bluuebunny/crossref_metadata_2025 huggingface dataset:
The direct observation, assignment, and partial deperturbation of ν5 and ν3+ν5 in Ã 1A
u acetylene (C2H2)
Authors: Tobiason J. D. , Utz A. L. , Crim F. F.
Published: July 2002

A pulsed-laser double resonance technique (vibrational overtone excitation combined with laser-induced fluorescence detection) provides previously unavailable spectroscopic data on the rovibrational structure of Ã 1Au acetylene (C2H2). We collect fluorescence excitation spectra of transitions to vibronic levels lying between 2800 and 4300 cm−1 above the Ã state origin. In this region, we observe only two vibronic levels that are relatively unperturbed, which we assign to the Ã state antisymmetric C–H stretching fundamental vibration ν′5 and its combination with the trans-bending vibration, ν′3 + ν′5. Parity and symmetry selection rules for the Ã←X̃ band, ab initio predictions for the ν′5 fundamental frequency, and the known frequencies of other Ã state vibrations permit an unambiguous assignment of the vibrations. The fit of ν′5 and ν′3 + ν′5 to a near-prolate asymmetric top Hamiltonian yields the observed vibrational frequencies (ν′5= 2857.4 ± 0.2 cm−1 and ν′3 + ν′5 = 3894.4 ± 0.1 cm−1) and rotational and centrifugal distortion constants. We deperturb the first-order a-axis Coriolis interactions of remote perturbers with ν′5 and ν′3 + ν′5 to obtain partially deperturbed a-axis rotational constants. We discuss other weak perturbations and present an energy level diagram for the vibrational states with ungerade symmetry in Ã state acetylene.
Collisional efficiencies for vibrational energy relaxation of C6F14 and C8F18: Dependence on deactivator mass
Authors: Tardy D. C.
Published: July 2002

Rate coefficients, kVT, for the deactivation of vibrationally excited C6F14 and C8F18 with average excitation energies ranging from 5000 to 30 000 cm−1 were measured via time resolved optoacoustics for a series of 13 deactivators. Relative collision efficiencies (β=kVT/kHS=〈〈ΔE〉〉/〈E〉), an intrinsic measure of the deactivator, were calculated. The average energy removed per collision, 〈〈ΔE〉〉, was linear with 〈E〉 since β was found to be independent of energy for all deactivator-substrate combinations. The trends for the deactivators with the 6 and 8 carbon atom substrates were similar except that β was ∼15% smaller for the larger substrate. The efficiency for the rare gases increases from helium to neon to argon and then decreases to krypton followed by a slight leveling off for xenon. This trend with deactivator mass was also observed for the polyatomic deactivators studied. The rapid rise and slow fall in β vs mass can be simulated by an encounter between the deactivator and a diatomic substrate containing a pseudo atom with a mass of ∼30 amu.
Microscopic theory of dielectric response for molecular multilayers
Authors: Munn R. W.
Published: July 2002

Microscopic molecular theory is used to derive detailed algebraic results for the linear dielectric response of a multilayer assembly of molecules in terms of the responses of the individual layers. Initially, the layers are assumed not to interact with one another. The extension to nonlinear response is outlined. Each layer’s contribution to the overall response is weighted by its volume fraction and by factors giving its relative contribution to the macroscopic electric field. The results are consistent with those derived previously for an assembly of layers characterized by their permittivity tensors. A full treatment of the linear response of interacting layers is then presented; the overall response is now an average, again weighted by volume fraction, over the mean responses in individual sets of identical layers, including interactions across the interfaces to other layers.
The vibrational predissociation of cis-methyl nitrite in the S1 state: A comparison of exact quantum mechanical wave packet calculations with classical trajectory calculations and detailed experimental results
Authors: Untch Agathe , Schinke Reinhard , Cotting René , Huber J. Robert
Published: July 2002

We present quantum mechanical wave packet calculations for the vibrational predissociation of cis-CH3ONO in the S1 state including three degrees of freedom—the CH3O–NO dissociation bond, the N=O stretching coordinate, and the CH3O–N–O bending angle. We calculate the autocorrelation function, the absorption spectrum, the lifetimes of the excited complex as a function of the internal excitation, and the final vibrational-rotational state distributions of the NO fragment. The lifetimes and the product state distributions are compared with experimental data as well as with previous results obtained from classical trajectory calculations. The calculated vibrational state distributions of the NO product satisfactorily reproduce the systematic variation with the initially prepared quasibound state of the CH3ONO(S1) complex found experimentally; however, they are considerably narrower than the experimental distributions. The theoretical rotational state distributions of NO, all being highly inverted and having the overall shape of a Gaussian, agree well with the experimental data; this is the case for several quasibound vibrational states of CH3ONO(S1) as well as several final vibrational states of the NO product. In general, the classical trajectory calculations parallel the quantum mechanical results. The existing differences have to be attributed to the inability of the purely classical treatment in reproducing subtle quantum effects if the dissociation proceeds through a relatively long-lived complex. While the calculations yield satisfactory agreement with the experimental NO state distributions including the envelope of the absorption spectrum, they disagree with the experiment in that the resonance widths are about one order of magnitude narrower than in the measured spectrum. Additional calculations for which the torsional angle of NO with respect to the intermolecular dissociation vector R is approximately taken into account as a fourth coordinate reveals that dephasing by out-of-plane motion can explain most of this discrepancy.
Ultrafast vibrational dynamics of nascent diiodide fragments studied by femtosecond transient resonance impulsive stimulated Raman scattering
Authors: Banin Uri , Ruhman Sanford
Published: July 2002

