bluuebunny/crossref_metadata_2025 huggingface dataset:
Validity of a hybrid quantum/classical approach in photodissociation/recombination of I2 in rare gas matrices
Authors: Liu Li , Guo Hua
Published: July 2002

In this work, we examine the validity of a hybrid quantum/classical method used to study the photodissociation/recombination dynamics of I2(A) in rare gas matrices. Our simplified model includes a I2 molecule embedded in a linear chain of rare gas atoms (Kr or Xe). The aggregate is partitioned into a quantum system and a classical bath and their dynamics are coupled self-consistently within the Ehrenfest framework. Two partitioning schemes are used. The first scheme treats the I–I coordinate quantum mechanically and the rare gas coordinates classically. The second and more reliable scheme includes in the quantum system both the I–I mode and the symmetric motion of the two nearest rare gas atoms. Both models show substantial energy transfer from the dissociating iodine to the solvent, followed by coherent vibrational motion in the recombined I2. It is found that the one-dimensional quantum/classical scheme is consistent with its higher dimensional counterpart, although the latter shows much faster dephasing.
Toeplitz matrices within discrete variable representation formulation: Application to collinear reactive scattering problems
Authors: Eisenberg Eli , Charutz David M. , Ron Shlomo , Baer Michael
Published: July 2002

Recently, a new approach based on the features of the Toeplitz matrix was introduced for reactive scattering problems. So far these features were used only along the reagents translational coordinate (either for Eckart-type models or for collinear scattering). In this work, we show how to employ the Toeplitz features for the two asymptotic regions of the collinear system.
The bridge function of a Lennard-Jones fluid calculated from a second-order Percus–Yevick equation
Authors: Henderson Douglas , Sokol/owski Stefan
Published: July 2002

Following Attard, we obtain a second-order Percus–Yevick equation from the theory of inhomogeneous fluids. Although we calculate the radial distribution function and thermodynamic functions, our main interest is in the calculation of the bridge function, a part of the radial distribution function either not included or simply approximated in the usual theories. A comparison is made with simulation results for the bridge function, correlation functions, and thermodynamic functions for the Lennard-Jones fluid with good results. A comparison is also made with recent binodal chain approximation results.
Photoelectron spectroscopy of mixed metal cluster anions: NiCu−, NiAg−, NiAg−2, and Ni2Ag−
Authors: Dixon-Warren St. J. , Gunion R. F. , Lineberger W. C.
Published: July 2002

Negative ion photoelectron spectra of NiCu−, NiAg−, NiAg−2, and Ni2Ag− are presented for electron binding energies up to 3.5 eV. The metal cluster anions were prepared in a cold cathode dc discharge flowing afterglow source. The dimer spectra exhibit three low lying electronic states; the ground electronic states are vibrationally resolved. The dimer electron affinities are determined to be EA(NiCu)=0.889±0.010 eV and EA(NiAg)=0.979±0.010 eV. Two excited state electron configurations are assigned as (d8Nid10Cuσ2σ*1) and as 4Δ (d9Nid10Cuσ1σ*1). The NiCu− ground state is assigned as 3Δ (d9Nid10Cuσ2σ*1), and has a vibrational frequency of 235±25 cm−1. The photoelectron spectrum of NiAg− strongly suggests that the electronic configurations of the three observed states are the same as those of NiCu−. The NiAg ground state vibrational frequency is 235±25 cm−1 and the NiAg− frequency is 185±25 cm−1. The chemical bonding in both NiCu and NiAg dimers is dominated by a sσ molecular orbital, and the extra electron in the anions has primarily sσ* character. The photoelectron spectra of the trimers, NiAg−2 and Ni2Ag−, are remarkably similar to those obtained for the coinage metal trimers, and are consistent with a transition between a linear anion ground state and a linear excited state of the neutral.
Curvature-based analysis of nonmonotonic forced Rayleigh scattering signals
Authors: Spiegel D. R. , Sprinkle M. B. , Chang T.
Published: July 2002

We propose a new method for analysis of nonmonotonic forced Rayleigh scattering (FRS) decay profiles. It is shown that the curvature near the local maximum of a decay–grow–decay profile or a grow–decay profile can be used to determine directly the geometric mean of the unperturbed and photoproduct diffusion coefficients, which will in many cases mitigate the need to apply multiparameter nonlinear fitting routines. FRS tracer-diffusion measurements of methyl red in poly(vinyl acetate)/toluene solutions at two different probe wavelengths, analyzed using the new method, are in good agreement with results of previous studies. Simulations are used to estimate the systematic error due to the approximation employed, and it is concluded that accuracies of a few percent or better should be routinely achievable in most cases. We have, in addition, investigated the probe-wavelength dependence of the profile shape for this system.
Dependence of quasielastic light scattering in poly(methyl methacrylate) on the concentration of free monomers
Authors: Surovtsev N. V. , Mermet A. , Duval E. , Novikov V. N.
Published: July 2002

