bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization of the X̃ 1A′ state of isocyanic acid
Authors: East Allan L. L. , Johnson Christopher S. , Allen Wesley D.
Published: July 2002

Characteristics of the ground electronic state of HNCO have been investigated theoretically in a series of eight ab initio analyses involving qualitative features of the electronic structure, the barrier to linearity, the NH(3Σ−)+CO fragmentation energy, the H–NCO bond dissociation energy, heats of formation of isomers of HNCO, fundamental vibrational frequencies and anharmonic force fields, the rovibrational spectrum of DNCO, and the precise Re structure of isocyanic acid. Sundry state-of-the-art electronic structure methods were employed in the study, including restricted and unrestricted Hartree–Fock (RHF and UHF), complete-active-space self-consistent-field (CASSCF), configuration interaction singles and doubles (CISD), Mo/ller–Plesset perturbation theory through fourth and occasionally fifth order (MP2–MP5), coupled-cluster singles and doubles (CCSD), and CCSD augmented by a perturbative contribution from connected triple excitations [CCSD(T)]. The one-particle basis sets ranged in quality from (9s5p1d/4s2p1d) to (13s8p3d2f/6s5p3d2f ) on the heavy atoms and from (4s1p/2s1p) to (6s2p1d/4s2p1d) on hydrogen. Several revisions of thermochemical data are proposed, in particular, a larger barrier to linearity of 5.7(3) kcal mol−1, an enhanced bond energy of 85.4(10) kcal mol−1 for D0(NH–CO), and more reliable relative energies for the isomers of HNCO, viz., γe(HOCN)=25.5(10), γe(HCNO)=70(2), and γe(HONC)=84.5(15) kcal mol−1. In addition, the experimental value D0(H–NCO)=113.0(2) kcal mol−1 is confirmed. These results lead to several new proposals for heats of formation (ΔH°f,0, kcal mol−1): HNCO(−26.1), HOCN(−0.7), HCNO(+43.0), HONC (+57.6), and NCO(+35.3). A complete quartic force field has been constructed for HNCO by combining RHF third- and fourth-derivative predictions with CCSD quadratic force constants subjected to the scaled quantum mechanical (SQM) optimization scheme.

This force field yields a set of ωi and χij vibrational constants which gives the following fundamental frequencies (with total anharmonicities in parentheses): ν1=3534(−186), ν2=2268(−45), ν3=1330(−9), ν4=778(−50), ν5=576(+9), and ν6=657(+21) cm−1, thus reproducing the observed band origins to 4 cm−1 or less. For DNCO the theoretical force field reveals misassignments of the low-frequency bending vibrations and predicts ν4(a′)=727, ν5(a′)=458, and ν6(a″)=633 cm−1. Finally, the theoretical vibration–rotation interaction constants (αi) for five isotopic species of HNCO have been used in conjunction with empirical rotational constants and the Kraitchman equations to determine re(N–H)=1.0030(20) Å, re(N–C)=1.2145(6) Å, re(C–O)=1.1634(4) Å, θe(H–N–C)=123.34(20)°, and θe(N–C–O)=172.22(20)°.
The structure of electrolytes in cylindrical pores
Authors: Yeomans Laura , Feller Scott E. , Sánchez Enrique , Lozada-Cassou Marcelo
Published: July 2002

The hypernetted chain/mean-spherical approximation (HNC/MSA) equations are derived for ion distributions within and surrounding a charged cylindrical pore for the restricted primitive model of electrolytes. The equilibrium density profiles, mean electrostatic potential profiles, as well as other electrical double-layer structural parameters are presented and compared with the results of the Poisson–Boltzmann (PB) equation and with grand Canonical Monte Carlo data. Quantitative and qualitative differences between the HNC/MSA and the PB theories are found which are especially large for small pore diameters. The effect of geometry is investigated through a comparison with HNC/MSA results for electrolyte confined within a slit pore formed by two parallel plates of infinite extension.
Ab initio prediction of the barrier height for abstraction of H from CH4 by OH
Authors: Dobbs Kerwin D. , Dixon David A. , Komornicki Andrew
Published: July 2002

The reaction CH4+OH→CH3+H2O has been investigated by ab initio molecular orbital theory with a variety of basis sets and levels of correlation. At the highest level of basis set (Dunning’s correlation consistent) and highest level of correlation [QCISD(T)], the value of ΔH(0) is −13.44 kcal/mol and the classical barrier height is 6.62 kcal/mol which is reduced to 5.24 kcal/mol if zero point effects are included. Transition state theory is used to predict the rate constants and good agreement with the experimental values is found over a broad temperature range.
Evidence for a second molecular channel in the fragmentation of formaldehyde
Authors: van Zee Roger D. , Foltz M. Frances , Moore C. Bradley
Published: July 2002

