bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental study of dynamic isotope effects in molecular liquids: Detection of translation-rotation coupling
Authors: Holz Manfred , Mao Xi-an , Seiferling Dieter , Sacco Antonio
Published: July 2002

How and to what extent do molecular motions in a liquid depend upon the molecular mass? Since this fundamental problem could not yet be satisfactorily resolved by theoretical approaches, we present in this paper an experimental approach. In continuation of previous work from our laboratory, the effects of isotopic H–D substitution on viscosity and self-diffusion have been measured for twelve molecular liquids in a certain temperature range near room temperature. These liquids are the often used solvents: methanol, ethanol, formamide, N,N-dimethylformamide, dimethylsulfoxide, acetone, acetonitrile, pyridine, nitromethane, tetrahydrofuran, dioxane, and benzene; we have found comparatively high dynamic isotopic effects between 3% and 14%. Beside the NMR self-diffusion measurements we have, for the first time, performed measurements of the effect of mass change on the molecular rotational diffusion via 17O and 14N intramolecular quadrupole relaxation. In all the given molecular liquids, except benzene, the dynamic isotope effects on viscosity, translational, and rotational diffusion follow a square-root of the moment of inertia law, revealing strong translation-rotation coupling in these liquids. However, in the two further liquids n-octanol and n-dodecanol this coupling seems to be absent, since we find here for the dynamic isotopic effect on viscosity a square-root of mass law. We have also measured D*, the tracer-diffusion coefficients of light (protonated) molecules in the heavy (deuterated) liquids. Here, we find a very weak mass dependence of D*, which obviously follows a square-root of reduced mass law, as predicted from simple gas kinetic theory. A ‘‘negative’’ dynamic isotope effect for CHCl3/CDCl3 postulated in the literature could not be confirmed by our measurements. The NMR techniques used for this study turned out to be excellently suited for investigations of the mass effects on dynamic quantities in liquid systems.
X-ray and neutron scattering studies of the structure of water at a hydrophobic surface
Authors: Bellissent-Funel Marie-Claire , Sridi-Dorbez Rachida , Bosio Louis
Published: July 2002

The structure of water in contact with a hydrophobic surface, namely, activated carbon, is studied using both x-ray and neutron scattering techniques. The structure factors and the corresponding radial distribution functions are determined for several levels of hydration, at room temperature and down to 77 K. At high water content (∼200%), the properties of water are very similar to those of bulk water; in partially hydrated samples (<50%), the molecular structure exhibits change of its local environment within particular a distortion at the level of the second neighbors, and water does not crystallize when the temperature is lowered.
Structures and energetics of Ni24–Ni55 clusters
Authors: Wetzel Thiele L. , DePristo Andrew E.
Published: July 2002

We predict stable geometrical structures and interaction energies of Ni clusters using non self-consistent electron density functional based corrected effective medium (CEM) and MD/MC-CEM methods. A plot of the reaction energies for the atomic ejection process, NiN+1→NiN+Ni, for Ni24–Ni55 displays a number of informative characteristics: (a) peaks and valleys represent internal structural rearrangement in which the number of core atoms increases by at least one and; (b) a plateau at N=50–54 is associated with the closing of the second MacKay icosahedron at Ni55. The lowest energy structures of NiN clusters for N=24–55 are dissimilar generally to those of both rare gas clusters and fragments of the bulk crystal lattice except where a stable icosahedral or bulk core is present. The growth scheme for N<50 is determined by the stability and structure of the changing number of core atoms. By contrast, the growth scheme for 51≤N≤55 is determined by the addition of surface atoms to a very stable and invariant 13-atom icosahedral core. The theoretical predictions are compared to available model growth schemes and experimental data.
Field-dependent isotropic shifts and limitations to linewidths in solid state nuclear magnetic resonance: A Floquet treatment
Authors: Zax D. B.
Published: July 2002

Previously we have provided a quantitative theory that explains the apparent field dependence observed for chemical shifts in high resolution solids state 13C nuclear magnetic resonance spectroscopy (where magic angle spinning is combined with high-power 1H decoupling). We have expanded that theoretical treatment so as to include the effect of homonuclear dipole–dipole couplings among the abundant nuclear spin species, and to the impact of off-resonance rf fields. These differing contributions to the residual linewidths are assessed in a unified theoretical treatment using the Floquet theory for time-dependent Hamiltonians. Some comments are provided as to the circumstances in which the various contributing terms dominate.
On the quantum nonequilibrium theory with density matrices
Authors: Ogul R. , Eren N.
Published: July 2002

Using density matrices we describe the time evolution of nonequilibrium states with a set of hierarchical of quantum kinetic equations. An analogous formalism to this equation is developed via the path integral method in a holomorphic representation of density matrices and related to the Green’s function formalism.
Conductivity in alkali doped polyacetylene. V. Reorganization energy and interchain coupling induced by the alkali ion
Authors: Rodriguez-Monge Lucia , Larsson Sven
Published: July 2002

The electronic structure of two ethene molecules and two allyl radicals in the presence of an alkali atom are studied using ab initio methods as a simplified model of electron transfer (ET) for an alkali-dopant site for ET in polyacetylene. At the optimized structure the systems are stable charge transfer complexes, where the charge is transferred to one of the monomers. Parameters of interest for the understanding of electron transfer steps in alkali doped polyacetylene, such as reorganization energy and electronic factor for ET, are calculated. We find for the two systems studied here an important additional contribution to the reorganization energy for ET between the chains (about 0.40 eV) due to the motion of the alkali ion.
Tunneling dynamics, symmetry, and far-infrared spectrum of the rotating water trimer. I. Hamiltonian and qualitative model
Authors: van der Avoird A. , Olthof E. H. T. , Wormer P. E. S.
Published: July 2002

