bluuebunny/crossref_metadata_2025 huggingface dataset:
Density profiles at a planar liquid-liquid interface
Authors: Toxvaerd So/ren , Stecki J.
Published: July 2002

Molecular dynamics of liquid-liquid interface in an immiscible binary mixture of simple particles at low temperature and high pressure reveals stable equilibrium oscillatory structures in the density profiles of both components. The layering vanishes albeit slowly with the increase of the surface area but is not affected by an increase of the depths of bulk liquid layers. A simple Gaussian broadening predicts scaling of the oscillatory portion of the density profiles and good agreement is found. The width of the dip in the total density is also examined.
State-selected ion-molecule reactions: Charge transfer and atomic rearrangement processes in thermal energy collisions of H2+(X;v)+N2 and of N2+(X,A;v) + H2
Authors: Uiterwaal C. J. G. J. , van Eck J. , Niehaus A.
Published: July 2002

Using the photo-electron-product-ion-coincidence method (PEPICO) we have measured state-selective cross sections for the following processes: (A) N+2(X,A;v)+H2→N2H++H, (B) H+2(X;v)+N2→N2H++H, (C) N+2(X,A;v)+H2→H+2+N2, and (D) H2+(X;v)+N2→N2++H2. The measurements were performed at thermal velocities (Ec.m.≊40 meV). We have found that the charge transfer processes (C) and (D) have cross sections that are at least an order of magnitude smaller than the cross sections for the rearrangement processes (A) and (B). The cross section for reaction (A) with N2+(A;v) as reactant is found to be (50.2±2.4)% of the cross section for the same reaction with N2+(X;v) as reactant. The cross section for reaction (B) is found to be independent of the internal energy of the reactant ion. The measured variation of the cross sections as a function of the internal energy of the reacting ion is compared with calculations based on a RRKM type statistical model and an electronic correlation diagram of the (N2–H2)+ system. Excellent agreement is found, indicating complete randomization of internal energy within the collision complex. Absolute cross sections are determined for the rearrangement reactions: for reaction (A) the cross section is 76.1 Å2 starting with N2+(X;v=0,1) and 38.05 Å2 starting with N2+(A;v=0–5). For reaction (B) the cross section is 114 Å 2 for H2+(X;v=0–6).
Surface phonons on Ru(001) observed by electron energy loss spectroscopy in the presence of ordered oxygen overlayers
Authors: Mitchell William J. , Wang Youqi , Schick Matthias , Weinberg W. Henry
Published: July 2002

We have investigated, using electron energy loss spectroscopy, the phonons on the Ru(001) surface that are rendered dipolar active in the specular direction by virtue of ordered p(2×2) and p(1×2) oxygen adatom overlayers. We show that two surface phonons are present in the specular EEL spectra, and we verify, using group-theoretical arguments that only two surface modes can be dipolar active. This analysis also allows polarizations to be assigned to these modes. Implications for the reconstruction of the Ru(001) surface induced by the oxygen adatom overlayers are discussed.
Site-specific fragmentation following Si:2p core-level photoexcitation of F3SiCH2Si(CH3)3 in the vapor phase
Authors: Nagaoka Shin-ichi , Ohshita Joji , Ishikawa Mitsuo , Takano Keiko , Nagashima Umpei , Takeuchi Takae , Koyano Inosuke
Published: July 2002

Site-specific fragmentation following Si:2p photoexcitation of (trifluorosilyl) (trimethylsilyl)methane [F3SiCH2Si(CH3)3 (FSMSM)] has been studied by means of photoelectron-photoion and photoion–photoion coincidence techniques. The total photoionization efficiency curve of FSMSM has only one broad peak near the Si:2p core-ionization threshold and no evidence is obtained for the occurrence of selective excitation of each of the two Si atoms. From the results of ab initio calculations of FSMSM, it is found that the magnitude of the Coulomb interaction between the valence electrons and the Si:2p core electrons is very similar at the two Si sites. As a result, the difference in the chemical shift of the 2p core level between the two Si sites is very small. However, the evidence for site-specific fragmentation of the molecule following the Si:2p core-level photoexcitation is obtained by analyzing the photoion–photoion coincidence spectra; the relative yields of H+–SiF+ and CH+n–SiF+3 (n=1–3) ion pairs are enhanced around 104.5 eV, and those of CH+3–F3SiCH2Si(CH3)+2, CH+3–SiCH3F+2, CH+3–Si(CH3)2F+, and H+–SiCH+3 are enhanced around 107 eV. These two thresholds are interpreted to correspond to the 2p core-level photoexcitation at the Si atom bonded to three F atoms and that at the Si atom bonded to three CH3 groups. Ionic fragmentation following excitation of these core electrons occurs selectively around the Si atom where the photoexcitation has taken place. The site-specific fragmentation scheme can be explained on the basis of the valence electron density around the excitation site.
Theory of universal fast orientational dynamics in the isotropic phase of liquid crystals
Authors: Sengupta Abhijit , Fayer M. D.
Published: July 2002