This paper reports CO(v,J) distributions from photolysis on the 2141, 45, 2161, 2143, and 2341 bands of H2CO and the 2243 band of D2CO. A significant fraction of the CO(v=0) photofragment is found in low rotational states (JCO<15) for excitation above the threshold of the H+HCO dissociation channel. Photolysis on the 2141 band, which lies below this radical threshold, shows no measurable population in low-JCO states. The fraction of the total population in low-JCO states increases with increasing photolysis energy. In contrast, the CO(v=1, J) distributions do not broaden significantly with increasing excitation energy. Similar results are found for D2CO. Two alternative models addressing the dynamics of this dissociation are proposed. First, anharmonic motion at the transition state may lead the molecule to dissociate from configurations with smaller impact parameters and thus produce a broadened rotational distribution. More likely, a second fragmentation path, related to the exit channel of the H+HCO→H2+CO abstraction reaction and accessed through the radical channel, may open.
Instabilities in the equations of state of hard-disk and hard-sphere fluids from the virial expansions
Authors: Maeso M. J. , Solana J. R.
Published: July 2002

Equations of state for hard-disk and hard-sphere fluids are obtained from a generalization of the Carnahan–Starling method of direct summation of the virial series. The equations of state thus obtained, besides reproducing all known virial coefficients, agree very accurately with simulation data for stable fluids. If appropriate values for the sixth and seventh virial coefficients are chosen within their uncertainty, the equations of state predict that the fluids become unstable at Kauzmann’s density.
Frequency-dependent hyperpolarizabilities in the Mo/ller–Plesset perturbation theory
Authors: Aiga Fumihiko , Sasagane Kotoku , Itoh Reikichi
Published: July 2002

A formulation for calculating frequency-dependent hyperpolarizabilities in the Mo/ller–Plesset perturbation theory is presented as the correlation correction to the TDHF approximation. Our quasienergy derivative (QED) method is applied, and the difference between the QED method and the pseudoenergy derivative (PED) method by Rice and Handy is discussed. The Lagrangian technique is utilized to obtain simple and practical expressions for response properties in which the TDHF orbital rotation parameters satisfy the 2n+1 rule and the Lagrange multipliers satisfy the 2n+2 rule. Explicit expressions for response properties up to third order [μ, α(−ω1;ω1), β(−ωσ;ω1,ω2)] are derived in the second-order Mo/ller-Plesset perturbation theory.
Systematic construction of efficient many-body perturbation series
Authors: Dietz Klaus , Schmidt Christoph , Warken Markus , Hess Bernd A.
Published: July 2002

A new procedure for the splitting of many-body Hamiltonians into ‘‘free’’ and ‘‘interaction’’ parts is proposed which leads to a rapidly converging perturbation expansion. The efficiency of this method is shown for the case of small molecules: Already first and second order perturbations turn out to produce very reasonable results even for excited states; higher terms rapidly converge to zero. An important point to notice is that our method allows for a priori estimates of the convergence (or divergence) behavior.
A comparison of an experimental unimolecular lifetime distribution with Rice–Ramsperger–Kassel–Marcus theory
Authors: Anicich Vincent G. , Sen Atish D. , McEwan Murray J. , Smith Sean C.
Published: July 2002

The ion–molecule association system (CH+3/CH3CN) has been reexamined by the ion cyclotron double resonance technique. An experimental distribution of lifetimes has been measured for the collision complex (CH3CNCH+3)* formed in the association reaction between CH+3 and CH3CN. The experimental mean lifetime of the association complex formed within the ICR cell was 140 μs. A theoretical examination of the distribution of complex lifetimes using an RRKM model was also undertaken. The matrix of lifetimes for the various values of the total energy of the system (E) and the total angular momentum of the system (J) was obtained. This information was used to visualize the canonical ensemble of collision complexes in the ICR experiment in terms of their lifetimes. Once the distribution of lifetimes predicted by the model was modified to conform to experimental constraints, it was found to give a good approximation of the lifetime distribution determined experimentally. As a result of the new measurements of the complex lifetimes, we report absolute values of the collisional stabilization efficiencies. We also report rate coefficients for unimolecular dissociation and radiative relaxation.
Theoretical studies on structure, thermochemistry, vibrational spectroscopy, and other features of ZrX2−6 (X=F,Cl,Br,I): Coulombic energy in inorganic and organic hexahalogenozirconates
Authors: Rak Janusz , Gutowski Maciej , Dokurno Pawel/ , Thanh Hoan Vu , Bl/ażejowski Jerzy
Published: July 2002

The geometries of ZrX2−6 (X=F,Cl,Br,I) were optimized at the ab initio Hartree–Fock SCF level using all-electron MIDI and effective core potential valence basis sets of double-zeta quality, supplemented with diffuse functions. In addition, geometries and energies of X2, as well as energies of gaseous Zr were determined in order to predict energies of formation of ZrX2−6. Effects of electron correlation were taken into account at the second order Mo/ller–Plesset level of theory. Vibrational frequencies were determined in the harmonic approximation and compared with available experimental data. Standard routines were employed to evaluate entropies, heat capacities, heats of formation and free enthalpies of formation of gaseous ZrX2−6. Electrostatic cohesive energies in hexahalogenozirconates were evaluated by the extended Ewald method. It was assumed that each ion gathers a formal charge +1, +2, or −2. Net atomic charges in complex ions were determined either from various population analyses or fitted to the ab initio quantum mechanical electrostatic potential. The Coulombic energies decrease gradually with the increase in volume of the simplest structural unit. A similar tendency is noted as regards distances between interacting centers. Theoretically determined properties are in a good agreement with available data, mostly of experimental origin.
A nonlocal density functional theory of electric double layer: Symmetric electrolytes
Authors: Patra Chandra N. , Ghosh Swapan K.
Published: July 2002

