bluuebunny/crossref_metadata_2025 huggingface dataset:
Two-pulse coherent control of electronic states in the photodissociation of IBr: Theory and proposed experiment
Authors: Levy Izak , Shapiro Moshe , Brumer Paul
Published: July 2002

It is shown how the principles of coherent control can be applied to a pulsed experiment aimed at controlling curve-crossing processes. A realistic computational study (incorporating the ground, A, B, and Y electronic states of IBr) of control of atomic states produced in the photodissociation of IBr is presented. The suggested scheme, which consists of applying an excitation pulse followed by a dissociation pulse, is theoretically shown to yield essentially total control over the Br*/Br atomic branching ratio. It is shown that the only external parameters that need to be varied are the central frequency of the excitation pulse and the time delay between the two pulses. A brief description of a proposed experiment is provided.
Multipolar electrolyte solution models. I. Computer simulation of the charged and dipolar hard sphere mixture
Authors: Eggebrecht John , Ozler Pelin
Published: July 2002

Mixtures of charged and dipolar hard spheres are studied using Monte Carlo computer simulation. The dependence of pair correlations and thermodynamic and static dielectric properties on ionic concentration and charge are examined at fixed total density. Approximate solutions of the Ornstein–Zernicke integral equation are tested by comparison. Analyses of solvation structure and ion clustering are made. A solubility limit is observed and large ion clusters above and below this limit form chains of alternating sign. Orientational order and compactness in the solvation sphere are diminished near this limit relative to both higher and lower concentrations.
Extensions to the instantaneous normal mode analysis of cluster dynamics: Diffusion constants and the role of rotations in clusters
Authors: Adams John E. , Stratt Richard M.
Published: July 2002

For the instantaneous normal mode analysis method to be generally useful in studying the dynamics of clusters of arbitrary size, it ought to yield values of atomic self-diffusion constants which agree with those derived directly from molecular dynamics calculations. The present study proposes that such agreement indeed can be obtained if a sufficiently sophisticated formalism for computing the diffusion constant is adopted, such as the one suggested by Madan, Keyes, and Seeley [J. Chem. Phys. 92, 7565 (1990)]. In order to implement this particular formalism, however, we have found it necessary to pay particular attention to the removal from the computed spectra of spurious rotational contributions. The utility of the formalism is demonstrated via a study of small argon clusters, for which numerous results generated using other approaches are available. We find the same temperature dependence of the Ar13 self-diffusion constant that Beck and Marchioro [J. Chem. Phys. 93, 1347 (1990)] do from their direct calculation of the velocity autocorrelation function: The diffusion constant rises quickly from zero to a liquid-like value as the cluster goes through (the finite-size equivalent of) the melting transition.
Molecular dynamics of a plastic crystal with two types of orientational disorder: Static and dynamic properties of plastic bicyclooctane
Authors: Duparc O. B. M. Hardouin , Meyer M.
Published: July 2002

A molecular dynamics simulation of a realistic model of bicyclo-octane [HC(CH2–CH2)3CH] in its plastic phase has been performed at three different temperatures. The interactions between the molecules of the simulated system are calculated with phenomenological exp-6 additive atom–atom potentials. The molecules are considered rigid and their equations of motion are integrated using a method of constraints. A special attention is given to the problem of the C–H bond value. The static analysis of the molecular dynamics (MD) results yields a clear description of two types of orientational order: It confirms the experimental results about the equipartition of the molecular threefold axes around the eight 〈111〉 crystallographic axes and it furthermore unambiguously shows the existence of six preferred orientations for the orientational order of the molecules around their threefold axes where a previous experimental work concluded in favor of a quasifree rotation. The mean-square displacement amplitudes of the molecular centers of mass reveal that our simulation model is not impeded by the abnormal behavior of the exp-6 potentials at small interactive distances. From a dynamical point of view, the molecular dynamics scheme clearly shows that the jumps are not instantaneous and are slightly more diverse than previously assumed. In order to achieve a precise description of these jumps, we propose to make a conceptual distinction between the mean residence times and the correlation times. By contrast with a former MD simulation, our model gives two sets of distinct characteristic times which perfectly agree with the experimentally deduced values. Given the simplifying assumptions entering the experimental models, the quality of this latter agreement also shows a certain lack of sensitivity vis ā vis the exact nature of the underlying microscopic mechanisms. We also examined the ability of our model to simulate the behavior of a monovacancy. We found the influence of such a defect on its immediate neighborhood to be very small even when considering the nature of the complex orientational (dis)order.
Orientation and shape of flexible polymers in a slit
Authors: van Vliet J. H. , ten Brinke G.
Published: July 2002

The shape and orientation of a polymer in a good solvent confined between two parallel plates is investigated by Monte Carlo simulations of self-avoiding random walks on a simple cubic lattice. Decreasing the width of the slit, the coil starts, due to its anisotropic shape, to orient long before its shape changes. Only after the longest principal axis is aligned considerably parallel to the plates, the coil is squeezed. Using a scaled distance between the plates, a corresponding state behavior as a function of chain length is found. The results are in good agreement with predictions based on an analytical rigid ellipsoid model with principal axes’ lengths set equal to the span lengths along the principal axes of inertia of the self-avoiding random walk.
Isomers of niobium clusters: Direct spectroscopic evidence
Authors: Knickelbein Mark B. , Yang Shihe
Published: July 2002

