bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotational magnetic moment of the Na2 molecule in A 1Σ+u state: Perturbation effects
Authors: Stolyarov A. V. , Klincare I. P. , Tamanis M. Ya. , Auzin’sh M. P. , Ferber R. S.
Published: July 2002

Values and signs of Landè factors (g) have been measured for four rovibronic levels of the A 1Σ+u state of the Na2 molecule. Because of a very small product gτ∼10−12 s, the level crossing signal of a dispersion shape was employed using circular light polarization and mutually orthogonal excitation, observation of laser induced fluorescence, and external magnetic field directions. The effects of the b 3Πu, B 1Πu, and a 3Σ+u states on the g factors of the Na2(A 1Σ+u), as well as their dependence on vibrational and rotational quantum numbers, have been theoretically analyzed. An analytical connection has been found between the constants of Λ doubling (q) and Landè factors of the interacting singlet states. Simple expressions have been found for estimating the effects of distant electronic states on q and the g factors without summing over bounded and integrating over continuum levels of the perturbing state.
Ab initio variational transition-state-theory reaction-rate calculations for the gas-phase reaction H+HNO→H2+NO
Authors: Soto Maribel R. , Page Michael
Published: July 2002

We report ab initio calculations of the potential-energy surface and variational transition-state-theory calculations of the temperature dependence of the reaction rate for the chemical reaction H+HNO→H2+NO. This reaction is very exothermic and proceeds nearly without activation. Steepest-descent reaction pathways on the potential-energy surface for the abstraction reaction are presented both at the five-electron, five-orbital complete-active-space self-consistent-field level, and at the single- and double-substitution configuration-interaction level. Multireference configuration-interaction calculations are reported for several points along each of the reaction pathways. All electronic structure calculations employ a correlation consistent polarized valence double-zeta basis set. Canonical variational transition-state-theory calculations using the calculated potential-energy surface information predict a rate constant for this reaction that is an order of magnitude greater than that predicted previously by flame experiments. Our calculated rate constant is well represented by the three-parameter expression, k(T)=7.406×10−13T0.720 exp(−0.655/0.001 99T) cm3 molecules−1 s−1 over the range 200–3000 K.
Determination of the internal state distribution of NO(X 2Π) produced in the O(3P)+NH(X 3Σ−) reaction
Authors: Huang Yu-Lin , Dagdigian Paul J.
Published: July 2002

The internal state distribution of the NO product from the O(3P)+NH(X 3Σ−) reaction has been determined from a laser fluorescence experiment in a cell at a total pressure of 60 mTorr. The O atom and the NH reagents were prepared in a microwave discharge in oxygen and by the two-photon 193 nm photolysis of ammonia, respectively. The NO product was observed in the vibrational levels v=1–8 by laser fluorescence excitation in A 2Σ+–X 2Π bands. The nascent vibrational state distribution was found to be monotonically decreasing vs increasing v. The v=1 rotational state distribution, extrapolated back to zero photolysis-probe delay, could be parametrized as a 1130±50 K Boltzmann distribution. Very little of the available energy is found as internal excitation of the NO product. The O+NH→H+NO reaction is expected to proceed by the formation and decay of a short-lived HNO complex. The observed NO vibrational state distribution is interpreted in terms of a Franck–Condon model involving the overlap of vibrational wave functions for the NO stretch coordinate in the HNO complex with those for vibration in the free NO product. The NO rotational state distribution is governed largely by kinematic constraints in this H+HL→HH+L reaction, where H and L are heavy and light atoms, respectively.
The role of inversion transitions in K(150p)/NH3, ND3 collisions
Authors: Ling X. , Frey M. T. , Smith K. A. , Dunning F. B.
Published: July 2002

Data for K(150p)/NH3, ND3 collisions are presented showing that near-resonant energy transfer associated with transitions between different inversion levels of the target is an important process leading to collisional state changing.
Observation of A 1Σu+∼b 3Πu interaction in g factors of weakly coupled Na2 A 1Σu+ state levels
Authors: Stolyarov A. V. , Klincare I. P. , Tamanis M. Ya. , Ferber R. S.
Published: July 2002

The b 3Πu–A 1Σu+ interaction in Na2 as manifested by A 1Σu+ state Lande factors was investigated. Landé factors gA were measured from the dispersion shape Hanle signal in fluorescence excited by 647.1 and 676.4 nm Kr+-laser lines. The partial contribution gAb due to weak A∼b interaction was extracted from measured g factors for vA, J levels with vA between 8 and 29. The electronic part of the spin–orbit (SO) interaction matrix element Heso = 5.86± 0.20 cm−1 was determined by deperturbation fitting of gAb values. Possible error sources in A 1Σu+ state g-factor treatment due to imperfection in the physical model employed, inaccuracy of existing set of b 3Πu state molecular constants and nonlinear magnetic field effects were analyzed.
Escape rates in bistable systems with position-dependent friction coefficients
Authors: Compiani Mario
Published: July 2002

