bluuebunny/crossref_metadata_2025 huggingface dataset:
Femtosecond real-time probing of reactions. V. The reaction of IHgI
Authors: Dantus M. , Bowman R. M. , Gruebele M. , Zewail A. H.
Published: July 2002

The dissociation reaction of HgI2 is examined experimentally using femtosecond transition-state spectroscopy (FTS). The reaction involves symmetric and antisymmetric coordinates and the transition-state is well-defined: IHgI*→[IHgI]‡*QS′Qa′q→HgI+I. FTS is developed for this class of ABA-type reactions and recurrences are observed for the vibrating fragments (symmetric coordinate) along the reaction coordinate (antisymmetric coordinate). The translational motion is also observed as a ‘‘delay time’’ of the free fragments. Analysis of our FTS results indicates that the reaction wave packet proceeds through two pathways, yielding either I(2P3/2) or I*(2P1/2) as one of the final products. Dissociation into these two pathways leads to HgI fragments with different vibrational energy, resulting in distinct trajectories. Hence, oscillatory behaviors of different periods in the FTS transients are observed depending on the channel probed (∼300 fs to ∼1 ps). These results are analyzed using the standard FTS description, and by classical trajectory calculations performed on model potentials which include the two degrees of freedom of the reaction. Quantum calculations of the expected fluorescence of the fragment are also performed and are in excellent agreement with experiments.
Consistency of capillary wave theory in three dimensions: Divergence of the interface width and agreement with density functional theory
Authors: Weeks John D. , van Saarloos Wim , Bedeaux Dick , Blokhuis Edgar
Published: July 2002

We compare the predictions of the capillary wave model for fluid interfaces with known exact expressions, due to Triezenberg, Zwanzig, and Wertheim, that relate the surface tension to the interfacial structure, and find agreement in all dimensions. It is shown that contrary to the case d<3, the interfacial pair correlation function does not obey scaling in d=3 dimensions on length scales less than the capillary length Lc. Nevertheless, once the sensitive dependence on the gravitational field and the short distance cutoff is properly taken into account, we find no evidence for recent allegations that the capillary wave model leads to inherently inconsistent results for the direct correlation function in d=3. Several issues regarding the connection with density functional theory and the interpretation of the Triezenberg–Zwanzig formula, on which these results touch, are discussed.
Electronic spectra of jet-cooled tropolone. Effect of the vibrational excitation on the proton tunneling dynamics
Authors: Sekiya Hiroshi , Nagashima Yusuke , Nishimura Yukio
Published: July 2002

The laser fluorescence excitation spectra and single vibronic level fluorescence spectra have been measured for jet-cooled tropolone (–OH) and tropolone (–OD). The ν′11(a1), ν12(a1), and ν′19(a2), modes have been newly identified in the excitation spectrum of tropolone (–OH), in addition to the previously observed ν13(a1), ν′14(a1), ν25(b1), and ν′26(b1) modes. The tunneling doublet splittings of these modes in the Ã 1B2 state have been determined to investigate the effect of the vibrational excitation on the proton tunneling dynamics. The tunneling splitting is highly mode specific. The ν13(a1) and ν′14(a1) skeletal deformation modes are strongly coupled to the proton transfer coordinate and enhance tunneling, whereas the ν19(a2) skeletal twisting mode as well as ν′25(b1) and ν26(b1) out-of-plane bending modes diminish tunneling. The ν′11(a1) C–C stretching and ν12(a1) C–C–C bending modes are considered to be weakly coupled to the proton transfer coordinate, because the tunneling splittings of these modes are similar to that of the zero-point level. The hydrogen/deuterium isotope dependence of the tunneling splitting of the vibronic level has been studied. In contrast with the ν′13 mode, the isotope effect on the tunneling splitting is much smaller in the ν14 mode, although the magnitude of the tunneling splitting of ν′14 is very similar to ν13. This implies that both the average O–O distance and the potential energy barrier are significantly different between these modes.
Vibrational dynamics of the bifluoride ion. I. Construction of a model potential surface
Authors: Epa V. C. , Choi J. H. , Klobukowski M. , Thorson W. R.
Published: July 2002

Construction of an extended model potential surface for the bifluoride ion [FHF−] is described, based on ab initio calculations for the free ion at the CID (configuration interaction, double replacement) level with a Huzinaga–Dunning double-zeta basis set. 710 data points were generated, for displacements in the three noncyclic vibrational coordinates exploring the potential surface to a height at least 30 000 cm−1 above its minimum, and giving a realistic account of the dissociation into HF+F−. Analogous calculations were made for HF and F− using the same basis. The predicted hydrogen bond energy (De) is 48.13 kcal/mol, with equilibrium F–F separation Re =4.2905 a.u., in good agreement with other recent calculations. A model potential has been constructed, based on a superposition of Morse potentials associated with each H–F distance plus a fairly structureless correction function expressible as a 36-term least-squares polynomial in the prolate spheroidal coordinates used to describe vibrational displacements. The resulting model surface fits all 710 ab initio data points with an r.m.s. deviation of 65.6 cm−1, and points less than 15 000 cm−1 above the minimum with a deviation of 26.3 cm−1. This surface provides the basis for a series of vibrational dynamics studies on the FHF− system being done in this laboratory.
An estimating formula for ion–atom association rates in gases
Authors: Chatterjee B. K. , Johnsen R.
Published: July 2002

