bluuebunny/crossref_metadata_2025 huggingface dataset:
Short time dynamics of polymer liquid and glass studied by molecular dynamics simulation
Authors: Roe Ryong-Joon
Published: July 2002

Molecular dynamics simulation of a united-atom polyethylene model was performed to study the short time dynamics of polymer liquid and glass. Simulation runs lasting up to 5 ns were performed at ten temperatures above and below the estimated glass transition temperature. Quantities evaluated for the purpose of studying the dynamics include: segmental mean square displacement, van Hove space–time correlation function, intermediate scattering function, dynamic structure factor, and velocity autocorrelation function. Many of the features observed agree well with those obtained from quasielastic neutron scattering measurements with polymeric and nonpolymeric liquids. The dynamics in the time scale between 0.01 and 103 ps clearly divides itself into two regimes; the fast process, occurring below about 1.5 ps, is Debye-like, while the slower process follows the Kohlrausch–Williams–Watts function. The former probably arises from motion of segments within a cage, and the latter from the α relaxation involving cooperative motion of groups of segments. The ‘‘apparent’’ glass transition temperature, noted from the behavior of the mean square displacement plotted against temperature, is shifted downward as the observation time is prolonged. The hopping process, seen with small molecule liquids at or below the glass transition temperature, has not been observed here at any temperature. Yet there is no indication that a nonergodicity, or a state of complete structural arrest, sets in even at the lowest of the temperatures studied.
A site–site Born–Green–Yvon equation for hard sphere dimers
Authors: Taylor Mark P. , Lipson J. E. G.
Published: July 2002

A Born–Green–Yvon (BGY) integral equation approach is used to study the structure and thermodynamics of a fluid of fused hard sphere dimers. Starting from a BGY equation for the dimer molecular pair distribution function, an equation for the simpler site–site distribution is derived. The multisite distribution functions appearing in this latter equation are related to the site–site function via a set of superposition approximations. Detailed results are presented for dimers with bond lengths of L=0.6σ and 1.0σ (where σ is the hard sphere site diameter). These results, which include the site–site distribution function over a range of densities, as well as the virial and compressibility pressure equations of state, are compared with analytic and Monte Carlo results.
An accurate ab initio potential energy surface of the He–H2 interaction
Authors: Tao Fu-Ming
Published: July 2002

An accurate potential energy surface of the He–H2 interaction is calculated with a large basis set at the complete fourth-order Mo/ller–Plesset approximation. The basis set—a combination of a nucleus-centered set 6s4p2d and a bond function set 3s3p2d centered at the midpoint between He and the H2 center of mass—is designed to give the optimal description of both the intra- and intersystem correlation effects. The validity of the basis set is confirmed by extensive preliminary calculations on the linear (orientation angle θ=0°), bent (45°), and T-shaped (90°) structures at a fixed separation (R=6.5a0) with a series of large basis sets containing different polarization functions and/or bond functions. Bond functions are found more effective than polarization functions in recovering the intersystem correlation energy and they are particularly useful in removing the geometric bias of a basis to give an accurate description for the potential anisotropy and the relative energies of different structures. The effect of bond functions is insensitive to the displacement of bond functions and the geometric midpoint of the van der Waals bond is a satisfactory choice for the center of bond functions. The potential energy surface of He–H2 is calculated at 15 values of R from 2.0 to 15.0a0 along each of the three main configurations (θ=0°, 45°, and 90°) with the vibrationally averaged H2 bond length r=1.449a0. Additional calculations are given for r=1.28 and 1.618a0 to show the effect of H2 zero-point vibration. While our potential at the self-consistent field (SCF) level is essentially the same as the previous calculations, our potential at the correlated level is globally deeper in the attractive region and less repulsive in the shorter range. Our calculated well depth (47.19 μhartrees) corresponding to the global minimum at θ=0° and R=6.5a0, is very close to the estimated experimental value of 48 μhartrees. In the Legendre expansion, our potential compares very well with the empirical potential of Rodwell and Scoles, but differs considerably from the empirical potential of Tang and Toennies and the previous ab initio potentials.
Ab initio calculation of [OH−;e+] system with consideration of electron correlation effect
Authors: Tachikawa Masanori , Sainowo Hiroshi , Iguchi Kaoru , Suzuki Kazunari
Published: July 2002

Ab initio calculations are made to examine theoretically the possibility of stable existence of [OH−;e+] system. Diffuse functions are added to the conventional 6-31G basis set, considering the wide spread of positron orbital. Moreover, the Mo/ller–Plesset perturbation of the second order is calculated to take the electron correlation into account. These two improvements are found to be very effective for the stable existence of the system. The positron affinity of OH− is computed to be 4.9 eV, and the binding energy of positronium to OH as 0.7 eV which is in good agreement with experimental estimate.
An unexpected packing of fluorinated n-alkane thiols on Au(111): A combined atomic force microscopy and x-ray diffraction study
Authors: Liu Gang-yu , Fenter Paul , Chidsey Christopher E. D. , Ogletree D. Frank , Eisenberger Peter , Salmeron Miquel
Published: July 2002

