bluuebunny/crossref_metadata_2025 huggingface dataset:
Development of a pure diffusion quantum Monte Carlo method using a full generalized Feynman–Kac formula. I. Formalism
Authors: Caffarel Michel , Claverie Pierre
Published: July 2002

This paper presents systematic developments in the previously initiated line of research concerning a quantum Monte Carlo (QMC) method based on the use of a pure diffusion process corresponding to some reference function and a generalized Feynman–Kac path integral formalism. Not only mean values of quantum observables, but also response properties are expressed using suitable path integrals involving the diffusion measure of the reference diffusion process. Moreover, by relying on the ergodic character of this process, path integrals may be evaluated as time-averages along any sample trajectory of the process. This property is of crucial importance for the computer implementation of the method. As concerns the treatment of many-fermion systems, where the Pauli principle must be taken into account, we can use the fixed-node approximation, but we also discuss the potentially exact release-node procedure, whereby some adequate symmetry is imposed on the integrand (of the generalized Feynman–Kac formula), associated with a possibly refined choice of the reference function.
Magnetic structure and Pu ground state in β-Pu2O3
Authors: Wulff M. , Lander G. H.
Published: July 2002

Neutron diffraction from a polycrystalline sample of 242Pu2O3 (La2O3 structure) has been used to solve the magnetic structure below TN=19 K. The magnetic unit cell requires a doubling in all three crystallographic directions of the chemical unit cell. Below 4 K the magnetic and chemical unit cells become identical. The moments point along the unique c axis at all temperatures below TN. The magnetic moment is 0.60(2)μB per Pu ion and the C2 coefficient in the Pu3+ form factor is 4.5(3). The value of the moment, its direction within the unit cell, and the C2 coefficients are all consistent with the ‖±3/2〉 Kramers doublet being the ground state of the Pu3+ ion.
Theoretical study of excited singlet states of H+3: Potential surfaces and transition moments
Authors: Talbi Dahbia , Saxon Roberta P.
Published: July 2002

A b initio calculations of 13 singlet states of H+3 in C2v symmetry are reported. The diabatic correlation diagram linking the states at the ground state equilibrium D3h symmetry to those at the H2+H and three-body asymptotes has been established by careful analysis of the excited state wave functions. Transition moments between the two lowest states and those connecting the diabatic states arising from H+2 +H(n=2) and the lower states have been evaluated. Radiative lifetimes of the excited states have been estimated. Implications for observing photodissociation of H+3 are discussed.
Deexcitation of He(2 1P) in collisions with rare gas atoms
Authors: Ukai Masatoshi , Yoshida Hiroaki , Morishima Yasumasa , Nakazawa Hidenobu , Shinsaka Kyoji , Hatano Yoshihiko
Published: July 2002

Deexcitation (Penning ionization) cross sections of He(2 1P) by Kr and Xe have been obtained in a region of mean collisional energy between 18–38 meV by a pulse radiolysis method. Fairly large cross sections of above 100 Å2 and their collisional energy dependence are interpreted by the Penning ionization cross sections based on a long-range dipole–dipole interaction. Validity of the theoretical formula for the Penning ionization cross section (the Watanabe–Katsuura formula) is discussed. Two kinds of the cross sections have been further calculated by means of the impact parameter method with experimentally simulated classical trajectories; in one procedure, the polarization axis of the p-state helium has been assumed to rotate in order to keep collinear or perpendicular configuration with respect to the interatomic axis, in the other procedure, the polarization axis is fixed in a certain direction. The classical motion of the particles have been shown to cause considerable influence on the absolute values and the collisional energy dependence of the cross sections. The influence has increased accordingly to the attractive force of the interatomic potential, i.e., in order of Ar<Kr<Xe. Modified form of the dipole–dipole autoionization width with the electron exchange interaction is also discussed. It has also been suggested that rotation of the p-state atomic polarization depends strongly on the van der Waals interaction with the target atoms. The effect of the rotation has been shown to be most prominent for Xe but small for Ar.
Cluster integrals for square well particles: Application to percolation
Authors: Sevick E. M. , Monson P. A.
Published: July 2002

We present a calculation of the cluster integrals which appear in the density expansion of the inverse mean cluster size for an assembly of spherical particles with a square well potential where the range of connectedness is equivalent to the interparticle interaction range. This percolation series is constructed to third order in density, corresponding to four-point graphs in the virial expansion, using a method first put forth by Katsura. We show, by virtue of this particular evaluation method, that the large number of integrals in the percolation problem reduces to the class of integrals already contained in the virial coefficient evaluation. Moreover, it can be shown that different particle systems, as for example square well particles and binary collection of hard particles, share the same class of integrals in percolation or virial solutions. In this paper, the method is developed and applied to the simplest percolation case, one where the attractive and connectedness ranges are equivalent. The results are used to demonstrate compact and ramified attractive clustering and to investigate the accuracy of the series representation of the inverse mean cluster size.
Isobaric diffusion constants in simple liquids and normal mode analysis
Authors: Seeley G. , Keyes T. , Madan B.
Published: July 2002

The isobaric temperature dependence of the self-diffusion constant is evaluated by computer simulation in normal and supercooled liquid Lennard-Jones argon. The simulation results are in agreement with a simple formula derived from normal mode analysis.
A new interpretation of the sticky hard sphere model
Authors: Menon S. V. G. , Manohar C. , Rao K. Srinivasa
Published: July 2002