We derive a simple estimating formula for rate coefficients of three-body ion atom association in gases and compare its predictions to experimental data on ion association and three-body radiative charge transfer reactions of singly- and doubly-charged rare-gas ions. The formula appears to reproduce most experimental data quite well. It may be useful for estimating the rates of reactions that have not been studied in the laboratory.
Dielectric relaxation of electrolyte solutions
Authors: Wei Dongqing , Patey G. N.
Published: July 2002

The dielectric relaxation theory of electrolyte solutions is formulated in terms of solvent–solvent, ion–ion, and ion–solvent van Hove time correlation functions. General wave vector frequency-dependent expressions are given for the longitudinal components of the relevant (i.e., polarization–polarization, current–current, current–polarization, polarization–current) time correlation functions and of the susceptibility, dielectric, and conductivity tensors. The Kerr theory relating the distinct and self parts of the van Hove functions is extended to mixtures of molecular fluids and solved explicitly in the k→0 limit for solutions of spherical ions (assuming that the self part of the van Hove functions is given by Fick’s law) immersed in polar solvents. At this level of theory, the van Hove functions, the time correlation functions and the susceptibilities are all found to depend upon coupled ion–solvent motion. However, the dynamical coupling terms are shown to cancel exactly in the final expressions for the conductivity and dielectric constant yielding relatively simple results. Specifically, the conductivity obtained is independent of frequency and is related to the self diffusion constants of the ions by the Nernst–Einstein expression. If a spherical diffusor model is chosen for the solvent, then the frequency-dependent dielectric constant is given by a Debye-type formula with a concentration dependent relationship connecting the Debye and self reorientational relaxation times of the solvent.These results are discussed in the context of previous theories and experimental observations. It is shown that, although obviously oversimplified, the present theory does qualitatively predict the correct concentration dependence of the observed relaxation times for a number of salt solutions.
The chain length dependence of the chemical potentials of macromolecular systems at zero density: Exact calculations and Monte Carlo simulations
Authors: Kumar Sanat K.
Published: July 2002

In a previous paper we illustrated a Monte Carlo method to calculate the incremental chemical potential between polymer chains of length ν and ν+1 at all densities. In this document we present a statistical mechanics model that allows for its quantitative evaluation as a function of chain length and temperature for macromolecular systems at zero density. Through this approach the incremental chemical potential at zero density is shown to be a strong function of chain length at low temperatures, but becomes chain length independent even for short chains (ν≥3) at higher temperatures. The temperature where the chemical potential becomes essentially chain length independent is a definition of the Boyle transition for finite chain length polymers, and we show that this transition temperature has a well defined chain length dependence. Chain dimensions and incremental entropies associated with chain segments are also found to decrease significantly when the temperature is reduced below this θ temperature thus proving that chain segments are localized in the vicinity of other segments in the collapsed state. The consequences of these results on the modeling of the phase equilibria of polymer solutions are also examined.
Memory effects in the relaxation of quantum open systems
Authors: Suárez Alberto , Silbey Robert , Oppenheim Irwin
Published: July 2002

A close examination of the validity of the Markovian approximation in the context of relaxation theory is presented. In particular, we examine the question of positivity of various approximations to the reduced dynamics of an open system in interaction with a heat reservoir. It is shown that the Markovian equations of motion obtained in the weak coupling limit (Redfield equations) are a consistent approximation to the actual reduced dynamics only if supplemented by a slippage in the initial conditions. This slippage captures the effects of the non-Markovian evolution that takes place in a short transient time, of the order of the relaxation time of the isolated bath.
A scaling perspective on quantum energy flow in molecules
Authors: Schofield Sarah A. , Wolynes Peter G.
Published: July 2002

We present an analysis of quantum energy flow and localization in molecules from the point of view of a scaling approach. This scaling approach is based on earlier scaling theories of Anderson localization in disordered metals. The picture provides a simple general framework for describing both energy flow and the localization of eigenstates. This framework can be applied to molecules whose Hamiltonians are described by local random matrices, in which transitions between states nearby in quantum space occur more easily than those between distant states. In this theory the central quantity of interest is the survival probability. It is the squared overlap of a wave function at a later time with its initial condition. Its long time behavior varies depending on whether the eigenstates are localized or not. In the scaling description the survival probability approaches its long time limit varying inversely with time raised to some power. Different power laws are valid in different time regimes, depending on the degree of localization and the size of the molecule’s phase space. Near the localization transition we find that the survival probability decays as t−1. This decay is much slower than when energy flows easily, for a large number of dimensions participating in the energy flow.
Further studies of prewetting transitions via Monte Carlo simulation
Authors: Fan Y. , Monson P. A.
Published: July 2002