Atomic force microscopy (AFM) and grazing incidence x-ray diffraction (GIXD) have been used to study the structure of self-assembled monolayers of CF3(CF2)n(CH2)2SH (n=11, 7, and 5) on the Au(111) surface. Surprisingly, although the nearest-neighbor fluorinated alkane thiol distance is very close to the lattice constant of a commensurate p(2×2) structure, the close-packed rows of molecules are rotated ∼30° with respect to the underlying gold lattice. That packing is incommensurate or at most only close to the high-order commensurate c(7×7) structure. The relative orientation of the organic monolayer and the Au(111) substrate has been determined unambiguously both with GIXD, and by AFM, taking advantage of an earlier finding (Ref. ) that AFM tips can reversibly displace the thiol molecules under high loads. In addition, we demonstrate that the two techniques provide complementary information on the order and the domain structures of these monolayers.
Electronic structure calculations and dynamics of methane activation on nickel and cobalt
Authors: Burghgraef H. , Jansen A. P. J. , van Santen R. A.
Published: July 2002

The dissociative chemisorption of CH4 on nickel and cobalt has been studied using different cluster models. Density functional theory is used to determine the structure and potential energy surface in the reactant-, transition state-, and product region. The transition state is explicitly determined on a single atom, a one layer 7-atom cluster and a spherical 13-atom cluster. We find transition state barriers of 41 kJ/mol for a single nickel atom, 79 kJ/mol for a single cobalt atom, 214 kJ/mol for the Ni7-cluster, 216 kJ/mol for the Co7-cluster, 121 kJ/mol for the Ni13-cluster, and 110 kJ/mol for the Co13-cluster. The overall reaction energies are −34, 6, 142, 135, 30, and 8 kJ/mol, respectively. The higher barrier for the single cobalt atom in comparison with the nickel atom can be attributed to the difference between both atoms in the occupation of the s-orbital in the lowest lying states. The higher and almost the same barrier for the 7-atom clusters can be attributed to the intrinsic lower reactivity of the central atom embedded in the cluster and the similar electronic nature of the atoms in the clusters; in both clusters the atoms have open s- and d-shells. The lower barrier for the 13-atom clusters compared with the 7-atom clusters is a result of each surface atom now having 5 bonds, which gives a more balanced description of the substrate model.
Photochemistry of ozone in solid mixtures with argon
Authors: Benderskii Alexander V. , Wight Charles A.
Published: July 2002

The photochemistry of solid ozone and solid solutions of ozone in argon has been investigated at 10–30 K using 266 nm laser irradiation and detection by ultraviolet and infrared absorption spectroscopies. Pure ozone is converted to O2 with a quantum yield of 2.0±0.3 (independent of temperature) due to reaction of each photolytically generated O atom with a second ozone molecule. The quantum yield decreases when O3 is diluted in argon because geminate recombination of O+O2 becomes competitive with the O+O3 reaction. Photodissociation of ozone monomers is thought to occur by direct cage escape, which forms a separated O+O2 pair. This is the first experimental study in which the temperature dependence of the escape probability has been systematically investigated. The apparent activation energy of the process, 0.38±0.08 kJ/mol, is consistent with excitation of librations of ozone in the argon lattice, which brings it into orientations that are favorable for direct cage escape of the O atom.
Beam investigations of D2 adsorption on Si(100): On the importance of lattice excitations in the reaction dynamics
Authors: Kolasinski Kurt W. , Nessler Winfried , Bornscheuer Karl-Heinz , Hasselbrink Eckart
Published: July 2002

The adsorption of D2 on Si(100) has been investigated by means of supersonic molecular beam techniques. We have succeeded in measuring the dependence of the molecular D2 sticking coefficient S on surface temperature Ts and nozzle temperature Tn. The sticking coefficient increases gradually in the range 300≤Tn≤1040 K. The influence of increased v=1 population has not been deconvoluted from the effects of translational energy alone. The dependence on Ts is more interesting. With an incident translational energy of 65 meV, S rises from a value insignificantly different from the background level to a maximum value of (1.5±0.1)×10−5 at Ts=630 K. The decrease in the effective sticking coefficient beyond this Ts is the result of desorption during the experiment. Having established that S increases with both increasing molecular energy and increasing sample temperature, we have demonstrated directly for the first time that the adsorption of molecular hydrogen on Si is activated and that lattice vibrational excitations play an important role in the adsorption process.
Thermochemical properties of the gaseous scandium, yttrium, and lanthanum fluorides
Authors: Hildenbrand D. L. , Lau K. H.
Published: July 2002

Equilibria involving the gaseous mono- and difluorides of Sc, Y, and La were studied by effusion-beam mass spectrometry, and reaction enthalpies were derived by second and third law methods. In addition, the sublimation pressures of the crystalline trifluorides ScF3, YF3, and LaF3 and related thermodynamics of the gaseous trifluorides were determined by the torsion-effusion method. Dissociation energies at 298 K in kcal mol−1 for the bonds M–F, FM–F, and F2M–F, respectively, were derived as follows: For Sc, 143.2±3.2, 148.8±4.1, and 155.7±6.4; for Y, 163.8±3.2, 145.6±3.2, and 151.6±4.7; and for La, 157.5±4.1, 153.2±3.2, and 149.2±5.4. These values do not fall into any obvious pattern. The dissociation energies of ScF and YF are in good agreement with the results of high-level theoretical calculations. Spectroscopic and molecular constants of the species are summarized, and the thermochemical results are compared with values in the literature.
Perturbation density functional theory and Monte Carlo simulations for the structure of hard triatomic fluids in slitlike pores
Authors: Phan S. , Kierlik E. , Rosinberg M. L. , Yethiraj A. , Dickman R.
Published: July 2002