Impulsive stimulated resonance Raman scattering performed with femtosecond pulses comprises an all time domain Raman technique providing vibrational spectra of the ground state chromophore. We report the first application of this technique to record Raman free induction decays of nascent diiodide ions, undergoing rapid vibrational relaxation following triiodide photodissociation in ethanol solution.
Kinetics of concentration fluctuations and spinodal decomposition in star/star and star/linear polymer blends
Authors: Clarke N. , Mcleish T. C. B.
Published: July 2002

Using the tube model we calculate the relaxation dynamics of density fluctuations in a concentrated solution of star polymers, in which the stars are assumed to relax by arm retraction. We find the surprising result that, in the long wavelength limit, the dynamics have a simple power law dependence on the arm length unlike the exponential dependence that one usually associates with star polymers. Consequently, the contribution of branch point diffusion to relaxation dynamics is negligible compared to the effect of arm retraction. We then apply our results to early time phase separation kinetics in star/linear and star/star blends, using the Cahn–Hilliard–Cook theory. This calculation requires a rederivation of Brochard’s theory for the mutual diffusion coefficient, in terms of polymer fluxes instead of polymer velocities. Following Brochard we assume incompressibility and use the concept of a tube velocity which is common to all polymers. We pay particular attention to the dependence of the growth rate of the fastest growing wavelength fluctuation on the molecular weight of the polymers.
Time-resolved nonpolar solvation dynamics in supercooled and low viscosity n-butylbenzene
Authors: Fourkas John T. , Benigno Andrea , Berg Mark
Published: July 2002

We have measured the time-resolved dynamics of the structural solvation of dimethyl-s-tetrazine in n-butylbenzene, a completely nonpolar system. A combination of transient hole burning and time-resolved fluorescence has been used to measure Stokes shift dynamics from 155–250 K, spanning a viscosity range of 6.6×105–2.5 cP. The decays have a nonexponential shape that is well described by a stretched exponential with β∼0.5. The time constants for solvation are equal to the shear relaxation times derived from viscosity and ultrasound measurements, suggesting that mechanical relaxation of the solvent is of prime importance in the structural solvation of nonpolar electronic states. A potential correlation with solute rotational dynamics is argued to be less plausible, based on the small size of the implied hydrodynamic volume.
An isotropic intermolecular potential for sulfur hexafluoride based on the collision-induced light scattering spectrum, viscosity, and virial coefficient data
Authors: Meinander N.
Published: July 2002

Recent effective intermolecular potentials of sulphur hexafluoride from the literature have been evaluated using the interaction-induced light scattering spectrum, viscosity, and pressure virial coefficient data. It was found that none of the potentials reproduce all three sets of experimental data. A Hartree–Fock-dispersion-type isotropic potential is proposed that reproduces all the experimental data reasonably well.
The algebra of effective Hamiltonians and operators: Truncated operators and computational aspects
Authors: Hurtubise Vincent , Freed Karl F.
Published: July 2002

We extend to finite orders of perturbation theory our previous analysis of effective Hamiltonians h and effective operators a which produce, respectively, exact energies and matrix elements of a time-independent operator A for a finite number of eigenstates of a time-independent Hamiltonian H. The validity of various properties is examined here for perturbatively truncated h and a, particularly, the preservation upon transformation to effective operators of commutation relations involving H and/or constants of the motion, of symmetries, and of the equivalence between dipole length and velocity transition moments. We compare formal and computational features of all a definitions and of the more limited Hellmann–Feynman theorem based ‘‘effective operators,’’ which provide only diagonal matrix elements of A in special cases. Norm-preserving transformations to effective operators are found to yield a simpler effective operator formalism from both formal and computational viewpoints.
Investigation of the gas-phase B̃–X̃ electronic spectra of CH–Ar by laser-induced fluorescence
Authors: Lemire G. W. , McQuaid M. J. , Kotlar A. J. , Sausa R. C.
Published: July 2002