We revisit the study of prewetting transitions via Monte Carlo simulation for a single component fluid in contact with a planar solid surface. The principal focus of the present work is a more precise location of a prewetting transition by equating chemical potentials and surface tensions for the states of the adsorbed film in equilibrium at the prewetting transition. The surface tension is calculated from the pressure tensor and also from the Gibbs adsorption isotherm (for states below the prewetting transition in pressure). For the system studied by Finn and Monson [Phys. Rev. A 39, 6402 (1989)], a fluid interacting via a truncated Lennard-Jones 12-6 potential in contact with a 9-3 wall, the new calculations confirm the location of the prewetting transition in that study. We have also considered a similar system in which the fluid molecules interact with a shifted force Lennard-Jones potential. In this case layering transitions are observed in addition to prewetting, and apparently above the bulk triple point temperature. This confirms density functional theory predictions of layering transitions above the triple point temperature. The surface phase diagram also bears some similarity to that found in mean field studies of a lattice gas in contact with a 9-3 wall.
Variational statistical study of the CN+O2 reaction employing ab initio determined properties for the transition state
Authors: Klippenstein Stephen J. , Kim Young-Woong
Published: July 2002

A combination of ab initio electronic structure and variational statistical calculations are employed in a study of the kinetics of the CN+O2 reaction. Interaction energies for the transition state region of the CN+O2 reaction are evaluated within a multiconfiguration self-consistent field framework. Optimized geometries and force fields are determined for six fixed CO separation distances (RCO) ranging from 1.7 to 3.0 Å and for the NCOO complex. The optimized NCO and COO bending angles are generally near 180° and 115°, respectively. A model analytical potential is fit to the ab initio data. This model potential is then used in variational statistical evaluations of the rate of complex formation employing a bond length reaction coordinate. A comparison between theoretical and experimental results indicates the importance of considering the deviations of the electronic interactions from those predicted by long-range expansions. In particular, variational statistical calculations employing a realistic potential energy surface which fully incorporates the short-range interactions are in quantitative agreement with the experimental data for temperatures ranging from 50 to 3000 K.
Using symmetry forbidden interactions to create small band gap polymers: Poly-aminosquaraine and related compounds
Authors: Tol A. J. W.
Published: July 2002

Materials with low band gaps, down to 0.5 eV, can be obtained from polymers containing squaraine or croconaine moieties. The properties of poly-aminosquaraine, a representative of this novel class of materials, and related compounds have been investigated by both semiempirical and ab initio quantum mechanical methods. Formally, poly-aminosquaraine is a donor/acceptor compound. The band gap in these compounds is usually determined by the interaction between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor. If the interaction is weak a small band gap results, but it will also lead to an unfavorably small dispersion of the bands. In poly-aminosquaraine the levels around the gap are well described by a three-level model, involving two acceptor levels and one donor level. Symmetry forbidden interactions allow for a small band gap and a significant dispersion at the same time. In principle, by choosing different donors and acceptors, the band gap can be manipulated. This is illustrated by results on poly-aminocroconaine and poly-amino-(2-propenolate).
Explicit nondivergent formulas for atomic and molecular dynamic hyperpolarizabilities
Authors: Bishop David M.
Published: July 2002

Two methods are used to develop nondivergent, compact, sum-over-states formulas for dynamic hyperpolarizabilities up to fifth order. Expressions for both atoms and molecules are given; those at fourth and fifth order for the first time. The fifth-order formula is related to such nonlinear optical phenomena as stimulated hyper-Raman scattering and the self-induced intensity-dependent refractive index. Questions of time-reversal invariance are also discussed. Importantly, the derivations explicitly take into account secular divergences in a simple and clear way.
Correlated electronic structure models suggested by the large-dimension limit
Authors: Loeser J. G. , Summerfield J. H. , Tan A. L. , Zheng Z.
Published: July 2002

We describe three simple models for electronic structure in many-electron atoms and ions. Each model is parametrized by the spatial dimensionality D, which is ordinarily set to 3. All procedures are motivated by the solution for D → ∞, a classical limit which can be solved exactly either with or without electron correlation. Each model modifies the D → ∞ limit treatment to reflect important finite-D effects, but retains the classical character of the limit; the modifications, in order of increasing realism, are designated post-scaling, pre-scaling, and pre-structuring. The three models yield pointlike electronic structures somewhat reminiscent of pre-quantal atomic models. However, all electronic positions have components outside the D-dimensional physical subspace, a feature which significantly enhances the ability of a localized structure to represent the true solution and to model it quantitatively. Specific calculations reported are total and correlation energies for atoms with Z ≤ 92 (computed by post-scaling and pre-scaling), correlation corrections to ionization potentials and electron affinities for Z ≤ 54 (post-scaling), and the asymptotic behavior of correlation energies for Z → ∞ (post-scaling).
On the lifetimes of Rydberg states probed by delayed pulsed field ionization
Authors: Merkt F. , Zare R. N.
Published: July 2002