A theoretical treatment is presented that demonstrates universal dynamical behavior in the isotropic phase of liquid crystals on ultrafast time scales and short distance scales. The theoretical development generates a temperature independent power law for the short time scale decay of the molecular orientational correlation function. This provides a theoretical rationale for the postulate of universal behavior based on recent experimental observations on two liquid crystal systems. A temperature independent power law decay with the identical exponent, 0.63, was observed for the two systems. First, an alternative theoretical approach reproduces the Landau de Gennes results for the long distance scale, slow time scale orientational dynamics in the isotropic phase. This approach is also capable of examining the short distance scale and short time scale dynamics, and yields a temperature independent power law decay with exponent 0.5. Then critical correlations of fluctuations and local symmetry considerations are included. The Ising model of critical systems is employed. This detailed analysis yields the experimentally observed exponent, 0.63, without recourse to adjustable parameters. Modern theories of dynamic critical phenomena like dynamic scaling theory, the kinetic Ising model and the stochastic model of Karder–Parisi–Zhang are considered as alternative approaches. While these theories can generate some of the features found in experiment, it is not possible to reproduce the observed experimental results without internal inconsistencies or unwarranted adjustable parameters.
Ab initio studies of the nuclear magnetic resonance chemical shifts of a rare gas atom in a zeolite
Authors: Jameson Cynthia J. , Lim Hyung-Mi
Published: July 2002

The intermolecular chemical shift of a rare gas atom inside a zeolite cavity is calculated by ab initio analytical derivative theory using gauge-including atomic orbitals (GIAO) at the Ar atom and the atoms of selected neutral clusters each of which is a 4-, 6-, or 8-ring fragment of the zeolite cage. The Si, Al, O atoms and the charge-balancing counterions (Na+, K+, Ca2+) of the clusters (from 24 to 52 atoms) are at coordinates taken from the refined single crystal x-ray structure of the NaA, KA, and CaA zeolites. Terminating OH groups place the H atom at an appropriate O–H distance along the bond to the next Si or Al atom in the crystal. The chemical shift of the Ar atom located at various positions relative to the cluster is calculated using Boys–Bernardi counterpoise correction at each position. The dependence of the rare gas atom chemical shift on the Al/Si ratio of the clusters is investigated. The resulting shielding values are fitted to a pairwise additive form to elicit effective individual Ar–O, Ar–Na, Ar–K, Ar–Ca intermolecular shielding functions of the form σ(39Ar, Ar...Ozeol)= a6r−6+a8r−8+a10r−10+a12r −12, where r is the distance between the Ar and the O atom. A similar form is used for the counterions. The dependence of the Ar shielding on the Al/Si ratio is established (the greater the Al content, the higher the Ar chemical shift), which is in agreement with the few experimental cases where the dependence of the 129Xe chemical shift on the Al/Si ratio of the zeolite has been observed.
Pairwise and nonpairwise additive forces in weakly bound complexes: High resolution infrared spectroscopy of ArnDF (n=1,2,3)
Authors: Farrell John T. , Davis Scott , Nesbitt David J.
Published: July 2002

High resolution infrared spectra of the vDF=1←0 stretch in ArnDF (n=1–3) have been recorded using a slit-jet infrared spectrometer. Analysis of the rotationally resolved spectra provides vibrationally averaged geometries and vibrational origins for a DF chromophore sequentially ‘‘solvated’’ by Ar atoms. Calculations using pairwise additive Ar–Ar and Ar–DF potentials predict lowest energy equilibrium structures consistent with the vibrationally averaged geometries inferred spectroscopically. Variational calculations by Ernesti and Hutson [A. Ernesti and J. M. Hutson, Faraday Discuss. Chem. Soc. (1994)] using pairwise additive potentials predict rotational constants which are in qualitative agreement with, but consistently larger than, the experimental values. The inclusion of nonpairwise additive (three-body) terms improves the agreement, though still not to within the uncertainty of the pair potentials. The vibrational redshifts of 8.696, 11.677, and 14.461 cm−1 for n=1–3, respectively, reflect a nonlinear dependence of the redshift on the number of Ar atoms. Both the variational calculations of Ernesti and Hutson and diffusion quantum Monte Carlo calculations [M. Lewerenz, J. Chem. Phys. (in press)] using pairwise additive potentials systematically overpredict the magnitude of these redshifts, further signifying the need for corrective three-body terms. Analysis of the ArnDF (n=2,3) rovibrational line shapes reveals an upper limit to homogeneous broadening on the order of 2–3 MHz, consistent with vibrational predissociation lifetimes in excess of 50 ns.
Preferential C–Cl bond rupture from 1-bromo-2-chloro-1,1,2-trifluoroethane following photoabsorption via n(Cl)→σ*(C–Cl) transition
Authors: Yokoyama Atsushi , Takayanagi Toshiyuki , Fujisawa Ginji
Published: July 2002

The photodissociation mechanism and dynamics of the title molecule have been studied at excitation wavelengths of 157 and 193 nm by using a photofragmentation translational spectroscopy. In the case of the excitation at 157 nm, the C–Cl and C–Br bond ruptures occur competitively with the branching ratio of 1.0:0.3, indicating the preferential C–Cl bond rupture over the weaker C–Br bond. The C–Br bond rupture occurred via two pathways; the dissociation on the excited repulsive potential energy surface and the dissociation of the vibrationally excited molecule in the ground electronic state following the internal conversion. In the case of 193 nm, only the C–Br bond rupture on the repulsive potential energy surface was observed.
Asymmetric partitioning of a polymer into a curved membrane
Authors: Baumgärtner A.
Published: July 2002

