bluuebunny/crossref_metadata_2025 huggingface dataset:
Predissociation dynamics of I2(B) in liquid CCl4 observed through femtosecond pump-probe measurements: Electronic caging through solvent symmetry
Authors: Zadoyan R. , Sterling M. , Ovchinnikov M. , Apkarian V. A.
Published: July 2002

Direct observations of the solvent induced electronic predissociation of I2(B) in liquid CCl4 are made using femtosecond pump–probe measurements in which fluorescence from spin–orbit excited I*I* pairs, bound by the solvent cage, is used as detection. Data is reported for initial preparations ranging from the B state potential minimum, at 640 nm, to above the dissociation limit, at 490 nm. Analysis is provided through classical simulations, to highlight the role of solvent structure on: recombination, vibrational relaxation, and decay of coherence. The data is consistent with an anisotropic I2(X)–CCl4 potential which, in the first solvent shell, leads to an angular distribution peaked along the molecular axis. The roles of solvent structure and dynamics on electronic predissociation are analyzed. The data in liquid CCl4 can be understood in terms of a curve crossing near v=0, at 3.05 Å<Rc<3.8 Å, and the final surface can be narrowed down to 2g or a(1g). This nonadiabatic u→g transition is driven by static and dynamic asymmetry in the solvent structure. The role of solvent structure is demonstrated by contrasting the liquid phase predissociation probabilities with those observed in solid Kr. Despite the twofold increase in density, predissociation probabilities in the solid state are an order of magnitude smaller, due mainly to the high symmetry of the solvent cage. The role of solvent dynamics is evidenced in the energy dependent measurements. Independent of the kinetic energy content in I2, electronic predissociation in liquid CCl4 proceeds with a time constant equal to the molecular vibrational period. A modified Landau–Zener model, in which the effective electronic coupling is taken to be a linear function of vibrational amplitude fits the data, and suggests that cage distortions driven by the molecule enhance its predissociation probability. A nearly quantitative reproduction of the observations is possible when using the recently reported off-diagonal DIM surface that couples the B(0u+) and a(1g) states [Batista and Coker, J. Chem. Phys. 105, 4033 (1996)].
C–Cl and C–H bond cleavage in 193 nm photodissociation of CH3CF2Cl and CH3CFCl2
Authors: Melchior A. , Bar I. , Rosenwaks S.
Published: July 2002

The relative amounts of Cl(2P3/2) [Cl], Cl(2P1/2) [Cl*], H, and HCl produced by 193 nm photodissociation of CH3CF2Cl and CH3CFCl2 were studied by a laser pump-and-probe technique. An excimer laser photodissociated the parent molecules and a frequency doubled tunable dye laser probed the photofragments via (2+1) resonantly enhanced multiphoton ionization coupled with a time-of-flight mass spectrometer. Only atomic products were observed; no HCl was found. The profiles were well fitted to Gaussians. The Cl*/Cl branching ratios were almost identical, 0.18±0.04 and 0.16±0.04, while the H/(Cl+Cl*) ratios were 0.67±0.21 and 0.18±0.07, for CH3CF2Cl and CH3CFCl2, respectively. The mechanism for the production of the atomic species is discussed.
An experimental investigation of the nf Rydberg states of carbon disulfide
Authors: Berger J.-P. , Couris S. , Gauyacq D.
Published: July 2002

The (2Πg3/2,1/2)nf Rydberg states of CS2 are investigated by means of (3+1) resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) spectroscopy. The excitation spectrum of jet-cooled carbon disulfide has been obtained in the 74 000–81 000 cm−1 energy region. From the CS2+ mass selected REMPI spectra, the nf Rydberg series have been clearly identified and they have been found converging to the two spin–orbit components of the CS2+ (X 2Πg) ground state corresponding to the n=4–11 members. Comparison of the CS2+, CS+, S+ mass selected REMPI spectra gave a better insight of the competition between dissociation and ionization processes. The use of linearly and circularly polarized laser light, selection rules and quantum defect considerations have led to a preliminary analysis of the measured nf complexes.
Characteristics and dynamics of superexcited states of diatomic molecules: General theoretical procedure
Authors: Hiyama Miyabi , Kosugi Nobuhiro , Nakamura Hiroki
Published: July 2002

A general theoretical procedure to analyze the characteristics and dynamics of superexcited states of diatomic molecules is developed. It utilizes the following three methods effectively: (1) quantum chemical methods to evaluate basic parameters of the superexcited states, (2) spectroscopic experiments to improve the basic information obtained theoretically, and (3)multi-channel quantum defect theory (MQDT) to analyze couplings among various kinds of states and to clarify the dynamics. This procedure is composed of seven steps, including a method to evaluate the electronic coupling of the first kind of superexcited state to ionization continuum which is newly proposed here. The two-center Coulomb function in the field of the corresponding diatomic molecular ion is used to evaluate this coupling. This can be done by incorporating the Coulomb functions into an ab initio quantum chemical code. The whole procedure is illustrated for the CO molecule.
Theoretical and experimental studies of CIDNP kinetics in recombination of radical pairs by the method of switched external magnetic field. I. Theory
Authors: Parnachev Andrei , Purtov Peter , Bagryanskaya Elena , Sagdeev Renad
Published: July 2002

