bluuebunny/crossref_metadata_2025 huggingface dataset:
Laser threshold photoelectron spectra of the cis and trans rotational isomers of p-dimethoxybenzene–Arn (n=0,1,2): Observation of the intermolecular van der Waals stretching and bending vibrational modes in the cation
Authors: Cockett Martin C. R. , Okuyama Katsuhiko , Kimura Katsumi
Published: July 2002

The cis and trans rotational isomers of p-dimethoxybenzene–Arn (n=0,1,2) have been studied in a supersonic free jet by two-color laser resonance enhanced multiphoton ionization threshold photoelectron spectroscopy. The two-color (1+1′) threshold photoelectron spectra recorded via the S1 state of the cis and trans isomers of the 1:1 and 1:2 argon complexes reveal well resolved vibrational structure characteristic of the low frequency bending and stretching van der Waals vibrational modes. In the case of the trans isomer of the 1:2 complex, a very low frequency progression (11 cm−1) in a nontotally symmetric van der Waals bending mode appears in single quanta in the spectrum. The equivalent spectrum recorded for the cis isomer exhibits structure characteristic of van der Waals stretching modes as well as double quanta excitation in both totally symmetric and nontotally symmetric van der Waals bending modes. The observation of single quantum excitation in formally forbidden van der Waals vibrational modes implies the possibility of a change in the overall symmetry of the complex in the ground cationic state when compared to that in the S1 state. The adiabatic ionization energies (Ia) for the cis and trans isomers of p-dimethoxybenzene–Arn (n=0,1,2) were measured as 60 774±7 (cis; n=0), 60 687±7 (cis; n=1), 60 509±7 (cis; n=2), 60 563±7 (trans; n=0), 60 479±7 (trans; n=1), and 60 295±7 cm−1 (trans; n=2).
Higher analytic derivatives. IV. Anharmonic effects in the benzene spectrum
Authors: Maslen Paul E. , Handy Nicholas C. , Amos Roger D. , Jayatilaka Dylan
Published: July 2002

This is the fourth in a series of papers on the ab initio calculation of the third and fourth derivatives of the energy of a molecule. In this paper we examine anharmonic effects in the infrared and Raman spectra of benzene. The following spectroscopic properties have been calculated; ab initio anharmonic corrections (ω−ν) and estimates of the harmonic frequencies ω for all 30 vibrational modes of C6H6 and C6D6, a complete set of anharmonic constants x and g for C6H6, intensities for the infrared spectrum of C6H6 up to 6148 cm−1, and anharmonic corrections to the Raman scattering factors for the fundamental modes of C6H6. In addition, we have improved on previous calculations of the equilibrium geometry of benzene, using Mo/ller–Plesset perturbation theory and a triple zeta plus double polarization (TZ2P) basis. We have also calculated a zero-point vibrationally averaged geometry which is in good agreement with the experimental R0 value. All these calculations are based on a Hartree–Fock quartic potential, cubic dipole surface, and quadratic polarizability surface, using a double-zeta plus polarization (DZP) basis. This is the first time a complete anharmonic potential has been obtained for a molecule of this size; the computer time required was minimized by the use of analytic derivative programs in favor of finite-difference programs. The quartic potential is presented in three coordinate systems.

We discuss efficient algorithms for the nonlinear transformation of the potential from normal coordinates to valence coordinates and for symmetry checking the potential. The approximations used in our calculations have been examined and we find that the use of a Hartree–Fock DZP potential together with a perturbative treatment of the vibrational Hamiltonian is just as accurate for D6h benzene as for smaller molecules. In order to examine correlation effects in the B2u modes 14 and 15, basis-set limit second-order Mo/ller–Plesset TZ2P+f harmonic frequencies have been calculated for these modes. It is suggested that, while these modes are very sensitive to correlation, anharmonicity has only a small effect, so a Hartree–Fock DZP anharmonic potential is adequate. Furthermore, experimental determination of anharmonic corrections to frequencies is very difficult for a molecule of this size so we hope our calculations will fill this gap.
Nonlinear aspects of band structure in liquids. I. Neat liquids
Authors: Chen Zhe , Stratt Richard M.
Published: July 2002

One can make significant inroads into the problems of identifying the collective linear excitations (band structure) of liquids using what have been called ‘‘single-site’’ theories. In the context of liquid-theory methods for band structure, such theories arise from linear liquid theories such as the mean spherical approximation. However, at low densities, and when certain levels of eigenvector information are required, these theories are manifestly inadequate. We show here how a nonlinear theory for band structure in liquids can be constructed based on the EXP liquid theory. When tested against simulations of electronic s bands, the predictions are found to be quantitatively accurate in both low-density fluids and in impurity-band situations with uncorrelated dopants. Beyond its ramifications for band structure, the calculation presented here is also of some technical interest as an example of a nonlinear treatment of fluctuating internal degrees of freedom in liquids.
State-to-state rotational cross sections in vibrational modes. Application to the infrared Q-branch profile for the (11 10)I←(00 0) 12C16O2 bending band
Authors: Bonamy L. , Bonamy J. , Temkin S. , Robert D. , Hartmann J. M.
Published: July 2002

