bluuebunny/crossref_metadata_2025 huggingface dataset:
The rotational spectrum of a highly vibrationally mixed quantum state. I. Intramolecular vibrational energy redistribution (IVR) exchange narrowing of the rotational spectrum
Authors: Pate Brooks H.
Published: July 2002

The description of the rotational spectrum of a single, highly vibrationally mixed molecular eigenstate is given using two formulations. The model studied is a set of anharmonically coupled normal-mode rovibrational states. In the first approach, the rotational problem is cast into the form of the single bright-state model of IVR. This eigenstate-level formulation reveals the fragmentation of the rotational spectrum as the magnitude of the anharmonic coupling is increased. It is also seen that the center frequencies of all of the molecular eigenstate rotational spectra approach the same value, determined by the ensemble average rotational constant, as the IVR rate is increased. Furthermore, this formulation provides a generalization to rotational spectroscopy. When there is extensive state mixing, the center frequency of the pure rotational spectrum of a single molecular eigenstate is determined by the expectation value of the rotational constant, and the width of the spectrum is determined by the quantum mechanical fluctuation of the rotational constant in the molecular eigenstate. The lineshape properties of the spectrum are addressed using the motional (exchange) narrowing formalism for the ensemble spectrum. This formulation provides a quantitative description of the narrowing of the rotational spectrum by an IVR exchange mechanism. Finally, the convergence of the line shape profile of the eigenstate rotational spectrum to the line shape of the ensemble spectrum is investigated using a statistical model Hamiltonian. Convergence is observed when the number of overlapping states, defined as Wrms*ρ, reaches 1. These results show that an experimental measurement of the rotational spectrum of single molecular eigenstates can provide important average properties of the rotational constant distribution, dipole moment distribution, and the IVR rate at a well-defined total energy.
Analyses of the local order in poly(ethylene terephthalate) in the glassy state by two-dimensional solid-state 13C spin diffusion nuclear magnetic resonance spectroscopy
Authors: Kaji Hironori , Horii Fumitaka
Published: July 2002

The locally ordered structure for poly(ethylene terephthalate) in the glassy state, quenched from the melt, has been investigated by two-dimensional solid-state 13C spin diffusion nuclear magnetic resonance spectroscopy. The orientational correlation is found to be successfully examined for different mixing times, which correspond to 13C spin diffusion times, through the 13C spin diffusion for carbonyl carbon 13C-labeled poly(ethylene terephthalate). For mixing times shorter than 0.8 s, the broadening of the diagonal peak occurs and its intensity decreases in the region from σ11 to σ22 with increasing mixing time. From a comparison with the simulated spectrum, it is suggested that such changes are due to the existence of the well ordered region even in the glassy state. For mixing times longer than 0.4 s, the intensities of the off-diagonal peaks are also found to increase with increasing mixing time. Detailed analyses of the off-diagonal peaks suggest that there is a stacked component for mixing times of around 1 s, in which the planes composed of the phenylene ring and two adjacent carbonyl groups are stacked with each other. The spectra for mixing times over 3.2 s, which reach the quasiequilibrium state, are in good accord with the spectrum simulated by assuming no long-range orientational correlation. From the calculation of the 13C spin diffusion rate, the radius of the region having the orientational correlation is estimated to be 0.5–1.4 nm.
Resonance affected scattering: Comparison of two hybrid methods involving filter diagonalization and the Lanczos method
Authors: McCormack Drew A. , Kroes Geert-Jan , Neuhauser Daniel
Published: July 2002

We apply two hybrid methods for solving scattering problems affected by resonances, to a four-dimensional reactive surface scattering system. In each method the solution of the problem is divided into two parts: a wave packet propagation, and a resonance calculation; results of the resonance calculation are used to extrapolate the long-time behavior of the system. In the first hybrid method, the propagation is by the multistep Chebyshev method, with calculation of resonances performed by the Lanczos method. In the second, the propagation is done using an implementation of the absorbing boundary condition (ABC) evolution operator, and the resonance calculation by filter diagonalization (FDG). Each method produces accurate scattering results in much less computation time than standard long-time wave packet propagation. The Chebyshev–Lanczos approach proves most capable for the calculation of resonances, but is computationally expensive. The ABC–FDG method is much cheaper to implement, but could not be made to extract accurate data for certain broad, overlapping resonances. This was overcome by propagating longer (still much shorter than for long-time propagation) to allow the elusive resonances time to decay.
A helium atom scattering study of the structure and dynamics of a Xe monolayer on NaCl(001)
Authors: Gerlach R. , Graham A. P. , Toennies J. P. , Weiss H.
Published: July 2002

The adsorption of xenon on the sodium chloride (001) surface has been studied using high-resolution helium atom scattering. From the temperature dependence of the specular intensity in the range between 27 and 70 K under isobaric conditions the heat of adsorption of the Xe monolayer was determined to be Eads=19.2±2 kJ/mol. At monolayer coverage, helium atom diffraction measurements reveal that a strained quasi-hexagonal superstructure is formed, which differs from the well-known hexagonal structure of the Xe bulk, and has a c(10×2) symmetry. The perpendicular frustrated translation vibrational energy was measured with helium atom time-of-flight spectroscopy to be 2.5 meV and showed an avoided crossing with the NaCl surface Rayleigh mode. A further dispersive mode of the monolayer has been assigned to a longitudinal in-plane mode of the Xe layer. As found for Xe monolayers on several low index copper surfaces the best fit radial force constant for this mode is a factor of four smaller than deduced from the Xe–Xe gas-phase potential.
Stabilized spin-polarized jellium model and odd-even alternations in jellium metal clusters
Authors: Payami M. , Nafari N.
Published: July 2002