The quasielastic light scattering (QELS) of poly(methyl methacrylate) was measured as a function of both temperature and free monomer concentration, which was varied by heating. The motion responsible for the QELS is not thermally activated (i.e., the corresponding relaxation time is independent of temperature), while the QELS intensity is strongly temperature dependent and proportional to the monomer concentration. The QELS is interpreted as due to a rapid localized motion released by thermally activated slow structural relaxation, which can create free volume.
Vibrational energy transfer from four levels below 410 cm−1 in S1 p-difluorobenzene. I. A strong collision partner dependence in state-to-state transfer by monatomics
Authors: Mudjijono  , Lawrance Warren D.
Published: July 2002

Collision-induced vibrational energy transfer has been studied from four levels [302 (Evib=240 cm−1), 82 (Evib=361 cm−1), 271 (Evib=403 cm−1) and 61 (Evib=410 cm−1)] in S1 p-difluorobenzene in supersonic free jet expansions of He, Ne, Ar, and Kr at ∼30–40 K. In broad terms the trends are similar to those observed previously in studies of aromatics: the transfer is highly selective, and one quantum changes in the low frequency modes are preferred. However, a significant collision partner dependence is observed, whereby changing from He through to Kr causes a substantial increase in multiple quanta (‖Δυ‖≳1) transfer. SSH-T calculations fail to capture this trend. The preference for ‖Δυ‖≳1 transfer appears to be enhanced as the interaction time and attractive force on the collision partner increase. Consequently, it is predicted that (i) differences in the state-to-state branching ratios between collision partners will increase as the temperature is lowered; (ii) for a particular collision partner there will be an increase in ‖Δυ‖≳1 transfer with decreasing temperature; and (iii) ‖Δυ‖≳1 transfers will be most important for collision partners with small velocities (i.e., large masses), large intermolecular potential well depths (ε) and size (σ). The nearly isoenergetic 271 and 61 levels have virtually identical state-to-state branching ratios for Ar and small differences are observed for He. This suggests that the branching ratios are not particularly sensitive to the initial vibrational motion. Relaxation of 61 and 271 is inefficient compared with relaxation from 302 and 82.
Response to ‘‘Comment on ‘Short time dynamics of glass-forming liquids’ ’’ [J. Chem. Phys. 104, 8169 (1996)]
Authors: Roland C. M. , Ngai K. L.
Published: July 2002

Our J. Chem. Phys. 103, 1152 (1995) article described synthetic data generated from the coupling model, which was analyzed in the framework of mode coupling theory (MCT). The purpose was to demonstrate that someone unaware of how the data was generated could carry out an MCT analysis and find some, though not necessarily all, of the features to be in conformance to MCT. The work thus served as a caution against fixation on only some features of short time experimental data, since this can lead to premature conclusions that MCT has been verified. Due to the limitations of experiments on real materials, most data either does not allow a test of all MCT predictions or, as has been found in polymers, is in disagreement with MCT. These points, which have evidently been missed, are reiterated in the present communication.
Classical simulation of a cage effect in the dissociation of I2Rgn clusters (Rg = Ar,Kr,Xe; n⩽5)
Authors: Schröder H. , Gabriel H.
Published: July 2002

The optical dissociation of I2 can be markedly suppressed, if the I2 molecule is weakly bound to one or more rare-gas (Rg) atoms (cage effect). A classical simulation of this process gives a fast disappearance of the cage effect and of the fluorescence intensity, as soon as the optical excitation energy Eω exceeds the dissociation energy of I2 by a certain amount of energy, which is controlled by the binding energy of I–Rgn in the ground state. For lower Eω the van der Waals potential between I2 and Rgn is strong enough in the asymptotic region to prevent the separating iodine atoms from dissociating. The oscillating I atoms can then transfer part of their vibrational energy to the Rg motion till the rare-gas atoms are evaporated. This mechanism gives a fluorescence spectrum of I2 with a cutoff at higher vibrational energies—smeared out only by the thermal excitation of the ground-state complex. The dependence of the spectrum on temperature and potential parameters has been investigated. At high excitation energies Eω a spectrum with an isolated peak can occur, if the van der Waals binding energy is increased or if more than one rare-gas atom is bound to I2. Other mechanisms which could result in a cage effect at higher Eω require a hard collision between an I and a rare-gas atom immediately after excitation. This is possible at high temperatures or for a linear conformation I–I–Rg. For an extended range of photon energies the simulations gave high yields of I–Rgn fragments from a I2–Rgn beam.
Rovibronic coupling in the Na3 B system
Authors: Mayer M. , Cederbaum L. S. , Köppel H.
Published: July 2002

The rovibronic spectrum of the Na3 B system is computed taking into account full rovibronic coupling between different vibronic states. The rovibronic coupling matrix elements are calculated using the vibronic states following from a pseudo Jahn–Teller model for the vibronic dynamics in the B system. Comparison with results of optical–optical double resonance measurements [W. E. Ernst and S. Rakowsky, Can. J. Phys. 72, 1307 (1994)] shows good agreement. In particular, certain properties of the Coriolis splittings, so far explained by an ad hoc spin–rotation interaction, are now well understood as the consequence of rovibronic coupling between two vibronic states. A general discussion of rovibronic dynamics on electronic potential energy surfaces with three equivalent minima is presented.
Molecular photoionization as a probe of vibrational–rotational–electronic correlations
Authors: Rao R. M. , Poliakoff E. D. , Wang Kwanghsi , McKoy V.
Published: July 2002