The partitioning of a flexible model polypeptide into curved and flat lipid bilayer model membranes has been investigated using Monte Carlo methods. It is shown that a curved lipid membrane exhibits a gradient of lipid density fluctuations across the bilayer. The fluctuations increase continuously from the side of lower curvature (outer layer) to the side of higher curvature (inner layer). It is found that curvature promotes partitioning of the polymer into the outer layer of the membrane. Moreover, it is shown that in the case of the curved bilayer, the polymer moves spontaneously from the outer to inner layer of the membrane, where it is subsequently localized. It is argued that the curvature-induced gradients of the fluctuations of the mean local lipid density and the local mean lipid orientation across the membrane is the entropical driving force for the observed translocation process.
The effects of local stiffness disparity on the surface segregation from binary polymer blends
Authors: Kumar Sanat K. , Yethiraj Arun , Schweizer Kenneth S. , Leermakers Frans A. M.
Published: July 2002

The surface segregation from free space polymer blends based on purely entropic effects is investigated using computer simulation and integral equation theory. Computer simulations are performed for tangent-hard-sphere chains of length ranging from short 10 bead chains to experimentally realistic 500 bead chains. The chain segments of one species experience a bending potential which is introduced between any two consecutive bonds and this serves to make this component stiffer than the other blend component. Computer simulations and numerical wall polymer reference interaction site model (wall-PRISM) integral equation calculations for finite hard core athermal chains demonstrate that at liquidlike densities the segments of the stiffer polymer always partition to a neutral surface, apparently independent of the length of the polymer chains in question. Although the primary factor affecting this segregation is the better local packing of the stiff chains at the surface, lattice mean-field calculations suggest that local conformational changes in the molecules also favor the stiff chains at the surface under these conditions. Further, nonlocal effects appear to be irrelevant in this context. Recently, field theoretic based models have suggested in the context of an incompressible approximation that stiffness disparity is the underlying cause for the experimentally observed surface segregation of branched molecules from blends of linear and branched hydrocarbon polymers (the branched molecules were considered more ‘‘flexible’’ or ‘‘conformationally smaller’’).

The segregation observed in the simulations, however, is both much smaller in magnitude and of the opposite sign to that seen in the field theoretic calculations. Coupled with results of independent work on the bulk behavior of these athermal mixtures, which do not capture the experimentally observed phase separation, we suggest that hydrocarbon blends, at least over the chain lengths examined, cannot be modeled in terms of purely entropic effects, but rather through the incorporation of energetics. Analytic wall-PRISM results for a thread like model of the polymer molecules are also presented, and show that the various approximations made in deriving analytical theories critically affect the magnitude and the sign of the predicted athermal segregation. The connections of our analytical work to recent field theoretic analyses is also discussed.
Competition between electron detachment and monomer evaporation in the thermal destruction of hydrated electron clusters
Authors: Arnold Susan T. , Morris Robert A. , Viggiano A. A.
Published: July 2002

We have examined the competition between electron detachment and monomer evaporation in the thermal destruction (dissociation plus detachment) of hydrated electron clusters by monitoring the products in a selected ion flow tube apparatus as (H2O)−n clusters, 14≤n≤24, were heated over the temperature range 100 to 150 K. The destruction of the smaller clusters is dominated by electron detachment, and the detachment occurs over the narrow temperature range 120–145 K. The larger clusters initially undergo sequential evaporation of neutral monomer units, forming smaller and smaller ionic clusters. As the temperature increases, the electron detachment channel begins to compete with monomer evaporation, and the smaller ions eventually decay by electron detachment. Second-order rate constants and activation energies were obtained for the thermal destruction of clusters 14≤n≤17 and 23≤n≤24. The activation energies for the destruction of the larger clusters, n≥17, are nearly constant at ∼0.34 eV, which is close to the energy required to evaporate a single water molecule from the clusters, ∼0.40 eV. The difference indicates we are in the low-pressure limit of thermal dissociation. The activation energy for the smaller cluster sizes, n<16, is significantly smaller than the monomer evaporation energy, and since the primary thermal destruction channel for these clusters’ is electron detachment, the activation energies determined here are a measure of the clusters adiabatic electron affinity. The estimated electron affinities for n=14 and n=15 are 0.12 and 0.23 eV, respectively. The electron affinities are in accord with that predicted by the dielectric continuum model. A model reported by Klots considering the temperature- and size-dependent kinetics for the evaporation of particles from van der Waals clusters is in accord with the experimentally observed competition between these two cluster thermal decay processes.
A new Jastrow factor for atoms and molecules, using two-electron systems as a guiding principle
Authors: Flad Heinz-Jürgen , Savin Andreas
Published: July 2002