In this paper, we have considered the mechanical stability of a jellium system in the presence of spin degrees of freedom and have generalized the stabilized jellium model, introduced by Perdew et al. [Phys. Rev. B 42, 11627 (1990)], to a spin-polarized case. By applying this generalization to metal clusters (Al, Ga, Li, Na, K, Cs), we gain additional insights about the odd-even alternations, seen in their ionization potentials. In this generalization, in addition to the electronic degrees of freedom, we allow the positive jellium background to expand as the clusters’ polarization increases. In fact, our self-consistent calculations of the energetics of alkali metal clusters with spherical geometries, in the context of density functional theory and local spin density approximation, show that the energy of a cluster is minimized for a configuration with maximum spin compensation (MSC). That is, for clusters with an even number of electrons, the energy minimization gives rise to complete compensation (N↑=N↓), and for clusters with an odd number of electrons, only one electron remains uncompensated (N↑−N↓=1). It is this MSC rule which gives rise to alternations in the ionization potentials. Aside from very few exceptions, the MSC rule is also at work for other metal clusters (Al, Ga) of various sizes.
Inducing and imaging single molecule dissociation on a semiconductor surface: H2S and D2S on Si(111)-7×7
Authors: Rezaei M. A. , Stipe B. C. , Ho W.
Published: July 2002

Using a variable-temperature, ultrahigh vacuum scanning tunneling microscope (STM), we have induced and imaged the dissociation of H2S and D2S on Si(111)-7×7. H2S and D2S adsorb dissociatively at low coverage, from 50 to 300 K. Individual HS (or DS) fragments can be further dissociated with the STM at low temperatures without affecting neighboring adsorbates. The hydrogen (deuterium) atom either desorbs or re-attaches to a nearby silicon atom. Near room temperature (297 K) and above, DS dissociates thermally, with an activation barrier of 0.73±0.15 eV. The activation barrier was calculated from atomistic studies of the dissociation rates at temperatures between 297 and 312 K.
Dissociative electron attachment and charging of SF6 adsorbed on rare-gas films
Authors: Weik Fritz , Illenberger Eugen
Published: July 2002

Electron stimulated desorption (ESD) of fragment ions in the energy range between 0 and 18 eV from SF6 adsorbed on rare-gas films (Kr, Xe) is reported. The ESD results are compared with previous experiments on dissociative electron attachment (DA) to gas-phase SF6. At energies characteristic for the respective rare-gas substrate strong resonant enhancements in the ESD yield of F− are observed. This enhancement is explained by the appearance of an “electron–exciton complex” in the rare-gas film (the analogue to the anionic Feshbach resonances in single atoms) which couples to the first dipole allowed excitation of the SF6 molecule. After electron and energy transfer, the highly excited SF6*− ion dissociates at the surface resulting in the desorption of F− fragments. At low electron energies (in the range from 0 to 0.6 eV) charging of the rare-gas film covered with SF6 is observed. From these experiments a charging cross section of 2.1(±1.8)×10−15 cm2 is derived.
Responses of an immobilized-catalyst Belousov–Zabotinsky reaction system to electric fields
Authors: Miyakawa Kenji , Mizoguchi Michiko
Published: July 2002

Effects of direct and alternating current electric fields on the chemical oscillator have been investigated. The chemical oscillator was realized by immersing the cation exchange bead loaded with the ferroin catalyst in the Belousov–Zabotinsky reaction solution. Various dynamic behaviors appeared depending on the direction, the frequency, and the strength of the electric field. Their time series were characterized in terms of power spectra. Observed behaviors were explained by assuming a field-induced transport of the inhibitor Br−. A phase diagram concerning dynamic behaviors was obtained as a function of the frequency and the strength of the electric field. The irregular oscillation was found to necessarily appear in the transition between 1/1 and 1/2 (or 2/1) entrainments. For further examination, a three-dimensional phase portrait was constructed using the time-delay method, and the largest Lyapunov exponent was calculated.
Restricted primitive model of an ionic solution confined to a plane
Authors: Weis J. J. , Levesque D. , Caillol J. M.
Published: July 2002

A model system of electrolytes, the three-dimensional restricted primitive model (RPM), is investigated for the case where the charged hard spheres are constraint to move on a two-dimensional surface. The properties of the thermodynamic states corresponding to the gas and liquid phases have been determined by computer simulation for a wide range of densities and temperatures. The simulations provide results for the equation of state, internal energy, and correlation functions of the 3D RPM confined to 2D, and allow a characterization of the local order of the charged spheres as a function of density and temperature. The most striking aspect of this order is the organization, at low, although supercritical, temperature and low to moderate densities, of the spheres into “clusters” where linear arrangements or chains of alternatively positive and negative charges dominate. The domain of validity of the hypernetted chain integral equation is investigated. The critical temperature of the gas–liquid transition is located with precision by means of a finite size scaling analysis of the simulation results in the critical region. With regard to the critical behavior of the 2D RPM, the limitations on system sizes which could be studied do not allow to conclude more than possible compatibility with the 2D Ising universality class.
Ab initio molecular dynamics simulations of liquid GaAs
Authors: Godlevsky V. , Chelikowsky James R.
Published: July 2002