In this paper we consider the generalization of the Kramers’ model of chemical reactions to the case that the friction coefficient γ(x) depends on the reaction coordinate x. Extending previous efforts the escape rate is exactly evaluated in the high-friction limit imposing on γ(x) much milder conditions than used so far in the literature. The ensuing rate retains the Kramers’ form and reproduces the renormalization effect of the damping coefficient which has been reported in laboratory experiments on chemical reactions. The origin of variable friction coefficients is then discussed within the framework of a multidimensional Markovian model and ascribed to the nonlinear coupling of the reaction coordinate with faster auxiliary variables. Finally, the implications of our results for the rate of ligands migration in proteins are briefly considered.
Bulk properties from finite-cluster calculations. VII. Accurate ab initio calculations on the Peierls distortions in polyacene
Authors: Cioslowski Jerzy
Published: July 2002

Properties of both the undistorted and the Peierls-distorted polyacene polymers are obtained with the help of Padé approximants from the results of HF/6–311G** and MP2/6–311G** electronic structure calculations on the respective finite clusters. At the Hartree–Fock level, the (E)-distorted structure has the lowest energy per unit cell. At the correlated level, the undistorted structure is slightly favored. The C–C bond alternation is found to be more pronounced in the (Z)-distorted isomer than in the (E)-distorted one.
Tungsten hexahydride (WH6). An equilibrium geometry far from octahedral
Authors: Shen Mingzuo , Schaefer Henry F. , Partridge Harry
Published: July 2002

Ab initio all-electron quantum mechanical methods were applied to the tungsten hexahydride (WH6) molecule using very large basis sets. Seven distinct structures were investigated with full geometry optimizations at the self-consistent field (SCF) level of theory. The effects of electron correlation were estimated with second-order Mo/ller–Plesset perturbation theory (MP2), modified coupled pair functional (MCPF) method, and the coupled cluster with single and double excitations (CCSD) method, and relativistic effects estimated using the Cowan–Griffin (mass–velocity and Darwin) corrections. Our results suggest that the ground state of the tungsten hexahydride (WH6) molecule is a closed-shell triangular prism belonging to the C3v point group, with a set of three hydrogens stacked on top of another set of three hydrogens. From the perspective of inorganic chemistry, it is truly remarkable that the octahedral structure lies 130 kcal/mol above the C3v ground state. Furthermore, these results imply major qualitative differences between WH6 and the well-known W(CH3)6 molecule. Another C3v structure, with one set of three hydrogens staggered with respect to the other set, is energetically nearby. With MCPF and relativistic corrections at the SCF optimized geometry, a third low-lying structure belonging to the C5v point group is virtually degenerate with the triangular prism C3v structure.
The zero-kinetic-energy photoelectron spectrum of nitrogen between 15.5 and 26.5 eV
Authors: Merkt F. , Guyon P. M.
Published: July 2002

The zero-kinetic-energy (ZEKE) photoelectron spectrum of nitrogen has been measured between 15.5 and 26.5 eV using synchrotron radiation at a resolution up to 5 meV. The rotational contours obtained for the various vibrational components of the X 2Σg+(v+=0−2)←X 1Σg+(v=0) transitions are in good agreement with the results obtained using extreme ultraviolet (XUV) laser sources [Merkt and Softley, Phys. Rev. A 46, 302 (1992)], and appear to be strongly perturbed by autoionization. The X and A states of the ion are observed up to v+=5 and 9, respectively, and the non-Franck–Condon intensity distribution of these progressions originates in part from autoionization of Rydberg states belonging to the series converging to the A and B states of the ion. In contrast, the intensities of the v+=0,1 components of the B state correspond well to Franck–Condon factors between neutral and ionic species. In addition, the rotational contour obtained for the B(v+=0) state is found to be identical to that measured by He i photoelectron spectroscopy (PES) by Baltzer et al. [Phys. Rev. A 46, 315 (1992)]. Autoionization does therefore not seem to be a significant source of intensity in this region. The ZEKE spectrum between 23 and 26.5 eV shows considerably more structure than expected on the basis of previous lower resolution photoelectron spectroscopic studies and reveals a number of differences compared to a recent high-resolution He ii photoelectron spectrum [Baltzer et al., Phys. Rev. A 46, 5545 (1992)]. At least three vibrational progressions are observed. The dominant one is attributed to the C state. Possible assignments for the other two progressions are discussed.
Microwave spectrum of chloroborane, BH2Cl
Authors: Kawashima Yoshiyuki , Takeo Harutoshi , Sugie Masaaki , Matsumura Chi , Hirota Eizi
Published: July 2002

The microwave spectrum of BH2Cl existing as an intermediate in the reaction between diborane and boron trichloride was observed. The lifetime of this molecule in a brass waveguide cell was found to be less than 1 s. The observed spectrum indicated the molecule to be planar with C2v symmetry. The rotational constants were determined for 11BH2 35Cl, 11BH2 37Cl, 10BH2 35Cl, and 11BD2 35Cl, and were employed to calculate the structure parameters (rs):r(B–H)=1.1916(2) Å, r(B–Cl)=1.7353(5) Å, and ∠HBH=124.22(3)°. The quadrupole coupling constants of Cl, an extraordinarily small χzz and a large η compared to related molecules, were obtained. The dipole moment was determined to be μa=0.75 (5) D for BH2Cl.
Exponential split operator methods for solving coupled time-dependent Schrödinger equations
Authors: Bandrauk André D. , Shen Hai
Published: July 2002