The theory of chemically induced dynamic nuclear polarization (CIDNP) of radical pairs in restricted volume formed in experiments with instantaneously switched external magnetic field is developed. For this purpose the theory of recombination of radical pairs with instantaneously changed spin Hamiltonian is extended to the case of rather weak magnetic fields. Using kinematic approximation it is shown that the recombination probability of radical pairs is fully expressed via the matrix formulas in terms of averages of Green’s functions of Liouville equation over both the reaction zone and that of energy level crossing. The effect of field switching on the CIDNP value is calculated for the typical parameters of the radical pairs. Simple biexponential expressions are derived for the CIDNP time dependencies for the case of strongly averaged S−T0 transitions and full mixing in configuration space.
Chirped pulse enhancement of multiphoton absorption in molecular iodine
Authors: Yakovlev Vladislav V. , Bardeen Christopher J. , Che Jianwe , Cao Jianshu , Wilson Kent R.
Published: July 2002

Chirp effects on three photon absorption yields in I2 vapor are investigated using femtosecond pulses with center wavelengths in the region 550–600 nm. Enhancements of as much as a factor of 3 are observed for chirped pulses with respect to transform-limited, zero chirp pulses. Theoretical considerations and model calculations suggest that wave packet dynamics play an important role.
Calculated paramagnetic hyperfine structure of pentagonal bipyramid Ag7 cluster
Authors: Arratia-Pérez Ramiro , Hernández-Acevedo Lucı́a , Alvarez-Thon Luis
Published: July 2002

Symmetry-adapted angular momentum basis functions have been generated for the D5h* molecular double point group to obtain the self-consistent Dirac cluster wave function Φ and the Dirac cluster orbitals. Once Φ is obtained, we proceed throughout a relativistic first-order perturbation procedure to calculate the magnetic hyperfine tensors of the Ag7 cluster. The calculated spin distribution and magnetic hyperfine tensors fully support the ESR assignment made by Weltner et al. of a cluster composed of seven silver atoms with a pentagonal bipyramid structure. The single unpaired electron spin spend 40.3% of its time on each axial silver atom.
Polarized fluorescence in α-sexithienyl single crystal at 4.2 K
Authors: Muccini M. , Lunedei E. , Bree A. , Horowitz G. , Garnier F. , Taliani C.
Published: July 2002

The polarized fluorescence spectra of the bc plane of α-sexithienyl (αT6) single crystal at T=4.2 K have been measured and analyzed. The spectra show a sharp component superimposed on a broad component. The origin of fluorescence at 18332 cm−1 with a rather narrow linewidth (FWHM=8 cm−1) is completely polarized along b. We assign it to a shallow X-trap 18 cm−1 below the bottom of the 1 1Bu exciton band. The 1 1Bu molecular electronic excited level splits in the crystal in four Davydov components (ag, bg, au, and bu) of which au and bu are dipole allowed. The calculated exciton band structure based on the Ewald sums, assuming the point dipole–dipole approximation, predicts two lowest degenerate Davydov components: an au b polarized component and a forbidden ag component. The fluorescence with the intrinsic sharp vibronic progressions is analyzed in terms of ground state totally symmetric modes which are in excellent agreement with Raman scattering data of the single crystal. Sharp c polarized false origins are identified at 18 167, 18 084, and 18 026 cm−1 and discussed either in terms of X-traps or Herzberg–Teller vibronic coupling with the second higher 2 1Bu molecular level polarized along the in plane short axis. A broad c polarized component is attributed to aggregates.
Energy switching approach to potential surfaces. III. Three-valued function for the water molecule
Authors: Varandas A. J. C. , Voronin A. I. , Caridade P. J. S. B.
Published: July 2002

A recently proposed two-valued energy switching potential energy surface for the water molecule [J. Chem. Phys. 107, 867 (1997)] is modified to incorporate the proper dissociation limits in all channels pertaining to the first excited A′1 electronic state, in addition to those of the ground state. A brief analysis of the loci of conical intersections is also presented.
Chain stretching effect on domain growth during spinodal decomposition of binary polymer mixtures under simple shear flow
Authors: Qiu Feng , Zhang Hongdong , Yang Yuliang
Published: July 2002

The chain stretching effect on domain growth during spinodal decomposition of binary polymer mixtures under simple shear flow is investigated by computer simulation. The simulation is based on a modified time-dependent Ginzburg–Landau equation, in which the chain stretching effect is introduced in the free energy functional. It is found that, for higher value of Rouse terminal relaxation time, the critical strain value for the burst of the domains is higher, thereby the domains are highly elongated. This may be responsible for the stringlike patterns observed experimentally under strong shear. When the chain stretching effect is introduced, the shear rate dependencies of the shear stress and first normal stress difference become stronger. The shear stress and first normal stress difference reach their maxima for the system of 1:1 mixture. The simulated results agree with the experimental observations qualitatively.
Inverse virial symmetry of diatomic potential curves
Authors: Herrick David R. , O’Connor Sean
Published: July 2002

The virial theorem for potential curves V(R) is recast into a linear second-order differential equation in the inverse picture R(V), which provides a framework for correlating the inner and outer walls of the potential well. Leading solutions extended for scaling define a new class of five-parameter potentials in terms of Gauss hypergeometric functions. Curves are classified with two labels (p,q) related to Dunham spectroscopic constants and long-range threshold behavior. Special limiting cases include the Morse potential, the harmonic oscillator, and the 2n:n inverse-power generalization of the Lennard-Jones 12:6 and Kratzer–Coulomb 2:1 potentials. Empirical maps of (p,q) values from experimental molecular data reveal distinct clustering of points correlated to covalent, van der Waals and ionic bonding. Semiclassical quantization gives hypergeometric formulas for energy levels and RKR potentials. Threshold behavior of exact molecular curves is consistent with a linear combination of elementary inverse potentials, with the extended hypergeometric basis as a first approximation.
Laser photolysis of OClO in solid Ne, Ar, and Kr. II. Site selectivity, mode specificity, and effects of matrix hosts
Authors: Lai Lih-Huey , Liu Chin-Ping , Lee Yuan-Pern
Published: July 2002

