bluuebunny/crossref_metadata_2025 huggingface dataset:
Full-dimensional quantum mechanical calculation of the rate constant for the H2+OH→H2O+H reaction
Authors: Manthe Uwe , Seideman Tamar , Miller William H.
Published: July 2002

The cumulative reaction probability (CRP) (the Boltzmann average of which is the thermal rate constant) has been calculated for the reaction H2+OH↔H2O+H in its full (six) dimensionality for total angular momentum J=0. The calculation, which should be the (numerically) exact result for the assumed potential energy surface, was carried out by a direct procedure that avoids having to solve the complete state-to-state reactive scattering problem. Higher angular momenta (J≳0) were taken into account approximately to obtain the thermal rate constant k(T) over the range 300<T<700 K; the result is significantly larger than the experimental values (a factor of ∼4 at 300 K), indicating that a more accurate potential energy surface is needed in order to provide a quantitative description of this reaction.
Rotational excitations of quantum liquid clusters: He7 and (H2)7
Authors: McMahon Michele A. , Barnett Robert N. , Whaley K. Birgitta
Published: July 2002

We have studied the quantum clusters, He7 and (H2)7, in their ground (L=0) and rotationally excited states (L=2–8 for He and L=2–12 for H2) by variational Monte Carlo, and selectively by diffusion Monte Carlo. The optimized trial wave functions are eigenfunctions of L2, and, as such, are orthogonal and yield upper bounds to the energy of each state. We report energies and structural results, including density profiles and moments-of-inertia distributions. Overall, the He7 cluster is more delocalized than the (H2)7 cluster, but the computed structures show that both He7 and (H2)7 are highly nonclassical and nonrigid, evolving from spherical to toroidal structures as the angular momentum increases. We find that by L=2 for He7 and by L=6 for (H2)7, the clusters become metastable with respect to dissociation, as determined by comparison of excited-state energies of the N=7 clusters with the ground-state energies for N=6. The root-mean-square bond length fluctuations indicate that both N=7 clusters are liquidlike in the L=0 ground state, according to the Lindemann criterion. These fluctuations increase with L, implying a further decrease in rigidity for the rotationally excited states.
A cell theory for solid solutions: Application to hard sphere mixtures
Authors: Cottin X. , Monson P. A.
Published: July 2002

We consider the application of the cell theory to the properties of solid solutions. In contrast with previous implementations of the cell theory for mixtures we include all types of cell partition function which arise from different nearest neighbor compositions and arrangements of the nearest neighbors, a feature which is necessary for a realistic treatment of substitutionally disordered solid solutions with components of different molecular sizes. An efficient algorithm for the simultaneous calculation of all contributing cell partition functions is presented. The theory is applied to the properties of binary hard sphere mixtures forming substitutionally disordered solid solutions. Solid–fluid equilibria are determined by using the cell theory for the solid phase together with an accurate fluid phase equation of state. Good agreement with Monte Carlo simulations is obtained.
Spin-dependent unitary group approach. II. Derivation of matrix elements for spin-dependent operators
Authors: Gould Mark D. , Battle James S.
Published: July 2002

This paper is concerned with matrix elements of spin-dependent operators. We present a detailed derivation of the matrix elements of the operators Δ(n)ij and Δ(n+1)ij shown in the article by Gould and Paldus [J. Chem. Phys. 92, 7394 (1990]) to lead to the matrix elements of all spin-dependent operators within the Gel’fand spin-adapted basis; as required for spin-dependent CI and the calculation of relativistic energy level shifts. Besides being useful for spin-dependent CI calculations, involving the incorporation of spin–orbit and spin–spin type electronic interactions, our results can be used for the calculation of spin-dependent properties coming from a wave function with well defined spin. Such wave functions can now be routinely computed using any one of numerous unitary group based program systems, hence the usefulness of our formalism for chemical properties. In particular we give an application of this formalism to the calculation of the spin density of a molecule using a spin-adapted wave function. In a future publication we will show how our formalism involving the Δ(n) operator leads to compact formulas for the shifts in the electronic energy spectrum of a molecule due to relativistic effects. We briefly describe our numerical implementation of these new matrix elements as used for the calculation of the spin density of a molecule.
Modeling adsorbate uptake: Coverage dependence of the iron cluster-ammonia binding energy
Authors: Parks E. K. , Riley S. J.
Published: July 2002

Experimental data on the uptake of ammonia by iron clusters in the polyicosahedral series Fe19, Fe23, Fe26, Fe29, Fe32, and Fe34 have been measured at a series of temperatures. The uptake data are analyzed with a statistical model that parametrizes both molecular adsorption and desorption in terms of collision cross sections, Rice–Ramsperger–Kassel (RRK) frequency factors, and cluster–ammonia binding energies. Fits to the data give a binding energy of 0.88±0.02 eV for the first NH3 molecule on all six clusters, and a linear decrease in binding energy with increasing number of molecules, with larger clusters showing a slower rate of decrease. When expressed as a function of coverage (the ratio of the number of molecules to the number of surface metal atoms), the binding energy is also linear and is essentially independent of cluster size. The observed behavior is discussed in terms of the probable nature of cluster–ammonia interactions.
On the intersection of two potential energy surfaces of the same symmetry. Systematic characterization using a Lagrange multiplier constrained procedure
Authors: Manaa M. Riad , Yarkony David R.
Published: July 2002

