bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurements of the structure of methyltrioxorhenium using microwave spectroscopy
Authors: Wikrent P. , Drouin B. J. , Kukolich S. G. , Lilly J. C. , Ashby M. T. , Herrmann W. A. , Scherer W.
Published: July 2002

The rotational spectra for six isotopomers of methyltrioxorhenium (MTO) were measured in the 6–14 GHz range using a Flygare–Balle-type pulsed-beam microwave spectrometer. The measured transition frequencies were analyzed to obtain rotational constants and quadrupole coupling strengths. Rotational constants from the new measurements were used, along with previous results for the normal and C13 isotopomers, to determine the complete, three-dimensional structure for MTO. The bond lengths obtained are r(Re–C)=2.074(4) Å, r(Re–O)=1.703(2) Å, and r(C–H)=1.088(7) Å. The interbond angles are ∠Re–C–H=108.9(2)° and ∠C–Re–O=106.4(4)°. This complex appears to have C3v symmetry in the gas phase, unlike the solid-state structure. Structural parameters are compared with neutron-diffraction, electron-diffraction, and density functional theory results. Spectra for the asymmetric-top isotopomers were much more difficult to analyze due to effects of off-diagonal quadrupole coupling terms and possible internal rotation. The Re quadrupole coupling strength is increased by 1.4% for CD3ReO3 compared with CH3ReO3, illustrating a secondary isotope effect.
Ionization energy of HSSH
Authors: Cheng Bing-Ming , Eberhard Jürg , Chen Wei-Chen , Yu Chin-hui
Published: July 2002

The photoionization efficiency (PIE) spectrum of HSSH was measured in the spectral range (110–140) nm by means of a discharge flow and a photoionization mass spectrometer coupled to a synchrotron radiation source. HSSH was generated from the self-reaction of HS radicals in the flow tube. HS radicals were produced by reacting Cl atoms with H2S. The adiabatic ionization energy of HSSH was determined to be (9.06±0.02) eV. GAUSSIAN-2 ab initio calculations predict 9.086 eV for ionization to the ground state of trans-HSSH+, in satisfactory agreement with the experimental result. The heat of formation of HSSH+, ΔfH°298(HSSH+), was derived to be (212.7±0.5) kcal mol−1.
Theory of nucleation from the gas phase by a sequence of reversible chemical reactions
Authors: Girshick Steven L.
Published: July 2002

A general theory is presented for the problem of condensed-phase particle nucleation from gas-phase precursors via a sequence of reversible chemical reactions, where no supersaturated vapor exists. We consider a system in which nucleation is initiated by the reaction between an “initiating species” and a “growth species.”Subsequent steps in cluster growth involve reversible reactions between a cluster and the growth species, producing a larger cluster and a volatile byproduct, which may be considered a “suppressing species.” Following the mathematical formalism of homogeneous nucleation theory, a steady-state nucleation rate is derived in the form of a summation over discrete cluster sizes. The resulting nucleation rate is linearly proportional to the product of the concentrations of the initiating species and the growth species, while the ratio of the concentrations of the growth species to the suppressing species, relative to a suitably defined equilibrium value, is seen to play a similar role as the vapor saturation ratio in homogeneous nucleation.
Adiabatic phase transformations in confinement
Authors: Umantsev Alexander
Published: July 2002

The phase diagram of small one-component particles has been analyzed under conditions of thermal insulation, i.e., conservation of energy. In large isolated systems the absolute stability belongs to heterogeneous states with phase separation. However, for small particles the global stability analysis shows a considerable extension of the single-phase regions into a two-phase zone of the phase diagram. Moreover, for very fine particles with sizes only 5-20 times exceeding interfacial thickness, phase separation does not occur at all and the equilibrium is achieved on homogeneous transition states that can never be obtained in bulk samples because of their absolute instability. The thermodynamic and dynamical explanations are presented. This type of a small-particle phase diagram may be relevant to the theory of amorphization, magnetocaloric effect, and nanophase composite materials where small particles or thin whiskers, capable of undergoing a transition, are immersed into a poorly conducting matrix. In case of small particles of solid solution, where mass conservation replaces the conservation of energy, present results predict the appearance of new stable phases with compositions deeply inside the miscibility gap.
Tight-binding recursion calculations of step energetics on the GaAs(110) surface
Authors: Chuan Kang H. , Chen X. F. , Tan H. S.
Published: July 2002

On the GaAs(110) surface steps perpendicular to the [001] direction may be either arsenic-terminated or gallium-terminated. We compute the energy difference between these steps using a tight-binding recursion method. We find that the arsenic-terminated step is more stable by approximately 0.5 eV. Our results suggest that some recent experimental observations on the shape of islands formed during homoepitaxy on GaAs(110) may be the consequence of an energy-driven rather than a kinetics-driven epitaxial growth.
The collisional deactivation of highly vibrationally excited pyrazine by a bath of carbon dioxide: Excitation of the infrared inactive (100), (020), and (0220) bath vibrational modes
Authors: Michaels Chris A. , Mullin Amy S. , Park Jeunghee , Chou James Z. , Flynn George W.
Published: July 2002