We report results for the electronic, structural and dynamical properties of liquid GaAs simulated by using ab initio molecular dynamics. Results for the total and partial structure factors, self-diffusion properties, pair correlation and angular distribution functions, and the electronic density of states are presented. Our calculated structure factor and pair correlation functions are in good agreement with available experimental data. A “common neighbor” cluster analysis is used to examine the structure of the liquid.
Multireference perturbation theory with flexible energy denominators
Authors: Finley James P.
Published: July 2002

Multireference perturbation theory (MRPT) is formulated with one-, two-, and higher-body shifts introduced into the energy denominators in a manner that preserves the linked diagram theorem (LDT) and the exponential form of the wave operator. Increased flexibility in the choice of energy denominators permits perturbative expansions to have improved convergence behavior and avoid intruder states. This flexible-energy-denominator method can be combined with modified version of previous formulations to yield methods that have improved convergence behavior and/or retain the LDT.
Theoretical interpretation of the electroabsorption spectrum of fullerene films
Authors: Pac Barbara , Petelenz Piotr , Slawik Michał , Munn R. W.
Published: July 2002

A three-dimensional analogue of the Merrifield model of the coupling between the Frenkel and charge transfer (CT) excitations of a molecular crystal is used to calculate the electroabsorption (EA) spectrum of buckminsterfullerene films. The approach is essentially nonempirical, with most of the input parameters estimated from theoretical calculations or from independent experiments, but in doubtful cases a comparison of the calculated and experimental EA spectra guides the choice between alternative sets of parameters. Semiquantitative agreement between the calculated and experimental EA spectra lends credence to the model, which can therefore be used to draw conclusions concerning the underlying physical mechanisms. The EA signal of the CT states is shown to be substantially affected by the off-diagonal CT interactions, and by interactions with the optically inactive Frenkel excitons, whose intramolecular parentage is tentatively assigned. The results confirm the validity of previous calculations of the electrostatic CT state energies.
A quantum electrodynamical treatment of second harmonic generation through phase conjugate six-wave mixing: Polarization analysis
Authors: Hands Ian D. , Lin Shujie , Meech Stephen R. , Andrews David L.
Published: July 2002

The theory underlying a six-wave mixing experiment is developed using the methods of molecular quantum electrodynamics. This general theory allows the intensity of the second harmonic radiation generated by the six-wave process to be found for arbitrary arrangements of the generating laser beams. Several different polarization geometries are treated in detail, and comparison is made to experiments performed using near-resonant conditions. The agreement is good in all cases and allows detailed information pertaining to the six-wave tensor to be extracted. The information thus obtained provides evidence of a marked departure from Kleinman symmetry.
Density- and density-matrix-based coupled Kohn–Sham methods for dynamic polarizabilities and excitation energies of molecules
Authors: Görling Andreas , Heinze Habbo H. , Ruzankin Sergey Ph. , Staufer Markus , Rösch Notker
Published: July 2002

Basis set methods for calculating dynamic polarizabilities and excitation energies via coupled Kohn–Sham equations within time-dependent density functional theory are introduced. The methods can be employed after solving the ground state Kohn–Sham equations with a fitting function approach. Successful applications of the methods to test molecules are presented. Coupled Kohn–Sham methods based on the linear response of the Kohn–Sham density matrix are derived from the standard coupled Kohn–Sham equation based on the linear response of the electron density and the relations between the two types of coupled Kohn–Sham equations are investigated. The choice of norm functions associated with basis set representations of the coupled Kohn–Sham equations is discussed and shown to be a critical point of basis set approaches to time-dependent density functional theory.
Electrical interaction energy between two charged entities
Authors: Hsu Jyh-Ping , Liu Bo-Tau
Published: July 2002

A systematic approach is proposed for the derivation of the electrical interaction energy between two charged entities under a general surface condition. The analysis provides necessary information which facilitates the description of various phenomena of practical significance such as the adsorption of particles to surfaces, boundary effect on the electrophoretic behavior of a colloidal particle, and the behavior of a dispersed phase, e.g., its stability and phase transition. Four examples are provided to justify the applicability of the present method: a sphere and a planar surface, a sphere in a planar slit, a sphere in a spherical pore, and a sphere in a cylindrical pore.
The melting line of diamond determined via atomistic computer simulations
Authors: Glosli James N. , Ree Francis H.
Published: July 2002

The diamond melting line was determined for a model system based on Brenner’s bond order potential for hydrocarbon systems. The location of this first-order phase boundary was found by a free energy calculation of the diamond and liquid phases, using atomistic simulation methods. The melting line was found to have a positive slope consistent with the present understanding. The location at lower pressure was also consistent with experiment and a number of other theoretical approaches. The slope is found to increase with pressure. The structure of the liquid is examined as a function of density and is suggestive of a liquid–liquid phase boundary.
Dipole moment of nonrigid molecular systems: The dipole moment of hydrochloric dimers
Authors: Naaman Ron , Vager Zeev
Published: July 2002

