bluuebunny/crossref_metadata_2025 huggingface dataset:
Calorimetric effects of intergranular water in ice
Authors: Salvetti G. , Tombari E. , Johari G. P.
Published: July 2002

The heat capacity, Cp, of microcrystalline and large or single crystal ice was measured from 270 to 272.6 K and the effect of the heat input (required before measuring the increase in temperature) on the measured Cp investigated. The measured Cp of microcrystalline solids was much higher than that of large and single crystal solids and increased further with increase in the heat input. At 271.16 K, the measured Cp was independent of the heat input, but for higher temperatures, it increased nonlinearly with the heat input. These effects are caused by the increase in the volume fraction of intergranular water in impurity free ice with increase in both its temperature and the heat input. A significant fraction of the heat used for Cp measurement is consumed in melting ice at grain junctions which does not raise the temperature. Thus, the measured Cp appears to be much greater than the true value. Quantitative analysis of the data yielded results which agree reasonably well with the prediction of a theory given in J. Chem. Phys. 100, 4548 (1994).
High-pressure study of the far-infrared collision-induced absorption of nondipolar liquids
Authors: Fujita Yoshitaka , Ikawa Shun-ichi
Published: July 2002

Far-infrared collision-induced absorption of liquid benzene, hexafluorobenzene, and 1,3,5-trifluoro- benzene were measured at pressures and room temperature with a synchrotron radiation source. The zeroth and second spectral moments of the absorption bands were obtained as a function of pressure. The collision-induced dipole moments and their time derivatives were obtained from the spectral moments for these liquids and previously measured carbon disulfide and carbon tetrachloride. The induced-dipole moments decrease with increasing pressure for the liquids studied except carbon disulfide for which the moment is nearly independent of pressure. This decrease in the moments is probably caused by increase in the cancellation effect of three-molecule correlation with an increase in pressure. The time derivatives of the collision-induced moments, on the other hand, increase with increasing pressure. This is explained by increase in collision frequencies with pressure.
Pressure- and temperature-variable viscosity dependencies of rotational correlation times for solitary water molecules in organic solvents
Authors: Wakai Chihiro , Nakahara Masaru
Published: July 2002

Deuteron nuclear magnetic resonance spin-lattice relaxation times T1 have been measured for solitary water molecules (D2O) at low concentrations in apolar and polar organic solvents at 30 °C at high pressures; D2O (30 mM) in C6H6 up to 90 MPa, D2O (60 mM) in CHCl3 up to 300 MPa, D2O (100 mM) in CH3CN up to 300 MPa. The rotational correlation times τ2R for D2O in the organic solvents increase with increasing pressure. The pressure effect on τ2R for D2O in solution is considerably larger than that on τ2R and η (viscosity) for the neat solvent. We have tested the two forms of modified Stokes–Einstein–Debye law; the linear and nonlinear forms are τ2R=τ02R+S(η/T) and τ2R=B(η/T)α, respectively. The rotational correlation times are linearly related to solvent viscosity divided by temperature (η/T) with a large positive intercept (τ02R≳0). It is shown that the linear form is practically better, and that the nonlinear form constrained at η/T=0 is invalid. The temperature-variable slope (ST) and the pressure-variable one (Sp) are markedly different, the ratios of Sp to ST being 0.2–0.3. The extended-diffusion models based on isolated binary collisions cannot be used to explain the observed pressure effect because of the neglect of the attractive solute–solvent interactions.
Nonadiabatic energies of the ground state of the hydrogen molecule
Authors: Wolniewicz L.
Published: July 2002

Possible sources of residual errors in the theoretical energies of the hydrogen molecule are investigated. Nonadiabatic corrections are computed for all bound, J≤10 X 1Σg+ ro-vibrational states of the six isotopic hydrogen molecules. The new results improve significantly the overall agreement with accurate experimental transition frequencies. In order to estimate the convergence errors of the Born–Oppenheimer energies generalized James–Coolidge functions with powers of the interelectronic distance, r12, up to 6 are used and the precision of the computations is increased. Except for the equilibrium separation, R=1.4011 bohr, the obtained potential energy curve is lower by a few thousandths of a wave number than any other reported variational result. This lowers the v=0 vibrational levels by 0.009 cm−1 and results in a dissociation energy of H2, D0=36118.069 cm−1.
Infrared spectroscopic studies on photolysis of methyl iodide and its clusters in solid parahydrogen
Authors: Momose Takamasa , Miki Masaaki , Uchida Mikio , Shimizu Takayuki , Yoshizawa Isamu , Shida Tadamasa
Published: July 2002

Methyl iodide is trapped as the monomer and as clusters in the parahydrogen, known as a quantum crystal, at temperatures below about 8 K. UV illumination of the deposited sample at about 5 K causes the dispersal of clusters and the production of the methyl radical, methane, and ethane as evidenced by their infrared absorption spectra. Thermal annealing of the photolyzed sample at temperatures up to 11 K results in the disappearance of the methyl radical, the enhancement of ethane, and the regeneration of methyl iodide. When the initial concentration of the iodide is small, the clusters in the deposited sample are suppressed. For such a sample the UV excitation produces the methyl radical and methane but the formation of ethane is negligibly small. Relevance of the present work to studies of photolysis in gaseous clusters of methyl iodide is discussed.
Determination of the intermolecular potential energy surface for (HCl)2 from vibration–rotation–tunneling spectra
Authors: Elrod M. J. , Saykally R. J.
Published: July 2002

