period T/3. The interaction time is estimated for a radiation X3/2→X1/2 transition in the F+H2 and Br+H2 systems.
Optical third-harmonic generation in the fluorinated methanes and sulfur hexafluoride
Authors: Ward J. F. , Elliott D. S.
Published: February 2003

Optical third-harmonic generation experiments are used to determine nonlinear polarizabilities for the fluorinated methanes and sulfur hexafluoride. An ambiguity present in previous measurements of third-harmonic generation in gases is resolved.
Vibrational circular dichroism and vibrational optical rotatory dispersion in molecular crystals. II. Selection rules
Authors: Lalov I. J. , Kotzev J. N.
Published: February 2003

The selection rules for the contribution of the electric and magnetic dipole and electric quadrupole moments of the optical active vibrations to the gyrotropy of molecular crystals are derived by group-theoretical analysis. The peculiarities of nondegenerate and degenerate vibration modes are discussed. The selection rules for k → 0 case are tabulated for all 32 crystal classes, taking into account the time-reversal symmetry and Wigner co-representation theory.
1H and 13C NMR study on rotation of congested methyl groups in methyl substituted phenanthrenes, fluorenes, and fluorenones
Authors: Takegoshi K. , Imashiro F. , Terao T. , Saika A.
Published: February 2003

We have investigated methyl rotation in 4-methyl and 4,5-dimethyl substituted phenanthrenes, fluorenes, and fluorenones by the temperature dependence of the 1H spin-lattice relaxation times and high-resolution solid-state 13C NMR spectra. Distinctly unsymmetric curves of the relaxation times with temperature are ascribed to the existence of inequivalent methyl groups in the unit cell. The inequivalence is considered to be brought about by distortion about the central C4a–C4b bonds and anisotropic intermolecular forces in the crystalline state. The steric effect of the protruding 5-hydrogen atom in a planar molecular structure gives rise to a remarkably high barrier (5.05±0.13 kcal mol−1) to rotation of the methyl group in 4-methylphenanthrene (1).
Semiclassical investigation of vibrational state and molecular orientation effects in electron transfer reactions for the H+2/H2 collision
Authors: Lee Chyuan-Yih , DePristo Andrew E.
Published: February 2003

An accurate interaction potential is used in the semiclassical energy conserving trajectory formulation to investigate electron transfer reactions in the H+2/H2 collision for initial ion vibrational states 0≤ν′0 ≤5. The state-to-state cross sections are calculated at several initial molecular orientations and ion kinetic energies. The relative total charge transfer cross sections as a function of ν0 are in good agreement with experimental data. At the state-to-state level, the cross section for the resonant channel at low energies (16 and 32 eV) contributes more than 75% of the total charge transfer cross section at ν′0 =0, but decreases with ν0 to less than 50% at ν′0 =5. At high energies (400 and 800 eV) the cross section of many off-resonant channels are as large as that of the resonant channel. These detailed state-to-state results depend on the initial molecular orientations. We also show the charge transfer probabilities as a function of impact parameter. The oscillatory variation suggests the number of electron jumps between two colliding ion cores.
Parametrization of molecular orbital transformations
Authors: King Harry F. , Camp R. Nicholas , McIver James W.
Published: February 2003

It is proved that any orthogonal matrix U can be represented in the form U=eX Ū where X is real, antisymmetric, and block off-diagonal, and Ū is orthogonal and block diagonal with arbitrary block structure. The unique elements of X constitute the orbital rotation subset of a larger set (p, q, X) of independent perturbational and variational parameters. These provide a convenient separation of variables for orbital optimization and for analytic computation of derivative properties of variational wave functions.
Ab initio valence-electron-only molecular electronic structure calculations: Theory and test applications
Authors: Leasure S. C. , Martin T. P. , Balint-Kurti G. G.
Published: February 2003