A Hamiltonian is derived for the rotating water trimer with three internal motions—the rotations of the monomers about their hydrogen bonds. We obtain an expression of the kinetic energy operator, which is a non-trivial extension of earlier heuristic forms used for the non-rotating trimer. The Coriolis coupling operator between the single-axis monomer angular momenta and the overall trimer rotation is given for the first time. To analyze the effects of the tunneling and Coriolis splittings on the energy levels of the trimer, we introduced a qualitative model for the pseudo-rotation and donor tunneling. By perturbation theory and application of the permutation-inversion groups G6 and G48 we obtain algebraic expressions for the splittings due to pseudo-rotation and donor tunneling, respectively. The pseudo-rotation does not produce any internal angular momentum and does not yield first order Coriolis splitting, but in second order the Coriolis coupling lifts various degeneracies and gives rise to observable J-dependent splittings. Donor tunneling splits every pseudo-rotation level into a quartet and those levels in this quartet that belong to the three-dimensional irreps of G48 into doublets. For J≳0 a rather complex pattern of larger (for the internal states with G6 labels k=±1 and ±2) and smaller (for the levels with k=0 and k=3) splittings is obtained, especially for the substates with |K|=1 which are Coriolis coupled to the K=0 substates. The results of calculations in the companion paper, together with the model introduced in the present paper, will be used to interpret all the tunneling splittings observed in high-resolution spectra of (H2O)3 and (D2O)3.
Mo/ller–Plesset perturbation theory applied to vibrational problems
Authors: Norris Lawrence S. , Ratner Mark A. , Roitberg Adrian E. , Gerber R. B.
Published: July 2002

Mo/ller–Plesset perturbation theory is employed to improve the accuracy of static mean field computations in molecular vibration problems. This method is a simple and efficient way to get nearly exact frequencies for few-mode model potentials. For more realistic potentials representing the dynamics of water and formaldehyde, the Mo/ller–Plesset treatment works equally as well. However, we find in general that MP2 level corrections give very accurate energies and additional corrections by higher level terms in the MP series are not substantial. Moreover, we find that for reference states on high energy manifolds degeneracies can result when higher level terms are included in the series. We discuss several ways to remove these degeneracies.
Calorimetric heats for CO and oxygen adsorption and for the catalytic CO oxidation reaction on Pt{111}
Authors: Yeo Y. Y. , Vattuone L. , King D. A.
Published: July 2002

Single crystal adsorption calorimetry was applied to investigate the heats of adsorption of CO and oxygen and the reaction heats for the CO oxidation process on Pt{111} at room temperature. Both sticking probabilities and heats of adsorption for CO and oxygen are presented as a function of coverage. These results are used to interpret the subsequent measurements taken for the CO oxidation process on the same surface. The initial heats of adsorption of CO and oxygen on Pt{111} are 180±8 and 339±32 kJ/mol, respectively. In addition the pairwise lateral repulsive interaction between CO molecules in a (√3×√3)R30° ordered layer at θ=1/3 is found to be 4 kJ/mol. A detailed Monte Carlo modeling of the dissociative adsorption and sticking probability of oxygen on Pt{111} is performed. The initial rapid fall in heat is attributed to adsorption on defect sites, and subsequent adsorption on the planar {111} surface proceeds with a third neighbor interaction energy between the oxygen adatoms ω3∼22 kJ/mol. When gaseous CO reacts with preadsorbed oxygen adatoms, the CO2 produced has an excess energy of 16±8 kJ/mol.
Similarity transformed equation-of-motion coupled-cluster study of ionized, electron attached, and excited states of free base porphin
Authors: Nooijen Marcel , Bartlett Rodney J.
Published: July 2002

The newly developed similarity transformed equation-of-motion coupled-cluster (STEOM-CC) method and its perturbative analog (STEOM-PT) are applied to obtain ionization potentials, electron affinities and singlet and triplet excitation energies of free base porphin. STEOM-CC results are found to agree nicely with the available experimental data. Ground-state correlation is not described adequately in lowest order of perturbation theory, although STEOM-PT results qualitatively agree with the infinite order STEOM-CC results. The STEOM computational strategy is operationally like CI singles for excited states, which leads to exceptional efficiency.
Eley–Rideal and hot-atom reaction dynamics of H(g) with H adsorbed on Cu(111)
Authors: Caratzoulas Stavros , Jackson Bret , Persson Mats
Published: July 2002

Quasiclassical trajectory studies have been performed for the reaction between an H (or D) atom incident from the gas phase and a H (or D) atom adsorbed onto a Cu(111) surface. Results from a density functional calculation of the interaction between H and a Cu(111) surface are used to construct a detailed potential energy surface which contains all six nuclear degrees of freedom. Impacts of the incident atom close to the adsorbate can lead to direct Eley–Rideal reactions and the dynamics of these reactions are explored. Interaction of the incident atom with the adsorbate also results in trapping, with a high probability. This adsorbate-mediated trapping mechanism is important for impacts within 2 Å of the adsorbate. At larger impact parameters scattering from the corrugation also leads to trapping. These trapped “hot” atoms can go on to react with an adsorbed species, and the dynamics of such hot-atom reactions are explored. The final-state distributions of the products are examined with regard to isotope effects for the direct and hot-atom pathways, and compared with experiment.
On the calculation of G tensors of organic radicals
Authors: Törring J. T. , Un S. , Knüpling M. , Plato M. , Möbius K.
Published: July 2002