In recent experiments on HCl dimers, three surprising observations were found. A “dipole moment” was measured in Stark-shift-related experiments despite the spectroscopic indication of fast tunneling motion. The measured dipole moment varied with the state probed, and the apparent first-order Stark effect was observed only in the case of the heterodimer H35Cl–H37Cl but not for the homodimers (H35Cl)2 or (H37Cl)2. We present physical arguments which explain all the observations and indicate that the spectroscopy has to be reinterpreted and that the isotope effect in the tunneling motion can not be inferred simply from the spectroscopy.
Photofragment angular momentum distributions in the molecular frame: Determination and interpretation
Authors: Rakitzis T. Peter , Zare Richard N.
Published: July 2002

Photolysis of a molecule typically yields open-shell photofragments having angular momenta. A procedure is described for the measurement of the photofragment angular momentum distribution in terms of polarization parameters aq(k)(p) which are expressed in the molecular frame and which may be related to the transition dipole matrix elements. The index (p) indicates either a parallel transition (∥), a perpendicular transition (⊥), or a mixed transition (∥,⊥) having both parallel and perpendicular character. This procedure has the advantage that it decouples the angular momentum distributions in the molecular frame from the photofragment angular distributions in the laboratory frame, which gives new insight into the photodissociation dynamics. For cases in which k⩽2 and with linearly polarized photolysis light, the photofragment angular momentum distribution arising from pure parallel transitions can be described with only one parameter, a0(2)(∥); photofragment angular momentum distributions arising from pure perpendicular transitions require only two parameters, a0(2)(⊥) and a2(2)(⊥); photofragment angular momentum distributions arising from mixed transitions, having both parallel and perpendicular character, can be described with five parameters: the two (coherent) interference terms Im[a1(1)(∥,⊥)] and Re[a1(2)(∥,⊥)] in addition to the three incoherent terms mentioned above. We describe procedures for the measurement of the complete angular momentum distribution of state-selected photofragments using laser detection (such as REMPI) and some form of laboratory velocity selection (such as time-of-flight mass spectrometry, Doppler spectroscopy, or ion imaging). The laser-detection probability of a single photofragment is presented in the form I=1+f[θε,Θ,Φ,β,aq(k)(p)], where θε is the angle between the recoil direction and the photolysis polarization, Θ and Φ are the spherical polar angles describing the orientation of the probe polarization about the recoil direction, and β is the spatial anisotropy parameter. The physical significance of the aq(k)(p) is discussed; in particular, the a0(k)(∥) and a0(k)(⊥) describe the photofragment m-state distribution along the recoil direction; the a2(k)(⊥) describe how broken cylindrical symmetry in the parent molecule is reflected in the photofragment angular momentum distribution in a plane perpendicular to the recoil direction; and the a1(k)(∥,⊥) are related to the asymptotic phase difference associated with the interfering channels, and are thus sensitive to the shapes of the dissociative surfaces.
On the Feynman path centroid density for Bose-Einstein and Fermi-Dirac statistics
Authors: Roy Pierre-Nicholas , Voth Gregory A.
Published: July 2002

Expressions are derived for the Feynman path centroid density of systems of identical particles, namely, Bosons and Fermions. The resulting expressions are applied to a system of two independent particles in a quartic oscillator and the corresponding centroid densities are computed, plotted and compared to the distinguishable particle case. In regions where the particle centroids are close to each other, the Boson centroid density displays an enhanced amplitude in comparison to the distinguishable case. This behavior is attributed to the attractive correlations in Boson systems. The Fermion centroid density, however, can have the peculiar property of being negative in those regions where the particle centroids are in close proximity. This feature is related to the Pauli exclusion principle. This property of the Fermion centroid density rules out its strict interpretation as a probability distribution. Equilibrium properties such as the canonical partition function and the average position were accurately computed using both densities. The Boson density was also used to compute the position autocorrelation function using the Centroid Molecular Dynamics method and the results are in excellent agreement with those of an exact quantum calculation.
Generalized symmetric Rayleigh–Ritz procedure applied to the closed-shell Hartree–Fock problem
Authors: Wadleigh Harold H. , Ionova Irina V. , Carter Emily A.
Published: July 2002

We present the Generalized Symmetric Rayleigh–Ritz (GSRR) procedure for finding approximate eigenfunctions and corresponding eigenvalues for a linear operator, L, in a finite function space, {φi}i=1N. GSRR is derived by minimizing the residual in the norm induced by an inner product, (⋅,⋅), under the constraint that the resulting eigenfunctions be mutually orthogonal with respect to another inner product, (⋅,⋅)a. When L is the closed-shell Fock operator, f, GSRR is a generalization of the Roothaan equations. We apply this method to f with (⋅,⋅) defined by a grid, {rk}k=1M, and (⋅,⋅)a defined by analytic integration, noting that a grid-defined (⋅,⋅) lends itself to faster operator evaluation (scaling as MN2) and effective parallelization. When a grid is used, GSRR scales as pseudospectral methods do; however, it is in the spirit of conventional spectral methods (e.g., GSRR does not use an inverse transform).
Analysis of the Pechukas description of mixed quantum-classical dynamics
Authors: McWhirter J. Liam
Published: July 2002