A new method for performing molecular electronic structure calculations using electronic wave functions which make explicit reference only to valence electrons is presented. The method contains no adjustable parameters and makes use of projection operators to project out the core function space from the valence-electron wave function. The energy calculated using this valence-electron-only method is guaranteed to converge, with increasing size of orbital basis, upon the valence energy which would be obtained from an equivalent all-electron calculation using a frozen SCF description of the core electrons. The method may be used in conjunction with any desired treatment of the valence-electron wave functions (i.e., SCF, CI, MC-SCF, VB, or VB-SCF). In this paper, test calculations are performed using an SCF treatment of the valence-electron wave function for the molecules NaCl, Na2, and Cl2. All-electron and valence-electron-only calculations are compared using differing (i.e., reduced) sizes of orbital basis sets for the valence-electron-only calculations. It is shown that valence-electron-only calculations can lead to considerable savings in computer time while still providing accurate potential energy curves and valence energies.
Dipole moments and polarizabilities of rare gas atom pairs
Authors: Pearson Erik W. , Waldman Marvin , Gordon Roy G.
Published: February 2003

The modified electron gas Drude model is applied to the calculation of rare gas pair dipole moments and pair polarizabilities. The dipole moment calculations, which incorporate both short range and dispersion effects, are in general agreement with both experimental results and results obtained by other, more complicated, theoretical models. The pair polarizability calculations, in which dispersion is neglected, are less successful. Predicted deviations from the dipole-induced dipole model are of the correct sign but of insufficient range and magnitude. This behavior is interpreted as a consequence of a computationally convenient but physically inadequate parametrization of the polarizability model, and improvements are suggested.
Lattice effects on bonding, covalency, and transferred hyperfine interaction in K2NaCrF6 and CrF3
Authors: Barandiarán Z. , Pueyo L.
Published: February 2003

The effects of the crystal lattice potential on bonding and covalency parameters of K2NaCrF6 and CrF3 have been investigated in terms of a partially relaxed lattice model described earlier. The theoretical values of different quantities in these two crystals are compared with the Hartree–Fock predictions on the CrF3−6 cluster in vacuo. In K2NaCrF6, the lattice potential reduces the orbital energies of the metal and ligand valence MO’s by 16.8 and 15.7 eV, respectively. In CrF3 these figures are 0.5 eV smaller. The 3d states, which have positive orbital energies in the cluster-in vacuo calculation, become bound states by the action of the lattice potential. The overlap populations suffer different changes in MO’s of different symmetries. The bonding MO’s turn out to be less bonding and the antibonding MO’s still less antibonding, the net effect being an increase in the total overlap population. The covalency parameters λπ and γπ are proportional to the square root of the ligand–metal π overlap and change with the internuclear distance RML as R−3ML and R−2ML , respectively. This calculation suggests that some 15% of the value of these parameters can be attached to the 3d radial expansion. CrF3 appears to be slightly less covalent than K2NaCrF6 according to the values of both γπ and the theoretical nephelauxetic ratio. The observed spin-transfer parameter fπ cannot be reproduced by simple models such as the metal–ligand orthogonalization or the first-order perturbation mixing of the 3dπ and 2pπ states. Our spin-restricted cluster calculation gives fπ values in reasonable agreement with neutron diffraction and resonance spectroscopy data and predicts that fπ varies as R−6ML . The cluster–lattice interaction gives a small contribution to fπ but it seems to be appropriate for explaining the differences in fπ which a given cluster shows in different lattices.
A molecular theory of crystal nucleation from the melt
Authors: Harrowell Peter , Oxtoby David W.
Published: February 2003

We construct a molecular theory for the homogeneous nucleation of crystals from pure liquids by combining an order parameter theory of freezing with a square gradient approximation for the nonlocal dependence of free energy on density. We show the results of our theory for the free energy, radius, and shape of crystal nuclei as a function of supercooling, and compare with the conventional nucleation theory based on the capillarity approximation. We argue that the simple picture given by the conventional approach is qualitatively reasonable, although calculated values of the liquid–solid surface free energy may be inaccurate. We point out some of the limitations of our approach and stress the need for additional structural data on supercooled liquids.
Thermal desorption of argon and neon from solid xenon. II. Sticking probabilities
Authors: Adams John E. , Doll Jimmie D.
Published: February 2003