Coherent excitation of molecules with laser pulses are usually described by coupled time-dependent linear parabolic partial differential equations, i.e., Schrödinger equations. Numerical solutions of these equations based on splitting (factorization) of the exponential form of the evolution operator or time-dependent propagator are examined for accuracy of amplitude and phase as a function of various unitary splitting schemes.
Unified kernel function approach to two-center integrations in quantum-chemical calculations
Authors: Hierse W. , Oppeneer P. M.
Published: July 2002

We present a unified kernel function treatment of the two-center integrals over atomic orbitals appearing in quantum chemical calculations on the basis of density functional theory. By using the Fourier transform method, we obtain analytical expressions for the kernel functions, from which actual integral values can be evaluated through twofold integration. As our only assumption is that the angular dependences of the atomic basis functions is given by spherical harmonics, our kernel functions are applicable to almost arbitrary types of analytically given or numerically tabulated atomic radial functions. In addition, we give some interesting new recurrence relations holding between parts of kernel functions, which enable an entirely recursive treatment of the angular momentum arguments. When applied to analytically given atomic basis functions, which contain a factor rν+l, a qualitatively new and general set of recurrence relations for auxiliary functions of two-center integrals and, in some cases, of the two-center integrals themselves, results. As an example, two-center overlap integrals over Slater-type orbitals are calculated recursively for higher angular momentum quantum numbers and compared with the results of a direct calculation. We expect the present recurrence relations to be superior to other existing recurrences with respect to angular momentum quantum numbers.
Diblock copolymer melts with rough surfaces
Authors: Tang Hai
Published: July 2002

The Ohta–Kawasaki free energy functional is used to study the influence of surface roughness on diblock copolymer melts above the bulk order–disorder transition temperature. First order perturbation calculations are presented for the concentration profile. For sinusoidal undulation, the profile correction is also sinusoidal along the surface, but oscillatory with exponentially decaying amplitude perpendicular to the surface. At the surface, this amplitude is proportional to both the substrate height variation amplitude and the surface concentration of the corresponding flat surface. The proportionality coefficient, which depends on the ‘‘bare’’ surface and the Flory interaction parameters, is always positive at high temperatures, but changes from positive to negative as the undulation wave vector goes from 0 to ∞ at lower temperatures. The two types of undulation response behavior is connected by a continuous transition, the first of such kind predicted for liquid–substrate systems to our knowledge. The proper undulation response behavior in the presence of the lamellar bulk phase is also suggested.
Applicability of symplectic integrator to classically unstable quantum dynamics
Authors: Takahashi Kin’ya , Ikeda Kensuke
Published: July 2002

Applicability of symplectic integrator (SI) to classically unstable (chaotic) quantum systems is examined, and accuracy and efficiency as a numerical integrator of Schrödinger equation are demonstrated. The second order SI is well known as the split operator method to molecular scientists. Recently, construction of higher order SIs has been developed by several authors. In the present paper, we compare systematically various higher order SI schemes by applying them to a simple quantum chaos system and emphasize the necessity of introducing higher order schemes. Although the higher order SIs have originally been invented for high precision computation of classical trajectories, it will more promisingly be applied to quantum systems. This is because the exponential instability is absent in quantum systems, but an extensive numerical test reveals that the accumulation of error accompanying a long time integration by SI reflects the stability of the system in the classical limit, and closer attention must be payed for classically chaotic quantum system in the semiclassical regime. It is in such a regime that higher order SI schemes work efficiently.
Acetonitrile pair formation in aqueous solution
Authors: Matsumoto Masakazu , Tanaka Hideki , Nakanishi Koichiro
Published: July 2002

Extended reference interaction site model (RISM) theoretical calculations and molecular dynamics simulation have been carried out for dilute aqueous solution of acetonitrile. Potential of mean force between two solute molecules was calculated. Two solute molecules tend to associate with each other by an attractive interaction between two negatively charged nitrogen atoms. It is found that ‘‘bifurcated hydrogen bonds’’ between a hydrogen atom on a water molecule and nitrogen atoms on acetonitrile molecules play an important role in the solute–solute interaction.
Some mathematical properties of the absolute area of a difference spectrum
Authors: Gaufrès R. , Arab Y.
Published: July 2002

Difference spectra may be used to measure wave number shifts or changes of bandwidths between two bands. Up to this date, the quantity taken into account to get the information embodied in a difference spectrum was the difference between the ordinates of the ‘‘peak’’ and the ‘‘valley.’’ A new treatment, well adapted to multichannel spectrometry and computer facilities, which uses the absolute area of the difference spectrum, has been proposed. Some mathematical properties of this quantity, especially its dependence upon the quantities of direct interest such as the wave number shift and/or the difference of bandwidth, are established, for every situation in which a spectral shift between two bands can be defined. This study has been carried out having in view Raman spectroscopy, but its results may be extended to any emission or scattering spectroscopy.
Spin-lattice relaxation of the hydrogen atom in a fused quartz
Authors: Ichikawa Tsuneki , Kurshev Vadim
Published: July 2002