A model fluid composed of semiflexible tangent hard sphere trimers is investigated using Monte Carlo simulations and perturbation density functional theory (PDFT), focusing on the density profile and conformational properties in the vicinity of a hard wall. The surface density is reduced (enhanced) at low (high) densities. We also observe preferential end absorption. Both features arise from competition between chain conformational entropy and packing constraints. Molecules adjacent to the wall tend to lie flat against it, particularly at high density and/or stiffness. But the bond angle distribution is only weakly affected by the presence of the wall. For rigid molecules, the density profiles depend strongly on the value of the bond angle. PDFT is in excellent agreement with simulation, and promises to be a successful means of elucidating the interfacial structure of complex fluids.
Electronic absorption spectra of cyano-substituted polyacetylene cations in neon matrices
Authors: Forney Daniel , Freivogel Patrick , Fulara Jan , Maier John P.
Published: July 2002

The A 2Π←X 2Π transitions and vibrational frequencies in the excited electronic states have been determined for the cyanopolyacetylene cations H–(C≡C)n–CN+ (n=2–6) and NC–(C≡C)n–CN+ (n=1–5). The spectroscopic information was derived by absorption measurements after cations from a mass selected beam were condensed together with excess of neon to form a 5 K matrix. Spectra have been also obtained for ions with odd number of π-centers and are tentatively assigned to H–C2n−1–CN+ (n=3–6) and NC–C2n−1–CN+ (n=3–5). The assignments are based on trends observed for the homologous series of these cations, known gas phase data on three of the smaller species, and the free electron molecular orbital model.
Recombinative desorption of hydrogen from the Ge(100)–(2×1) surface: A laser-induced desorption study
Authors: Lewis Lisa B. , Segall Jeffrey , Janda Kenneth C.
Published: July 2002

The recombinative desorption of H2 from Ge(100)–(2×1) is studied by temperature programed desorption (TPD) and laser-induced desorption (LID). In contrast to what is observed for the Si(100)–(2×1) surface, the TPD spectra for Ge(100) do not appear to show appreciable formation of a stable dihydride species. Both the TPD and LID results are consistent with the first-order recombinative desorption kinetics. Analysis of the LID results yield an activation energy, Ea=40±2 kcal/mol and preexponential factor, ν=4×1013±1 s−1. The results are discussed in terms of several mechanisms that have been proposed for the first-order recombinative desorption of hydrogen from Si(100)–(2×1).
Integral equation algorithm for fluids of fully anisotropic molecules
Authors: Lado F. , Lomba E. , Lombardero M.
Published: July 2002

We outline a practical algorithm for the solution of liquid-state integral equations for fluids of fully anisotropic rigid molecules requiring three Euler angles for their configurational description and leading to pair functions of five angular variables. The method is suitable for all potentials. We illustrate the technique with sample results for SO2.
The electronic structure of LaO: Ligand field versus ab initio calculations
Authors: Schamps Joël , Bencheikh Mohammed , Barthelat Jean-Claude , Field Robert W.
Published: July 2002

The potentially pathological example of LaO has been chosen to test the application of ligand field theory (LFT) to metal monoxides. The test consists of a comparison of closed-shell ligand LFT (CSLLFT) results (in which a 2+/2− ionic M2+O2− model is a priori postulated with a point-charge ligand) against ab initio multiconfiguration self-consistent-field–multireference configuration interaction (MCSCF-MRCI) results (in which no ionicity is assumed a priori and an internal structure is allowed for the ligand). Special care has been devoted to the determination of a La3+ pseudopotential and its associated atomic basis set in order to keep the ab initio model close to the LFT one, yet at the same time capable of clearly exhibiting the consequences and the importance of the restrictions imposed in the CSLLFT model. The ab initio calculations reveal that the effective (Mulliken) ionicity in LaO is not La2+O2− but quite close to La+O−. Despite this, the (2+/2−) ionic CSLLFT model leads to the correct orbital occupations in the ground state but this model cannot account for the significant covalency contribution via the nominal 2pσ and 2pπ oxygen orbitals. The CSLLFT calculations correctly reproduce the excitation energies of the four lowest-lying observed states of LaO: these states are calculated to within 2000 cm−1 and globally better than via ab initio calculations. However, the (2+/2−) ionic CSLLFT model is shown to ignore the existence of a manifold of low-lying quartet (and doublet) states of (1+/1−) ionicity that might be relevant for interpreting features of the spectrum. This result exemplifies the need for developing, beyond the first attempts made in this direction, a computationally manageable open-shell ligand theory for the frequently encountered case of predominantly ionic structures with an open-shell ligand.
Overcoming the zero-point dilemma in quasiclassical trajectories: (He,H+2) as a test case
Authors: Kumar Sanjay , Sathyamurthy N. , Ramaswamy Ramakrishna
Published: July 2002