We present a simple model to evaluate the degree of l and ml mixing in high Rydberg states that results from perturbations caused by weak, homogeneous dc electric fields and static ions. This model predicts the lifetime of these states qualitatively and explains several seemingly contradictory observations obtained using zero-kinetic-energy (ZEKE) photoelectron spectroscopy. The presence of a small homogeneous dc electric field and a few ions in the sample volume causes ml mixing in general as well as l mixing, both of which contribute to the lengthening of the lifetimes. Consequently, the lifetime lengthening appears to be insensitive to the sample pressure. The effect of the dc electric field on the lifetime is complex. Although the electric field results in l mixing, with increasing field strength it inhibits ml mixing, and, at still higher field strength, induces ionization. The variation of the lifetimes with ion concentration is also complicated. At low ion concentration, the ml mixing varies across the Stark manifold of Rydberg states that belong to the same principal quantum number, so that different states have different lifetimes. At higher ion concentration, l and ml mixing are more uniform, which lengthens the lifetimes and makes them more similar across the Stark manifold. At still higher concentrations, collisional ionization dominates, which shortens the lifetimes.
A multireference coupled-cluster study of the ground state and lowest excited states of cyclobutadiene
Authors: Balková A. , Bartlett Rodney J.
Published: July 2002

The electronic structure of the ground state and several low-lying excited states of cyclobutadiene are studied using the new state-universal multireference coupled-cluster method with single and double excitations (MR-CCSD) augmented by a noniterative inclusion of the triple excitations [MR-CCSD(T)]. Two possible ground state configurations are examined, namely the square and the distorted rectangular geometries, and the multireference coupled-cluster energy barrier for the interconversion between the two rectangular ground state structures is estimated to be 6.6 kcal mol−1 compared with the best theoretical value, 6.4 kcal mol−1 obtained using the highly accurate coupled-cluster method with full inclusion of the triple excitations (CCSDT). The ordering of electronic states for the square geometry is determined, with the ground state singlet being located 6.9 kcal mol−1 below the lowest triplet electronic state. We also examine the potential energy surface for the interconversion between the two equivalent second-order Jahn–Teller rhombic structures for the first excited singlet state. When comparing the MRCC energies with the results provided by various single- and multireference correlation methods, the critical importance of including both the dynamic and nondynamic correlation for a qualitatively correct description of the electronic structure of cyclobutadiene is emphasized. We also address the invariance properties of the present MRCC methods with respect to the alternative selections of reference orbital spaces.
Quantum rates for a double well coupled to a dissipative bath: Accurate path integral results and comparison with approximate theories
Authors: Topaler Maria , Makri Nancy
Published: July 2002

We present accurate fully quantum calculations of thermal rate constants for a symmetric double well system coupled to a dissipative bath. The calculations are performed using the quasiadiabatic propagator path integral (QUAPI) methodology to evaluate the flux–flux correlation function whose time integral determines the rate coefficient. The discretized path integral converges very rapidly in the QUAPI representation, allowing efficient calculation of quantum correlation functions for sufficiently long times. No ad hoc assumption is introduced and thus these calculations yield the true quantum mechanical rate constants. The results presented in the paper demonstrate the applicability of the QUAPI methodology to practically all regimes of chemical interest, from thermal activation to deep tunneling, and the quantum transmission factor exhibits a Kramers turnover. Our calculations reveal an unusual step structure of the integrated reactive flux in the weak friction regime as well as quantum dynamical enhancement of the rate above the quantum transition state theory value at low temperatures, which is largely due to vibrational coherence effects. The quantum rates are compared to those obtained from classical trajectory simulations. We also use the numerically exact classical and quantum results to establish the degree of accuracy of several analytic and numerical approximations, including classical and quantum Grote–Hynes theories, semiclassical transition state theory (periodic orbit) estimates, classical and quantum turnover theories, and the centroid density approximation.
Effects of vibrational excitation on multidimensional tunneling: General study and proton tunneling in tropolone
Authors: Takada Shoji , Nakamura Hiroki
Published: July 2002

Tunneling energy splittings of vibrationally excited states are calculated quantum mechanically using several models of two-dimensional symmetric double well potentials. Various effects of vibrational excitation on tunneling are found to appear, depending on the topography of potential energy surface; the symmetry of the mode coupling plays an essential role. Especially, oscillation of tunneling splitting with respect to vibrational quantum number can occur and is interpreted by a clear physical picture based on the semiclassical theory formulated recently [Takada and Nakamura, J. Chem. Phys. 100, 98 (1994)]. The mixed tunneling in the C region found there allows the wave functions to have nodal lines in classically inaccessible region and can cause the suppression of the tunneling. The above analysis is followed by the interpretation of recent experiments of proton tunneling in tropolone. Ab initio molecular orbital calculations are carried out for the electronically ground state. A simple three-dimensional model potential is constructed and employed to analyze the proton tunneling dynamics. Some of the experimentally observed intriguing features can be explained by the typical mechanisms discussed above.
A neutron scattering study of liquid D2O under pressure and at various temperatures
Authors: Bellissent-Funel M.-C. , Bosio L.
Published: July 2002