The results of calculations of doublet state G tensors of organic radicals often show unsatisfactory agreement with the respective experimental data. This led us to reconsider the results of the Rayleigh–Schrödinger perturbation theory approach found in the literature. We came to the conclusion that neither the expressions given for restricted nor for unrestricted Hartree–Fock functions are fully correct. In this paper new expressions for the G tensor to be used with restricted Hartree–Fock functions will be introduced and results of calculations employing these new formulas in conjunction with molecular states from semiempirical Hartree–Fock-type calculations are discussed.
State-specific lifetime determination of the a 3Π state in CO
Authors: Jongma Rienk T. , Berden Giel , Meijer Gerard
Published: July 2002

Two different techniques were applied to measure the lifetime of the lowest rotational levels in the metastable a 3Π1(v=0) state of CO. First, measurement of the absolute absorption cross-section for several absorption lines of the a(v=0)←X(v=0) transition yields an Einstein coefficient of A0,0=97±3 s−1. In combination with the experimentally determined branching ratios for the a→X transition, the lifetime of each component of the a 3Π1(v=0,J=1) Λ-doublet is determined to be 3.67±0.20 ms. Second, detection of the spin-forbidden fluorescence at two positions in the molecular beam downstream from the excitation region, as a function of velocity of the molecules directly probes the exponential decay. With this technique the lifetime of the lower component of the same a 3Π1(v=0,J=1) Λ-doublet is determined to be 3.4±0.4 ms, while for the upper component a value of 3.8±0.5 ms is found.
Photoelectron spectra of the C2nH− (n=1–4) and C2nD− (n=1–3) anions
Authors: Taylor Travis R. , Xu Cangshan , Neumark Daniel M.
Published: July 2002

Anion photoelectron spectra of the carbon monohydrides, C2nH− for n=1–4 and C2nD− for n=1–3, have been measured. The spectra were recorded at a wavelength of 266 nm (4.657 eV) and yield electron affinities for each species. The spectra are vibrationally resolved, and some of the vibrational modes in the neutral C2nH(D) radicals are assigned. In addition, photoelectron angular distributions allow one to distinguish between photodetachment transitions to the Σ+2 and Π2 states of the neutrals. The spectra confirm previous work showing that C2H and C4H have Σ+2 ground states, while C6H and C8H have Π2 ground states. In addition, we observe the low-lying Π2 or Σ+2 excited states for all four radicals. The photoelectron angular distributions also serve as a probe of vibronic coupling between the Σ+2 and Π2 states. These effects are particularly prominent in the C2H− and C4H− spectra.
Dynamic properties of dioctanoyl peroxide guest molecules constrained within the urea tunnel structure: A combined incoherent quasielastic neutron scattering and solid state 2H nuclear magnetic resonance investigation
Authors: Girard Pascale , Aliev Abil E. , Guillaume François , Harris Kenneth D. M. , Hollingsworth Mark D. , Dianoux Albert-José , Jonsen Paul
Published: July 2002

The dynamic properties of dioctanoyl peroxide guest molecules within the urea host tunnel structure in the dioctanoyl peroxide/urea inclusion compound have been investigated by incoherent quasielastic neutron scattering (IQNS) and solid state 2H nuclear magnetic resonance (NMR) techniques. The IQNS investigations were carried out on samples of urea inclusion compounds containing perdeuterated urea to ensure that the incoherent scattering is dominated by the dioctanoyl peroxide guest molecules. Using semioriented polycrystalline samples, translational motions of the guest molecules along the tunnel were investigated separately from reorientational motions of the guest molecules about the tunnel axis. The 2H NMR experiments used dioctanoyl peroxide deuterated selectively in both the α CD2 groups and urea with natural isotopic abundance. The dynamic models that have been found to describe the translational and reorientational motions of the guest molecules from the IQNS and 2H NMR data are discussed in detail. The reorientational dynamics of the guest molecules about the tunnel axis can be described by a model of uniaxial rotational diffusion in a twofold potential, and the translations of the guest molecules along the tunnels can be interpreted by a model of translational jumps between sites with unequal probabilities of occupation. These models differ markedly from those found previously to describe the dynamic properties of alkane guest molecules within the urea tunnel structure.
Response of the elastic properties of colloidal crystals to phase transitions and morphological changes
Authors: Schöpe H. J. , Decker T. , Palberg T.
Published: July 2002

We report on the shear modulus G of colloidal crystals formed from thoroughly deionized suspensions of charged latex spheres. G is measured as a function of particle number density n. Body- and face-centered-cubic (bcc and fcc) crystal structures are observed by simultaneously performed static light scattering, and a broad coexistence region is found between (2.7±0.1)×1018 m−3⩽n⩽(4.8±0.2)×1018 m−3. Below n=1019 m−3, G closely follows theoretical predictions for both bcc and fcc, while it stays constant throughout the transition region. Above n=1019 m−3, G still observes the predicted n-dependence but with values larger than expected. While in that region, an upper bound of particles per crystallite is estimated from scattering data to be on the order of 104; the abrupt change in G cannot be solely attributed to the gradual morphological transition from polycrystalline to nanocrystalline materials.
Equilibration and sampling in molecular dynamics simulations of biomolecules
Authors: Stella L. , Melchionna S.
Published: July 2002