We present a new technique for circumventing the problem of the zero-point leak in classical trajectories by extending the action-billiard approach of de Aguiar and Ozorio de Almeida [Nonlinearity 5, 523 (1992)]. In addition to demonstrating its utility in a model problem, we examine the application of various methods of overcoming the zero-point leak in the case of collinear He+H+2 collisions. We also show that not neglecting leaky trajectories gives, on an average, good agreement with quantal results for collinear as well as 3-dimensional collisions.
Three-color triple resonance spectroscopy of highly excited ng Rydberg states of NO: Decay dynamics of high-l Rydberg states
Authors: Fujii Asuka , Morita Norio
Published: July 2002

With three-color triple resonance excitation, rotational-state-resolved laser multiphonon ionization spectra of highly excited ng Rydberg states (n=11–67, v=0 and 1) of NO have been observed. Determining the term values of the observed states, we have found that those values are in good agreement with the theoretical values calculated on the basis of the long range force model. Through detailed analysis of the spectral intensity distributions, we have proved, for the first time, the theoretical prediction that even in g Rydberg states there are efficient vibrational and rotational autoionization processes much faster than the radiative decay. In addition, we have also found the presence of non-negligible predissociation processes competing with the autoionization processes, roughly estimating both predissociation and autoionization rates.
Potential symmetry breaking, structure and definite vibrational assignment for azulene: Multiconfigurational and density functional results
Authors: Kozlowski Pawel M. , Rauhut Guntram , Pulay Peter
Published: July 2002

A systematic ab initio determination of the geometry and harmonic force field with inclusion of correlation energy for the ground state of azulene is presented. The calculations were carried out using the unrestricted natural orbital-complete active space (UNO-CAS) method and density functional theory (DFT) using the compound Becke–Lee–Young–Parr (B3-LYP) exchange-correlation functional. The 6-31G* basis set has been used in both methods. The geometry optimized with DFT leads to a structure with approximately equal C–C bond lengths and C2v symmetry, while UNO-CAS results show an alternating bond structure with Cs symmetry and a very low (∼32 cm−1) barrier height. Agreement of the calculated vibrational frequencies with experiment is very good for both methods, except in the B2 symmetry species which contains the bond alternation mode. In this species, the density functional results agree better with experiment than those based on a multiconfigurational wave function, showing that the minimum has C2v symmetry. All vibrations are assigned for azulene and its perdeuterated analog.
Photodissociation of ICN in liquid chloroform: Molecular dynamics of ground and excited state recombination, cage escape, and hydrogen abstraction reaction
Authors: Benjamin Ilan
Published: July 2002

The photodissociation of ICN in liquid chloroform on different electronic states of the Ã band is studied using molecular dynamics simulations. By taking into account nonadiabatic transitions to the ICN ground state and by using a simple statistical model for the reaction between the CN radical and a chloroform molecule, the competition between recombination, cage escape and hydrogen abstraction reaction with the solvent is examined. Good agreement with the cage escape results of a recent experiment by Raftery et al. [J. Chem. Phys. 101, 8572 (1994)] is found. Simulations which did not include the nonadiabatic transitions to the ground state overestimated the probability for cage escape by about a factor of two. Very fast translational relaxation of the CN fragment and very low probability for achieving a transition state configuration for the abstraction reaction are calculated. This supports the suggestion that the abstraction reaction proceeds thermally and is controlled by a structural (entropic) barrier.
Observation of the β-methylallyl radical by resonance Raman spectroscopy
Authors: Getty James D. , Liu Xianming , Kelly Peter B.
Published: July 2002

The resonance Raman spectrum of the β-methylallyl (2-methyl-allyl) radical is detected with excitation from 223 nm to 243 nm. The first observation of four fundamentals, the symmetric CH2 rock, the symmetric CCC stretch, the C–CH3 stretch, and the CCC bend, as well as an overtone, the symmetric CH2 twist of the β-methylallyl radical are reported. The assignments of the resonance Raman spectra are based upon the previous force field calculation, and comparisons with molecules having similar characteristic vibrations. Results from the resonance Raman spectra are compared with frequencies obtained by infrared spectroscopy using matrix isolation and our previous work on the allyl radical.
Molecular-orbital decomposition of the ionization continuum for a diatomic molecule by angle- and energy-resolved photoelectron spectroscopy. I. Formalism
Authors: Park Hongkun , Zare Richard N.
Published: July 2002

A theoretical formalism is developed for the quantum-state-specific photoelectron angular distributions (PADs) from the direct photoionization of a diatomic molecule in which both the ionizing state and the state of the ion follow Hund’s case (b) coupling. The formalism is based on the molecular-orbital decomposition of the ionization continuum and therefore fully incorporates the molecular nature of the photoelectron–ion scattering within the independent electron approximation. The resulting expression for the quantum-state-specific PADs is dependent on two distinct types of dynamical quantities, one that pertains only to the ionization continuum and the other that depends both on the ionizing state and the ionization continuum. Specifically, the electronic dipole-moment matrix element rlλ exp(iηlλ) for the ejection of a photoelectron with orbital angular momentum quantum number l making a projection λ on the internuclear axis is expressed as ΣαλŪlαλλ exp (iπτ̄αλλ) Mαλλ, where Ūλ is the electronic transformation matrix, τ̄αλλ is the scattering phase shift associated with the αλth continuum molecular orbital, and Mαλλ is the real electronic dipole-moment matrix element that connects the ionizing orbital to the αλth continuum molecular orbital.