Irradiation of low-lying vibronic levels of Ã 2A2 of OClO isolated in solid Ne, Ar, or Kr produces ClOO. Destruction of OClO and production of ClOO in various matrix sites were monitored with IR absorption lines near 1100 and 1440 cm−1, respectively. For OClO in solid Ar at 5 K, site selectivity is observed after irradiation at the vibronic line associated with a specific site; decreased intensity of the IR line associated with a specific site of OClO correlates well with increased intensity of the corresponding line of ClOO. Similar behavior is observed for OClO in solid Kr except that formation of ClOO in one site (1412.1 cm−1) is invariably dominant. The nature of the carriers of two major sites of ClOO (corresponding to 1416.7 and 1442.5 cm−1 in solid Ar, 1412.1 and 1440.7 cm−1 in solid Kr) is discussed. Temporal profiles of absorbance AOClO and AClOO were probed after various periods of irradiation until approximately 20%–60% of OClO was destroyed. Excitation of the ν3 vibrational mode of OClO Ã 2A2 enhances photodissociation of OClO appreciably. Rates of destruction of OClO in solid Ar or Kr are much smaller than that in solid Ne. The apparent conversion factor, ΔAClOO/(−ΔAOClO) is greater for OClO in solid Ne than that in solid Ar or Kr. Effects of matrix host on photolysis are discussed.
Outside and inside the critical region of the Lennard-Jones fluid
Authors: Tang Yiping
Published: July 2002

The Lennard-Jones (LJ) fluid is studied for states both inside and outside the critical region. Outside the critical region, the mean spherical approximation (MSA) theory is found best for calculating the phase diagram. Inside the critical region, the MSA yields the classical exponents as anticipated. The failure of the MSA and other mean field theories is resolved by a new renormalization group (RG) theory. The new theory is based on a combination of White’s RG transformation, the density functional theory, and the superposition approximation. The new RG theory reduces to White’s RG theory for the van der Waals equation of state and is applicable for other integral equation theories as well as the MSA. An implementation of the MSA+RG transformation for the LJ fluid indicates that nonclassic behavior can be described satisfactorily. An accurate global theory is achieved in this work for the Lennard-Jones fluid.
Determination of photofragment ion translational energy and angular distributions in an octopole ion guide: A case study of the Ar2+ and (N2O⋅H2O)+ cluster ions
Authors: Williams S. , Chiu Y.-H. , Levandier D. J. , Dressler R. A.
Published: July 2002

The first measurement of ion photodissociation product recoil velocity and angular anisotropy in an octopole ion guide are presented. The experimental and numerical procedures required to obtain photofragment ion translational energy and angular distributions are discussed. Cluster ions are photodissociated in an octopole ion guide, and photofragment ion velocity distributions are measured using time-of-flight (TOF). The instrumental discrimination function is determined using guiding field variation (VAR). A validation study using the Σg+2←2Σu+ transition of Ar2+ probed at 300 nm and a photodissociation dynamics study of (N2O⋅H2O)+ to form N2OH++OH, N2O++H2O, and H2O++N2O in the 458–657 nm range are presented. The H2O+ and N2O+ photofragment translational energy and angular distributions are derived, and new information regarding the photodissociation of the (N2O⋅H2O)+ cluster ion is obtained.
A combined molecular dynamics–ab initio study of H2 adsorption on ideal, relaxed, and temperature-reconstructed MgO(111) surfaces
Authors: Hermansson Kersti , Baudin Micael , Ensing Bernd , Alfredsson Maria , Wojcik Mark
Published: July 2002

2D periodic ab initio calculations have been performed for H2 interacting with a temperature-reconstructed MgO(111) surface, created from a molecular dynamics (MD) simulation at 300 K. The temperature effects involve both a long-time reconstruction of the surface and instantaneous vibrational displacements. Chemisorption energies of ∼60 μJ/mol were found. Comparisons with adsorption on ideal and relaxed MgO(111) slabs show that the physi- and chemisorption properties for the H2/MgO(111) system are very sensitive to small topological details in the surface structure. The large surface reconstruction at 300 K involves drastic changes of the surface’s adsorption properties. We also find that a low coordination number of the surface ions is not sufficient to make the site attractive for chemisorption.
Glass formation and local disorder: Amorphization in planar clusters
Authors: Sim Eunji , Patashinski Alexander Z. , Ratner Mark A.
Published: July 2002

Structural aspects of amorphization in a cluster model of a disordered phase are presented. Molecular dynamics simulations were done within a “mobile cluster static boundary” model, in which static, disordered particles at the boundary of the simulation cell cause particles within the cell to adjust themselves structurally. With sufficient boundary disorder, both the radial distribution function and the orientational order parameter show onset of amorphization behavior. For Lennard-Jones particles, which are not good glass formers, the amorphization occurs only within a certain healing distance of the disordered boundary. With increasing boundary disorder, the orientational correlation function and the radial distribution function indeed show orientational phase scrambling, disappearance of long-range order, and the onset of glass-like amorphized structure.
Size dependence of transfer free energies: A hard-sphere-chain- based formalism
Authors: Shimizu Seishi , Ikeguchi Mitsunori , Nakamura Shugo , Shimizu Kentaro
Published: July 2002