The basic results of the sticky hard sphere model are derived using a perturbative solution of the factorized form of the Ornstein–Zernike equation and the Percus–Yevick closure relation. The perturbation parameter is Δ/(σ+Δ), where Δ and σ are, respectively, the width of the attractive square well pair potential and the hard core diameter. This derivation leads naturally to an expression for the stickiness parameter, different from the one conventionally used, without invoking the concept of an infinitely deep potential. The theoretical structure factor is compared with two sets of Monte Carlo simulation data and excellent agreement is observed in both cases without the scaling of the square well potential suggested in literature.
Vibrational energy transfer of CO/Cu(100): Nonadiabatic vibration/electron coupling
Authors: Morin M. , Levinos N. J. , Harris A. L.
Published: July 2002

Vibrational energy relaxation of the internal C–O stretching mode of carbon monoxide in the c(2×2) overlayer on the Cu(100) surface at 120 K is measured by picosecond pump–probe spectroscopy. A resonant 1.5 ps infrared pulse at ν=2085 cm−1 pumps the C–O stretching mode. The energy relaxation is monitored by sum frequency generation from a delayed pair of 1.5 ps infrared and visible pulses. A single component decay, with a decay time of 2.0 ±0.5 ps, is reported. Uncertainties in the actual excited state lifetime are discussed, and the actual lifetime is estimated to be 2.0 ±1.0 ps. This lifetime is close to the lower limit of 1.2 ps set by the observed vibrational linewidth of 4.5 cm−1. The energy relaxation process is interpreted to occur by nonadiabatic energy transfer to the electrons (electron-hole pair excitations) of the copper substrate, and the measurement supports previous assertions that the nonadiabatic energy transfer rate for this system is very rapid. The nonadiabatic energy transfer lifetime of this mode has previously been estimated by density-functional calculations [T. T. Rantala and A. Rosen, Phys. Rev. B 34, 837 (1986)], and has recently been calculated by extrapolation of ab initio Hartree–Fock electronic structure calculations for CO on copper clusters [M. Head-Gordon and J. Tully, preceding paper, J. Chem. Phys. 96, 3939 (1992)]. The calculated lifetimes in both cases are in the 1–3 ps range, in good agreement with the experimentally measured value.
Decomposition of the chemisorption bond by constrained variations: Order of the variations and construction of the variational spaces
Authors: Bagus Paul S. , Illas Francesc
Published: July 2002

Constrained variations provide a way to analyze electronic wave functions and to theoretically decompose the chemical bond into contributions from different bonding mechanisms. The constrained space-orbital variation method has been used extensively to decompose the chemisorption bond for adsorbates on surfaces into, in particular, intraunit polarizations and interunit donations or covalent bonding. The results obtained for different orders of the intraunit and interunit constrained variations are compared for the representative examples of CO/Cu and O/Cu. For CO/Cu, both orders indicate the same dominant bonding mechanism, but for O/Cu they give very different views of the importance of the Cu 3d contribution to the Cu–O covalent bond. When the results for the two orders are not the same, a different division of the variational space for the constrained variations is needed. An appropriate division of the space for the Cu–O bond into contracted orbitals and diffuse orbitals is described.
Cumulative reaction probability from Siegert eigenvalues: Model studies
Authors: Ryaboy Victor , Moiseyev Nimrod
Published: July 2002

A simple approach to cumulative reaction probability, N(E), calculation is described and tested using one-dimensional symmetric and nonsymmetric Eckart potential barriers. This approach combines semiclassical transition state theory formulated by Miller [Faraday Discuss. Chem. Soc. 62, 40 (1977)] and reviewed recently by Seideman and Miller [J. Chem. Phys. 95, 1768 (1991)] and the complex coordinate method for calculations of Siegert eigenvalues. Siegert eigenvalues calculated numerically and analytically are found in excellent agreement with each other. It is demonstrated that corresponding eigenfunctions are localized in the potential barrier region and can be counted by their nodes. Perfect agreement between semiclassical N(E) dependence and exact quantum mechanical results was found in a broad energy range.
Intermolecular multiple-quantum coherences and cross correlations in solution nuclear magnetic resonance
Authors: He Qiuhong , Richter Wolfgang , Vathyam Sujatha , Warren Warren S.
Published: July 2002

It was recently reported that multiple-quantum NMR coherences could apparently be observed in water and other concentrated samples, in direct violation of established theory. These results were previously explained in a dressed-state framework as manifestations of the coupling between the spins and the coil (quantized radiation damping). Here we provide details of previously communicated experimental explorations of these effects [J. Chem. Phys. 96, 1659 (1992)], and we extend these results to multicomponent samples. We observe cross peaks between independent molecules in solution in two-dimensional experiments, including spectra with multiple-quantum coherence transfer echoes; we also demonstrate coherence transfer between solvent and (dilute) solute molecules. However, we show that these intermolecular cross peaks are induced by a mechanism which is local in nature, and thus radiation damping (either classical or quantized) cannot provide the bulk of the explanation for their occurrence. Simulations and analytical results show that the dipolar demagnetizing field can account for many of these surprising effects, although a complete picture must be more complex.
A Born–Oppenheimer approximation for path integrals with an application to electron solvation in polarizable fluids
Authors: Cao J. , Berne B. J.
Published: July 2002