Two nonrelativistic Born–Oppenheimer potential energy surfaces of the same space-spin symmetry may intersect on a surface of dimension N−2, where N is the number of internal nuclear degrees of freedom. Characterization of this entire surface can be quite costly. An algorithm, employing multiconfiguration self-consistent-field (MCSCF)/configuration interaction(CI) wave functions and analytic gradient techniques, is presented that avoids the determination of the full N−2 dimensional surface, while directly locating portions of the crossing surface that are energetically important. The algorithm determines extrema of the Lagrangian function LIJ(R,ξ,λ) = EI(R) + ξ1[EI(R) − EJ(R)] + ξ2HIJ(R)/2+ ∑Mk=1λkCk(R), where Ck(R) is any geometric equality constraint such as RKL2−αKL2=0, or RKL2−RMN2=0, RKL=‖RK−RL‖ and the ξ and λ are Lagrange multipliers. The efficacy of this algorithm is demonstrated using a MCSCF/first order CI description of 1,22A′ states of HCO.
Femtosecond real-time probing of reactions. XI. The elementary OClO fragmentation
Authors: Baumert T. , Herek J. L. , Zewail A. H.
Published: July 2002

Femtosecond reaction dynamics of OClO in a supersonic molecular beam are reported. The system is excited to the A 2A2 state with a femtosecond pulse, covering a range of excitation in the symmetric stretch between v1=17 to v1=11(308–352 nm). A time-delayed femtosecond probe pulse ionizes the OClO, and OClO+ is detected. This ion has not been observed in previous experiments because of its ultrafast fragmentation. Transients are reported for the mass of the parent OClO as well as the mass of the ClO. Apparent biexponential decays are observed and related to the fragmentation dynamics:   OClO+hν→(OClO)‡*→ClO+O                 →Cl+O2 .   Clusters of OClO with water (OClO)n (H2O)m with n from 1 to 3 and m from 0 to 3 are also observed. The dynamics of the fragmentation reveal the nuclear motions and the electronic coupling between surfaces. The time scale for bond breakage is in the range of 300–500 fs, depending on v1; surface crossing to form new intermediates is a pathway for the two channels of fragmentation: ClO+O (primary) and Cl+O2 (minor). Comparisons with results of ab initio calculations are made.
On the spatial and temporal distribution of clusters
Authors: Milchev Alexander
Published: July 2002

The paper considers some basic stochastic properties of assemblies of objects randomly distributed in space or appearing in time as a nonstationary flux of random events. General formulas are derived for the probability distribution function of the distances between nth neighbours in a ν-dimensional space. The theoretical predictions are compared with experimental data obtained by studying the kinetics of first order phase transitions.
Influence of vibrational excitation and collision energy on the ion-molecule reaction NH+3(ν2)+ND3
Authors: Posey Lynmarie A. , Guettler Robert D. , Kirchner Nicholas J. , Zare Richard N.
Published: July 2002

The influence of vibrational excitation and collision energy on the ion-molecule reaction NH+3(ν2)+ND3 has been investigated using a recently constructed quadrupole-octopole-quadrupole mass spectrometer. The NH+3 reagent ions are prepared state selectively with 0–7 quanta in the ν2 umbrella bending mode by (2+1) resonance enhanced multiphoton ionization through the B̃ or C̃′ Rydberg states of ammonia. Reactive collisions between the mass-filtered ion beam and a thermal distribution of neutral reagent molecules occur with controlled collision energies (0.5–10.0 eV center of mass) within the octopole ion guide, enabling product ions to be collected independent of scattering dynamics. The reaction of NH+3 with ND3 has three major product channels: (1) deuterium abstraction, (2) charge transfer, and (3) proton transfer. Each of these channels exhibits a strong dependence on ion vibrational excitation and collision energy. Product branching ratios and relative cross sections are reported and compared with previous results. Briefly, both deuterium abstraction and charge transfer are enhanced by vibrational excitation, whereas proton transfer is suppressed. As the collision energy increases, the branching fraction for charge transfer increases sharply, that for proton transfer decreases, and that for deuterium abstraction remains nearly unchanged. These results point to a short-lived collision complex in which vibration and translation play inequivalent roles.
Reactions of velocity-aligned atoms probed by Doppler profiles: H+O2→OH+O
Authors: Kim Hong Lae , Wickramaaratchi M. A. , Zheng Xiaonan , Hall G. E.
Published: July 2002

Final state resolved vector properties of the reaction H (2S)+O2(3Σ−g)→OH(2Π)+O(3P) have been measured using Doppler spectroscopy. Velocity-aligned H atoms were generated by photodissociation of H2S with linearly polarized ArF excimer laser light. The correlated velocity and angular momentum distribution of nascent OH reaction products was probed by laser induced fluorescence. Techniques are described for analyzing the line shapes, modeling the bipolar moments of the OH velocity and angular momentum distribution as parametric functions of the scattering angle. Distinctive Doppler-broadened lineshapes indicate scattering angle distributions with a strong forward and a weaker backward maximum. A qualitatively different angular momentum polarization for the two lambda doublet components of the OH product is required to explain the Doppler profiles.
Free energy and phase equilibria for the restricted primitive model of ionic fluids from Monte Carlo simulations
Authors: Orkoulas Gerassimos , Panagiotopoulos Athanassios Z.
Published: July 2002