The paramagnetic spin-lattice relaxation of hydrogen atoms in γ-irradiated fused quartz at 77 K was measured by an electron spin echo method. The relaxation was induced by dipole interactions between electron spins, so that the relaxation rate increased with the concentration of the hydrogen atoms. The observed relaxation was compared with theoretical one in which the spin diffusion and the phonon-induced simultaneous transition of the electron spin state and the local vibrational state of the hydrogen atoms in trapping sites were taken into account. The theoretical relaxation behavior was in perfect accordance with the observed one. The relaxation was ten orders of magnitude accelerated by the spin diffusion of the hydrogen atoms, which indicates that the spin diffusion plays a significant role on the spin-lattice relaxation of paramagnetic species with small g anisotropy.
Near-resonant vibrational energy transfer in ozone. Double-resonance measurements and calculations in the temperature range 200–300 K
Authors: Boursier C. , Menard-Bourcin F. , Menard J. , Doyennette L.
Published: July 2002

The near-resonant vibrational energy transfer mtlm;cO3(001)+O3(010)mtlm;c →mb;2qkmb;0 O3(011)+O3(000)+Δν=17 cm−1 has been investigated in the temperature range 200–300 K by means of a time-resolved double-resonance method using two CO2 lasers. The transfer rate coefficient k deduced from the measurements is compared to values calculated using a semiclassical model based on dipole (ν3)–dipole (ν3) interaction. The temperature dependences of the calculated and experimental values are in good agreement, the first ones being a little smaller than the second ones.
Complex formation between water and formamide
Authors: Engdahl Anders , Nelander Bengt , Åstrand Per-Olof
Published: July 2002

The infrared spectrum of the water–formamide complex in argon matrices has been recorded from 10 to 4000 cm−1. The interaction energy of the complex forming molecules has been calculated from a theoretical potential. One global and three different local minima have been found for this potential. Intermolecular vibration frequencies have been calculated for each minimum. The results are compared with the experimentally observed far infrared spectrum. In agreement with microwave measurements and ab initio calculations, the global minimum of the complex is found, both from calculations and experiment, to have a cyclic structure with water forming a hydrogen bond to the amide oxygen and receiving a hydrogen bond from an amide hydrogen. In addition to the cyclic complex, we observe one of the local minimum structures of the complex, where water accepts a hydrogen bond from the amide NH on the CH side of the amide.
115In hexadecapole interaction upper limit in InP: 31P–115In exchange and pseudodipolar coupling constants
Authors: Sears R. E. J. , Ni Q. W.
Published: July 2002

Using NMR multipulse methods an upper limit of 20 kHz was established for the 115In hexadecapole coupling constant in InP. From the orientation dependence of the 31P second moment the magnitude of the 31P–115In exchange coupling constant and the pseudodipolar coupling constant were obtained. The results are 0.17±0.04 kHz, and −0.30±0.07 or −0.99±0.07 kHz, respectively.
Ab initio study on quartet states of non-Kekulé-type molecules 1,3,5-trimethylenebenzene and 1,3,5-triaminobenzene trication
Authors: Yoshizawa Kazunari , Hatanaka Masashi , Matsuzaki Yoichi , Tanaka Kazuyoshi , Yamabe Tokio
Published: July 2002

The electronic structures of non-Kekulé-type isoelectronic molecules 1,3,5-trimethylenebenzene (TMB) and 1,3,5-triaminobenzene trication (TAB3+) having a threefold axis are discussed with the ab initio molecular orbital (MO) method. The quartet state with a D3h geometry is predicted to be the ground state of TMB and TAB3+. According to the Jahn–Teller theorem, the doublet 2E″ states of TMB and TAB3+ are subject to the first order distortions which remove the degeneracy. The quartet–doublet energy splittings are calculated for these non-Kekulé-type molecules at the second order Mo/ller–Plesset (MP2) perturbation level of theory. In TMB the 4A2″ state with a planar D3h geometry is predicted to lie well below the 2A2 state with a planar C2v geometry in which one of the methylene groups has a longer C–C bridge bond connecting with the benzene ring. Moreover, in TAB3+ the 4A2″ state with a planar D3h geometry also lies well below the 2A2 state. The 2A2–2B1 energy splittings on the pseudorotation potential surface overestimated at the unrestricted Hartree–Fock (UHF) level are corrected with the MP2 method.
The dissociative ionization of ethylene dimers, trimers, and tetramers studied by photoelectron photoion coincidence
Authors: Booze Jon A. , Feinberg Thomas N. , Keister Jeffrey W. , Baer Tomas
Published: July 2002

The dissociation dynamics of energy selected ethylene dimer ions is investigated by photoelectron photoion coincidence. The derived dissociation rates as a function of the ion internal energy were found to be identical to those previously measured for rotationally cold C4H+8 ions. This demonstrates that the ethylene dimer ions rearrange to the 2-butene isomers prior to dissociation. The ethylene dimer ions are produced by ionization of neutral ethylene dimer as well as by dissociative ionization of the neutral trimer. The statistical theory analysis of the kinetic energy released in the dissociative photoionization of the ethylene trimers indicates that these ions dissociate to the 2-butene ion. At the energies investigated, the evaporation of a single monomer unit from the trimer ion removes sufficient energy to prevent further dissociation of the C4H+8 ions. All of the ethylene trimer ion signal was a result of dissociative ionization of higher order clusters. Finally, we show that the ethylene tetramer ion evaporates an ethylene unit as the core polymerizes to a straight chain hexene ion.
Missing mode effect in resonance Raman excitation profile
Authors: Lee Soo-Y. , Chang Hau-Y.
Published: July 2002