Gas-phase methyidyne–argon (CH–Ar) van der Waals complexes have been detected spectroscopically by laser-induced fluorescence (LIF) in the region of the CH B 2Σ−–X 2Πr (0,0) and (1,0) bands near 363.5 and 388.9 nm, respectively. They are formed by a supersonic free-jet expansion of argon gas seeded with CH radicals generated from the 248 nm photolysis of CHBr2Cl. The excitation spectra reveal a number of rovibronic bands which are assigned to various stretching and/or bending motions of the CH–Ar complex. From the excitation spectra, lower limits for the ground and exited state binding energies are estimated. Rotational analysis based on combination differences and computer simulations of eight of the rovibronic bands yields an average ground state value of B″av = 0.174 ± 0.004 cm−1 and excited state constants ranging from B′=0.086–0.116 cm−1. This indicates that the CH–Ar van der Waals bond is lengthened considerably upon electronic excitation. A splitting of the ground state rotational energy levels, related to the anisotropy of the intermolecular potential and Coriolis coupling, is also observed. Based on an analysis of the rovibronic structure of the CH–Ar bands in terms of a hindered internal rotational model describing the interaction of Ar(1S0) atom with a CH monomer, a linear equilibrium geometry is inferred for the excited state and a ‘‘T’’ shaped geometry for the ground state. These results are compared to those obtained from gas-phase LIF studies of OH–Ar complexes recorded near the OH A 2Σ+–X 2Πi (0,0) and (1,0) bands.
Exchange energy of H2 calculated by the surface integral method in zeroth order approximation
Authors: Tang K. T. , Toennies J. Peter , Yiu C. L.
Published: July 2002

The surface integral method of Holstein and Herring is applied to the calculation of the exchange energy of the H2 molecule. This theory provides a means for calculating the exchange energy by taking into account the physical exchange of the two electrons with respect to the nuclei. Problems associated with symmetrization of the polarized wave functions which have encumbered previous attempts at developing a perturbation theory of the chemical bond are circumvented. Whereas the previous calculations using this method by Gor’kov and Pitaevski and Herring and Flicker were restricted to the asymptotic (R→∞) limit, in the present calculation we have used an extended formula to examine the validity also for the short range region down to R=0.5 a.u. In order to compare with the results of Heitler–London theory we have used the undisturbed zeroth order wave function of the H atoms in the calculations. An analytic expression is obtained for the exchange energy and the numerical results are found to be in good agreement with the exact calculations of Kolos and Wolnicwicz with an error of less than 20% for R≤8 a.u., where the zeroth order approximation is best justified. The agreement with the exact results is comparable to that of the original Heitler–London theory. The good agreement indicates that the basic assumption of the theory that electrons exchange places with respect to the nuclei is on a good physical basis and refutes previous textbook wisdom that the exchange energy cannot be given a simple physical interpretation.
Temperature dependence of dielectric relaxation in H2O and D2O ice. A dissipative quantum tunneling approach
Authors: Bruni Fabio , Consolini Giuseppe , Careri Giorgio
Published: July 2002

We have measured the dielectric relaxation time of orientational defects for several H2O and D2O polycrystalline ice samples, in the temperature range 200–270 K, and over the frequency range 0.3–1000 kHz. Results are in good agreement with previous studies, and at T<240 K, departures from the familiar Arrhenius law have been observed. We show that these deviations from classical rate theory can be well described within the framework of dissipative quantum tunneling (DQT) theory, assuming impurity-generated Bjerrum defects responsible for the observed dielectric relaxation process over the entire temperature range investigated. The temperature regions where quantum tunneling, crossover to thermal hopping, and quantum corrections to classical laws, respectively, prevail are clearly identified, and experimental data have been successfully fitted with theoretical predictions. Particularly significant is the perfect agreement, near the crossover temperature Tc, of all our different samples with a universal scaling law, as predicted by DQT theory, and here experimentally verified for the first time. The crossover temperature Tc, where quantum tunneling and thermal hopping merge, has been found close to 240 and to 220 K for H2O and D2O ices, respectively, thus showing the higher relevance of quantum effects in H2O ice, as expected. It is shown that the dielectric relaxation time of orientational defects for both H2O and D2O ice samples never attains a fully classical behavior, even at their melting temperature. Implications of these findings for the mechanism of migration for orientational defects associated with impurities and water interstitials in ice physics are discussed.
The effects of quantum modes on the energy gap law for electron- transfer reactions
Authors: Tang Jau
Published: July 2002

The effects of quantum modes on the free energy relationship for electron-transfer reactions formulated by the ‘‘spin-boson’’ model are examined. The modified energy gap law for exothermic electron transfer reactions with quantum modes is derived in a simple analytical form, using the method of steepest descent to avoid the usual multiple convolution integrals.
Variational approach to the conformation of flexible polymers in solution
Authors: Melenkevitz J. , Curro J. G. , Schweizer K. S.
Published: July 2002

Approximate solutions to the self-consistent reference interaction-site model (RISM) integral equation theory of polymer liquids are employed to study the conformational properties of flexible polymers as a function of density and chain length. The nonlinear nature of the expression describing the medium-induced potential between two sites along the polymer chain necessitates the use of approximate methods to determine the intrachain pair-correlation function consistent with the interchain packing. In the limiting case where the polymer segment hard-core diameter approaches zero and the number of interaction sites comprising the chain goes to infinity, with the segment volume fraction held fixed, the intrachain pair correlation function is determined through a Feynman variational procedure for the effective free energy of a single chain in solution. The screening of the excluded volume interactions and the corresponding reduction of the polymer dimensions as a function of both polymer density and chain length is systematically studied with the results being in good agreement with scaling predictions. In addition, the variational method is successfully extended to chains having finite hard-core diameters, with the predictions of the theory comparing favorably with existing theories for polymer solutions based on field theoretic approaches.
The origin of ‘‘glassy’’ dynamics at solid–oligomer interfaces
Authors: Bitsanis Ioannis A. , Pan Chongmin
Published: July 2002

