bluuebunny/crossref_metadata_2025 huggingface dataset:
Solvation energies and electronic spectra in polar, polarizable media: Simulation tests of dielectric continuum theory
Authors: Bader Joel S. , Berne B. J.
Published: July 2002

A dielectric continuum theory for the solvation of a polar molecule in a polar, polarizable solvent is tested using computer simulations of formaldehyde in water. Many classes of experiments, for example those which measure solvent-shifted vertical transition energies or electron transfer rates, require an explicit consideration of the solvent electronic polarization. Due to the computational cost of simulating a polarizable solvent, many simulation models employ non-polarizable solute and solvent molecules and use dielectric continuum theory to relate the properties of the non-polarizable system to the properties of a more realistic polarizable system. We have performed simulations of ground and excited state formaldehyde in both polarizable and non-polarizable water, and the solvation energies and solvent-shifted electronic spectra we obtained are used to test dielectric continuum, linear response predictions. Dielectric continuum theory correctly predicts that free energy differences are the same in polarizable and non-polarizable water. The theory wrongly predicts that the reorganization energy in a polarizable solvent is 30% smaller than the reorganization energy in a polar, non-polarizable solvent; in the simulations, the reorganization energies differ by only 6%. We suggest that the dielectric continuum theory fails because it assumes that both solute electronic states exist in the same size cavity in the solvent, whereas in the simulation the cavity radius increases by 20% after the electronic transition. We account for the change in the cavity size by adding a non-linear solute–solvent coupling to the dielectric continuum theory, and find that the resulting predictions are just outside the error bounds from the simulation. The cavity size corrections have the undesired and incorrect side-effect of predicting fluctuations far smaller than seen in the simulations. This reveals the inherent difficulty in devising a simple, fully self-consistent dielectric continuum theory for solvation.
Thermal rate constants for R+N2H2→RH+N2H (R=H, OH, NH2) determined from multireference configuration interaction and variational transition state theory calculations
Authors: Linder Douglas P. , Duan Xiaofeng , Page Michael
Published: July 2002

Ab initio electronic structure calculations were performed to determine features of the potential energy surface for abstraction of a hydrogen atom from N2H2 by H, OH, and NH2. Based on multireference configuration interaction calculations with basis sets up to correlation consistent polarized valence triple zeta, the barrier heights determined for these reactions are 4.3, 3.0, and 4.4 kcal/mol, respectively. Using features of the potential energy surface along minimum energy paths determined at the complete active space self-consistent-field level of theory, variational transition state theory calculations were performed to determine the rate coefficients over the temperature range 300–3000 K. The temperature dependent computed rate coefficients for the three reactions are well represented by the following three-parameter expressions: kH(T) =1.41×10−19T2.63 exp(115.8/T) cm3 molec−1 s−1, kOH(T)=9.84×10−23 T3.40 exp(686.3/T) cm3 molec−1 s−1, and kNH2(T)=1.46×10−25T4.05 exp(810.5/T) cm3 molec−1 s−1. Abstraction from N2H2 is predicted to occur at a significantly slower rate than analogous abstractions from the isoelectronic HNO.
Photodissociation dynamics of Ã state ammonia molecules. I. State dependent μ-v correlations in the NH2(ND2) products
Authors: Mordaunt David H. , Ashfold Michael N. R. , Dixon Richard N.
Published: July 2002

The H(D) Rydberg atom photofragment translational spectroscopy technique has been applied to a further detailed investigation of the photodissociation dynamics of NH3 and ND3 molecules following excitation to the lowest two (v2=0 and 1) vibrational levels of the first excited (Ã 1A2″) singlet electronic state. Analysis of the respective total kinetic energy release spectra, recorded at a number of scattering angles Θ [where Θ is the angle between the ε vector of the photolysis photon and the time-of-flight (TOF) axis], enables quantification of a striking, quantum state dependent, μ-v correlation in the NH2(ND2) products. The spatial distribution of the total flux of H(D) atom photofragments is rather isotropic (βlab∼0). However, more careful analysis of the way in which the TOF spectra of the H(D) atom photofragments vary with Θ reveals that each H+NH2(D+ND2) product channel has a different ‘‘partial’’ anisotropy parameter, βlab(v2,N), associated with it: The H(D) atom ejected by those molecules that dissociate to yield NH2(ND2) fragments with little rotational excitation largely appear in the plane of the excited molecule (i.e., perpendicular to the transition moment and the C3 axis of the parent, with β tending towards −1). Conversely, the H(D) atoms formed in association with the most highly rotationally excited partner NH2(ND2) fragments tend to recoil almost parallel to this C3 axis (i.e., β→+2). Such behavior is rationalized in the context of the known potential energy surfaces of the Ã and X̃ states of ammonia using a classical, energy and angular momentum conserving impact parameter model in which we assume that all of the product angular momentum is established at the ‘‘point’’ of the conical intersection in the H–NH2(D–ND2) dissociation coordinate. We conclude by reemphasizing the level of care needed in interpreting experimentally measured β parameters in situations where there is averaging over either the initial (parent) or final (product) quantum states.
Relativistic four-component multiconfigurational self-consistent-field theory for molecules: Formalism
Authors: Jo/rgen Aa. Jensen Hans , Dyall Kenneth G. , Saue Trond , Fægri Knut
Published: July 2002