In this work, we investigate the liquid–vapor phase transition of the restricted primitive model of ionic fluids. We show that at the low temperatures where the phase transition occurs, the system cannot be studied by conventional molecular simulation methods because convergence to equilibrium is slow. To accelerate convergence, we propose cluster Monte Carlo moves capable of moving more than one particle at a time. We then address the issue of charged particle transfers in grand canonical and Gibbs ensemble Monte Carlo simulations, for which we propose a biased particle insertion/destruction scheme capable of sampling short interparticle distances. We compute the chemical potential for the restricted primitive model as a function of temperature and density from grand canonical Monte Carlo simulations and the phase envelope from Gibbs Monte Carlo simulations. Our calculated phase coexistence curve is in agreement with recent results of Caillol obtained on the four-dimensional hypersphere and our own earlier Gibbs ensemble simulations with single-ion transfers, with the exception of the critical temperature, which is lower in the current calculations. Our best estimates for the critical parameters are T*c=0.053, ρ*c=0.025. We conclude with possible future applications of the biased techniques developed here for phase equilibrium calculations for ionic fluids.
The stabilization theory of scattering
Authors: Mandelshtam V. A. , Ravuri T. R. , Taylor H. S.
Published: July 2002

A new method of doing scattering calculations is presented and illustrated. Reactive state-to-state transition amplitudes, microcanonical rate, resonance parameters, and related quantities are computed for the collinear H+H2→H2+H exchange reaction. The method only involves diagonalizations of a real symmetric system Hamiltonian placed in a series of enclosing boxes.
Formation and decay of C−60 following free electron capture by C60
Authors: Matejčik Štefan , Märk Tilmann D. , Španěl Patrik , Smith David , Jaffke Thomas , Illenberger Eugen
Published: July 2002

The results of a detailed crossed electron/molecular beam study of electron attachment to C60 molecules and electron detachment from C−60 over the range of electron energies from near zero to about 15 eV are described. It is shown by comparing the experimental data for the attachment cross sections (normalized to the absolute thermal cross sections determined using the flowing afterglow/Langmuir probe apparatus) with quantum calculations that attachment occurs at low energies in the p-wave channel, and in the d- and f-wave channels (and probably higher-order partial waves) at the higher electron energies. At electron energies above 7 eV, thermal detachment of electrons from the hot C−60 negative ions is seen to occur, and the unimolecular rate coefficients for detachment, kd, have been determined as a function of the energy of the attaching electron. Hence, by relating kd to the derived temperature of the hot C−60 ions, the electron detachment energy, Ed, has been determined as 2.6 eV, which is close to the electron affinity of C60 as measured by photodetachment from cold C−60 ions. Additionally, by combining the measured attachment rate coefficients, ka, from the previous flowing afterglow/Langmuir probe study with the kd data determined in this study, equilibrium constants for the detachment/attachment reactions have been obtained which are reconciled with those calculated using total partition functions. An important conclusion to be drawn from all these studies is that C60 very efficiently captures electrons over the wide electron energy range from about 0.2 eV to around 15 eV and retains them if the energy released in the electron capture process can be removed before thermal detachment can occur.
Nonadiabatic effects on the charge transfer rate constant: A numerical study of a simple model system
Authors: Shin Seokmin , Metiu Horia
Published: July 2002

We use a minimal model to study the effects of the upper electronic states on the rate of a charge transfer reaction. The model consists of three ions and an electron, all strung on a line. The two ions at the ends of the structure are held fixed, but the middle ion and the electron are allowed to move in one dimension, along the line joining them. The system has two bound states, one in which the electron ties the movable ion to the fixed ion at the left, and the other in which the binding takes place to the fixed ion at the right. The transition between these bound states is a charge transfer reaction. We use the flux–flux correlation function theory to perform two calculations of the rate constant for this reaction. In one we obtain numerically the exact rate constant. In the other we calculate the exact rate constant for the case when the reaction proceeds exclusively on the ground adiabatic state. The difference between these calculations gives the magnitude of the nonadiabatic effects. We find that the nonadiabatic effects are fairly large even when the gap between the ground and the excited adiabatic state substantially exceeds the thermal energy. The rate in the nonadiabatic theory is always smaller than that of the adiabatic one. Both rate constants satisfy the Arrhenius formula. Their activation energies are very close but the nonadiabatic one is always higher. The nonadiabatic preexponential is smaller, due to the fact that the upper electronic state causes an early recrossing of the reactive flux. The description of this reaction in terms of two diabatic states, one for reactants and one for products, is not always adequate. In the limit when nonadiabaticity is small, we need to use a third diabatic state, in which the electron binds to the moving ion as the latter passes through the transition state; this is an atom transfer process. The reaction changes from an atom transfer to an electron transfer, as nonadiabaticity is increased.
External vibrations of hydrocarbons on Cu(100)
Authors: Witte G. , Wöll Ch.
Published: July 2002