We determine the rotationally state-resolved 2σu−1 photoionization of N2 into alternative vibrational channels as a function of energy over a 200 eV range. Experiment and theory reveal that Cooper minima highlight the coupling between electronic, vibrational, and rotational degrees of freedom over this very wide range.
Dynamic properties of homopolymer layers adsorbed on a solid surface
Authors: Wang Yongmei , Rajagopalan Raj
Published: July 2002

Dynamic properties of homopolymer layers adsorbed on a solid surface from a dilute solution are examined through dynamic Monte Carlo simulations over a range of surface/segment interaction energies and at different bulk concentrations. In particular, we studied the relationship between the detachment rate of adsorbed polymers and the first Rouse relaxation rate of adsorbed chains. The power-law dependence of the detachment rate on the chain length which we have reported earlier [Phys. Rev. Lett. 74, 2503 (1995)] is seen to correspond to the power-law dependence of the first Rouse relaxation rate of the adsorbed chains, which is also the same as the power-law dependence of the first Rouse relaxation rate of the free chains (nonadsorbed chains). However, the detachment rate is usually much slower than the first Rouse relaxation rate of adsorbed chains. Increasing the surface/segment interaction energy (ε in units of kBT) increases the difference between the two. More interestingly, when ε∼1.0 or higher, the detachment rate decreases significantly as one lowers the bulk concentration, but the first Rouse relaxation rate does not decrease significantly. The results seem to suggest that the detachment rate may become effectively zero (i.e., detachment time goes to infinite) as one continuously decreases the bulk concentration in the strong adsorption limit (ε≥1.0).
Shear viscosity of model mixtures by nonequilibrium molecular dynamics. II. Effect of dipolar interactions
Authors: Lee Song Hi , Cummings Peter T.
Published: July 2002

In a recent paper [Lee and Cummings, J. Chem. Phys. 99, 3919 (1993)], we presented nonequilibrium molecular dynamics calculations of the shear viscosity of pure liquid argon, pure liquid krypton, and argon/krypton liquid mixtures at constant temperature (T=135 K) and pressure (P=40 bar) as the base case for a consistent study of the role of intermolecular potentials on the transport properties of molecular liquids and their mixtures. In this paper, nonequilibrium molecular dynamics is used to study the shear viscosity of pure dipolar fluids, polar/nonpolar fluid mixtures, and polar/polar fluid mixtures at the same temperature and pressure. The fluids are assumed to interact via Lennard-Jones intermolecular potentials augmented by point dipole/point dipole potentials in the case of polar/polar interactions. By comparing the computed shear viscosity with that for corresponding nonpolar pure fluids and mixtures, the effect of dipolar interactions on the shear viscosity can be clearly delineated.
Dynamic specific heat of a supercooled liquid
Authors: Menon Narayanan
Published: July 2002

The dynamic specific heat Cp(ν) and thermal conductivity, κ, of the simple glass-forming liquid di-n-butylphthalate are measured over a wide frequency range (4 mHz–8 kHz) by a nonadiabatic ac technique due to Birge and Nagel. The relaxation frequency obtained from the specific heat is found to be non-Arrhenius down to the lowest temperatures attained in this measurement. The temperature dependence of the relaxation frequency is in close agreement with that obtained from shear modulus and dielectric susceptibility measurements indicating that all motions in the supercooled liquid are strongly coupled to one another. The shape of the relaxation spectrum changes with temperature. The width of the relaxation is similar to that of the dynamic specific heat of other supercooled liquids. The thermal conductivity is frequency independent through the calorimetric glass transition. The static specific heat is reported over a wide range in temperature extending up to 100 K above the melting point. No evidence is found for thermodynamic anomalies accompanying the proposed formation of spatial inhomogeneities in the liquid. The high frequency relaxation shape exhibits a power law which crosses over to a weaker frequency dependence as seen in the dielectric response.
Assigning spectra of chaotic molecules with diabatic correlation diagrams
Authors: Rose John P. , Kellman Michael E.
Published: July 2002

An approach for classifying and organizing spectra of highly excited vibrational states of molecules is investigated. As a specific example, we analyze the spectrum of an effective spectroscopic fitting Hamiltonian for H2O. In highly excited spectra, multiple resonance couplings and anharmonicity interact to give branching of the N original normal modes into new anharmonic modes, accompanied by the onset of widespread chaos. The anharmonic modes are identified by means of a bifurcation analysis of the spectroscopic Hamiltonian. A diabatic correlation diagram technique is developed to assign the levels with approximate ‘‘dynamical’’ quantum numbers corresponding to the dynamics determined from the bifurcation analysis. The resulting assignment shows significant disturbance from the conventional spectral pattern organization into sequences and progressions. The ‘‘dynamical’’ assignment is then converted into an assignment in terms of ‘‘nominal’’ quantum numbers that function like the N normal mode quantum numbers at low energy. The nominal assignments are used to reconstruct, as much as possible, an organization of the spectrum resembling the usual separation into sequences and progressions.
Transformations of styrene at high pressure
Authors: Gourdain D. , Chervin J. C. , Pruzan Ph.
Published: July 2002