Direct connection between spectroscopic relaxation cross sections and appropriate state-to-state rotational cross sections in vibrational modes enables one to introduce convenient energy corrections to the sudden approximation. Since the angular momentum coupling photon–rotation–vibration is properly taken into account, this approach applies to stretching as well as bending bands. Diode laser experiments have been made for pure 12C16O2 in the infrared Q branch of the (11 10)I←(00 00) band in order to investigate the effect of collisional line mixing on the spectral profile. The energy corrected sudden scaling law is used within the present theoretical approach for both the inversion of j→0 basis cross sections from broadening data and the calculation of line-coupling coefficients. Resulting absorption profiles are in good agreement with measured values without the need for the introduction of any ad hoc decoupling factor for the considered bending band.
Quantum dynamics of overtone relaxation in benzene. IV. Relaxation from CH(v=4)
Authors: Wyatt Robert E. , Iung Christophe
Published: July 2002

The first large scale quantum mechanical calculations of the benzene CH(v=4) overtone spectrum are reported. These results are compared with recent experimental studies.
Beyond the Bloch equations: A wave function-based approach to selective excitation in condensed media
Authors: Gross Peter , Neuhauser Daniel , Rabitz Herschel
Published: July 2002

An optimization procedure for producing selective excitation of molecules undergoing stochastic dephasing collisions is presented. The collisional effects are incorporated by propagating the Schrödinger equation with random perturbations. The optimization procedure utilizes gradients of the perturbation-averaged physical objective with respect to the laser field parameters; we demonstrate on a model problem that very few averages over collisional events suffice to yield converged results. Therefore, this method is computationally efficient because large averaging of dynamical simulations is not necessary. These results show the viability of the optimal control procedure for application to controlling chemical reactions in solution.
Nuclear magnetic relaxation study of poly(ethylene oxide)–lithium salt based electrolytes
Authors: Donoso J. P. , Bonagamba T. J. , Panepucci H. C. , Oliveira L. N. , Gorecki W. , Berthier C. , Armand M.
Published: July 2002

We have studied the low-temperature NMR line shape for three nuclei (1H, 7Li, and 19F) in poly(ethylene oxide)-lithium salts (LiClO4, LiBF4, and LiAsF6) solid polymer ionic conductors and measured their spin–lattice relaxation rates as functions of frequency and temperature. The three nuclei probe the dynamics of the polymer segments, the cations, and the anions. We find that the Li+ cations follow the segmental motion of the chain, while the anions move independently. Homonuclear interactions and heteronuclear interactions with the polymer and the anion contribute to the 7Li line shape. When the heteronuclear contributions were selectively eliminated by the decoupling method, it was found that the Li–H interaction accounts for 80%–90% of the linewidth. Additional evidence for the correlation between the cationic and the polymeric motions is provided by the remarkably similar temperature dependences of the measured relaxation rates for 7Li and 1H, which differ significantly from the dependence for 19F. The frequency dependence of the relaxation rates is poorly described by the Bloembergen, Purcell, and Pound model; a recently developed graphical procedure nevertheless shows that the motion of the protons and the anions is governed by a single time scale, while the 7Li ions are affected by an additional scale, associated with the coupling of its quadrupolar moment to the electric-field gradient.
Stochastic dynamics simulations of polymethylene melts confined between solid surfaces
Authors: Winkler R. G. , Matsuda T. , Yoon D. Y.
Published: July 2002

Stochastic dynamics simulations of polymethylene chains (CnH2n+2 for n=13, 28, 60) represented by united atom models have been carried out in order to investigate both the equilibrium and dynamic properties of polymer melts confined between flat solid surfaces. The packing manner of monomer segments, segment orientation and local conformations of chains are found to be independent of chain length, and agree very well with the results of the previous Monte Carlo simulations of C13H28 melts. The chains are extremely flattened close to the surface and many molecules assume essentially two-dimensional train configurations even in the case of C60H122 melts. However, the local chain conformations are only slightly affected by the surfaces. The apparent self diffusivities are found to be independent of the distance from the surface as the result of a reduced mobility normal to the surface and an enhanced mobility parallel to the surface. The absolute magnitudes of self-diffusion constants are larger than experimental values, most likely due to the application of united atom model.
Pairwise and many-body contributions to interaction potentials in Hen clusters
Authors: Parish Carol A. , Dykstra Clifford E.
Published: July 2002

High level ab initio calculations have been carried out to assess the pairwise additivity of potentials in the attractive or well regions of the potential surfaces of clusters of helium atoms. A large basis set was employed and calculations were done at the Brueckner orbital coupled cluster level. Differences between calculated potentials for several interacting atoms and the corresponding summed pair potentials reveal the three-body and certain higher order contributions to the interaction strengths. Attraction between rare gas atoms develops from dispersion, and so helium clusters provide the most workable systems for analyzing nonadditivity of dispersion. The results indicate that the many-body or nonpairwise contributions tend to be less than a few percent of the attractive interaction across regions around the minima of the potential energy surfaces of small clusters. Dipole–dipole–dipole dispersion and dipole–dipole–quadrupole dispersion are noticeable parts of the small three-body terms.
Cumulative reaction probability by the complex coordinate scattering theory
Authors: Rom Naomi , Ryaboy Victor , Moiseyev Nimrod
Published: July 2002