To get an idea of the qualitative behavior of an optimal Jastrow factor we have considered a Hylleraas-type wave function of very high accuracy for the helium atom. Owing to the lack of nodes this wave function can be easily interpreted as a general type of Jastrow factor. As a result we obtained a simple parameter dependent ansatz for a Jastrow factor, which incorporates the essential features of the observed behavior. We have optimized the parameters with respect to the variance of the local energy, using variational Monte Carlo techniques, for the atoms He through Ne and for the simple molecules H2, LiH, and Li2. Finally, we compare our approach with other types of Jastrow factors discussed in the literature.
The GeOH+–HGeO+ system: A detailed quantum mechanical study
Authors: Yamaguchi Yukio , Richards Claude A. , Schaefer Henry F.
Published: July 2002

The ground state potential energy hypersurface of the GeOH+–HGeO+ system has been investigated employing ab initio electronic structure theory. First, geometries of two equilibrium and isomerization (1,2 hydrogen shift) reaction transition state were determined at the self-consistent-field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] levels of theory using four basis sets. A qualitatively incorrect geometry is predicted for GeOH+ unless f functions are included in the basis set. Second, physical properties including dipole moments, harmonic vibrational frequencies, and infrared (IR) intensities of three stationary points were evaluated at the optimized geometries. The effects of electron correlation reduce the dipole moment of HGeO+ by 1.25 Debye. At the highest level of theory employed in this study, CCSD(T) using the triple zeta plus double polarization with diffuse and higher angular momentum functions [TZ2P(f,d)+diff] basis set, linear GeOH+ is predicted to be more stable than linear HGeO+ by 71.7 kcal/mol. After correction for zero-point vibrational energies (ZPVEs), this energy difference becomes 70.3 kcal/mol. With the same method the classical barrier height for the exothermic isomerization (1,2 hydrogen shift) reaction HGeO+→GeOH+ is determined to be 30.3 kcal/mol and the activation energy (with the ZPVE correction) is 28.0 kcal/mol. The predicted dipole moments of GeOH+ and HGeO+ are 0.61 and 4.64 Debye, respectively. Thus, the HGeO+ ion may be suitable for a microwave spectroscopic investigation. On the other hand, the GeOH+ ion may be suitable for an IR spectroscopic study due to the strong IR intensities of the three vibrational modes. The geometrical and energetic features are compared with those of the valence isoelectronic HCO+–COH+ and SiOH+–HSiO+ systems.
Molecular and phenomenological rate coefficients of fast reactions in solutions
Authors: Naumann Wolfgang , Molski Andrzej
Published: July 2002

The paper is concerned with the problem of formulating chemical rate equations for reversible reactions in solution in terms of concentration-independent, phenomenological rate coefficients. These time-dependent rate coefficients approach, after an initial transient, the rate constants that can be obtained in a relaxation experiment. We start with the coupled evolution equations for the macroscopic concentrations, and for the two-particle distribution functions describing association–dissociation (A+B=C), bimolecular isomerization (A+B=B+C), and double decomposition (A+B=C+D). The effects of interparticle forces and long-ranged reactivity are included. We derive general identities linking the reactants and products radial distribution functions. For association–dissociation this leads to relations among the molecular rate coefficients which are valid for both contact and long ranged reactivities. For the other two reaction types, we were able to derive analogous relations only for contact reactivities. We demonstrate how the phenomenological rate coefficients can be defined via the solutions of the corresponding diffusional boundary-value problems. This approach is quite general, and valid for both contact and long-ranged reactivities and if interaction forces are included.
Core extraction for measuring state-to-state differential cross sections of bimolecular reactions
Authors: Simpson William R. , Orr-Ewing Andrew J. , Rakitzis T. Peter , Kandel S. Alex , Zare Richard N.
Published: July 2002

We describe a method we call core extraction for measuring the speed distributions of products from photoinitiated bimolecular reactions for the purpose of determining state-to-state differential cross sections. Core extraction is demonstrated by determination of the state-to-state differential cross section for the reaction Cl+CH4(υ3=1)→HCl(υ=1, J=1)+CH3. The method of core extraction measures three-dimensional projections of the velocity distribution using a time-of-flight mass spectrometer equipped with a mask to reject off-axis scattered products. This three-dimensional projection is then converted to a state-to-state differential cross section via simple transformations. Competition between instrumental resolution and signal in core extraction is discussed, and the behavior of our system is checked with simple velocity distributions that result from photodissociation of Cl2. Core extraction is compared with other methods for the measurement of state-resolved differential cross sections.
Statistical thermodynamics of double-stranded polymer molecules
Authors: Chen Shi-Jie , Dill Ken A.
Published: July 2002

We develop a matrix method to compute the configurational partition functions of chain molecules that have intramolecular double strands, including RNA and DNA and antiparallel β-sheet regions in proteins. An accurate theory has not been available for these types of molecules due to the nature of their self-contacts: (i) biomolecules have many self-contacts, and (ii) contacts are sequence-specific. The present theory, which we call the firehose model, can treat such conformations, including specific loops, bulges, and stems. The method is based on the use of polymer graphs. Tests against exact enumeration results on the two-dimensional (2D) square lattice show that conformational entropies are predicted quite accurately. As an example, we compute the denaturation profile of a 2D chain designed to resemble an RNA structure.
Direct measurement of the transition dipole moment of the v3 asymmetric C–H stretching vibration of the CH3 radical
Authors: Bethardy G. A. , Macdonald R. Glen
Published: July 2002