The Born–Oppenheimer approximation is introduced into the path integral expression for the canonical partition function. It is difficult to apply this to simulations. We devise a new adiabatic expression which we call the free energy Born–Oppenheimer approximation which is much easier to simulate. It is shown that this approximation deviates from the exact path-integral expression by no more than does the standard Born–Oppenheimer approximation and we apply it to the problem of an excess electron dissolved in a polarizable fluid.
Hyperfine structures of the triplet states of 23Na85Rb
Authors: Matsubara Kensuke , Wang You-Chang , Ishikawa Kiyoshi , Baba Masaaki , McCaffery Anthony J. , Katô Hajime
Published: July 2002

The sub-Doppler high resolution excitation spectrum of NaRb was measured by selectively monitoring the fluorescence intensity from perturbed levels to the (1)3Σ+ lower state. Many transitions were observed to the (2)3Σ+ state perturbed by B 1Π. Energy shifts of the perturbed levels were analyzed and the spin–orbit coupling constant between the (2)3Σ+ and B 1Π states was determined, together with molecular constants of each state. Fully resolved hyperfine splittings, induced by the coupling of electron spin with both 23Na and 85Rb nuclear spins, were observed in the transitions. By analysis of the observed splittings, the mechanism of perturbation was confirmed as due to Fermi contact interaction and hyperfine constants of the 23Na85Rb (2)3Σ+ state were determined. The ratio of electron spin densities at 23Na and 85Rb in the (2)3Σ+ state is estimated to be 0.84:0.16.
Friction and mobility of many spheres in Stokes flow
Authors: Cichocki B. , Felderhof B. U. , Hinsen K. , Wajnryb E. , Bl/awzdziewicz J.
Published: July 2002

An efficient scheme is presented for the numerical calculation of hydrodynamic interactions of many spheres in Stokes flow. The spheres may have various sizes, and are freely moving or arranged in rigid arrays. Both the friction and mobility matrix are found from the solution of a set of coupled equations. The Stokesian dynamics of many spheres and the friction and mobility tensors of polymers and proteins may be calculated accurately at a modest expense of computer memory and time. The transport coefficients of suspensions can be evaluated by use of periodic boundary conditions.
Intramolecular collisional transfer in NO (a 4Π→B 2Π, b 4Σ−): Gateway-type, resonant versus direct, nonresonant mechanisms
Authors: Ottinger Ch. , Vilesov A. F.
Published: July 2002

A beam containing NO in the long-lived a 4Π state was allowed to interact with target particles in a collision cell. Intramolecular collision-induced transitions a 4Π→B 2Π and a 4Π→b 4Σ− were observed via the subsequent emissions in the β bands (B 2Π→X 2Π) and the Ogawa bands (b 4Σ−→a 4Π), respectively. In the ultraviolet part of the spectrum long β band progressions originating from the B-state vibrational levels v=0 and 3 were observed. In each band only a few lines appear, which were assigned to transitions from the rotational levels 2Π3/2(10.5) in v=0 and 2Π1/2(17.5) in v=3. These particular B 2Π levels are perturbed by specific levels of the a 4Π state, serving as so-called gateways to allow the otherwise spin-forbidden a 4Π→B 2Π collision-induced transition. An external magnetic field has a strong effect on the collision-induced emission from the NO(B, v=0) level. With paramagnetic target gases, direct spin-changing NO(a→B) collisional transfer was also observed in addition to the gateway transitions. The selectivity for the final state level is much weaker here. The collision-induced Ogawa band emission in the near-infrared showed smooth, partially resolved rotational contours with all target gases. This indicates that the collisional transfer NO(a→b) within the quartet system is also not of the highly restrictive gateway type, but results from direct collisional coupling.
Study of sol-state properties of aqueous gelatin solutions
Authors: Bohidar Himadri B. , Jena Sidhartha S.
Published: July 2002

Aqueous gelatin solutions have been studied in a systematic and exhaustive manner in a neutral buffer medium, in the temperature range of 30–60 °C, by static (SLS) and dynamic laser light scattering. It has been clearly established that upon cooling the sol from 60 °C, the gelatin chains form thermoreversible aggregates until the gelation is encountered below 30 °C. The radius of gyration of the chains (Rg) at any temperature was found to scale with corresponding molecular weight as Rg∼M0.57±0.03w. The ratio of Rg to hydrodynamic radius RH has been found to be Rg/RH=1.82±0.03 as expected for random coils. The translational diffusion coefficient of the chains Dz(c) exhibited linear concentration dependence; Dz(c)=D0(1+KDc) and D0∼M−0.57±0.03. The second virial coefficient (A2) of osmotic pressure obtained through SLS could be excellently related to KD through KD≂2A2Mw for all temperatures. The Flory–Huggins interaction parameter χ was determined to be χ≂0.48 and it showed negligible temperature dependence. The temperature dependence of sol viscosity η(T) could be fitted to Doolittle equation ln η(T)=A+B/T. At any given temperature, the concentration dependence of specific viscosity ηsp(c) followed ηsp(c)/c=1/([η]+k√c). The intrinsic viscosity [η] was found to scale with Mw as [η]∼M0.69±0.08w also yielding a typical overlap concentration of c*≂2.5% (w/w). The Flory–Fox constant Φ was found out to vary between 2.66×1021 at 35 °C and 1.27×1021 at 60 °C, against the standard value of 2.21×1021. The density values of the sol have been measured by a simple but novel method. Both the sol density and refractive index show monotonous increase as the gel point is approached from the hot sol state of gelatin.
Cavity ring-down spectroscopy for quantitative absorption measurements
Authors: Zalicki Piotr , Zare Richard N.
Published: July 2002