The practical difficulties associated with full molecular dynamics simultations of thermal desorption processes have necessitated the finding of more efficient ways of studying these phenomena. Transition state theory provides part of the solution, but in order to obtain exact rate constants, one must also calculate dynamical corrections, in this case sticking probabilities. The present work proposes a means for computing sticking probabilities that permits the inclusion of both the thermal motion of the solid atoms and also the interactions among adsorbed species. Using this formalism we have made a study of the effects of changes in temperature and adsorbate coverage on sticking probabilities for Ar, Ne–Xe(111), with the results suggesting that these two effects can be indeed significant but that they are not necessarily independent of one another.
Variational solutions of simple quantum systems subject to variable boundary conditions. I. A model for physisorption
Authors: MacMillen Donald B. , Landman Uzi
Published: February 2003

A variational solution to a model problem of a hydrogen atom near a sharp planar surface of a semi-infinite substrate is studied. Results are presented for the ground and excited eigenstates and dipole moments of the hydrogen atom as a function of distance from the surface for two models: (a) a system in which the half-space occupied by the solid is represented by an infinite potential step; and (b) a system in which the semi-infinite substrate is perfectly imaging subject to a boundary condition that excludes the atomic electron from the substrate. Model (b) is considered as an idealized model for physisorption, where the exclusion of the electron simulates the exchange repulsion originating from the overlap of the atomic and substrate electrons. Results for the physisorption holding potential as a function of distance from the plane are presented and discussed.
Theory of chemical reactions of vibronically excited H2(B 1Σ+u). I. Prediction of a strongly bound excited state of H4
Authors: Nicolaides C. A. , Theodorakopoulos G. , Petsalakis I. D.
Published: February 2003

MRD–CI calculations were carried out on the potential energy surfaces of the ground and first excited singlet of the same symmetry of H4, which predict a bound excited state with respect to H2+H*2 (B 1Σ+u) of 3.1 eV at a geometry of trigonal pyramid. This geometry was predicted theoretically according to a model of chemical bonding for ‘‘maximum ionicity excited states.’’ At the minimum and the three dimensional space surrounding it, there is a near touching of the excited with the ground state hypersurfaces. This fact has implications for the spectroscopy and photochemistry of the H2+H*2 system.
Polarization and M-dependent effects in rotationally inelastic collisions of open-shell diatomic molecules: Ar–NO(X 2Π1/2)
Authors: Alexander Millard H. , Orlikowski Tadeusz
Published: February 2003

Our earlier quantum study of collisions of NO(X 2Π) with Ar [T. Orlikowski and M. H. Alexander, J. Chem. Phys. 79, 6006 (1983)] is here extended to the calculation of M-dependent cross sections in the case of laboratory-frame quantization, as would be appropriate to collision cell experiments. The calculations confirm the earlier prediction of Alexander and Davis [J. Chem. Phys. 79, 227 (1983)] that the JM→J, -M cross sections will be vanishingly small for transitions which preserve the e or f Λ-doublet symmetry. This propensity rule is here shown to be rigorous within the coupled-states (CS) limit. The effect of collisions on the initial rotational density matrix can be expressed in terms of multipole transfer efficiencies. The calculated values for Ar–NO collisions indicate that e/f changing collisions are much more strongly depolarizing, in the sense of destroying the multipole moments of the initial density matrix, than those collisions which preserve the e/f Λ-doublet label. Since the circular or linear polarization of the fluorescence from a laser excited molecule can be written in terms of these multipole transfer efficiencies, we are able to use the Ar–NO(X 2Π) cross sections reported here to model qualitatively the depolarization effects one might expect to see in experiments involving excited molecules in 2Π electronic states.
Brownian dynamics simulation of diffusion-influenced bimolecular reactions
Authors: Northrup Scott H. , Allison Stuart A. , McCammon J. Andrew
Published: February 2003

A method is developed and tested for extracting diffusion-controlled rate constants for condensed phase bimolecular reactions from Brownian dynamics trajectory simulations. This method will be useful when highly detailed model systems are employed, such as those required to explore the complicated range of interactions between enzymes and their substrates. The method is verified by comparing with exact analytical results for simple cases of spheres with uniform reactivity subject to various centrosymmetric Coulombic and Oseen slip hydrodynamic interactions. The utility of the method is illustrated for more complicated cases involving anisotropic reactivity and rotational diffusion.
Theoretical studies of photoexcited state Cu atom reactions. I. Excited state responsible for H2 capture
Authors: Ruíz M. E. , García-Prieto J. , Novaro O.
Published: February 2003