We have reproduced kinetics results indicating that niobium clusters in the size range Nb9–Nb12 are produced in two isomeric forms which possess widely different rates reactivity toward D2. We have exploited this difference in reactivity to record photoionization efficiency spectra of the unreactive forms of Nb9, Nb10, and Nb12 as well of a mixture containing predominantly the reactive forms. We find that the photoionization thresholds measured for Nb9 and Nb12 with the reactive forms of these species depleted via reaction with D2 are shifted to higher energies compared those recorded without D2 present. The photoionization threshold energies for both unreactive and reactive forms of Nb10 are approximately equal.
Resonance-mediated dissociative adsorption of hydrogen
Authors: Darling G. R. , Holloway S.
Published: July 2002

Selective adsorption has been examined theoretically using time-dependent quantum-mechanical methods. In the simple case of a potential with no open channel to dissociation, we find that the description in terms of bound states of the laterally averaged potential holds, and perturbation theory gives adequate results for the lifetimes of the states. A more complicated situation, in which dissociation and resonance interact strongly, is beyond the reach of perturbation theory. Nevertheless, the resonances still exist at energies determined by kinematics. They decay more rapidly than before into both the dissociation and scattering channels. The most striking feature of the results is that the dissociation probability, as well as the diffraction intensities, exhibits a Fano profile around the resonance. Thus resonance diminished dissociation can be seen. We examine the possible inclusion of inelastic effects in our calculations, and the direct comparison with experiment this would allow.
Second harmonic generation and thermally stimulated current measurements: A study of some novel polymers for nonlinear optics
Authors: Köhler Werner , Robello Douglas R. , Dao Phat T. , Willand Craig S. , Williams David J.
Published: July 2002

The relaxation of electric field-induced polar orientation in novel side and main chain polymers was measured by simultaneous second harmonic generation and thermally stimulated discharge current techniques. Using this combination of experiments, individual microscopic relaxation mechanisms could be discriminated and identified. The relaxation of a polymer bearing polar chromophores on a side chain polymer was found to occur through local reorientation. For polymers containing the polar chromophore within the main chain, both a local rotation of the chromophores and the reorientation of the end-to-end vectors of the chains could be detected. Physical aging near the Tg of the polymers greatly increased the stability of the poling-induced orientation below Tg. Nonlinear dependencies on the poling field were observed for low fields and are discussed in terms of screening due to space charges. Kirkwood–Frohlich correlation factors were found to be very close to unity.
Direct observation of the picosecond dynamics of I2–Ar fragmentation
Authors: Breen J. J. , Willberg D. M. , Gutmann M. , Zewail A. H.
Published: July 2002

Picosecond real-time observations of the dynamics of I2–Ar fragmentation are reported. The state-to-state rates, k(νi′;νf′), are directly measured and related to the homogeneous broadening of the initial state, and to product state distributions in the exit channel. Comparisons with different theories of vibrational (and electronic) predissociation are made.
An experimental study of radiation-induced pure dephasing: ArF excited emission of O2
Authors: Zhang Y. P. , Ziegler L. D.
Published: July 2002

The effects of incoherent incident light on both nonresonant and resonant secondary radiation (RSR) are demonstrated for the spontaneous emission of O2. ArF excimer radiation (FWHH∼120 cm−1) is resonant with several rovibronic features of the v′=4 band of the Schumann–Runge absorption system. Off-resonant contributions to the RSR spectrum are Raman-like (v″=1) but carry the linewidth of the incident incoherent radiation. Purely resonant emission features are found to be entirely fluorescence-like (v″≥6). Other RSR vibrational bands of O2 exhibit contributions of both types of emission including interferences between Raman (off-resonant) and fluorescence (resonant) amplitudes. The observed depolarization ratios also reflect these various emission characters. The RSR spectra of O2 excited by incoherent (ArF) driving fields are contrasted with that due to monochromatic excitation. Convolution of the incident spectral density with a rovibronic Kramers–Heisenberg irreducible tensor treatment of resonance Raman scattering cross sections is shown to capture all the observed RSR emission characteristics.
Fourier transform–electron paramagnetic resonance spectroscopy of correlated radical pairs
Authors: Kroll G. , Plüschau M. , Dinse K.-P. , van Willigen H.
Published: July 2002

Utilizing the high time and phase resolution of Fourier transform electron paramagnetic resonance, the buildup and decay kinetics of the transient radical pair Zn tetraphenylporphyrin+/Duroquinone− in ethanol could be studied by detecting the dispersive component in the Fourier transformed free induction decays of the fully separated radical anions. The dispersive line components originate from the exchange coupling J of the radical pair in its metastable state trapped by an attractive temperature dependent Coulomb interaction U(rc,T) at an equilibrium distance rc. The decay rate constant kd into free ions was studied over the temperature range of 225–298 K and varied from kd=3.2×106 to 10×106 s−1. This change in kd could be fully rationalized in terms of the temperature dependence of the solvent dielectric constant (leading to a variation of the cage potential from 0.07 to 0.12 eV) and the diffusion constant. The buildup rate constants were consistent with the assumption of a diffusion-controlled electron transfer process as a primary step.
Rotor synchronized three-dimensional 13C magic angle spinning nuclear magnetic resonance: Correlation between molecular structure, order, and dynamics in solids
Authors: Yang Y. , Hagemeyer A. , Zemke K. , Spiess H. W.
Published: July 2002