A formalism for relativistic four-component multiconfigurational self-consistent-field calculations on molecules is presented. The formalism parallels a direct second-order restricted-step algorithm developed for nonrelativistic molecular calculations. The presentation here focuses on the differences required by the use of the Dirac Hamiltonian with the incorporation of time-reversal symmetry and point group symmetry for D2h and subgroups, providing the expressions in this framework which correspond to the nonrelativistic expressions. It is found that an efficient algorithm requires only twice the memory used by the largest nonrelativistic calculation in the equivalent basis, due to the complex arithmetic. The feasibility of the calculations is then determined more by the disk space for storage of integrals and N-particle expansion vectors.
The nuclear Fukui function and Berlin’s binding function in density functional theory
Authors: Baekelandt Bart G.
Published: July 2002

The recently introduced nuclear Fukui function φα is formally identified as a reactivity index of the density functional theory (according to the postulated criterion of |dμ|) and is shown to constitute the conformational contribution to a change in the molecular electronic chemical potential μ, through the relation dμ|N=∫ f(r)dν(r)dr=−∑αφαdRga, with φα=(∂Fα/∂N)ν=−(δμ/δRα)N, where N is the number of electrons, f(r) the electronic Fukui function, ν(r) the external potential at point r, Rα the space coordinate of nucleus α, and Fα the force on nucleus α. Scaling of the nuclear coordinates with a factor λ, as a particular conformational change, links the nuclear Fukui function with Berlin’s binding function B(r) for polyatomic molecules, dμλ|N=dλ∫ f(r)B(r)dr=−∑αφα. This relation is instructing for interpretative purposes: changes in electron density are weighted by the binding function, which, according to Berlin’s theorem, separates the system in binding and antibinding regions.
Electrorheological effect in the nematic phase of 4-n-pentyl-4′-cyanobiphenyl
Authors: Negita Keishi
Published: July 2002

Rheological properties in the nematic phase of 5CB (4-n-pentyl-4′-cyanobiphenyl) are investigated under applications of electric fields. When an electric field of a few kV mm−1 is applied, an increase in the apparent viscosity is observed with its amount being saturated at higher electric fields, which is accompanied by a change of the flow from a Newtonian to a non-Newtonian and again to a Newtonian fluid. Such an electrorheological effect is observed only in the nematic phase with the effect being dependent on shear rate and temperature. On the basis of the theory of Leslie and Ericksen, the electric field dependence of the viscosity is calculated as a function of shear rate in good quantitative agreement with the experimental one. These results show that the observed viscosity change is due to a change of the director orientation, determined by the balance between the electric and the viscous torques exerted on the director.
Dynamics of the formation of CD4 from the direct reaction of incident D atoms with CD3/Cu(111)
Authors: Rettner C. T. , Auerbach D. J. , Lee J.
Published: July 2002

Using molecular beam techniques we find that incident D atoms can abstract CD3 from a Cu(111) surface to yield CD4 in a direct (Eley–Rideal) gas–surface reaction with a cross section of ∼10−16 cm2/D atom. Dynamical evidence for a direct reaction includes the observation of an extremely sharp angular distribution that is clearly displaced from the surface normal, and the determination of a very high translational energy of the product, Ef, which is ∼2 eV. For a 0.25 eV D-atom beam incident at 45° on a 95 K surface, this energy varies with the detection angle, θf, as Ef(θf)=(1.8+θf/45) eV, where θf<0° in the ‘‘backscattering’’ direction. For these conditions, the angular distribution approximately follows the function cos70(θf−5.5), being peaked 5.5° from the normal with a full width at half maximum of <17°. Lowering the beam energy to 0.07 eV gives a broader angular distribution peaked at about 1.5° from the normal, consistent with cos60(θf−1.5). The reaction with 0.25 eV H incident at 45° gives a similar distribution peaked at ∼3.5° from the normal. The shifts in the angular distributions are approximately consistent with parallel momentum conservation. The CD3/Cu(111) surface was prepared by thermal dissociation of CD3I on the surface or by adsorbing CD3 directly from a CD3 beam produced by the pyrolysis of azomethane.
The electron affinity of iridium
Authors: Davies B. J. , Ingram C. W. , Larson D. J. , Ljungblad U.
Published: July 2002

The electron affinity of iridium has been measured to be 12 613(4) cm−1. A pulsed dye laser beam perpendicular to an ion beam photodetached the negative iridium ions, and the relative cross section near threshold was measured by detecting the fast neutrals. The threshold was determined by fitting the cross section to the Wigner law.
Extended diffusion theory of reorientation of symmetric top molecules with internal rotation
Authors: Jang Joonkyung , Shin Kook Joe
Published: July 2002

The extended diffusion (ED) theory for the reorientational dynamics of a symmetric top molecule with internal rotation is reinvestigated. Based on a unified ED picture, we express, by using the convolution theorem of the Laplace transform, the reorientational correlation time with overall and internal free rotor correlation functions. In this work, Bull’s “equal angular momentum correlation time assumption” for both overall and internal rotations is removed. Limiting expressions, for diffusion limit of the overall rotation, are derived and shown to reduce to the results of previous works. As an application, our numerical calculations are compared with experiments on 13C nuclear-magnetic-resonance dipolar relaxation times of toluene. Angular momentum correlation times, thus obtained from the comparison, show that our calculations agree with the previous work of Chung et al.[Chem. Phys. Lett. 93, 499 (1982)].
Detection and characterization of scandium imide, ScNH
Authors: Steimle T. C. , Xin J. , Marr A. J. , Beaton S.
Published: July 2002