A new computational approach for the calculation of the cumulative reaction probability N(E) is introduced. As distinct from the optical potential method, recently applied by Seideman and Miller [J. Chem. Phys. 96, 4412 (1992)], we use the complex coordinate scattering theory to overcome the numerical difficulties in calculating the Green’s operator. Illustrative numerical examples for the 1D H+H2 collinear collision (Eckhart barrier) using complex scaling, in which both the physical potential and the flux matrix elements are perturbed, and smooth exterior complex scaling, in which both are left unchanged as in the Seideman–Miller procedure, are presented.
Ultrafast transient-absorption spectroscopy of the aqueous solvated electron
Authors: Alfano Joseph C. , Walhout P. K. , Kimura Y. , Barbara Paul F.
Published: July 2002

We have performed the first direct pump-probe transient-absorption measurements on the near-infrared (IR) band of the equilibrated aqueous solvated electron. The pump pulse was centered at 780 nm. The absorption spectrum of the excited state is observed to be red-shifted relative to the ground-state absorption. The radiationless transition from the excited state to the ground state occurs with an average time constant of 550±170 fs. In observing a subpicosecond lifetime and red-shifted absorption for the excited p-states, these experiments are in accord with a growing body of experimental and theoretical work, serving to provide a consistent picture of the photophysics of the solvated electron.
Ultrafast stimulated emission spectroscopy of perylene in dilute solution: Measurement of ground state vibrational population relaxation
Authors: Hambir S. A. , Jiang Y. , Blanchard G. J.
Published: July 2002

We report ultrafast stimulated emission measurements of perylene in a series of polar and nonpolar solvents. In all solvents the perylene stimulated emission spectra evolve in time. We observe individual features corresponding to distinct vibronic resonances in the stimulated emission spectra. The intensities of these features increase subsequent to excitation and persist for hundreds of picoseconds. The fast build-up seen at short delay times is related directly to the vibrational population relaxation time, T1 , of the ground vibrational state that is the lower energy state of the stimulated transition. The measured T1 times for perylene vary with both ground vibrational state and solvent. The slow decay rates for these data, the sum of the stimulated and spontaneous decay rates for the particular transition, depend critically on the particular transition that is resonant with the probe laser electric field.
Photochemical hydrogenation of methyl radicals on Pt(111)
Authors: Ukraintsev V. A. , Harrison I.
Published: July 2002

Submonolayers of methyl bromide and hydrogen were coadsorbed on a 25 K Pt(111) surface and laser irradiated at 308 nm. Photoinduced dissociative electron attachment of methyl bromide produced energetic methyl radicals that reacted with hydrogen to form methane. The photoreaction dynamics is discussed.
Competition between adsorbate fragmentation and desorption on laser-heated surfaces
Authors: Weaver R. V. , Zeiri Y. , Uzer T.
Published: July 2002

We present a theoretical study of the consequences of rapid laser heating of a model adsorbate–substrate system. The adsorbate in this study contains one weak bond and a number of strong bonds. The impetus for our theoretical model comes from the desorption/dissociation experiments on labile organic molecules adsorbed on heated surfaces. Among the numerous interesting results of these experiments are nonstatistical processes where depending on the heating rate, stronger bonds break before the weaker ones. We model these processes by means of a stochastic trajectory approach using the generalized Langevin method. Results are given for model polyatomics of different lengths and configurations as well as different heating rates. The strengths of the weak bonds inside the molecule are changed and reactivities of adsorbates in parallel and perpendicular adsorption geometries are contrasted. The branching ratio between desorption and dissociation is found to depend sensitively on a number of these variables. We also find that small changes in the relative bond strengths of the weak bonds can change the reactivity from statistical to nonstatistical.
Mode-specific chemical branching ratios in the photodissociation of OClO
Authors: Bishenden Elizabeth , Donaldson D. J.
Published: July 2002

The chemical identity (i.e., Cl+O2 vs OCl+O) of the products of the photolysis of OClO depends on the vibrational mode excited in the parent molecule. The ratio of OCl:Cl production is enhanced when combination bands associated with antisymmetric stretching are excited in the OClO Ã electronic state.
Reorientation cross sections in collisions of He(1S)+H2(B 1Σu+)
Authors: Odutola J. A. , Lester W. A. , Grimes R. M.
Published: July 2002

Reorientation cross sections have been computed for the He(1S)+H2(B 1Σu+) system using a potential energy surface and model previously introduced for rovibrational energy transfer. The model consists of restricting the scattering solely to the excited state potential energy surface and the use of the coupled-channel method. The calculated reorientation cross section is 41.6 Å2 for the transition (vjmj≡0,1,0 → v′j′m′j≡0,1, ± 1) at room temperature; the corresponding Boltzmann average is 39.2 Å2. The latter result is in very good agreement with a recent measurement by Pibel and Moore of 29.7±7.4 Å2 [J. Chem. Phys. 93, 4804 (1990)].
Geometrical shell structure of clusters
Authors: Näher U. , Zimmermann U. , Martin T. P.
Published: July 2002