We examine under what conditions cavity ring-down spectroscopy (CRDS) can be used for quantitative diagnostics of molecular species. We show that CRDS is appropriate for diagnostics of species whose absorption features are wider than the spacing between longitudinal modes of the optical cavity. For these species, the absorption coefficient can be measured by CRDS without a knowledge of the pulse characteristics provided that the cavity ring-down decay is exponential. We find that the exponential ring-down decay is obeyed when the linewidth of the absorption feature is much broader than the linewidth of the light circulating in the cavity. This requirement for exponential decay may be relaxed when the sample absorption constitutes only a small fraction of the cavity loss and, consequently, the sample absorbance is less than unity during the decay time. Under this condition the integrated area of a CRDS spectral line approximates well the integrated absolute absorption coefficient, which allows CRDS to determine absolute number densities (concentrations). We determine conditions useful for CRDS diagnostics by analyzing how the absorption loss varies with the sample absorbance for various ratios of the laser pulse linewidth to the absorption linewidth for either a Gaussian or a Lorentzian absorption line shape.
Liquid–vapor asymmetry in pure fluids: A Monte Carlo simulation study
Authors: Wilding N. B. , Müller M.
Published: July 2002

Monte Carlo simulations within the grand canonical ensemble are used to obtain the joint distribution of density and energy fluctuations pL(ρ,u) for two model fluids: a decorated lattice gas and a polymer system. In the near critical region the form of pL(ρ,u) is analyzed using a mixed field finite-size-scaling theory that takes account of liquid–vapor asymmetry. Field mixing transformations are performed that map pL(ρ,u) onto the joint distribution of critical scaling operators p̃=(x,y) appropriate to the Ising fixed point. Carrying out this procedure permits a very accurate determination of the critical point parameters. By forming various projections of p̃=(x,y), the full universal finite-size spectrum of the critical density and energy distributions of fluids is also obtained. In the sub-critical coexistence region, an examination is made of the influence of field mixing on the asymmetry of the density distribution.
Slow motion electron spin resonance line shapes of lyotropic liquid crystals in hexagonal phase
Authors: Liang Zhichun , Wikander Göran , Westlund Per-Olof
Published: July 2002

In this paper the slow motion electron spin resonance (ESR) line shape theory is extended to hexagonal mesophases. The stochastic Liouville equation is applied to the dynamic description of both local molecular reorientation and azimuthal surface translational diffusion of the lipid around the cylinder axis. The established ESR line shape models allow a separation of contributions to the electron spin relaxation from the two motions, if macroscopically aligned hexagonal spectra are simulated. Such simulations can yield, not only the dynamic parameters such as local ordering parameter, local motional rate, and azimuthal surface diffusion coefficient, but also a disorder parameter accounting for residual cylinder disorder and the disorder effects due to finite cylinder length and curved surface along the cylinder axis. To test these models, they are applied to the analyses of X-band ESR spectra of the hexagonal phase of the sodium dodecyl sulphate (SDS)/decanol/water ternary system. The obtained surface diffusion coefficient and disorder parameter are in good agreement with previously reported values in a deuterium nuclear magnetic resonance (NMR) study. The local dynamic properties, which are not readily available by NMR method, are compared with those obtained for the micellar phase of the SDS/water binary system. Such a comparison reveals that while the local orderings are similar, the local dynamics is much slower in the hexagonal phase than in the micellar phase.
Substitutionally ordered solid solutions of hard spheres
Authors: Cottin X. , Monson P. A.
Published: July 2002

A recently developed cell theory for multicomponent solid solutions is applied to the study of substitutionally ordered binary hard sphere solid solutions. Solid phase thermodynamic properties and solid–fluid phase diagrams have been determined for a range of particle size ratios. The theoretical predictions agree well with available Monte Carlo simulation results. From the phase diagrams we draw some conclusions about the domains of stability of some simple compounds such as AB, AB2, and AB13 in terms of the size ratio, density, and overall composition of the mixture.
Simulated polymer melt stress relaxation. I. Plateau behavior
Authors: Gao J. , Weiner J. H.
Published: July 2002

The computer simulation of stress relaxation in a model polymer melt is performed with a nonequilibrium molecular dynamics algorithm. The chains are freely jointed with N=30, 100, and 200 bonds and with both intra- and interchain excluded volume interactions. For N=200, the model exhibits incipient plateau behavior as evidenced by an inflection point in the stress relaxation history. Comparison is made between the stress history as computed on the atomic level by the virial stress formula and on the molecular level using the entropic spring formulation. The two histories are in agreement only for the case of N=200, with entanglement length Ne=40, and only for the time period following the inflection point.
Cumulative reaction probability from time independent scattering theory
Authors: Moiseyev Nimrod
Published: July 2002

The formally exact expression for the cumulative reaction probability, N(E), was first derived by Miller, Schwartz and Tromp (MST) [J. Chem. Phys. 79, 4889 (1983)] from the flux–flux time-dependent expression for the thermal rate constant. We derive the MST formula for N(E) from the time-independent expression for the state-to-state transition probability. Our proof shows that the flux can be calculated at a dividing surface which can be located at any point, including a point where bound states have nonzero probability density.
The optical and optical/Stark spectrum of iridium monocarbide and mononitride
Authors: Marr A. J. , Flores M. E. , Steimle T. C.
Published: July 2002