A supersonic molecular beam of scandium imide, ScNH, and deuterated imide, ScND, were produced using a laser ablation/reaction source and interrogated by laser induced fluorescence and dispersed fluorescence spectroscopy. The field free fine and magnetic hyperfine interactions and Stark effect in the (0,0) Ã 2Πr–X̃2Σ+ transition of ScNH (origin=15 023.033 cm−1) were analyzed. Ground state vibrational spacing for ScNH and ScND was measured. The determined permanent electric dipole moments are μ(Ã 2Π)=4.08(7)D and μ(X̃ 2Σ+)=2.28(15)D. Plausible molecular orbital descriptions are given for the Ã 2Πr and X̃ 2Σ+ states based upon the interpretation of the spectroscopic parameters and a comparison with other isovalent molecules is made.
Polarization and nonadiabatic effects on resonance enhanced stimulated Raman scattering in H2
Authors: Ghosh Swaralipi , Bhattacharyya S. S. , Saha Samir
Published: July 2002

Resonance enhanced stimulated Raman scattering (RESRS) from the ground X 1Σg+ (vg=0, Jg=0–2) state of a H2 molecule via intermediate B 1Σu+ (vi=14, Ji=0–3) and C 1Πu (vi=3, Ji=1–3) states coupled nonadiabatically with each other is studied theoretically in the presence of laser and Stokes fields applying the second-order perturbation technique. The present study includes the calculation of Raman gain (gR) of Stokes intensity resulting from various Q- and S-branch transitions considering the parallel–parallel, parallel–perpendicular, and circular (both same and opposite senses) polarizations of the laser and Stokes beams. The gR profiles are investigated for the ranges of incident photon frequency (ν̄L) about the resonance values [ν̄L(res)] for the two perturbed vibronic states (B,C). For the frequencies considered both the large resonance enhancement and the polarization dependence of the stimulated Raman gain spectra can be computed by taking into account the resonant intermediate levels only. In general, the Franck–Condon vibrational overlap and the configuration coupling determine the gain. The angular parts of transition matrix elements determine the polarization effect for the specified range of ν̄L and a particular set of rotational–vibrational quantum numbers. For Q-branch transitions, in most of the cases, the maximum and minimum gains occur, respectively, for the same- and opposite-sense circular polarizations of two fields while for S-branch transitions the results are reversed. The calculated gR values for RESRS in some cases are found to be larger by about ten orders of magnitude than the values obtained for nonresonant stimulated Raman scattering in H2. The variation of gR with gas temperature at constant density is also studied in some specific cases using the thermal Boltzmann distribution for vibrational–rotational states at different temperatures and polarizations. This study indicates that the thermal dependence of RESRS gain is different for different cases of transitions depending on the polarizations of the laser and Stokes lights.
Open Newton–Cotes differential methods as multilayer symplectic integrators
Authors: Chiou J. C. , Wu S. D.
Published: July 2002

Open Newton–Cotes differential methods that possess the characteristics of multilayer symplectic structures are shown in this paper. In numerical simulation, volume-preservation plays an important role in solving the Hamiltonian system. In this regard, developing a numerical integrator that preserves the volume in the phase space of a Hamiltonian system is a great challenge to the researchers in this field. Symplectic integrators were proven to be good candidates for volume-preserving integrators (VPIs) in the past ten years. Several one-step (single-stage or multistages) symplectic integrators have been developed based on the symplectic geometric theory. However, multistep VPIs have seldom been investigated by other researchers for the lack of an advanced theory. Recently, Zhu et al. converted open Newton–Cotes differential methods into a multilayer symplectic structure so that multistep VPIs of a Hamiltonian system are obtained. Mainly, their work has concentrated on the issue of achieving both the accuracy and efficiency by solving the quantum systems. But, there exist some unclear aspects in deriving this result in their paper. In this regard, we resolve their problem and provide a different aspect in connecting the relationship between open Newton–Cotes differential methods and symplectic integrators. A numerical example has been carried out to show the effectiveness of the present differential method.
Structural, vibrational, and optical properties of silicon cluster assembled films
Authors: Mélinon P. , Kéghélian P. , Prével B. , Dupuis V. , Perez A. , Champagnon B. , Guyot Y. , Pellarin M. , Lermé J. , Broyer M. , Rousset J. L. , Delichère P.
Published: July 2002