The main purpose of this paper is to present a theoretical scheme which describes the solvation and transfer free energies of small molecules and relate them to solvent contributions in the biomolecular processes. Several proposals, based originally on Flory–Huggins theory, have been made recently that there is a non-negligible solute’s volume-proportional term in solvation free energy and the term should be subtracted to obtain solute/solvent contact free energy for biochemical applications. These proposals have resulted in the revision of the magnitude of the hydrophobic effect in biomolecules. The validity has been controversial, since the existence, physical origin, and magnitude of the volume-proportional term have been model dependent. In this paper, we cleared up this problem by using an accurate fused-hard sphere model and a perturbation scheme in which the compensation between the repulsive and attractive interactions has been clarified. The solvation free energy is shown to be dependent on the solute’s surface area and curvature: the volume-proportional term is shown to be negligibly small. This disproves the basic assumption of the previous theories whose purpose is to “correct” the magnitude of the solvation free energy by subtracting volume-proportional terms. The relationship of our theory to previous theories is also discussed.
A novel quantum Monte Carlo strategy: Surplus function approach
Authors: Huang Hongxin , Xie Qingji , Cao Zexing , Li Zelin , Yue Zeng , Ming Lei
Published: July 2002

A concept of surplus function for the Schrödinger equation is put forward. A novel quantum Monte Carlo approach, namely, the surplus function method, is suggested with use of a novel trial function of significant physical meaning which is based on the proposed surplus function. The trial function is an iteration type, as given in the text. It is theoretically proved that the energy expectation value obtained from the proposed trial function decreases step by step in iterations. In addition, computation formulas and concrete procedures for energy expectation value are presented. Calculations for H2, LiH, Li2, and H2O molecules indicate that the energy expectation values after only 4–5 iterations achieve over 90% of the correlation energy, indicating that the convergence rates are rapid. The trial function used in the present paper requires no parameter optimization and is of the highest accuracy.
Dissipations and polarizations in irreversible electrode processes: A unite formalism of stochastic thermodynamics of both concentration polarization and activation polarization
Authors: Luo Jiuli
Published: July 2002

A generalized stochastic model of the irreversible electrode processes is proposed. By means of it the dissipation and the fluctuation around nonequilibrium steady states of the electrode chemical reaction systems is analyzed. It is shown that the polarizations, in spite of the concentration polarization or the electrochemical polarization, originate from the nontrivial dissipations together with the non-Poissonian fluctuations in irreversible electrode processes. Furthermore a dissipation-fluctuation formula of electrode polarization is deduced explicitly. This formula is a dominant nonequilibrium thermodynamic equality, useful for the theoretic analysis of the polarization in irreversible electrode processes when the validity of the classical equilibrium thermodynamics breaks down. It turns out that the well-known Nernst formula followed by concentration polarization and the famous Tafel equation valid for electrochemical polarization are only the macroscopic approximations of two special limit cases in the formalism presented in this paper. If the fluctuation departs from the Gaussian a revision will be inevitable.
Spectral diffusion in glasses under high pressure: A study by time-resolved hole-burning
Authors: Lock A. J. , Creemers T. M. H. , Völker S.
Published: July 2002

We have studied optical dephasing and spectral diffusion of the S1←S0 0–0 transition of bacteriochlorophyll-a (BChl-a) in the glass 2-methyltetrahydrofuran (MTHF) at ambient (Δp=0) and high pressure (Δp=3.6 GPa) between 1.2 and 4.2 K by time-resolved hole-burning. The “effective” homogeneous linewidth Γhom′ follows a power law dependence on temperature, Γhom′=Γ0′+aT1.3±0.1, where Γ0′=Γ0+Γ0ET+Γ0ET→SD(td) is the residual linewidth and a=aPD+aSD(td)+aET→SD(td) is the coupling constant. The separate contributions to Γ0′ and a are the fluorescence decay rate Γ0=(2πτfl)−1, the “downhill” energy-transfer rate Γ0ET, the coupling constants due to “pure” dephasing aPD and “normal” spectral diffusion aSD(td), and two terms related to “extra” spectral diffusion induced by energy transfer, Γ0ET→SD(td) and aET→SD(td). We have quantitatively analyzed these contributions at ambient and high pressure. The results show that “normal” SD, “extra” SD, and ET→SD are strongly influenced by pressure. We have interpreted our findings in terms of a change in the number of two-level-systems, the low-frequency modes characteristic for the glassy state.
Nuclear magnetic resonance proton dipolar order relaxation in the selectively deuterated nematic para-azoxyanizole
Authors: Mensio O. , Zamar R. C. , Pusiol D. J. , Becker S.
Published: July 2002