Supersonic molecular beam samples of iridium monocarbide IrC and iridium mononitride IrN were generated using a laser ablation/reaction source and characterized using high resolution (Δν<30 MHz FWHM) laser induced fluorescence spectroscopy. This is the first identification of gaseous IrN. Numerous strong band systems in the 18 800 to 14 360 cm−1 spectral range were assigned as the (v′,0) progression of the A 1Π–X 1Σ+ band system of IrN. The (1,0) and (0,0) bands were analyzed to produce a set of fine and hyperfine parameters. The electric field induced effects on the R(0) line of these bands were analyzed to produce permanent electric dipole moments: A 1Π(v=0) 2.78(2) D, A 1Π(v=1) 2.64(2) D, X 1Σ+(v=0)=1.66(1) D. The (0,0) band of the D2 Φ7/2−X 2Δ5/2 system of IrC was recorded and analyzed to produce a set of fine and hyperfine parameters. The electric field induced effects on the R(2.5) branch feature were analyzed to produce permanent electric dipole moments: D 2Φ7/2(v=0) 2.61(6) D and X 2Δ5/2(v=0) 1.60(7) D. Plausible electronic configurations consistent with the experimental observations are given.
Reaction pathways in the photodetachment of an electron from aqueous chloride: A quantum molecular dynamics study
Authors: Staib Arnulf , Borgis Daniel
Published: July 2002

Reaction and relaxation processes induced by photoexcitation of an aqueous chloride ion are studied with quantum molecular dynamics simulations. A predominant channel leading to a metastable hydrated electron-chlorine pair is found. By means of theoretical transient and stationary absorption spectra, the solvent reorganization involved in the charge repartitioning is discussed. The dissipation of excess electron kinetic energy by surrounding water molecules plays an essential role in the equilibration of an electron-atom pair. For this intermediate species, two competing reaction pathways are identified. One is the barrier-impeded dissociation yielding a hydrated electron. Shape and height of the free energy barrier determined by quantum umbrella sampling point to a diffusion controlled electron photodetachment. The other channel is the geminate recombination via a nonadiabatic transition for which a self-consistent and fully dynamical treatment of the solvent electronic polarization is found to be important. From the rate constants computed for the individual channels, a kinetic model is derived to explain time-dependent spectral signatures and electron escape yields recently observed in photodetachment experiments on aqueous halides.
Many-body effects in weakly bound anion and neutral clusters: Zero electron kinetic energy spectroscopy and threshold photodetachment spectroscopy of ArnBr− (n=2–9) and ArnI− (n=2–19)
Authors: Yourshaw Ivan , Zhao Yuexing , Neumark Daniel M.
Published: July 2002

The anion zero electron kinetic energy (ZEKE) spectra of the van der Waals clusters Ar2-3Br− and Ar2-3I− have been measured, and partially discriminated threshold photodetachment (PDTP) experiments have been performed on Ar4-9Br− and Ar8-19I−. The experiments yield size-dependent adiabatic electron affinities (EAs) and electronic state splittings of the halogen atom in the neutral clusters formed by photodetachment. These results are compared with simulated annealing calculations using model potentials for the anion and neutral clusters, making use of the neutral and anion pair potentials determined from previous work on the diatomic rare gas–halide atom complexes [Y. Zhao, I. Yourshaw, G. Reiser, C. C. Arnold, and D. M. Neumark, J. Chem. Phys. 101, 6538 (1994)]. A simple first-order degenerate perturbation theory model [W. G. Lawrence and V. A. Apkarian, J. Chem. Phys. 101, 1820 (1994)] of the neutral cluster potentials was found to agree well with the size-dependent splitting of the halogen 2P3/2 state observed in the ZEKE spectra. However, the binding energies calculated from the pair potentials alone were found to be inconsistent with the experimental electron affinities, and it was necessary to include various nonadditive terms in the simulated annealing calculations to obtain reasonable agreement with experiment. Many-body induction in the anion clusters was found to be the dominant nonadditive effect. The exchange quadrupole effect—i.e., the interaction of the exchange induced electron charge distribution distortion among argon atoms with the halide charge—was also found to be important. This comparison between experiment and theory provides a sensitive probe of the importance of nonadditive effects in weakly bound clusters.
Mid-infrared spectra of the proton-bound complexes Nen–HCO+ (n=1,2)
Authors: Nizkorodov Sergey A. , Dopfer Otto , Meuwly Markus , Maier John P. , Bieske Evan J.
Published: July 2002

The ν1 band of Ne–HCO+ has been recorded for both 20Ne and 22Ne containing isotopomers by means of infrared photodissociation spectroscopy. The rotational structure of the band is consistent with a parallel Σ–Σ type transition of a linear proton-bound complex. The following constants are extracted for 20Ne–HCO+: ν0=3046.120±0.006 cm−1, B″=0.099 54±0.000 05 cm−1, D″=(5.30±0.30)×10−7 cm−1, H″=(1.1±0.9)×10−11 cm−1, B′=0.100 03±0.000 05 cm−1, D′=(4.89±0.30)×10−7 cm−1, H′=(1.6±0.9)×10−11 cm−1. The ν1 band is redshifted by 42.5 cm−1 from the corresponding ν1 transition of free HCO+ indicating that the Ne atom has a pronounced influence on the proton motion. Linewidths for individual rovibrational transitions are laser bandwidth limited, demonstrating that the lifetime of the ν1 level is at least 250 ps. An approximate radial potential for the collinear Ne...HCO+ interaction is constructed by joining the mid-range potential obtained from a Rydberg–Klein–Rees inversion of the spectroscopic data to the theoretical long-range polarization potential. Based on this potential, the estimated dissociation energy (D0) for Ne–HCO+ is 438 cm−1 in the (000) state and 454 cm−1 in the (100) excited state. The rotationally unresolved ν1 band of 20Ne2–HCO+ is slightly blueshifted with respect to that of 20Ne–HCO+. The observed frequency shift is compatible with a trimer structure where the second Ne atom is attached to the linear Ne–HCO+ dimer core.
Phase diagram of Yukawa systems near the one-component-plasma limit revisited
Authors: Hamaguchi S. , Farouki R. T. , Dubin D. H. E.
Published: July 2002