An accurate and detailed semiempirical intermolecular potential energy surface for (HCl)2 has been determined by a direct nonlinear least-squares fit to 33 microwave, far-infrared and near-infrared spectroscopic quantities using the analytical potential model of Bunker et al. [J. Mol. Spectrosc. 146, 200 (1991)] and a rigorous four-dimensional dynamical method (described in the accompanying paper). The global minimum (De=−692 cm−1) is located near the hydrogen-bonded L-shaped geometry (R=3.746 Å, θ1=9°, θ2=89.8°, and φ=180°). The marked influence of anisotropic repulsive forces is evidenced in the radial dependence of the donor–acceptor interchange tunneling pathway. The minimum energy pathway in this low barrier (48 cm−1) process involves a contraction of 0.1 Å in the center of mass distance (R) at the C2h symmetry barrier position. The new surface is much more accurate than either the ab initio formulation of Bunker et al. or a previous semiempirical surface [J. Chem. Phys. 78, 6841 (1983)].
Solid-to-solid isostructural transition in the hard sphere/attractive Yukawa system
Authors: Rascón C. , Mederos L. , Navascués G.
Published: July 2002

A thermodynamically consistent density functional-perturbation theory is used to study the isostructural solid-to-solid transition which takes place in the hard sphere/attractive Yukawa system when the Yukawa tail is sufficiently short ranged. A comparison with results for the square well potential allows us to study the effect of the attractive potential form on the solid–solid transition. Reasonable agreement with simulations is found for the main transition properties as well as for the phase diagram evolution with the range of the attractive potential.
A new molecular dynamics method combining the reference system propagator algorithm with a fast multipole method for simulating proteins and other complex systems
Authors: Zhou Ruhong , Berne Bruce J.
Published: July 2002

An efficient molecular dynamics (MD) algorithm is presented in this paper for biomolecular systems, which incorporates a novel variation on the fast multipole method (FMM) coupled to the reversible reference system propagator algorithm (r-RESPA). A top-down FMM is proposed which calculates multipoles recursively from the top of the box tree instead of from the bottom in Greengard’s original FMM, in an effort to be more efficient for noncubic or nonuniform systems. In addition, the use of noncubic box subdivisions of biomolecular systems is used and discussed. Reversible RESPA based on a Trotter factorization of the Liouville propagator in generating numerical integration schemes is coupled to the top-down FMM and applied to a MD study of proteins in vacuo, and is shown to be able to use a much larger time-step than the standard velocity Verlet method for a comparable level of accuracy. Furthermore, by using the FMM it becomes possible to perform MD simulations for very large biomolecules, since memory and CPU time requirements are now nearly of order of O(N) instead of O(N2). For a protein with 9513 atoms (the photosynthetic reaction center), the efficient MD algorithm leads to 20-fold reduction in CPU time for the Coulomb interaction and approximately 15-fold reduction in total CPU time over the standard velocity Verlet algorithm with a direct evaluation of Coulomb forces.
The dynamics of succinonitrile in the plastic and liquid phases from the depolarized Rayleigh spectra
Authors: Wròż T. , Kubicki J. , Naskrȩcki R. , Bancewicz T.
Published: July 2002

The paper reports the determination of the times of the orientational relaxation of a succinonitrile trans isomer in the plastic and liquid phase in the temperature range from 292.3 to 348.9 K, by the depolarized Rayleigh light scattering method. The activation energy of the process of the trans isomer reorientation in the plastic phase was found to be 11.6±4.2 kJ/mol. The decay constants of the interaction-induced processes in the liquid and plastic phase as well as the squares of the effective anisotropy of the optical polarizability of both trans and gauche isomers and the anisotropy due to the interaction-induced processes were determined.
Photodissociation of HF in ArnHF (n=1–14,54) van der Waals clusters: Effects of the solvent cluster size on the solute fragmentation dynamics
Authors: Schröder Thomas , Schinke Reinhard , Liu Suyan , Bac̆ić Zlatko , Moskowitz Jules W.
Published: July 2002

A comprehensive study of the photodissociation of HF in ArnHF van der Waals clusters, with n=1−14,54, for an ultrashort δ(t)-pulse excitation, is presented. The emphasis is on the dependence of the photodissociation dynamics of the HF solute molecule on the size and geometry of the Arn solvent cluster. This cluster size range encompasses formation and closing of the first solvation shell, which occurs for n=12, the addition of the complete second solvent layer (n=54), as well as the change of the HF location in the cluster, from a surface site for n≤8 to the interior of a cage for n≥9 clusters. Evolution of the fragmentation dynamics is revealed by following how the H-atom kinetic energy and angular distributions, the survival probability, and cluster fragmentation patterns change as a function of the cluster size and structure. Classical trajectories are used to simulate the photodissociation dynamics. The probability distributions of the initial coordinates and momenta of the H and F atom are defined by accurate quantum five-dimensional eigenstates of the coupled, very anharmonic large amplitude intermolecular vibrations of HF in the cluster. All aspects of the dissociation process studied here are found to exhibit a strong dependence on the size and geometry of the ArnHF clusters.
Activated rate processes: Anharmonic corrections to the quantum rate
Authors: Georgievskii Yuri , Pollak Eli
Published: July 2002