The determination of the electronic excited state of Cu which is responsible for H2 dissociation has been carried out by ab initio effective potential configuration interaction calculations. Energy surfaces for the interaction of H2 and Cu atoms in their ground (2S) and excited (2P and 2D) states are reported. The single determinant and CI calculations are analyzed. The results enlighten and support previous experimental work.
Model studies of atom and molecule diffusion on surfaces
Authors: Park Seung C. , Bowman Joel M.
Published: February 2003

We report a classical trajectory study of the diffusive dynamics of a rigid-rotor N2 trapped on a rigid but corrugated LiF(001) surface. An analogous study is also made for a fictitious atom with the atomic weight of N2. The ensemble-averaged square displacement as well as the velocity autocorrelation functions are computed for a range of kinetic energies, each exhibiting the standard signatures for diffusive behavior. The ‘‘temperature’’ dependence of the diffusion coefficients determined show Arrhenius behavior with an activation energy which can be approximately related to barriers on the three-dimensional potential surface.
Monte Carlo studies of a general model for lipid bilayer condensed phases
Authors: Scott H. L.
Published: February 2003

The sequence of phases exhibited for a two-dimensional lattice model with two coupled degrees of freedom has been obtained by Monte Carlo simulation. For a certain range of the parameter space of the model, modulated spatial correlations exist. Within this region, spatially modulated structures (ripples) appear on runs started in a disordered phase. When the ordered phase is used as the initial state, the time scale required for the appearance of modulated structures is extremely long. This model is designed to describe lipid bilayers below their main phase transition temperatures. As such, it provides explanations, in terms of intermolecular interactions, for the ripple phase and the chain-tilted ground state observed in phosphatidylcholine multilayers.
On the theory of self-diffusion in a polymer gel
Authors: Altenberger Andrzej R. , Tirrell Matthew
Published: February 2003

The problem of self-diffusion in a random medium consisting of a solvent and a fixed set of scattering centers (obstacles) is discussed. A new equation describing diffusion of a Brownian particle in such a system is derived. We find that the self-diffraction coefficient of the particle depends on the concentration of scattering centers like D/D0=1−A√C0−BC0+⋅⋅⋅, where A and B are positive constants, depending on the size of particle and repulsive range of scatterers, and D0 is the diffusion coefficient in a pure solvent.
A high pressure photon correlation study of bulk poly(methylacrylate). Comparison with relaxation processes in poly(ethylacrylate) and related polymethacrylates
Authors: Fytas G. , Patkowski A. , Meier G. , Dorfmüller Th.
Published: February 2003

We report results of the photon correlation polarized light scattering study on the dynamics of bulk polymethylacrylate (PMA) in the temperature range of 7–76 °C and in the pressure range 1–2100 bar. A single relaxation process with a distribution of relaxation times represents the experimental photon correlation functions. The temperature and pressure dependence of the mean relaxation time are analyzed in terms of the VFT and WLF equations. The parameters obtained are discussed in terms of the rotational isomerization model and the free volume model. The results obtained in light scattering agree well with dielectric and mechanical relaxation data. In the case of polyethylmethacrylate (PEMA) and polybutylmethacrylate (PBMA), two overlapping relaxation modes are found. The values of the activation parameters and the relaxation time at the glass transition temperature (Tg) thus obtained characterizing the dynamics of a polymer melt above Tg are compared for PMA, PEA, PEMA, and PBMA.
Molecular beam measurements of differential cross sections for the reaction D+H2→HD+H at E=1.0 eV
Authors: Götting Rüdiger , Mayne Howard R. , Toennies J. Peter
Published: February 2003