Periodic oscillations in the mass spectra of large clusters often indicate the existence of geometric shell structure. There is, in fact a unique relation between the period of the oscillations and the geometry of the cluster. Applying this rule to the data available for aluminum clusters in the size range of 200–15 000 atoms shows that these clusters probably have an octahedral shape.
Dynamics of the N(4S
u)+NO(X 2Π)→N2(X 1Σ+g)+O(3P
g) atmospheric reaction on the 3A″ ground potential energy surface. II. The effect of reagent translational, vibrational, and rotational energies
Authors: Gilibert Miquel , Aguilar Antonio , González Miguel , Sayós R.
Published: July 2002

The effect of translational vibrational, and rotational energies on the dynamics of the N(4Su)+NO(X 2Π)→N2(X 1Σg+)+O(3Pg) reaction has been examined using a Sorbie–Murrell analytical fitting of a grid of ab initio configuration interaction (CI) points for the 3A″ ground potential energy surface reported by the authors in a previous work. Translational energy is shown to increase total reaction cross section for all the initial rovibrational states of reactants considered. The reaction mode analysis points towards a direct mechanism and a strong influence of the shape of the potential energy surface on the reactivity, especially at low relative collision energies. Vibrational excitation of the NO reactant molecule changes the total reaction cross section values moderately, while increasing the initial rotational states of NO at low fixed relative collision energies decreases the reaction cross section sharply, eventually becoming zero for the highest J values explored. By comparing with model calculations on the same surface involving extreme H+HL and L+LH mass combinations, the microscopic reaction mechanism is shown to imply product molecules being created with rotational angular momentum (J′) oriented preferentially antiparallel with respect to their orbital angular momentum (l′) at low relative energies, with loss of orientation for higher relative energies. Thus, the surface used indicates a strong vector correlation between l′ and J′ and also an important influence in equipartitioning total angular momentum between the rotational and orbital angular momenta of products. Comparison with unfortunately scarce experimental data (e.g., fraction of vibrational energy in products and rate constants) shows a very good agreement.
Chebyshev scheme for the propagation of quantum wave functions in phase space
Authors: Torres-Vega Go.
Published: July 2002

The propagation scheme based on the Chebyshev polynomial expansion of evolution operators is adapted for use in the propagation of quantum mechanical wave functions in phase space. This shows, once more, that one can do true quantum mechanics in phase space, in a similar way as it is done in abstract and coordinate or momentum spaces.
Statistical thermodynamics of curvature elasticity in surfactant monolayer films: A molecular approach
Authors: Cantor Robert S.
Published: July 2002

A statistical thermodynamic theory is developed to study curvature elastic properties of self-assembled monolayer films formed by nonionic surfactants at the interface between water and oil domains in microemulsions. A mean-field approximation to the conformational entropy is obtained by sequential placement of chain segments on a lattice, such that bond-correlated excluded volume effects are properly incorporated. The resulting free energy is minimized at zero lateral (osmotic) pressure with respect to a set of layer- and orientation-dependent statistical weights, from which the splay and Gaussian bending elastic moduli, spontaneous curvature, molecular area, and chain segment distributions are predicted. Calculations are reported for monolayers comprised of a single surfactant and for mixtures of surfactants of different lengths and stiffness. We examine the origins of the significant differences between our predictions and those of Szleifer et al. In particular, for self-assembled monolayers, we find that the bending moduli of mixtures of chains of different lengths are not lower than those of a single-component monolayer with a surfactant of corresponding number-average length. The approach to long-chain behavior of the elastic moduli is predicted to occur for chains much longer than those normally used experimentally as nonionic surfactants. However, extrapolation of our predictions to this limit yields power-law exponents very similar to those predicted by Wang and Safran.
Effective diffusivities and conductivities of random dispersions of nonoverlapping and partially overlapping unidirectional fibers
Authors: Tomadakis Manolis M. , Sotirchos Stratis V.
Published: July 2002

Computer simulation results are presented for the effective bulk, transition, and Knudsen diffusion coefficients in structures consisting of parallel nonoverlapping (impenetrable) or partially overlapping (penetrable) fibers. The numerical procedure is based on a discrete step-by-step random walk mechanism used to determine the mean square displacement of molecules traveling in the interior of the porous medium. The computed transition regime diffusivities are compared with the predictions of the reciprocal additivity effective diffusivity expression. The bulk diffusivity results are also expressed as formation factors describing other transport properties (e.g., thermal and electrical conductivity, magnetic permeability, and dielectric constant) of the fibrous structures or in general of the corresponding arrays of cylinders (either nonconducting or infinitely conducting with respect to the matrix phase). The formation factors of nonoverlapping cylinders are compared with variational bounds and analytical and numerical results of the literature.
The Ar–C2H2 intermolecular potential from high resolution spectroscopy and ab initio theory: A case for multicenter interactions
Authors: Bemish R. J. , Block P. A. , Pedersen L. G. , Yang Weitao , Miller R. E.
Published: July 2002