In this paper we present an equilibrium molecular dynamics study of oligomer (5-mer) films at liquid densities confined between planar simple-cubic surfaces. The systems studied were wide enough to develop a bulklike region in the middle half of the film and two well-separated interfaces. Our work focused on the effect of surface–segment attraction (εws) on chain dynamics. We studied separately the dynamics of ‘‘free’’ and ‘‘adsorbed’’ chains. ‘‘Free’’ chains have no segments inside the first layer next to each surface, while ‘‘adsorbed’’ chains have one or more of their segments inside these layers. ‘‘Free’’ chains exhibited bulk dynamics, irrespectively, of the value of εws and their proximity to a surface. ‘‘Adsorbed’’ chains exhibited two distinctly different dynamical patterns depending on the strength of surface segment attraction (εws). Chains ‘‘adsorbed’’ on weakly physisorbing surfaces (εws≤1.0 kT) were moderately affected by the surface. Their relaxation time was at most 10 times higher than bulk for collapsed chains and became gradually bulklike with decreasing number of surface–segment contacts. In sharp contrast, chains ‘‘adsorbed’’ on strongly physisorbing surfaces (εws=2.0, 3.0) experienced a dramatic increase of their relaxation times (by a factor of ∼1500 for εws=3.0) and a severe decrease of their mobility. Furthermore, chain relaxation was quite insensitive to the exact number of surface–segment contacts. Our findings indicate that the extremely sluggish dynamics of strongly physisorbed chains result from the severe slowdown of segmental motions inside the first layer. This slowdown is caused by the very high density of the first layer, rather than the bare surface–segment attraction.
The reactions of O(1D) with CH4 and C3H8 monomers and clusters
Authors: Rudich Y. , Hurwitz Y. , Frost G. J. , Vaida V. , Naaman R.
Published: July 2002

Reactions of O(1D) with hydrocarbon monomers and clusters were investigated via a cross molecular beam experiment applying laser induced fluorescence for the detection of the OH product. The translational, vibrational, rotational, spin–orbit, and Λ-doubling state populations were analyzed. Based on this information the mechanisms for the reactions of O(1D) with methane, propane, and their clusters were established. Nonstatistical distributions are observed even for the reaction of large clusters and are discussed in terms of nonadiabatic effects induced by the long lived collision complex.
Limited effects of polarization for Cl−(H2O)n and Na+(H2O)n clusters
Authors: Jorgensen William L. , Severance Daniel L.
Published: July 2002

Monte Carlo statistical mechanics simulations have been carried out for Cl− and Na+ in clusters of 1–14 water molecules. OPLS potential functions were used including the TIP4P model of water that do not include explicit polarization effects. In contrast to a prior report with the same potential functions, the computed enthalpies of binding for Cl− are shown to agree closely with experimental data, while the values for Na+ are about 10% too exothermic. The latter discrepancy is largely removed by using Aqvist’s Na+ parameters with TIP3P water. Moreover, the structures of the clusters show the expected nonspherical distribution of water molecules.
A comparative analysis of two methods for the calculation of electric-field-induced perturbations to molecular vibration
Authors: Martí Josep , Bishop David M.
Published: July 2002

Two common methods of accounting for electric-field-induced perturbations to molecular vibration are analyzed and compared. The first method is based on a perturbation-theoretic treatment and the second on a finite-field treatment. The relationship between the two, which is not immediately apparent, is made by developing an algebraic formalism for the latter. Some of the higher-order terms in this development are documented here for the first time. As well as considering vibrational dipole polarizabilities and hyperpolarizabilities, we also make mention of the vibrational Stark effect.
Chemisorption of carbon monoxide on platinum cluster anions
Authors: Ren Xiaoli , Hintz Paul A. , Ervin Kent M.
Published: July 2002

Reactions of platinum cluster anions with carbon monoxide are investigated using a flow tube reactor (flowing afterglow) instrument. The platinum cluster anions are prepared with a cathodic discharge source. The cluster anions, Pt n−(n≤7), react with carbon monoxide in a flowing helium/argon buffer gas at 0.5 Torr total pressure and at 300 K. Clusters with four or more platinum metal atoms react by addition of CO with an effective bimolecular reaction rate of greater than 70% of the calculated collision rate, while for n=3 the reaction rate is 16% of the collision rate. At high CO concentrations, additional CO molecules attach to the clusters to form platinum carbonyl anion intermediates, with little loss of metal atoms. Saturation limits were found, above which no further CO addition occurred. The saturated cluster species are Pt3(CO)6−, Pt4(CO)8−, Pt5(CO)9−, and Pt6(CO)10−. The application of simple geometry arguments and electron counting rules to infer possible skeletal structures of the metal clusters is discussed.
The anisotropic interaction of He–C2H2 from differential scattering experiments
Authors: Buck Udo , Ettischer Ingo , Schlemmer Stephan , Yang Moonbong , Vohralik Peter , Watts Robert O.
Published: July 2002