A search for external vibrations of hydrocarbons physisorbed on Cu(100) has been carried out with high-resolution inelastic He-atom scattering. For the saturated hydrocarbons n-hexane, cyclohexane, n-octane, and n-decane, the frustrated translation normal to the surface could be detected with energies of 6.8 meV, 7.3 meV, 7.0 meV, and 7.0 meV, respectively. A very similar value of 7.3 meV for the same vibration is observed for the unsaturated hydrocarbon benzene. No vibrational modes corresponding to frustrated translations parallel to the surface could be detected.
Molecular reorientation in ortho-carborane studied by dielectric spectroscopy
Authors: Lunkenheimer P. , Loidl A.
Published: July 2002

The dielectric properties of ortho-carborane have been investigated in a broad frequency range of 20 Hz≤ν≤1 GHz and at temperatures 10 K≤T≤380 K. At T≳275 K the ortho-carborane molecules undergo fast and isotropic reorientations with frequencies larger than 1 GHz. Below a phase transition at Tc≊275 K the reorientation is partly restricted and the dynamics of the remaining reorientational motion is reduced. At 160 K, the dielectric results reveal a small anomaly which we assign to a second phase transition. At temperatures 150 K<T<275 K the dipolar active orientational degrees of freedom freeze in on the time scale of the measuring frequencies. The corresponding peaks in the frequency dependence of the dielectric loss can well be described using the Fourier transform of the Kohlrausch–Williams–Watts (KWW) function with a stretching parameter β≊0.8. However, for frequencies well above the peak maximum we find significant deviations from KWW behavior. In the temperature and frequency range investigated, the dynamics of the dipolar relaxation process behaves thermally activated with an activation energy of 0.52 eV. In the classification scheme of Angell et al. the fragility of ortho-carborane can be estimated as m≊20 characterizing it as a strong glass former. Ortho-carborane fits well into the correlation scheme of Böhmer et al. where polydispersivity and fragility are related to each other. At high temperatures and low frequencies conductivity processes (hopping) dominate the dielectric response of ortho-carborane.
Phase behaviors of supercooled water: Reconciling a critical point of amorphous ices with spinodal instability
Authors: Tanaka Hideki
Published: July 2002

The anomalies of supercooled water in thermodynamic response functions at atmospheric pressure, the phase transition between low and high density amorphous ices (LDA and HDA), and a predicted fragile–strong transition are accounted for in a unified manner by reconciling an idea due to Stanley and co-workers introducing a second critical point separating LDA and HDA ices with a conjecture proposed by Speedy that LDA is a different phase from a normal water, called water II. The reconciliation is made on the basis of results from extensive molecular dynamics simulations at constant pressure and temperature. It is found that there exist large gaps around temperature 213 K in thermodynamic, structural, and dynamic properties at atmospheric pressure, suggesting liquid–liquid phase transition. This transition is identified with an extension of the experimentally observed LDA–HDA transition in high pressure to atmospheric pressure. Thus, we propose a new phase diagram where the locus of the second critical point is moved into negative pressure region. With this simple modification, it becomes possible to account for the divergence of the thermodynamic response functions at atmospheric pressure in terms of the critical point and the spinodal-like instability of HDA. The unstable HDA undergoes a transition to LDA phase in lower temperature. The transition is also observed in high pressure region such as 200 MPa while it disappears at negative pressure, −200 MPa. This reinforces our proposed phase diagram in which there is no continuous path from a supercooled state to LDA at atmospheric pressure. It is argued that the HDA–LDA transition is accompanied by a fragile–strong transition. A possible mechanism of avoiding crystallization of aqueous solutions is also discussed in terms of a difference in hydrogen bond number distribution between LDA and HDA.
New conformational search method based on local torsional deformations for cyclic molecules, loops in proteins, and dense polymer systems
Authors: Baysal Canan , Meirovitch Hagai
Published: July 2002

We propose a conformational search method, based on local torsional deformations (LTD) for locating the low energy structures of cyclic peptides, loops in proteins or dense polymer systems. LTD is applied preliminarily to cycloundecane modeled by the MM2 force field, and is found to be more efficient than other techniques.
Two-exciton spectroscopy of photosynthetic antenna complexes: Collective oscillator analysis
Authors: Kühn O. , Chernyak V. , Mukamel S.
Published: July 2002

The linear and third-order polarizabilities of the light-harvesting antennae of photosynthetic bacteria and green plants are calculated using an equation of motion approach which maps the system onto a coupled set of anharmonic excitonic oscillators. The oscillator representation is shown to have several advantages over the traditional picture based on properties of individual global eigenstates of the aggregate; besides a considerable reduction of computational effort, the dynamics of excitations in the two-exciton band is conveniently analyzed in terms of single-exciton Green’s functions and the two-exciton scattering matrix.
Copolymer brushes
Authors: Ferreira P. G. , Leibler L.
Published: July 2002