We review the Pechukas stationary phase analysis which yields the semiclassical reduced propagator. This propagator describes the dynamics of a mixed quantum-semiclassical system. In addition, we review a variant of this analysis, presented by us in an earlier article [J. Chem. Phys. 108, 5683 (1998)], which yields a semiclassical reduced propagator written in terms of connected and partially connected “classical” paths. These paths are obtained by solving the concatenation of several short time interval Pechukas equations. We argue and then demonstrate numerically that the “energy” along these paths is generally piecewise conserved: conserved across one short time interval, but not across several such intervals. In our review of these analyses, we relax the assumption made by Pechukas that the magnitude of the transition amplitude associated with the quantum subsystem varies much more slowly with changes in the classical subsystem’s trajectory than its phase. As our analyses demonstrate, this assumption serves to simplify the evaluation of the normalization path integral; the stationary phase paths are not affected by the making of this assumption. Solving the Pechukas equation subject to the initial configuration and velocity of the classical subsystem yields a collection of “classical” paths; the solution is nonunique. We provide a short time uniqueness theorem pertaining to the class of functional differential equations to which the Pechukas equation belongs; then, we review the sample problem that Pechukas used to first demonstrate this nonuniqueness. The theorem and the sample problem allow us to identify which parts within the Pechukas equation’s structure are responsible for the nonuniqueness in its solution. This nonuniqueness is verified numerically. Here, we show that the “energy” is conserved along each of the “classical” paths in the collection. However, the “energies” of any two paths in this collection will be the same only if the initial state specified in the Pechukas equation is an instantaneous adiabatic eigenstate of the quantum subsystem Hamiltonian which contains the interaction potential between the quantum and classical subsystems.
Guided-ion beam study of the O2++C2H2 charge-transfer and chemical reaction channels
Authors: Chiu Yu-hui , Dressler Rainer A. , Levandier Dale J. , Williams Skip , Murad Edmond
Published: July 2002

Guided-ion beam cross section and product ion time-of-flight (TOF) measurements are presented for the O2+(2Πg)+C2H2 reaction over the center-of-mass collision energy range of 0.05–12 eV. The reaction exhibits a strong charge-transfer (CT) channel and several weak chemical reaction channels leading to the following product ions: C2H2O+, CH2+, COH+/HCO+, CO+, CH+, and C+. The magnitude of the charge-transfer cross section is comparable to the capture cross section at low collision energies. The charge-transfer products are characterized predominantly by a direct, long-range mechanism that is accompanied by smaller impact parameter collisions involving longer-lived complexes, whose lifetimes are several times shorter than the complex rotational period. The TOF analysis indicates that the complexes also decay to chemical reaction products at higher energies. A density functional survey of intermediates is conducted. The chemical reaction products are postulated to be formed via an endothermic transition state associated with a H2CCO2+ intermediate.
Infrared-laser driven vibrational excitation of relaxing adsorbates: Quantum dynamical aspects
Authors: Paramonov G. K. , Saalfrank Peter
Published: July 2002

As a first step to the active manipulation of adsorbates by external, time-dependent electromagnetic fields, the infrared-laser driven selective excitation of molecular vibrations of adsorbates at metal surfaces is investigated here in the framework of time-dependent open-system density matrix theory. Special emphasis is given to the inclusion of vibrational damping, caused by the coupling of the adsorbate vibrations to possibly electronic substrate degrees of freedom. For the example system NH3/Cu, a non-Markovian, two-mode open-system Liouville–von Neumann model for the vibrational relaxation of an excited adsorbate is proposed. After studying the field-free decay of excited adsorbates, it is shown that even in rapidly relaxing environments optimal IR laser pulses in the picosecond domain can be designed which lead to temporarily high populations of selected target states of adsorbates at metal surfaces.
Determination of hydrodynamic properties in highly charged colloidal systems using static and dynamic light scattering
Authors: Härtl W. , Beck Ch. , Hempelmann R.
Published: July 2002

The structure and dynamics of highly charged colloidal silica systems has been investigated at low concentrations by means of light scattering. The index of refraction of the colloidal particles was matched using a mixture of water/glycerol. By simultaneously measuring the dynamic and static properties of the colloidal systems, we were able to extract the hydrodynamic function H(Q) at different concentrations. Our results are in excellent agreement with theoretical calculations over a large concentration range.
Stereodynamics of the vibrational channel O(1D)+H2O→OH(v′=2)+OH
Authors: Tsurumaki Hiroshi , Fujimura Yo , Kajimoto Okitsugu
Published: July 2002