Escape of a particle from a metastable potential, whose motion is governed by the generalized Langevin equation, is a common model of many chemical and physical activated processes in condensed phase. In the intermediate-to-strong damping regime the rate of escape is controlled by the particle dynamics near the barrier top. Since Kramers, the parabolic barrier approximation is commonly used to get the expression for the rate in this regime. We consider the influence of anharmonic corrections to the potential barrier on the quantum rate and get leading order corrections in terms of the inverse barrier height. New terms appearing in the quantum expression for the rate are associated with tunneling through the barrier and become important at low temperatures. The analytic theory is compared with recent numerically exact quantum simulations [M. Topaler and N. Makri, J. Chem. Phys. 101, 7500 (1994)].
Resonant states of helium atoms scattered from the Pt(110)-(1×2) surface
Authors: Krzyzowski Michael A. , Zeppenfeld Peter , Comsa George
Published: July 2002

The specular intensity of helium atoms scattered from the missing-row reconstructed clean Pt(110)-(1×2) surface is measured as a function of the helium beam energy. Sharp resonance features were observed from which four eigenvalues of the He–Pt interaction potential could be determined; ε 0=−6.05±0.05 meV, ε1=−2.93±0.05 meV, ε2=−1.05±0.05 meV, and ε3=−0.32±0.05 meV. Several model potentials were examined and the potential parameters were fitted to these bound states. Excellent agreement with the experimental results is obtained with the shifted-Morse-hybrid (SMH)-potential with a well depth D=−8.17 meV. The shape of the so determined potential is discussed and compared to results from previous studies of the same system.
Surface enhanced Raman spectroscopy of mass-selected carbon clusters deposited on silver surfaces
Authors: Haslett T. L. , Fedrigo S. , Moskovits M.
Published: July 2002

Mass-selected C, C2, and C4 cations were ‘‘soft-landed’’ on cold-deposited Ag films and the SERS spectra of the resulting deposits measured as a function of coverage. Very little amorphous or graphitic carbon formation was observed even at ∼1 monolayer coverage. A band at 760 cm−1 observed only for deposited C+ and C+2 is assigned to isolated Ag–C moieties. A band at 2010 cm−1 is ascribed to a cluster or clusters larger than C3.
Approximative calculation of short-pulse pump–probe ionization signals
Authors: Braun Michael , Meier Christoph , Engel Volker
Published: July 2002

Approximative calculations of pump–probe ionization signals induced by short laser pulses are compared to numerically exact calculations performed in first-order perturbation theory. The motivation is to demonstrate that in cases where the wavepacket prepared by the pump pulse does not move over a region where the difference between the potentials of the involved electronic states changes significantly, the computational effort can be reduced dramatically. This is of great importance for quantum mechanical simulations of systems with several degrees of freedom.
Experimental determination of conjugation lengths in long polyene chains
Authors: Kohler Bryan E. , Samuel I. D. W.
Published: July 2002

Since the low energy absorption spectrum of a linear polyene is dominated by the strongly allowed 1 1Ag→1 1Bu transition (S0 to S2), it can be reconstructed for any chain length if the 0–0 band excitation energy, the band profile, and intensity are known. From this premise, this paper uses information developed in high resolution experiments on well-defined linear polyenes to extract the distribution of effective conjugation lengths from absorption spectra of solutions of long linear polyenes. These distributions are found to be dominated by short conjugation lengths.
The caloric effect of adsorption at the electrified metal/electrolyte interface
Authors: Mulder Willem H.
Published: July 2002

A thermodynamic treatment of the polarizable metal/electrolyte interface is presented and generalizes the classical electro-capillary equation, which interrelates variations in interfacial tension, cell potential, and solution composition, to include temperature changes. The result can be used in investigations of the structure of electrified interfaces; for example, in the calculation of entropy changes which occur in the solvent layer adjacent to the metal surface (inner or Stern layer) and, in particular, in the study of phase transitions in insoluble organic monolayers adsorbed at an electrochemical interface.
Induction of optical transitions through complexation within Hg–rare gas van der Waals systems
Authors: Krim L. , Jouvet C. , Soep B. , Onda K. , Yamanouchi K. , Visticot J. P.
Published: July 2002

The high repulsive states of HgAr and HgNe van der Waals complexes, correlating with Hg 6s6d atomic states have been investigated by double resonance spectroscopy, through the first excited state A 30+ and B 31 of the complexes. The repulsive potentials have been fitted through numerical Franck–Condon simulations. They have been characterized by perturbative calculation as quasi-pure 6dΣ potentials in Hund’s case a. The strong Hg–rare gas electrostatic interaction potential overruns the spin–orbit interaction at distances shorter than 7 Å. These observed repulsive states are mostly of Ω=1 character correlating with 3D3 at infinite distances. The contribution from the potential of Ω=0− symmetry correlating with 1D2 is of minor importance. Therefore, the absorption in the repulsive states of the complex arises mostly from proximity induced absorption in an optically forbidden transition 3P1→3D3. A perturbative model accounts well for the bound free absorption intensities experimentally observed.
Thermal transport generated by an external force in a sheared dilute gas
Authors: Garzó V.
Published: July 2002