Theory and application are described of a new rotor-synchronized 3D-NMR (three-dimensional nuclear magnetic resonance) experiment, that correlates molecular structure, order, and dynamics in partially oriented solids. General expressions for the sideband intensities in three frequency dimensions are developed for uniaxially ordered samples undergoing ultraslow molecular motions. For an isotropic powder the exchange pattern is confined to a single plane perpendicular to one of the frequency axes. For a partially ordered solid without exchange the sideband pattern is confined to a cross-diagonal plane. Sidebands in the full 3D cube result only if the residue giving rise to the signal is involved in both, molecular order and dynamics. The experiment, therefore, correlates these two properties and relates them to the chemical structure. Moreover, it offers a means to exploit the superior information about the geometry of the molecular motions provided by partially ordered samples under high resolution magic angle spinning (MAS) conditions. The technique is illustrated by 3D MAS spectra of highly oriented poly(oxymethylene), where strong positive and negative sidebands in the full 3D cube result from the helical jump motion.
Surface reactions between O2 and hydrocarbons induced by dissociative electron attachment
Authors: Sanche L. , Parenteau L.
Published: July 2002

Desorption of O− and OH− ions induced by low-energy (4–20 eV) electron impact on O2 and hydrocarbon molecules (CnH2n+2, n=5 and 8; CnH2n, n=2, 3, and 4) coadsorbed on Pt is reported. The magnitude of the O− and OH− signals is investigated as a function of incident electron energy and substrate coverage. Beyond monolayer coverage, results are provided for two types of coadsorption: a single hydrocarbon layer physisorbed on a multilayer O2 film and a multilayer film containing 25% volume O2 mixed with hydrocarbon molecules. For all experiments, the OH− yield function can be correlated with that of the O− signal from pure O2 and hydrocarbon–O2 mixture films. This result indicates that the OH− ions are produced by the abstraction reactions O−+CnH2n+2→OH−+CnH2n+1 and O−+CnH2n→OH−+CnH2n−1 where O− ions are generated by the dissociative attachment reaction e+O2 (3∑−g)→O−2 (2∏u,2∑+g,2∑+u)→O− (2P)+O(3P,1D). The observed reaction efficiency for OH− formation, defined as the ratio of the OH− intensity to that of O−, is found to increase with coverage of the substrate by C4H8–O2 and C5H12–O2 mixtures. It reaches values of 3% and 8%, respectively, above 3 monolayers for incident electrons of 13 eV. The energetics involved in those reactions as well as the behavior of the OH− intensity as a function of incident electron energy and coverage strongly suggest that OH− arises from dissociation of the intermediate quasi-bound anions CnH2n+2 O− and CnH2n O− into the limits OH−+CnH2n±1.
The kinetics of CO desorption from Ni(110)
Authors: Feigerle C. S. , Desai S. R. , Overbury S. H.
Published: July 2002

The kinetics of CO desorption from clean Ni(110) have been measured by thermal desorption mass spectrometry (TDMS) using the method of variation of heating rates. Analysis of the desorption spectra gives the coverage dependence of the preexponential factor νd, and the activation energy Ed, of the desorption rate constant. For 0<θ<0.5, Ed is constant at 30.6 kcal/mol, allowing analysis of the coverage dependence of the preexponential factor under conditions where the compensation effect is absent. We observe only small changes in the preexponential factor under these conditions, where log[νd(s−1)] rises slowly from 14.6 to 15.3 from 0<θ<0.65. Near θ=0.8, a synchronous drop in νd by 0.6 orders of magnitude and in Ed by 5 kcal/mol is observed at the onset of formation of the (2×1)p2mg structure. These results are discussed in light of the structures of CO in Ni(110) and the effects of ordering transitions on the desorption kinetics. The experimental results for νd(θ) are compared with those predicted for desorption by three precursor mechanisms and for desorption of a lattice gas with lateral interactions.
Translational basis set contraction in variational reactive scattering
Authors: Manolopoulos David E. , D’Mello Michael , Wyatt Robert E.
Published: July 2002

A new translational basis set is introduced for quantum reactive scattering calculations that use the log derivative version of the Kohn variational principle. This basis set, which is similar in many respects to that used in electron–atom scattering calculations by Burke and Robb, is obtained by contracting a primitive basis of Lobatto shape functions to the box eigenfunctions of a one-dimensional reference Hamiltonian H0. In addition, a single energy-dependent scattering function is included in the variational expansion to ensure completeness at the boundary of the box. One fairly obvious choice for the reference Hamiltonian in an atom–diatom reaction is suggested, and all of the equations which are actually needed to implement the method in this context are carefully described. Example applications to the three-dimensional F+H2 reaction are then chosen to illustrate the practical potential of the approach.
Analysis of translational, rotational, and vibrational energy transfer in collisions between CO2 and hot hydrogen atoms: The three-dimensional ‘‘breathing’’ ellipsoid model
Authors: Kreutz Thomas G. , Flynn George W.
Published: July 2002