Phase transformations of styrene (C8H8) at room temperature and up to 32 GPa have been studied by visual observations, Fourier transform infrared spectroscopy, and Raman scattering. At room temperature, styrene freezes at 0.3 GPa and undergoes a solid-to-solid transformation around 3.5 GPa. Around 15 GPa the extent of the reaction monostyrene→polystyrene becomes significant. At 32 GPa the reaction is nearly completed and, contrary to the general trend, the phenyl group is still stable at this pressure. This uncommon behavior is very likely correlated with the amorphous character of the polystyrene formed under pressure.
Coherent control of bimolecular collisions: Collinear reactive scattering
Authors: Holmes Daniel , Shapiro Moshe , Brumer Paul
Published: July 2002

A recently proposed approach to the coherent control of bimolecular reactions is applied to collinear models of H+H2, D+H2, and F+H2 scattering. Reactive scattering probabilities above the reaction threshold are shown to be controllable in these systems over a wide range, often nearing total yield control.
A simple physical picture of liquid–glass transition
Authors: Tanaka Hajime
Published: July 2002

We demonstrate here a simple physical picture of liquid–glass transition, which qualitatively explains most of the dynamic features from the strong-glass to the fragile-glass limit. We classify the dynamic environment of a molecule into loose and tight cages. Glass transition is then characterized by two transitions: (i) the appearance of tight cages at the crossover temperature Tc and (ii) the disappearance of loose cages at the Vogel–Fulcher temperature T0. We propose that the fragility of glass is strongly correlated with the relative importance of the topological confinement against the anisotropic attractive intermolecular interactions for the formation of tight cage.
On the ν4 bending frequency of gaseous ThF4
Authors: Konings R. J. M.
Published: July 2002

The ν4 bending frequency of ThF4 was measured by high-temperature infrared spectroscopy. Its value, 116 cm−1, is considerably different from estimates used in the analysis of thermochemical studies. Based on this value and the previously reported ν3, a new set of vibrational frequencies for ThF4 has been derived, which is consistent with thermochemical equilibrium studies.
Steady-state and time-resolved fluorescence study of some dyes in polymer microspheres showing morphology dependent resonances
Authors: Bisht Prem B. , Fukuda Kazuhiro , Hirayama Satoshi
Published: July 2002

Fluorescence emission spectra of N,N′-bis(2,5-di-tert-butylphenyl)-3,4:9,10- Perylenebis(dicarboximide) (DBPI), rhodamine 6G (R6G), and cresyl violet (CV) in spherical polymer beads of less than 20 μm diameter show sharp ripple structures. The observed peak positions and the intervals of the structures are consistent with the calculations of the morphology dependent resonances (MDR). Observed intensities of the MDR in the fluorescence emission spectra are found to show excitation energy dependence. The fluorescence spectra have also been measured as a function of the refractive indexes of the medium and the bead. These MDR in the beads up to 4 μm diameter do not appear to affect the fluorescence decay of the dyes, since the fluorescence lifetime remains constant irrespective of the size of the bead and the refractive index of a surrounding medium. Simulations based on the Lorentz–Mie theory for the microspheres of different refractive indexes have been used to quantify the observed effect on the basis of the available data on the homogeneous widths of the dye molecules. A fluorescence study of microcrystals of DBPI is also presented here from the point of view of comparison with fluorescence decay of dye impregnated beads. The microcrystals exhibit a size effect in the fluorescence decay which has been attributed mainly to the self-absorption effect.
Observation and spectroscopy of high-lying states of the CaOH radical: Evidence for a bent, covalent state
Authors: Pereira R. , Levy Donald H.
Published: July 2002

The CaOH radical has been produced in a supersonic molecular beam by the reaction of water vapor with laser desorbed calcium. Three new electronic states, the D 2Σ+, E 2Σ+, and F states have been observed by laser induced fluorescence and resonance enhanced multiphoton ionization spectroscopy. The D and E states are linear, but the F state is bent, the first bent and strongly covalent state of CaOH to be observed. Vibrational constants for the D state have been determined and a partially rotationally resolved spectrum has provided the rotational constant and Ca–OH bond length in this state. The D and E states are more strongly bound than the ground state, perhaps indicating some covalent contribution to the bonding. Additional vibrational constants of the ground electronic state including the CaO–H vibrational frequency have been determined from the emission spectra.
Theory of ultrafast laser control of isomerization reactions in an environment: Picosecond cope rearrangement of substituted semibullvalenes
Authors: Korolkov M. V. , Manz J. , Paramonov G. K.
Published: July 2002

An efficient approach to control isomerization reactions by ultrashort infrared laser pulses in the presence of a thermal environment is developed and demonstrated by means of model simulations within the reduced density matrix formalism beyond a Markov-type approximation for a picosecond Cope rearrangement of 2,6-dicyanoethyl-methylsemibullvalene coupled to a quasi-resonant environment. The population transfer from the reactant state via the delocalized transition state to the product state is accomplished by two picosecond infrared laser pulses with a probability up to 80% despite the rather strong coupling to the environment, which reduces the lifetime of the transition state into the femtosecond time domain. Simulations, carried out for helium (4 K), nitrogen (77.2 K) and room (300 K) temperatures, show that low temperatures are preferable for state-selective laser control of isomerization reactions.
Study of the low-lying electronic states of CCO by photoelectron spectroscopy of CCO− and ab initio calculations
Authors: Zengin V. , Joakim Persson B. , Strong K. M. , Continetti R. E.
Published: July 2002