Distributions of neutral Si-clusters centered around mean sizes of 50 and 200 atoms have been produced using a laser vaporization source and deposited on various substrates at room temperature in ultrahigh vacuum. The Si-cluster assembled films obtained, resulting from the random stacking of incident free nanosize clusters, were subsequently coated by appropriate protective layers before removing in air to perform ex situ infrared, visible, Raman, and photoluminescence spectrometry measurements, as well as transmission electron microscopy observations. The main characteristics of the cluster films are comparable to those observed for amorphous hydrogenated silicon and quite different to those of conventional nanoporous structures or clusters larger than 2–3 nm. The observed intense photoluminescence signal and band gap suggest the presence of a low number of dangling bonds probably due to surface reconstruction effects, connections between adjacent clusters, and oxygen contamination. As for the oxygen contamination, infrared and x-ray photoemission spectrometry measurements agree with the assumption of oxygen atoms trapped at the cluster surface. Finally, all the results on the vibrational and optical properties tend to confirm the failure of the classical confinement model in a diamond lattice to explain the behavior of such nanostructured materials with grain size typically in the nanometer range. The presence of five-membered rings characteristic of the Si-cluster structures in this size range with the subsequent rehybridization effects, as well as the connection process between adjacent clusters seem to be a track which is discussed for a better interpretation of the results.
The unimolecular dissociation of H2CO on the lowest triplet potential-energy surface
Authors: Yamaguchi Yukio , Wesolowski Steven S. , Van Huis Timothy J. , Schaefer Henry F.
Published: July 2002

The unimolecular dissociation reaction of H2CO on the triplet potential-energy surface has been studied via ab initio electronic structure theory. The stationary point geometries for the equilibrium and transition state are determined employing the configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] levels of theory with large basis sets up to the correlation consistent (cc)-pVQZ basis. With the best method, cc-pVQZ CCSD(T), the first excited triplet (ã 3A″) state lies 72.2 kcal/mol (25 260 cm−1) above the ground (X̃ 1A1) state of H2CO, which is in excellent agreement with the experimental observation of 72.03 kcal/mol (25 194 cm−1). The dissociation limit (H⋅+HCO⋅) is located at 86.3 kcal/mol (30 170 cm−1) above the ground state of H2CO, which is again in good agreement with the two experimentally determined values of 86.57 kcal/mol (30 280 cm−1) and 86.71 kcal/mol (30 328.5 cm−1). With the same method the triplet dissociation transition state lies 92.4 kcal/mol (32 300 cm−1) above the ground state. Consequently, the activation energy for the dissociation reaction of H2CO on the triplet surface is determined ab initio to be 18.9–20.1 kcal/mol (6620–7040 cm−1) (including an estimated error bar of 1.2 kcal/mol or 420 cm−1). The zero-point vibrationally corrected exit barrier height is predicted to be 4.9–6.1 kcal/mol (1710–2130 cm−1). These newly predicted energies are consistent with the recent experimental observations by the Moore group at University of California-Berkeley (1987) and by the Wittig group at University of Southern California (1997).
Fock-space coupled-cluster method in the intermediate Hamiltonian formulation: Model with singles and doubles
Authors: Meissner Leszek
Published: July 2002

The intermediate Hamiltonian technique is applied to reformulate the Fock-space coupled-cluster (FSCC) method with singles and doubles. A version of FSCC designated to direct calculation of excitation energies is considered. The presented scheme offers several advantages over those used so far like simplification of the equations, smooth convergence, and the possibility of one-root calculation. Results of some illustrative numerical examples are also shown where comparison with other methods as well as with the full configuration interaction results is available.
Very strong hydrogen bonds in neutral molecules: The phosphinic acid dimers
Authors: González Leticia , Mó Otilia , Yáñez Manuel , Elguero José
Published: July 2002

Ab initio molecular orbital and density functional theories have been used to study the structures and binding energies of the dimers of phosphinic acid (PA) and its dimethyl derivative (DMPA). For the first compound we have located all possible minima of the potential energy surface, while for the second only the most stable dimer was considered. The geometries were fully optimized at the MP2(full)/6-31+G(d,p) and B3LYP/6-31+G(d,p) levels of theory. The harmonic vibrational frequencies were evaluated at the same levels, while the final energies were obtained using a B3LYP/6-311+G(3df,2p) approach. Both phosphinic acid and its dimethyl derivative form cyclic dimers in the gas phase, where the two monomers are held together by hydrogen bonds (HBs) which are significantly stronger than those found for their carboxylic analogs. The estimated dimerization enthalpies for PA (23.2) and DMPA (23.2 kcal/mol) are the highest reported so far for neutral homodimers in the gas phase and almost twice those measured for formic and acetic acid dimers. For the particular case of DMPA this estimated value is in very good agreement with the experimental one (23.9±6 kcal/mol). As a consequence of the large strength of the HBs, the activation barriers associated with the concerted double proton transfer are also sizably smaller than those predicted for their carboxylic analogs. These barriers become negligibly small when zero point energy (ZPE) corrections are taken into account and therefore tunneling must be very efficient. The calculated harmonic vibrational frequencies for the most stable DMPA dimer are consistent with its experimental infrared (IR) spectrum in the gas phase, which shows a characteristic ABC structure of the ν(OH) band, typically associated with strongly hydrogen bonded complexes. Internal cooperative effects are not negligible in the case of phosphinic acid dimers, and the P=O⋯H hydrogen bonds in the global minimum are about 1.0 kcal/mol stronger than those found in other stable dimers where only one of these linkages exists.
Diffusion and reaction in regular arrays of spheres
Authors: Lu Shih-Yuan
Published: July 2002