The collisional quenching of highly vibrationally excited pyrazine, C4H4N2, by CO2 has been investigated using high resolution infrared transient absorption spectroscopy at a series of cell temperatures. Attention is focused on collisions which result in excitation of the Fermi-mixed bath vibrational states (1000) and (0200), along with the unmixed overtone bend state (0220). The vibrationally hot (Evib≈5 eV) pyrazine molecules are formed by 248 nm excimer laser pumping, followed by rapid radiationless decay to the ground electronic state. The nascent rotational and translational product state distributions of the CO2 molecules in each vibrationally excited state are probed at short times following the excitation of pyrazine. The temperature dependence of this process, along with the CO2 product state distributions and velocity recoils, strongly suggest that the vibrational excitation of CO2 is dominated by a long-range electrostatic interaction despite the fact that the dipole transition matrix elements connecting the CO2 ground state to the excited states vanish for the isolated molecule. The vibrational energy transfer is accompanied by very little rotational and translational excitation and displays the characteristic strong, inverse temperature dependence (probability of transfer increases with decreasing temperature) expected of energy transfer mediated by a long range attractive interaction. A number of possible explanations for this apparent anomaly are considered, of which energy transfer mediated by dipole/quadrupole forces appears to be the most consistent with the data.
Binary collision contribution to transverse current correlation function of dense fluids
Authors: Sharma Rajneesh K. , Tankeshwar K. , Pathak K. N. , Ranganathan S. , Johnson R. E.
Published: July 2002

An expression for the binary collision contribution to the first order memory function of the transverse current correlation function has been obtained using the cluster expansion technique for particles of a fluid interacting via a continuous potential. The resulting expression involves the static pair correlation function and the time dependence of position, momentum, and acceleration of a particle. In the long wavelength limit our expression determines the time development of the autocorrelation function of transverse stress. These time-dependent functions have been evaluated numerically for the Lennard-Jones potential for some thermodynamic states and results have been compared with molecular dynamics data. It has also been found that for a hard sphere interaction our expression for the shear viscosity reduces to the Enskog result.
Untangling the physical contributions to instantaneous normal mode approximations: Inhomogeneous broadening, motional narrowing, and energy relaxation
Authors: Kalbfleisch T. , Keyes T.
Published: July 2002

An instantaneous normal mode (INM) approach to vibrational lineshapes, including motional narrowing, is presented. Simulations and calculations are carried out for a diatomic in Lennard-Jones solvent as a function of vibrational frequency, with an emphasis on determining the contributions of different physical relaxation mechanisms. The velocity correlation of a bond is easily related to a bond-weighted INM density of states, containing both resonant energy relaxation (ER) and unnarrowed inhomogeneous broadening. An effective weighted density of states or static spectrum, the distribution of an effective time-dependent frequency Ω(t), is introduced and proposed as a measure of the inhomogeneous linewidth only. It is found that the vibrational INM are in the motionally narrowed or fast modulation limit; motional narrowing of INM cannot be ignored. A dynamic spectrum containing only the motionally narrowed inhomogeneous spectrum and corresponding pure dephasing relaxation is isolated. Reintroducing energy relaxation results in excellent agreement with simulation. The validity of INM approximations and the relative importance of different relaxation mechanisms as a function of vibrational frequency is analyzed. It is suggested that, through INM, a role may be found for motional narrowing in intermolecular dynamics.
Theoretical assignments of the photo-dissociation excitation spectra of Mg+ ion complexes with water clusters: Multi-reference CI studies
Authors: Watanabe Hidekazu , Iwata Suehiro
Published: July 2002

Implementing the ab initio molecular orbital (MO) method, electronic excitation energies of Mg+(H2O)n clusters have been calculated in order to analyze the photo-dissociation excitation spectra reported by Fuke and his co-workers. Observed bands for n⩽2 are assigned to the s→p transitions of the most stable isomer of Mg+(H2O)n, with all waters directly bonded to the metal ion. For n=3, n=4 and 5, the dominant bands are also those of the most stable isomer, [Mg+(H2O)3](H2O)n−3, which has three water molecules in the first hydration shell. There are, however, shoulders and weak peaks in the experimental spectra, and calculations prove that these are due to the s→p transitions of less stable isomers. The calculated spectra strongly indicate the coexistence of a few isomers for n⩾3 in molecular beam experiments.
Wave packet interferometry without phase-locking
Authors: Leichtle C. , Schleich W. P. , Averbukh I. Sh. , Shapiro M.
Published: July 2002

We apply the method of “coherence observation by interference noise” (COIN) to molecular wave packet interferometry. In contrast to conventional wave packet interferometry, where one has to control precisely the relative phase between the pair of time-delayed laser pulses, COIN employs a sequence of randomly phased laser pulses. Whereas in this case the mean signal does not reflect anymore the coherent evolution of a localized wave packet, the fluctuations around this mean still mimic the dynamics of the propagating wave packet. We demonstrate the feasibility of wave packet interferometry without phase–locking by simulating fluorescence interferograms for the Na2– molecule.
Response to “Comment on ‘On the direct complex scaling of matrix elements expressed in a discrete variable representation: Application to molecular resonances’ ” [J. Chem. Phys. 109, 1201 (1998)]
Authors: Leforestier Claude , Museth Ken
Published: July 2002