A structural investigation of liquid D2O was performed by neutron scattering at pressures up to 6 kbar and in a range of temperatures from 53 down to −65 °C. Some comparison is made with Monte Carlo (MC) and molecular dynamics (MD) simulations. In particular, the results are used to test some recent MD simulations leading to a phase diagram which provides a complete picture of the stable and metastable behavior of water and incorporates the two amorphous ices of water.
Diffusion as a chemical reaction: Stochastic trajectories between fixed concentrations
Authors: Eisenberg R. S. , Kl/osek M. M. , Schuss Z.
Published: July 2002

Stochastic trajectories are described that underly classical diffusion between known concentrations. The description of those experimental boundary conditions requires a phase space using the full Langevin equation, with displacement and velocity as state variables, even if friction entirely dominates the dynamics of diffusion, because the incoming and outgoing trajectories have to be told apart. The conditional flux, probabilities, mean first-passage times, and contents (of the reaction region) of the four types of trajectories—the trans trajectories LR and RL and the cis trajectories LL and RR—are expressed in terms of solutions of the Fokker–Planck equation in phase space and are explicitly calculated in the Smoluchowski limit of high friction. With these results, diffusion in a region between fixed concentrations can be described exactly as a chemical reaction for any potential function in the region, made of any combination of high or low barriers or wells.
Dielectric spectroscopy of a polymerizing liquid and the evolution of molecular dynamics with increase in the number of covalent bonds
Authors: Parthun M. G. , Johari G. P.
Published: July 2002

Dielectric spectroscopy and calorimetry studies of a low viscosity, initially monomeric liquid undergoing spontaneous chemical reaction, to form a linear chain polymer while maintaining isothermal conditions, have been used to determine how the number of covalent bonds formed during the growth of a linear chain affects the dielectric permittivity, relaxation time, and the spectral shape. During this reaction, the static permittivity decreased and the relaxation time increased towards limiting values. As the number of covalent bonds increased towards the Avogadro number, the change in the complex permittivity as measured for a fixed frequency was phenomenologically similar to that observed on varying the frequency, although the exact formalisms in both cases differed. In both cases the relaxation function could be well described by a stretched exponential or sum of exponentials, with a width that decreased as the liquid’s state changed from monomeric liquid to a fully reacted chain polymer. The observed increase in the relaxation time with the number of bonds formed seems consistent with the decrease in the configurational entropy or the number of accessible configurations available to the structure, under isothermal conditions. It decreases progressively more slowly as the number of covalent bonds in the structure increases. As this occurs, a second relaxation process at higher frequencies is revealed. The dielectric manifestation of the irreversible process of covalent bond formation is remarkably similar to that observed on supercooling a molecular or polymeric liquid. The study demonstrates how negative feedback between molecular diffusion and chemical reaction vitrifies a liquid isothermally.
Photofragment translational spectroscopy of IBr at 304 nm: Polarization dependence and dissociation dynamics
Authors: Jung Kwang-Woo , Griffiths Jennifer A. , El-Sayed Mostafa A.
Published: July 2002

The photodissociation dynamics of IBr has been studied at 304 nm by state-selective photofragment translational spectroscopy. Velocity distributions, anisotropy parameters, and relative quantum yields are obtained for the ground I(2P3/2) and spin–orbit excited state I*(2P1/2) iodine atoms, which are produced from photodissociation of IBr at this wavelength. Two sharp velocity distributions observed for the I channel suggest the two dissociation pathways that correlate with ground-state iodine formation. Based on the expected translational energy release and the energy separation between the peaks, the two distributions have been assigned to dissociation of IBr to form I(2P3/2)+Br(2P3/2) and I(2P3/2)+Br*(2P1/2) with the former channel appearing at higher translational energy. The I* distribution shows one strong peak indicating that there is one dominant channel for formation of I* atoms at this wavelength which has been assigned to dissociation of IBr to form I*(2P1/2)+Br(2P3/2) with a quantum yield of 0.1. The I* signal formed from the I*(2P1/2)+Br*(2P1/2) dissociation channel is observed very weakly.

The observed anisotropy parameter indicates that the I+Br* product (β=−0.7) is formed mainly from the perpendicular 1Π1(2341)←X transition while the I*+Br channel (β=1.8) is formed predominantly from the parallel 3Π0+(2341)←X transition followed by curve crossing to the 3Σ0+−(2422) state. The recoil energy dependence of the anisotropy parameter in the I atom produced in the I+Br channel shows a positive β value above maximum of the peak recoil energy and a negative value below the peak maximum of the recoil energy distribution. These results are interpreted in terms of the presence of more than one path for the formation of I+Br photoproduct with opposite polarization for their absorbing transitions, most likely the 3Π0+(2341)←X and the 3Π1(2341)←X transitions. The possible excited state dynamics which give the observed results are discussed in terms of the previously proposed potential energy diagrams for IBr and ICl.
High-pressure range of the addition of HO to HO. III. Saturated laser-induced fluorescence measurements between 200 and 700 K
Authors: Fulle D. , Hamann H. F. , Hippler H. , Troe J.
Published: July 2002