The density functional theory of inhomogeneous classical neutral fluids is extended to study the structure of electric double layer using the restricted primitive model as well as the nonprimitive three-component molecular solvent model. The formalism is based on a weighted density approach where the hard-sphere contributions to the excess free energy density and the one-particle correlation function are evaluated nonperturbatively using a position-dependent effective density while the corresponding ionic part is obtained through a second-order functional Taylor expansion around this effective density. The calculated results for the density profiles of the ions and the mean electrostatic potential are in good agreement with the available simulation results for the continuum solvent primitive model. The corresponding results for the nonprimitive molecular solvent model provide insight into the layering effect due to hard-sphere exclusion and the charge inversion phenomena.
Comment on ‘‘A multiconfiguration time-dependent Hartree approximation based on natural single-particle states’’ [J. Chem. Phys. 99, 4055 (1993)]
Authors: Manthe Uwe
Published: July 2002

It is shown that the multiconfigurational time-dependent Hartree (MCTDH) equations presented in Jansen’s work are a special case of generalized MCTDH equations presented in earlier work. The convergence with respect to the number of expansion functions employed is unaltered compared to the simple original MCTDH scheme, no reduction in the number of expansion functions required can be gained by employing natural single-particle functions. Also, the computational effort for the numerical integration of the equations of motion is found to be similar for both sets of MCTDH equations.
Effect of lateral interactions on the kinetics of the oxidation of carbon monoxide on palladium
Authors: Araya Paulo , Cortés Joaquin
Published: July 2002

Monte Carlo simulations are used to study the effect of lateral repulsion between CO molecules on the kinetics of the oxidation of CO and O2 on a Pd catalyst. The results of CO coverage and reaction rate are compared using three models of lateral interaction: (i) short-range lateral interaction (nearest neighbors); (ii) long-range lateral interaction (dipole–dipole type); and (iii) lateral interaction energy equal to β θCO mean field approximation (MFA). The results show that the MFA model predicts reaction rates and CO coverage higher than other models. However, the disagreement is significantly less between the MFA model and the long-range lateral interaction model. The results may be interpreted in terms of the different topography of the adsorbed CO phase that is generated when using the various models.
Fourier transform emission spectroscopy of HfH and HfD
Authors: Ram R. S. , Bernath P. F.
Published: July 2002

The electronic emission spectra of HfH and HfD were investigated in the 476 nm to 1 μm spectral region using a Fourier transform spectrometer. The bands were excited in a hafnium hollow cathode lamp in the presence of a mixture of neon and H2 or D2. For HfH, two ΔΩ=0 electronic transitions with a common Ω″=3/2 lower state, most probably the X 2Δ3/2 ground state, were identified at 14 495 and 19 147 cm−1. These bands have been assigned as the [14.5]3/2−X 2Δ3/2 and the [19.1]3/2−X 2Δ3/2 transitions. The rotational analysis of the 0–0 and 1–1 bands of the [14.5]3/2−X 2Δ3/2 system and the 0–0 band of the [19.1]3/2−X 2Δ3/2 system provides the following equilibrium constants for the ground state of HfH: Be″ = 5.019 11(8) cm−1, αe″= 0.120 26(11) cm−1, and re″ = 1.830 691(15) Å. For HfD, only the 0–0 band of the [19.1]3/2−X 2Δ3/2 transition at 19 134 cm−1 was identified. This work represents the first observation of the HfH and the HfD molecules.
Rotational autoionization dynamics in high Rydberg states of nitrogen
Authors: Merkt F. , Mackenzie S. R. , Softley T. P.
Published: July 2002

The decay dynamics of the high Rydberg states of N2 converging on the first few rotational levels (N+=0,1,2,3) of the ground vibronic X 2Σ+g (v+=0) state of the N+2 cation have been investigated by delayed pulsed field ionization (PFI) following two-photon enhanced (2+1′) three-photon excitation via the a″ 1Σ+g (v′=0) state of N2. The experiments were carried out in the presence of a weak homogeneous dc electric field and at typical ion densities of 200–2000 ions/mm3. All Rydberg states in the range of principal quantum number n=140–200 exhibit extreme stability against autoionization and predissociation and some have lifetimes which exceed 30 μs. The decay of the highest Rydberg states beyond n=200 is induced by external perturbations (field ionization and collisional ionization) and no Rydberg states beyond n=350 can be observed by delayed PFI. The Rydberg states which converge on the N+=0 and 1 rotational levels of the ion, and which therefore are not subject to rotational autoionization, decay into neutral products (by a process presumed to be predissociation) in less than 7 μs in the range n<100. The importance of predissociation is greatly reduced beyond n=100 and becomes negligible on our experimental timescale (30 μs) above n=140. The decay of the Rydberg states converging on the N+=2 and 3 rotational levels of the ion is more complex. Below n=100, only 30%–40% of the Rydberg population decays by fast rotational autoionization whereas 60%–70% decays by predissociation.