The anisotropic potential energy surface of He–C2H2 is determined by analyzing total differential cross sections measured at E=71.3 meV and differential energy loss spectra obtained at two energies, E=102.9 and 62.0 meV. Calculations are carried out using infinite-order-sudden, coupled states, and close coupling methods. The data evaluation clearly demonstrates that the experimental results are much better reproduced by a spindle-shaped contour in the repulsive potential than by the more usual hard ellipsoidal shape. This change also explains the unusually small damping of the diffraction oscillations in the total differential cross section.
Simulation of diffusion-limited step-growth polymerization in 2D: Effect of shear flow and chain rigidity
Authors: Agarwal U. S. , Khakhar D. V.
Published: July 2002

Multimolecule Brownian dynamics simulation results for diffusion controlled polymerization of bead–rod chain molecules in 2D solution are presented. Reaction between any two molecules undergoing Brownian diffusion takes place if the reactive chain ends approach each other to within a certain reaction radius, and if the chain end carrying segments are collinear within certain specified limits. The second order reaction rate constant is found to decrease with time as the molecular lengths increase and the diffusivities decrease. Application of a shear flow is seen to result in alignment of the molecules along the flow direction, thereby enhancing the concentration of molecular pairs with parallel orientation of reactive-end carrying chain segments, and hence the overall reaction rate. This effect is found to be more pronounced in the case of long rigid molecules as compared to flexible molecules because of the slow rotation and high level of orientation under flow of the former. Even the molecular weight distribution (MWD) obtained during polymerization may be affected. For example, longer molecules have lower diffusivities and hence lower reactivities, resulting in a narrower MWD in the absence of flow, as compared to the results with the usual assumption of molecular reactivity being independent of chain length. Furthermore, in the presence of an external flow, the longer molecules orient to a higher degree and hence display a higher enhancement in reactivity. This results in a wider MWD of the polymer. The simulation results are in qualitative agreement with previous experimental data for solution polymerization of rod-like molecules.
Strong shifts in diabatic nondynamic electron correlations cause conical intersection between low-lying closed-shell adiabatic singlets of like symmetry in ozone
Authors: Atchity Gregory J. , Ruedenberg Klaus
Published: July 2002

Using the quantum chemical construction of diabatic states from adiabatic states formulated in the subsequent paper, the lowest two 1A1 states of ozone are expressed in terms of its diabatic components and the interaction matrix Hij between them is determined. An analysis of the electronic structure of the diabatic states leads to an understanding of the reasons for the sign changes in ΔH and H12 and, hence, for the conical intersection between these two low-lying closed-shell adiabatic states of like symmetry.
Laser spectroscopy of the low-lying electronic states of NbN: Electron spin and hyperfine effects in the states from the configurations σδ and δπ
Authors: Azuma Y. , Huang G. , Lyne M. P. J. , Merer A. J. , Srdanov V. I.
Published: July 2002

Rotational and hyperfine analyses have been carried out for the (0,0) bands of the C 3Π–X 3Δ, e 1Π–X 3Δ, and f 1Φ–a 1Δ transitions of gaseous NbN from laser excitation spectra taken at sub-Doppler resolution. The δπ C 3Π and e 1Π states lie only 102 cm−1 apart in zero order but the spin–orbit matrix element between them, which is the sum of the spin–orbit constants for the δ and π electrons, is 698 cm−1; as a result the 3Π1 spin component lies below both the 3Π0 and 3Π2 components, and its hyperfine structure is highly irregular. This irregularity is an extreme example of how cross terms between the spin–orbit interaction and the Fermi contact hyperfine operator alter the apparent value of the hyperfine a constant, the coefficient of I⋅L in the magnetic hyperfine Hamiltonian. Molecular parameters for the C 3Π and e 1Π states have been obtained from a combined fit to the two of them. Including data for the B 3Φ state recorded earlier [Azuma et al., J. Chem. Phys. 91, 1 (1989)], detailed information is now available for all six of the electronic states from the electron configurations σδ and δπ. It has been verified that the spin–orbit/Fermi contact cross terms cause roughly equal and opposite shifts in the hyperfine a constants for the singlet states and the Σ=0 components of the triplet states. After allowing for this effect, it has been possible to interpret the hyperfine a constants in terms of one-electron parameters for the δ and π electrons, in similar fashion to spin–orbit parameters. Wavelength resolved fluorescence, following selective laser excitation of the C 3Π, e 1Π, and f 1Φ states, has led to the discovery of three new electronic states, δ2 c 1Γ, δ2 A 3Σ−, and σ2 b 1Σ+, besides giving the absolute position of a 1Δ. Strong configuration interaction mixing is found to occur between the σ2 b 1Σ+ and δ2 d 1Σ+ states. The low-lying electronic states of NbN are now well understood.
Fluctuations in the unimolecular decomposition of jet-cooled NO2: Implications for overlapping resonances and the transition state
Authors: Reid S. A. , Robie D. C. , Reisler H.
Published: July 2002