Transition inverse temperatures (or Γ values) at the fluid–solid phase boundary of Yukawa systems near the one-component-plasma (OCP) limit have been evaluated by molecular dynamics simulations. These values are systematically smaller than those obtained in an earlier study by Farouki and Hamaguchi [J. Chem. Phys. 101, 9885 (1994)]. The discrepancy is attributed to the fact that, in the earlier study, the harmonic entropy constants were approximated by that of the OCP, whereas the new results are based on more accurate harmonic entropy constants obtained from lattice-dynamics calculations. The new molecular dynamics simulations also confirm that the bcc–fcc phase transition curve is in good agreement with that of the quasiharmonic theory in the regime κ≤1.4, where κ is the ratio of the Wigner–Seitz radius to the Debye length. Examples of Yukawa systems include dusty plasmas and colloidal suspensions.
Superradiance of polaritons: Crossover from two-dimensional to three-dimensional crystals
Authors: Agranovich V. M. , Basko D. M. , Dubovsky O. A.
Published: July 2002

In spite of the relative simplicity of the structure of Frenkel excitons in molecular crystals some questions concerning the theory of polaritons in such crystals remain controversial—especially those concerning the crossover from the two-dimensional to the three-dimensional case. In the present work a detailed microscopic study of Frenkel exciton-polaritons in crystal slabs of arbitrary thickness is performed for the states with the tangential wave vector k∥=0. Starting from the microscopic quantum theory we have obtained two basic equations. One of them relates the complex energy of a polariton E to the quantity q, which in the limiting case of bulk crystal comes to the normal component of the wave vector. When the number N of the monolayers in the pile is large, N≫1, this equation is reduced to the dispersion equation of the macroscopic electrodynamics which uses the dielectric function ε(ω). The other equation of our microscopic theory has the meaning of Ewald’s extinction rule and for N≫1 is reduced to Maxwell’s boundary conditions. Using the equations obtained we found the complete set of polariton terms for the arbitrary N from N=1 to N→∞. We have traced the rise and evolution of two branches of polariton terms with increasing N. Special attention was paid to the study of polariton superradiance—enormous radiative damping or very short corresponding lifetimes for some states. At small N≪λ/a, with λ being the light wavelength and a the lattice constant, the superradiant linewidth is proportional to N(λ/a)2Γ(0), where Γ(0) is the molecular radiative width. After further increasing the thickness this linewidth is monotonously decreased to zero. We also show that for the macroscopic slabs the radiative broadening may be obtained as a result of taking into account multiple reflections of the polariton from the surfaces of the crystal. Illustrative calculations were performed using parameters of the anthracene crystal.
Potential of mean force and reaction rates for proton transfer in acetylacetone
Authors: Hinsen Konrad , Roux Benoı̂t
Published: July 2002

The intramolecular proton transfer in the enol form of acetylacetone is investigated at various temperatures both classically and quantum-mechanically using computer simulations. The potential energy surface is modeled using the empirical valence bond (EVB) approach of Warshel and fitted to the results of ab initio calculations. Quantum-statistical results are obtained via discretized Feynman path integral simulations. The classical and centroid potential of mean force for the reaction coordinate is obtained using umbrella sampling. The proton transfer rate is calculated based on classical and on Feynman path integral quantum transition state theory. For the classical system, the transmission coefficient is obtained from activated dynamics. Two different reaction coordinates are compared, the first one involving explicitly the transferring proton and the second one involving only heavy atoms in the molecules. The influence of isotopic substitutions is investigated by considering a fully deuterated version of acetylacetone. It is observed that there are significant differences between classical and quantum-mechanical calculations caused mainly by the lack of tunneling effects in the former. The quantum fluctuations of heavy atoms are found to have a considerable influence on the magnitude of the proton transfer rate.
Monte Carlo studies of self-assembled monolayers using simple generalized models. II. A two-site molecular model
Authors: Pertsin A. J. , Grunze M.
Published: July 2002

A simple two-site molecular model is suggested for computer simulation of self-assembled monolayers (SAMs) formed by long-chain functionalized molecules on the surface of solid substrates. In this model, one force site, placed at the functional headgroup, is responsible for the interaction of the headgroup with the periodic substrate field and the other headgroups. The second force site, positioned in the middle of the molecule, describes the chain–substrate and chain–chain interactions. For the latter interaction, a Gay–Berne–Pechukas orientation-dependent potential is used. The parameters of the model potentials are chosen so as to mimic the interactions in alkanethiol/Au(111) SAMs. The phase and structural behavior of the model system is explored using the Monte Carlo technique as a function of temperature, coverage, and the magnitude of surface corrugation. Simulations reveal a commensurate–incommensurate solid-phase transition preceding two-dimensional melting. At incomplete coverages, a two-phase region is observed, in which the usual crystalline phase characteristic of saturation coverage coexists with a low-density translationally disordered phase. The surface-aligned “striped” phase observed in STM experiments [G. E. Poirier and E. D. Pylant, Science 272, 1154 (1996)] can only be modeled at low temperatures assuming head-to-head dimerization of the molecules.
Ab initio statistical thermodynamical models for the computation of third-law entropies
Authors: East Allan L. L. , Radom Leo
Published: July 2002