The low-lying electronic states of CCO have been investigated by photoelectron spectroscopy of CCO− at wavelengths of 266 and 355 nm in conjunction with ab initio calculations. Photodetachment is observed to occur to the X̃ 3Σ−, Ã 3Π, ã 1Δ, and b̃ 1Σ+ electronic states of CCO. This marks the first observation of the low-lying singlet states. A revised value for the electron affinity of CCO is found to be 2.289±0.018 eV. These results are compared with CASPT2 ab initio calculations of the energetics and structure of the ground and excited states of CCO and CCO−. Using the measured electron affinity of CCO, the heats of formation Δf H0298(CCO)=3.99±0.20 eV and ΔfH0298(CCO−)=1.67±0.20 eV are determined. In addition, the C–C bond dissociation energies in CCO and CCO− are determined, as well as the H–CCO bond energy in HCCO.
Spectroscopic studies and crystal-field analysis of U3+ ions in RbY2Cl7 single crystals
Authors: Karbowiak M. , Drozdzynski J. , Murdoch K. M. , Edelstein N. M. , Hubert S.
Published: July 2002

Single crystals of U3+ diluted into a matrix of RbY2Cl7 have been grown by the Bridgman–Stockbarger method. There are two different intrinsic U3+ sites in this crystalline host, each with an approximate C2v site symmetry. A third minor U3+ site was also found. Absorption, fluorescence, and excitation spectra of these crystals were obtained at 300 and 4.2 K. Transitions belonging to each U3+ site were distinguished by time-gated site-selective spectroscopy. The energy levels of the two intrinsic sites were assigned and fitted to a semiempirical Hamiltonian representing the combined atomic and crystal-field interactions for a 5f3 ion at a C2v symmetry site. Quite different 4F9/2 fluorescence lifetimes were measured for the three sites, due to a multiphonon decay mechanism which is affected by differences in their respective crystal-field strengths. These results are analyzed quantitatively using the theory for the electron–phonon interaction.
Coherent control of multiphoton driven processes: A laser-induced catalyst
Authors: Garrett W. R. , Zhu Yifei
Published: July 2002

A well-known quantum interference phenomenon is combined with laser-induced collisional energy transfer to create a coherently controlled excitation involving two species. From one viewpoint the process involves a laser induced catalyst. An experimental demonstration is performed in mixtures of Xe and Kr gases.
Convergence of many-body perturbation methods with lattice summations in extended systems
Authors: Sun Jun-Qiang , Bartlett Rodney J.
Published: July 2002

Recently the possible divergence of many-body perturbation theory or many-body green function (propagator) methods with lattice summations in extended systems has been raised. The convergence of these methods with lattice summations is not only the key to establishing their correct scaling properties in inhomogeneous systems, but is also a necessity if numerical calculations are to be meaningful. In this report, it is rigorously shown that many-body perturbation theory (MBPT), coupled cluster theory (CC), and many-body green function (MBGF) methods all converge uniformly with lattice summations, although the integrand for the integration over the reciprocal lattice vector, k, could become infinite at special k values. Our proof is given not only for infinite polymers but also for crystals. In our proof, only the continuity of the zeroth-order band structure and Bloch orbitals with k is used. We show that MBPT, CC, and MBGF methods converge with the radius of the lattice summation range R at least as fast as 1/R1/n where n is the dimensionality of the system. It could be much faster if the zeroth-order band structure and Bloch orbitals have continuous first- or even higher-order derivatives with k. In practical numerical calculations, one should not pursue the convergence of the integrand, but the convergence of the correction considered with lattice summations.
Excluded volume anisotropy and two-cavity distribution functions in hard sphere fluids
Authors: Ben-Amotz Dor
Published: July 2002

A new model for the distribution of spherical cavity pairs in a hard sphere fluid is derived by extending exact low density results to include the effects of cavity pair anisotropy at finite solvent density. In particular, the unitless excluded surface-area-to-volume ratio is used to quantitate the anisotropy dependence of the excess chemical potential of a solute cavity pair. The Carnahan–Starling contact radial distribution function of a single component hard sphere fluid is used to fix this anisotropy dependence. Predictions of the resulting excluded volume anisotropy (EVA) model for the two-cavity distribution function are found to compare favorably with previous simulation and analytical results. Generalization of the EVA model to predict solvation thermodynamics of arbitrary shaped solutes in molecular liquids, as well as multi-cavity distribution functions in hard sphere fluids are suggested.
Theoretical study of the water tetramer
Authors: Wales David J. , Walsh Tiffany R.
Published: July 2002