Energy and momentum transport in a strongly sheared dilute gas is analyzed in the context of the nonlinear Boltzmann equation. Thermal transport is generated in the system by the action of a nonconservative external force which tries to mimick the effects of a temperature gradient. By performing a perturbation expansion in powers of the field strength, we obtain expressions for the field susceptibility tensor and the shear viscosity coefficient up to second order in the force. They are highly nonlinear functions of the shear rate. It is shown that the choice of the heat field used in computer simulations yields a field susceptibility tensor different from the thermal conductivity tensor. In order to get equivalent results, a new shear-rate dependent force is suggested. The calculations presented here extend previous results derived from the Bhatnagar–Gross–Krook approximation [J. Chem. Phys. 101, 1423 (1994)].
Investigation of the Ã 3E↔X̃ 3A2 system of methylnitrene radical by laser spectroscopy
Authors: Shang Hairong , Yu Cheng , Ying Liming , Zhao Xinsheng
Published: July 2002

The vibrationally resolved laser-induced fluorescence spectra of the Ã 3E↔X̃ 3A2 system of CH3N radical were obtained experimentally. The dispersed spectra at different vibrationally excited states confirm that the system studied belongs to the CH3N radical. From the vibrational progressions in the excitation spectrum, following vibrational frequencies of the Ã 3E state were determined: ν1′=2943 cm−1, ν2′=1239 cm−1, ν3′=759 cm−1, and ν5′=1500 cm−1. In addition, the lifetimes of the Ã 3E state at four vibrational states (ν3′=0, 3, 4, and 6) were measured, which are, respectively 292.8±1.2, 203.4±1.2, 188.2±0.5, and 157.2±0.5 ns at a total pressure of 1.7 Torr. Our experiments show that the Ã 3E state is bound at least up to 4800 cm−1 and is relatively free from photochemistry.
The temperature dependence of the Cl2/GaAs(110) surface product distribution
Authors: Simpson W. C. , Tong W. M. , Weare C. B. , Shuh D. K. , Yarmoff J. A.
Published: July 2002

The reaction of Cl2 with GaAs(110) is studied with soft x-ray photoelectron spectroscopy (SXPS). The temperature dependence of the surface product distribution, in the range of 300–650 K, is derived from SXPS core-level and valence-band spectra and compared to known gas-phase product distributions. It is found that both Ga and As chlorides are formed at room temperature. Following reaction at temperatures above ∼400 K, no Cl remains on the surface. Instead, for temperatures up to ∼600 K an As overlayer is formed, whereas reaction at ∼650 K leads to the stoichiometric removal of Ga and As. These findings indicate that there is a direct correlation between the surface and gas-phase product distributions.
The solvation reaction field for a hydrogen atom in a dielectric continuum
Authors: Chipman Daniel M.
Published: July 2002

A reaction field exists even for a nonpolar solute embedded in a spherical cavity within a surrounding homogeneous dielectric continuum. This arises from the tail of the electronic wave function that penetrates beyond the cavity boundary into the dielectric region. This effect, which is neglected or treated only in cursory fashion in most reaction field implementations, is examined in detail for the simple case of a ground state hydrogen atom, where very accurate solutions of the relevant equations can be obtained. Properties considered include the penetration of the electron outside the cavity, the electronic density at the nucleus, the electron binding energy, the electrostatic free energy of solvation, the polarizability, and the vertical 1s→2p excitation energy. Also, the effect of the common approximation of neglecting the volume polarization and treating only the surface polarization contribution to the reaction field is critically evaluated.
A Monte Carlo study of wetting transitions in polymer blends confined to a capillary
Authors: Pereira Gerald G. , Wang Jian-Sheng
Published: July 2002

We investigate the problem of wetting transitions in polymer blends confined to a slitlike adsorbing capillary of thickness H by Monte Carlo methods. Two paths for capillary wetting are considered, either along a path of increasing temperature, T, or increasing surface chemical potential, μ1. We find that H can be thought of as an additional thermodynamic parameter which controls the nature of the transitions. We find that there exists a capillary critical separation, Hc. For separations less than Hc the blend falls in the one phase region of the phase diagram. Above Hc there exists a separation Hbulk, where the polymer blend begins to show bulk behavior. Hbulk is shown to separate regions of first order transitions (H<Hbulk) from critical wetting transitions (H≥Hbulk). Along a path of constant T and increasing μ1 we find for 2ξ<H1<H2<Hbulk that the first-order transitions between the two separations is shifted according to μc1(H2)−μc1(H1)∝1/H1−1/H2. We discuss the implications of these results for the case of H→∞ and compare them with theoretical results.
Collisional removal of OH (X 2Π,ν=7) by O2, N2, CO2, and N2O
Authors: Knutsen Karen , Dyer Mark J. , Copeland Richard A.
Published: July 2002