Infrared spectra have been obtained for the Ar–C2H2 complex, which include a combination band associated with the low frequency bending mode. These data are used, together with ab initio calculations and the results of previous studies of this system, to construct a two-dimensional Hartree–Fock plus damped dispersion (HFD) intermolecular potential surface corresponding to the C–H stretch excited vibrational state. A high quality SCF surface, which includes ghost orbital corrections, has been used to fix the repulsive part of the potential. The remaining potential parameters were initially estimated with the aid of various combining rules and the collocation technique was used to solve the bound state problem for this potential and to calculate the spectrum of the Ar–C2H2 complex. To obtain good agreement between the calculated and experimental spectra it was necessary to distribute the dispersion interaction over the length of the acetylene subunit. The result is a double minimum potential upon which the complex executes wide amplitude bending motion.
Structure of zinc phthalocyanine by gas phase electron diffraction
Authors: Mihill A. , Buell W. , Fink M.
Published: July 2002

Gas phase electron diffraction data have been successfully recorded for zinc phthalocyanine. Molecular structure parameters have been determined by least square fitting a geometrically consistent model of C4v symmetry to the data. The results agree well with previous x-ray diffraction studies of crystals. However, the zinc atom was found to be slightly out the plane of the molecule in contrast with the x-ray results.
Theory of polarizable liquid crystals: Optical birefringence
Authors: Cao Jianshu , Berne B. J.
Published: July 2002

The theory of polarizable anisotropic fluids is developed on the basis of a model fluid consisting of anisotropic Drude oscillators with embedded permanent dipoles. A matrix theory is developed. The dielectric tensor is derived and a matrix Monte Carlo method is used to calculate the optical absorption and birefringence of ordered fluids.
The application of Toeplitz matrices to scattering problems
Authors: Gilibert M. , Baram A. , Last I. , Szichman H. , Baer M.
Published: July 2002

This work describes a new approach to treating quantum mechanical scattering problems. It is based on expanding the wave function in terms of an infinite set of localized Gaussian functions and employing the features of a Toeplitz matrix.
Photoionization electronic spectroscopy of AlAg
Authors: Robbins D. L. , Yeh C. S. , Pilgrim J. S. , Lang G. L. , Duncan M. A.
Published: July 2002

AlAg is produced in a supersonic molecular beam by laser vaporization of alloy samples in a pulsed nozzle cluster source. Electronic spectroscopy is studied with resonant two-photon photoionization. In addition to the two electronic excited states previously reported by Clements and Barrow, we have observed ten new states. Vibrational analyses are presented for each of these states, and rotational analyses are given for selected states. The number and characteristics of these excited states are compared to the predictions of recent ab initio calculations.
Integral expressions for the semiclassical time-dependent propagator
Authors: Kay Kenneth G.
Published: July 2002

Rather general expressions are derived which represent the semiclassical time-dependent propagator as an integral over initial conditions for classical trajectories. These allow one to propagate time-dependent wave functions without searching for special trajectories that satisfy two-time boundary conditions. In many circumstances, the integral expressions are free of singularities and provide globally valid uniform asymptotic approximations. In special cases, the expressions for the propagators are related to existing semiclassical wave function propagation techniques. More generally, the present expressions suggest a large class of other, potentially useful methods. The behavior of the integral expressions in certain limiting cases is analyzed to obtain simple formulas for the Maslov index that may be used to compute the Van Vleck propagator in a variety of representations.
From concepts to algorithms for the characterization of reaction mechanisms. H2CS as a case study
Authors: Minichino Camilla , Barone Vincenzo
Published: July 2002

All the stationary points on the S0 surface of H2CS and their quadratic force fields have been determined by correlated methods using large multiply polarized basis sets. Complete quartic force fields of all stationary points and reaction paths connecting each pair of minima through a first-order saddle point have been obtained at the MP2 level using smaller polarized basis sets. These data allow the computation of reliable isomerization and fragmentation rates by semiclassical expressions which take into account tunneling as well as curvature and mode–mode couplings. Further insight into the characteristics of the different reaction channels has been gained from the parameters of the so-called reaction path Hamiltonian (RPH) both in adiabatic and diabatic representations. The bookkeeping and processing of the large body of data involved in this study has been possible through the development of a package which is able to perform anharmonic and reaction path computations. The results are discussed with special reference to the H2CO system and to the interplay of potential, kinetic, and statistic effects in determining reaction mechanisms and rates.
Equation of state for hard convex body fluids from the equation of state of the hard sphere fluid
Authors: Maeso M. J. , Solana J. R.
Published: July 2002

A simple and accurate equation of state for fluids of hard convex molecules is derived from the pressure equation and the equation of state of the hard sphere fluid. The equation of state provides theoretical support to some equations of state used in perturbation theories for real molecular liquids. The equation of state reproduces the simulation data with an accuracy comparable to that derived from density functional theory.
Theoretical study of electron spin resonance parameters: H2CN and H2CO+
Authors: Suter H. U. , Engels B.
Published: July 2002