We report rearrangement mechanisms and new stationary points for the water tetramer and deduce the associated tunneling splitting patterns and nuclear spin weights when different processes are assumed to be feasible. The basis sets employed for the ab initio calculations are double-zeta plus polarization (DZP) and DZP with additional diffuse functions (DZP+diff), and results have been obtained within both the Hartree–Fock (HF) and density functional theory frameworks employing the Becke exchange and the Lee–Yang–Parr correlation functionals (BLYP). The results are compared with those found for a relatively sophisticated empirical rigid-body intermolecular potential. One direct degenerate rearrangement of the cyclic global minimum was characterized in the HF calculations, but disappears when density functional theory is applied. The latter mechanism involves a larger barrier than pathways mediated by higher index saddle points belonging to the torsional space. In principle, doublet splittings could result from tunneling via a number of possible routes, and further calculations will be needed to elucidate the dynamics for this system.
Application of the semi-empirical method to determine the spatial distribution function for thermalized photoelectrons created by vacuum ultraviolet or high-energy irradiation of some nonpolar dielectric liquids
Authors: Guelfucci J. P. , Fitte-Rey J. , Casanovas J. , Baird J. K.
Published: July 2002

A semi-empirical method is tested to determine the spatial distribution function of the thermalized photoelectrons, created by vacuum ultraviolet (VUV) or γ irradiations of some liquid alkanes. It seems that modified exponentials must be associated to the thermalization process on VUV irradiation. A Gaussian distribution function could be used for high-energy irradiation. The partial inadequacy of the method in the case of high-energy irradiation can be imputed to the existence of multiple ion pair recombinations.
A high level theoretical investigation of the cyclic hydrogen fluoride trimer
Authors: Tschumper Gregory S. , Yamaguchi Yukio , Schaefer III Henry F.
Published: July 2002

A high level ab initio theoretical investigation of the cyclic hydrogen fluoride trimer was carried out. The structures of the hydrogen fluoride monomer, dimer, and trimer were fully optimized at the coupled-cluster level of theory including single, double, and perturbatively applied connected triple excitations [CCSD(T)] using three large basis sets. Geometrical parameters, dipole moments, harmonic vibrational frequencies, infrared intensities, and total energies are reported for each equilibrium structure. Changes in bond lengths and shifts in HF stretching frequencies relative to the monomer, as well as the dissociation energies corresponding to various fragmentation pathways, are given for the dimer, trimer, and their deuterated isotopomers. The theoretical results presented here are compared to the available experimental data and to those obtained from empirically refined potential energy surfaces.
Frozen phases with re-entrant transition for random heteropolymers with composition specific and annealed cross-links
Authors: Gutman Lorin , Shakhnovich Eugene
Published: July 2002

A field theoretic formalism of random heteropolymers (RHPs) with composition specific cross-links that are annealed, is developed in this work. This system can serve as a simple meaningful model of proteins with disulfide bonds. The agent that cross-links the macromolecules, as in real proteins, is assumed to maintain an equilibrium cross-linking density within the heteropolymer globule; this situation is reminiscent to in vivo reagent induced cross-linking, and protrudes the experimental desired control on formation and dissolution of cross-links. The starting point of our analysis, i.e. the random heteropolymer Hamiltonian, captures both the chain connectivity and the essentially quenched nature of amino-acids distribution reminiscent of linear uncross-linked polypeptides. The conformational statistics of the RHP is determined within the replica approach. For experimentally realizable values of the RHP interaction parameters, we predict the appearance of two frozen phases of RHPs wherein a small number of chain conformations of order O(1) are being sampled. At the interim between the two phases our calculation predicts the appearance of a re-entrant transition wherein the number of chain conformations sampled is of order O(N). The occurrence of the re-entrant transition is due to a tight competition between the intersegment interactions (measured by χF), and the propensity of composition specific and annealed cross-links to form (determined by the chemical potential of the cross-linking agent). We suggest how to traverse the re-entrant transition line, by chemically manipulating the competition of dissimilar interactions of the disparate segments, and the specific propensity of the peptides to cross-link, and also, we provide an explanation for the observation that in nature cross-linking between alike rather than dissimilar segments is observed.
High-pressure neutron diffraction on fluid carbon tetrafluoride and interpretation by reverse Monte Carlo simulations
Authors: Waldner I. , Bassen A. , Bertagnolli H. , Tödheide K. , Strauss G. , Soper A. K.
Published: July 2002

Neutron scattering experiments on carbon tetrafluoride (CF4) at high pressure were performed along the 370 K isotherm at three supercritical densities, covering a density range from ρ=1.07 to 1.26 g cm−3. The structure factors of the investigated thermodynamic states and the weighted sums of the atom pair correlation functions are presented. The variation of the density has only a weak effect on the structure factors. The experimentally obtained total atom pair correlation functions are interpreted with reverse Monte Carlo simulations. The atom pair correlation functions and angular distribution functions indicate a completely disordered arrangement of the molecules in fluid CF4 with no significant short-range orientational order, except for very close distances.
Kramers theory of chemical reactions in a slowly adjusting environment
Authors: Berezhkovskii A. M. , Zitserman V. Yu. , Sheu S.-Y. , Yang D.-Y. , Kuo J. , Lin S. H.
Published: July 2002