The angular and velocity distributions of HD formed in the cross molecular beam reaction D+H2 have been measured at an effective center-of-mass energy of 1.0 eV. The results are compared with calculations based on quasiclassical trajectories for the Siegbahn–Liu–Truhlar–Horowitz potential. The calculations are in reasonable agreement with the experiment, which probes the potential energy surface at 0.85 eV in the repulsive saddle point region.
Resonant interactions of normal modes in rotating classical molecules
Authors: Combs J. Andrew , Hoover William G.
Published: February 2003

Vibrational motions of isolated rotating classical molecules exhibit resonance due to Coriolis and centrifugal coupling. Analytical solutions for normal mode amplitudes are obtained under the assumptions of constant rotation rate and harmonic intramolecular forces. Degenerate modes resonate most strongly, completely exchanging energy after a rotation through an angle θr=π/2ζ, where 0<ζ≤1 is a measure of the Coriolis interaction.
Vibrational predissociation resonances for a model H–C–C→H+C=C Hamiltonian by the complex coordinate methoda)
Authors: Moiseyev N. , Bar-Adon R.
Published: February 2003

The resonances of a H–C–C that represent a protype alkyl radical dissociation reaction are studied by the complex coordinate method. It is shown that even in energy above the dissociation limit, the local mode approximation is applicable and the mixing of the two modes is very small. These results are in complete agreement with the classical and semiclassical quasiperiodic trajectories recently studied by Hase et al., and support the possible existence of narrow resonances above the potential barrier, or even in its absence, as proposed by Bloemergen et al. for the mechanism of the dissociation of HN3 to NH[3π] and N2.
Biaxial nematic phase, multiphase critical point, and reentry transition in binary liquid crystal mixtures
Authors: Chen Zhong-Ying , Deutch J. M.
Published: February 2003

A model considering both short-range and long-range interactions for liquid crystals mixtures of rod-like and plate-like molecules is presented in this paper. The model is treated in the framework of a mean field, van der Waals-type theory. The relationship of the model to a Landau-type treatment is discussed in an Appendix. It can be shown for this model that a multiphase critical point exists between the isotropic, rod nematic, plate nematic, and biaxial nematic phases. This point can be located from a set of analytic equations. The fluctuations and the magnetically induced birefringence around this point are calculated. The model predicts the phase diagrams of binary liquid crystal mixtures both with and without reentry transition. The reentrant phase transitions from the biaxial to the uniaxial nematic phase and from the uniaxial nematic to the isotropic phase have not received prior theoretical attention but have been demonstrated in recent experiments.
Classical dynamics of a coupled double well oscillator in condensed media. II
Authors: Fonseca Teresa , Gomes J. A. N. F. , Grigolini Paolo , Marchesoni Fabio
Published: February 2003

The influence exerted on reaction processes by the coupling between the reaction coordinate x and transverse nonreactive modes is discussed. Attention is mainly focused on the synergism of inertia and multiplicative fluctuation, which enhances the reaction rate throughout a wide domain, ranging from the high to the low-friction region. The high-friction region is explored by applying the adiabatic elimination procedure (AEP) described in the first paper of this series: When the relaxation time of x is larger than that of the nonreactive mode, a new term occurs. This term is shown to be responsible for making the ‘‘noise-induced phase transition effects’’ discussed in the first paper more pronounced. The low-friction region is explored with a first-passage time procedure by using energy as the slow variable. The synergism of inertia and multiplicative noise is then shown to produce finite reaction rates even in conditions where Kramers theory predicts them to be vanishingly small.
Collisional deactivation of vibrationally highly excited polyatomic molecules. IV. Temperature dependence of 〈ΔE〉.
Authors: Heymann M. , Hippler H. , Troe J.
Published: February 2003

The collisional loss of energy from vibrationally highly excited toluene molecules (excitation energy 52 000 cm−1) was monitored directly by time-resolved UV absorption spectroscopy after laser excitation. Experiments were performed in a variety of bath gases heated up to 870 K with a modulated cw CO2 laser. Comparison with experiments at 300 K yields information on the temperature dependence of the average energy 〈ΔE〉 transferred per collision. The temperature coefficient of 〈ΔE〉∝T n was found to be close to zero (n=0.0±0.2) for monoatomic and diatomic bath gases; for larger bath gases small negative n values were found (n=−0.3±0.3). The results confirm earlier experiments on the steady-state photoisomerization of cycloheptatriene which were reevaluated. They are in marked disagreement with recent conclusions on a strong decline of 〈ΔE〉 with increasing temperature.
Experimental excitation cross sections by electron impact of n
p′ [1/2]0 (n=3,4,5) levels of Ne
Authors: Shaw M. , Borge M. J. G. , Campos J.
Published: February 2003