A direct measurement of the transition dipole moment, μ3, of the degenerate v3 in-plane asymmetric C–H stretching vibration of the methyl radical has been made. The measurements were carried out in a flow reactor using laser-photolysis transient infrared absorption spectroscopy. Cyano (CN) radicals (and Cl atoms) were produced by laser photolysis of BrCN (or ClCN) at 193 nm and reacted with methane to give both CH3 and HCN (and HCl). The intensities of 18 rotational lines of the v3 fundamental band were measured relative to the R(8) line of the C–H stretching vibration (v3) of HCN(001←0). The best estimate of the transition dipole moment of the CH3 (00110←0) transition was provided by the measured line intensity for the CH3 (00110←0)rR(3,3) transition and was determined to be μ3=0.0327±0.0021 D.
Ion-pair formation in the photodissociation of HF and DF
Authors: Yencha Andrew J. , Hopkirk Andrew , Grover J. Robb , Cheng Bing-Ming , Lefebvre-Brion Hélène , Keller Françoise
Published: July 2002

The excitation functions for ion-pair formation in supersonically cooled HF and DF have been measured using synchrotron radiation with photon resolutions of 0.12 and 0.06 nm. The excitation functions for both molecules are characterized by an intense, sharp peak, essentially coincident with the thermodynamic onset for ion-pair formation, followed by weaker, broader structure to higher energy. All of the structure is interpreted as arising from the photoexcitation of Rydberg states of the neutral molecules that are predissociated by the V 1Σ+ ion-pair state potential. Ab initio calculations using multichannel quantum defect theory to include both predissociation and autoionization enabled assignments of the observed structure to be made and the construction of simulated ion-pair excitation function spectra in an energy region encompassing ∼0.25 eV of the lowest energy part of the experimental excitation functions. The intense first peak in the ion-pair excitation function of both molecules is assigned to a high vibrational level of the 3sσ Rydberg state converging to the A 2Σ+ ion state, while most of the structure immediately following the first peak is assigned to d Rydberg complexes converging to the v+=1 level of both X 2Π ion substates.
van der Waals vibrations and isomers of 2,3-dimethylnaphthalene⋅Ne: Experiment and quantum three-dimensional calculations
Authors: Droz Thierry , Leutwyler Samuel , Mandziuk Margaret , Bačić Zlatko
Published: July 2002

We report a combined experimental and theoretical study of the van der Waals isomers and intermolecular vibrations of the 2,3-dimethylnaphthalene⋅Ne complex in the S1 electronic state. The two-color resonant two-photon ionization spectrum exhibits eight bands within ≊40 cm−1 of the electronic origin. Theoretical considerations in combination with hole-burning spectroscopic measurements show that the transition closest to the electronic origin (at 000+5 cm−1) arises from an isomer which is different from that responsible for the other seven bands in the spectrum. The latter involve excitations of the intermolecular vibrations of the main isomer of 2,3-dimethylnaphthalene⋅Ne. Accurate three-dimensional quantum calculations of the van der Waals vibrational levels of the complex were performed using a discrete variable representation method. Combination of theory and experiment led to a complete assignment as well as to a quantitative theoretical reproduction of the experimental intermolecular vibrational level structure, and a parametrization of the intermolecular potential energy surface, modeled as sum of atom–atom Lennard-Jones pair potentials. This potential surface exhibits a global minimum above (and below) the aromatic ring plane of 2,3-dimethylnaphthalene and a shallower local minimum at C2v geometry, on the C2 axis of the molecule, adjacent to the two methyl groups. The main and minor isomers identified experimentally are associated with the global and the local minimum, respectively. The quantum calculations were extended to ≊1000 van der Waals vibrational states, i.e., to energies up to 78% of D0. These include levels localized either in the global or local minima, as well as highly excited vibrational states delocalized over all three potential minima, providing comprehensive insight into the quantum dynamics of the high-lying van der Waals states of an atom–large aromatic molecule complex.
Efficient absorption line shape calculations for an electron coupled to many quantum degrees of freedom: Applications to an electron solvated in dry sodalite and halo-sodalites
Authors: Blake Nick P. , Metiu Horia
Published: July 2002