The state-selected differential cross section (DCS) and rotational angular momentum polarization for the reaction O(1D)+H2O→OH+OH have been measured by utilizing the polarized Doppler-resolved laser-induced fluorescence probing technique. Stereodynamics of the reaction channel forming the newly formed OH in the specific vibrational level v′=2 is discussed on the basis of the vector properties. A nearly isotropic DCS for the product OH(2Π3/2, v′=2, j′=5.5) most probably indicates that the reaction is dominated by an insertion mechanism involving a collisional HOOH complex with a lifetime comparable to its rotational period. The extremely asymmetrical energy partitioning between the two OH fragments, therefore, suggests that the redistribution of the available energy does not occur on a time scale comparable to the rotational period of the complex. Furthermore, it has been found that the product rotational angular momentum vector j′ is predominantly perpendicular to the collision plane spanned by k and k′ (the relative velocity vectors of the reactants and products, respectively) both for the forward- and backward-scattered products. It suggests that the initially excited bending motion of the H–O–O moiety in the collisional HOOH complex primarily contributes to the product rotation.
An ab initio study of the torsional-puckering pseudorotation in the cyclic water pentamer
Authors: Graf Stephan , Mohr Wolfgang , Leutwyler Samuel
Published: July 2002

The intermolecular potential energy surface and the torsional-puckering (TP) motion of the cyclic homodromic water pentamer were theoretically investigated at the level of second-order Møller–Plesset perturbation theory using double- and triple-zeta correlation-consistent basis sets including diffuse functions. Extensive searches yielded three cyclic homodromic stationary points: (i) the puckered global minimum structure GS, (ii) a torsional-puckering saddle point, 62 cm−1 higher, denoted TS1, and (iii) a planar C5h symmetric structure (Hessian index 7), 800 cm−1 higher in energy. Harmonic vibrational frequencies and normal modes were calculated for all stationary points. Based on the energies and second derivatives of the GS and TS1 structures a cyclic minimum energy path for the large-amplitude TP motion with ten permutationally equivalent minima was constructed. The ground and all excited states are delocalized by TP pseudorotation, giving vibrationally averaged C5h symmetry. The first pseudorotational TP level lies at ≈1 cm−1 (≈0.6 cm−1) and the ten lowest TP states within a band of ≈20 cm−1 (8 cm−1) for (H2O)5 [(D2O)5]. The corresponding TP dipole moment functions were also calculated, and intensities for the far-infrared torsional-puckering transitions of (H2O)5 and (D2O)5 evaluated up to 300 cm−1.
Scalar and angular correlations in CF3NO photodissociation: statistical and nonstatistical channels
Authors: Spasov Julia S. , Cline Joseph I.
Published: July 2002

Polarized 1+1′ REMPI combined with TOFMS detection was used to obtain recoil speed distributions, NO product rotational distributions, and μ–v–jNO angular correlations in the photofragmentation of CF3NO between 585 and 650 nm. At wavelengths longer than 610 nm dissociation occurs only on the S0 potential energy surface of CF3NO. At 585 nm the product recoil speed distribution is bimodal, with the higher speed component attributed to dissociation across a potential barrier on the T1 surface. Measurements of the T1 channel NO product trajectory show it has no laboratory frame rotational alignment or recoil anisotropy, but has a significant preference for v⊥jNO angular correlation, reaching a maximal value of β00(22)=−0.308±0.052. The results are interpreted using the results of statistical phase space theory and classical trajectory calculations. It is concluded that S0 channel dissociation occurs by a statistical dissociation mechanism whereas the T1 channel dynamics is dominated by exit channel recoil forces due to a potential barrier.
Hybrid quantum/classical simulation and kinetic study of the vibrational predissociation of Cl2⋯Nen (n=2, 3)
Authors: Bastida Adolfo , Miguel Beatriz , Zuñiga José , Requena Alberto , Halberstadt Nadine , Janda Kenneth C.
Published: July 2002

A hybrid quantum/classical method is applied to the vibrational predissociation of van der Waals clusters containing a diatomic molecule and several rare gas atoms, Cl2⋯Nen (n=2, 3). The vibrational degree of freedom of the diatomic is treated quantum mechanically while all the other degrees of freedom are treated classically. A kinetic mechanism is proposed in order to interpret the dynamics in terms of the following elementary steps; vibrational predissociation (VP), intramolecular vibrational redistribution (IVR), and evaporative cooling (EC). The resulting lifetimes are in very good agreement with the experimental linewidth measurements of Janda and co-workers, and with the quantum mechanical reduced-dimension results of Le Quéré and Gray on Cl2⋯Ne2. The final rotational state distributions agree very well with the experimental results and exhibit a quasistatistical behavior. The final vibrational distributions reproduce the main experimental features.
Unimolecular dissociation of NO2. I. Classical trajectory and statistical calculations on a global potential energy surface
Authors: Grebenshchikov S. Yu. , Beck C. , Flöthman H. , Schinke R. , Kato S.
Published: July 2002

The title reaction is examined with classical and statistical (RRKM, SACM) mechanics on an ab initio three-dimensional global potential energy surface of the lowest adiabatic electronic state of NO2. The energy-resolved rate constants obtained within different theoretical frameworks are compared with each other and with the available experimental data. The question of the transition state definition in this barrierless reaction is analyzed and the applicability of the statistical hypothesis is discussed. Although the rate of internal vibrational energy redistribution is estimated to be larger than 5 ps−1 at the quantum dissociation threshold, which is much larger than the dissociation rate, a substantial fraction of classical trajectories remains regular. This causes statistical transition-state-like theories to overestimate the actual dissociation rate defined from the full classical dynamics calculations.
Density functional investigation on the ferromagnetic coupling of spins in phenylenevinylene-bridged nitroxide radicals: Monomer and polymer cases
Authors: Mitani Masaki , Yamaki Daisuke , Yoshioka Yasunori , Yamaguchi Kizashi
Published: July 2002