The importance of predissociation decreases rapidly above n=100 and becomes negligible beyond n=140. The decay by rotational autoionization can be observed at all n values but becomes noticeably slower beyond n=100. In the range n=140–200 it exhibits a marked biexponential decaying behavior with 30% of the population decaying within a few microseconds and 70% displaying long term stability (τ≳30 μs). The branching between predissociation and autoionization is explained by the effect of the dc electric field which mixes strongly the optically accessible p Rydberg series with the high l manifold beyond n=100. The long lifetimes observed experimentally indicate that ml mixing becomes important as soon as l mixing sets in.
Charge stabilization by chloromethane molecules on multilayer Kr films
Authors: Bass A. D. , Gamache J. , Ayotte P. , Sanche L.
Published: July 2002

We have measured the charge trapping cross section for the chloromethane molecules CCl4, CHCl3, and CH2Cl2 physisorbed onto Kr films. Within the 0–10 eV range investigated, charge is stabilized at the surface as atomic anions which are formed by dissociative electron attachment (DEA) to the molecules. Intermediate anion states previously observed in gas phase experiments are apparent. Below ≊4 eV electron impact energy, the charge trapping cross section becomes an absolute cross section for DEA because of the absence of a desorbed ion signal. At higher energies, our results provide a lower limit for this same quantity. Comparisons with gas phase cross sections, show substantial and systematic differences that can be understood in terms of the interaction between the intermediate anion states and electronic polarization field they induce on the Kr surface.
Time resolved heat propagation in a gold crystal by means of picosecond x-ray diffraction
Authors: Chen P. , Tomov I. V. , Rentzepis P. M.
Published: July 2002

Characteristic Cu Kα x-ray pulses of less than 10 ps duration at 300 Hz repetition rate are generated in a x-ray diode driven by 1.8 ps, 193 nm pulses. Both light and x-ray pulses are utilized in pump–probe experiment to study the heat propagation in a Au(111) crystal lattice. Changes in the x-ray diffraction pattern in the first 100 ps are clearly observed with about 10 ps resolution. The system allows for detection of temperature changes of 20 °C in the 8–20 ps time frame.
Matched two-pulse electron spin echo envelope modulation spectroscopy
Authors: Jeschke Gunnar , Schweiger Arthur
Published: July 2002

The theory of nonideal microwave pulses acting on electron–nuclear spin systems is extended and applied to optimize the two-pulse electron spin echo envelope modulation (ESEEM) experiment. A superoperator approach for a computationally efficient simulation of experiments involving non-ideal pulses is introduced and the corresponding unitary transformation superoperator is given analytically for a system consisting of one electron spin S=1/2 and one nuclear spin I=1/2. Density operator single-element transfers are divided into allowed and forbidden ones and are classified according to their functioning in pulse ESR. By increasing the efficiency of forbidden transfers by Hartmann–Hahn matching during prolonged pulses, the sensitivity of the conventional two-pulse ESEEM experiment may drastically be improved and discrimination between basic, hyperfine, and combination frequencies becomes possible. The implications of the theory for spin systems with an arbitrary number of nuclear spins 1/2 are investigated by deriving and discussing a general condition for Hartmann–Hahn matching of forbidden transitions. It is shown that the product rule valid for two-pulse echo modulations caused by more than one nucleus does not hold for nonideal pulses. A method is developed that allows one to reduce the thus arising large dimensionality of the diagonalization problem in numerical simulations. The theoretical conclusions are verified by experiments on two transition metal complexes in single crystals and on a spin-label-doped polymer sample.
Bimolecular reactions observed by femtosecond detachment to aligned transition states: Inelastic and reactive dynamics
Authors: Zhong Dongping , Cheng P. Y. , Zewail Ahmed H.
Published: July 2002

With fs radical detachment and kinetic energy-resolved time-of-flight (KETOF) mass spectrometry, we are able to study the transition state dynamics of the bimolecular reaction CH3I+I, inelastic and reactive channels; the collision complex is coherently formed (1.4 ps) and is long lived (1.7 ps). We also report studies of the dynamics of I2 formation. Direct clocking of the CH3I dissociation, hitherto unobserved, gives 150 fs for the C–I bond breakage time and 0.8 Å for the repulsion length scale.
UV photodissociation of oxalyl chloride yields four fragments from one photon absorption
Authors: Ahmed Musahid , Blunt David , Chen Daniel , Suits Arthur G.
Published: July 2002

The photodissociation of oxalyl chloride, (ClCO)2, has been studied near 235 nm using the photofragment imaging technique. Observed products include both ground state Cl (2P3/2) and spin-orbit excited Cl*(2P1/2) chlorine atoms and ground electronic state CO molecules. The rotational distribution obtained for the CO v=0 product is peaked at about J=30 and extends beyond J=50. Photofragment images were recorded for both chlorine atom fine structure components as well as many rotational levels of the CO v=0, yielding state-resolved angular and translational energy distributions. The recoil speed distribution for the ClCl* exhibits a dominant fast component, with a translational energy distribution peaking at about 48 kJ/mol. The ground state chlorine atom showed two components in its speed distribution, with the slow component dominant. The corresponding translational energy distribution peaked at 10 kJ/mol but extended to 80 kJ/mol. The total average translational energy release into the Cl product is 34 kJ/mol. Similarly, the low rotational levels of the CO showed only a slow component, the intermediate rotational levels showed a bimodal speed distribution, and the highest rotational levels showed only the fast component. The fast components of both chlorine atom product and the higher rotational levels of the CO show an anisotropic angular distribution, while all slow fragments show a nearly isotropic angular distribution. These observations suggest a novel dissociation mechanism in which the first step is an impulsive three-body dissociation yielding predominantly Cl*, rotationally excited CO and chloroformyl radical ClCO, with only modest momentum transfer to the latter species. Most of the remaining ClCO undergoes subsequent dissociation yielding low rotational levels of CO and little translational energy release.
Ab initio quartic force fields for anions: A benchmark study on OH−16, OH−18, and OD−16
Authors: Lee Timothy J. , Dateo Christopher E.
Published: July 2002