Classical scattering of a particle from a three-dimensional ‘‘breathing’’ ellipsoid is used to model translational, rotational, and vibrational energy transfer in collisions between hot hydrogen atoms and CO2. The model combines the theoretical formalism which describes rotational scattering between a particle and a three-dimensional rigid ellipsoid with a simple scheme to account for the effects of CO2 vibrational excitation. The calculations are used to analyze experimental data (measured using time-domain diode laser absorption spectroscopy) concerning the state-specific deposition of energy among the translational, rotational, and vibrational degrees of freedom of CO2 following collisions with translationally hot H atoms. The model provides substantial insight into the features of the final-state-resolved experimental data, indicating which ‘‘types’’ of trajectories, e.g., ‘‘end-on’’ vs ‘‘broadside’’ collisions, are responsible for scattering into particular final states. Despite its simplicity, the model is shown to predict virtually all of the major features of the experimental data with remarkable accuracy. In addition, it reproduces highly complex behavior seen in the state–to–state collision cross sections which had been previously obtained using a data-inversion procedure.
Rotational level dependence of the electronic quenching of NH2Ã by helium
Authors: Wysong Ingrid J. , Jeffries Jay B. , Crosley David R.
Published: July 2002

The rotational level dependence of the electronic quenching of the Ã2A1 state of the NH2 radical is investigated by time-resolved laser-induced fluorescence. Quenching of ∑(0,9,0) is found to increase monotonically with rotational level from 2×10−11 cm3s−1 for N′=0 to 7×10−11 cm3 s−1 for N′=7. Measurements on the neighboring ∑(0,7,0), Π(0,8,0), and Π(0,10,0) vibronic levels show no significant rotational level dependence. The behavior in ∑(0,9,0) cannot be satisfactorily explained by either molecular collision dynamics or by state mixing.
Rydberg and ion-pair state mixing in the 1 1Σ+u and 2 3Πu vibronic states of Cl2 as studied by vacuum ultraviolet laser spectroscopy
Authors: Tsuchizawa Tai , Yamanouchi Kaoru , Tsuchiya Soji
Published: July 2002

Fluorescence excitation and resonantly enhanced photoionization spectra of jet-cooled Cl2 in the 71 000–77 500 cm−1 region are measured using a tunable vacuum ultraviolet light generated by four-wave difference frequency mixing of two lasers in Kr. For the 1 1Σ+u state, vibrational term values for 35Cl2, 35Cl37Cl, and 37Cl2 and rotational constants for 35Cl2 and 35Cl37Cl are determined with high precision by the measurement of the 1 1Σ+u –X 1Σ+g transitions. It is shown clearly that the v′=39 vibrational wave functions of the 1 1Σ+u state of 35Cl2 and 35Cl37Cl are localized in the Rydberg well while the vibrational wave functions for the v′<39 levels are delocalized in the ion-pair well and those for the v′>39 levels are between the inner Rydberg wall and the outer ion-pair wall. A double minimum potential composed of the Rydberg and the ion-pair states explains well the observed vibrational and rotational structures. The molecular constants for the Rydberg well of 2 3Π(0+u) determined from the rovibrational structures of the 2 3Π(0+u)–X 1Σ+g transitions are ωe=647.3(33) cm−1, ωexe=3.37(59) cm−1 and re=1.833(20) Å, respectively.
Photodissociation dynamics of water containing clusters. I. Kr⋅H2O+
Authors: Kim H-S. , Kuo C-H. , Bowers M. T.
Published: July 2002

The mass selected Kr⋅H2O+ cluster is photodissociated in the range 514 to 357 nm using lines from an argon ion laser. Product branching ratios are measured and shown to be a strong function of photon wavelength; Kr+/H2O products dominate at 357 nm (90%) but are equal in intensity to H2O+/Kr products at 514 nm. A small KrH+/OH product is observed at all wavelengths (∼5%), representing the first observation of a photoinduced, intracluster proton transfer reaction. The total cross section is estimated to be ∼2×10−19 cm2 at 514 nm. Laser polarization studies indicated the Kr+/H2O products come from direct accessing of a repulsive upper state (intracluster charge–transfer reaction). Both Kr+(2P3/2) and Kr+(2P1/2) spin–orbit states are formed, but their branching ratio is very strongly dependent on wavelength: 100% Kr+(2P3/2) at 514 nm, 100% Kr+(2P1/2) at 357 nm, and variable amounts of each in between. Analysis of the kinetic energy distribution of Kr+/H2O products indicates H2O is strongly rotationally excited (0.18 to 0.23 eV). This fact, coupled with analysis from an impulsive model for Kr+–H2O dissociation suggests the Kr atom is above (or below) the H2O+ plane in the Kr⋅H2O+ ground state, situated closer to the O end of the molecule. Further analysis of the Kr+/H2O kinetic energy distribution yields the binding energy D00(Kr–H2O+) =0.33± 0.1 eV. Polarization studies indicate H2O+/Kr products arise from a bound upper state. Phase space theory modeling of the kinetic energy distribution indicates the H2O+ product is formed with ∼1.3 eV internal energy. Two models are discussed, one that suggests H2O+(Ã 2A1) is formed and a second that suggests H2O+ is the chromophore, internally converts to vibrationally hot H2O+(X̃ 2B1) and slowly leaks vibrational energy to the c
Vibrational predissociation of a rotating collinear triatomic model molecule
Authors: Jiang Jun , Hutchinson John S.
Published: July 2002