Because Ūλ and τ̄αλλ depend only on the dynamics in the ionization continuum, this formalism allows maximal exploitation of the commonality between photoionization processes from different ionizing states. It also makes possible the direct experimental investigation of scattering matrices for the photoelectron–ion scattering and thus the dynamics in the ionization continuum by studying the quantum-state-specific PADs, as illustrated in the companion article on the photoionization of NO.
Quantum effects on the surface tension of simple fluids
Authors: Frisch H. L. , Nielaba P.
Published: July 2002

Quantum corrections to the surface tension of fluids such as H2 and He, which can be approximated as hard spheres, are found from the scaled particle theory. In this theory the surface tension depends on the effective pressure which is larger due to the quantum diffraction effect.
Investigation of the uniqueness of the reverse Monte Carlo method: Studies on liquid water
Authors: Jedlovszky P. , Bakó I. , Pálinkás G. , Radnai T. , Soper A. K.
Published: July 2002

Reverse Monte Carlo (RMC) simulation of liquid water has been performed on the basis of experimental partial pair correlation functions. The resulted configurations were analyzed in various aspects; the hydrogen bond angle distribution, three body correlation and orientational correlation were calculated. The question of the uniqueness of the RMC method was also examined. In order to do this, two conventional computer simulations of liquid water with different potential models were performed, and the resulted pair correlation function sets were fitted by RMC simulations. The resulted configurations were then compared to the original configurations to study how the RMC method can reproduce the original structure. We showed that the configurations produced by the RMC method are not uniquely related to the pair correlation functions even if the interactions in the original system were pairwise additive. Therefore the difference between the original simulated and the RMC configurations can be a measure of the uncertainty of the RMC results on real water. We found that RMC produces less ordered structure than the original one from various aspects. However, the orientational correlations were reproduced rather successfully. The RMC method exaggerates the amount of the close packed patches in the structure, although these patches certainly exist in liquid water.
Molecular dynamics studies of self-interstitials in ice Ih
Authors: Itoh Hidenosuke , Kawamura Katsuyuki , Hondoh Takeo , Mae Shinji
Published: July 2002

We carried out molecular dynamics simulations of interstitial water molecules in ice to clarify behavior and vibrational properties of self-interstitials in ice. We used an atom–atom potential model developed by Kumagai et al. [N. Kumagai, K. Kawamura and T. Yokokawa, Mol. Sim. 12, 177 (1994)] which allows intramolecular motions. We confirmed high reliability of the potential model for reproducing the structure and vibrational spectra of ice. From trajectory observations of self-interstitials, we found that a stable interstitial site is an uncapped trigonal site. By comparing power spectra of vibrational motions for self-interstitials with that of matrix ice, we found that the large formation entropy of the interstitial molecule is attributed to much higher density of states in a low frequency region for the interstitial than that for the matrix.
Nonadditive, three-body dipoles and forces on nuclei: New interrelations and an electrostatic interpretation
Authors: Li X. , Hunt K. L. C.
Published: July 2002

This work establishes the relation between three-body forces and the polarization P(r), for a group of interacting molecules A...B...C with weak or negligible charge overlap. Using nonlocal polarizability and hyperpolarizability densities, we derive new equations for the three-body forces and dipoles due to dispersion and induction–dispersion interactions. For clusters containing at least one polar molecule, we identify a nonadditive induction–dispersion energy that was not included explicitly in previous analyses of van der Waals cluster spectra. Numerical estimates show that this effect is significant for Ar2...HCl and Ar2...DCl. Separately, we determine the effects of linear and nonlinear induction, dispersion, and induction–dispersion interactions on the polarization P(r); then we relate each contribution to P(r) to the corresponding component of the intermolecular force. Our work provides a classical electrostatic interpretation for all of the three-body forces at long range, including dispersion: The polarization of a molecular cluster produces these forces. This result holds even though particular interaction effects may generate a force but no net dipole, or a dipole but no net force.
Surface laser light scattering studies of the air/poly(N-vinyl-2-pyrrolidone)-water solution interface
Authors: Huang Q. R. , Wang C. H.
Published: July 2002

An experimental study of poly(N-vinyl-2-pyrrolidone) (PVP) in water solution has been carried out by using the surface laser light scattering (SLLS) technique. The concentration dependence data of the peak frequency and linewidth are obtained from the SLLS spectra of the PVP/water solutions with two PVP molecular weights over a wide concentration range varying from the dilute to semidilute regime. The equilibrium surface tension of the solution is also measured as a function of polymer concentration and molecular weight. In the dilute solution, the peak frequency and surface tension show only a small decrease with increasing concentration. However, in the semidilute concentration regime, the surface tension and the frequency shift decrease rapidly with increasing concentration, accompanied by the dramatic increase in the spectral linewidth. The surface tension, SLLS spectral peak frequency and linewidth show an important molecular weight dependence. Over the entire concentration range investigated, the SLLS data of the PVP/water solutions do not show evidence of the capillary to Rayleigh wave evolution.
A wave packet approach to the Liouville–von Neumann equation for dissipative systems
Authors: Gerdts Thorsten , Manthe Uwe
Published: July 2002