The singles and doubles coupled-cluster method that includes a perturbational correction for connected triple excitations, denoted CCSD(T), is used in conjunction with several one-particle basis sets to determine quartic force fields for OH−. Basis set convergence properties of the various rovibrational spectroscopic constants is studied. To assure numerical stability of the quartic force fields and to investigate variational calculations of vibrational energy levels, sextic force fields have also been evaluated. The largest one-particle basis set employed contains up through h functions on oxygen and g functions on hydrogen. Agreement between experiment and theory is excellent and it is shown that the CCSD(T)/spdfg approach performs as well for OH− as it previously did for H2O. Due to the inherent difficulty in high resolution studies on molecular anions, it is concluded that currently the CCSD(T)/spdfg approach may be the most reliable technique for the determination of accurate rovibrational spectroscopic properties of small- to medium-sized anions.
Decoupling of tracer diffusion from viscosity in a supercooled liquid near the glass transition
Authors: Bhattacharyya Sarika , Bagchi Biman
Published: July 2002

Several experimental and computer simulation studies have found a partial decoupling between the diffusion coefficient of a small tagged particle from the viscosity of the medium in the supercooled liquid regime. In order to understand the microscopic origin of this decoupling, we have carried out detailed theoretical calculations of both quantities by using a self-consistent mode-coupling theory (MCT) which provides a microscopic treatment of coupled solute-solvent dynamics. We find that at low degree of supercooling, both the tracer diffusion coefficient (Dt) and the viscosity (η) of the medium vary similarly with a ratio close to the value given by the Stokes–Einstein relation (with the slip boundary condition). However, at higher supercooling the viscosity increases faster than the decrease in the diffusion coefficient. This decoupling is found to depend strongly on both the size of the solute and the degree of supercooling, in agreement with the recent experimental results. The decoupling starts at a lower degree of supercooling for smaller sized tracers. The physical origin of the decoupling can be traced back to the faster increase of viscosity due to the appearance of a long-time tail in the dynamic structure factor. While the appearance of the long-time tail leads to a rapid increase of viscosity, the friction on the tracer molecule, whose motion occurs on a shorter time scale and smaller length scale, increases slowly, thus leading to the partial decoupling. However, when the size of the solute is the same as that of the solvent molecules, the straightforward application of MCT fails to predict the observed decoupling. It is shown here that this decoupling could be explained semi-quantitatively by extending the mode-coupling theory to include solvent inhomogeneity.
Effect of solvent quality on the conformation and relaxation of polymers via dissipative particle dynamics
Authors: Kong Y. , Manke C. W. , Madden W. G. , Schlijper A. G.
Published: July 2002

Dissipative particle dynamics (DPD), a new simulation technique appropriate at mesoscopic length scales, has been applied to a dilute solution of polymer in solvents of varying quality. Unlike earlier simulations, the solvent is represented in the form mobile particulate packets, so that potential interferences of solvent flow about neighboring beads of the polymer are explicitly included. We establish that the mechanism used to vary the solvent quality produces a collapse transition, as judged both by static conformational and by dynamical criteria. The scaling of the polymer radius of gyration and of its longest dynamical relaxation time are in good agreement with accepted theory. The condition for transition from good to poor solvent is consistently predicted by several static and dynamical measures. Though the model that underlies DPD is not atomically detailed, the results presented strongly suggest that both excluded volume and hydrodynamic interaction must present.
Crossover of domain-growth behavior from percolation to cluster regime in phase separation of an off-critical polymer mixture
Authors: Takeno Hiroyuki , Hashimoto Takeji
Published: July 2002

Time-resolved light scattering has been used to investigate phase-separation dynamics for off-critical mixtures of polybutadiene (PB) and polyisoprene (PI) with a well defined phase diagram. Percolation-to-cluster transition (PCT) occurs in the course of the phase separation process for an off-critical mixture of the PB and the PI in the shallow quenches covered in this work. Before the onset of PCT occurs, the phase separation dynamics is fast, typically obeying qm∼t−0.9 and Im∼t2.7 (“percolation regime”), where qm is the scattering vector at the maximum scattered intensity Im. After the onset of PCT the time changes in qm and Im become very slow, and the phase separation process might appear to stop, if we observe the dynamics in a short time scale after the onset of PCT. The behavior corresponds to the results which Hashimoto et al. reported for the off-critical mixtures of poly(styrene-ran-butadiene) (SBR) and PB, and SBR and PI (spontaneous pinning of the domain growth) [J. Chem. Phys. 97, 679 (1992)]. However, it is shown in this paper that the droplets formed after PCT eventually grow according to the scaling law of qm∼t−n and Im∼t3n with n=1/4 to 1/3 (“cluster regime”), if we keep observing the phase separation process in the much longer time scale, beyond the time period over which the domain growth appeared to be pinned. To our knowledge, it is for the first time that such crossover of domain-growth behavior from percolation to cluster regime was observed via the intermittent time period over which the growth kinetics are apparently pinned.
Dissociative chemisorption of CH4 on Ni: The role of molecular orientation
Authors: Carré Marie-Noëlle , Jackson Bret
Published: July 2002