The addition of HO to HO was studied by saturated laser induced fluorescence at temperatures between 200 and 700 K and at pressures of the bath gas helium up to 100 bar. In combination with earlier measurements at 298 K, a set of falloff curves is constructed for the given temperature range. The limiting high-pressure rate constant for the reaction HO+HO(+He)→H2O2(+He) follows as k1,∞=(2.6±0.8)×10−11 (T/300 K)0±0.5 cm3 molecule−1 s−1, practically independent of the temperature between 200 and 400 K. At higher temperatures, k1,∞ decreases. These results serve as a reference for statistical adiabatic channel model calculations of the recombination rate.
A theory of vibrational energy relaxation in liquids
Authors: Egorov S. A. , Skinner J. L.
Published: July 2002

A microscopic statistical mechanical theory of the vibrational energy relaxation of a diatomic solute in an atomic solvent is presented. The diatomic is treated as a breathing Lennard-Jones sphere. The relaxation rate is obtained from the Fourier transform of the force–force time-correlation function. The latter is expanded in powers of time (up to t4), and expressions for the expansion coefficients are derived using equilibrium statistical mechanics. These coefficients are used to determine the parameters of an analytic ansatz for this correlation function, which can be evaluated at all times (and thus can be Fourier transformed). The resulting theory for the time-correlation function is compared to numerical results from a molecular dynamics simulation. Theoretical results for the vibrational relaxation rate are compared to experiments on I2 in Xe over a wide range of densities and temperatures.
Effect of location of energy barrier on the product alignment of reaction A+BC
Authors: Han Ke-Li , He Guo-Zhong , Lou Nan-Quan
Published: July 2002

The trajectory calculations of heavy heavy–light, light light–light, heavy light–light, and light heavy–light mass combination reactions on attractive and repulsive potential surfaces have been carried out to study the dependence of the product rotational alignment on collision energies. The calculated results for heavy heavy–light mass combination reaction are compared with the predictions from the constrained product orbital angular momentum model. The final rotational angular momentum was found to be perpendicularly polarized with respect to the reagents’ relative velocity vector on either attractive or repulsive potential surface. There is similar behavior of the product rotational alignments as a function of collision energies for heavy heavy–light and heavy light–light reactions, i.e., the more anisotropic the distribution of the product rotational angular momentum vector is, the higher the collision energies are, whether the potential surface is attractive or repulsive. However, the calculations for light light–light mass combination reaction predict that the product rotational alignments depend strongly on collision energies for an attractive potential surface, but this behavior is not observed on the repulsive potential surface. For light heavy–light mass combination, the product rotational alignments hardly depend on collision energies on both potential surfaces.
A study of conical intersection effects on scattering processes: The validity of adiabatic single-surface approximations within a quasi-Jahn–Teller model
Authors: Baer Roi , Charutz David M. , Kosloff Ronnie , Baer Michael
Published: July 2002

Conical intersections between Born–Oppenheimer potential energy surfaces create singularities which are known to have a direct effect on the symmetry of the nuclear wave functions. In this article is presented a quasi-Jahn–Teller model to study the symmetry effects of these singularities on nonreactive and reactive scattering processes. Applying this model, we were able to determine in what way and to what extent the conical intersection affects the relevant S-matrix elements. Having the results of this study available, conclusions concerning more realistic systems were derived.
Comparison of the perturbative convergence with multireference Möller–Plesset, Epstein–Nesbet, forced degenerate and optimized zeroth order partitionings: The excited BeH2 surface
Authors: Chaudhuri Rajat K. , Finley James P. , Freed Karl F.
Published: July 2002

High order perturbation energies are computed for excited 1A1 states of BeH2 at geometries near the Be→H2 symmetric insertion transition state. The equations of multireference perturbation theory are solved through 30th order to study the difficulties in selecting the appropriate zeroth order Hamiltonian, orbitals, orbital energies, and reference functions for the computations of smooth molecular potential energy surfaces. The origin of the perturbative divergence produced by Möller–Plesset and Epstein–Nesbet partitionings is analyzed using a conceptually simple two-state model constructed using one state each from the reference and orthogonal spaces. The optimized zeroth order partitioning scheme (OPT) for double reference space computations with configurations 1a122a123a12 and 1a122a121b22 produces a truly convergent perturbation expansion through 30th order. The OPT energies are accurate in low orders as compared to the exact (197 dimensional) solution within the basis. The forced valence orbital degeneracy partitioning method (FD) also generates a truly convergent expansion for the same double reference space calculation, with slightly poorer low order energies than the OPT scheme. The BeH2 system facilitates the consideration of larger reference spaces (constructed using three through six orbitals) where the FD method produces highly accurate energies in low orders despite the asymptotic nature of the FD perturbation expansion. The “delayed’’ perturbative divergence behavior with the FD partitioning scheme (for large reference spaces) is shown to occur due to the incorrect ordering between the zeroth order energies of some reference and complementary space levels.
Electronic states and nature of bonding in the molecule MoC by all electron ab initio calculations
Authors: Shim Irene , Gingerich Karl A.
Published: July 2002