Cross sections have been measured for excitation of np′[ 1/2  ]0(n=3,4,5) levels of Ne i by electron impact. The electron energy ranged from the threshold to 400 eV. For the aforesaid levels the experimental cross sections are 202, 43, and 20 (×10−20 cm2), respectively, for an electron energy of 100 eV.
Entropy bottlenecks in ion–molecule reactions
Authors: Dodd James A. , Golden David M. , Brauman John I.
Published: February 2003

The significance of entropy bottlenecks in dissociation and recombination pathways in the prototype ionic system CH3+CH+3 has been investigated. Ion–molecule systems are shown to react through an entirely different dynamics than neutral systems, due to intrinsic differences in the shapes of the relevant potential surfaces. Consequences with regard to the interpretation of experimental rate parameters in the ion–molecule area are discussed.
The reaction of CO+2 with atomic hydrogen
Authors: Tosi P. , Iannotta S. , Bassi D. , Villinger H. , Dobler W. , Lindinger W.
Published: February 2003

Using a quasistatic drift tube, a constant rate coefficient k=2.9×10−10 cm3 s−1±30% has been obtained for the reaction of CO+2 with atomic hydrogen at relative energies 0.06 eV≤KEcm≤0.14 eV. Due to the presence of molecular hydrogen in the reaction region, the title reaction is competing with the reaction of CO+2 with H2. A method of separating the two reactions is presented.
Studies of the photoionization cross sections of acetylene
Authors: Lynch D. , Lee M.-T. , Lucchese R. R. , McKoy V.
Published: February 2003

We have studied the photoionization cross sections and photoelectron asymmetry parameters for all molecular orbitals of acetylene. These cross sections were obtained using accurate frozen-core Hartree–Fock continuum orbitals in the final-state wave function. The Hartree–Fock continuum equations were solved using the iterative Schwinger variational method. These fixed-nuclei cross sections are compared with available experimental data and with results obtained using the Stieltjes moment theory approach and the continuum multiple scattering method. The possible role of a shape resonance in the 1πu→kπg channel and the resonant structure of the σu continuum are discussed in some detail.
Kinetics of the reactions of atomic bromine with HO2 and HCO at 298 K
Authors: Poulet G. , Laverdet G. , Le Bras G.
Published: February 2003

The rate constants for the reactions Br+HCO→HBr+CO (2) and Br+HO2→HBr+O2 (4) have been measured at 298 K by the discharge flow technique coupled to laser induced fluorescence and mass spectrometry for detection of reactive species. HO2 radicals were generated by reacting atomic bromine with formaldehyde using an excess of O2, and were monitored after conversion into OH and NO2 with added excess NO. k2 and k4 were found to be (2.8±1.2)×10−10 and (7.6±0.9)×10−13cm3 molecule −1 s−1, respectively. The experimental kinetic curves were also compared to the predicted profiles obtained from computer simulation of the chemical system used. Although k4 is found to be 3–4 times higher than an earlier direct measurement, this result confirms that HBr is not an efficient reservoir for stratospheric bromine.
The electronic structure of one-to-one and defect scandium sulfide
Authors: Misemer D. K. , Nakahara J. F.
Published: February 2003

Self-consistent electronic structure calculations have been performed on two compositions of scandium sulfide ScS and Sc3S4. The results of the calculation of ScS are similar to those obtained for other transition metal chalcogenides and are in excellent agreement with heat capacity and reflectance measurements as well as UPS experiments. The calculation of the defect structure indicates the creation of sulfur p nonbonding states in metal-deficient ScS. The valency of the metal ions remains unchanged upon the creation of vacancies.
Numerical calculation of four-center Coulomb integrals
Authors: Talman James D.
Published: February 2003