The hyperfine structure of the two isoelectronic molecules H2CN and H2CO+ in their electronic ground state (X 2B2) is studied. The influence of the atomic orbital (AO) basis sets, of the correlation treatment, and of the equilibrium geometry on the obtained hyperfine properties is investigated. It is found that the multireference double excitation-configuration interaction (MRD-CI)/BK treatment in which an MRD-CI wave function is corrected by a modified BK method yields equivalent results to quadratic CI [QCISD(T)], coupled cluster single doubles [CCSD(T)], or Brueckner doubles [BD(T)]. Uncertainties in the equilibrium geometries are found to be the major source for discrepancies between theoretically and experimentally determined isotropic hyperfine coupling constants (hfccs). For the heavier centers, the calculated values of the isotropic hfccs agrees nearly perfectly with experimental values (≊1%–2%). The calculated values for the hydrogens are too low, but using the equilibrium structure suggested by Yamamoto and Sato [J. Chem. Phys. 96, 4157 (1992)], the best estimate deviates by less than 3%.
The new photoisomerization mechanism of stilbene
Authors: Kawaguchi Yoshihisa
Published: July 2002

This paper presents evidence that both hydrogens of the ethylenic and phenylic groups of trans-stilbene interact to stabilize the planar structure of the molecule. Also, it proposes a new model that the photoisomerization (PI) arises from the cutting off process of these intramolecular hydrogen interactions (IMHI). The photoisomerization dynamics of stilbene has been investigated by semiempirical quantum chemistry calculations. This new model yields good agreement with experimental results and specifies the PI/IMHI mechanism in detail.
Hyperchaos and chaotic hierarchy in low-dimensional chemical systems
Authors: Baier Gerold , Sahle Sven
Published: July 2002

Chemical reaction chains with feedback of one of the products on the source of the chain are considered. A strategy is presented in terms of ordinary differential equations which creates one, two, and three positive Lyapunov exponents as the finite dimension of the system is increased. In particular, a nonlinear inhibition loop in a chemical reaction sequence controls the type of chaos. The bifurcation scenarios are studied and chaos and hyperchaos are found for broad regions of bifurcation parameter. Some implications for the occurrence of higher chaos in real systems are discussed.
Intermolecular vibrations of the 2,3-dimethylnaphthalene⋅Ar van der Waals complex: Experiment and quantum three-dimensional calculations
Authors: Mandziuk Margaret , Bačić Zlatko , Droz Thierry , Leutwyler Samuel
Published: July 2002

A combined experimental and theoretical study of the intermolecular vibrations of 2,3-dimethylnaphthalene⋅Ar (2,3-DMN⋅Ar), for the first excited electronic state (S1), is reported. Methyl groups at C2 and C3 positions of naphthalene lower the symmetry of the complex, so that transitions involving excitation of the intermolecular long-axis in-plane x mode become allowed in electronic spectra, in addition to the out-of-plane z mode. Two-color resonant two-photon ionization (2C-R2PI) spectrum of the van der Waals (vdW)-mode region (000+70 cm−1) of 2,3-DMN⋅Ar exhibits six bands to the high-frequency side of the electronic origin 000, which arise from excitation of low-frequency intermolecular vibrations of the complex in the S1 state. Accurate quantum three-dimensional (3D) calculations of vdW vibrational (J=0) levels of S1 2,3-DMN⋅Ar have been performed, using a recently developed quantum method based on the 3D discrete variable representation. Since no approximation is made in the treatment of coupled, very anharmonic vdW vibrations, the calculated eigenstates are essentially exact for the intermolecular potential energy surface (PES) employed, thus enabling direct comparison between theory and experiment. The intermolecular PES was modeled as a sum of atom–atom Lennard-Jones (LJ) pair potentials. Some of the initial LJ parameters were modified until very good agreement was achieved between the calculated and measured vdW frequencies of S1 2,3-DMN⋅Ar. This allowed assignment of the vdW bands to the blue side of 000, and resulted in an improved intermolecular PES of the complex. In addition, the quantum 3D calculations provided a quantitative description of the vdW vibrational level structure and floppiness of S1 2,3-DMN⋅Ar up to ∼60–70 cm−1 above the ground vdW state. The wave functions of all vdW states below ∼49 cm−1, relative to the ground state, are sufficiently regular to allow assignment of vibrational quantum numbers. At higher excitation energies, mode coupling becomes stronger, and irregular vdW states whose assignment is uncertain, are common.
A wave operator description of molecular photodissociation processes using the Floquet formalism
Authors: Jolicard Georges , Killingbeck John P. , Durand Philippe , Heully Jean Louis
Published: July 2002

During the last few years, the Floquet matrix approach to the theory of intense-field multiphoton dissociation processes has undergone a revival of interest which is due to the emergence of new powerful algorithms within the complex quasivibrational energy formalism. The method presented here shows that the Bloch wave operator theory is, in the framework of this theory, an efficient tool to select the active space which participates principally in the dissociation process. An illustrative numerical example reveals that the one dimensional active space model, used mainly to reproduce cw laser experimental results, fails when the laser is tuned to bound state–bound state transitions.
Quadratic steepest descent on potential energy surfaces. IV. Adaptation to singular Hessians
Authors: Ruedenberg Klaus , Sun Jun-Qiang
Published: July 2002