Collisional removal rate constants for the OH (X 2Π,ν=7) radical are measured for the colliders O2, CO2, and N2O, and an upper limit is established for N2. OH(ν=4) molecules, generated in a microwave discharge flow cell by the reaction of hydrogen atoms with ozone, are excited to ν=7 by the output of a pulsed infrared laser via direct vibrational overtone excitation. The temporal evolution of the ν=7 population is probed as a function of the collider gas partial pressure by a time-delayed pulsed ultraviolet laser. The probe laser light is resonant with the (0,7) band of the B 2Σ+−X 2Π transition. Fluorescence from the B 2Σ+ state is detected in the visible spectral region. We measure rate constants for CO2, (6.7±1.0)×10−11; N2O, (3.0±0.6)×10−11; O2, (7±2)×10−12; and N2, <6×10−13 (all in units of cm3 s−1).
Interaction of NO2 with Ru(001): Formation and decomposition of RuOx layers
Authors: Mitchell William J. , Weinberg W. Henry
Published: July 2002

We have investigated, using temperature-programed desorption, high-resolution electron energy loss spectroscopy and low-energy electron diffraction, the formation and decomposition of the suboxides of Ru which form after exposure of NO2 to the Ru(001) surface. The NO2 (radical) molecule is not limited by any significant kinetic barrier to its decomposition even at the high oxygen coverages considered. It is this barrier which restricts measurable rates of dissociative chemisorption of O2 to surface coverages below 0.5 under ultrahigh vacuum conditions. We have identified a subsurface state which is occupied after saturation of the surface overlayer. Utilizing isotopically labeled molecular oxygen and a detailed mixing/segregation model, we have been able to elucidate and quantify the nature of mixing between the surface and subsurface oxygen as a function of total oxygen uptake.
Structure and electronic properties of quinizarin chemisorbed on alumina
Authors: Frank Irmgard , Marx Dominik , Parrinello Michele
Published: July 2002

The anthraquinone dye molecule quinizarin is known to allow for persistent spectral hole burning up to liquid nitrogen temperatures after chemisorption on alumina surfaces. The mechanism underlying these improved hole-burning properties is not known, though is has been speculated that it might be related to intrinsic surface effects on the electronic structure of the dye. We approach this problem theoretically using gradient corrected density functional theory. The chemisorbed compound system is modelled by a periodically replicated nine layer slab which represents the (0001) surface of α-Al2O3. The chemisorption geometry obtained by geometry optimization and confirmed by Car–Parrinello molecular dynamics runs at room temperature is shown to be a perpendicular arrangement of quinizarin on the surface, where a chelate-like bond is formed with one exposed surface aluminum atom. In order to get information about the electronic structure, the frontier orbitals that are relevant for the description of the electronic excitation to the first excited state are evaluated for the isolated molecule, the chemisorbed molecule, and a quinizarin-aluminum-water complex. The strong red shift of the absorption frequency found in experiment upon chemisorption is reproduced. However, the results show that the shape of the frontier orbitals and hence the properties of the electronic excitation remain essentially unchanged by chemisorption. Thus, the differences in the behavior of the isolated and the chemisorbed dye observed in persistent spectral hole-burning experiments cannot be explained by genuine surface induced effects on the molecular electronic structure.
Semiempirical MNDO, AM1, and PM3 direct dynamics trajectory studies of formaldehyde unimolecular dissociation
Authors: Peslherbe Gilles H. , Hase William L.
Published: July 2002

Direct dynamics calculations are performed, using the semiempirical neglect of diatomic differential overlap (NDDO) molecular orbital theory, to explore the level of electronic structure theory required to accurately describe the product energy partitioning when formaldehyde dissociates into hydrogen and carbon monoxide. Trajectories are initiated at the saddlepoint and are propagated for the short time needed to form products, by obtaining the energy and gradient directly from the NDDO theory. The resulting product energy partitioning is compared to available experimental data and the findings of two previous trajectory studies, including one ab initio trajectory study at the HF/6-31G** level of theory [Chem. Phys. Lett. 228, 436 (1994)]. The MNDO, AM1, and PM3 semiempirical Hamiltonians are studied, as well as Hamiltonians based on specific reaction parameters (SRP). For the latter, the original PM3 and AM1 parameters are adjusted to reproduce some ab initio potential energy surface properties, such as stationary points and part of the reaction path. A series of NDDO-SRP Hamiltonians are chosen by fitting different features of a HF/6-31G** potential energy surface. Only qualitative agreement is found between the product energy distributions of the NDDO-SRP Hamiltonians and that of the HF/6-31G** Hamiltonian. This result is consistent with the well known difficulty of reproducing a HF/6-31G** Hamiltonian with a NDDO-SRP model, since dynamic correlation is not treated in ab initio SCF, but is incorporated into semiempirical methods. Trajectory results with NDDO-SRP Hamiltonians, which reproduce a few experimental and/or high-level ab initio stationary points, are in poor agreement with the experimental product energy partitioning. Reparameterizing the NDDO Hamiltonian is laborious, and only a few properties of the potential energy surface can be reproduced at the same time. This indicates the limitations of the NDDO-SRP approach, which might be well suited for locally interpolating ab initio data, but not for quantitatively describing global potential energy surfaces.
Energy and angular momentum control of the specific opacity functions in the Ba+HI→BaI+H reaction
Authors: Kalogerakis Konstantinos S. , Zare Richard N.
Published: July 2002