We study a diblock AB copolymer brush in good solvent using the self-consistent field approximation. The copolymer chains of the brush are end grafted to a planar surface always by the same type of monomers. Depending on the Flory–Huggins interaction parameter χAB, we found two different regimes. If χAB≳0 a demixing transition occurs when the solvent is poor. An exclusion zone appears close to the grafting surface from where one type of monomers is expelled. In this region a steplike brush is formed by the other monomers. At the extremity of this brush appears then a conventional parabolic brush formed by the excluded monomers. If χAB<0 the monomers are miscible and backwards twisted configurations become very important as |χAB| increases. For larger values of |χAB| the collapse of the system may occur.
CO overlayers on Ru(0001) studied by helium atom scattering: Structure, dynamics, and the influence of coadsorbed H and O
Authors: Braun J. , Kostov K. L. , Witte G. , Wöll Ch.
Published: July 2002

Ordered phases of CO on a Ru(0001) surface have been characterized with regard to structural and dynamical properties using high resolution helium atom scattering. In the energy regime below 10 meV a vibrational mode corresponding to a frustrated translation parallel to the surface (FTx) could be identified, the energy amounts to 5.9 meV for isolated CO molecules and to 5.75 meV for the (∛×∛)R30°CO structure. The formation of the more compressed (2∛×2∛)R30°CO and (5∛×5∛)R30°CO structures is accompanied by significant changes of the low energy external vibrations, in pronounced contrast to the gradual frequency increase of the CO internal ν1-vibration. Coadsorption of hydrogen or oxygen was found to result in substantially larger FTx energies. Implications of these findings on the character of the molecule-surface interaction will be discussed, as well as the connection between the FTx-dispersion and the strength and type of the adsorbate-adsorbate interaction.
Remarks on total and elastic cross sections for electron and positron scattering from CO2
Authors: Kimura M. , Sueoka O. , Hamada A. , Takekawa M. , Itikawa Y. , Tanaka H. , Boesten L.
Published: July 2002

There remain discrepancies in the determination of the total and elastic cross sections for electron and positron collisions with carbon dioxide at impact energies from 0.3 to 100 eV. We have carried out a joint experimental and theoretical study in an attempt to resolve the differences. Our measurements for total cross sections for electron impact agree extremely well with those of Hoffman et al. [Phys. Rev. A 25, 1393 (1982)] above 2 eV, while those for positron impact agree to better than 15% in magnitude, and are in an excellent accord for the energy-dependence. The present observations, along with an analysis using a knowledge of the dominant inelastic processes, lead us to conclude that the total cross section determined in earlier studies is a considerable underestimate at intermediate energies, where inelastic processes are not negligible. The analysis provides a rationale for understanding the discrepancies. Furthermore, we present that the total cross section by positron impact below 2 eV becomes larger by 20% than that of electron impact, and provide a rationale as well as implication to a possible application.
Xe
129
adsorbed in AlPO4-11 molecular sieve: Molecular dynamics simulation of adsorbate dynamics and NMR chemical shift
Authors: Kantola Juha-Heikki , Vaara Juha , Rantala Tapio T. , Jokisaari Jukka
Published: July 2002

Molecular dynamics simulation of 129Xe adsorbed in the AlPO4-11 molecular sieve has been carried out in the constant NVT ensemble at T=300 K. The host material was modeled as a fully dynamic framework. Single-particle and pair distribution functions of the adsorbed gas in the unidirectional channels of the material with different adsorbate loadings are presented. Self-diffusion coefficients, DXe, have been calculated for the dynamics of the adsorbates between the adsorption cells in AlPO4-11. The dynamic framework is shown to decrease the effective potential barrier for intercellular jumps. An estimate is given for the jump frequency. The NMR chemical shifts of 129Xe were evaluated using parametrizations based on first principles calculations of Xe–Xe and Xe-OH2 model systems. No adjustable parameters were used to improve agreement with experimental results. The results of a phenomenological three-site dynamic averaging model proposed earlier by others were reproduced with a good accuracy using chemical shift parameters from gradient corrected density-functional theory. Finally, the approximations inherent to the present computational approach are discussed.
Relativistic and correlated calculations on the ground, excited, and ionized states of iodine
Authors: de Jong W. A. , Visscher L. , Nieuwpoort W. C.
Published: July 2002

The electronic structure, spectroscopic, and bonding properties of the ground, excited, and ionized states of iodine are studied within a four-component relativistic framework using the MOLFDIR program package. The experimentally determined properties of the Σg+1 ground state are well reproduced by our results calculated at the CCSD(T) level of theory. Relativistic effects and core–valence correlation need to be included in order to get reliable results, but the Gaunt interaction can be neglected. The photoelectron spectrum and the potential energy curves of the ionized and excited states are calculated using relativistic configuration interaction and coupled cluster methods. The calculated properties of the excited states are generally in good agreement with the experimental data, as well as with the earlier theoretical results of Teichteil and Pelissier. An alternative assignment of some recently measured, low lying, ionized states is proposed.
Determination of differential-cross-section moments from polarization-dependent product velocity distributions of photoinitiated bimolecular reactions
Authors: Rakitzis T. Peter , Kandel S. Alex , Zare Richard N.
Published: July 2002