It is shown that the quadratic steepest descent procedure developed in three previous papers goes smoothly and naturally into a limiting form, presenting no computational problems, when the Hessian becomes singular. This is in contrast to quasi-Newton algorithms which, in the case of singular or near-singular Hessians, yield unrealistic search directions and are kept from breaking down only by introducing arbitrary additional assumptions. The reason is that quasi-Newton algorithms give inadequate weight to the first order variation of the energy, i.e., its gradient.
Comment on ‘‘Electron flow and electronegativity equalization in the process of bond formation’’ [J. Chem. Phys. 99, 5151 (1993)]
Authors: Sukumar N.
Published: July 2002

The implications of the concepts of bond hardness and electronegativity equalization to the density functional theory of Jahn–Teller systems and superconductivity are discussed.
The bridge function expansion and the self-consistency problem of the Ornstein–Zernike equation solution
Authors: Vompe A. G. , Martynov G. A.
Published: July 2002

We propose self-consistent solutions to the Ornstein–Zernike equation where the approximate closure is replaced by a bridge function expansion, whose main advantage is the improvement of correlation functions. Unknown coefficients of this expansion are found from the principle of total thermodynamic consistency. The latter includes not only the conventional pressure–compressibility equation but also the relation between internal energy and pressure. We show that utilizing only the first equation one may face a nonunique partially consistent solution conditioned by noncomplete formulation of the consistency problem. At the same time the suggested set of equations is sufficient to determine a true and unique physical solution regardless of the number of unknown coefficients. In this paper we expand the bridge function in powers of potential of mean force and perform the example of building the approximate self-consistent closure. Moreover, the approach via total thermodynamic consistency introduces the value of residual inconsistency as the internal criterion of accuracy, which is equal to zero only for the exact closure relation and the corresponding solution. This value is found to be in agreement with the deviation of thermodynamic quantities from numeric simulation data. The proposed method is tested on the classical model of a Lennard-Jones fluid in a wide range of temperatures and high densities.
Transitions to periodic structures: Higher harmonic corrections with concentration fluctuations
Authors: Jones J. L. , Olvera de la Cruz M.
Published: July 2002

Weakly first order transitions from a disordered state to periodic ordered phases, are observed in many physical systems, such as diblock copolymer melts, weak crystallization in liquids and micellar systems. In the weak segregation regime, successful theories have used a Landau–Ginzburg form of the free energy. It is well established that a complete description of the system necessitates the inclusion of thermally induced concentration fluctuations in the theory. In some cases it may also be important to construct the density in the periodic structure from higher harmonic vectors other than simply the first ones. Recently, exotic structures have been observed experimentally which cannot be distinguished from simple morphologies unless higher harmonics and their phases are included. In this paper we present a general theory where the order parameter is expanded up to the second harmonic vectors with phase factors, and where the concentration fluctuations are included self-consistently in the analysis.
Laser spectroscopy of perturbed levels in N2(B 3Πg,v=10) and the first experimental determination of the N2(A′ 5Σ+g) term energy
Authors: Ottinger Ch. , Vilesov A. F.
Published: July 2002

Using both a pulsed and a narrow-band cw dye laser, laser-induced fluorescence excitation spectra of N2(B,v=10) were observed on the N2(B 3Πg←A 3Σ+u) transition from the metastable N2(A) component of a molecular beam. Lifetime measurements were made for some of the B,v=10 rotational/fine structure levels, including one which was recently shown to exhibit very specific kinetic effects due to a perturbation by the N2(A′ 5Σ+g) state [Ch. Ottinger, L. G. Smirnova and A. F. Vilesov, J. Chem. Phys. 100, 4848 (1994)]. The perturbation manifested itself in the present work by a significantly longer lifetime of the 3Πe2(12) level relative to other B,10 levels, as well as by line shifts. The S/O interaction matrix element is obtained to be 0.35 cm−1. From the required very close accidental coincidence between well-identified levels in the B and A′ states the term energy of the latter could be precisely determined as Te(A′ 5Σ+g)=75 990.0 cm−1. This is the first measurement of this value, and the result is ≂440 cm−1 lower than was predicted by ab initio calculations.
Interaction of an aluminum atom with a closed subshell metal atom: Spectroscopic analysis of AlZn
Authors: Behm Jane M. , Blume Thorsten , Morse Michael D.
Published: July 2002

Resonant two-photon ionization spectroscopy has been employed to investigate diatomic AlZn produced by laser vaporization of a 1:2 Al:Zn alloy target disk in a supersonic expansion of helium. Several discrete transitions are reported in the energy range from 18 400 to 19 100 cm−1. Most of these are assigned as members of the B 2Π←X 2Π system, although an isolated band has been observed and assigned as the 2-0 band of the A Ω′=0.5←X 2Π1/2 system. A pair of strongly mixed levels are identified as resulting from a homogeneous spin–orbit perturbation between the A Ω=0.5, v′=3 and the B 2Π1/2, v′=1 levels, and the perturbation matrix element has been deduced to be 8.11 cm−1 for 27Al64Zn, 8.23 cm−1 for 27Al66Zn. The ground state has been unambiguously identified as a 2Πr state with a bond length of 2.6957±0.0004 Å. Comparisons to the results of the preceding article on the spectroscopy of AlCa are also provided, along with a discussion of the chemical bonding in AlZn in relation to AlCa, AlAr, and AlKr.
Monte Carlo evaluation of ensemble averages involving particle number variations in dense fluid systems
Authors: Kaminsky R. D.
Published: July 2002