Crossed-beam and beam-gas experiments on the reaction Ba+HI→BaI+H have been performed, in which the most probable collision energy ranges from 3 to 17 kcal/mol. The results, combined with previous experimental studies on this reaction system, show a remarkable collision energy dependence. Between low and high collision energies, a transition occurs in the intensity, width, and peak location of the product vibrational and rotational population distributions. The onset of this transition is estimated to occur at approximately 5 kcal/mol. For collision energies smaller than 5 kcal/mol, the product vibrational distribution is bell shaped and peaks at v=12. For collision energies larger than 5 kcal/mol, a second maximum appears at v=0 in the vibrational distribution. The rotational distributions of the crossed-beam experiments are extremely narrow but broaden at lower collision energies. As the collision energy is increased above 5 kcal/mol, the BaI rotational excitation is very near the energetic limit, and the maximum for the BaI(v=0) rotational population distribution moves from J=415.5 to J=538.5. In contrast, below the transition onset, the maximum remains unchanged around J=420.5. Moreover, the peaks of the BaI(v=1) and BaI(v=2) rotational distributions appear at successively lower J values, as expected from energy conservation arguments. The nature of the kinematic constraints for this reaction allows the determination of the opacity functions for the production of the BaI product in a specific vibrational level v. Detailed analysis of the collision energy dependence of the specific opacity functions offers insight into the role of conservation of energy and angular momentum in influencing this reaction. At low collision energies, the maximum reactive impact parameter, bmax, is determined by an angular momentum (centrifugal) barrier. At collision energies larger than 5 kcal/mol, conservation of energy dictates the value of bmax. These two processes are identified as the mechanisms that control the Ba+HI reaction cross section. The transition between the two mechanisms provides an interpretation for the bimodal character of the BaI product internal-state distribution.
Velocity correlation functions, Fickian and higher order diffusion coefficients for ions in electrostatic fields via molecular dynamics simulation
Authors: Koutselos Andreas D.
Published: July 2002

The dynamic and transport properties of swarms of ions in a uniform electrostatic field are studied by using a molecular dynamics method. For a representative system, K+ in Ar, using a universal interaction model potential, second and third order ion-velocity correlation functions are determined at various field strengths. From them, Fickian diffusion coefficients parallel and perpendicular to the field, as well as higher order diffusion coefficients, Qzzz, are obtained within estimated overall accuracy 5% and 7%, respectively. Comparisons of the Fickian diffusion coefficients against results of the moment solution of Boltzmann kinetic equation and a Monte Carlo simulation method using the same interaction potential as well as against experimental data, reveal consistency among all calculation procedures and in addition agreement with drift tube measurements. These comparisons provide new tests for the accuracy of the employed interaction potential. The method has been applied for up to third order velocity correlations and diffusion coefficients but it is extendible to higher order dynamic and transport properties.
Application of the reference interaction site model theory to analysis on surface-induced structure of water
Authors: Kinoshita Masahiro , Hirata Fumio
Published: July 2002

We consider the extended simple point charge (SPC/E) model water near uncharged and charged macroparticles. It is shown that the size of the macroparticle tested is sufficiently large and it acts in effect as a planar wall. The reference interaction site model (RISM) theory is employed, and a robust and very efficient algorithm has been developed for solving the basic equations. The algorithm is a hybrid of the Picard and Newton-Raphson methods. The Jacobian matrix is just part of the input data and need not be recalculated at all. Sufficiently accurate solutions are obtained in only 7 to 16 iterations. The reduced density profile of oxygen atoms near an uncharged macroparticle indicates significant dewetting and a clear contribution of the icelike structure. The surface potential calculated is positive but very small. When the macroparticle is charged, the number of water molecules in the close vicinity of the surface increases regardless of the sign of the charge. The interaction between uncharged macroparticles with sufficiently large sizes immersed in pure water is attractive and very strong at small separations. The interaction between negatively charged macroparticles is ‘‘more repulsive’’ than between positively charged macroparticles. On the whole, however, the asymmetry of the SPC/E water in responding to positively and negatively charged surfaces is not high. It has been shown that the results obtained from the RISM theory combined with the SPC/E water are in qualitative accord with those calculated by the reference hypernetted-chain (RHNC) theory for hard spheres with embedded dipoles and tetrahedral quadrupoles, in terms of reduced density profiles, surface potential, and macroparticle interactions.
Molecular radiative transport. II. Monte-Carlo simulation
Authors: Nunes Pereira E. J. , Berberan-Santos M. N. , Martinho J. M. G.
Published: July 2002