The dissociation of CH4 on a Ni surface is examined quantum mechanically. The molecule is treated as a quasidiatomic R–H, where R=CH3, and vibrational, rotational, and translational motion normal to the surface are included. The metal surface is assumed to be flat and the potential energy surface is based on ab initio studies of dissociation over the Ni(111) atop site. Lattice motion is introduced via the surface mass model. Bessel–Legendre and fast Fourier transform pseudospectral techniques are used to evolve the wave function in time, and energy resolved reactive fluxes are extracted via a time-to-energy transform. Agreement with experiment is good, particularly with regard to the dependence of the dissociation probability on incident energy and surface temperature. The dynamics of the dissociation reaction for the various initial states of the molecule are examined.
From many monomers to many polymers: Soft ellipsoid model for polymer melts and mixtures
Authors: Murat Michael , Kremer Kurt
Published: July 2002

We present an extremely efficient and rather general model in which whole polymer chains are represented as soft particles. The particles are characterized by their overall sizes and shapes, as given by the conformations of the underlying chains. The probability of occurrence of a particle with a given size determines its internal free energy. The density of monomers within each particle is calculated from all conformations that have the same size. The interaction between two particles is taken to be proportional to the spatial overlap of their monomer density distributions. When a large number of such particles are brought into contact, as is the case for a polymer melt, the interactions between the particles force them to shrink and modify the equilibrium size distribution. We show by simulations that this model leads to a Gaussian statistics of the chains in melt. Since the internal degrees of freedom of a chain are integrated out, a large number (of order 104) of long (e.g., N=100) chains can be simulated within reasonable computer time on a single work-station processor. A straightforward extension of this model is used to study symmetric polymer blends.
The microwave spectrum of the NF radical in the second electronically excited (b 1Σ+) state: Potentials of three low-lying states (X 3Σ−,a 1Δ,b 1Σ+)
Authors: Kobayashi Kaori , Saito Shuji
Published: July 2002

The pure rotational transition of the NF radical in the second electronically excited state, b 1Σ+, was detected by microwave spectroscopy. The NF radical was generated by dc-discharge through a mixture of NF3 and H2 at around 90 K. Six rotational transitions for v=0 and four for v=1 were observed in the 73–442 GHz region. In addition to the rotational and centrifugal distortion constants, the quadrupole coupling constant of the nitrogen nucleus of NF(b 1Σ+) was determined for the first time from the lowest two transitions of NF in the v=0 level. The spectrum of NF(a 1Δ) in the first vibrational excited state was also observed in order to derive highly accurate potential parameters for comparison with the data of NF(X 3Σ−,b 1Σ+).
Vector parametrization of the N-atom problem in quantum mechanics. I. Jacobi vectors
Authors: Gatti Fabien , Iung Christophe , Menou Michel , Justum Yves , Nauts André , Chapuisat Xavier
Published: July 2002

Within the framework of an adequate spectral representation, the geometrical description of an N-atom molecular system by n=N−1 Jacobi relative position vectors is shown to be particularly advantageous with regard to the criterion of prediagonalization of the matrix representing the kinetic energy operator.
Fast structural relaxation of polyvinyl alcohol below the glass-transition temperature
Authors: Murakami Hiroshi , Kushida Takashi , Tashiro Hideo
Published: July 2002

In order to obtain information about structural relaxations of polymers within a time window of several nanoseconds, the absorption, site-selective steady-state fluorescence and time-resolved fluorescence spectra have been measured for polyvinyl alcohol doped with rhodamine 640 in the 150–300 K temperature range. The temperature dependence of the absorption and fluorescence spectra has been analyzed on the basis of one- and two-dimensional configuration-coordinate models. In spite of the measurement below the glass-transition temperature of the matrix, the existence of a fast relaxation process which is completed within a few hundred ps has been clarified. The magnitude of this relaxation increases with increasing temperature, while the relaxation mechanism cannot be ascribed to the thermal crossing of static energy barriers. It has been found that the experimental results are not explained by the two-dimensional configuration coordinate model in which the fast and slow structural relaxations are assumed to occur independently along the two axes. A relaxation process triggered by temperature-dependent release from the constraint preventing the structural change is shown to account for the experimental results well using a one-dimensional configuration coordinate model.
A comparison of the efficiency of Fourier- and discrete time-path integral Monte Carlo
Authors: Chakravarty C. , Gordillo M. C. , Ceperley D. M.
Published: July 2002