In this Communication we analyze the relaxation to equilibrium of a biomolecule and its sampling of the equilibrium ensemble, by using simple collective observables. We demonstrate that the customary use of the root mean square positional deviation parameter from the initial structure of a molecular dynamics trajectory, to determine the attainment of a stationary state in a simulation of biomolecules, necessarily leads to an overestimation of the relaxation time; this causes a loss of precious data that otherwise could be used in the calculation of equilibrium properties. A simple and reliable alternative is suggested, by computing the root mean square deviation from several different reference conformations along the trajectory.
Pseudorotation tunneling in several water trimer isotopomers
Authors: Geleijns M. , van der Avoird A.
Published: July 2002

The tunneling dynamics of several water trimer isotopomers is investigated by using an extension of the Hamiltonian that was previously derived for pure (more symmetric) trimers. This Hamiltonian takes into account the overall rotation of the trimer and three internal, torsional, or pseudorotational, motions of the monomers. Quantitative calculations of the torsional levels are presented, and the small effects of internal, i.e., hydrogen bonded, H/D substitutions and the much larger effects of external H/D substitutions are discussed. Transition line strengths are computed as well, and the assignment of most of the bands observed for the various water trimer isotopomers is confirmed. In one case we suggest a different assignment.
Quantum/classical time-dependent self-consistent field treatment of Ar+HCO inelastic and dissociative scattering
Authors: Whittier Gregory S. , Light John C.
Published: July 2002

A quantum/classical time-dependent self-consistent field (Q/C TDSCF) approach is used to simulate the dynamics of collisions of Ar with HCO. We present state-to-state cross sections and thermal rate constants for vibrational transitions. Using this model together with assumptions about the rotational energy transfer and a master equation treatment of the kinetics, the low-pressure thermal rate of collision-induced dissociation (CID) was calculated over the 300–4000 K temperature range. A comparison with experiment shows good agreement at high temperatures and poor agreement at low temperatures. The high temperature results were sufficient to obtain an Arrhenius expression for the rate that agrees with all experimental results of which we are aware.
Rotational constants of all H/D substituted water trimers: Coupling of intermolecular torsional and symmetric stretching modes
Authors: Sabo Dubravko , Bačić Zlatko , Graf Stephan , Leutwyler Samuel
Published: July 2002

A four-dimensional (4D) intermolecular potential energy surface (IPES) for the water trimer was calculated in the subspace of the three torsional coordinates and the symmetric intermolecular stretch coordinate, employing high-level ab initio theory. Torsionally adiabatic potential energy curves for the intermolecular symmetric stretching vibration were constructed based on this IPES. They were used to calculate the symmetric stretch fundamentals and stretching-averaged interoxygen R(O⋯O) distances 〈RH〉 and 〈RD〉, for the n=0–6 torsional levels of (H2O)3 and (D2O)3. 〈RH〉 and 〈RD〉 increase with n up to n=5, and decrease for n=6. Torsionally averaged rotational constants A, B, and C of all 20 isotopomers of water trimer, for the torsional levels n=0, 5 and 6, were obtained by averaging the inverse inertia tensor over the 3D torsional wave functions. Two approaches were examined: (i) setting the interoxygen R(O⋯O) distance to a fixed value, independent of torsional excitation; (ii) effectively incorporating the vibrational averaging due to the intermolecular symmetric stretching mode by using the appropriate 〈RH〉 and 〈RD〉 values to define the R(O⋯O) distances for the isotopomers in the torsional state n. Both approaches yielded n=0, 5 rotational constants in good agreement with experiment. However, only approach (ii) reproduced the experimentally observed decrease in the rotational constants A and B upon 5←0 torsional excitation. Fixing the R(O⋯O) distances to RH and RD values obtained by fitting the 3D torsionally averaged rotational constants to the experimental values for (H2O)3 and (D2O)3, only marginally improved the agreement with experiment for other isotopomers.
The transferability of the carbonyl group in aldehydes and ketones
Authors: Graña Ana M. , Mosquera Ricardo A.
Published: July 2002

The atomic and bond properties of the carbonyl group of a series of 42 aldehydes and ketones were calculated in order to analyze the transferability of this group. This was done by using the theory of atoms in molecules (AIM) on 6-31++G**/6-31G* wave functions. We found that the magnitudes ρ(rc), μ(O), μ(C), v(C), and λ3 differ between aldehydes and ketones, and can be said to be transferable within each of these series, with the exception of the formaldehyde molecule, which behaves in a specific way. We considered N(O), N(C), r1(O), r1(C), v(O), R, r, ε, and H(rc) as transferable dividing them into three groups: Aldehydes, methylketones, and ketones of greater length; we omitted formaldehyde, acetaldehyde, and acetone molecules, which behave in a specific way. Both the total and potential energies, either absolute or by unit of population of the C and O atoms, together with their summation, varied in accordance with molecule size and, therefore, cannot be considered transferable properties of the carbonyl group in any group of compounds. However, the molecular energies of the compounds studied can be reproduced extremely accurately by means of a group contribution model which distinguishes the classic fragments: H aldehyde, C=O, CH2, and CH3. The AIM study proves that this reproduction stems from the complementary variation which the values of the energies of the different fragments undergo throughout the series of compounds. At the level of accuracy with which we determined the maximum surface of the oxygen atom, none of the integrated properties are affected by the value of L(Ω). However, for the carbon atom the population and the first moment depend linearly on L(Ω) within each of the established groups, which prevents the values obtained for the above properties from being used directly in the analysis of transferability or evolutions of properties. The relationships between the concrete property and L(Ω), however, were found useful for both studies.
Exact method for the simulation of Coulombic systems by spherically truncated, pairwise r−1 summation
Authors: Wolf D. , Keblinski P. , Phillpot S. R. , Eggebrecht J.
Published: July 2002