The overall rate constant of reaction of diffusing species in regular arrays of spherical particles is investigated. The surface reaction occurring on the particle surface is of finite rate. Calculation results for three common spherical arrays: Simple cubic (SC), face-centered cubic (fcc), and body-centered cubic (bcc) arrays, are illustrated and studied. The normalized overall rate constant, k/k0, is found to be a function of the particle volume fraction, a dimensionless parameter P characterizing the relative rate of diffusive transport versus surface reaction, and the array structure. When the process is diffusion limited (P=0), results from the present development agree very well with those from first passage time simulations. When it is surface reaction limited (P→∞), the normalized overall rate constant is shown to exactly equal 1/(1−f ), independent of the system structure. For general P, results of the present study conform well with Torquato and Avellaneda’s upper bound [J. Chem. Phys. 95, 6477 (1991)]. The normalized overall rate constant increases with increasing f, but decreases with increasing P. For the three arrays, the magnitudes of k/k0, at the same f and P, are in the order of bcc≈fcc>SC. As P increases, the structural effect on k/k0 weakens.
Charge-overlap effect on the electronic transitions in moderate-energy collisions between closed-shell particles with rare-gas structure
Authors: Kita S. , Gotoh S. , Hasegawa T. , Shimakura N.
Published: July 2002

A potential model, which is represented by the overlap of electron clouds of interacting particles, has been applied to discuss the diabatic potential crossings which lead to one- and two-electron transitions in moderate-energy collisions of closed-shell particles. The crossings evaluated with the potential model interpret reasonably well both differential scattering experiments and ab initio calculations. In the model, the potential crossings originate from the hole(s) produced by the promotion of electron(s) in the outermost shell(s) of incident particle(s).
Phase coexistence of neutral polymer gels under mechanical constraint
Authors: Suzuki A. , Ishii T.
Published: July 2002

We have measured the equilibrium diameter of cylindrical poly(N-isopropylacrylamide) (NIPA) gels with submillimeter diameters under a mechanical constraint along the uniaxial direction. The linear swelling curve, diameter vs temperature, was obtained in the vicinity of the volume phase transition temperature under isometric constraint: both ends of the gel were fixed at rest (stress was zero) in a swollen state at 30 °C (a few degrees below the transition temperature). Thereafter, the temperature was gradually changed by keeping the uniaxial length constant (to the fixed length). It was demonstrated that the gel could take a coexistent state at the transition point and remain stable for several days. This time was much longer than the characteristic relaxation time of the phase transition in this tiny gel. The phase coexistence observed here was caused by stress inhomogeneity along the uniaxial direction due to the mechanical constraint. A collapsed to swollen phase transition induced by uniaxial stress at a fixed temperature was also presented. It was shown that the ratio of the swollen portion to the total length could be controlled by the degree of elongation: when the elongation was slightly increased or decreased, the phase boundary between the swollen and collapsed phases was accordingly shifted to increase or decrease the swollen state, respectively. The ratio of the swollen to the collapsed phase in the case of stress-induced coexistence is discussed herein in terms of a phase diagram (diameter vs elongation), and a simple phase selection rule is presented.
Self-consistent description of a metal–water interface by the Kohn–Sham density functional theory and the three-dimensional reference interaction site model
Authors: Kovalenko Andriy , Hirata Fumio
Published: July 2002

We have developed a self-consistent description of an interface between a metal and a molecular liquid by combination of the density functional theory in the Kohn–Sham formulation (KS DFT) for the electronic structure, and the three-dimensional generalization of the reference interaction site model (3D RISM) for the classical site distribution profiles of liquid. The electron and classical subsystems are coupled in the mean field approximation. The procedure takes account of many-body effects of dense fluid on the metal–liquid interactions by averaging the pseudopotentials of liquid molecules over the classical distributions of the liquid. The proposed approach is substantially less time-consuming as compared to a Car–Parrinello-type simulation since it replaces molecular dynamics with the integral equation theory of molecular liquids. The calculation has been performed for pure water at normal conditions in contact with the (100) face cubic centered (fcc) surface of a metal roughly modeled after copper. The results are in good agreement with the Car–Parrinello simulation for the same metal model. The shift of the Fermi level due to the presence of water conforms with experiment. The electron distribution near an adsorbed water molecule is affected by dense water, and so the metal–water attraction follows the shapes of the metal effective electrostatic potential. For the metal model employed, it is strongest at the hollow site adsorption positions, and water molecules are adsorbed mainly at the hollow and bridge site positions rather than over metal atoms. Layering of water molecules near the metal surface is found. In the first hydration layer, adsorbed water molecules are oriented in parallel to the surface or tilted with hydrogens mainly outwards the metal. This orientation at the potential of zero charge agrees with experiment.
Dissociative excitation of HCOOH by single-vacuum ultraviolet and two-ultraviolet photon
Authors: Tabayashi Kiyohiko , Aoyama Jun-ichi , Matsui Masafuyu , Hino Takashi , Saito Ko
Published: July 2002