We compare the efficiency of Fourier and discrete time path integral Monte Carlo (PIMC) methods on a cluster of 22 hydrogen molecules at 6 K. The discrete time PIMC with a pair density matrix approximation to the path action is shown to be the most efficient for evaluating all the observables studied here. The Fourier PIMC technique has a comparable efficiency for observables diagonal in the coordinates but is significantly worse for estimating the kinetic and total energies. The superior performance of the discrete time PIMC is shown to be due to the more accurate treatment of the path action using the pair density matrix approach; the discrete time PIMC simulation within the primitive approximation is much less efficient. Complete details of the implementation of all algorithms are given.
Effect of dipolar interactions on the phase behavior of the Gay–Berne liquid crystal model
Authors: Houssa Mohammed , Rull Luis F. , McGrother Simon C.
Published: July 2002

A computer simulation study of the phase behavior of the dipolar Gay–Berne liquid crystal model is presented. The phase transitions are determined with isothermal–isobaric (NPT) Monte Carlo simulations, utilizing the reaction field method. The electrostatic forces are found to have a considerable effect on the nature of the observed phases, but the density at which the isotropic fluid becomes unstable with respect to partially ordered phases is seen to be remarkably insensitive to the strength of the dipole. We pay particular attention to the structure of the mesophases, combining information from several singlet and pair distribution functions to build up an accurate picture of the molecular arrangement of the systems.
Fluid transport properties by equilibrium molecular dynamics. I. Methodology at extreme fluid states
Authors: Dysthe D. K. , Fuchs A. H. , Rousseau B.
Published: July 2002

The Green-Kubo formalism for evaluating transport coefficients by molecular dynamics has been applied to flexible, multicenter models of linear and branched alkanes in the gas phase and in the liquid phase from ambient conditions to close to the triple point. The effects of integration time step, potential cutoff and system size have been studied and shown to be small compared to the computational precision except for diffusion in gaseous n-butane. The RATTLE algorithm is shown to give accurate transport coefficients for time steps up to a limit of 8 fs. The different relaxation mechanisms in the fluids have been studied and it is shown that the longest relaxation time of the system governs the statistical precision of the results. By measuring the longest relaxation time of a system one can obtain a reliable error estimate from a single trajectory. The accuracy of the Green-Kubo method is shown to be as good as the precision for all states and models used in this study even when the system relaxation time becomes very long. The efficiency of the method is shown to be comparable to nonequilibrium methods. The transport coefficients for two recently proposed potential models are presented, showing deviations from experiment of 0%–66%.
Discrimination of product isomers in the photodissociation of propyne and allene at 193 nm
Authors: Sun Weizhong , Yokoyama Keiichi , Robinson Jason C. , Suits Arthur G. , Neumark Daniel M.
Published: July 2002

The photodissociation dynamics of propyne and allene are investigated in two molecular beam/photodissociation instruments, one using electron impact ionization and the other using tunable vacuum ultraviolet (VUV) light to photoionize the photoproducts. The primary dissociation channels for both reactants are C3H3+H and C3H2+H2. Measurement of the photoionization efficiency curves on the VUV instrument shows that the C3H3 product from propyne is the propynyl (CH3CC) radical, whereas the C3H3 product from allene is the propargyl (CH2CCH) radical. The dominant C3H2 product from both reactants is the propadienylidene (H2CCC) radical. We also observe a small amount of secondary C3H2 product from photodissociation of the C3H3 radicals in both cases.
First experimental photoelectron spectra of superhalogens and their theoretical interpretations
Authors: Wang Xue-Bin , Ding Chuan-Fan , Wang Lai-Sheng , Boldyrev Alexander I. , Simons Jack
Published: July 2002

Photoelectron spectra of the MX2− (M=Li, Na; X=Cl, Br, I) superhalogen anions have been obtained for the first time. The first vertical detachment energies (VDEs) were measured to be 5.92±0.04 (LiCl2−), 5.86±0.06 (NaCl2−), 5.42±0.03 (LiBr2−), 5.36±0.06 (NaBr2−), 4.88±0.03 (LiI2−), and 4.84±0.06 eV (NaI2−), which are all well above the 3.61 eV electron detachment energy of Cl−, the highest among atomic anions. Experimental photoelectron spectra have been assigned on the basis of ab initio outer valence Green function (OVGF) calculations. The corresponding theoretical first VDEs were found to be 5.90 (LiCl2−), 5.81 (NaCl2−), 5.48 (LiBr2−), 5.43 (NaBr2−), 4.57 (LiI2−), and 4.50 eV (NaI2−), in excellent agreement with the experimental values. Photodetachment from the top four valence molecular orbitals (2σg22σu21πu41πg4) of MX2− was observed. Analysis of the polestrength showed that all electron detachment channels in this study can be described as primarily one-electron processes.
Determination of highly excited rovibrational states for N2O using generalized internal coordinates
Authors: Zúñiga José , Alacid Mercedes , Bastida Adolfo , Carvajal Francisco J. , Requena Alberto
Published: July 2002