Contrary to Bludsky et al.’s claim, it is shown that the numerical continuation procedure of matrix elements previously proposed [K. Museth and C. Leforestier, J. Chem. Phys. 107, 7008 (1996)] is able to achieve high accuracy. For the simple one-dimensional model considered here, resonances can be obtained up to 10 and 5 significant digits for the real and imaginary parts, respectively.
Properties of electron density and other one-electron observables derived from generalized Hiller–Sucher–Feinberg identities
Authors: Cioslowski Jerzy , Lopez-Boada Roberto
Published: July 2002

A generalization of the Hiller–Sucher–Feinberg (HSF) and Rassolov–Chipman identities for the electron density is presented. A companion expression for the electrostatic potential is derived. Properties of the electron density furnished by the generalized HSF identity are discussed. In particular, the behavior in the vicinity of an arbitrary potential singularity and the long-range asymptotics are analyzed in detail. A simple numerical example is provided to illustrate the new theoretical results.
Static structure of electrolyte systems and the linear response function on the basis of a dressed-ion theory
Authors: Varela L. M. , Perez-Rodriguez M. , Garcia M. , Sarmiento F. , Mosquera V.
Published: July 2002

The static structure of a bulk electrolyte solution or colloid system is investigated in the framework of a dressed-ion theory (DIT). The number–number, charge–number, and charge–charge static structure factors are calculated and are seen to depend only on the linear response function of the DIT α̂(k), the α function therefore determining the charge structure of the fluid in what is an expression of the fluctuation–dissipation theorem. The expression of the static structure factors for one-component charged spheres (OCCS) is evaluated in the random-phase approximation and in a modified version of the mean-spherical approximation (MSA), using the hard-sphere fluid as a reference system, and an explicit expression for the linear response function and dielectric function is obtained. The effective screening length (κ−1) and the transition from monotonic exponential to oscillatory behavior obtained from the modified MSA expression of the α function are seen to improve the ones derived from the second moment condition at intermediate concentrations. The internal charge density distribution of a dressed ion and the renormalized ion charges (q*) are also investigated. The oscillatory behavior of the charge distributions suggests an “onionlike model,” with the central ion surrounded by spherical charge shells. The effective charges calculated from the modified MSA are seen to diverge in the neighborhood of the transition from the monotonic exponential regime to the oscillatory regime. In the limit of vanishing concentration, Debye–Hückel (DH) results are recovered.
Phase diagram analysis of random heteropolymers with composition specific and quenched cross-links
Authors: Gutman Lorin , Shakhnovich Eugene
Published: July 2002

We study the conformational organization of a novel class of soft matter, random heteropolymers (RHPs) with composition specific and quenched cross-links by replica field theory. These technologically relevant materials carry an essentially quenched sequence distribution of disparate segments, and also a fixed distribution of composition specific cross-links. The chain connectivity of the linear RHPs is captured most generally by a continuous microscopic RHP Hamiltonian, while the quenched character of the cross-links is enforced by spatial constraints on segments of a prescribed composition, and by allowing fluctuations in the total number of composition specific cross-links around an experimentally controlled average. The replica theory, a formalism widely used in the study of linear RHPs, is extended in the present work to analysis of systems with multiform disorder that have interdependent disorder components. By numerically analyzing the free energy and its stability we predict the occurrence of frozen phase formation wherein few conformations are sampled. By systematically varying the sequence/cross-link fluctuations, temperature, and the RHP interaction parameters we show that two conformational transitions can occur in the frozen phase. The low temperature conformational transition resembles to a large extent the reduction in chain conformations which occurs during the folding of proteins with fixed disulfide bonds. The frozen phase domain microstructure formation is discussed in context of the recently elucidated phase diagram of a close related system, RHPs with annealed cross-links. Explicit connection is made with possible experiments and computer simulations methods to test our predictions.
Calculation of H+H2 and H+D2 reaction probabilities within the multiconfiguration time-dependent Hartree approach employing an adiabatic correction scheme
Authors: Jäckle A. , Meyer H.-D.
Published: July 2002

The multiconfiguration time-dependent Hartree (MCTDH) method is employed to calculate initial-state selected reaction probabilities for the two isotopic reactions H+H2(D2) with initial states ν=0,j=0–3 and total angular momentum J=0. To compute the reaction probabilities, an initial wave packet is prepared and propagated in time employing the recently developed constant mean-field integrator, thus reducing the computational effort by an order of magnitude. An adiabatic correction scheme is introduced which allows the initial wave packet to be moved from the asymptotic region of the educt channel close to the interaction region. The calculations are performed on the Liu-Siegbahn-Truhlar-Horowitz (LSTH) potential surface which is expanded in products of one-dimensional functions of the Jacobian coordinates. Initial-state selected reaction probabilities are computed for total energies up to 2.5 eV utilizing a combined flux operator/complex absorbing potential approach.
Theoretical studies of the structures and vibrational frequencies of actinide compounds using relativistic effective core potentials with Hartree–Fock and density functional methods: UF6, NpF6, and PuF6
Authors: Hay P. Jeffrey , Martin Richard L.
Published: July 2002