Dissociative excitation processes of HCOOH in the vacuum ultraviolet (VUV) region were studied by single-VUV photon with synchrotron radiation source and by two-ultraviolet (UV) photon with KrF excimer laser. In the VUV dissociation, fluorescence excitation cross sections for the OH(A) and HCOO* were separately determined in the 106–155 nm region. The branching fraction was found to be a function of the VUV excitation wavelength. The magnitude is σOH(A)/[σOH(A)+σHCOO*]=0.13 at 124.5 nm and gradually increases to 0.39 at 110 nm. In the UV multiphoton dissociation at 249 nm, OH(A) and HCOO* fragments were also identified by a fluorescence spectrum. The production of OH(A) was shown to take place in the two-UV photon absorption of HCOOH. Nascent rotational and vibrational (V/R) state distributions of OH(A 2Σ+) produced via the photodissociation at a single excitation energy of 9.96 eV (124.5×1/249 nm×2), HCOOH+nhν(n=1,2)→HCO+OH(A 2Σ+), were determined by simulation analysis of the dispersed fluorescence spectra. The internal state distributions were found to be of the relaxed type, and rotational distribution could be approximated by a Boltzmann distribution. One-VUV photon excitation gave the best-fit rotational temperature Tr(v′=0)=3000 K and vibrational population ratio Nv′=1/Nv′=0=0.14, while two-UV photon excitation showed Tr(v′=0)=2000 K with Nv′=1/Nv′=0=0.12. Possible mechanisms for the OH(A) formation by both excitation sources were examined based on simple theoretical models. The degree of internal excitation is not consistent with a direct dissociation on a repulsive surface, and neither is a dissociation from a long-lived intermediate state. The formation of OH(A 2Σ+) is interpreted as dissociation of an electronically excited intermediate state, leading to the formation of OH(A)+CHO, populated competitively via an electronic predissociation process. The substantially different V/R distributions observed are dependent on the excited precursor state initially accessed, and may result from the constraint in the competing predissociation step that follows.
Theoretical study of ammonia activation by M+ (M=Sc, Ni, Cu)
Authors: Nakao Yoshihide , Taketsugu Tetsuya , Hirao Kimihiko
Published: July 2002

The reactions of the first-row transition metal cations, Sc+ (3D,1D), Ni+ (2D), Cu+ (1S), with NH3 have been studied by the multiconfigurational and multireference-based theories, to clarify the similarities and differences in the reactivity of early (Sc+) and late (Ni+, Cu+) transition metal cations. In all the cases, the ion–dipole complex, MNH3+, is initially formed with a C3v symmetry structure, which is the most stable complex in the respective potential energy surfaces except for Sc+ (1D). The M+–NH3 binding energy was evaluated as 42.4, 37.8, 50.9, and 48.1 kcal/mol for Sc+ (3D), Sc+ (1D), Ni+, and Cu+, respectively. In the second step, M+ is expected to activate one N–H bond of NH3, leading to the insertion complex, HMNH2+. In Sc+ (3D,1D), three different stationary points of HScNH2+, i.e., Cs (in-plane), Cs (out-of-plane), and C2v structures, were located, which correspond to a minimum point, a first-order saddle point, and a second-order saddle point, respectively. In these complexes, the singlet state originating from Sc+ (1D) is largely stabilized compared to the triplets. The singlet HScNH2+ (in-plane) is calculated to be the most stable compound. There are three dissociation channels from HScNH2+, i.e., →ScNH2++H, →ScH++NH2, and →ScNH++H2. The third dissociation occurs through the transition state of a four-centered structure, with a small activation barrier of 23 kcal/mol, in both singlet and triplet surfaces. As to the late transition metal cations Ni+ and Cu+, there is no intermediate complex of HMNH2+, thus, all the dissociations occur through highly vibrational excitations of MNH3+. The calculated results are consistent with experimental observations.
Vibrational spectroscopy of methanol and acetonitrile clusters in cold helium droplets
Authors: Behrens Michael , Fröchtenicht Ralf , Hartmann Matthias , Siebers Jörg-Gerald , Buck Udo , Hagemeister Fredrick C.
Published: July 2002

Infrared molecular beam depletion spectroscopy of small methanol and acetonitrile clusters embedded in large helium clusters has been studied in the spectral region of the CO stretch and the CH3 rock mode from 1023 to 1059 cm−1. The results are compared with the experimental spectra of the corresponding free clusters generated in adiabatic expansions and calculations based on density functional theory or empirical potential models. For methanol clusters, the two types of experimental results are the same for the dimer and trimer structure. Different isomers are found in cold helium for the tetramer and pentamer, namely a monomer and dimer attached to a cyclic trimer. For acetonitrile clusters in helium, aside from the dimer, different structures are observed. The spectra from the trimer to the hexamer are dominated by structures which contain the antiparallel dimer as building block with D2d symmetry for the tetramer. They do not correspond to the minimum configurations observed for the free clusters. The fragmentation of the two cluster groups in helium droplets by electron impact ionization is discussed.
Simulation of a single polymer chain in solution by combining lattice Boltzmann and molecular dynamics
Authors: Ahlrichs Patrick , Dünweg Burkhard
Published: July 2002