When describing the reaction dynamics in a slowly relaxing environment, one has to include slow nonreactive modes of the environment in an explicit consideration along with the “chemical” mode intrinsically responsible for the chemical transformation. This is done within the framework of the Kramers approach to condensed phase chemical reaction dynamics. The problem is studied under the condition of high friction of the nonreactive mode (slow adjustment) while friction of the chemical mode covers the whole range from weak to high friction. It is found that the reaction dynamics and, hence, the kinetics depend strongly on the strength of the coupling of the reactive and the nonreactive modes. For strong mode coupling the rate constant monotonically decreases with the increase of the friction of the chemical mode. Such behavior is quite distinctive from one for fast adjustment of the environment when the rate constant demonstrates a turnover behavior. Turnover behavior takes place for moderate strength mode coupling. This case has its own interesting specific features: (1) When friction of the chemical mode tends towards zero, the reaction rate remains finite due to the energy diffusion of the chemical mode induced by the motion of the nonreactive mode. (2) For a certain range of the friction coefficients particles escape the reactant domain on a path that avoids the saddle on the potential surface. This saddle-point avoidance is accompanied by violation of the Arrhenius law in the sense that the activation energy becomes a function of the friction intensity. (3) There is a range of the friction coefficients, where the kinetics is multiexponential. Analytical expressions for the rate constant are obtained for those conditions when the kinetics is single exponential. They show how the rate constant depends on the friction coefficients as well as on the parameters of the potential surface.
Eigenstate assignments and the quantum-classical correspondence for highly-excited vibrational states of the Baggot H2O Hamiltonian
Authors: Keshavamurthy Srihari , Ezra Gregory S.
Published: July 2002

In this paper we study the classical and quantum mechanics of the 3-mode Baggot vibrational Hamiltonian for H2O. Our aim is to classify and assign highly-excited quantum states based upon a knowledge of the classical phase space structure. In particular, we employ a classical template formed by the primary resonance channels in action space, as determined by Chirikov resonance analysis. More detailed analysis determining the exact periodic orbits and their bifurcations and families of resonant 2-tori for the Baggot Hamiltonian confirms the essential correctness of the Chirikov picture. It is emphasized that the primary periodic orbits alone do not define a suitable phase space skeleton; it is important to consider higher dimensional invariant structures, such as 2-tori and 3-tori. Examining the manifold of quantum states for a given superpolyad number P=n1+n2+nb/2 reveals sequences of eigenstates that progress along the classical resonance zones. These sequences provide insight into the nature of strongly mixed states found in the vicinity of the resonance junction. To further explore the classical-quantum correspondence, we have also computed eigenstate Husimi phase space distribution functions and inverse participation ratios. It is thereby possible to provide dynamically based assignments for many states in the manifold of states with superpolyad number P=16.
High-accuracy discrete path integral solutions for stochastic processes with noninvertible diffusion matrices. II. Numerical evaluation
Authors: Drozdov Alexander N.
Published: July 2002

We present a fast, high precision and easily implementable path integral method for numerically solving Fokker–Planck equations. It is based on a generalized Trotter formula, which permits one to attain an adequate description of dynamical and equilibrium properties even though the time increment τ=t/N is rather large. A remarkable property of the symmetric Trotter splitting is used to systematically eliminate the lower-order errors resulting from time discretization. This means a significant reduction of the number of time steps that are required to retain a given accuracy for a given net increment t=Nτ, and, therefore, significantly increasing the feasibility of path integral calculations. Yet another attractive feature of the present technique is that it allows for equations with singular diffusion matrices that are known to present a special problem within the scope of the path integral formalism. The favorable scaling of the fast Fourier transform is used to numerically evaluate the path integral on a grid. High efficiency is achieved due to the Stirling interpolation which dynamically readjusts the distribution function every time step with a mild increase in cost and with no loss of precision. These developments substantially improve the path integral method and extend its applicability to various time-dependent problems which are difficult to treat by other means. One can even afford to extract information on eigenvalues and eigenfunctions from a time-dependent solution thanks to the numerical efficiency of the present technique. This is illustrated by calculating the propagator and the lowest eigenvalues of a one-dimensional Fokker–Planck equation. The method is also applied to a two-dimensional Fokker–Planck equation, whose diffusion matrix does not possess an inverse (a so-called Klein-Kramers equation). The numerical applications show our method to be a dramatic improvement over the standard matrix multiplication techniques available for evaluating path integrals in that it is much more efficient in terms of speed and storage requirements.
On the ground and electronically excited states of Na3O: Theory and experiment
Authors: Hampe Oliver , Koretsky Geoffrey M. , Gegenheimer Martin , Huber Christian , Kappes Manfred M. , Gauss Jürgen
Published: July 2002

Na 3 O has been generated by reacting preformed sodium clusters with O2 and N2O in a crossed beam pickup arrangement. This “superalkali” species was probed by photodepletion spectroscopy coupled with one-photon ionization mass spectroscopy to yield: (i) a first measure of visible/NIR region photodissociation cross sections showing several broad absorption features, (ii) a rough determination of the ground state dissociation energy (1.48±0.04 eV), as well as (iii) a remeasurement of the ionization potential (3.69±0.15 eV). The experimental investigations were supplemented by quantum chemical ab initio calculations employing coupled-cluster methods for ground and excited states of Na3O. Experiment and theory are in good agreement, allowing a tentative assignment of the Na3O depletion spectrum while providing further evidence for the computed D3h ground state. Observed and calculated dipole-allowed electronic transitions are discussed in terms of the unusual electronic structure of this nominally one excess-electron species.
A complete active space valence bond method with nonorthogonal orbitals
Authors: Hirao Kimihiko , Nakano Haruyuki , Nakayama Kenichi
Published: July 2002