Conventional quantum mechanical studies of molecular vibrations usually neglect the effects of rotation. When a rotating molecule has high enough energy to dissociate, this approximation is generally inaccurate. In this study, the effect of molecular rotation on vibrational predissociation is investigated for a model collinear H–C–C molecule. The vibrational–rotational interaction in this simple model is solely centrifugal, which introduces additional coupling of the two stretching modes. For a large range of angular momenta J, vibrational predissociation resonance widths (which are proportional to unimolecular state-specific reaction rates) are computed quantum mechanically using the complex coordinate method. The resonance widths are found to have substantial dependence on J. The results of corresponding classical trajectory studies show that the J-dependence of the resonance widths is a purely quantum effect. The quantum results are verified by a comparison of the complex coordinate method with the R-matrix propagation method. The resonance widths given by the two procedures are found to be in very good agreement.
Quantum-mechanical oligomer approach for the calculation of vibrational spectra of polymers
Authors: Cui C. X. , Kertesz Miklos
Published: July 2002

The force-constant matrix of an oligomer, composed of five or more repeat units with appropriate terminal groups, can be used to construct the force-constant matrix of the corresponding planar or helical polymer. The calculations on some typical polymers (polymeric sulfur, polyacetylene, and polyethylene) show that the oligomer approach is accurate enough to duplicate the vibrational frequencies of frozen-phonon calculations. The oligomer approach has much more predictive power if used in conjunction with some form of empirical scaling (scaled quantum-mechanical oligomer force field for polymers). ir and Raman selection rules for helical polymers are also discussed. Vibrational frequencies and intensities of polymeric sulfur and polyethylene have been calculated at the ab initio STO-3G, 4-31G, 6-31G, and 6-31G* basis-set levels. The agreement of frequencies with experiment is excellent. The quality of calculation is limited by the basis set and theoretical model used rather than the oligomer approach.
Local structure of fluids containing chain-like molecules: Polymer reference interaction site model with a Yukawa closure
Authors: Yethiraj Arun , Hall Carol K.
Published: July 2002

The local structure of fluids composed of chain-like molecules (modeled as a pearl necklace of freely jointed hard spheres) is investigated via an integral equation approach. A Yukawa closure for the direct correlation function is used in the framework of polymer-RISM theory. This introduces two free parameters which are chosen by matching the theory’s predictions for the compressibility and the contact value of the site–site distribution function, to an equation of state and simulation data, respectively. The theory shows good agreement when compared to Monte Carlo simulations of tangent diatomics, and freely jointed 4-mers, and 8-mers. Methods for estimating the contact value of the site–site distribution function are discussed.
Shape resonance effects in the rotationally resolved photoelectron spectra of O2
Authors: Braunstein M. , McKoy V. , Dixit S. N. , Tonkyn R. G. , White M. G.
Published: July 2002

We report the results of theoretical and experimental studies of the rotationally resolved photoelectron spectra of O2 at low temperature leading to the v+=0, 1, and 2 levels of the X 2Πg state of O+2. A delayed, pulsed field ionization technique is used in conjunction with a coherent VUV radiation source to obtain high resolution spectra near threshold. The data are compared with theoretical results obtained using static-exchange photoelectron orbitals and a full description of the mixed Hund’s case (a)–(b) ionic ground state. Agreement with experiment is good, especially for the v+=1 and v+=2 levels. Analysis of the rotational branch intensities yields detailed information on the angular momentum composition of the shape resonance near threshold. We also show that the dependence of the electronic transition moment on internuclear distance caused by the shape resonance leads to a significant dependence of the rotational branch intensity on ion vibrational level.
Quantum scattering study of rotational energy transfer in OH(A 2Σ+, v′=0) in collisions with He(1S)
Authors: Jörg Andreas , Esposti Alessandra Degli , Werner Hans-Joachim
Published: July 2002

A quantum mechanical study of rotational energy transfer (RET) in OH(A 2Σ+, v′=0) in thermal collisions with He(1S) has been performed. The interaction potential of OH(A)+He was computed using the coupled electron pair approximation (CEPA) and a very large basis set. An analytical fit of the resulting OH–He potential was employed in close-coupling (CC) and coupled states (CS) calculations of integral RET cross sections for collision energies up to 5000 cm−1. The cross sections were integrated over a Boltzmann energy distribution to yield thermally averaged rate coefficients. State-to-state RET coefficients for the lowest 11 fine structure levels of OH(A, v′=0) were calculated as a function of the temperature. The agreement between the theoretical and recently measured values at 300 K is very good. The data for the OH(A)+He system are compared to the results of a previous theoretical study of the OH(A)+Ar system [A. Degli Esposti and H.-J. Werner, J. Chem. Phys. 93, 3351 (1990)]. The theoretical findings fully confirm the qualitatively different behavior of the OH–He and OH–Ar systems, which has been found experimentally by A. Jörg, U. Meier, and K. Kohse-Höinghaus [J. Chem. Phys. 93, 6453 (1990)]. For rotationally inelastic collisions with He the calculations predict a strong propensity for conserving the Fi fine structure levels in OH. In contrast, only a weak propensity for Fi conservation was reported for OH+Ar. In addition, our calculations for OH+He show a preference for transitions with ‖ΔJ‖=‖ΔN‖=2, whereas a strong preference of the nearly isoenergetic transitions with ‖ΔJ‖=1 and ΔN=0 was reported for OH+Ar.
Excitation shifts of parallel conjugated polymers due to π-electron dispersion forces
Authors: Soos Z. G. , Hayden G. W. , McWilliams P. C. M. , Etemad S.
Published: July 2002