In this paper we establish a new efficient method for simulating polymer–solvent systems which combines a lattice Boltzmann approach for the fluid with a continuum molecular-dynamics (MD) model for the polymer chain. The two parts are coupled by a simple dissipative force while the system is driven by stochastic forces added to both the fluid and the polymer. Extensive tests of the new method for the case of a single polymer chain in a solvent are performed. The dynamic and static scaling properties predicted by analytical theory are validated. In this context, the influence of the finite size of the simulation box is discussed. While usually the finite size corrections scale as L−1 (L denoting the linear dimension of the box), the decay rate of the Rouse modes is only subject to an L−3 finite size effect. Furthermore, the mapping to an existing MD simulation of the same system is done so that all physical input values for the new method can be derived from pure MD simulation. Both methods can thus be compared quantitatively, showing that the new method allows for much larger time steps. Comparison of the results for both methods indicates systematic deviations due to nonperfect match of the static chain conformations.
Solute molecular dynamics in a mesoscale solvent
Authors: Malevanets Anatoly , Kapral Raymond
Published: July 2002

A hybrid molecular dynamics (MD) algorithm which combines a full MD description of solute–solute and solute–solvent interactions with a mesoscale treatment of solvent-solvent interactions is developed. The solvent dynamics is modeled on a mesoscale level by coarse graining the system into cells and updating the velocities of the solvent molecules by multiparticle collisions within each cell. The solvent dynamics is such that the correct hydrodynamic equations are obtained in the macroscopic limit and a Boltzmann distribution of velocities is established in equilibrium. Discrete-time versions of the hydrodynamic equations and Green–Kubo autocorrelation functions are derived. Between the discrete-time solvent–solvent collisions the system evolves by the classical equations of motion. The hybrid MD scheme is illustrated by an application to the Brownian motion of a nanocolloidal particle in the mesoscale solvent and concentrated nanocolloidal suspensions.
Theoretical principles of near-field optical microscopies and spectroscopies
Authors: Dereux Alain , Girard Christian , Weeber Jean-Claude
Published: July 2002

This paper deals with the principles of detection of optical signals near a surface in a manner permitting the mapping of the distribution of the fields close to various kinds of illuminated samples. We begin with a discussion of the main physical properties of the optical fields near a surface in the absence of any probe tip. This mainly concerns phenomena involving evanescent waves for which the local decay lengths are governed not only by the sizes but also by the intrinsic properties of the surface structures. The interpretation of the detection process is reviewed on the basis of a discussion about the possibility of establishing direct comparisons between experimental images and the solutions of Maxwell equations or the electromagnetic local density of states.
Perturbation of Schrödinger Hamiltonians by measures—Self-adjointness and lower semiboundedness
Authors: Brasche Johannes
Published: July 2002

We study the Hamiltonians for nonrelativistic quantum mechanics in one dimension, in terms of energy forms ∫‖df/dx‖2 dx+∫‖ f ‖2 d( μ −ν), where μ and ν are positive, not necessarily finite measures on the real line. We cover, besides regular potentials, cases of very singular interactions (e.g., a particle interacting with an infinite number of fixed particles by ‘‘delta function potentials’’ of arbitrary strengths). We give conditions for lower semiboundedness and closability of the above energy forms, which are sufficient and, for certain classes of potentials (e.g., μ−ν a signed measure), also necessary. In contrast to the results in other approaches, no regularity conditions and no restrictions on the growth of the measures μ and ν at infinity are needed.
On the stable analytic continuation with a condition of uniform boundedness
Authors: Stefanescu I. Sabba
Published: July 2002

It is shown that, if h(x) is any continuous function defined on some interval [−a,b]⊆(−1,1) of the real axis, then, in general, its best L2 approximant, in the class of functions holomorphic and bounded by unity in the unit disk of the complex plane, is a finite Blaschke product. An upper bound is placed on the number of factors of the latter and a method for its construction is given. The paper contains a discussion of the use of these results in performing a stable analytic continuation of a set of data points under a condition of uniform boundedness, as well as some numerical examples.
The fusion of R-matrices using the Birman–Wenzl–Murakami algebra
Authors: MacKay N. J.
Published: July 2002

The action of the Birman–Wenzl–Murakami algebra is used as the centralizer of SOq(N) and Spq(N) to fuse Jimbo’s trigonometric R-matrices and thus to construct new solutions of the Yang–Baxter equation.
Reshetikhin’s formula for the Jones polynomial of a link: Feynman diagrams and Milnor’s linking numbers
Authors: Rozansky L.
Published: July 2002

Feynman diagrams are used to prove a formula for the Jones polynomial of a link derived recently by Reshetikhin. This formula presents the colored Jones polynomial as an integral over the coadjoint orbits corresponding to the representations assigned to the link components. The large k limit of the integral can be calculated with the help of the stationary phase approximation. The Feynman rules allow one to express the phase in terms of integrals over the manifold and the link components. Its stationary points correspond to flat connections in the link complement. A relation between the dominant part of the phase and Milnor’s linking numbers is conjectured. It is checked explicitly for the triple and quartic numbers by comparing their expression through the Massey product with Feynman diagram integrals.
The polygon quantum-billiard problem
Authors: Liboff Richard L.
Published: July 2002