Larmor frequency dependent measurements of Zeeman (T1Z) and dipolar order (T1D) relaxation times provide experimental information on the molecular dynamics in liquid crystals. However, at present, a comprehensive theoretical expression relating T1D with the spectral densities of the molecular motions is not available for liquid crystals. In fact, recent relaxation studies in nematic thermotropic liquid crystals have shown that the traditional model of isolated phenyl proton pairs predicts a relaxation of the dipolar order noticeably slower than the observed one. In this work we show that the failure cannot be assigned exclusively to the assumption of isolated spin-pairs. With this aim, we study the dipolar order relaxation in the nematic PAAd6 (methyl deuterated para-azoxyanizole). After calculating a generalized expression for T1D valid for an arbitrary number of spins, we found that the contributions from multispin interactions and correlations are negligible. This means that, from the point of view of the traditional weak collision theory of relaxation, PAAd6 provides an example of an ensemble of isolated spin-pairs. Nevertheless, after measuring T1D over a broad frequency range (103–3×107 Hz), we found a faster relaxation than predicted by the two-spin model, even in this compound. We conclude, therefore, that the failure of the model should be ascribed to basic assumptions of the traditional semiclassical model of spin-lattice relaxation in liquid crystals
Condensation of isolated metal clusters studied with a calorimeter
Authors: Bachels T. , Tiefenbacher F. , Schäfer R.
Published: July 2002

We describe the combination of molecular beam techniques with a calorimeter in order to investigate thermal properties of isolated metal clusters. A size distribution of neutral SnN clusters with 6⩽N⩽600 is deposited on a bimetallic micromechanical sensor and the released heat during the cluster deposition can be determined from the time-dependent bending of the sensor. The size distribution of the deposited clusters coming from the pulsed source has been probed with a quasicontinuous time-of-flight mass spectrometer. The formation enthalpy per atom of the investigated SnN cluster size distribution is obtained from the measured calorimetric heat to Δhf,298=0.45–0.53 eV.
Crossover scales at the critical points of fluids with electrostatic interactions
Authors: Moreira André G. , Telo da Gama M. M. , Fisher Michael E.
Published: July 2002

Criticality in a fluid of dielectric constant D that exhibits Ising-type behavior is studied as additional electrostatic (i.e., ionic) interactions are turned on. An exploratory perturbative calculation is performed for small ionicity as measured by the ratio of the electrostatic energy e2/Da (of two univalent charges, ±e, separated by the atomic/ionic diameter a) to kBTc0 which represents the strength of the short-range nonionic (i.e., van der Waals) interactions in the uncharged fluid. With the aid of distinct transformations for the short-range and for the Coulombic interactions, an effective Hamiltonian with coefficients depending on the ionicity is derived at the Debye-Hückel limiting-law level for a fully symmetric model. The crossover between classical (mean-field) and Ising behavior is then estimated using a Ginzburg criterion. This indicates that the reduced crossover temperature depends only weakly on the ionicity (and on the range of the nonionic potentials); however, the trends do correlate with the, much stronger, dependence observed experimentally.
Molecular dynamics simulation study of water near critical conditions. II. Dynamics and spectroscopy
Authors: Bursulaya Badry D. , Kim Hyung J.
Published: July 2002

The dynamic and spectroscopic properties of water with density d⩾0.66 g cm−3 at elevated temperatures (T) are studied with the Molecular Dynamics computer simulation technique. The electronic polarizability and hyperpolarizability effects and related many-body interactions are explicitly incorporated into the simulations via the truncated adiabatic basis-set representation. The rotational and translational dynamics associated with individual and collective motions of water molecules are examined. Under the thermodynamic conditions studied here, dielectric relaxation becomes accelerated with increasing T and decreasing d, while water librations become weaker and slower. The far-IR absorption, depolarized Raman scattering (DRS) and optically heterodyne detected optical Kerr effect (OKE) spectroscopies are analyzed. The DRS structure near 50–60 cm−1 observed in the supercritical phase is attributed to the collective excitations, i.e., acoustic wave propagation in the short wavelength limit. Its asymmetric band shape is mainly due to the water librational motions. As for OKE, the nuclear Kerr response becomes slower and less oscillatory with growing T and lowering d. Spectroscopic scales couched in the spectral shift of far-IR and DRS librational bands are introduced to characterize hydrogen bonding. Comparison with the results of the preceding paper [B. D. Bursulaya and H. J. Kim, J. Chem. Phys. 110, 9646 (1999)] shows that simple geometric criteria could significantly overestimate the extent and strength of H-bonding in supercritical water.
An ab initio potential energy surface for Ne–CO
Authors: McBane George C. , Cybulski Slawomir M.
Published: July 2002

A new ab initio two-dimensional potential energy surface for the Ne–CO interaction is described. The surface was obtained by the supermolecule method at the CCSD(T) level of theory. It is compared with several experimental data sets and with the symmetry-adapted perturbation theory (SAPT) surface of Moszynski et al. [J. Phys. Chem. A 101, 4690 (1997)]. The new surface gives modestly better predictions of experimental results that depend on close approach of Ne to CO, but does not describe the ground state geometry as well as the SAPT surface.
The 5a1−1 photoionization of oriented CF3I molecules: Angular distributions of the ka1 and ke photoelectron continua
Authors: Downie Peter , Powis Ivan
Published: July 2002

Molecule-frame photoelectron angular distributions are obtained by the measurement of electron–ion recoil vector correlations in the dissociative 5a1−1 photoionization of CF3I. The laboratory frame orientation of an ionized molecule is inferred from the CF3+ fragment ion recoil direction and the contributions of radiation polarized perpendicular and parallel to the molecular axis can be estimated. This allows the degenerate ka1 and ke continua to be distinguished, each having a distinctive angular distribution. Experimental data are compared with calculated fixed-molecule photoelectron angular distributions.
About surface reaction kinetics on probabilistic and fractal substrates
Authors: Moiny F. , Dumont Martine
Published: July 2002