Magnetic interactions in phenylenevinylene-bridged organic radicals are investigated as realistic models of pendant-type ferromagnetic polymers, since several molecules having some types of phenylenevinylene skeletons with phenoxy or nitroxide radicals were recently synthesized and their ferromagnetic characters were also confirmed by experimental works. In the present study, a spin source of NOH nitroxide radical is examined as a model of stable tertiary-butyl nitroxide in this system. According to NOH orientations, it is possible to form hydrogen bond between radical O and vinylene H atoms which may work as additional effects on the ferromagnetic spin coupling. To elucidate this point, density-functional calculations are carried out for both monomers and polymers by using Becke’s exchange with Lee–Yang–Parr correlation functional. It is detected that the influence of hydrogen bond is emphasized in polymers by the cooperative interaction arising from one-dimensional network along chain. Implications of the hydrogen-bonding effects are briefly discussed in relation to hydrogen abstraction reactions by nitroxide groups and stabilizations of ferromagnetic polymers by selections of substitution positions of radical groups. Computational results were also compared with available experimental results by Nishide, Tsuchida, and others. Finally, a new proposal for synthesis of thermal and photo-induced organic ferrimagnets is presented.
Wetting of a fluid interface by a homopolymer: A system with a rich prewetting behavior
Authors: Leermakers F. A. M. , Dorrepaal C. , Besseling N. A. M.
Published: July 2002

A lattice-based self-consistent-field theory is used to reveal the wetting characteristics of a polymeric phase at an interface between two solvent phases. Both solvents are monomeric and the polymer chains are modeled as freely jointed chains of N=100 similar segments. From the polymer point of view we limit our investigations to the symmetric case; both solvents are equally poor for the polymer. We show that the polymer wets the interface both in the limit of strongly and weakly segregated liquids, but that at intermediate values of the L/L demixing the polymer phase can be in a lens configuration (partial wetting). The wetting transitions at strong segregation of the two solvents is always first order, the ones at weak segregation can be either first or second order, depending on the polymer/solvent interactions. For a particular value of the polymer–solvent interaction parameter the two wetting transitions as generated by changing the L/L demixing (then both of the first-order type) merge. At this point the two prewetting lines still exist and obviously merge as well. It is further possible to find detached “prewetting” lines in the regime where the polymer wets the interface at all relevant (i.e., consistent with the presence of three phase coexistence) values of the L/L demixing. Physically this means that there is a range of interfacial widths for which the formation of a new (polymeric) phase is hindered by a free energy barrier at a certain film thickness, but that there is no wetting transition associated with this barrier.
Response to “Comment on ‘Reversible work of formation of an embryo of a new phase within a uniform macroscopic mother phase’ ” [J. Chem. Phys. 111, 3769 (1999)]
Authors: Debenedetti Pablo G. , Reiss Howard
Published: July 2002

External constraints are necessary in order to calculate the energetics of embryo formation when the embryo is not a critical nucleus. The expression for the reversible work of formation obtained in by Debenedetti and Reiss [J. Chem. Phys. 108, 5498 (1998)] is rigorous and valid regardless of the relative densities of the embryo and mother phase.
Simulation of mutation: Influence of a “side group” on global minimum structure and dynamics of a protein model
Authors: Vekhter Benjamin , Berry R. Stephen
Published: July 2002

The 46-bead, three-color model of a β-barrel-forming protein is modified by the addition of a single side group, represented by a bead which may be hydrophilic or hydrophobic. Molecular dynamics and quenching simulations show how the nature and location of the bead influence both the structure at the global minimum of internal energy and the relaxation processes by which the system finds its minima. The most drastic effects occur with a hydrophobic side group in the middle of a sequence of hydrophobes.
Two-exciton states and spectroscopy of phenylacetylene dendrimers
Authors: Chernyak Vladimir , Poliakov Evgeni Y. , Tretiak Sergei , Mukamel Shaul
Published: July 2002

The two-exciton wave functions of conjugated dendrimers with fractal geometries are calculated using the Frenkel-exciton model. Self-similarity and the high degree of symmetry make it possible to express the two-photon spectra of these chromophore aggregates in a compact form using irreducible representations of optical excitations, single-exciton states, and an effective two-exciton transition dipole moment. The explicit calculation of the complete manifold of two-exciton states which involves an expensive l3×l3 diagonalization, l being number of generations, is totally avoided. A real space analysis shows that the two-exciton states and resonances are dominated by periphery chromophores due to their exponentially large number.
Hydrogen bond network fluctuations in small (H2O)n clusters (n=8–12–24)
Authors: Masella Michel , Flament Jean-Pierre
Published: July 2002