Vibrational structure in the resonance Raman excitation profile (REP) of adenosinetriphosphate (ATP) in aqueous solution has been observed for both the 1333 and 1482 cm−1 modes, even though the absorption spectrum is structureless. The ∼2100 cm−1 spacing of the vibrational structure in both cases is much larger than the normal mode frequencies of the molecule. It is possible for such a vibrational structure to develop in a quasidiatomic (single mode) case, based on the reflection principle of the product wave functions ‖〈Q‖1〉〈0‖Q〉‖2 appropriate for the fundamental REP, if the (dimensionless) displacement Δ between excited and ground state potentials is sufficiently large (Δ≳1) and the absorptive component of the REP dominates. However, this is not the case for ATP, where we show that the observed vibrational structure is due to an interplay of multiple modes. This ‘‘missing mode effect’’ (MIME) for REP, as in the case of luminescence spectra of large molecules, can be explained using the time-dependent correlation function picture of electronic transitions. The harmonic model is used and it is shown how the MIME frequency in the REP arises. The phenomenon is illustrated with ATP, where the harmonic parameters are derived from its resonance Raman spectrum at an excitation frequency of 260 nm, which are then used to calculate the absorption and REPs to compare with the experimental results.
Bipolarons and the temperature dependence of exchange in the metal trihydrides
Authors: Hiller Eric M. , Harris Robert A.
Published: July 2002

We analyze a two-particle, two-site, tight-binding model linearly coupled to a harmonic bath in order to describe the temperature dependence of exchange in the metal trihydrides. We present a variational calculation that predicts two regimes: coherent, motionally narrowed exchange, and incoherent, quenched exchange. We show for an ohmic bath that the behavior rapidly switches from unquenched to quenched at a finite temperature that scales linearly with the correlation energy U and logarithmically with both bath coupling parameters and the zero-temperature exchange 4t2/U. We show that Zilm’s description of the temperature dependence of the spin coupling (Ref. 1) can be derived asymptotically and shows a geometric dependence of the ‘‘activation’’ energy that is absent in a finite basis approximation. The model proposed explains the temperature dependence of the exchange splittings observed by nuclear magnetic resonance for the metal trihydrides.
A modified Kirkwood coupling approach for free energy simulations in asymmetric molecular systems
Authors: Li T. W. , Munoz F. , Chimowitz E. H.
Published: July 2002

In this short note we describe a novel method, based upon the Kirkwood equation, for doing free energy simulations in asymmetric molecular systems typical of those encountered with supercritical fluid mixtures involving large organic species solvated in a lighter supercritical gas. The rationale behind this modified Kirkwood approach has been to develop simulation procedures that are both accurate and stable, by minimizing the perturbation to the system that is implicit to all perturbation-based simulation methods. A good example of an algorithm in this latter class of methods is the free energy perturbation method (FEP) which has been used in this work for comparative purposes with our proposed technique which we refer to as the modified Kirkwood coupling method (MKC). The MKC approach described here effectively makes the perturbation process in the simulation process vanishingly small and appears to result in a stable, accurate algorithm for free energy calculations in chemical mixtures. Numerical results supporting these conclusions are provided in two Lennard-Jones model systems at supercritical conditions.
Structure model of liquid water as investigated by the method of reciprocal space expansion
Authors: Iijima Takao , Nishikawa Keiko
Published: July 2002

A comparison between experimental and theoretical x-ray scattering intensities of liquid water has been carried out. In the region of 0–2.5 Å−1 of the scattering parameter, the theoretical intensities for a structure model were calculated by means of reciprocal space expansion. It has been shown that the ice I model by Narten and co-workers cannot account for the experimental intensities in this region where they had difficulty in calculating the theoretical intensities. The structure model has been revised so that the experimental intensities may be accounted for in the whole region of the scattering parameter. In the revised model, the six-membered ring lying perpendicular to the c axis is flapping, and as a result, the unit cell in the local-lattice structure is reduced to nearly a half of that for the ice I model. The lattice parameters of the hexagonal unit cell have been determined as a=4.47 Å and c=4.49 Å with the shortest O...O distance 2.91 Å. Water molecules occupy both frame sites and interstitial sites by equal chance. The hydrogen bonding distance between two neighboring frame sites is equivalent to that between a frame site and the nearest interstitial site. In calculating scattering intensities for a system composed of both vacant and occupied sites, the scattering theory for binary alloys has been applied and the finite zero-angle intensity due to density fluctuation has also been explained satisfactorily.
On the ratio T2/T1 for non-Ohmic spectral densities
Authors: Laird Brian B. , Chang Tsun-Mei , Skinner J. L.
Published: July 2002