Two models of bimolecular surface reactions A+(1/n)Bn⟶ lim surfacecatalystAB (n=1,2) occurring on irregular substrates (probabilistic supports and Sierpinski carpets) are investigated by means of Monte Carlo Simulations. Steady-state regimes controlled by the reaction exhibit fractal kinetics features which are compared to those observed when these reactions are simulated on a regular lattice. Noninteger rate orders and modifications of the phase diagram (case n=2) are presented as a function of the concentration of the inactive sites of the surface. These fractal kinetics properties are tentatively interpreted in terms of structural specifics that characterize the ramification of both the substrate and the reactive adsorbate.
Vapor–liquid equilibria of vibrating square well chains
Authors: Hu Liegi , Rangwalla Hasnain , Cui Jingyu , Elliott J. Richard
Published: July 2002

Vibrating square well (SW) 2-mer, 4-mer, and 8-mer with average reduced bond lengths of 0.97±0.03, 0.60±0.03, and 0.40±0.03 were studied by discontinuous molecular dynamics (DMD) simulation in the NVE ensemble. Average bond angles for the reduced bond length of 0.4 were constrained to 127±16° while the longer bond lengths were freely jointed. Vapor–liquid equilibria of the vibrating SW fluids were determined based on DMD simulation by isochoric integration and compared to that of rigid SW chains from Gibbs ensemble Monte Carlo (MC) simulation. The binodals of vibrating chains show a shift to higher temperatures relative to rigid chains, reflecting their less repulsive (more attractive) nature. Vapor pressures of the vibrating chains were computed through isochoric integration with Clausius–Clapeyron consistency to 5% or better. Vapor pressure behavior for each chain model was characterized in terms of critical temperature, critical pressure, and acentric factor. The trend in acentric factor vs. chain length showed that shorter bond lengths gave improved agreement with the experimental trend for n-alkanes. Nevertheless, the trends in acentric factor did not support any molecular model for alkanes which represented methylene segments as individual SW interaction sites. If SW chains are to be applied as models of alkanes, each interaction site must be assigned more than one methylene segment.
Pressure-induced dynamic homogeneity in an athermal diblock copolymer melt
Authors: Floudas G. , Fytas G. , Reisinger T. , Wegner G.
Published: July 2002

We report on the effect of pressure on the dual segmental relaxation of an athermal poly(isoprene-b-vinylethylene) (PI-PVE) diblock copolymer melt using dielectric spectroscopy. The quantity (∂ log τ/∂P)T is employed as a probe of the state of dynamic miscibility in the system. We find that pressure, unlike temperature, induces dynamic homogeneity.
Resonant and antiresonant transport through a fluctuating cage
Authors: Bénichou Olivier , Gaveau Bernard , Moreau Michel
Published: July 2002

We consider an idealized model of diffusion in a cage. The boundary of the cage can be occupied by various correlated obstacles which fluctuate in time according to a dynamic of a Glauber-type. A particle enters the cage and travels through it according to a given stochastic process. Its exit may be blocked by the presence of the obstacles at the surface of the cage. We show that the probability of transmission through the cage can have a maximum and a minimum as a function of the frequency of fluctuation of the obstacles, provided a certain simple condition on the dynamics inside the cage is satisfied.
Dielectric behavior of ferroelectric liquid crystals in the vicinity of the transition into the hexatic phase
Authors: Rychetský I. , Pociecha D. , Dvořák V. , Mieczkowski J. , Górecka E. , Glogarová M.
Published: July 2002

Collective molecular dynamics was studied in ferroelectric smectic C* and hexatic smectic I* phases by the dielectric method. In the frequency range 1 Hz to 10 MHz one relaxation process was observed in both phases. In the SmC* phase the relaxation frequency of this process linearly decreases and its dielectric strength increases when approaching the SmC*–SmI* phase transition temperature TI. In the SmI* phase the relaxation frequency is one order of magnitude lower and further slightly decreases on cooling. A phenomenological theory has been developed which describes static and dynamic dielectric properties near TI and satisfactorily explains the experimental results. The observed relaxation has been attributed to fluctuations of the bond orientational order coupled to the molecular tilt. A parameter describing the softening of this mode is determined by fitting theoretical results to the experimental data. The model also describes an observed anomalous increase in the spontaneous polarization as a result of the change in the strength of bond orientational order when passing to the SmI* phase.
Stochastic bi-resonance without external signal in the CO+O2 catalytic oxidation reaction system
Authors: Hou Zhonghuai , Yang Lingfa , Xin Houwen
Published: July 2002

The noisy dynamic behavior of a surface catalytic reaction model to describe the oxidation of carbon monoxide is investigated when the control parameter is perturbed by external noise near a supercritical Hopf bifurcation point. Noise induced coherent oscillation (NICO) is observed and the NICO strength goes through two maxima with the increment of the noise intensity D from zero, characteristic of the occurrence of stochastic multiresonance without external signal. The frequency of the NICO also increases with the increment of D.
Colloidal suspensions in charged cylindrical pores: A perturbative density functional approach
Authors: Choudhury Niharendu , Ghosh Swapan K.
Published: July 2002