We present quantum mechanical calculations of the absorption line shape of an electron ‘‘solvated’’ in several sodalites. Photon absorption by the electron modifies the forces acting on the nuclei, setting the counterions in motion. This nuclear motion causes broadening and gives vibrational structure to the absorption spectrum of the electron. The major effort in the computation of the absorption spectrum is directed toward the evaluation of an overlap integral that evolves in time because of nuclear motion. The systems considered here have a very large number of nuclear degrees of freedom, and this makes a brute-force quantum mechanical calculation of the overlap impossible. Good results can be obtained with a method that exploits the fact that in a system with many degrees of freedom the overlap integral decays rapidly to zero, and can therefore be evaluated accurately and efficiently by short-time methods. The short-time method that seems most advantageous is the Gaussian wave packet (GWP) procedure proposed some time ago by Heller. This simplifies the nuclear dynamics and also substantially diminishes the number of electron energy calculations needed for determining the forces acting on the nuclei. When the GWP method is used, the electronic wave function is calculated only for a small number of nuclear configurations along the classical trajectory on which the center of the nuclear wave packet evolves. The present calculation is the first use of this method to compute the absorption spectrum of a complex system. We study the absorption line shape for an electron solvated in a dry sodalite, and in chloro-, bromo-, and iodo-sodalite. We find that the homogeneous linewidth due to the nuclear motion is narrower than that observed experimentally. This implies that the measured linewidth is due to inhomogeneous broadening. For the dry sodalite the main inhomogeneity is the disorder in the position of the counterions, and for halo-sodalites, the presence of defects introduced during synthesis. Our results imply that a careful synthesis can improve the contrast in displays based on the cathodochromic effects in zeolites.
Integral equation theory of molecular liquids: Kirkwood hierarchy approach to diatomic and polyatomic liquids
Authors: Farhat Hikmat , Chan Eu Byung
Published: July 2002

A set of integral equations is derived for site-site pair correlation functions for diatomic fluids from the Kirkwood hierarchy of correlation functions. The integral equations obtained are self-consistent field equations which are generalizations of the Percus-Yevick integral equation for simple liquids. In this theory the intra- and intermolecular correlation functions are self-consistently coupled. The integral equations reduce to the integral equations in the reference site interaction model (RISM) theory of diatomic liquids if an approximation is made to the direct correlation functions and the molecule is assumed to be rigid. The theory is also generalized to polyatomic fluids. The predictions of the theory are compared with those by the RISM theory and Monte Carlo simulations available in the literature. The computational requirement for the integral equations of the present theory is comparable to that of the RISM theory, but its predictions are more accurate than those by RISM theory at low densities and are practically the same as the latter at high densities.
Monte Carlo simulations of a liquid of mesogenic molecules
Authors: Vacatello Michele , Iovino Mariacristina
Published: July 2002

Monte Carlo computer simulations of a liquid of model anisotropic molecules at various temperatures have been performed for the first time with a system large enough to be representative of a liquid crystalline domain. Although the molecules are not confined to a lattice, the behavior of the simulated system is found to be in surprisingly good agreement with the predictions of the Flory–Ronca–Irvine lattice theory. Our results indicate that reliable computer experiments on the relationships between molecular constitution and properties of liquid crystalline materials are nowadays feasible, provided that the absence of spurious effects related to the system size is carefully checked in each case.
Spin–orbit configuration interaction study of potential energy curves and transition probabilities of the mercury hydride molecule and tests of relativistic effective core potentials for Hg, Hg+, and Hg2+
Authors: Alekseyev Aleksey B. , Liebermann Heinz-Peter , Buenker Robert J. , Hirsch Gerhard
Published: July 2002

Ab initio CI calculations have been carried out for the low-energy states of the mercury hydride molecule HgH and its isotopomers. A relativistic effective core potential (RECP) given by Ross et al. [J. Chem. Phys. 93, 6654 (1990)] is employed to describe all but the Hg 5d and 6s valence electrons. Tests for a series of low-lying states of Hg, Hg+, and Hg2+ demonstrate that 0.1 eV accuracy is obtained at the SCF level with a high-quality basis set for this RECP in comparison with all-electron Dirac–Fock results up to 32 eV excitation energy. The DF values are themselves in error by 1–3 eV on the average compared to experiment, but the present CI calculations based on this RECP lead to considerably higher accuracy because of the importance of correlation effects in such determinations. Energy differences (12 cases) between states with the same number of electrons are computed to an accuracy of 0.1–0.2 eV in all cases after the spin–orbit interaction is included. These results compare favorably with those obtained by Häussermann et al. [Mol. Phys. 78, 1211 (1993)] with a ... 5s2 5p6 5d10 6s2 RECP and a corresponding larger AO basis to describe the more tightly bound electrons. Good agreement is found for the spectroscopic constants of the HgH molecule in its lowest four electronic states: X 2Σ+1/2, A1 2Π1/2, A2 2Π3/2, and B 2Σ+1/2 (maximal errors of 1000 cm−1 for Te, 0.03 Å for re and 150 cm−1 for ωe).

An RKR curve reported for the A1 state is shown to be in error beyond r=4.0 a0 because of its failure to describe a key avoided crossing with the B state. Radiative lifetimes computed for the A 2Π multiplets are both found to agree with values deduced from experiment to within 40%. The calculations find no difference in the HgH and HgD radiative lifetimes for either the A1 or the A2 states, whereas a large distinction in the measured A1 lifetimes of the two isotopomers is observed, thereby supporting the previous experimental conclusion that strong predissociation occurs in the HgH A1 state. Numerous higher-lying electronic states are also studied, with Te values up to 60 000 cm−1, and on this basis it is argued that earlier assignments for the HgH C–X and D–X transitions are incorrect, as previously concluded by Nedelec et al. [Chem. Phys. 134, 137 (1989)].
Stark effect and dipole moments of the ammonia dimer in different vibration–rotation–tunneling states
Authors: Cotti Gina , Linnartz Harold , Meerts W. Leo , van der Avoird Ad , Olthof Edgar H. T.
Published: July 2002