Molecular exciton methods are extended to obtain excited-state shifts of parallel conjugated strands arising from interchain Coulomb interactions. A method is developed to evaluate second-order corrections involving π-electron dispersion forces, with exact Pariser–Parr–Pople states of isolated segments as zeroth-order functions. Crystal shifts are estimated for polyacetylene (PA) at interchain separation d=3.9 Å; the two-photon excitation to the covalent 2 1Ag state is hardly shifted, but the one-photon excitation to 1 1Bu is red shifted by ∼0.30 eV and specific higher n 1Ag states are strongly red shifted by ∼1 eV. Their stabilization correlates with charge separation and ionicity in the excited state. Excitation shifts due to π-electron dispersion forces do not parallel changes in the static polarizability and show unusual long-range behavior of d−3 in the interchain separation.
The microwave spectrum of the dihydronitrosyl radical, H2NO (2B1)
Authors: Mikami Hitomi , Saito Shuji , Yamamoto Satoshi
Published: July 2002

The microwave rotational spectrum of the H2NO radical in the X̃ 2B1 ground vibronic state was observed by using a source-modulation spectrometer and a 1 m long free-space cell. The H2NO radical was generated in the cell by a reaction of hydroxylamine (NH2OH) with the products of 2450 MHz discharge in CF4. The spectral lines of H2NO were observed for the a-type R-branch transitions of N=1←0 to N=6←5, most of which showed resolved hyperfine structures due to the hydrogen and nitrogen nuclei. The 19 molecular constants including the hyperfine coupling constants of both the nuclei were precisely determined by the least-squares fit of 129 observed lines. The observed hyperfine structure shows that the H2NO radical in the ground electronic state is essentially planar with C2v symmetry. The nitrogen hyperfine coupling constants determined indicate that an unpaired electron occupies the pπ orbital perpendicular to the molecular plane, and the electronic wave function in the ground electronic state belongs to 2B1 symmetry. However, the small but negative inertial defect (Δ=−0.0051 uÅ2 ) suggests a possibility that H2NO may have a double-minimum potential function for the out-of-plane vibration.
Collision-induced dissociation of CS+2. Heat of formation of the CS radical
Authors: Prinslow Douglas A. , Armentrout P. B.
Published: July 2002

Guided-ion beam mass spectrometry is used to probe the reaction of CS+2 with Xe as a function of ion kinetic energy from thermal to 16 eV. S+, CS+, and Xe+ are the only observed reaction products. Formation of S+(4Su) is observed, even though it is spin forbidden, providing evidence for the importance of spin–orbit coupling in the dissociation of CS+2. The threshold for formation of S++CS, 4.74±0.04 eV, leads to a heat of formation of the CS radical ΔfH00(CS) of 65.8±0.9 kcal/mol. From this value, we derive the bond energies D00(CS)=169.8±0.9 kcal/mol, D00(S–CS)=103.8±0.9 kcal/mol, and D00(O–CS)=158.7±0.9 kcal/mol. The onset for production of CS+ by collision-induced dissociation of CS+2 is observed at 6.16±0.07, 0.40 eV above the thermodynamic threshold, but coincident with the threshold for excitation to the C̃ state of CS+2. The cross section for charge transfer to form Xe+ displays a threshold of ≊2 eV, corresponding to the difference in ionization energies of Xe and CS2. This process is significantly enhanced at higher energies where the concomitant dissociation of CS2 to CS and S becomes accessible.
Boron cluster ion oxidation: Reactions with CO2, dissociation of boron cluster oxide (BnO+) ions, and sequential oxidation
Authors: Ruatta Stephen A. , Hintz Paul A. , Anderson Scott L.
Published: July 2002

Cross sections for oxidation reactions of boron cluster ions (B+1–14) with CO2 are reported as a function of collision energy. The results give insight into the oxidation mechanism and are a sensitive probe of the B+n–O bond energy. In addition, collision-induced dissociation (CID) was used to measure the stabilities of boron monoxide cluster ions (BnO+) and energetic and structural information is obtained that indicates that at least, in some cases, oxidation causes structural rearrangement of the clusters. Sequential oxidation of boron cluster ions by CO2 was also studied and the results give further insight into the reaction mechanism and energetics.
Structure and vibrational properties of water hydrogen halide complexes
Authors: Hannachi Y. , Silvi B. , Bouteiller Y.
Published: July 2002

A series of self-consistent field (SCF) calculations has been carried out on the 1:1 hydrogen bonded complexes of water and hydrogen halide (HF,HCl,HBr,HI) using core pseudopotentials and PS-31G** basis sets. The geometry optimization confirm the Cs structure predicted by the Legon–Millen rules. Electron density difference (complex superposition of isolated molecules) emphasize the role of polarization effects in this type of hydrogen bond. Finally, the HX frequency shifts with respect to monomeric species, the intensity ratios of the corresponding IR lines and the intermolecular stretching frequencies have been calculated by a variational method which takes into account both coupling and anharmonicity effects. An overall agreement with experimental data is obtained for these spectroscopic properties.
Quantum thermal rate constants for the exchange reactions of hydrogen isotopes: D+H2
Authors: Park Tae Jun , Light J. C.
Published: July 2002