Molecular dynamic studies of three (H2O)n clusters (n=8–12–24) were performed using our recently developed many-body model TCPE [J. Chem. Phys. 107, 9105 (1997)] in the microcanonical ensemble. The trajectories were analyzed using a new structural local index derived from one of the many-body energetic term of TCPE. In the energy domain where the clusters are in a liquidlike state, a dynamical equilibrium is theoretically predicted to exist among molecules in the PHB1 and the PHB2 state (i.e., among molecules which one of their protons is involved in one hydrogen bond, the PHB1 state, and molecules which both of their protons are involved in two hydrogen bonds, the PHB2 state). The enthalpy and entropy changes corresponding to that equilibrium for the three clusters range from 0.75 to 1.10 kcal mol−1 and from −7 to −3.8 cal mol−1 K−1. Such an equilibrium between two species of hydrogen bonded molecules could be related to that experimentally observed in the case of liquid water at ambient conditions. In particular, the entropy changes corresponding to PHB2/PHB1 equilibrium in the case of the three clusters are very close to those experimentally reported for liquid water (about 6.6±0.5 cal mol−1 K−1), suggesting that the equilibrium observed in the case of liquid water could correspond to a PHB2/PHB1 equilibrium. The analysis of hydrogen bond networks in terms of PHBm states appears thus to be an encouraging way in characterising the dynamical properties of water systems.
Long-range effects in optimizing the geometry of stereoregular polymers. I. Formalism
Authors: Jacquemin Denis , André Jean-Marie , Champagne Benoı̂t
Published: July 2002

We develop an original methodology to calculate analytically the long-range (LR) Coulombic effects to be included in Hartree–Fock forces computed on stereoregular polymers. The technique presented, based on multiple Taylor series expansions, is completely general and can be easily extended to all orders of expansion and to geometrical derivatives of higher order. In the McMurchie Davidson or similar schemes, the LR terms are added directly to Hermite integrals and LR effects are naturally considered during the computation of the energy and its derivatives. Each type of derivative (nuclear-repulsion energy, overlap and kinetic energy integrals, electron–nuclear attraction and two-electron integrals) is examined to quantify the impact of LR corrections. It turns out that the lattice sums of the gradients evaluated on a model macromolecule converge much faster when including those corrections. In addition, the dependence of the energy with respect to the unit cell length is for the first time considered in full details.
Structure of binary hard-sphere mixtures near a hard wall: A simple weighted-density-functional approach
Authors: Patra Chandra N.
Published: July 2002

The structure of binary hard-sphere mixtures near a hard wall is studied using a density functional theory. The formalism is based on a simple weighted density approach for the one-particle correlation functions of the nonuniform system, and requires as input only the one- and two-particle direct correlation functions of the corresponding uniform system. The approach is designed in a way, where the weight function is decoupled from the weighted density. Numerical results on the density profiles are shown to compare well with available simulation data.
Rotational and vibrotational transfer in H*–CO2 collisions: The influence of stereokinematic restrictions
Authors: Clare Suzanne , Marks Alison J. , McCaffery Anthony J.
Published: July 2002

We describe a quantitative angular momentum (AM) model for predicting rotational transfer (RT) and vibrotational transfer (VRT) in collisions between CO2 and hot H atoms. This molecule is important in several contexts, not least as a bridge between the relative simplicity of diatomic molecules and the complexities of polyatomic RT and VRT. We show that for pure RT, an AM constraint dominates but that this changes to a dominant energetic constraint in the case of VRT. The requirement that the (001) vibrational channel be opened simultaneously with the generation of AM imposes special restrictions which effectively limit the trajectories that lead to VRT. The origin of this is a constraint-induced restriction on the effective impact parameter (bnmax) for individual Δj channels and the effect is manifest as reduced probability for populating low Δj channels. In CO2–H* this leads to a shift in the peak of (VRT) Δj probabilities away from zero as found experimentally for the (001) vibrational mode. We report a Monte Carlo trajectory calculation similar to that of Kreutz and Flynn [J. Chem. Phys. 93, 452 (1990)] but predict an exponential-like dependence of pure RT on Δj. For VRT to (001) the constraint-induced restrictions on bnmax are incorporated quantitatively and the vibrational channel-opening velocity is treated as a vector quantity. The results of these calculations are in good agreement with experiment. The underlying mechanism, likely to be general in VRT, is clearly revealed in plots of relative velocity versus rotational AM change.
Optically detected electron paramagnetic resonance by microwave modulated magnetic circular dichroism
Authors: Börger Birgit , Bingham Stephen J. , Gutschank Jörg , Schweika Marc Oliver , Suter Dieter , Thomson Andrew J.
Published: July 2002

Electron paramagnetic resonance (EPR) can be detected optically, with a laser beam propagating perpendicular to the static magnetic field. As in conventional EPR, excitation uses a resonant microwave field. The detection process can be interpreted as coherent Raman scattering or as a modulation of the laser beam by the circular dichroism of the sample oscillating at the microwave frequency. The latter model suggests that the signal should show the same dependence on the optical wavelength as the MCD signal. We check this for two different samples [cytochrome c-551, a metalloprotein, and ruby (Cr3+:Al2O3)]. In both cases, the observed wavelength dependence is almost identical to that of the MCD signal. A quantitative estimate of the amplitude of the optically detected EPR signal from the MCD also shows good agreement with the experimental results.