A method for the numerical solution of the dissipative Liouville–von Neumann equation is presented. The reduced density operator is expanded in a basis of time-dependent wave functions. This guarantees that the size of the basis required in the representation of the density operator is minimal. Equations of motion for the expansion wave functions and the density matrix elements are obtained from the Dirac–Frenkel variational principle. The numerical effort of the method scales proportional to N, where N is the dimension of the relevant Hilbert space. As a first example, the dynamics of a three-mode system with vibronic coupling (with N≈50 000) coupled to a dissipative bath has been studied, modeling the S1/S2 states of pyrazine. For the cw-absorption spectrum, fast convergence with respect to the number of expansion wave functions has been obtained.
Rotationally resolved photoelectron spectra from vibrational autoionization of NO Rydberg levels
Authors: Park Hongkun , Zare Richard N.
Published: July 2002

Rotationally resolved photoelectron spectra from vibrational autoionization of individual rotational levels of the nlR (v=1, 11⩽n⩽15, 0⩽lR⩽3) Rydberg states of NO are measured by combining two-color double-resonance excitation via the NO A 2∑+ (vi=1, Ni=19) state with time-of-flight photoelectron spectroscopy. The photoelectron spectra show that both even-l and odd-l continuum partial waves are generated by the autoionization events, and thus provide evidence for angular momentum exchange between the outgoing electron and the molecular-ion core. We interpret these observations as caused by the multipolar interactions between the outgoing electron and the vibrating nuclear core, which appear to be brought about both by the dependence on internuclear distance of the electron-ion-core electronic interaction and by Rydberg-valence state couplings.
Self-assembled molecular rectifiers
Authors: Dhirani A. , Lin P.-H. , Guyot-Sionnest P. , Zehner R. W. , Sita L. R.
Published: July 2002

Self-assembled monolayers (SAMs) of C6H5–C≡C–C6H4–C≡C–C6H4SH (1) on Au(111) and Ag(111) exhibit electrical rectifying behavior when probed by the scanning tunneling microscope whereas SAMs derived from decanethiol, benzenethiol and 4-phenylethynylbenzenethiol do not. We suggest that rectification arises from the conjugated length of the molecular framework of 1 and an increase of the donor character of the sulfur atom to this system upon adsorption to the metal. This is supported by surface second-harmonic generation measurements which show a significantly larger second order optical nonlinearity of SAMs of 1 compared to the other systems.
Dynamics and relaxation in interacting systems: Semigroup methods
Authors: Kosloff Ronnie , Ratner Mark A. , Davis William B.
Published: July 2002

The dynamical study of chemical systems whose evolution is governed by quantum mechanics can now be computed fairly effectively for small systems in which the evolution is entirely Hamiltonian. When such Hamiltonian systems interact with their environment, however, relaxation and dephasing terms are introduced into the evolution. To include the effect of these terms, several methods are in current use. This paper is devoted to an exposition, analysis, and several simple applications of the semigroup technique for dealing with these non-Hamiltonian evolution terms. We discuss the nature of the semigroup terms, how they arise and how they are applied, and some of their advantages and disadvantages compared to other methods including dissipation. Specific applications to three simple two-site problems are given.
Indirect determination of the composition of the coexisting phases in a demixed colloid polymer mixture
Authors: Bodnár Igor , Oosterbaan Wibren D.
Published: July 2002

In order to determine the coexisting phases in a colloid polymer mixture, exhibiting a gas liquid phase transition, a new method is presented making use of volume measurements. By combining the information from the volume measurements along at least three different paths through the phase diagram, and the lever rule, the binodal and the associated tie lines are reconstructed. Four samples with different concentration ratios of hard sphere colloidal silica particles (radius 47 nm) and nonadsorbing polymer (polydimethylsiloxane, radius of gyration 23 nm) were studied. The general shape of the phase diagram is qualitatively in agreement with other experiments and theory.
Monte Carlo simulation study of the effects of nonequilibrium chemical reactions during pulsed laser desorption
Authors: Itina T. E. , Tokarev V. N. , Marine W. , Autric M.
Published: July 2002

Monte Carlo simulation is used to study the role of chemical reactions in the gas flow of particles laser desorbed from the target into a vacuum. The influence of recombination and dissociation processes on the properties of the gas flow is considered. It was found that chemical reactions have a significant effect on the composition of the desorption jet and on the angular and mean energy distributions of the desorbed particles. The study of these phenomena is of a particular interest for the understanding of the process of thin film deposition by pulsed laser ablation.
Comparison of the Na(4p)+H2 and Na(3p)+H2 reactive/quenching systems studied with CARS, resonance-enhanced CARS, and DFWM
Authors: Motzkus M. , Pichler G. , Kompa K. L. , Hering P.
Published: July 2002