The theory of radiative transport allows in principle the accurate calculation of the fluorescence intensity and anisotropy decays, and of the fluorescence spectrum and macroscopic quantum yield, under given conditions. However, most of the coefficients of the theoretical expressions are in general not amenable to analytic form, and even their numeric computation is quite difficult. Given the probabilistic nature of the underlying processes of absorption and emission, a Monte-Carlo (MC) simulation built upon the basic theoretical equations is particularly well suited for the task. In this work, we discuss and carry out detailed simulations for a realistic system (rhodamine 101 in ethanol) in a finite three-dimensional volume that reproduces a common fluorescence cell. The two usual geometries of detection are considered: front face and right angle. The MC simulation method developed allows, for the first time, the accurate calculation of the effect of radiative transport on fluorescence intensity and anisotropy decays, time-resolved and steady-state spectra, as well as on the values of the macroscopic quantum yield and steady-state anisotropy. Because the spatial distribution of each generation of excited molecules can also be obtained with this method, a direct and clear picture of the spatial evolution of the excitation is also obtained.
Self-consistent determination of fullerene binding energies BE (C+n–C2), n=58⋅ ⋅ ⋅44
Authors: Wörgötter R. , Dünser B. , Scheier P. , Märk T. D. , Foltin M. , Klots C. E. , Laskin J. , Lifshitz C.
Published: July 2002

Using recently measured accurate relative partial ionization cross section functions for production of the C60 fragment ions C+58 through C+44 by electron impact ionization, we have determined the respective binding energies BE(C+n–C2), with n=58,...,44, using a novel self-consistent procedure. Appearance energies were determined from ionization efficiency curves. Binding energies were calculated from the corresponding appearance energies with the help of the finite heat bath theory. Then using these binding energies we calculated with transition state theory (TST), the corresponding breakdown curves, and compared these calculated ones with the ones derived from the measured cross sections. The good agreement between these breakdown curves proves the consistency of this multistep calculation scheme. As the only free parameter in this procedure is the binding energy C+58–C2, we studied the influence of different transition states chosen in the determination of this binding energy via TST theory and iterative comparison with breakdown curve measurements. Based on this study we can conclude that extremely loose transition states can be confidently excluded, and that somewhat looser transition states than those used earlier result in an upward change of the binding energy of less than 10% yielding an upper limit for the binding energy C+58–C2 of approximately 7.6 eV.
Potential barriers, tunneling splittings, and the predicted J=1←0 spectrum of CH+5
Authors: Kolbuszewski M. , Bunker P. R.
Published: July 2002

It has been established from earlier ab initio calculations that in its equilibrium configuration the CH+5 molecular ion consists of an H2 moiety bound to the apex of a pyramidal CH+3 group; the H2 group is approximately perpendicular to the C3 axis of the CH+3 group, and the binding energy is about 15000 cm−1. Two internal motions, the torsion and the flip, provide connections with low barriers between all 120 symmetrically equivalent minima on the potential energy surface so that all proton permutations are feasible. We present the results of new high level ab initio calculations of the parts of the potential energy surface associated with these two motions, and in particular we determine the continually optimized structure, and associated electronic energy, for the CH+5 molecular ion as it undergoes the flip motion. For the flip motion we numerically integrate the one-dimensional Schrödinger equation for the tunneling to determine the splitting. Since this splitting is small (1.4 cm−1) we can incorporate it into a 120×120 matrix treatment of the simultaneous torsion-flip dynamics to determine the energy level splitting pattern in the J=0 and 1 states, in the approximation of neglecting other tunneling pathways, and we calculate the positions of the lines in the J=1←0 millimeter wave spectrum.
Analytical potential energy surface for the CH4+Cl→CH3+ClH reaction: Application of the variational transition state theory and analysis of the kinetic isotope effects
Authors: Espinosa-García J. , Corchado J. C.
Published: July 2002

We present a potential energy surface for the CH4+Cl→CH3+Cl reaction, based on the analytical function J1 for the analog CH4+H→CH3+H2 reaction by Joseph et al. To calibrate the new surface we chose the reactant and product experimental properties as reference data. The forward and reverse rate constants were calculated using variational transition state theory with large curvature transmission coefficients over a wide temperature range, 200–1000 K. The variational effects were concluded to be small for this reaction, and good agreement with experimental rate constants was found in both forward and reverse reactions. The kinetic isotope effects (KIEs) at different temperatures for the forward and reverse reactions were also analyzed showing always a ‘‘normal’’ behavior. The factor analysis of the KIEs in the forward reactions indicated high vibrational and tunneling contributions at low temperatures.
On the calculation of dynamical properties of solvated electrons by maximum entropy analytic continuation of path integral Monte Carlo data
Authors: Gallicchio E. , Berne B. J.
Published: July 2002

The maximum entropy analytic continuation method, to determine the dynamical properties of a solvated electron from equilibrium path integral Monte Carlo data, is applied to the calculation of the optical absorption spectra, real time correlation functions, and transport coefficients of an excess electron in water, supercritical helium, and supercritical xenon. Comparisons with experiments and with analytical theories are presented.
Binary hard chain mixtures. I. Generalized Flory equations of state
Authors: Wichert John M. , Gulati Harpreet S. , Hall Carol K.
Published: July 2002

In this series of two papers we study the thermodynamics of binary hard chain mixtures. Here, a generalized Flory-dimer (GF-D) equation of state is derived for binary hard chain mixtures composed of chains of variable length and segment diameter. Compressibility factors predicted by the GF-D equation of state developed here and by the previously derived generalized Flory equation of state are compared to previous Monte Carlo results for hard monomer/hard chain mixtures, and to new molecular dynamics (MD) hard monomer/hard chain and hard chain/hard chain mixture simulation results. Compared to the MD simulations, the GF-D theory is found to be quite accurate, with an average error of about 3% at liquid-like densities.
High resolution spectroscopic study of arsine: 3ν1 and 2ν1+ν3 dyad: The tendency of symmetry reduction
Authors: Ulenikov Oleg N. , Sun Fu-ge , Wang Xiao-gang , Zhu Qing-shi
Published: July 2002