Third-law gas-phase statistical entropies are computed for a variety of closed-shell singlet state species using standard formulae based upon canonical partition functions. Molecular parameters are determined ab initio, and sensitivity analyses are performed to determine expected accuracies. Several choices for the canonical partition function are examined for internal rotations. Three general utility procedures for calculating the entropies are developed and designated E1, E2, and E3 in order of increased accuracy. The E1 procedure adheres to the harmonic oscillator approximation for all vibrational degrees of freedom other than for very low barrier internal rotations, these being treated as free rotations, and yields entropies to an accuracy of better than 1 J mol−1 K−1 for molecules with no internal rotations. For molecules with internal rotations, errors of up to 1.8 J mol−1 K−1 per internal rotation are observed. Our E2 procedure, which treats each individual internal rotation explicitly with a simple cosine potential, yields total entropies to an accuracy of better than 1 J mol−1 K−1 for species with zero or one internal rotation, and better than 2 J mol−1K−1 for species with two internal rotation modes. Rotor–rotor coupling is found to contribute on the order of 1 J mol−1 K−1 for a third-law entropy. Our E3 procedure takes this into account and, with the aid of new ab initio two-dimensional torsional potential energy surfaces of state-of-the-art accuracy, improves the accuracy of the predicted entropy for species with two internal rotation modes to approximately 1 J mol−1 K−1.
Diffusion of potassium on Re(001) investigated by coverage grating-optical second-harmonic diffraction
Authors: Zhao W. , Verhoef R. W. , Asscher M.
Published: July 2002

Using the coverage grating-optical second-harmonic diffraction method, we have measured the diffusion rate of potassium on Re(001) in the coverage range of 0.6–1 ML. We observe a minimum in both the activation energy for diffusion and the diffusion preexponential at an initial potassium coverage of 0.75 ML. For potassium coverages of 0.93, 0.84, 0.73, and 0.59 ML, the activation energies for diffusion are determined to be 5.0±0.2, 3.9±0.7, 3.6±0.2, and 5.0±1.0 kcal/mol, and D0 is 5.6×10−3±0.3, 3.5×10−4±1.1, 5.8×10−4±0.3, and 3.7×10−1±1.9 cm2/s, respectively. The results are discussed in terms of electrostatic dipole–dipole repulsive interactions among neighboring adsorbates.
Dynamics of polyisoprene in star block copolymers confined in microstructures: A dielectric spectroscopy study
Authors: Floudas G. , Paraskeva S. , Hadjichristidis N. , Fytas G. , Chu B. , Semenov A. N.
Published: July 2002

Dielectric spectroscopy has been employed to study selectively the local and global dynamics of polyisoprene (PI) in three microphase separated star diblock copolymers (SI)4, where PI and polystyrene (PS) form the core and corona, respectively. The thermally induced order-to-disorder transition (ODT) has been identified by rheology and small-angle x-ray scattering (SAXS). All dielectric measurements were made at temperatures well below the ODT and the polystyrene glass transition temperatures, where the four PI chain ends are tethered by the glassy polystyrene domains. The ordered state morphology, studied by SAXS, revealed the formation of PS spheres (fPS=0.25, where fPS is the polystyrene volume fraction), polyisoprene cylinders (fPS=0.68), and a lamellar structure (fPS=0.41) in the three copolymers. In contrast to the local segmental motions, the chain orientational dynamics associated with the amplitude and the characteristic relaxation time are strongly influenced by the spatial confinement. The main effects are the suppressed amplitude and the enhanced chain mobility, which are predicted and found to be stronger if the star topology is accompanied by topological (entanglement) effects of the PI block, irrespective of the ordered star morphology.
Coherent anti-Stokes Raman spectroscopy of shock-compressed liquid carbon monoxide–oxygen and nitrogen–oxygen mixtures
Authors: Schmidt S. C. , Moore D. S. , Shaw M. S.
Published: July 2002