Defining T1 and T2 to be the population and phase relaxation times, respectively, for a two-level system coupled to a bath, previous work for Ohmic spectral densities has shown, surprisingly, that in some instances T2≳2T1. This note shows that this inequality can also be realized for non-Ohmic spectral densities.
Packing rods on d-dimensional lattices: From direct enumeration to series expansions
Authors: Nemirovsky A. M. , Huston Shawn E. , Graham R. L. , Freed Karl F.
Published: July 2002

We consider the statistical properties associated with the packing of p self-avoiding rods of length M on a d-dimensional hypercubic lattice with N sites and periodic boundary conditions. The exact treatment for few (p≤4) rods is combined with information derivable from the lattice cluster theory (LCT) to obtain the exact analytic form for the free energy f per site. The thermodynamic limit of this free energy f is reexpressed as a series expansion about the zeroth order Flory mean field approximation. The expansion is in powers of the rod volume fraction φ=pM/N and contributions are retained through order φp (with p=4) for any M and d. The theory is compared with previous diagram based LCT calculations and with the DiMarzio approximation. Departures (in the thermodynamic limit) from the latter successful approximation appear at order φ4 and arise from correlations of four rods in configurations where the rods are not all parallel, correlations which are absent in the DiMarzio approximation. Our method uses computer enumerations to replace the time consuming task of evaluating the many-body diagrams of the LCT. The series for d≳1 are ill behaved in the large M limit and strongly indicate that resummations are required to obtain physically meaningful results.
Four component regular relativistic Hamiltonians and the perturbational treatment of Dirac’s equation
Authors: Sadlej A. J. , Snijders J. G. , van Lenthe E. , Baerends E. J.
Published: July 2002

By combining the ideas of the direct perturbation theory approach to the solution of the Dirac equation with those underlying the regular expansion as used to obtain the two-component Chang–Pélissier–Durand Hamiltonian, a four-component form of the regular expansion is proposed. This formulation lends itself naturally to systematic improvement by a nonsingular form of perturbation theory. Alternatively it can be viewed as a double perturbation version of direct perturbation theory, where relativistic effects on the Hamiltonian and the metric are considered separately and the Hamiltonian perturbation is summed to infinite order. The scaling procedure that was earlier shown to be exact in the case of a hydrogenic potential and that greatly improved the core orbital energies, is found to follow naturally from the current formulation. The accuracy of the various approximations to the wave functions is assessed with respect to several radial expectation values weighing different regions in the uranium atom as a test case.
Isotope and temperature effects on the hyperfine interaction of atomic hydrogen in liquid water and in ice
Authors: Roduner Emil , Percival Paul W. , Han Ping , Bartels David M.
Published: July 2002

The Fermi contact hyperfine interaction of hydrogen isotopes in liquid and solid water is below the vacuum value, shows a mass dependence, and has a negative temperature coefficient in the liquid. Furthermore, it shows a pronounced solvent isotope effect in H2O/D2O mixtures. This behavior is analyzed in terms of Dalgarno–Lewis perturbation theory for the atom in a spherical parabolic solvent potential and with a phenomenological model for spin delocalization onto solvent molecules. The results support previous suggestions that hydrogen atoms induce clathrate-like cages in liquid water. These may resemble the static structures of noble gas clathrates except that they are of transient dynamic nature in liquid water.
Application of a distributed nucleus approximation in grid based minimization of the Kohn–Sham energy functional
Authors: Iyer Karthik A. , Merrick Michael P. , Beck Thomas L.
Published: July 2002

In the distributed nucleus approximation we represent the singular nucleus as smeared over a small portion of a Cartesian grid. Delocalizing the nucleus allows us to solve the Poisson equation for the overall electrostatic potential using a linear scaling multigrid algorithm. This work is done in the context of minimizing the Kohn–Sham energy functional directly in real space with a multiscale approach. The efficacy of the approximation is illustrated by locating the ground state density of simple one electron atoms and molecules and more complicated multiorbital systems.
Perturbation treatment of pump–probe laser–molecule interactions: An application to the fluorescence from the S1 state of α-NPO
Authors: Jagatap B. N. , Meath William J. , Tittelbach-Helmrich Dietrich , Steer Ronald P.
Published: July 2002

Time-dependent perturbation theory, together with a (minimal) molecular model consisting of three energy levels (S0,S1,S3), is used to investigate the spectroscopy and the dynamics of fast time resolved, two-photon, two-color, pump–probe experiments, involving the direct S0→S1 two-photon excitation of α-NPO. In particular the theory is used to examine the Θ-dependence of the fluorescence signal from the S1 state, where Θ is the angle between the polarization vectors of the pump and probe lasers, for fixed (zero) time delay between the laser pulses. It is predicted, in contradistinction to the cos2 Θ dependence of the fluorescence signal from the S2 state of azulene arising from the sequential two-photon S0→S1→S2 transition, that the signal from the S1 state of α-NPO can vary between pure cos2 Θ and pure cos4 Θ dependencies and that secondary maxima in the signal, as a function of Θ, can occur for certain laser intensities. Also reported is a new series of experiments for α-NPO, motivated by the theory, that yields results for the fluorescence intensity of the S1 state, as a function of Θ and laser intensity, in agreement with the theoretical predictions. Comparison of experiment and theory is used to estimate the relative orientations of the relevant transition and permanent dipole moments, and the transition moment between the S1 and S3 states of α-NPO. The important role played by the permanent dipoles of the S0 and S1 states, and the importance of including averages over the relative laser phase, the jitter in the time-delay, and the orientations of the absorbing molecules, is emphasized in the theoretical analysis of the problem.
Reaction of N on a Ni(110) surface with H atoms
Authors: Takehiro Naoki , Mukai Kozo , Tanaka Ken-ichi
Published: July 2002