The high resolution spectrum of AsH3 3ν1 and 2ν1+ν3 stretching overtone dyad was recorded and analyzed. The major vibration-rotation parameters of these overtones were obtained. The result indicates that these overtone vibrational states are close to the local mode limit and the rotational levels show the tendency to approach an asymmetric top.
Contributions to the temperature dependent band gap in conjugated polymers: A Monte Carlo simulation
Authors: Severin Mattias , Inganäs Olle
Published: July 2002

The electronic and conformational properties of a single polyacetylene chain in solution has been studied as a function of temperature. Monte Carlo simulations, using the Su–Schrieffer–Heeger Hamiltonian, show that bond-length variations, and rotations about the bonds both give important, but opposing, contributions to the temperature dependent band gap. For the case of polyacetylene, the net effect is shown to be an increase of the band gap with temperature, similar to experimental findings on other conjugated polymers. The electronic structure is coupled to the global conformation via the conjugation length. Temperature induced off-diagonal disorder is also found to produce tails in the density of states that reaches into the band gap of the ground state polymer, which could be of experimental significance.
Measurement of the ν3 fundamental transition moment and vibrational relaxation rates of the CD3 radical
Authors: Rudolph R. N. , Hall G. E. , Sears T. J.
Published: July 2002

Diode laser absorption spectroscopy was used to measure the transition dipole moment for the ν3 (degenerate asymmetric stretch) fundamental vibration of CD3 produced by the 193-nm photodissociation of acetone-d6. The time evolution of the ground state absorption was used to measure the vibrational relaxation rates from the manifold of excited vibrational states to the ground state of CD3 following collisions with various bath gases. The transition dipole moment was determined to be 0.030±0.002 D and the vibrational relaxation coefficients were determined to be (2.5±0.2)×10−13 for argon, (2.6±0.2)×10−13 for helium, (3.15±0.2)×10−13 for nitrogen, and (4.3±0.5)×10−12 for acetone-d6 in units of cm3 molecules−1 s−1. Results are compared to literature values for CH3 and the mechanism of the relaxation is discussed.
Ab initio study of van der Waals interaction of formamide with a nonpolar partner. Ar⋅⋅⋅H2NCOH complex
Authors: Kukawska-Tarnawska Beata , Chal/asiński Grzegorz , Szçześniak Mal/gorzata M.
Published: July 2002

The potential energy surface (PES) of the ground state of the Ar–formamide van der Waals complex is calculated by using the supermolecular Moller–Plesset perturbation theory and the related perturbation formalism of intermolecular interactions. Six stationary points (five minima) are identified on the PES. The structure with the Ar atom coplanar with formamide and located between O and H atoms of the COH fragment is found to correspond to the global minimum. The best estimates of Re and De are 3.75 Å, and 217 cm−1, respectively. The structure with Ar located above the C atom proved to be another important minimum, only slightly shallower than the global one. The best estimates of related Re, and De are 3.625 Å, and 210 cm−1, respectively. Behavior of the different fragments of the formamide molecule in van der Waals interactions is discussed in the context of different components of the interaction energy, in particular the exchange repulsion term. The relationship between the lowest energy structures of the Ar–formamide complex and the local depletions of the formamide charge density is elucidated.
Neutron, nuclear magnetic resonance, and dielectric study of ion motion in pyridinium hexafluorophosphate
Authors: Wa̧sicki J. W. , Kozak A. , Paja̧k Z. , Czarnecki P. , Belushkin A. V. , Adams M. A.
Published: July 2002

Extensive investigation of polycrystalline pyridinium hexafluorophosphate and its perdeuterated analog was performed by neutron scattering, NMR, and dielectric methods in a wide temperature range. The results of different spectroscopic methods enabled us to propose a general model of both ion reorientations and to determine their activation parameters. The aging effects observed slightly change the hindering barriers and significantly affect the phase transition temperature and the pyridinium cation dynamics.
Photofragmentation dynamics of Mg2H2O+
Authors: Chen J. , Cheng Y. C. , Kleiber P. D.
Published: July 2002

We report studies of the structure and dissociation dynamics of the Mg2H2O+ ion–molecule complex. The weakly bound clusters are formed in a supersonic molecular beam equipped with a laser vaporization source, mass-selected, and studied by laser photofragmentation spectroscopy in a tandem time-of-flight mass spectrometer. Broad structureless molecular absorption bands are observed in the red (610–745 nm), the green (515–595 nm), and the near UV (335–390 nm) spectral regions, and are assigned to transitions localized on the Mg2+ chromophore. Three daughter ions (Mg+, Mg2+, and MgH2+) have been observed. We have studied the competitive branching into accessible product channels as a function of photolysis wavelength in these bands. In order to understand the dissociation mechanisms we have carried out ab initio calculations of the ground and low lying excited states of the Mg2H2O+ complex. These studies give insight into the dissociation dynamics. Results from these experiments are compared and contrasted with previous work on the photodissociation of Mg2CO2+.