A two-stage light gas gun and single-pulse multiplex coherent anti-Stokes Raman scattering (CARS) have been used to obtain carbon monoxide, nitrogen, and oxygen vibrational spectra for several high-pressure/high-temperature, dense fluid, carbon monoxide–oxygen, and nitrogen–oxygen mixtures. The experimental spectra were compared to synthetic spectra calculated with a semiclassical model for CARS intensities and using best fit vibrational frequencies, peak Raman susceptibilities, and Raman linewidths for each mixture component. Up to a maximum shock pressure of 6.75 GPa for carbon monoxide–oxygen mixtures, the CO and O2 vibrational frequencies were found to increase monotonically with pressure and depended on the carbon monoxide–oxygen mixture ratio. For the nitrogen–oxygen mixtures, the N2 vibrational frequency increased monotonically with pressure to a maximum experimental pressure of 12.9 GPa, however the O2 vibrational frequency increased with pressure to about 11 GPa and then appeared to decrease slightly as the pressure increased to the experiment maximum of 12.9 GPa. Empirical fits of the measured Raman frequencies incorporating previously published neat nitrogen, carbon monoxide, and oxygen data and using a functional form dependent on pressure, temperature, and mixture ratio, accurately describe the N2 , CO, and O2 vibrational frequency shifts for both the carbon monoxide–oxygen and the nitrogen–oxygen mixtures. The transition intensity and linewidth data suggest that thermal equilibrium of the vibrational levels is attained in less than 10 ns at these shock pressures. The vibrational temperatures obtained for the nitrogen–oxygen mixtures were used to improve the oxygen potential function used to calculate equation-of-state pressures and temperatures. The measured linewidths for CO, N2 , and O2 were different for the different mixtures and did not appear to depended significantly on mixture ratios. The broadening of all spectral lines suggested that the vibrational dephasing time for each species decreased to a few ps at the highest pressure shock states.
Early time dynamics of trans-azobenzene isomerization in solution from resonance Raman intensity analysis
Authors: Biswas Nandita , Umapathy Siva
Published: July 2002

Resonance Raman spectra have been recorded for trans-azobenzene in carbon tetrachloride using 16 excitation wavelengths in the region from 355–600 nm. It has been observed that for many totally symmetric fundamentals viz. C–N, N=N stretch, etc., the resonance Raman intensities decrease near the maxima of the resonant electronic (2 1Ag←1 1Ag) transition. This is attributed to interference due to preresonant scattering from the strongly allowed (1 1Au←1 1Ag) electronic transition. The Raman excitation profiles (REPs) for the ten Franck–Condon active fundamentals have been successfully modeled using Heller’s time-dependent approach with the inclusion of interference effect from higher electronic state. The short time isomerization dynamics is then examined from a priori knowledge of ground-state normal mode descriptions to convert the wave packet motion in dimensionless normal coordinates to internal coordinates. It is observed that within 5–30 fs of photoexcitation, the major changes experienced by trans-azobenzene are on N=N and C–N stretching vibrations, while N=N suffers reduction, C–N bond elongates, and with time the ring C atoms distort relatively out of the plane.
The dipole bound-to-covalent anion transformation in uracil
Authors: Hendricks J. H. , Lyapustina S. A. , de Clercq H. L. , Bowen K. H.
Published: July 2002

Nucleic acid base anions play an important role in radiation-induced mutagenesis. Recently, it has been shown that isolated (gas-phase) nucleobases form an exotic form of negative ions, namely, dipole bound anions. These are species in which the excess electrons are bound by the dipole fields of the neutral molecules. In the condensed phase, on the other hand, nucleobase anions are known to be conventional (covalent) anions, implying the transformation from one form into the other due to environmental (solvation) effects. Here, in a series of negative ion photoelectron spectroscopic experiments on gas-phase, solvated uracil cluster anions, we report the observation of this transformation.
Simple liquids confined to molecularly thin layers. II. Shear and frictional behavior of solidified films
Authors: Kumacheva Eugenia , Klein Jacob
Published: July 2002

Using a surface force balance with high sensitivity in measuring shear forces we investigated the mechanical properties of thin layers of cyclohexane and octamethylcyclotetrasiloxane (OMCTS) in the gap between two smooth solid surfaces at discrete thicknesses n=6–3 molecular layers. At these layer thicknesses the films have undergone solidification due to their confinement (see preceding paper) and are capable of sustaining a finite yield stress upon being sheared. The sliding of the confining surfaces at mean velocity vs across the films is characterized by a shear or frictional force Fs which varies with a characteristic stick-slip pattern. We investigate comprehensively the dependence of Fs on n, vs, and on the applied normal forces F across the films. We find that transitions in film thickness from n→(n−1), with a consequent increase in Fs, may occur spontaneously during sliding with no change in F, corresponding to a multivalued friction force between the surfaces for a given load. The critical yield stress S for sliding at a given film thickness n increases monotonically with applied normal pressure P as S=S0+CP where S0 is a constant of order 105 Pa (depending on n) and C is roughly constant and of order 1. A simple model for friction across such films which can account semiquantitatively for this behavior is introduced, based on a shear-melting mechanism using the Lindemann criterion. We find that the characteristic stick-slip behavior persists over the range of film thicknesses and the entire (large) range of mean shear velocities studied, and that over most of this range the mean shear forces are independent of vs.
Systematic location of intersecting seams of conical intersection in triatomic molecules: The 1 2A′–2 2A′ conical intersections in BH2
Authors: Gordon Mark S. , Glezakou Vassiliki-Alexandra , Yarkony David R.
Published: July 2002

Points of conical intersection are continuously connected forming seams. Recently a quite unanticipated situation has been found in which two distinct seams of conical intersection—one symmetry-allowed and one same-symmetry—originating from the same two states intersect each other. The identification of these confluences, based on ab initio electronic wave functions has been somewhat serendipitous. A systematic approach for locating such confluences, based solely on information obtained on the symmetry-allowed portion of the seam, has been suggested. In this work that approach is applied to identify the point where a Cs seam of conical intersection intersects a symmetry-allowed C2v seam of conical intersection for the 1 2A′ and 2 2A′ states of BH2, states that correlate with B(1s22s22p,2P)+H2. It is suggested, based on this and previous work, that this unexpected situation, which has fundamental implications for our understanding of nonadiabatic processes, is not at all uncommon.