We report a detailed examination of fluctuations in the internal state distributions of the NO (2II) fragment formed in the monoenergetic unimolecular decomposition of jet-cooled NO2, utilizing photofragment yield (PHOFRY) spectroscopy. The NO(v,J,Ω,Λ) PHOFRY spectra at E°<3000 cm−1 are highly structured, and we examine correlations among these spectra on the basis of the angular momentum, spin–orbit, parity, and vibrational labels of the monitored NO rovibrational levels. We find that levels of similar total angular momentum in the same vibrational manifold are more strongly correlated with specific resonances in the activated molecule, as well as levels of the same N in the two spin–orbit states of NO. The observed PHOFRY correlations, as well as those found in the NO(2ΠΩ; Ω=1/2,3/2) rotational state distributions, are interpreted in terms of projections of coherently excited overlapping molecular eigenstates onto the manifold of final states via levels of the transition state. The implications of the results to the transition state and the adiabatic evolution of the NO degrees of freedom are discussed.
Trapping of guests in a rare gas matrix: A molecular dynamics simulation
Authors: Fraenkel Ruchama , Haas Yehuda
Published: July 2002

A molecular dynamics simulation of the trapping of a guest molecule in a rare gas matrix deposition is presented. Using Lennard-Jones pairwise potentials, the shape and size of trapping sites are seen to depend on the preparation conditions, particularly the temperature and the cooling rate. The proper way to obtain a defect-free matrix is by depositing at temperatures that are somewhat lower than the annealing temperature.
Spatiotemporal motions due to global interaction
Authors: Middya Usuf , Luss Dan , Sheintuch Moshe
Published: July 2002

The global interaction between the spatial average of the autocatalytic variable in a reactive (catalytic, electrochemical, gel) medium and a surrounding mixed fluid in a continuously fed vessel may induce a rich structure of spatiotemporal patterns that would not exist otherwise. Patterns may form when the local kinetics are either excitable, oscillatory, or bistable and the reaction rate ascends with reactant concentration. Thus, a local change in the surface reaction rate may affect the reactant concentration in the surrounding well-mixed vessel, so that it arrests moving fronts on the surface. External control of the average temperature (or rate) of a catalytic ribbon by electrical heating is another form of interaction between a spatial average of a local oscillator and a space-independent variable that induces pattern formation. We study various patterns and bifurcations that can develop in a ring or a ribbon due to global interaction using a simple cubic kinetic expression. The predominant pattern on a catalytic ring is a rotating pulse. Other patterns, such as antiphase oscillations and crossing pulses, similar to those found previously on a controlled catalytic ribbon, may coexist for a sufficiently strong interaction. Several of the transitions between regions with qualitatively different patterns are via global bifurcation.
A computational study of the reaction of the FO radical with H2
Authors: Francisco Joseph S.
Published: July 2002

The reaction of the fluorine oxide radical, FO, with molecular hydrogen, H2 has been studied using ab initio molecular orbital methods. The heat of reaction is found to be 3.0±2 kcal mol−1 with an activation barrier of 17.4±2 kcal mol−1 using quadratic configuration interaction theory with single and double excitations with triple corrections [QCISD(T)]. Atmospheric implications of the results are discussed.
Nonadiabatic photodissociation dynamics of ICN in the A continuum: A semiclassical study
Authors: Wang Yifei , Qian Charles X. W.
Published: July 2002

The photodissociation dynamics of ICN in the A continuum was studied at several selected photolysis wavelengths using the semiclassical method. The calculations were performed on a set of new potential energy surfaces based on recent ab initio calculations. Classical trajectory calculations were carried out on these surfaces while the nonadiabatic interactions were treated by the surface-hopping model. The absorption cross section and the I*/I branching ratio were calculated as functions of the photolysis laser wavelength. The CN fragment rotational state population, alignment, and spatial anisotropy parameters were calculated for the dissociation at 266 and 249 nm. The results of our calculations agree well with the corresponding experimental observations. Our calculations were performed based on a model in which five excited state surfaces [i.e., 3Π0+(A′), 1Π1(A′), 1Π1(A″), 3Π1(A′), and 3Π1(A″)] are involved in the A continuum absorption and the consequent dissociation dynamics. At the low energy side of the A continuum, the initial excitation involves the 3Π0+ and 3Π1 states, whereas the 3Π0+ and 1Π1 states are mainly responsible for the absorption in the shorter wavelength region. Different nonadiabatic interactions of the Renner–Teller pair of the 1Π1 states with the 3Π0+ state, as well as the reduction of the CN rotational excitation on the diabatic 3Π0+(A′) surface at large internuclear distance are the key features of this model.
Integral equation theory for charged liquids: The structure of macroions in solution and the inversion of experimental data
Authors: Duh D.-M. , Haymet A. D. J.
Published: July 2002

The experimental small angle neutron scattering (SANS) data of Kunz et al. [J. Chem. Phys. 99, 2074 (1993)] for bulky ions in solution are reanalyzed to show that the potential energy fitting functions used to extract pair correlation functions from the data are not unique. This emphasizes a well-known difficulty is interpreting such data.
The effect of reagent translation on the reaction dynamics and the absolute reaction cross section of H+H2O→OH+H2
Authors: Jacobs A. , Volpp H.-R. , Wolfrum J.
Published: July 2002