A complete active space self-consistent field (SCF) wave function is transformed into a valence bond type representation built from nonorthogonal orbitals, each strongly localized on a single atom. Nonorthogonal complete active space SCF orbitals are constructed by Ruedenberg’s projected localization procedure so that they have maximal overlaps with the corresponding minimum basis set of atomic orbitals of the free-atoms. The valence bond structures which are composed of such nonorthogonal quasiatomic orbitals constitute the wave function closest to the concept of the oldest and most simple valence bond method. The method is applied to benzene, butadiene, hydrogen, and methane molecules and compared to the previously proposed complete active space valence bond approach with orthogonal orbitals. The results demonstrate the validity of the method as a powerful tool for describing the electronic structure of various molecules.
Brillouin–Wigner based multi-reference perturbation theory for electronic correlation effects
Authors: Wenzel Wolfgang , Steiner Matthew M.
Published: July 2002

We investigate basis set reduction (BSR), a configuration-based multi-reference perturbation theory using an implicit effective Hamiltonian motivated by Brillouin-Wigner perturbation theory. This approach avoids the intruder-state and level-crossing problems by construction and yields a rapidly converging perturbation expansion. Formulated to systematically approximate multi-reference configuration interaction, BSR yields accurate results in second order, because it includes relaxation effects of the primary space wavefunction in the presence of the perturbation. We benchmark the method for molecules (CH2,O3) in which both dynamical and non-dynamical correlation effects are known to be important, obtaining accuracies of the order of 1 kcal/mol across the potential energy surface in second-order perturbation theory. We address the critical issues of perturbative orbital optimization for the primary orbital space, the choice of the secondary orbital space and the effects of single excitations.
Formation of the trimer ion core in the heterogeneous rare gas cluster ions
Authors: Hiraoka Kenzo , Shimizu Akitaka , Minamitsu Akihito , Nasu Masayuki , Wasada Hiroaki , Yamabe Shinichi
Published: July 2002

Thermochemical stabilities of the cluster ions composed of mixed rare gases were measured using a pulsed-electron beam high pressure mass spectrometer. The formation of trimer ion cores, i.e., A+(B)2 and A2+(B)1, was found when A and B are next to each other in the Periodic Table. This trend is similar to that for the pure rare gas cluster ions, i.e., the formation of ion core Rg3+ for Rg=He, Ne, Ar, Kr, and Xe. Although Ar and Xe are not next to each other in the Periodic Table, the formation of the trimer ion core is found for Xe+(Ar)2. This may be due to the fact that the ionization potentials of Ar and Xe are close to each other. The bond energies of larger cluster ions A+(B)2(B)n−2 and A2+(B)1(B)n−1 were found to be similar to those of homogeneous cluster ions (B)3+(B)n−3. The experimental bond energies were confirmed by ab initio calculations with a modified G2(MP2) method [e.g., 0.2 kcal/mol (expt) and 0.3 kcal/mol (theory) for Ar2+⋅He].
Universal equation of state for an interacting multicomponent mixture of polymers
Authors: Gujrati P. D.
Published: July 2002

We present a closed form universal equation of state for an interacting multicomponent mixture of polymers of any architecture and dispersity. The equation is obtained by solving the model on a Bethe lattice and goes beyond the random mixing approximation. The latter property endows our theory with features that are consistent with real systems. The equation of state, though an approximate one, is thermodynamically consistent and is valid even in the incompressible limit. The predictions of the equation are consistent with simulations and experiments, as discussed.
Localized low-frequency dynamics in SiO2 glass
Authors: Uchino Takashi , Yoko Toshinobu
Published: July 2002

We have carried out ab initio molecular orbital calculations on a cluster of atoms modeling the medium-range structure in SiO2 glass to investigate its low-frequency vibrational properties. The model cluster is composed of several types of n-membered silica rings (n=3, 4, 5, and 6), and its geometry was completely optimized at the Hartree–Fock/3-21G(*) level. We have shown that the optimized structural parameters (the average Si–O, O–O, and Si–Si bond distances Si–O–Si and O–Si–O bond angles) are in good agreement with the observed ones. The three- and four-membered rings resulted in regular geometries, namely, a nearly planar and a puckered configuration, respectively, whereas the optimized geometries of the five- and six-membered rings were rather distorted. The frequency calculations on the cluster have demonstrated that relative rotations of the SiO4 tetrahedra occur in the low-frequency (⩽150 cm−1) vibrational region. The calculated vibrational density of states exhibits a maximum at ∼45 cm−1 that matches the observed “boson peak” of SiO2 glass. These rotational motions of the SiO4 tetrahedra have shown to be localized in the four-, five-, and six-membered rings, and the resonant frequencies increase with decreasing ring size. We have also found that collective rotations of the SiO4 tetrahedra exhibit transverse-type acoustic modes. These localized dynamics on the medium-range length scale is a possible mechanism for the anomalous low-frequency harmonic excitations in SiO2 glass called the boson peak.