Accurate thermal rate constants for the D+H2 reactions are determined for the Liu–Siegbahn–Truhlar–Horowitz potential energy surface over the temperature range 300–1500 K. We evaluate the rate constants via the quantum flux–flux autocorrelation function formulation of Miller [J. Chem. Phys. 61, 1823 (1974)] using the adiabatically adjusted principal axis hyperspherical coordinates of Pack [Chem. Phys. Lett. 108, 333 (1984)] and a symmetry adapted discrete variable representation used earlier for the H+H2 reaction [T. J. Park and J. C. Light, J. Chem. Phys. 91, 974 (1989)]. The initial L2 basis of ∼15 000 functions is sequentially diagonalized and truncated, with a final reduction to ∼420 accurate eigenvectors of the symmetry adapted (C2v) Hamiltonians for J=0. Direct products of these functions with symmetry adapted rotation functions are then used as the basis for the J>0 Hamiltonians. Nuclear spin symmetries are also included. For J>0, the individual J, KJ blocks of the Hamiltonian are diagonalized, the Coriolis coupling is neglected, and the KJ±2 coupling is included by perturbation theory. The thermal rate constants are evaluated for each total angular momentum from the flux–flux autocorrelation function. Angular momenta up to J=25 are required to converge the rate constants at 1500 K to ∼5%. Thermal rate constants as functions of T (and J) are presented for the D+H2 reaction and compared with experiment and other calculations. Agreement with experiment for D+H2 is excellent up to about 1000 K and remains within a factor of 2 of the experimental rate constant up to 1500 K. Thus agreement of the rates over more than four orders of magnitude is quite reasonable.
Bifurcation analysis of chemical reaction mechanisms. I. Steady state bifurcation structure
Authors: Olsen Robert J. , Epstein Irving R.
Published: July 2002

The vocabulary and techniques of numerical bifurcation analysis are described, with an emphasis on steady state bifurcations of codimension one and two. The direct computation of bifurcation sets is shown to be of considerable utility in analyzing and comparing complex chemical reaction mechanisms. The systems chosen for analysis are the chlorite–iodide and the mixed Landolt reactions. The calculation of a simple hysteresis loop for a mechanism of the chlorite–iodide reaction using both numerical bifurcation analysis and numerical integration begins an extended comparison between the methods advocated in this paper and more familiar methods. The systematic identification of the existence of isolated branches of steady states is described for a second mechanism of the chlorite–iodide reaction. Two mechanisms for the mixed Landolt system are contrasted. It is found that the alternative negative feedback pathway mechanism, which reproduces the periodic behavior more successfully at a selected point in parameter space, is less satisfactory than the EOE mechanism in describing the overall bifurcation behavior of the system in the parameter plane for which experimental data are available. Finally, we examine another mechanism for the chlorite–iodide reaction. This mechanism was designed with the intent of suppressing oscillatory behavior on a subset of parameter space. Numerical integration revealed only bistability of steady states, as desired, but numerical bifurcation analysis reveals a sizeable region in the previously investigated subset of parameter space on which oscillatory behavior exists.
Molecular flexibility and orientational ordering of nematic liquid crystals
Authors: Photinos Demetri J. , Samulski Edward T. , Toriumi Hirokazu
Published: July 2002

The ‘‘chord model’’ of flexible alkane solute ordering in the nematic phase [D. J. Photinos, E. T. Samulski, and H. Toriumi, J. Phys. Chem. 94, 4688 (1990); 94, 4694 (1990)] is extended to prolate mesogen molecules with a single alkyl tail, and the physical properties of two homologous series, 4,4′-n-alkylcyanobiphenyls and 4,4′-n-alkyloxycyanobiphenyls, in neat nematic phases are evaluated. The chord model of the mesogen’s anisotropic interactions yields a consistent description of the essential features of the chain order profile (derived from deuterium nuclear magnetic resonance) and the even–odd variation of thermodynamic quantities associated with the nematic–isotropic transition. Very accurate agreement with experimental observations is obtained using the standard rotational isomeric state description of the tail–chain’s flexibility along with a minimal form of the self-consistent anisotropic potential (i.e., only two coupling constants determine the strength of orientation dependent interactions, one for the biphenyl core and one for the aliphatic chord segments). There is remarkable compatibility among the values of optimized coupling constants obtained from modeling the chain segmental order parameters and the mesophase transition temperatures. Moreover, these coupling constants evaluated for aliphatic chord segments in the neat mesogens show quantitative agreement with the values obtained for simple alkane solutes in nematic solvents. These findings suggest that the chord model, which explicitly represents molecular shape anisotropy (excluded volume effects) in conjunction with molecular flexibility, provides a very good description of complex molecules in liquid crystals.
Emission polarization in the S3 state of CS2 vapor as a probe of predissociation: Consideration of the finite bandwidth of the incident field
Authors: Li Bulang , Myers Anne B.
Published: July 2002

Depolarization ratios of resonantly excited light emission have been measured for low pressure CS2 vapor at room temperature in three distinct spectral regions on resonance with the intense predissociative S3←S0 electronic transition. While the emission spectra have the general appearance of resonance Raman spectra, the non-negligible bandwidth of the laser source relative to the rovibronic linewidths implies that the emission should properly be described as a combination of incoherent ‘‘fluorescence’’ and true Raman processes. The effect on the emission polarization of a finite excitation spectral bandwidth arising from phase fluctuations in the incident radiation field is developed in detail. The depolarization dispersion curves are analyzed to estimate the J-averaged excited state lifetimes, which vary from 1.5 ps near 212 nm to 0.44 ps near 200 nm. The predissociation lifetimes appear to be at most weakly dependent on J based on analysis of the emission rotational band contours. Some discrepancies between the present lifetime estimates and those previously obtained through photofragment anisotropy measurements are discussed.
Deexcitation of He(2 1P) in a collision with a hydrogen molecule
Authors: Morishima Y. , Yoshida H. , Ukai M. , Shinsaka K. , Kouchi N. , Hatano Y.
Published: July 2002