The calculated bond lengths and vibrational frequencies of the octahedral compounds UF6, NpF6, and PuF6 from Hartree–Fock and various density functional calculations are compared. The calculations employ relativistic effective core potentials (RECPs) derived from one-component relativistic Hartree–Fock atomic wave functions. The structures and frequencies of the hexafluoride molecules were obtained using analytic gradient and second derivative techniques. Of the methods examined here the local density (SVWN) and hybrid functional (B3LYP) have the best performance in terms of the errors compared to experiment for the bond lengths and vibrational frequencies. The analytic representations of the RECPs for U, Np, and Pu are given along with Gaussian basis sets for the valence electrons of these actinide elements.
Microwave spectra and structure of an isoxazole–CO Van der Waals complex
Authors: McGlone S. , Bauder A.
Published: July 2002

The microwave spectra of an isoxazole–CO complex and four of its isotopomers have been measured between 7 and 18 GHz with a pulsed nozzle Fourier transform microwave spectrometer. Rotational constants, centrifugal distortion constants, and N14 quadrupole coupling constants have been fitted to the measured transition frequencies of each isotopomer. The permanent electric dipole moment has been determined from Stark effect splittings. A unique structure has been found for the complex by taking both the moments of inertia of the isotopomers and the quadrupole splittings into account. The complex is planar, with CO lying approximately radially away from nitrogen in the isoxazole ring.
Product-operator formalism in I⩾1/2 and S⩾3/2 J-coupled nuclear spins: Application to J assessment via SEDOR experiment
Authors: Capuani S. , De Luca F. , Maraviglia B.
Published: July 2002

Product-operator formalism has been extended to weakly coupled I⩾1/2 and S⩾3/2 nuclear spins to describe the result of nuclear SEDOR (spin echo double resonance) experiments for such spin numbers. By SEDOR modulation, the indirect nuclear spin–spin coupling constant JIS can be measured also when the quadrupolar broadening hides the high resolution evaluation of JIS and the method based on scalar relaxation of the second kind cannot be utilized because of the low quadrupolar relaxation efficiency. Furthermore, the nuclear SEDOR modulation can be performed in low intensity and homogeneity magnetic field. The theory has been applied to estimate JIS of the coupled nuclear spins I=1/2 (F19) and S=9/2 (Nb93) in NbF6− (octahedral symmetry) and of I=1/2 (H1) and S=3/2 (B11) in Na2B1211H11SH (icosahedral symmetry).
Unanticipated confluences of seams of conical intersection: Reinvestigating intersecting potential-energy surfaces using new tools. I. C(3P)+H2
Authors: Yarkony David R.
Published: July 2002

Recently a quite unexpected situation has been found in which two distinct seams of conical intersection originating from the same two states—one the accidental intersection of two states of different symmetries and one the accidental intersection of two states of the same symmetry—intersect. These confluences can now be systematically identified using an algorithm that relies solely on information obtained from the symmetry-allowed seam. It is necessary to ask whether, in the absense of such an algorithm, these confluences have been overlooked in the past. In this work the well studied 1 3A″–2 3A″ seam of conical intersection in C(3P)–H2 is reinvestigated using the above noted algorithm with surprising results.
The near ultraviolet dissociation dynamics of azomethane: Correlated V-T energy disposal and product appearance times
Authors: Bracker Allan S. , North Simon W. , Suits Arthur G. , Lee Yuan T.
Published: July 2002

We have measured the translational energy release for methyl radical photofragments with zero and one quantum of umbrella vibration, which were produced by the ultraviolet dissociation of azomethane. Translational energy distributions exhibit bimodal structure and are similar for both methyl radical vibrational states and for two dissociation wavelengths (355 and 330 nm). Compared to earlier non-state-selective measurements, these distributions exhibit a stronger preference for high kinetic energies. We have also observed that the appearance times for both methyl radical vibrational states are fast compared to the experimental time resolution, thus resolving an outstanding mechanistic disagreement. These observations are consistent with a previously proposed mechanism in which a concerted three-body dissociation follows randomization of parent internal energy.
Solvation state selective excitation in resonance Raman spectroscopy. II. Theoretical calculation
Authors: Yamaguchi T. , Kimura Y. , Hirota N.
Published: July 2002