A simple theoretical scheme based on a perturbative density functional approach using an approximate three-particle correlation function in conjunction with a sum rule relating the bulk pressure with the free energy functional of an inhomogeneous system has been proposed to study the structure of a model colloidal suspension (with screened Coulomb interaction) confined in a charged cylindrical pore. The calculated density profiles for the confined colloidal suspension for different values of the wall charges are shown to compare well with the available computer simulation results. The importance of the third order term has been demonstrated through the significantly better agreement of the present results as compared to those based on second order perturbative approach. The relative contribution of the third order term, however, is shown to depend on the screening parameter and hence the salt concentration in the suspension. The effect of the diameters of the cylindrical pores on the density profiles is also investigated.
Self-diffusion of a semiflexible polymer measured across the lyotropic liquid-crystalline-phase boundary
Authors: Russo Paul S. , Baylis Michael , Bu Zimei , Stryjewski Wieslaw , Doucet Garrett , Temyanko Elena , Tipton Debbie
Published: July 2002

The self-diffusion of fluorescently tagged poly(γ-benzyl-α,L-glutamate), a helical, semiflexible synthetic homopolypeptide, has been measured in isotropic and cholesteric liquid-crystalline solutions by pattern fluorescence photobleaching recovery. On the isotropic side of the sharp isotropic–liquid-crystalline (ISO–LC) phase boundary, the rodlike polymers assume all possible orientations in a three-dimensional space, becoming enmeshed. In liquid-crystalline solutions, as first shown by Robinson [Trans. Faraday Soc. 52, 571 (1956)], spontaneous alignment of the cholesteric screw axis parallel to the optical (z) axis of the instrument produces small monodomains in which parallel rodlike polymers are organized into planes. Each horizontal plane is twisted slightly compared to its neighbors. Over the thickness of the sample, the rodlike polymers assume all possible orientations in this two-dimensional space. Despite the small size of the monodomains, it was possible to determine the self-diffusion coefficient of the semiflexible rods, orientationally averaged in two dimensions. Crossing the sharp ISO–LC phase boundary corresponds to the sudden release of any putative topological constraints active in the isotropic phase, and produced a modest but significant increase in diffusion. A relationship developed by Hess, Frenkel, and Allen [Mol. Phys. 74, 765 (1991)] is used to show that diffusion perpendicular to the rod axis is about ten times slower than diffusion parallel to the rod axis in the liquid-crystalline phase. In dilute solution, the comparable number would be 2. The perpendicular diffusion had decreased to about 8% of its initial value in dilute, isotropic solution. The parallel diffusion decreased to about 40% of its initial value. These results were obtained by neglecting the uncertain effects of semiflexibility. Likewise, the effects of modest polydispersity have not been treated explicitly.
Surface tension in terms of direct correlation function c(r,r′): Proposed modelling of c(r,r′) through liquid–vapor interface
Authors: March N. H. , Tosi M. P.
Published: July 2002

A simple model is proposed for the moments of the inhomogeneous direct correlation function entering the theory of surface tension and liquid–vapor interfacial profile for a classical fluid.
Theoretical determination of the absolute electron impact ionization cross-section function for silver clusters Agn (n=2–7)
Authors: Deutsch H. , Pittner J. , Bonacic-Koutecky V. , Becker K. , Matt S. , Märk T. D.
Published: July 2002

A recently developed semiclassical approach (DM formalism) was used to calculate the absolute total electron-impact single ionization cross sections for silver clusters Agn (n=2–7) from threshold to 1000 eV. Similar to other cluster properties, a clear odd–even effect has been observed for the calculated cross sections and is ascribed to the effective electron numbers in the valence shell. The only available experimental data for comparison are partial cross sections for the production of the singly charged parent ions for the monomer up to the tetramer. No experimental data are available as yet for any larger clusters nor for the total ionization cross sections. Nevertheless, a comparison of the results of the present calculations with calculations using additivity rules applied to molecules and clusters gives valuable insight into the ionization behavior of clusters as a function of cluster size and thus elucidates the transition from a molecular-type situation where the cross sections of the constituents are additive to a cluster-type situation where geometric effects lead to a n2/3-dependence in the limit of large cluster sizes n.
Bonding in hypohalous acids HOX (X=F, Cl, Br, and I) from the topological analysis of the electron localization function
Authors: Berski Sławomir , Silvi Bernard , Latajka Zdzisław , Leszczyński Jerzy
Published: July 2002

The bonding in hypohalous acids has been investigated from the topological analysis of the electron localization function (ELF) at the Becke3LYP and Hartree–Fock levels. The interaction between halogen and oxygen atoms has been characterized by the presence of bonding, disynaptic attractors V(O,X) X=F, Cl, Br, and I with the mean electron population N̄ of 0.32, 0.61, 0.45, and 0.35e, respectively. In the case of HOBr, the possibility of a strong contribution of 3d bromine core electrons to the valence shell has been observed. On the base of the bonding evolution theory (BET), the O–F bond has been recognized as a covalent, polarized one whereas, the bonding between O and Cl, Br, and I atoms is of the electron donor–acceptor-type with halogen donating the electron density to valence shell of oxygen. The observed difference between HO+F− and HO−X+ (X=Cl, Br, and I) polarizations is reflected in topology of ELF maps with a large localization domain surrounding the V(F) and V(F,O) attractors in HOF and a common superbasin encompassing the V(O,H), V(O), and V(X,O) attractors in HOCl, HOBr, and HOI. The very large values of the relative quantum fluctuation (λ), above 0.8, found for V(O,X) suggest that the covalent electron density is almost entirely delocalized over other basins. The comparison of the mean electron population (N̄) of the V(H,X) and V(H,O) basins computed for hydrohalic (HX) and hypohalous acids (HOX) has revealed that the population alters in line with values of ΔEacid, therefore can be used as a approximate measure of acidic properties of molecules. The topological analysis of the ELF function supports the concept of probonded electronegativity and its usefulness as a tool for prediction of the nature of the oxygen–halogen linkage.
Excess properties of mixtures of n-alkanes from perturbation theory
Authors: Vega C. , MacDowell L. G. , López-Rodríguez A.
Published: July 2002