We describe procedures for the measurement of the differential-cross-section moments of the velocity distribution of the state-selected products of photoinitiated bimolecular reactions using resonance-enhanced multiphoton ionization (REMPI) detection and some form of laboratory velocity selection such as time-of-flight mass spectrometry. The relative ionization probability of a single product molecule is presented in the form I=1+f(Θ,Φ,θε,θu,Aq(k)stf), where the angles Θ, Φ, and θε describe the orientation of the product’s laboratory velocity with the photolysis and probe laser polarizations, θu is the product laboratory scattering angle, and the Aq(k)stf are the stationary target frame (STF) differential-cross-section moments. The STF is a reference frame defined by the laboratory velocity and the scattering plane. From the ionization probability, I, we derive a method to measure all five parameters with k⩽2, the differential cross section, 1/σ(dσ00/dΩr), and the four polarization parameters A1(1)stf, A0(2)stf, A1(2)stf, and A2(2)stf [where the Aq(k)stf are equal to the polarization-dependent differential cross sections normalized by the differential cross section, (dσkqstf/dΩr)/(dσ00/dΩr)]. The five parameters can be determined using only one rotational branch and several experimental geometries. We present simulations that show the effects of product polarization on experimental signals, and we discuss the effectiveness and limitations of inverting the measured signals to the Aq(k)stf.
Electro-optic response of chiral helicenes in isotropic media
Authors: Beljonne D. , Shuai Z. , Brédas J. L. , Kauranen M. , Verbiest T. , Persoons A.
Published: July 2002

Correlated quantum-chemical calculations are used to estimate the second-order electro-optic response of chiral helicene molecules in isotropic media. Dephasing-induced terms are included in a sum-over-states (SOS) perturbation formalism derived from the density-matrix expression. Besides the usual resonances associated with intrinsic electronic excitations of the material, resonant peaks are found at optical frequencies corresponding to energy differences between intermediate excited states. The mechanism for such resonance phenomena in helicenes is analyzed in the framework of a configuration interaction description of the singlet excited states. The possibility is discussed that in such a resonance regime the optical field can experience gain without linear loss.
Predissociation of the B 3Σu− state of S2
Authors: Wheeler Martyn D. , Newman Stuart M. , Orr-Ewing Andrew J.
Published: July 2002

Predissociation of the B 3Σu− state of S2 has been investigated by a combination of cavity ring-down spectroscopy and model calculations. The experimental spectra of the B 3Σu−−X 3Σg−(v′,0) bands for 10⩽v′⩽22 span the wavenumber range 35 480–39 860 cm−1. Extensive variation is observed in the degree of rotational structure within the vibrational bands because of lifetime broadening caused by predissociation. Fits to the band contours give homogeneous linewidths for transitions to the B-state vibrational levels for 10⩽v′⩽17 that vary from ⩽1 cm−1 for the (10,0) band to 7±1 cm−1 for the (17,0) band with a maximum linewidth of 14±1 cm−1 for the (13,0) band. For v′⩾18, all bands are completely diffuse, indicating linewidths in excess of 15 cm−1. The experimental results are compared with the results of a theoretical model that uses a Rydberg–Klein–Rees (RKR) potential for the B 3Σu− state, ab initio calculations of the repulsive potentials that cross the B state, and Fermi golden rule calculations of the predissociation rates for the different repulsive potentials. Minor adjustments to the ab initio potentials, and an estimate of the spin-orbit coupling between the bound and repulsive states enable us to calculate predissociation rates in excellent agreement with the experimental observations. We deduce that the predissociation for v′⩽16 is predominantly via a 1Πu state, whereas for v′⩾17, coupling to a second repulsive state, suggested to be either a 5Σu− or 5Πu state, provides the primary mechanism for predissociation.
A high resolution helium atom scattering and far infrared study of the dynamics and the lateral potential energy surface of CO molecules chemisorbed on Cu(001)
Authors: Graham Andrew P. , Hofmann Frank , Toennies J. Peter , Williams Gwyn P. , Hirschmugl Carol J. , Ellis John
Published: July 2002

Inelastic helium scattering (HAS) and infrared reflection adsorption spectroscopy (IRAS) have been used to measure the isotope shifts of the frequencies of both the parallel and perpendicular frustrated translation modes, as well as the frustrated rotation mode of CO molecules at on top sites on Cu(001). The measured isotope shifts for four different isotopomers indicates a significant rotational contribution to the parallel frustrated translation (T-mode), where the vibrational amplitude of the oxygen atom is significantly larger than for the carbon atom. Conversely, for the frustrated rotation the vibrational amplitude of the carbon atom was observed to be larger than for the oxygen atom. At surface temperatures above Ts=100 K a careful analysis of the peak shape of the HAS quasielastic peak shows a small broadening, which is attributed to a rapid diffusion of the CO molecules. The measured dynamic diffusion barrier of 31±10 meV is compatible with the shape of the potential at the on-top site and makes it possible to extend the potential energy surface to the region between the on-top sites.
Collapse of random copolymers
Authors: Ganazzoli Fabio
Published: July 2002

We propose a theoretical approach to the description of the coil–globule transition of random copolymers having a fixed sequence of units. For simplicity, we consider copolymers formed by two different units only, although the generalization to any other number is straightforward. The theory is based on self-consistent minimization of the intramolecular free energy, which includes two-body attractive interactions among units of a given type, two- and three-body repulsive interactions among all the units, and configurational entropy. Chain connectivity is accounted for throughout. Considering copolymers with 20%–60% mutually attractive units, we predict in all cases a first-order coil–globule transition, unlike the analogous homopolymer. The monomolecular micelle formed by the collapsed copolymer consists of two basic conformations: (a) stable compact globules, having the mutually attractive units clustered in a dense core, wherefrom the other units are expelled; (b) metastable open globules, where most attractive units are still within the core, but a few of them are outside, interspersed with the other units. Possible connections with ionomer behavior in apolar solvents and with current results on globular proteins are also discussed.
Water confined in Vycor glass. I. A neutron diffraction study
Authors: Bruni Fabio , Ricci Maria Antonietta , Soper Alan K.
Published: July 2002