Generalized internal vibrational coordinates are optimized and used to describe highly excited vibrational motions in the N2O molecule. These coordinates are defined as the magnitudes of two vectors, which are expressed as linear combinations of the internal displacement vectors and the angle formed between them. They depend on two parameters and contain, as particular cases, valence and orthogonal (Jacobi, Radau, etc.) coordinate systems. The coordinates are optimized by minimizing unconverged variationally computed vibrational energies with respect to the external parameters. A comparison of the optimal internal coordinates derived for N2O with valence and hyperspherical normal coordinates is made. The optimal internal coordinates are also used to determine a new potential energy function for N2O from the observed vibrational frequencies up to 15 000 cm−1 using fully variational calculations. The quality of the adjusted potential energy function is checked by computing vibrational-rotation energy levels and rotational constants for Σ, Π, Δ, Φ, and Γ states and comparing them with the observed values.
The mechanism of thickness selection in the Sadler-Gilmer model of polymer crystallization
Authors: Doye Jonathan P. K. , Frenkel Daan
Published: July 2002

Recent work on the mechanism of polymer crystallization has led to a proposal for the mechanism of thickness selection which differs from those proposed by the surface nucleation theory of Lauritzen and Hoffman and the entropic barrier model of Sadler and Gilmer. This has motivated us to reexamine the model used by Sadler and Gilmer. We again find a fixed-point attractor which describes the dynamical convergence of the crystal thickness to a value just larger than the minimum stable thickness, lmin. This convergence arises from the combined effect of two constraints on the length of stems in a layer: it is unfavorable for a stem to be shorter than lmin and for a stem to overhang the edge of the previous layer. The relationship between this new mechanism and the explanation given by Sadler and Gilmer in terms of an entropic barrier is discussed. We also examine the behavior of the Sadler-Gilmer model when an energetic contribution from chain folds is included.
Accurate intermolecular ground state potential of the Ar–N2 complex
Authors: Fernández Berta , Koch Henrik , Makarewicz Jan
Published: July 2002

The ground state potential energy surface of the Ar–N2 van der Waals complex is evaluated using the coupled cluster singles and doubles model, including connected triples corrections (CCSD(T)) and the aug-cc-pVTZ basis set extended with midbond functions. From the calculated ab initio potential the rovibronic spectroscopic properties are determined and compared with the available experimental data. Considerable improvement is obtained when four of the potential parameters are refined based on the Ar–14N2 rotational transition frequencies. The small discrepancies remaining demonstrate that the coupled cluster method can be used to predict the spectroscopic properties of van der Waals complexes.
Chiral symmetry breaking. I. Cooperative effects in solutions
Authors: Bersuker Gennadi
Published: July 2002

A mechanism that may lead to asymmetry between the concentrations of levo and dextro enantiomers is suggested. Under certain conditions, chiral symmetry breaking is possible due to the cooperative effect in solution when the chiral configuration of a molecule is affected by the chirality of others. This effect may lead to a kind of “phase transition” of the system to the state where a presence of only one type of enantiomers, either levo or dextro, is energetically advantageous. Cooperativity results from an increase of the effective radius of the intermolecular interaction due to mobility of molecules in the liquid and gas phases. The kinetics of the reactions, as well as possible ways to the synthesis of the compounds with desirable chirality, are considered. An experiment on chiral symmetry breaking in sodium chlorate crystallization, an example of chiral symmetry breaking in a macro system, is discussed.
Vibration-rotation emission spectra and combined isotopomer analyses for the coinage metal hydrides: CuH & CuD, AgH & AgD, and AuH & AuD
Authors: Seto Jenning Y. , Morbi Zulfikar , Charron Frank , Lee Sang K. , Bernath Peter F. , Le Roy Robert J.
Published: July 2002

High resolution infrared emission spectra have been measured for AuH and AuD and for two isotopomers of each of CuH, CuD, AgH, and AgD. The molecules were made in a carbon tube furnace (King furnace), and in spite of intense background thermal emission from the furnace (at >2000 °C), vibration–rotation emission data could be recorded. Together with high resolution measurements taken from the literature, the data for each species were treated using two types of combined-isotopomer analysis: One based on fits to empirical molecular parameters, and the other based on direct fits to the underlying potential energy functions, both of which take account of mass-dependent Born–Oppenheimer breakdown correction terms. Accurate isotopically related Dunham parameters and Born–Oppenheimer breakdown parameters are obtained for each species, as well as accurate analytic potential functions and adiabatic and nonadiabatic radial correction functions.
Surface enhanced Raman scattering in the near infrared using metal nanoshell substrates
Authors: Oldenburg Steven J. , Westcott Sarah L. , Averitt Richard D. , Halas Naomi J.
Published: July 2002

A metal nanoshell is a composite nanoparticle consisting of a dielectric core coated by a thin metal shell; its peak plasmon resonance wavelength is determined by the ratio of the core diameter to the shell thickness. When p-mercaptoaniline (p-MA) is in solution with gold nanoshells that have their plasmon resonance near a 1.06 μm excitation source, significant surface enhanced Raman scattering (SERS) is observed. The strongest Raman enhancements are obtained when enough gold is deposited on the silica cores to form a nearly complete metal shell. Correlations between transmission electron microscopy (TEM)-defined structure, ultraviolet (UV)-visible spectra, SERS signal strength, and electromagnetic theory show that the SERS signal is due to both the local enhancement of the dielectric field via the plasmon resonance of the nanostructure and to the localized regions of high field intensity provided by the nearly completed gold shell. Comparison with SERS enhancements on completed nanoshell structures indicates the relative contribution of these two effects.