With H atoms from ultraviolet laser photolysis of H2S and HI, the influence of the translational excitation of the reagents on the reaction dynamics and the absolute value of the reaction cross section of H+H2O→OH+H2 has been studied in the center of mass (c.m.) energy range from the reaction threshold up to 2.2 eV. To determine the OH product rotational fine-structure distributions, the nascent OH radicals were detected with quantum state resolution by laser-induced fluorescence (LIF). It was found that at all c.m. collision energies, the OH radicals are produced exclusively in the vibrational ground state. The measured OH(v=0) rotational fine-structure distributions can be described by Boltzmann distributions, with rotational temperatures which increase only slightly with increasing collision energy. Near the threshold, the OH fine structure rotational temperatures are almost equal; at higher collision energies, the rotational temperature of the OH(A′) fine structure distribution is about a factor of 1.5 higher than the rotational temperature of the corresponding OH(A″) fine-structure distribution, leading to preferential population of the symmetric Π(A′) state at high rotational quantum numbers. To investigate the influence of the reagents’ translational energy on the reactivity, absolute reaction cross sections were measured at different collision energies. Using a calibration method to measure absolute number densities of nascent OH product radicals under single-collision conditions, the following absolute reaction cross sections were obtained: σR(1.0 eV)=(0.03±0.02) Å2, σR(1.5 eV)=(0.16±0.05) Å2, σR(1.8 eV)=(0.18±0.06) Å2, σR(2.2 eV)=(0.25±0.07) Å2. The experimental absolute reaction cross sections and OH rotational distributions are compared to the results of recent quasiclassical and quantum scattering calculations on an ab initio potential energy surface.
Symmetry-adapted perturbation theory of the intramonomer correlation effects in intermolecular forces
Authors: Tachikawa Masanori , Suzuki Kazunari , Iguchi Kaoru , Miyazaki Tomoo
Published: July 2002

Molecular interaction energy is studied in terms of the double symmetry-adapted perturbation theory, taking account of both the electronic exchange between molecules and the intracorrelation fluctuation for individual monomers. The energy is divided into physically meaningful components, such as electrostatic, first-order exchange, second-order polarization, and second-order exchange terms. The algebraic expressions of second-order component energy terms, especially second-order exchange ones, are derived for the interaction of two-electron systems by considering only single-electronic exchanges between molecules. Our result for the He dimer is compared with that produced when the explicitly correlated Gaussian-type geminal is employed. The ratio of intracorrelation energy to Hartree–Fock energy in the second-order exchange is larger than those in the second-order polarization as well as in the first-order energies. The interaction energies of the H2 dimer including intracorrelation effect are computed in four orientations, i.e., linear, parallel, T, and X types.
Conformational properties for linear nematic and cholesteric polymers in the bend elastic chain model
Authors: Varichon L. , ten Bosch A.
Published: July 2002

The chain conformation of a liquid crystal polymer will be modified at the isotropic–anisotropic phase transition. Numerical results for the end-to-end distance 〈R2〉 and the radius of gyration 〈S2〉 of the polymer are presented using the bend elastic chain to model the polymer and including anisotropic mean field interactions. Anisotropy of conformation is calculated in nematics in the direction of preferred orientation as given by the component 〈R2z〉. In cholesterics, anisotropy occurs perpendicular to the director as well and 〈R2x〉≠〈R2y〉. The conformation is studied as a function of temperature, degree of polymerization, and persistence length. In cholesterics, a modification of the conformation is also found at those values of parameters (temperature, molecular weight) for which the pitch diverges and changes sign.
Solvation dynamics in electrolyte solutions
Authors: Chandra A. , Patey G. N.
Published: July 2002

A microscopic theory of the dynamics of ion solvation in electrolyte solutions is given. Consistent with the pure solvent case, the theory predicts a very fast and important inertial relaxation at short times. This is followed by oscillations and an extremely slow long-time decay associated with the formation of an equilibrium ‘‘ion atmosphere’’ about the newly charged particle.
Photodissociation of CH2. III. Two-dimensional dynamics of the dissociation of CH2, CD2, and CHD through the first excited triplet state
Authors: Beärda Robert A. , Kroes Geert-Jan , van Hemert Marc C. , Heumann Bernd , Schinke Reinhard , van Dishoeck Ewine F.
Published: July 2002

We present quantitative results on photodissociation of CH2 (X̃ 3B1) and its isotopomers CHD and CD2 through the first excited triplet state (1 3A1). A two-dimensional wave packet method employing the light–heavy–light approximation was used to perform the dynamics. The potential energy surfaces and the transition dipole moment function used were all taken from ab initio calculations. The peak positions in the calculated CH2 and CD2 spectra nearly coincide with the positions of unassigned peaks in experimental CH2 and CD2 3+1 resonance enhanced multiphoton ionization spectra, provided that the experimental peaks are interpreted as two-photon transitions. Comparing the photodissociation of CH2 and its isotopomers to photodissociation of water in the first absorption band, we find these processes to be very similar in all aspects discussed in this work. These aspects include the origin of the diffuse structure and the overall shape of the total absorption spectra of vibrationless and vibrationally excited CH2 , trends seen in the fragment vibrational level distribution of the different isotopomers, and selectivity of photodissociation of both vibrationless and vibrationally excited CHD. In particular, we find that the CD/CH branching ratio exceeds two for all wavelengths in photodissociation of vibrationless CHD.
A general parallel solution to the integral transformation and second-order Mo/ller–Plesset energy evaluation on distributed memory parallel machines
Authors: Limaye Ajay C. , Gadre Shridhar R.
Published: July 2002