We have reported in the previous paper [J. Chem. Phys. 109, 9075 (1998)] that the Raman Stokes shifts of the C=O and the C=N stretching modes in the resonance Raman spectra of a solvatochromic dye, phenol blue (PB), appear to depend on the excitation photon energy in polar or protic solvents. The reason was ascribed to the solvation state selective excitation due to the inhomogeneous distribution of the solvation environment. In this paper we have made a theoretical model calculation to demonstrate that the solvation state selective excitation indeed brings about the excitation energy dependence of the resonance Raman Stokes shift in solution. In our model, both the electronic and the vibrational transitions are linearly coupled to the same harmonic bath, to embody the coupling between the fluctuations of the electronic and the vibrational transition energies. The absorption and the resonance Raman cross sections are formulated for this model on the basis of the time dependent path integral method. In the formulation, the finite relaxation time of the bath mode and the vibrational progressions in the absorption spectra are also taken into account. We have calculated the resonance Raman spectra at various excitation photon energies for the model system probable for PB in methanol. The parameters for the calculation are estimated from the analysis of the absorption and the resonance Raman spectra. It is demonstrated that the Raman Stokes shift indeed depends on the excitation energy for this model. It is noted that the vibrational dephasing due to the coupling with the solvent bath mode should be faster than the relaxation time of the bath mode for the excitation energy dependence of the Raman Stokes shift to be observed.
Theoretical and experimental adsorption studies of polyelectrolytes on an oppositely charged surface
Authors: Mashl Robert J. , Gro/nbech-Jensen Niels , Fitzsimmons M. R. , Lütt M. , Li DeQuan
Published: July 2002

Using self-assembly techniques, x-ray reflectivity measurements, and computer simulations, we study the effective interaction between charged polymer rods and surfaces. Long-time Brownian dynamics simulations are used to measure the effective adhesion force acting on the rods in a model consisting of a planar array of uniformly positively charged, stiff rods and a negatively charged planar substrate in the presence of explicit monovalent counterions and added monovalent salt ions in a continuous, isotropic dielectric medium. This electrostatic model predicts an attractive polymer-surface adhesion force that is weakly dependent on the bulk salt concentration and that shows fair agreement with a Debye–Hückel approximation for the macroion interaction at salt concentrations near 0.1 M. Complementary x-ray reflectivity experiments on poly(diallyldimethyl ammonium) chloride monolayer films on the native oxide of silicon show that monolayer structure, electron density, and surface roughness are likewise independent of the bulk ionic strength of the solution.
A CASSCF-MRCI study of the electronic excited states of FClO and FOCl
Authors: Li Yumin , Francisco Joseph S.
Published: July 2002

The potential energy curves for the low-lying excited states of FClO and FOCl have been computed using the complete active space self-consistent field (CASSCF) with cc-pVTZ basis sets. The vertical excitation energies for the excited states were calculated using the multireference configuration interaction (MRCI) method with cc-pVTZ and cc-pVTZ+sp basis sets. The vertical excitation energies were also obtained using the EOM-CCSD (equation of motion-coupled-cluster single double) method with cc-pVTZ basis sets. Results show that Rydberg character is not present in the excited states studied here for both FClO and FOCl. For FClO, all the excited states studied are repulsive along the F–Cl coordinate, but some of the excited states are bound along the Cl–O coordinate. For FOCl, all the excited states studied are dissociative along both the F–O and O–Cl coordinates.
Optimal coordinates for separable approximations in quantum dynamics of polyatomic systems: Coordinate choice criteria and error estimates
Authors: Jungwirth Pavel , Roeselová Martina , Gerber R. Benny
Published: July 2002

A considerable effort has been recently directed toward developing separable (mean-field) approximations for quantum molecular dynamics, such as the time-dependent self-consistent field (TDSCF) or the classical separable potential (CSP) methods. Unlike numerically exact solutions of the time-dependent Schrödinger equation, the accuracy of separable quantum dynamical simulations crucially depends on the choice of the coordinate frame. Since the approximate methods replace exact interactions between individual degrees of freedom by mean-field couplings, the goal is to work with coordinates which separate modes as well as possible. Unfortunately, for a larger system no practical way to optimize coordinates for mean-field quantum dynamics exists. Here, we suggest a simple and practical method for estimating the error of separable simulations, which allows us to select from a given set the optimal coordinate frame, or to identify modes, the couplings between which have to be treated more accurately. In the spirit of the CSP method, the time-dependent error estimate is based on differences between the exact and mean-field Hamiltonians along a swarm of classical trajectories. This makes it possible to very simply determine optimal coordinates for CSP or TDSCF propagation before actually performing any quantum simulation. The present methodology is applied to realistic and experimentally relevant systems, namely to the ultrafast relaxation following electron photodetachment in I−Arn (n=2 and 12) and Cl−H2O clusters. It is shown that the accuracy of separable quantum methods is strongly system and coordinate dependent. Comparison with numerically exact results shows that the suggested error measure correlates well with the actual error of the approximate quantum propagation, the accuracy of which can be consequently improved significantly, practically without additional computational effort. Finally, the feasibility of the proposed method for simulations of large polyatomic systems is demonstrated.
A hierarchical model for surface effects on chain conformation and rheology of polymer solutions. II. Application to a neutral surface
Authors: Mavrantzas Vlasis G. , Beris Antony N.
Published: July 2002