In the present work all electron ab initio multiconfiguration self-consistent-field (CASSCF) and multireference configuration interaction (MRCI) calculations have been carried out to determine the low-lying electronic states of the molecule MoC. The relativistic corrections for the one electron Darwin contact term and the relativistic mass-velocity correction have been determined in perturbation calculations. The electronic ground state is predicted as Σ3−. The spectroscopic constants for the Σ3− electronic ground state and eight low-lying excited states have been derived by solving the Schrödinger equation for the nuclear motion numerically. Based on the results of the CASSCF calculations the Σ3− ground state of MoC is separated from the excited states Δ3,Σ5−,Γ3,Δ1,Π5,Σ1+ and Π3 by transition energies of 4500, 6178, 7207, 9312, 10 228, 11 639, and 16 864 cm−1, respectively. The transition energy between the Σ3− ground state and the Π3 state as derived in the MRCI calculations is 15 484 cm−1. For the Σ3− ground state the equilibrium distance has been determined as 1.688 Å, and the vibrational frequency as 997 cm−1. The chemical bond in the Σ3− electronic ground state has triple bond character due to the formation of delocalized bonding π and σ orbitals. The chemical bond in the MoC molecule is polar with charge transfer from Mo to C, giving rise to a dipole moment of 6.15 D at 3.15 a.u. in the Σ3− ground state.
A grid representation for spherical angles: Decoupling of the angular momentum operator
Authors: Dai Jiqiong , Light John C.
Published: July 2002

The angular momentum operator which is a function of the orientational angle θ and the azimuthal angle φ may be split into the φ-dependent and φ-independent parts so that the split exponential operator method can be exactly implemented (with orthogonal transformations) in a direct product discrete variable representation of θ and φ. Although one loses the exact representation of the angular momentum in the spherical harmonic basis, the direct product representations have been proved to converge and to be stable and efficient. An advantage is that computational time for a wave-packet propagation (for a matrix-vector product) is reduced for split exponential propagators since a direct product representation is preserved for all the angles.
State to state recoil anisotropies in the photodissociation of deuterated ammonia
Authors: Mordaunt David H. , Ashfold Michael N. R. , Dixon Richard N.
Published: July 2002

The near ultraviolet photodissociation of deuterated ammonia, ND3, allows particularly clear observation and quantification of the quantum state dependent angular anisotropy of the recoiling D+ND2(X̃) photoproducts. The recoil anisotropy is shown to depend upon five quantum numbers: The rotational quantum numbers of the parent molecule selected in the absorption process, and the rotational and vibrational quantum numbers of the resulting products.
Assessment of the single-root multireference Brillouin–Wigner coupled- cluster method: Test calculations on CH2, SiH2, and twisted ethylene
Authors: Pittner Jiřı́ , Nachtigall Petr , Čársky Petr , Mášik Jozef , Hubač Ivan
Published: July 2002

Recently developed single-root multireference Brillouin–Wigner coupled-cluster (MR BWCC) theory, which belongs to a broad family of state-selective multireference coupled-cluster methods, has been implemented in the ACES II program package at the CCSD level of approximation. The method represents a new approach to quasidegenerate problems, which is able to continuously switch between the single-reference CC in a nondegenerate situation and the Hilbert-space MRCC in a degenerate case. An assessment of the method has been carried out by means of a comparison with the full configuration interaction (CI) treatments of CH2, SiH2, and twisted ethylene diradicals. The problem of size-extensivity is discussed.
Product rotational polarization: Stereodynamics of the reaction Cl(2P3/2)+CD4(v=0,j=0)→DCl(v′=0,j′=1)+CD3
Authors: Zhang Li , Chen Mao-Du , Wang Ming-Liang , Han Ke-Li
Published: July 2002

The angular momentum polarization of the products of the reaction Cl(2P3/2)+CD4(v=0,j=0)→DCl(v′=0,j′=1)+CD3 is calculated via the quasiclassical trajectory method (QCT) based on extended London–Eyring–Polanyi–Sato (LEPS) potential energy surface (PES) at a collision energy of 0.28 eV (6.46 kcal/mol). In the stationary target frame (STF), the rotational alignment factor A0(2)stf has been calculated. In the meantime, we also present four polarization dependent “generalized” differential cross sections (PDDCS) (2π/σ)(dσ00/dωt), (2π/σ)(dσ20/dωt), (2π/σ)(dσ22+/dωt), and (2π/σ)(dσ21−/dωt) in the center of mass frame. Furthermore, the distribution of dihedral angle P(φr), the distribution of angle between k′ and J′, P(θtr), and the angular distribution of product rotational vectors in the form of polar plots in θr and φr are calculated as well. The calculated results are in good agreement with the experimental data.
Quantum state specific dynamics for the O(1D)+HD→OD+H reaction
Authors: Liu Xianghong , Lin Jim J. , Harich Steven A. , Yang Xueming
Published: July 2002