Three different nonlinear optical techniques, CARS (coherent anti-Stokes Raman scattering), resonance-enhanced CARS, and DFWM (degenerate four-wave mixing), were used to compare the reactive and quenching behavior of the two different electronically excited sodium atoms, Na(3p) and Na(4p), in a collision with H2. In the chemical reaction channel both excited sodium states produce NaH molecules, but in the case of Na(3p) it is shown that the reaction is not a direct formation process and involves more than one step. Both sodium states induce a population of the vibrational levels v″=0–3 of NaH. For the Na(3p) excitation the population of the NaH molecules is concentrated at v″=0, whereas for the excitation of the Na(4p) state NaH has its maximum population in the v″=1 level. The differences are attributed to different potential energy surfaces. The quenching investigations were focused on the behavior of the Na(4p) state and the results were compared with the well-known behavior of the Na(3p) state. Because no quenching products were detected in the CARS spectra with Na(4p) excitation, the upper limit for the integrated quenching cross section was estimated to be less than 5×10−17 cm2, as based on the detection sensitivity of the CARS apparatus.
Laser spectroscopy and density functional calculations on niobium monocarbide
Authors: Simard Benoit , Presunka Paul I. , Loock Hans Peter , Bérces Attila , Launila Olli
Published: July 2002

A survey of the jet-cooled Nb12C and Nb13C radicals has been carried out between 13 500 and 18 000 cm−1 using laser-induced fluorescence and resonant two-photon ionization spectroscopy. Several vibronic bands belonging to at least six band systems have been identified. Three of these systems appear to belong to 2Π1/2–2Δ3/2 transitions in which the lower 2Δ3/2 state is the ground electronic state of the molecule. The other three systems also terminate to the same three 2Π1/2 upper states, but originate from a state lying 830 cm−1 above the X 2Δ3/2 state. This state is assigned as the A 2Σ+ state. The ionization potential has been determined to be 56 402±15 cm−1 or 6.9929±0.0018 eV using two-color photoionization efficiency spectroscopy. This value, combined with the ionization potential of Nb and the bond energy of NbC+, yields an improved bond energy of 5.39±0.15 eV for NbC. The (4,0) band of the B 2Π1/2–X 2Δ3/2 system has been studied at a resolution of approximately 0.005 cm−1 using laser-induced fluorescence spectroscopy. The nuclear magnetic hyperfine structure has been resolved in both states, and an analysis confirms that the 2Δ ground state arises from the σ2δ1 electron configuration in which the unpaired δ electron is a pure Nb 4d electron associated with the 4F term arising from the excited 5s24d3 electron configuration. Density functional calculations have been carried out on the lowest 2Δ, 2Σ+, 4Δ, 2Π, and 4(Π,Φ) states of the neutral and the 1Σ+, 3Δ, and 3(Π,Φ) states of the cations. These calculations fully support the experimental evidence for the ground state.
Phase transition in strongly stretched polymer gels
Authors: Suzuki A. , Sanda K. , Omori Y.
Published: July 2002

We will present the volume phase transition of cylindrical poly N-isopropylacrylamide (NIPA) gels under large distortion along the uniaxial direction. The neutral and slightly ionized NIPA gels were synthesized in submillimeter diameter. The gel was stretched along the uniaxial direction with the longitudinal deformation ratio, α∥ (the ratio of the stretched length to the equilibrium length in the swollen state at 30 °C). By keeping the longitudinal length constant, we have simultaneously measured the equilibrium diameter and the force as functions of temperature. The swelling curves of the neutral NIPA gels including the volume phase transition temperature were obtained for several deformations in the range between α∥=1 and α∥=6. With increasing α∥, the transition temperature increased up to 1 °C in the small deformation below α∥∼3.5, and it saturated and slightly decreased in the large α∥ above α∥∼4. At the transition temperature on heating, the force to keep the length constant increased discontinuously in the smaller region below α∥∼3.5. The magnitude of this steplike change in the force began to decrease in the vicinity of α∥∼3.5, and hereafter exhibited negative change above α∥∼4. In the case of the slightly ionized NIPA gels, these effects came into play at an earlier stage, that is, at smaller α∥, because of the prestretching due to the ionic pressure. The present observations, especially for the strong deformation above α∥∼4, were discussed by the equation of states of strongly stretched hydrophobic gels on the basis of the extended Flory-type free energy taken into account the non-Gaussian effect.
Size correlations for condensation clusters produced in free-jet expansions
Authors: Knuth E. L.
Published: July 2002

For clusters produced by condensation from a gas phase, a basis for identifying the dimensionless thermodynamic and kinetic parameters which characterize cluster formation is provided by arranging an expression for the mass rate of production of critical-size clusters into dimensionless form. The experimentally observed dependence of the cluster size on source pressure dictates the choice between two alternative classical expressions for the nucleation rate. An earlier analysis of dimer formation in free jets guides the combining of the thermodynamic and kinetic parameters into a single scaling parameter which facilitates correlating the size of clusters in free jets for all rare gases, including He4, using a single curve. Important facets of the correlation are (a) the explicit dependence of the scaling parameter on the surface tension and, for the rare gases other than He4, the evaluation of the surface tension at the triple point, and (b) the use of real-fluid properties for the density and sound speed of the He4 in the source. An earlier conclusion that size distributions for large condensation clusters can be correlated with dimer partial pressure in the source is found to be fortuitous.
Methods for constraining zero-point energy in classical Monte Carlo transition-state theory
Authors: Marks Alison J.
Published: July 2002