In this part, the general formulation described in Part I is applied to the modeling of the behavior of a dilute polymer solution near a purely repulsive, planar solid surface, i.e., near a noninteracting wall. The static equilibrium problem is considered first. The model equations here reduce to a minimization problem for the Helmholtz free energy of the system, which results into the well known equilibrium condition that the chemical potentials of all chain conformations in the interfacial area should be equal to each other. The numerical results show that the loss of polymer conformational entropy in the interfacial region gives rise to a strong polymer depletion which extends up to a distance about three times the equilibrium root-mean-square polymer end-to-end distance. Next, the problem of a polymer solution flowing past the wall is investigated. Here, the full model equations need to be considered; these are solved numerically with a spectral collocation technique. The numerical results show that the flow field enhances polymer depletion phenomena near the wall relative to those observed under equilibrium (static) conditions: By increasing the shear stress, the polymer concentration in the interfacial area decreases, in full agreement with available experimental data. Moreover, the flow field is found to affect significantly the chain conformations near the wall: The applied shear stress is seen to extend the chains along a primary direction, ξ, and to depress them in the transverse direction, η. The depletion of the interfacial region in polymer molecules is further seen to lead to the formation of a boundary layer close to the wall, where the macroscopic fluid velocity increases rapidly from its zero value exactly at the wall to its asymptotic bulk profile, resulting into an apparent macroscopic slip at the wall. The theoretically calculated slip coefficient is found to be of the same order of magnitude with the experimentally measured one, as reported in the literature for a dilute polymer solution of polymethylacrylate flowing near a glass surface [H. Mueller-Moehnssen et al., J. Rheol. 34, 223 (1990)].
Cyclopropane⋯sulfur dioxide and ethylene⋯sulfur dioxide van der Waals complexes: A theoretical study
Authors: Rayón V. M. , Sordo J. A.
Published: July 2002

Ab initio calculations at different levels of theory [HF, MP2, MP4SDTQ, and QCISD(T)] and using different basis sets (Pople’s 6-31G**, Dunning-Huzinaga’s D95**, and Dunning’s aug-cc-pVDZ) were carried out to analyze the potential energy hypersurfaces of the cyclopropane⋯sulfur dioxide and ethylene⋯sulfur dioxide van der Waals complexes. Both the theoretical methods employed and the basis set chosen exert a great influence on the geometry and nature of the stationary points located: The MP2/aug-cc-pVDZ structures are consistent with experimental data from microwave spectroscopy. Binding energies, dipole moments, and rotational constants are in reasonable agreement with the corresponding experimental values. Calculations predict the existence of tunneling pathways between equivalent structures in accord with the experimentally observed splittings in the rotational spectra: The QCISD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ energy barrier in the case of ethylene⋯sulfur dioxide is 48 cm−1, in good agreement with the experimental estimate (30 cm−1). For the cyclopropane⋯sulfur dioxide complex where an experimental estimate cannot be done, the computed barrier is 69 cm−1. Although covalent forces make appreciable contributions to the stabilization of both van der Waals complexes, the electrostatic component of the intermolecular interaction seems to play an especially relevant role in determining the relative orientation of the two subunits in each complex.
The infrared spectrum of FeH2, studied in the gas phase by laser magnetic resonance
Authors: Körsgen Helga , Urban Wolfgang , Brown John M.
Published: July 2002

The infrared spectrum of the FeH2 radical in the gas phase has been studied in detail between 1605 and 1730 cm−1, by the technique of carbon monoxide laser magnetic resonance. The molecule is formed in an electric discharge through a mixture of H2 and Fe(CO)5 in helium. Many resonances are observed and assigned to transitions in the fundamental band of the antisymmetric stretching vibration 301 and in the associated hot band 211301. In the latter case, each vibrational level is split into two vibronic components. The measurements are used to determine the parameters of an effective Hamiltonian which is capable of modeling the data to within experimental error. The values determined for the band origin and zero-point vibrational constant are: ν3=1674.7203(3) cm−1 and B0=3.075 23(5) cm−1. The data show that the molecule is linear in its ground state and are consistent with an assignment of this state as 5Δg.
Computer simulation and mode coupling theory study of the effects of specific solute–solvent interactions on diffusion: Crossover from a sub-slip to a super-stick limit of diffusion
Authors: Srinivas Groundla , Bhattacharyya Sarika , Bagchi Biman
Published: July 2002

In many experimental situations, the interaction potential between the tagged solute and the solvent molecules is often different from that between the two solvent molecules. In such cases, the Stokes–Einstein relation attempts to describe the self-diffusion of the solute in terms of an effective hydrodynamic radius which, along with the hydrodynamic boundary condition (slip or stick), are varied to fit the experimental results. Extensive molecular dynamics (MD) simulations have been carried out to obtain the diffusion coefficient by varying interaction between the solute and the solvent. It is found that when this interaction is more repulsive than that between solvent–solvent, the diffusion can be significantly faster, leading to a complete breakdown of the Stokes–Einstein relation. In the limit of strong attractive interaction, we recover a dynamic version of the solvent–berg picture. The diffusion coefficient of the solute is found to depend strongly and nonlinearly on the magnitude of this specific interaction. The velocity correlation function also shows an interesting dependence on the sign and magnitude of the specific interaction. Another potentially important observation is that the specific solute–solvent interaction can induce a crossover from a sliplike to a stick-like diffusion, if one still uses the hydrodynamic language. Mode coupling theory analysis of the friction shows that the change in it originates largely from the modification of the binary component of the total friction. This is because the cage structure around the solute is modified due to the specific solute–solvent interaction, which directly affects the binary dynamics.
The potential energy surface and rovibrational states of He–HCO+
Authors: Meuwly Markus
Published: July 2002