Rydberg “tagging” time-of-flight (TOF) techniques has been applied to the investigation of the O(1D)+HD→OD+H reaction with very high translational energy resolution and sensitivity. High resolution TOF spectra for the H atom product at different laboratory angles have been measured for the first time, making the determination of the OD product quantum state distributions at different scattering angles possible. Preliminary experimental results show that different vibrationally excited OD products at low rotational levels have strikingly different angular distributions, indicating that intriguing dynamics are involved in this reaction. The experimental results in this work also provide a solid test ground for quantitative theoretical investigations of this benchmark system for insertion mechanism.
6D vibrational quantum dynamics: Generalized coordinate discrete variable representation and (a)diabatic contraction
Authors: Luckhaus David
Published: July 2002

A new discrete variable representation (DVR) in generalized vibrational coordinates is proposed together with a new mixed diabatic/adiabatic contraction technique for the treatment of multidimensional vibrational problems up to high vibrational excitations. Formally based on the equidistant Chebyshev DVR in the grid index the new formulation is particularly suitable for multidimensional minimum energy paths. The new Z-matrix DVR proposed in this paper encompasses usual valence coordinates as well as nonlinear maps of coordinates on optimal nonequidistant grids. The pointwise numerical calculation of all kinetic energy terms avoids the algebraic derivation of specialized analytical forms of the kinetic energy adding to the flexibility of the method. With efficient truncation schemes the generalized DVR allows for a compact representation of the time-dependent wave-packet dynamics in up to six dimensions. Vibrationally adiabatic approaches to the detailed modelling of multidimensional quantum-dynamics usually are hampered by the typically large number of (avoided) crossings in dense spectra. This problem is particularly severe for discrete variable representations. A solution is provided by the new technique of diabatic rotations leading to a systematic construction of locally diabatic channels. This allows the treatment of very dense spectra where conventional truncation techniques fail. Applying the new approach to the vibrational problem of tetratomic molecules demonstrates its flexibility and efficiency. The examples of formaldehyde, ammonia, and hydrogen peroxide cover the whole range from semirigid (CH2O) to large amplitude inversion (NH3) and torsional tunnelling dynamics (H2O2). In solving the full six-dimensional vibrational eigenvalue problems for CH2O and NH3 the Z-matrix DVR shows at least comparable if not superior numerical efficiency compared with specialized techniques. In the case of H2O2 the technique of diabatic rotations and adiabatic contraction for the first time allows the treatment of the tunneling dynamics significantly above the dissociation threshold up to the fifth OH stretch overtone. The calculated decrease of the tunneling rate by about one order of magnitude agrees well with experimental observations.
Gauge transformation and the higher order Korteweg–de Vries equation
Authors: Zheng Yu-kun , Chan W. L.
Published: July 2002

To the family of higher order Korteweg–de Vries (KdV) equations a new family of higher order mKdV equations depending on a parameter η is constructed. They are connected by an η2-dependent Miura transformation. A Bäcklund transformation is also established. Furthermore, a new gauge transformation is associated to this Bäcklund transformation. It enables one to derive auto-Bäcklund transformations for the families of higher order η2-mKdV and KdV equations, respectively. For illustration, four generations of explicit solutions of the second order KdV equation are presented.
Superfield and matrix realizations of highest weight representations for osp(m/2n)
Authors: Le Blanc R. , Rowe D. J.
Published: July 2002

A differential representation of the classical Lie superalgebra osp(m/2n) acting on superfield functions is given. This representation is used to construct matrix representations of the finite-dimensional irreducible representations of the algebra. Inner products on the irreducible spaces are discussed and classes of star and grade-star equivalent representations are identified.
Vector-coherent-state theory as a theory of induced representations
Authors: Rowe D. J. , Repka J.
Published: July 2002

A general definition is given of vector-coherent state (VCS) representation theory. It is shown that the theory is more general than suggested by previous applications and that it incorporates the standard theories of induced representations as special cases. The associated K-matrix theory is also given a fuller treatment than hitherto and shown to provide a rather general algorithm both for projecting VCS representations from larger representations in which they are embedded and for determining the Hermitian form, with respect to which an isometric-equivalent representation is, in fact, isometric.
Infrared singular fields and nonregular representations of canonical commutation relation algebras
Authors: Acerbi F. , Morchio G. , Strocchi F.
Published: July 2002

Infrared singular variables which often enter in the formulation of models in quantum field theory, many-body theory, and quantum statistical mechanics are described in terms of nonregular representations of canonical commutation relations algebras. General properties of such representations are discussed. In this way one also obtains insight into the construction and representation of charged field algebras 𝒜 as canonical extensions of a neutral (observable) algebra 𝒜0. The charged state representations of 𝒜0 are obtained through nonregular vacuum representations of the extended field algebra 𝒜. The above structures are proven to work in several explicit models.
Correlation properties of quantum measurements
Authors: Busch Paul , Lahti Pekka J.
Published: July 2002

The kind of information provided by a measurement is determined in terms of the correlation established between observables of the apparatus and the measured system. Using the framework of quantum measurement theory, necessary and sufficient conditions for a measurement interaction to produce strong correlations are given and are found to be related to properties of the final object and apparatus states. These general results are illustrated with reference to the standard model of the quantum theory measurement.