Neutron diffraction experiments with isotopic substitution on water confined in porous Vycor glass at two hydration states are presented and analyzed in terms of the experimentally accessible site-site distribution functions. The bias on these functions as well as their limitations, due to the presence of regions where water molecules are not allowed (excluded volume effects), and to the contribution of water–Vycor interference to the measured cross sections, are discussed. In particular the relative weight of these cross correlation terms is estimated for the first time. It is shown that the traditional analysis of diffraction data of these kinds which ignore cross correlations may be erroneous. A full account of the excluded volume effects is reported in paper II [A. Soper, F. Bruni, and M. A. Ricci, J. Chem. Phys. 109, 1486 (1998), following paper].
The metastable T−P phase diagram and anomalous thermodynamic properties of supercooled water
Authors: Ponyatovsky E. G. , Sinitsyn V. V. , Pozdnyakova T. A.
Published: July 2002

The metastable T−P phase diagram and the anomalies of the thermodynamic properties of supercooled water are calculated on the basis of a two-level thermodynamic model. Water is considered as a mixture of two components which differ in atomic configurations and correspond to low-density amorphous (lda) and high-density amorphous (hda) ice. The expression for the Gibbs potential of water is written in the form which is analogous to that of usual regular binary solutions. But this model considers the concentration, c, of the components, as a pressure and temperature-dependent internal parameter. There are only four constants in the expression for the Gibbs potential: the differences in the specific volumes, entropies, and energies of the two components and the mixing energy of the components whose values are ΔV0=−3.8 cm3/mol, ΔS0=4.225 J/mol, ΔE0=1037 J/mol, and U=3824 J/mol, respectively. The lda−hda phase equilibrium line terminates at the critical point, Tcr=230 K and Pcr=0.173 kbar, the second critical point in the phase diagram of water. The anomalous thermal dependence of the specific volume, the heat expansion coefficient, and the specific heat of water calculated for the atmospheric pressure is in a good quantitative agreement with the available experimental data. Thus anomalous properties of supercooled water are well explained by the occurrence of the second critical point close to the atmospheric pressure. The absolute value of parameter c is not crucial for the thermal behavior of properties, instead, the anomalies in water are due to the dependence on pressure and temperature. The parameter c behavior is analyzed in various pressure and temperature ranges around the second critical point. The thermal dependence of parameter c is very weak in the temperature range of 290–350 K at atmospheric pressure. As a consequence, the thermodynamic properties of water behave in this range like those of a normal liquid though water stays a mixture of two components, lda-like and hda-like, in an approximate proportion 2:3.
Two-dimensional localization of adsorbate/substrate charge-transfer excited states of molecules adsorbed on metal surfaces
Authors: Kambhampati Patanjali , Foster Michelle C. , Campion Alan
Published: July 2002

Electron energy loss experiments provide evidence for the two-dimensional spatial localization of the metal/molecule charge-transfer excited state of pyromellitic dianhydride adsorbed on copper single-crystal surfaces in ultrahigh vacuum. Codosing the surface with atomic cesium splits the interfacial loss peak into a doublet, whose energies are independent of cesium coverage. This behavior is in contrast to the smooth, continuous shift in energy expected from the classical picture of alkali metal adsorption in which the substrate Fermi energy increases with coverage at low coverage. Line shape analysis suggests that the localized region includes the four adsorbed molecules nearest each cesium adatom.
Analysis on conformational stability of C-peptide of ribonuclease A in water using the reference interaction site model theory and Monte Carlo simulated annealing
Authors: Kinoshita Masahiro , Okamoto Yuko , Hirata Fumio
Published: July 2002

Solvation structure and conformational stability of the C-peptide fragment of ribonuclease A in pure water have been analyzed using the full reference interaction site model (RISM) theory. The charged groups in the side chains of Lys-1+, Glu-2−, Lys-7+, Arg-10+, and His-12+ (in particular, the four like-charged groups) play substantial roles in stabilizing the conformations. The solvation free energy and the conformational energy are governed by the contribution from the electrostatic interaction with water and the intramolecular Coulombic energy, respectively, and the conformational stability is determined by competition of these two factors. The contributions from the hydrophobic hydration and the van der Waals and torsion terms in the conformational energy are less important, which is in contrast to the result for Met-enkephalin. The Monte Carlo simulated annealing combined with the RISM theory has been applied to the C-peptide using an almost fully extended conformation as the initial one. The conformation first changes in the direction that the charged groups in the side chains are more exposed to water, and in particular, the positively charged groups are closer together. Thus, the solvation free energy decreases greatly in the initial stage. Although this leads to a significant increase in the intramolecular Coulombic repulsion energy, the decrease in the solvation free energy dominates. In the later stage, however, a further decrease in the solvation free energy gives rise to an even larger increase in the intramolecular Coulombic repulsion energy, and the conformational change is greatly decelerated. The conformations thus stabilized in four different runs of the combined program are quite similar. The peptide conformation in water is stabilized far more rapidly than in the gas phase.
Dynamic correlation effect in reversible diffusion-influenced reactions: Brownian dynamics simulation in three dimensions
Authors: Kim Hyojoon , Yang Mino , Shin Kook Joe
Published: July 2002