Based on a recent result showing that the net Coulomb potential in condensed ionic systems is rather short ranged, an exact and physically transparent method permitting the evaluation of the Coulomb potential by direct summation over the r−1 Coulomb pair potential is presented. The key observation is that the problems encountered in determining the Coulomb energy by pairwise, spherically truncated r−1 summation are a direct consequence of the fact that the system summed over is practically never neutral. A simple method is developed that achieves charge neutralization wherever the r−1 pair potential is truncated. This enables the extraction of the Coulomb energy, forces, and stresses from a spherically truncated, usually charged environment in a manner that is independent of the grouping of the pair terms. The close connection of our approach with the Ewald method is demonstrated and exploited, providing an efficient method for the simulation of even highly disordered ionic systems by direct, pairwise r−1 summation with spherical truncation at rather short range, i.e., a method which fully exploits the short-ranged nature of the interactions in ionic systems. The method is validated by simulations of crystals, liquids, and interfacial systems, such as free surfaces and grain boundaries.
Combined photoelectron spectroscopy and ab initio study of the hypermetallic Al3C molecule
Authors: Boldyrev Alexander I. , Simons Jack , Li Xi , Chen Wenwu , Wang Lai-Sheng
Published: July 2002

The chemical structure and bonding of the hypermetallic Al3C and Al3C− species have been studied by photoelectron spectroscopy and ab initio calculations. Al3C− is found to have a planar triangular (D3h, 1A1′) structure (when averaged over zero-point vibrational modes) and Al3C is found to have a triangular distorted planar structure (C2v, B22) with one elongated Al–C bond. Four peaks in the photoelectron spectra of Al3C− were identified at 2.56, 2.69, 3.23, and 4.08 eV. Assignment of the observed features was made on the basis of the ab initio calculations. The experimental adiabatic electron affinity of Al3C was measured to be 2.56±0.06 eV, compared to 2.47 eV calculated at the CCSD(T)+OVGF/6-311+G(2df) level of theory. The excellent agreement between the calculated and experimental electron affinity, vibrational frequencies, and excitation energies allowed us to completely elucidate the geometrical and electronic structure of the Al3C molecule and its anion.
Optimized constant pressure stochastic dynamics
Authors: Kolb A. , Dünweg B.
Published: July 2002

A recently proposed method for computer simulations in the isothermal–isobaric (NPT) ensemble, based on Langevin-type equations of motion for the particle coordinates and the “piston” degree of freedom, is rederived by straightforward application of the standard Kramers–Moyal formalism. An integration scheme is developed that reduces to a time-reversible symplectic integrator in the limit of vanishing friction. This algorithm is hence expected to be quite stable for small friction, allowing for a large time step. We discuss the optimal choice of parameters, and present some numerical test results.
Collision-induced intersystem crossing from NH(a 1Δ,b 1Σ+) to NH(A 3Π): Gateway-mediated and direct mechanisms
Authors: Mo Y. , Ottinger Ch. , Shen G.
Published: July 2002

Metastable NH* radicals in a molecular beam, generated in a discharge, were allowed to collide with target particles (He through Xe rare gas atoms, and H2, CO, N2, NO, O2) in a cell or a crossed jet. Optical emission was observed issuing from the collision zone (and in the case of the jet also from different points along the primary beam). Spectral analysis (∼0.13 nm FWHM resolution) revealed two components; (a) a pair of sharp P, R lines (“spikes,” originating from the (perturbed) level NH(A 3Π, v=2, J=5, F3, Λ-component “e”; (b) broad NH(A 3Π→X 3∑−) emission in the (0, 0), (1, 1), and (2, 2) bands. Component (a) was shown to be due to a gateway coupling with the (perturbed) level NH(b 1∑+, v=5, J=5). From the collision gas pressure dependence of the “spike” intensity, relative cross sections were derived. They varied by less than a factor of 3 between He and NO. Weak spike emission was also observed issuing from the NH* beam without collisions. From the exponential decay of this “afterglow” intensity along 20 cm of the beam, the lifetime of the long-lived gateway emission component was found to be 52 μs, with a beam speed of ∼1220 m/s (measured using a chopper wheel and a particle multiplier detector). There is also a fast gateway component, having a (calculated) lifetime of ∼0.21 μs. It is too close (∼1 cm−1) to the slow component to be spectrally resolved and is, moreover, much weaker. The calculated branching ratio of the fast and the slow component is 1:247. Experimentally an upper limit of 1:20 was derived from simulations of the observed emission intensity profile downstream from the beam/jet crossing point. It is pointed out that only the weak, fast component of the “spike” intensity should properly be termed “gateway” emission, while the dominant, slow component is better described as being due to an “emission window” at a particular level of the otherwise dark NH(b) state. The broadband component (b) of the NH(A–X) emission is due to direct spin-changing energy transfer from (mainly) NH(a 1Δ) to NH(A 3Π). Surprisingly all target gases except He were effective, although the relative cross sections varied here by a factor of 120 between Ne and NO. NH(a) was identified as the dominant reactant species from the different beam attenuation in the target cell, compared to that of NH(b) (as measured using the spike attenuation). The contours of the intense NH(A–X) bands observed with Xe, O2, and NO were computer-simulated, yielding high rotational “temperatures” and, with O2, a striking excess population of the “f” Λ component (e:f=1:5).
Rotational-resolved pulsed field ionization photoelectron study of NO+(X 1Σ+,v+=0–32) in the energy range of 9.24–16.80 eV
Authors: Jarvis G. K. , Evans M. , Ng C. Y. , Mitsuke K.
Published: July 2002