In this paper we present Stark measurements on the G:K=−1 vibration–rotation–tunneling (VRT) transition, band origin 747.2 GHz, of the ammonia dimer. The observed splitting pattern and selection rules can be explained by considering the G36 and G144 symmetries of the inversion states involved, and almost complete mixing of these states by the applied electric field. The absolute values of the electric dipole moments of the ground and excited state are determined to be 0.763(15) and 0.365(10) D, respectively. From the theoretical analysis and the observed selection rules it is possible to establish that the dipole moments of the two interchange states must have opposite sign. The theoretical calculations are in good agreement with the experimental results: The calculated dipole moments are −0.74 D for the lower and +0.35 D for the higher state. Our results, in combination with the earlier dipole measurements on the G:K=0 ground state and the G:K=1 transition with band origin 486.8 GHz, confirm that the ammonia dimer is highly nonrigid. Its relatively small and strongly K-dependent dipole moment, which changes sign upon far-infrared excitation, originates from the difference in dynamical behavior of ortho and para NH3.
Angular distribution of molecular Auger electrons
Authors: Bonhoff K. , Nahrup S. , Lohmann B. , Blum K.
Published: July 2002

A general theory is developed for the angular distribution of Auger electrons emitted in the decay of molecular vacancies created by electron impact. The molecules are assumed as freely rotating. General expressions are derived where the angular distribution of the emitted Auger electrons is related to the anisotropy of the molecular axis distribution and to the shape and spatial orientation of the electronic orbitals prior to the Auger emission. Particular emphasis is placed on the correct formulation of the coherences produced during the ionization which requires an extension of previously derived formulas. The obtained equations are a necessary first step for future numerical calculations.
Normal mode theory of two step relaxation in liquids: Polarizability dynamics in CS2
Authors: Keyes T.
Published: July 2002

An instantaneous normal mode (INM) theory is given for relaxation in liquids by a fast β process followed by a slow α process. The β process is harmonic dynamics in the wells of the N-body potential, while the α process is structural relaxation coincident with barrier crossing to a neighbor well. The theory introduces a new parameter, the ‘‘harmonic fraction’’ denoted FH, which is the fraction of the mean-square fluctuations of a dynamical variable capable of being relaxed by the harmonic β process. Theory and computer simulation are compared for the polarizability correlation function, PC(t), and the polarizability time derivative correlation function, DPC(t), in a model of CS2 including internal degrees of freedom. Agreement is good, with the INM theory clearly showing the ‘‘signature’’ time dependence of a correlation function undergoing αβ relaxation in a low temperature liquid; there are no adjustable parameters in the theory. The polarizability is calculated in the ‘‘point atomic polarizability approximation’’ (PAPA) which is sensitive to molecular vibrations, so a preliminary classical INM treatment of Raman scattering is obtained. The PAPA overestimates the derivative of the polarizability with respect to the internal coordinates, and in reality the vibrations behave quantum mechanically, so the Raman intensities are inaccurate, but otherwise a plausible description is obtained for several features of the spectrum. It is explained how an improved PAPA will be combined with a quantum INM theory in future Raman calculations.
Theoretical investigations of molecular triple ionization spectra
Authors: Handke G. , Tarantelli F. , Sgamellotti A. , Cederbaum L. S.
Published: July 2002

Triple ionization of molecular systems is investigated theoretically by means of the three-particle propagator. This enables us to efficiently calculate the very dense triple ionization spectra. To be able to interpret these spectra an atomic three-hole population analysis is developed which provides information about the charge distribution in the molecular trication. In exemplary applications on CO and a series of fluorides the use of the approach is demonstrated. A large number of triply ionized electronic states are energetically accessible in the available particle impact ionization experiments and it is shown that many of these states contribute to the observations. Triply ionized states are also produced by Auger decay. In particular, shake-off satellite lines of molecular Auger spectra can be reproduced using the triple ionization energies from the propagator calculation and an estimate of the transition rates based on the three-hole population analysis. In general a dramatically growing complexity of the triple ionization spectra with increasing molecular size is demonstrated. In spite of this complexity the three-hole population analysis is of valuable help for the interpretation of the spectra and often a simple picture in terms of various hole-localization patterns arises.
Direct absorption observation of the van der Waals bending band of ArHCN by millimeterwave spectroscopy combined with pulsed-jet expansion technique
Authors: Uemura Keisuke , Hara Atsushi , Tanaka Keiichi
Published: July 2002

Millimeterwave absorption spectroscopy combined with the pulsed-jet expansion technique was applied to the measurement of rovibrational transitions in the van der Waals band of ArHCN near 200 GHz. Observations were extended to the higher millimeterwave frequency region up to 260 GHz, and 17 rovibrational transitions split into hyperfine components due to the nitrogen nucleus were newly observed for both the Σ1–Σ0 and Π1–Σ0 bands. An improved set of molecular constants, including the band origins, rotational constants, quadrupole coupling constants, and the Coriolis coupling constant between the Σ1 and Π1 bending substrates, was determined.
The GeOH–HGeO system: Are the 3d electrons core or valence?
Authors: Yamaguchi Yukio , Schaefer Henry F.
Published: July 2002