Excess properties of binary mixtures of n-alkanes have been evaluated from perturbation theory. A good equation of state for the reference system mixture is combined with a simple approximation to the perturbation term and with a reasonable set of potential parameters to yield a qualitatively correct description of the trends of excess volumes and excess Gibbs energies of n-alkane mixtures without the need for any adjustable parameter. Moreover, the theory can be made quantitative by introducing two adjustable parameters for each temperature. These two parameters have a clear molecular origin and they could be removed if some of the approximations of the theory proposed here were replaced by a more rigorous evaluation. In this sense this paper is just a first step toward a fully molecular theory of excess properties of alkanes. Excess properties estimated from perturbation theory by using these two adjustable parameters are in excellent agreement with experiment and are clearly superior to those obtained from the classic FOV theory proposed by Flory, Orwoll and Vrij [J. Am. Chem. Soc. 86, 3507, 3515 (1964)]. It is our view that the theory of this work is also conceptually superior to the FOV theory, since it rests on a more rigorous molecular basis.
A Monte Carlo study of model electrorheological fluids
Authors: Blair M. J. , Patey G. N.
Published: July 2002

Monte Carlo results for both spherical and nonspherical electrorheological (ER) particles are described. The ER particles are modeled as hard spheres or hard ellipsoids of revolution with much smaller charged hard spheres (ions) constrained to their inner surface. NVT Monte Carlo calculations show that these models display the strong polarization and chain formation characteristic of ER fluids. Both prolate and oblate ER particles align with their longest axis directed along the applied field allowing for larger induced dipole moments than those obtained for spherical ER particles of equal volume. In all cases it is found that chaining occurs at relatively high fields where the induced dipole moments are approaching their maximum value. An interesting feature of nonspherical ER particles is the field-induced orientational order. It is found that even slightly nonspherical ER particles are highly ordered by fields which are weaker than those required to induce chain formation. Very weak fields are sufficient to generate strong orientational order in prolate ER particles with moderate aspect ratios. Further, field-ordered oblate ER particles tend to align their symmetry (short) axes to form a biaxial phase at high densities. The field-induced, liquid-crystal-like behavior of nonspherical ER particles is discussed in detail.
Random walks with nearest neighbors prohibited on the diamond lattice
Authors: Tchijov Vladimir , Rodrı́guez-Romo Suemi
Published: July 2002

Random walks with nearest neighbors prohibited, of lengths N up to 1024 steps, are studied on the three-dimensional diamond lattice. This is a model to describe steric effects given by geometrical constraints on the conformation of macromolecules. Short walks (N⩽20) are exactly enumerated, whereas longer walks are examined by Monte Carlo simulation. A crossover is revealed between two distinct behaviors of the mean values (end-to-end distance and gyration radius), which is due to a short-range influence of the imposed geometrical constraints. The shapes of random walks with nearest neighbors prohibited are also studied. The results are compared to those for self-avoiding random walks on the diamond lattice.
Shear induced fluctuations and fluctuation-length transition in the quasiflexible polymer in a solution of a hydrogen-bond enhancing solvent
Authors: Tsunashima Yoshisuke , Kawanishi Hiroyuki
Published: July 2002

Dynamics of cellulose diacetate (a typical polysaccharide) in N,N-dimethylacetamide (a hydrogen-bond enhancing solvent) at 27 °C was investigated in a shear field of velocity gradient γ=0–5.3 s−1 through dynamic light scattering under circular Couette flow. The polymer mass concentration was set around the crossover of dilute and semidilute regimes. Growth of extra-large length-scale fluctuations and their transitions with shear were detected, which were originated by the long-ranged interaction, or the hydrogen bond, operating on the OH and CO groups in the glucose residues of cellulose acetates via high electronegative solvent. Two modes of fluctuations with fast and slow decay rates Γf and Γs were induced and each Γ value changed stepwise and linearly with γ, respectively. The decay rate distribution, which was estimated from the electric-field time correlation function expressed in the KWW-type of stretched exponential decay form g(1)(t)=A exp[−(t/τ)β] with β the index of decay rate distributions, was changed from 1/2 (one-sided stable distribution) to 3/2 (Holtzmark distribution) around γ=0.8–1 s−1 for the fast mode, while β=2 (Gaussian, or the sharpest distribution) over the full range of 0.40<γ<5.3 s−1 for the slow mode. The usual Cauchy distribution (β=1) at γ=0 (the quiescent state) was destroyed above a threshold γ*=0.033 s−1. At γ*<γ<0.2 s−1, fluctuations of various length-scale modes (0<β<1/2) were created but finally converged to two modes of fixed β(=1/2, 3/2, 2) at γ>0.4 s−1. In addition, oscillatory feature was conspicuous in g(1)(t) at every γ of 0.40<γ<5.3 s−1 and periodic fluctuations became remarkable in the scattered intensity IVv at γ>3.7 s−1, which may come from the competition between shear flow distortion and the Brownian diffusion of polymer chain in solution and can be grasped qualitatively by the initial stage of spinodal decomposition under the control of long-ranged attractive interactions.