The p(2×3) Ni(110)-N surface structure was produced by the reaction of NO with highly excess  H2 (1:150) on a Ni(110) surface at 650 K. The N atoms on Ni(110) surface were inactive for the hydrogenation with  H2. They, however, reacted with H atoms in the presence of  H2, and the formation of NH species was detected by high resolution electron energy loss spectroscopy (HREELS). In the temperature range between 300 K and 450 K, the amount of N on Ni(110) surface decreased by reacting with H atoms in the zero order kinetics with respect to the amount of N and the decreasing rate did not depend on the temperature. When the temperature is higher than a critical temperature of 500 K, however, the amount of N on the Ni(110) surface does not decrease even if the surface is exposed to H atoms. This critical temperature corresponds to the decomposition temperature of the NH species on Ni(110) surface determined by the HREELS. One one hand, the NH species were produced by the reaction, hydrogenation, of N with H atoms on the Ni(110) suface. On the other hand, it was reported that  NH2 species were preferentially formed by the decomposition, dehydrogenation, of  NH3 on the Ni(110) surface. Taking these facts into account, it was deduced that there is a rather high activation barrier in an elementary reaction path from NH to  NH2 on the Ni(110) surface.
Theory of resonance Raman scattering in benzene derivatives
Authors: Zgierski Marek Z. , Pawlikowski Marek , Hudson Bruce S.
Published: July 2002

A simple model is formulated which allows for study of the Raman excitation profiles and depolarization dispersion curves of fundamentals and overtones of Franck–Condon and pseudo-Jahn–Teller active modes in substituted benzenes in the resonance with the analogs of the 1 A1g→1 B2u and 1 A1g→1 B1u transitions of benzene. Three excited electronic states and three vibrational modes are taken into account. Of the former, one represents the B state whereas the other two are the components of the degenerate 1 E1u state of benzene. Of the latter, one is the analog of the ring breathing mode ν1 whereas the two others are analogs of the two components of the e2g mode responsible for the vibronically induced moment of the optical transition to the B state (ν8 mode for the 1 B1u state). The substituent effect is introduced by an electronic matrix element which mixes the B state under consideration with one of the orbital components of the E state. Resonance Raman spectra resulting from the model are discussed in the light of available experimental data. The effect of the relative position of identical substituents on these spectra is also investigated. Ab initio calculations of the normal modes of chloro derivatives of benzene and of their resonance Raman activity at the S0→S2 transition are also presented and discussed in terms of the model.
A theory for the 1-1/2 fluid
Authors: Chiu Hung-chang , Kofke David A.
Published: July 2002

The 1-1/2 fluid is a conformal solution in which one species has a size parameter (diameter) of zero. This ‘‘point-particle’’ species nevertheless interacts with the other component of the mixture, as the collision diameter of a point particle and a finite particle is nonzero. A great simplifying feature of this model mixture is that the point particles do not interact with each other. For hard repulsive potentials, the properties of a 1-1/2 fluid can be obtained exactly in terms of the properties of the pure fluid obtained upon removal of all the point particles. For other potentials, the properties of the 1-1/2 fluid can be obtained only approximately. We develop two approaches to the description of the 1-1/2 fluid, both based on the methods of diagrammatic expansion and topological reduction. The first approach is an extended virial treatment, in which the free energy is expanded in the density ρ1 of the full-sized species, keeping to all orders terms in the density ρ2 of the point particles. A complementary approach takes the pure full-sized fluid as a reference, keeping all terms to ρ1 while expanding in ρ2. Monte Carlo simulation is used to show that, properly formulated, an expansion containing only terms first order in ρ2 is capable of describing 1-1/2 fluid properties over a very broad range of conditions.
An analytical Hückel-type approach to the relationship between Peierls instability in polyenes and interchain interaction
Authors: Aoki Yuriko , Imamura Akira
Published: July 2002

We propose a convenient method to estimate the magnitude of Peierls instability in finite one- and two-dimensional (1D and 2D) polyenes from the view point of orbital symmetry. The formulas are derived in terms of in-phase and out-of-phase interactions between adjacent carbon atoms on the basis of the analytical Hückel orbitals for polyene with an arbitrary length. The stabilization energies due to bond alternation are defined for the individual energy levels. It is visually shown that bond alternation gives rise to stabilizing the occupied orbitals and destabilizing the vacant orbitals without using the k space based on periodic boundary condition in infinite polymer. This treatment is further extended to the Peierls instability in 2D polyene on the basis of the analytical Hückel orbitals derived for its regular structure. Total π and σ energies are provided as functions of bond alternation and interchain interaction. It is demonstrated that bond alternation is strongly suppressed under the existence of interchain interaction, in which interchange between occupied and unoccupied orbitals plays a crucial role. This treatment would provide a first step to investigate the relationship between crystal structure of more complicated polymer and its electronic property in connection with interchain interaction.
Flory and Guggenheim lattice statistics reinterpreted and extended to include molecular clustering and chemical dissociation
Authors: Martinez Gregory M.
Published: July 2002