The 2A′ ground state of GeOH–HGeO system has been investigated by ab initio electronic structure theory. The equilibrium geometries and physical properties including dipole moments, harmonic vibrational frequencies, and associated infrared (ir) intensities for GeOH, HGeO, and the isomerization (1,2 hydrogen shift) transition state are determined at the self-consistent-field (SCF) and configuration interaction with single and double excitations (CISD) levels of theory with four basis sets. There appear to be two minima for the bent HGeO (isomers A and B) on its SCF and CISD potential energy hypersurfaces. At the Hartree–Fock level the structure with HGeO angle near 90° (isomer B) lies lower, but correlated methods show that the structure with HGeO angle near 120° (isomer A) actually lies lower. At the optimized CISD geometries, the single point energies of coupled cluster with single and double excitations (CCSD) and CCSD with perturbative triple excitations [CCSD(T)] methods are also determined. In the correlated procedures three different types of frozen core orbital approximation (15 frozen core, 10 frozen core, and 6 frozen core orbitals) have been examined. The energetics based on the first (15 frozen core orbitals) approximation present errors of about 1 kcal/mol compared to more accurate second (10 frozen core orbitals) and third (6 frozen core orbitals) approximations. At the highest level of theory employed in this research, CCSD(T) with triple zeta plus double polarization with diffuse and higher angular momentum functions [TZ2P(f,d)+diff] basis set, the bent GeOH molecule is predicted to be lower in energy than the bent HGeO molecule by 28.5 kcal/mol. This energy separation becomes 25.7 kcal/mol with the zero-point vibrational energy (ZPVE) correction. The classical energy barrier for the exothermic isomerization reaction [HGeO(B)→GeOH] is determined to be 11.8 kcal/mol and the activation energy (with the ZPVE correction) 10.7 kcal/mol. The theoretically predicted isotope shifts for the GeO stretching vibrational frequency of GeOH agree very well with experimental assignments by Withnall and Andrews [J. Phys. Chem. 94, 2351 (1990)].
On the behavior of Padé approximants in the vicinity of avoided crossings
Authors: Dunn Martin , Watson Deborah K. , Walkup John R. , Germann Timothy C.
Published: July 2002

When linear Padé summation is applied to eigenvalue perturbation expansions near regions of parameter space where those eigenvalues undergo an avoided crossing, the Padé approximants may yield levels which cross diabatically, rather than displaying the proper avoided behavior. The purpose of this study is to elucidate the reasons for the peculiar behavior of Padé approximants in such situations. In particular, we demonstrate that the diabatic crossing is a natural consequence of using the (single-valued) Padé rational approximant to successfully resum series expansions of the multivalued energy function over much of the parameter space. This is illustrated with a perturbative treatment of the Barbanis Hamiltonian.
The importance of the anisotropic energy term for the structure of the solid phases of nitrogen
Authors: Mulder A. , Michels J. P. J. , Schouten J. A.
Published: July 2002

We have performed Monte Carlo simulations on the solid phases of nitrogen and improved the nitrogen–nitrogen intermolecular site–site potential derived by Etters et al. Previous simulations with the Etters potential reproduced the properties of dense fluid nitrogen and the structures of the α, β, and δ phase, but failed to describe the low temperature, high pressure γ and ε phases. We have examined the influence of the Coulomb term on the stability of the ε phase with various values for the quadrupole and hexadecapole moments. When the quadrupole is taken about 15% larger than the experimental value, the ε phase could be stabilized, although the volume and box angle did not correspond to the experimental values. The ε phase could also be stabilized by introducing an anisotropic term, which influences the shape of the molecule. The two anisotropy parameters have been determined with respect to the ε-phase structure, and resulted also in the stabilization of the γ phase. The anisotropic term changes the shape of the nitrogen molecule by changing the major diameter of the prolate spheroid, which is in agreement with electron density calculations.
A quantitative theory of linear chain polymer dynamics in the melt. I. General scaling behavior
Authors: Herman Michael F. , Panajotova Biliana , Lorenz K. Thomas
Published: July 2002

A theory of melt polymer dynamics for linear chain systems is developed. This theory generalizes recent work, which considers the lateral motion of the chains. A description is provided of the short time dynamics and of the crossover from this early time regime to a highly entangled dynamics. In both of these regimes, an effective friction coefficient for the lateral motion is evaluated by considering the extent of correlation between the displacements of the beads. This correlation is required due to the chain connectivity and the noncrossability of the chain backbones. The crossover time between these two regimes is found to be independent of chain length. In the early time regime, the bead mean squared displacement is found to have a time dependence between g∼t0.4 and g∼t0.5. In the highly entangled regime, g has a t2/7 dependence. The reptative motion of the chains along their own backbones and the coupling between this motion and the lateral chain motion is also included. It is found that the inclusion of these features results in a shorter terminal time in the long chain limit than would be the case otherwise. Long range correlated many chain motions are also considered in this work. These motions are expected to dominate the chain diffusion in the long chain limit. This theory predicts a terminal time that scales as N3.3 and a diffusion constant that scales as N−2.1, where N is the number of monomer units per chain.