Deexcitation cross sections of He(2 1P) by H2 have been measured using a pulse radiolysis method in a region of the mean collisional energy between 18 and 38 meV and compared with those obtained by theoretical calculations. Experimental cross-section values and their collisional energy dependence are interpreted in terms of the Penning ionization cross section based on a long-range dipole–dipole interaction between He(2 1P) and H2. Validity of the theoretical formula for the Penning ionization cross section, i. e., the Watanabe–Katsuura formula, is discussed. The cross section has been further calculated by means of an impact-parameter method with experimentally simulated classical trajectories; a hydrogen molecule has been assumed to be spherical to neglect the rotational effect of H2; it is also assumed that the polarization axis of the p-state helium rotates in order to keep collinear or perpendicular configuration with respect to the axis of He–H2. A modified form of the dipole–dipole autoionization width with an electron exchange interaction is also discussed.
Study of the density and temperature dependences of the vibrational Raman transition in compressed liquid N2
Authors: Oksengorn B. , Fabre D. , Lavorel B. , Saint-Loup R. , Berger H.
Published: July 2002

Accurate values of linewidth and line shift in the isotropic vibrational Raman spectrum of compressed liquid N2 have been obtained by using inverse Raman spectroscopy. Experiments have been performed for eight isotherms, from the normal boiling point to the critical point temperatures of N2, the number density varying between the value on the coexistence line, and the maximum value of 2.1×1022 cm−3. Minima of the linewidth have been observed above 86 K, showing the increasing influence of vibration–rotation coupling (motional narrowing) competing with the broadening due to pure vibrational dephasing. Moreover, for the first time, maxima of the red line shifts have been found, exhibiting increasing values as temperature increases. As far as the linewidth is concerned, the comparison with two existing models shows that negative interference of the attractive and repulsive forces must be taken into account for obtaining reasonable agreement between the experimental and theoretical parameters. The experimental density dependence of the linewidth can be described well enough with the Schweizer–Chandler model by taking only into account a soft-core, hard-sphere approximation for the calculation of the radial distribution functions. In the case of the frequency shifts, there is only qualitative agreement between the experimental and theoretical density dependences for the effective repulsive contributions calculated within the framework of the Schweizer–Chandler model.
Argon mediated electron impact ionization of Arn(CH3OH)m heteroclusters
Authors: Vaidyanathan Gopal , Coolbaugh M. Todd , Peifer William R. , Garvey James F.
Published: July 2002

In this paper we have contrasted electron impact cluster mass spectra of a methanol/argon expansion with a methanol/helium expansion and have measured appearance potentials for several of the prominent ions in each case. We have observed a number of ionic species in the argon expansion (i.e., CHxO+ where x=0–4) which are not observed in the helium expansion. Appearance potentials for these species fall in the region 11.3–11.8 eV indicating that the threshold ionization of Arn(CH3OH)m heteroclusters is mediated by the Ar 4s excited states (3P2,0;11.55 and 11.72 eV). This represents the first time that intracluster Penning ionization has been positively identified using electron impact measurements.
The 6d→5f fluorescence spectra of PaCl2−6 in a Cs2ZrCl6 crystal
Authors: Piehler David , Kot Wing K. , Edelstein Norman
Published: July 2002

Emission spectra from the parity-allowed electronic transitions between the lowest crystal field level of the 6d1 configuration to all the electronic levels of the 5f1 configuration of Pa4+ diluted into single crystals of Cs2ZrCl6 at 4.2 K show highly structured vibronic sidebands. Vibronic progressions are seen along the 310 cm−1 totally symmetric stretch of the PaCl2−6 complex. Most vibronic peaks correspond to even parity vibrations of the PaCl2−6 complex or the host lattice. Additionally, electronic Raman scattering was observed between the two lowest crystal field levels of the 5f1 configuration by exciting with an argon ion laser in near resonance with the lowest level of the 6d1 configuration.
A kinetic approach to the theory of nucleation in gases
Authors: Nowakowski B. , Ruckenstein E.
Published: July 2002

A theory of nucleation in gases is developed in which the rate of evaporation of the molecules from the cluster is calculated on the basis of a diffusion equation in the energy space. In contrast to the classical theory of nucleation, the theory does not employ macroscopic thermodynamics. Numerical calculations were performed by assuming an intermolecular potential which combines the dispersive attraction with the rigid core repulsion. The predictions of the theory are consistent with the classical theory in the limit of small supersaturations, i.e., for large critical clusters. For small critical clusters the present theory provides much higher rates of nucleation than the classical one. This difference can be attributed to the use of the macroscopic surface tension in the classical theory, which overpredicts the surface energy of small clusters, and consequently provides lower values for the nucleation rate. The theoretical results are compared with experimental data for nucleation of supersaturated vapors of lower alcohols carried in argon.