A new method is presented for counting the number of distinguishable arrangements available to a mixture of molecules in a lattice model. Chain molecules are used and these are allowed to undergo molecular clustering and chemical dissociation. The resulting equation for the configurational degeneracy is shown to reduce to the equations of Flory and Guggenheim in the appropriate limits. The Guggenheim equation is shown to be exact in one dimension; extension to higher dimensions requires allowing the lattice coordination number to increase beyond 2. The Flory equation is shown to be the low volume fraction approximation of the Guggenheim equation. The counting method developed can be used to model entropy effects associated with chemical reactions. The equation of state derived using the new configurational degeneracy term is shown to reduce to a number of well known equations. Comparison to van der Waals equation of state shows that the improved ability of the new equation to model observed behavior, results from a redefinition of the van der Waals free volume.
Dipole broadened line shape of impurity chromophores isolated in a transparent matrix
Authors: Ovchinnikov Mikhail A. , Wight Charles A.
Published: July 2002

The inhomogeneous broadening caused by dipole–dipole interactions between molecules having strongly allowed optical transitions is considered for the case in which the chromophores are randomly distributed at low concentration in a transparent matrix. Model calculations show that absorption in the wings of the spectrum is caused almost exclusively by strongly interacting pairs of chromophores. This result has important implications for the interpretation of recent experiments in which the spectra of single dye molecules have been observed in organic matrices. The generalized lineshape in these wings is shown to be proportional to the inverse square of the detuning from line center. The line shape is also somewhat asymmetric, with the red wing being more intense than the blue.
The potential energy function for a ligand substitution reaction of square-planar platinum (II) complex in water: The important role of three-body effect
Authors: Muguruma Chizuru , Koga Nobuaki , Kitaura Kazuo , Morokuma Keiji
Published: July 2002

The analytical potential energy function for interaction of [Pt(NH3)3]2+, Cl−, and H2O has been determined to describe the ligand substitution reaction: [Pt(NH3)3(H2O)]2+ +Cl−→[Pt(NH3)3Cl]++H2O in solution. The Honda–Kitaura potential function is used as the two-body potential function. Although the Honda–Kitaura potential reproduces the ab initio two-body interaction energy very well, the potential function that assumes pairwise additivity cannot reproduce the potential energy of the entire three-body complex because of a large repulsive three-body interaction in the strongly interacting region. We analyzed the origin of three-body energies, derived the physically meaningful potential functional forms from a perturbation theory, and fitted the ab initio three-body energies into an analytical form. The full potential function, the sum of pairwise two-body potential functions and the three-body terms, can reproduce very well the ab initio interaction energies in the entire geometrical space as well as the structures of the reaction intermediates and transition states. The results of the preliminary reaction coordinate and Monte Carlo calculation for the reaction in a cluster of water molecules are also presented.
Perturbation theory using series expansions and the Riccati equation
Authors: Bessis N. , Bessis G.
Published: July 2002

An algebraic procedure is proposed for the analytical solution of Schrödinger equations that can be viewed as a factorizable equation with an adequately chosen perturbation. This procedure relies on the solution of the Riccati equation associated with the given eigenequation and the use of power series of suitable functions which are specific to each factorization type. As illustrative examples, analytical solution of the symmetric anharmonic oscillator, perturbed Morse oscillator and singular anharmonic oscillator equations are carried out. Further applications are pointed out.
Two-dimensional orientational motion as a multichannel reaction by computer simulation
Authors: Müser M. H. , Ciccotti G.
Published: July 2002

The orientational diffusion of a nitrogen molecule N2 in a three-dimensional crystal of argon atoms is studied by using the reaction coordinate formalism and the Bennett–Chandler approach. To study the two-dimensional orientational diffusion the reaction coordinate formalism has to be generalized to multichannel reactions. The rate coefficient for the motion between the six stable orientational states is determined from correlation function expressions, which include corrections to transition-state theory due to recrossings of free energy barriers. At high temperatures the simulation results are compared with results obtained by standard molecular dynamics methods. At low temperatures the transmission coefficient is computed and the dependence of the plateau value upon the density of the surrounding argon matrix and upon the isotopes, chosen to form the N2 molecule, is investigated.
The dimerization process in a potassium beam in case of significant heat of formation
Authors: Obrebski A. , Cerny U. , Kaps T.
Published: July 2002

Dimer mole fractions β in a potassium beam were measured for a wide range of stagnation parameters. A comparison with a theoretical fitting formula was made. Very good results were obtained for the latter using as the stagnation temperature an effective stagnation temperature which includes a contribution from the heat released due to dimer formation in the gas expansion. This interpretation of the stagnation temperature allows a convenient description of dimerization processes for species with high binding energy and heat of formation, further an easy comparison with those species, which binding energy is low. For the theoretical formula we are able to give an assumption-free scaling constant for potassium.