We have obtained rotationally resolved pulsed filed ionization photoelectron (PFI-PE) spectra of NO in the energy range of 9.2–16.8 eV, covering ionization transitions of NO+(X 1Σ+,v+=0–32,J+)←NO(X 2Π3/2,1/2,v″=0,J″). The PFI-PE bands for NO+(X 1Σ+,v+=6–32) obtained here represent the first rotationally resolved spectroscopic data for these states. The simulation using the Buckingham–Orr–Sichel model provides accurate molecular constants for NO+(X 1Σ+,v+=0–32), including ionization energies, vibrational constants (ωe+=2 382.997±0.122 cm−1, ωe+χe+=17.437 84±0.000 90 cm−1, ωe+ye+=0.063 209 5±3.2×10−6 cm−1, and ωe+ze+=−0.001 400 0±7.2×10−8 cm−1), and rotational constants (Be+=1.996 608±0.006 259 cm−1, αe+=0.020 103±6.3×10−5 cm−1, and γe+=−(7.22±2.26)×10−6 cm−1). For v+=0–15, the rotational branches are ΔJ=J+−J″=±1/2, ±3/2, ±5/2, ±7/2, and ±9/2, which correspond to the formation of photoelectron angular momentum states l=0, 1, 2, and 3. The ΔJ=±1/2, ±3/2, ±5/2, ±7/2, ±9/2, and ±11/2 rotational branches are observed in the spectra for v+=16–32, revealing the production of continuum photoelectron states l=0, 1, 2, 3, and 4. The maximum ΔJ value and intensities for high ΔJ rotational branches are found to generally increase as v+ is increased in the range of 0–32. This observation is attributed to an increase in inelastic cross sections for collisions between the outgoing photoelectron and the nonspherical molecular ion core as the bond distance for NO+ is increased. Thus, this observation can be taken as strong support for the electron-molecular-ion-core scattering model for angular momentum and energy exchanges in the threshold photoionization of NO.
Freezing of simple fluids in microporous activated carbon fibers: Comparison of simulation and experiment
Authors: Radhakrishnan Ravi , Gubbins Keith E. , Watanabe Ayumi , Kaneko Katsumi
Published: July 2002

We study the freezing of CCl4 in microporous activated carbon fibers (ACF), using Monte Carlo simulation and differential scanning calorimetry (DSC). Microporous activated carbon fibers are well characterized porous materials, having slit-shaped pores due to the voids formed between graphitic basal planes. They serve as highly attractive adsorbents for simple nonpolar molecules, the adsorbent–adsorbate interaction being mostly dispersive (of the van der Waals-type). Recent molecular simulation studies have predicted an upward shift in the freezing temperature (ΔTf=Tf,pore−Tf,bulk>0) for simple fluids confined in such highly attractive carbon slit pores. Our DSC experiments verify these predictions about the increase in Tf. The results also indicate significant deviation from the prediction of ΔTf based on the Gibbs–Thomson equation (simple capillary theory). We employ a recently developed free energy method to calculate the exact freezing temperature in these confined systems using molecular simulation, in order to address the failure of the simple capillary theory.
Molecular calculations of excitation energies and (hyper)polarizabilities with a statistical average of orbital model exchange-correlation potentials
Authors: Schipper P. R. T. , Gritsenko O. V. , van Gisbergen S. J. A. , Baerends E. J.
Published: July 2002

An approximate Kohn–Sham exchange-correlation potential νxcSAOP is developed with the method of statistical averaging of (model) orbital potentials (SAOP) and is applied to the calculation of excitation energies as well as of static and frequency-dependent multipole polarizabilities and hyperpolarizabilities within time-dependent density functional theory (TDDFT). νxcSAOP provides high quality results for all calculated response properties and a substantial improvement upon the local density approximation (LDA) and the van Leeuwen–Baerends (LB) potentials for the prototype molecules CO, N2, CH2O, and C2H4. For the first three molecules and the lower excitations of the C2H4 the average error of the vertical excitation energies calculated with νxcSAOP approaches the benchmark accuracy of 0.1 eV for the electronic spectra.
Crossed beams studies of Mo(a 7S3) and Mo*(a 5S2) collisions with CH4 and C2H6
Authors: Hinrichs Ryan Z. , Willis Peter A. , Stauffer Hans U. , Schroden Jonathan J. , Davis H. Floyd
Published: July 2002