The potential energy surface for the electronic ground state of He–HCO+ is calculated using ab initio QCISD(T) calculations. The frozen core potential energy surface is corrected using an improved variant of the adiabatical correction scheme. The resulting potentials have well depths of 281 cm−1 for HCO+(v1=0) and 310 cm−1 for HCO+(v1=1). Variational calculations are performed to assess spectroscopic observables. The dissociation energies are D0=170 cm−1 for HCO+ in its ground state, and D0=185 cm−1 in the v1=1 state, and consume an appreciable amount of the well depth. The theoretically determined vibrational red shift of 15.4 cm−1 is close to the experimental value of 12.4 cm−1. It is proposed that the discrepancy originates mainly from the fact that the HCO+ bending is not included in the adiabatic correction.
Spectroscopic study of the C 1Σ+ state of 7LiH
Authors: Chen Jye-Jong , Luh Wei-Tzou , Jeung Gwang-Hi
Published: July 2002

Forty-two vibrational (v=2–43) levels of the 7LiH C 1Σ+ excited electronic state have been observed by a pulsed optical–optical double resonance fluorescence depletion spectroscopic technique. The absolute vibrational numbering of the C 1Σ+ state is identified with the measurements of the isotopic shifts between 7LiH and 6LiH among several rotation-vibration bands of the C 1Σ+−A 1Σ+ electronic system, and it is further demonstrated by the consistency between observed spectral intensities and calculated ones. The highest observed quasi bound rotational level for the v=43 level is J=8. The spectral term values for those vibrational levels lying above v=33 occur in an irregular order. The level of v=34 lies mainly in the inner, tiny well. The term values for the observed rovibrational levels (2⩽v⩽32, 0⩽J⩽11) in the outer ionic well are described by a set of Dunham-type coefficients, with which a Rydberg–Klein–Rees (RKR) potential energy curve is constructed. This RKR potential is combined with an ab initio potential to construct an effective hybrid potential. The calculated electronic transition moment function for the C-A transition is also given.
Nonadiabatic wave packet dynamics of NO2 on the X̃ 2A′/Ã 2A′ conical intersection
Authors: Santoro Fabrizio , Petrongolo Carlo
Published: July 2002

The NO2 optical spectrum and wave packet dynamics are deeply influenced by the conical intersection between the first two adiabatic potentials X̃ 2A′ and Ã 2A′ and by the (ν1,ν2) Fermi resonance of the upper electronic state 2B2. By considering a Franck–Condon excitation, we have expanded the NO2 wave packet in the basis of the nonadiabatic eigenstates and have studied the quantum dynamics on the coupled potentials. We have calculated the time evolution of the survival probability of different initial packets, of the populations of the diabatic and adiabatic electronic states and of the vibrational ones, of the energy distribution among the various degrees of freedom, of the average position, and of the probability density. The packet is mainly created on the upper diabatic or adiabatic potential, jumps three times between the adiabatic states up to about 58 fs, whose populations oscillate with a period close to that of the 2B2 bending mode, but it passes almost monotonically to the ground diabatic state, where it mainly moves for t>83 fs. During the time evolution, the energy is transformed from potential to kinetic and from electronic to vibrational, and flows from the bending to the stretching modes. The packet moves along both the symmetric stretching and bending directions, without splitting along the antisymmetric stretching, and the trajectory thus crosses the intersection seam four times. The dynamics becomes more and more complex and incoherent as time increases: no appreciable motion of the wave packet average position can be seen after 100 fs and all the available phase space is filled at about 200 fs, but the packet still feels the 2B2 Fermi resonance to about 350 fs.
On the high-temperature phase transitions of CsH2PO4: A polymorphic transition? A transition to a superprotonic conducting phase?
Authors: Ortiz E. , Vargas R. A. , Mellander B.-E.
Published: July 2002

X-ray diffraction, thermogravimetric (TGA), differential scanning calorimetric (DSC), and impedance analysis were used to study the reported high-temperature phase transitions at 107, 149, 230, and 256 °C in crystals of cesium dihydrogen phosphate, CsH2PO4 (CDP). Our results show strong evidence that at all these temperatures, the observed DSC or differential thermal analysis (DTA) endothermic effects appear only as a consequence of a dehydration process starting on the surface of the crystal. Our results thus show that the reported transition at 230 °C is not a polymorphic transition. This means that the monoclinic symmetry, stable at room temperature, with space group P21/m–C2k2, is maintained up to the final decomposition. Moreover, since we have not found any evidence for the existence of a superprotonic high-temperature phase above 230 °C, the high conductivity above 230 °C is thus only a consequence of the dehydration of the crystal surface.
Aqueous solvation dynamics studied by photon echo spectroscopy
Authors: Lang M. J. , Jordanides X. J. , Song X. , Fleming G. R.
Published: July 2002