We present here a parallel algorithm for four-index (integral) transformation and second-order Mo/ller–Plesset (MP2) energy evaluation, primarily designed for multiple instruction multiple data (MIMD) machines. It is a general algorithm designed to work with equal efficiency on any inhomogeneous network and any architecture. This algorithm works with only a twofold redundancy in integral storage, whereas some previously reported strategies demand a fourfold redundancy. The parallel transformation and sorting algorithm has been implemented on a 128 node inhomogeneous ring network, 64 of the processors being about 30%–40% slower. The present parallel scheme is seen to perform excellently in integral transformation processes even in such an inhomogeneous environment due to dynamic load balancing strategies. It has been found that integral transformation along with MP2 energy evaluation takes typically 120–200 min for molecules with 80–90 atomic orbitals. However, time taken for such systems reduces to ∼30–60 min when only requisite partial transformation is carried out for MP2 energy evaluation.
Change of bond length in free-energy simulations: Algorithmic improvements, but when is it necessary?
Authors: Wang Lu , Hermans Jan
Published: July 2002

Various methods are compared for calculating the constraint free energy in simulations with change of bond lengths. Bond forces can be evaluated as restraint forces with the use of flexible bonds, and also, when bond lengths are held fixed, as constraint forces calculated either from SHAKE displacements, or else from Gauss’ principle of least constraint, and these forces can then be used to calculate the potential of mean force or free energy for the bond length changes. An alternative is to use so-called mean-potential forces. Analysis of the equations derived from Gauss’ principle shows that the constraint force (and also the restraint force) contain a ‘‘dynamic’’ term, that depends on the molecular motion and for simple molecules averages to 2 kT/r, where r is the bond length. Integration of this term, which is not present with use of the mean-potential force, or in a Monte Carlo simulation, gives a free energy that corresponds to the change in conformational entropy of two particles maintained at a certain distance, when this distance is changed. It is unnecessary to compute this free energy term by simulation, as was done in earlier work. The accuracy and precision of these methods, and of methods in which the bond lengths do not change, are tested in a series of model calculations of increasing complexity. First, the equivalence of these methods is tested in a model system with small molecules whose motion is maintained by Brownian dynamics, and which can be subjected to very simple external forces, in order to permit very long simulations.

Second, a comparison is made of techniques for calculating free energy change with and without change of bond lengths, applied to the transformation of ethane into ethanol in water. A third test system is the conversion of an alanine residue into a serine residue in an α-helix in water. It is found that results are much improved if the ‘‘vanishing’’ atoms (in this case hydroxyl O and H) are coupled to a heat bath via Brownian forces. Statistical errors are estimated from four successive slow-growth simulation cycles. Calculations with twofold change of C–O and O–H bond lengths and linear coupling of the nonbonded energy terms are more precise than calculations without change of bond lengths and nonlinear coupling. The free energy change for the system with flexible bonds contains a significant contribution (circa 3 kJ mol−1) from elongation of the bond lengths as a result of interaction of the polar OH group with water. The constraint forces calculated from the SHAKE displacements and from Gauss’ principle turn out to be almost identical. The three methods for calculating the constraint forces achieve similar precision in the ethane/ethanol system as well as in the α-helix system. We also discuss the application of free energy calculations with changing bond lengths to changes of a molecule’s chemistry in the middle of a chain.
Accurate quantum mechanics from high order resummed operator expansions
Authors: Schwartz Steven D.
Published: July 2002

In this paper we report new developments in the expansion and partial resummation of the evolution operator. Higher order resummations allow derivation of an effective one-dimensional potential which accurately represents quantum dynamics for even strongly coupled low-frequency modes. This allows a system bath approximation which can accurately reproduce multidimensional quantum mechanics. In addition the formulation presented in this paper should prove significantly easier to extend to many-body problems than previous formulations we have derived. The accuracy of the method for even highly nonadiabatic applications, and the ease of implementation suggests that this approach will be useful in the calculation of the quantum dynamics of many dimensional systems.
Local structure of microemulsion
Authors: Ciach A. , Poniewierski A.
Published: July 2002

Correlation function between surfactant particles aligned along common direction is calculated in the Ciach–Ho/ye–Stell lattice model. We find that in the longitudinal direction antiferromagnetic ordering occurs in the whole region of stability of the homogeneous phase. A typical distance between neighboring oppositely oriented amphiphiles is different from the characteristic size of water domains. In the transversal direction, the form of the correlation function is related to the structure of the ordered low-temperature phase in coexistence with the microemulsion. In the proximity of the lamellar phase the correlation function decays monotonically indicating local ‘‘lamellar’’ ordering, while near the cubic bicontinuous phase it exhibits damped oscillations around zero.