A numerical method for the calculation of an arbitrary four-center Coulomb integral for angular momentum wave functions is developed. The method involves expanding the product of two angular momentum wave functions at two different centers as a linear combination of angular momentum wave functions centered at a point on the line segment joining the two centers. A formalism is developed that reduces the storage requirements for the functions defining the expansion of the product. The integral is therefore expressed as a sum of two-center integrals that are calculated in momentum space. A recently developed method for calculating spherical Bessel transforms is valuable in the final step. Sample results for a number of calculations are given and the possible application of the method in molecular structure calculations is considered.
Construction of a multidimensional potential energy surface from an energy spectrum
Authors: Waterland R. L. , Delos J. B.
Published: February 2003

A method for the systematic construction of a multidimensional potential energy surface from an energy spectrum is presented. We show that if the Hamiltonian and the energy spectrum satisfy certain conditions then there is an inversion technique which gives a potential energy function from the spectrum. The method proceeds via a sequence of canonical transformations and leads to a potential energy which is symmetric in all its variables. We test the procedure for one- and two-dimensional examples and show that it gives reasonable results. However, the method does not give a unique result and we discuss two types of nonuniqueness exhibited by the procedure.
Dirac-type functions as basis set for relativistic SCF calculations
Authors: Malli G.
Published: February 2003

Dirac-type functions (DTF) are proposed as basis set for the Dirac–Fock–Roothaan (DFR) SCF calculations on atoms. Expressions for all the matrix elements using DTF’s as basis set in DFR SCF formalism for atoms are also given. It is pointed out that the well-known Slater-type functions (STF) with noninteger principal quantum number are a special case of the DTF’s and that the matrix elements occurring in the DFR formalism for atoms using such STF’s as basis set can be easily derived from the corresponding matrix elements given in this paper.
Kinetic theory and irreversible thermodynamics of nonlinear transport processes in quantum systems
Authors: Eu Byung Chan
Published: February 2003

A theory of nonlinear irreversible processes in quantum systems (gases) is developed by using a quantum mechanical Boltzmann equation which is a generalization of the equation originally used by Nordholm, Uehling, and Uhlenbeck. The Boltzmann equation is solved by the modified moment method and quantum mechanical extensions are thereby obtained for various macroscopic equations, including the extended Gibbs relation, derived previously by means of the classical Boltzmann equation. The theory can yield a nonmonotonic entropy production as a function of fluxes in the system and thus provides us with possibility of describing instability in transport processes in the nonlinear regime. In order to demonstrate it with a simplified model, we consider the threshold switching phenomenon in semiconductors at low temperatures and show that, when the impact ionization recombination of donors is postulated as an underlying microscopic process for the phenomenon, the theory can indeed give a qualitatively correct result for the current-field characteristic and thus furnishes a nonequilibrium statistical mechanical description of the phenomenon. It also lays a foundation for studying the electron gas model for various nonlinear transport processes in metals and semiconductors at low temperatures in full conformation with the requirement of the thermodynamic laws.
Molecular dynamics studies of complexing in binary molten salts. I. Molten MAX4
Authors: Saboungi Marie-Louise , Rahman A. , Blander Milton
Published: February 2003

Molecular dynamics calculations were carried out for molten MX, AX3, and MAX4 for four different ionic radius ratios. The aim was to emulate expected properties of binary mixtures of complexing molten salts such as chloroaluminates and fluoroyttriates. The radial distribution functions were analyzed along with the distributions of the coordination numbers and of the angles of the A–A–A, A–X–A, and X–A–X triplets; these calculations yield detailed information on the nature and extent of the species in solution. The influence of changes of cation/anion radius ratios on the overall structure as well as on the formation and stability of complexes has been examined. For the two extreme values of these ratios, the nearest neighbor coordination number of A+3 ions changed from four to about six. One of the most significant conclusions is that the structure and ordering characteristic of complexing can extend beyond the local configurations of nearest neighbors and involve more distant ions. In addition, typical behavior often ascribed in the literature to covalency is detected in these simulated purely ionic melts.