The interactions of Mo(a 7S3) and Mo*(a 5S2) with methane, CH4, and ethane, C2H6, were studied under single collision conditions using the crossed molecular beams technique. Ground state Mo(a 7S3) atoms were found to be unreactive at all collision energies studied up to 〈Ecoll〉=35.4 kcal/mol. Nonreactive scattering of Mo(a 7S3) with methane and ethane was studied and compared to collisions with Ne and Ar. A forward peaking center-of-mass angular distribution, T(Θ), was necessary to simulate the elastic collisions with inert gases as well as inelastic collisions with the alkanes. At a collision energy of 14.4 kcal/mol with CH4 and 21.0 kcal/mol with C2H6, inelastic collisions were found to transfer ∼10% and ∼19% of the initial kinetic energy into alkane internal energy, respectively. For collisions of Mo*(a 5S2)+CH4, the dehydrogenation product, MoCH2, was observed at all collision energies studied down to 2.1 kcal/mol. The reaction Mo*(a 5S2)+C2H6→MoC2H4+H2 was observed down to 〈Ecoll〉=4.5 kcal/mol. For a given total energy (electronic+translational), it was found that electronic energy is highly effective in promoting this reaction whereas translational energy is ineffective.
Two-step phase separation of a polymer mixture. II. Time evolution of structure factor
Authors: Hayashi Masaki , Jinnai Hiroshi , Hashimoto Takeji
Published: July 2002

Nonlinear time evolution of phase-separating structures in the two-step phase separation process was investigated for a deuterated polybutadiene–polyisoprene mixture by using a time-resolved light scattering technique. The mixture studied has a lower critical solution temperature type phase diagram with a spinodal temperature of 36 °C. The first-step phase separation via spinodal decomposition (SD) was conducted by a temperature jump (T-jump) from 23 °C to 42 °C, and to the late stage of the SD for varying time periods, t0, in order to develop phase-separated domains with varying characteristic size Λm,1. This phase separation was followed by the second-step T-jump to a higher temperature of 70 °C so that each phase-separated domain is again quenched into thermodynamically unstable region. Nonlinear time evolution processes of phase-separating structures after the second-step SD were explored as a function of size of the initial structures Λm,1. We found the following intriguing effects of the initial structures on further evolution of phase-separating structure via the second-step SD: (1) When Λm,1≫Λm,0 (characteristic length of composition fluctuations developed in the early stage SD after quenching the system from a single-phase state to 70 °C), small domains were evolved within the initial domains (defined as large domains) developed during the first-step SD process, while (2) when Λm,1⩽Λm,0, the small domains were not developed, but only the large domains grew at a growth rate larger than that at 42 °C. In the former case (1), we succeeded in separating the scattering due to the small domains and that due to large domains from the observed scattering profile. The separation allows us to investigate a coupling of the time evolution of the large and small domains and nonlinear pathways for the system to achieve a new equilibrium structure after the second-step SD process.
The dynamic disorder of azulene: A single crystal deuterium nuclear magnetic resonance study
Authors: Bräuniger Thomas , Poupko Raphy , Luz Zeev , Gutsche Peter , Meinel Christoph , Zimmermann Herbert , Haeberlen Ulrich
Published: July 2002

Deuterium nuclear magnetic resonance measurements on single crystals of azulene, specifically deuterated in the 1 and 3 positions, are reported. The quadrupole coupling tensor of these deuterons was determined by rotation experiments, yielding Qzz=182.2 kHz and η=0.056, with Qxx, the intermediate component (magnitude wise), oriented perpendicular to the molecular plane. The deuterium signals are inhomogeneously broadened and their widths are strongly anisotropic. This is quantitatively interpreted in terms of alignment disorder, induced by polar (up–down) disorder already known to be present in azulene from earlier x-ray measurements. It is shown that the alignment disorder is due to a planar distribution in the orientation of the molecules about the short axis, with a root mean square deviation of ±1°. The linewidths are strongly temperature dependent and reduce from about 8.9 kHz (maximum width) at room temperature to 1.6 kHz at 67 °C. This effect is interpreted in terms of molecular “up–down” flips, which average out both the polar and the alignment disorder. The rate of this process is found to be 104 s−1 at 40 °C, with an activation energy of 65 kJ/mol. Magnetization transfer experiments were performed by selectively inverting the magnetization of one of the deuterium doublets, followed by monitoring the subsequent approach to equilibrium of the whole spectrum. The results show the presence of additional dynamic processes in the ultraslow motion regime. These include molecular π flips about their long axes, as well as jumps between different sites in the lattice. The rate of both processes is about 0.084 s−1 at 57 °C. The intersite jumps are predominantly of the flip type, which interchange crystallographic symmetry related deuterons. From the rate of this process, a self-diffusion constant of 0.35×10−22 m2 s−1, at 57 °C, is estimated.
On Komar integrals in asymptotically anti-de Sitter space-times
Authors: Magnon Anne
Published: July 2002

Recently, boundary conditions governing the asymptotic behavior of the gravitational field in the presence of a negative cosmological constant have been introduced using Penrose’s conformal techniques. The subsequent analysis has led to expressions of conserved quantities (associated with asymptotic symmetries) involving asymptotic Weyl curvature. On the other hand, if the underlying space-time is equipped with isometries, a generalization of the Komar integral which incorporates the cosmological constant is also available. Thus, in the presence of an isometry, one is faced with two apparently unrelated definitions. It is shown that these definitions agree. This coherence supports the choice of boundary conditions for asymptotically anti-de Sitter space-times and reinforces the definitions of conserved quantities.
A new perturbative approach to nonlinear problems
Authors: Bender C. M. , Milton Kimball A. , Pinsky Stephen S. , Simmons L. M.
Published: July 2002