A Brownian dynamics (BD) simulation for a pseudo-first-order diffusion-influenced reversible association–dissociation reaction of a target system in three dimensions with spherical symmetry is presented. The exact Green function for a reversible geminate dissociation that we obtained recently is utilized in the simulation. We compare the results of simulation with two successful theoretical predictions, the enhanced version of the superposition approximation approach (SA) and the more rigorous kinetic theoretical approach (KT). The KT predicts the correct power law behavior of ∼t−3/2 with a slightly higher amplitude in the long-time region, but it is in good agreement with the BD result in the transient region. On the other hand, a faster relaxation is observed in the transient region for the SA, but the correct power law behavior with numerically exact amplitude is predicted for the exact target system. An interesting interplay between the mobility of the system and the dynamic correlation effect incorporated with many-body problems is also revealed.
The vibronically-resolved emission spectrum of disulfur monoxide (S2O): An algebraic calculation and quantitative interpretation of Franck–Condon transition intensities
Authors: Müller T. , Vaccaro P. H. , Pérez-Bernal F. , Iachello F.
Published: July 2002

Emission spectra obtained from jet-cooled disulfur monoxide (S2O) molecules have been interpreted by means of a novel Lie algebraic formalism that makes possible the facile evaluation of multidimensional Franck–Condon factors. Fluorescence accompanying selective excitation of isolated vibronic bands in the S2O C̃ 1A′←X̃ 1A′(π*←π) absorption system has been dispersed under moderate spectral resolution, allowing assignment of ground state levels possessing up to 20 quanta of vibration in the ν2 S–S stretching mode [Evib(X̃)⩽13 900 cm−1]. Aside from providing a rigorous and economical description for the inherently anharmonic nature of highly-excited polyatomic species, our algebraic approach enables quantitative information on molecular wavefunctions to be extracted directly from spectroscopic data. The emerging picture of S2O vibrational dynamics suggests that the X̃ 1A′ potential surface is substantially more “local” in character than the C̃ 1A′ manifold. While the observed pattern of X̃ 1A′ vibrational energies could be reproduced well through use of model Hamiltonians that include only diagonal anharmonicities in the local algebraic basis, successful treatment of the C̃ 1A′ state necessitated explicit incorporation of off-diagonal anharmonicities that lead to pervasive mixing of local vibrational character. This disparate behavior is manifest strongly in measured C̃–X̃ transition strengths, thereby allowing detailed investigations of Franck–Condon intensities to discern the underlying dynamics. Structural parameters deduced from algebraic analyses are in good accord with previous predictions of the change in S2O geometry accompanying π*←π excitation.
Core-softened potentials and the anomalous properties of water
Authors: Jagla E. A.
Published: July 2002

We study the phase diagram of a system of spherical particles interacting in three dimensions through a potential consisting of a strict hard core plus a linear repulsive shoulder at larger distances. The phase diagram (obtained numerically, and analytically in a limiting case) shows anomalous properties that are similar to those observed in water. Specifically, we find maxima of density and isothermal compressibility as a function of temperature, melting with volume contraction, and multiple stable crystalline structures. If in addition a long range attraction between the particles is included, the usual liquid–gas coexistence curve with its critical point is obtained. But more interestingly, a first order line in the metastable fluid branch of the phase diagram appears, ending in a new critical point, as it was suggested to occur in water. In this way the model provides a comprehensive, consistent and unified picture of most of the anomalous thermodynamical properties of water, showing that all of them can be qualitatively explained by the existence of two competing equilibrium values for the interparticle distance.
Theoretical analysis of alkali metal trapping sites in rare gas matrices
Authors: Ahokas Jussi , Kiljunen Toni , Eloranta Jussi , Kunttu Henrik
Published: July 2002

The rare gas (Ne, Ar, Kr, Xe)–alkali metal (Li, Na) ground-state pair interaction potentials and distance-dependent isotropic hyperfine coupling constants are evaluated by coupled-cluster approaches at the van der Waals region of the dimers. The computed properties are further utilized in classical molecular dynamics simulations of rare gas lattices doped with alkali atoms. Atomic trajectories and time averaged hyperfine constants are obtained from the simulations and exploited to provide theoretical insights into experimentally observed atomic trapping and dynamics of alkali metal atoms in rare gas matrices. The simulations support our previous electron paramagnetic resonance (EPR) data [Chem. Phys. Lett, 310, 245 (1999)], suggesting that alkali metal atoms, while generated by laser vaporization, do trap in single substitutional sites, whereas thermal atom sources yield trapping in multiple substitutional sites. In order to theoretically reproduce the EPR spectra for the latter case, more than six neighboring vacancies had to be included in the model system. Based on the simulations, the trapped atoms are able to move rather freely within the extended cage.