The present work addresses the quantum polygon billiard problem with attention given to analytic and degeneracy properties of energy eigenstates. The ‘‘polygon ground-state theorem’’ is proven which states that the only polygons that contain respective ground states that are analytic in the closed domain of the entire polygon are the ‘‘elemental polygons’’ (defined in the text). The ‘‘polygon first excited-state theorem’’ is established which states that for every N-sided regular polygon, N equivalent first excited states exist, each of which contains a nodal curve that is a line of mirror symmetry of the related polygon. A vector description of nodal diagonal eigenstates is introduced to establish the second component of this theorem which indicates that the space of first excited states for the N-sided regular polygon is spanned by any two of these N nodal-diagonal eigenstates (i.e., the first excited state is twofold degenerate). At various levels of the discussion attention is drawn to the correspondence between the quantum and classical solutions for these configurations. Thus, for example, correspondence is demonstrated in the common source of integrability in the classical and quantum domains for three inclusive, nonoverlapping classes of polygons. Stemming from the preceding conclusions, a discussion is included on the possibility of employing transverse magnetic (TM) modes in a metal waveguide to determine if an equivalent quantum billiard configuration is chaotic.
On special classes of n-algebras
Authors: Vainerman L. , Kerner R.
Published: July 2002

We define n-algebras as linear spaces on which the internal composition law involves n elements: m:V⊗n■V. It is known that such algebraic structures are interesting for their applications to problems of modern mathematical physics. Using the notion of a commutant of two subalgebras of an n-algebra, we distinguish certain classes of n-algebras with reasonable properties: semisimple, Abelian, nilpotent, solvable. We also consider a few examples of n-algebras of different types, and show their properties.
Deriving the standard model from the simplest two-point K cycle
Authors: Wulkenhaar R.
Published: July 2002

Basing on a differential algebra over the simplest two-point K cycle and graded Lie algebras of homomorphisms of finite projective modules, we derive the classical action of the standard model. This construction uses both the general framework of noncommutative geometry developed by Connes and ideas of the Mainz–Marseille approach to model building. We get a prediction of the Weinberg angle and constraints between the fermion masses and the masses of the W and Higgs bosons on tree level, which differ from the relations obtained by Kastler and Schücker for the quaternionic K cycle.
On the geometric quantization of Jacobi manifolds
Authors: de León Manuel , Marrero Juan C. , Padrón Edith
Published: July 2002

The geometric quantization of Jacobi manifolds is discussed. A natural cohomology (termed Lichnerowicz–Jacobi) on a Jacobi manifold is introduced, and using it the existence of prequantization bundles is characterized. To do this, a notion of contravariant derivatives is used, in such a way that the procedure developed by Vaisman for Poisson manifolds is naturally extended. A notion of polarization is discussed and the quantization problem is studied. The existence of prequantization representations is also considered.
Kontsevich integral for Vassiliev invariants from Chern–Simons perturbation theory in the light-cone gauge
Authors: Labastida J. M. F. , Pérez Esther
Published: July 2002

We analyze the structure of the perturbative series expansion of Chern–Simons gauge theory in the light-cone gauge. After introducing a regularization prescription that entails the consideration of framed knots, we present the general form of the vacuum expectation value of a Wilson loop. The resulting expression turns out to give the same framing dependence as the one obtained using nonperturbative methods and perturbative methods in covariant gauges. It also contains the Kontsevich integral for Vassiliev invariants of framed knots.
Exactly solvable models of δ′-sphere interactions in nonrelativistic quantum mechanics
Authors: Hounkonnou M. N. , Hounkpe M. , Shabani J.
Published: July 2002

We introduce and perform a systematic study of a new exactly solvable model of sphere interactions in quantum mechanics : the δ′ interaction, formally given by −Δ+αδ′(|x|−R),  x∈R3,  R>0,  α∈R. We also consider the cases of a δ′ plus a Coulomb interaction and finitely many δ′-sphere interactions with support on concentric spheres. For all these models, we provide basic properties and discuss the stationary scattering theory. We also briefly discuss the δ′-sphere interaction of the second type.
Topological tensor current of p̃-branes in the φ-mapping theory
Authors: Duan Yishi , Fu Libin , Jia Guang
Published: July 2002

We present a new topological tensor current of p̃-branes by making use of the φ-mapping theory. It is shown that the current is identically conserved and behaves as δ(φ⃗), and every isolated zero of the vector field φ⃗(x) corresponds to a “magnetic” p̃-brane. Using this topological current, the generalized Nambu action for multi p̃-branes is given, and the field strength F corresponding to this topological tensor current is obtained. It is also shown that the magnetic charges carried by p̃-branes are topologically quantized and labeled by Hopf index and Brouwer degree, the winding number of the φ mapping.
Absolute and convective instability of a cylinder wake
Authors: Yang X. , Zebib A.
Published: July 2002

Local linear stability theory is used to investigate the stability characteristics of a circular cylinder wake at values of the Reynolds number, Re, up to 45. Both the Orr–Sommerfeld and Rayleigh analyses for complex frequency and wavenumber are applied to computed basic, steady wake profiles. These velocity profiles are Navier–Stokes solutions of a uniform, incompressible viscous flow around a cylinder, which we obtain by a spectral method. The distributions of convective and absolute instabilities in the near wake behind the cylinder are determined for different Reynolds numbers and locations. The study shows that an absolutely unstable region begins to form at Re of about 20, and grows with increasing Re. Hence the onset of global response of instability, which is known to occur at Re of about 40, must be characterized by a critical length of an absolutely unstable region. A criterion to determine the critical Reynolds number Rec and the preferred frequency, from local linear stability analysis, is proposed. The predicted values of Rec and frequency for global response agree well with the prediction of nonparallel stability analysis and experimental results.