A recently proposed perturbative technique for quantum field theory consists of replacing nonlinear terms in the Lagrangian such as φ4 by (φ2)1+δ and then treating δ as a small parameter. It is shown here that the same approach gives excellent results when applied to difficult nonlinear differential equations such as the Lane–Emden, Thomas–Fermi, Blasius, and Duffing equations.
Realization of exceptional superalgebras in terms of fermion–boson creation–annihilation operators
Authors: Della Selva A. , Sciarrino A.
Published: July 2002

Operators satisfying Clifford algebra (fermions) and Weyl algebra (bosons) are introduced. F(4) and G(3) are realized in terms of bilinears of fermions (the even part) and bilinears of one boson and one fermion (the odd part). D(2,1;α) is realized as bilinears (the even part) and trilinears (the odd part) of bosons.
Canonical representations of sp(2n,R)
Authors: Rangarajan Govindan , Neri Filippo
Published: July 2002

In this paper a rather unconventional real basis for the real symplectic algebra sp(2n,R) is studied. This basis is valid for representations carried by homogeneous polynomials of the 2n phase-space variables. The utility of this basis for practical computations is demonstrated by giving a simple derivation of the second- and fourth-order indices of irreducible representations of sp(2n,R).
Discrete differential calculus: Graphs, topologies, and gauge theory
Authors: Dimakis Aristophanes , Müller-Hoissen Folkert
Published: July 2002

Differential calculus on discrete sets is developed in the spirit of noncommutative geometry. Any differential algebra on a discrete set can be regarded as a ‘‘reduction’’ of the ‘‘universal differential algebra’’ and this allows a systematic exploration of differential algebras on a given set. Associated with a differential algebra is a (di)graph where two vertices are connected by at most two (antiparallel) arrows. The interpretation of such a graph as a ‘‘Hasse diagram’’ determining a (locally finite) topology then establishes contact with recent work by other authors in which discretizations of topological spaces and corresponding field theories were considered which retain their global topological structure. It is shown that field theories, and in particular gauge theories, can be formulated on a discrete set in close analogy with the continuum case. The framework presented generalizes ordinary lattice theory which is recovered from an oriented (hypercubic) lattice graph. It also includes, e.g., the two-point space used by Connes and Lott (and others) in models of elementary particle physics. The formalism suggests that the latter be regarded as an approximation of a manifold and thus opens a way to relate models with an ‘‘internal’’ discrete space (à la Connes et al.) to models of dimensionally reduced gauge fields. Furthermore, a ‘‘symmetric lattice’’ is also studied which (in a certain continuum limit) turns out to be related to a ‘‘noncommutative differential calculus’’ on manifolds.
A symmetry of massless fields
Authors: Liu Yu-Fen , Keller Jaime
Published: July 2002

It is proved that there exists an additional intrinsic symmetry in the left-handed and right-handed fermions (and other fields). The corresponding group of transformations is induced by the Poincaré translations in the space–time manifold. This symmetry predicts an additional intrinsic energy-momentum for fermions. Considering this symmetry as local leads to introduction of a gauge field and of a nonintegrable phase angle, the corresponding Berry-type phase depends on the topology of the Riemannian space–time manifold as determined by the vierbein. This additional symmetry provides us with the possibility of considering the fermions as gauge fields on the nonvector bundle.
On quantum field theory with nonzero minimal uncertainties in positions and momenta
Authors: Kempf Achim
Published: July 2002

We continue studies on quantum field theories on noncommutative geometric spaces, focusing on classes of noncommutative geometries which imply ultraviolet and infrared modifications in the form of nonzero minimal uncertainties in positions and momenta. The case of the ultraviolet modified uncertainty relation which has appeared from string theory and quantum gravity is covered. The example of Euclidean φ4-theory is studied in detail and in this example we can now show ultraviolet and infrared regularization of all graphs.
Integrable representations of the ultra-commutation relations
Authors: Dubin D. A. , Hennings M. A. , Solomon A. I.
Published: July 2002

We consider *-representations of the unital complex *-algebra generated by the identity and two elements, α and ν, with ν=ν* and one relation, αν−να=α, the ultra-commutation relations (ucr). In general, we do not impose any commutation relation between α and α*. This is a very general scheme, encompassing many important physical examples, inter alia: the ccr, car, q-deformed bosons and fermions. The representations of interest in physics have a diagonal number operator π(ν) whose spectrum is contained in the positive integers (together with some other technical conditions). Our principal result is that every *-representation in this class is completely determined, up to unitary equivalence, by the sequence of numbers [n+1]=|〈Ωn+1,π(α+)Ωn〉|2 for n⩾0, with [0]=0. Here Ωn is the normalized eigenvector of π(ν)¯ corresponding to the eigenvalue n if the dimension of that eigenspace is 1. If the carrier Hilbert space is infinite dimensional, this representation is irreducible if and only if [n]>0 for n⩾1. Finally, we consider spatial representations of some of these representations by kernels and differential operators.
A family of exact travelling wave solutions to nonlinear evolution and wave equations
Authors: Gudkov V. V.
Published: July 2002

A family {q} of the multicomponent special functions is defined for obtaining the exact travelling wave solutions to nonlinear evolution and wave equations. It is shown that the functions qn from {q} for some n=2,4,… are closely related to the special unitary groups SU(n). The necessary and sufficient conditions for existence of a family of the exact multicomponent travelling wave solutions to a quasilinear evolution equation are given. An efficiency of the method based on q-functions is demonstrated on several classes of the nonlinear partial differential equations.