A general formalism is presented for the calculation of thermodynamic properties via statistical ensembles augmented with nonphysical states. These augmented ensembles can be readily incorporated into Monte Carlo molecular simulation methods. Of particular interest is evaluation of thermodynamic properties by Monte Carlo applications which involve particle number variations. The augmented states can be constructed to facilitate Markov transitions between states with varying number of particles. This in turn can greatly increase the convergence rates of property averages, especially for high density systems. This benefit is analogous to chemical catalysis, which by introducing low activation energy reaction pathways increases reaction rate (i.e., convergence rate) without affecting the overall equilibrium. The utility of augmented ensembles is demonstrated through novel Monte Carlo implementations which are efficient regardless of system density of grand ensemble simulation and test particle activity evaluation. Applications to pure hard-sphere and 12-6 Lennard-Jones fluid systems are specifically considered.
Sub-Doppler laser spectroscopy of the 1B2(1Δu) state of CS2 in a cold molecular beam
Authors: Habib A. , Görgen R. , Brasen G. , Lange R. , Demtröder W.
Published: July 2002

Sub-Doppler excitation spectra of CS2 in the spectral range 30200−32300 cm−1 have been extensively measured with a frequency-doubled single mode cw dye laser in a cold collimated argon beam, seeded with CS2. From the rotational analysis of the measured spectra, the geometry and the position of the barrier to linearity of the CS2 molecule in the excited 1B2(1Δu) state has been determined. The observed bands are heavily perturbed and cluster around some intense vibrational transitions. A statistical analysis of the distribution of these bands showed that no chaotic behavior is present in the considered energy region and therefore a non-linear interaction with highly excited vibrational levels of the ground state cannot be responsible for this irregular vibronic structure, although chaotic regions for vibrational energies E≳12000 cm−1 in the electronic ground state have been reported in the literature. Different coupling mechanisms causing the observed perturbations of the rotational and vibrational structure are also discussed.
Comment on ‘‘The rheological behavior of concentrated colloidal dispersions’’ [J. Chem. Phys. 99, 567 (1993)]
Authors: Cichocki B. , Felderhof B. U.
Published: July 2002

We comment on a claim by Brady that the viscoelastic response of colloidal dispersion is a universal function of the diffusive relaxation time, as given by the short-time self-diffusion coefficient. The purported function was taken from an exact calculation for a semidilute suspension of hard spheres without hydrodynamic interactions. The claim is in disagreement both with an exact calculation for the same system with hydrodynamic interactions, and with experimental data on dense suspensions of hard spheres.
Multichannel quantum defect theory simulation of the zero-kinetic-energy photoelectron spectrum of H2
Authors: Softley T. P. , Hudson A. J.
Published: July 2002

The multichannel quantum defect theory (MQDT) is applied to the simulation of the v+=2 band of the zero-kinetic-energy (ZEKE) photoelectron spectrum of molecular hydrogen and also to the photoionization spectrum involving autoionizing Rydberg states in the region between the v+=1 and v+=2 thresholds. The results of the calculations are compared with previously reported experimental results [J. Chem. Phys. 96, 4149 (1992)]. The calculations are in very good agreement with experiment and provide quantitative confirmation of the previously proposed mechanisms for intensity perturbation.
Monte Carlo simulation of the glass transition in two- and three-dimensional polymer melts: Influence of the spatial dimension
Authors: Lobe B. , Baschnagel J.
Published: July 2002

This Monte Carlo simulation was undertaken to provide some qualitative insight into the dependence of the glass transition of polymer melts on the spatial dimension. To this end, two- and three-dimensional systems were simulated, in which we kept the external conditions, such as the density or the cooling schedule, the same for both dimensions. The melts, simulated by a lattice model (bond-fluctuation model), undergo a kinetic freezing, while being continuously cooled from the equilibrium liquid to the nonequilibrium glassy phase. The resulting glass transition as well as the other simulation results indicate that the spatial constraints are stronger in three than in two dimensions. This finding is reminiscent of the influence of the spatial dimension on the ability of a liquid to crystallize.
Picosecond time-resolved adsorbate response to substrate heating: Spectroscopy and dynamics of CO/Cu(100)
Authors: Germer T. A. , Stephenson J. C. , Heilweil E. J. , Cavanagh R. R.
Published: July 2002

The response of the molecular stretch mode of CO/Cu(100) near 2086 cm−1 (ν1) to resonant infrared, and nonresonant visible and ultraviolet pumping is measured on a picosecond time scale. Fourier transform infrared measurements establish that ν1 is anharmonically coupled to the frustrated translation near 32 cm−1 (ν4), so that transient shifts in ν1 indicate population changes in ν4. The ν1 response to visible and ultraviolet pumping is characterized by a spectral shift near zero delay time, which decays with a ≊2 ps time constant to an intermediate value, which then decays on a ≊200 ps time scale. The data agree well with a model whereby ν4 couples to both the photogenerated hot electrons and to the heated phonons. The characteristic coupling times to these two heat baths are found to both be a few picoseconds.