Two microcanonical sampling methods for constraining zero-point energy (ZPE) within classical Monte Carlo transition-state theory (MCTST) are described. Each is based on the efficient microcanonical sampling method [H. W. Schranz, S. Nordholm, and G. Nyman, J. Chem. Phys. 94, 1487 (1991)], with exclusion of phase space points not satisfying imposed ZPE constraints. Method 1 requires extensive sampling of phase space points, but method 2 relies on configurational sampling using a microcanonical statistical weight adjusted to account for the reduced local density of momentum states arising from ZPE restrictions. Simple applications to HenI2 clusters and a linear model for H–O–D dissociation show that there are significant deviations from standard MCTST. Methods 1 and 2 give similar results, but method 2 is the more efficient algorithm.
Quantum control of the yield of a chemical reaction
Authors: Pastirk Igor , Brown Emily J. , Zhang Qingguo , Dantus Marcos
Published: July 2002

Order of magnitude enhancement in the concerted elimination pathway leading to I2 product formation in the photodissociation reaction of CH2I2 by the use of positively chirped 312 nm femtosecond laser pulses is demonstrated. The maximum yield is found for chirps of 2400 fs2 while the minimum is found near −500 fs2. Multiphoton excitation with 624 nm pulses results in the opposite effect, where the maximum yield is found near −500 fs2. The enhancement as a function of chirp is found to depend on the wavelength and intensity of the laser pulses. These results offer new experimental evidence for quantum control of chemical reactions.
Comparing quantum and classical dynamics: H2 dissociation on W(100)
Authors: Kay M. , Darling G. R. , Holloway S.
Published: July 2002

A quantitative comparison of the dissociation probabilities of H2 on the W(100) surface obtained from quantum and classical dynamical simulations is presented. Four dimensions are employed, and the vibrationally adiabatic approximation is used to circumvent problems in classical mechanics associated with nonconservation of zero-point energy. For normal incidence, we find good agreement between the classical and quantum dissociation probabilities. Both are dominated by strong steering into geometries favorable for dissociation. Exciting either initial rotations or parallel translations reduces the effectiveness of steering, thus reducing the dissociation probability. For the former, the good agreement between classical and quantum still holds, however, for the latter this is not so, and the disagreement becomes progressively greater as the initial parallel momentum is increased. To understand this, we employ two new visualization techniques. For the classical dynamics, we use swarms of color-coded trajectories to illustrate the detailed motion of an ensemble. While for the quantum system, we project time-dependent wave packets onto a local basis set, adiabatic in the internal (rotational and parallel translational) degrees of freedom, plotting the results on the correlation diagram. Examination of these shows that the behavior of adiabatic states populated for normal incidence is dominated by a combination of steering and orientational hindering, both classical phenomena. In contrast, there are very large avoided crossings in the states populated at off-normal incidence, leading to motion which cannot be reproduced by a classical system.
Elastic properties of homopolymer-homopolymer interfaces containing diblock copolymers
Authors: Laradji Mohamed , Desai Rashmi C.
Published: July 2002

We study the elastic properties of homopolymer/homopolymer interfaces containing diblock copolymers by means of a theory of Gaussian fluctuations. The interfacial tension and the bending rigidity of the interface in the two-phase coexistence region are calculated from the power spectrum of capillary modes. Our theory shows that in the limiting case of a pure binary homopolymer mixture, while the interfacial tension increases monotonically with increasing χN (where χ is the Flory-Huggins parameter and N is the homopolymer molecular weight) the bending rigidity does not. The bending rigidity increases rapidly at first for small values of χN, but then decreases with further increase of χN. In the presence of diblock copolymers, the interfacial tension always decreases with increasing diblock copolymer volume fraction at a given χN. However, the bending rigidity can show either a decrease or an increase depending on χN and the ratio γ between the molecular weights of a diblock copolymer and that of a homopolymer. Our results for the surface pressure and the bending rigidity are further compared with results based on scaling arguments of wet polymer brushes.
Chemical structure and surface phonons associated with H on Si
Authors: Watanabe Satoru
Published: July 2002

The vibrational characteristics of H atoms on Si surfaces were investigated by infrared spectroscopy while surface electric fields were controlled on several vicinal (111) surfaces. These surfaces were prepared in solution. By analyzing the response to the electric fields, the orientation of the dynamic polarizations, Pdyns, for specific vibration modes such as Si–H stretching and wagging were determined. These vibration modes are associated with specific bulk-terminated chemisorption sites for H atoms on such surfaces as the top layer of Si atoms of the (111) terrace, the [111̄] micro facet of the [112̄] step edge, and the [001] micro facet of the [1̄1̄2] step edge. The anisotropy of Pdyns also showed that these step edges were atomically straight along [1̄10] and had the translational symmetry of a Si crystal, meaning that these vibrations were not isolated but collective, and thus should be treated as parts of surface phonons. The deduced surface structures confirm previous infrared analyses. The scissoring vibration of dihydride at the [1̄1̄2] step edge, which had been missed previously, was also observed. It was found that the [1̄1̄2] step edge had a strained vertical dihydride and a strained monohydride complex. By adopting a three-layer model with an experimentally determined screening factor when analyzing Pdyns, we concluded that the dihydride was pulled up at an orientation angle of 21° from its bulk-terminated position, in contrast with the other bulk-terminated structures. This is consistent with a Raman result [M. A. Hines et al., J. Chem. Phys. 101, 8055 (1994)] and explains the discrepancy with the previous results of energy loss and infrared spectra regarding the step-edge structure.