Three-pulse photon echo peak shift measurements were employed to study aqueous solvation dynamics. A new perspective of dielectric continuum theory [X. Song and D. Chandler, J. Chem. Phys. 108, 2594 (1998)] aided in characterizing the system-bath interactions of eosin in water. Application of this theory provides solvation energies, which were used within the spectral density representation ρ(ω), to calculate the experimental peak shift. Simulations with only solvation contributions to ρ(ω), where a substantial amplitude of the solvation occurs within ∼30 fs, are remarkably consistent with our data. Furthermore, simulations using this theoretical solvation spectral density and an experimentally determined intramolecular spectral density yield an excellent total simulation of the peak shift data over the entire dynamic range. Our experimental results substantiate predictions that interaction-induced polarizability effects, contributing via a ∼180 cm−1 band in the spectral density, influence the initial dynamics.
Efficient calculation of matrix elements in low storage filter diagonalization
Authors: Chen Rongqing , Guo Hua
Published: July 2002

Efficient extraction of frequency information from a discrete sequence of time signals can be achieved using the so-called low storage filter diagonalization approach. This is possible because the signal sequence can be considered as a correlation function associated with a quantum Hamiltonian. The eigenvalues of the Hamiltonian (i.e., the frequencies in the signal) in a pre-specified energy range are obtainable from a low-rank generalized eigenequation in a subspace spanned by the filtered states. This work presents an efficient and accurate method to construct the Hamiltonian and overlap matrices directly from correlation functions for several types of propagators. Emphasis is placed on a recurrence relationship between the Hamiltonian and overlap matrices. This method is similar to, but more efficient than, several existing methods. Numerical testing in a triatomic system (HOCl) confirms its accuracy and efficiency.
Observation of linear isomers of the ionized rare gas tetramers Ar4+ and Xe4+
Authors: Issendorff B. v. , Hofmann A. , Haberland H.
Published: July 2002

The ionized rare gas clusters Ar4+ and Xe4+ have been studied by photofragmentation. Center-of-mass velocities of the neutral and ionized photofragments have been measured for photon energies between 1.6 and 4.4 eV. In almost all of the examined photon energy region, both clusters exhibit fragment velocity distributions which show that they consist of a linear ionized trimer with an additional atom loosely attached to its side. For the lowest photon energies, however, fragmentation patterns are observed which can only be explained by a linear tetramer structure. This demonstrates that stable linear isomers of the ionized rare gas tetramers exist.
Giant enhancement of electron-induced dissociation of chlorofluorocarbons coadsorbed with water or ammonia ices: Implications for atmospheric ozone depletion
Authors: Lu Q.-B. , Madey Theodore E.
Published: July 2002

The Cl− yield produced by dissociative electron attachment of a submonolayer of CF2Cl2 is enhanced by factors of 102 and 104 when CF2Cl2 is coadsorbed with water ice and ammonia ice, respectively, on a surface at ∼25 K. Moreover, the magnitude of Cl− enhancement increases strongly with decreasing CF2Cl2 concentration. This enhancement is attributed to dissociation of CF2Cl2 by capture of electrons self-trapped in polar water or ammonia molecules. This process may be an unrecognized sink for chlorofluorocarbons in the atmosphere. Cl− ions produced may be directly or indirectly converted to Cl atoms, which then destroy ozone.
Aqua–potassium(I) complexes: Ab initio study
Authors: Lee Han Myoung , Kim Jongseob , Lee Sik , Mhin Byung Jin , Kim Kwang S.
Published: July 2002

A number of conformers of aqua-K+ complexes, K+(H2O)n (n=1–10) have been investigated using high level ab initio calculations, to elucidate the structures and thermodynamic energies of the hydrated potassium ions. Since the coordination number of K+ is around six in the bulk water, the focus of the present study has been the n=5 and 6 clusters. In contrast to previous studies which have used only the enthalpies to compare against the experimental numbers, the present study also employs free energies. As a result, the predictions of a number of hitherto unknown conformers are in excellent agreement with the experimental results. The maximum coordination number for K+ in ligands containing O atoms is evaluated to be around eight from the energetics of structures possessing only the first hydration shell of water molecules around the K+ ion. It is of interest to note that the hydration of the K+ ion is less structured than that of the Na+ ion, since the water–water interaction becomes more important in the aqua–K+ clusters. The predicted vibrational frequencies of the aqua–K+ clusters reflect the H-bonding signature, and hence, could be utilized in the identification of the hydration structures of K+ in experiments.
Collisional dynamics of Bi2 A(0u+). I. Quantum-resolved vibrational energy transfer for v′=0–4
Authors: Franklin Robert E. , Perram Glen P.
Published: July 2002

Vibrational-to-translational energy transfer between the lowest vibrational levels (v′=0–4) of the A(0u+) state of Bi2 has been investigated using spectrally resolved, laser-induced fluorescence techniques. The small vibrational spacing (ωe′≃132 cm−1) leads to highly nonadiabatic conditions, particularly for the Bi2(A)–He collision pair. However, the Δv=−1 transition probabilities for collisions with the rare gases range from 0.75% to 1.75% per collision, considerably lower than would be anticipated from standard vibrational energy transfer theory. Multiquantum (Δv′=±2) transfer rates are low, consistent with the low anharmonicity of the A(0u+) state. The rates for Δv′=±1 transitions scale linearly with vibrational quantum number as expected near the bottom of this nearly harmonic potential.