bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermodynamic properties of fluid helium at high pressures determined from speeds of sound
Authors: ten Seldam C. A. , Biswas S. N.
Published: July 2002

The equation of state and heat capacity of fluid helium have been determined at temperatures from 98.15 to 298.15 K and at pressures from 100 to 1000 MPa using a new computational method. This is based on stepwise construction of ρT isobars starting from an experimental reference pρ isotherm and using the previously reported sound speed data of helium as a function of pressure and temperature. In the region of overlap, the calculated equation-of-state data show significant discrepancies from those obtained previously with direct measurements.
Sideband modeling in molecular crystals N2 and CO2
Authors: Cardini G. , Righini R. , Löwen H. W. , Jodl H.-J.
Published: July 2002

Vibron-phonon excitation bands, phonon sidebands to the zero phonon line, mirror the one-phonon density of states g(ω) calculated in the harmonic approximation. The origin of bands in the sideband is investigated, e.g., phonon contributions from librational and translational modes or phonons at special points of the Brillouin zone. The temperature-dependent structure of the sideband (frequency shifts, line broadening) is due to anharmonic processes which modify g(ω): Temperature-dependent frequency shifts of maxima in the sideband are shown to depend mainly on the volume effect, whereas line broadening is due to phonon–phonon interactions, which are simulated in the lattice dynamics calculations of g(ω) by a special modeling procedure. Compared to the CO2 solid, effects are more pronounced in the N2 crystal due to the presence of strong mechanical and electrical anharmonicities. The latter give rise to multiphonon contributions in the vibron-phonon excitation process. Intensity changes with temperature can be explained by the thermal weighting of the one-phonon density of states.
Rotationally resolved photoionization of molecular oxygen
Authors: Braunstein M. , McKoy V. , Dixit S. N.
Published: July 2002

We report the results of theoretical studies of the rotationally resolved photoelectron spectra of ground state O2 leading to the X 2Πg state of O+2 via the absorption of a single vacuum ultraviolet photon. These studies elaborate on a recent report [M. Braunstein et al., J. Chem. Phys. 93, 5345 (1990)] where we showed that a shape resonance near threshold creates a significant dependence of the rotational branching ratios on the ion vibrational level. We also showed that analysis of the rotational branches yields detailed information on the angular momentum composition of the shape resonance. We continue this analysis giving a comprehensive derivation of the rotationally resolved cross sections and photoelectron angular distributions. We discuss the selection rules implied by these expressions and present very high resolution cross sections (J→J+) obtained using static-exchange photoelectron orbitals and explicitly taking into account the internuclear distance dependence of the electronic transition moment. These cross sections illustrate the selection rules and show more explicitly the angular momentum composition of the shape resonance. We also present rotationally resolved photoelectron angular distributions which would be expected at low energy.
Finite-element multiconfiguration Hartree–Fock calculations on the excitation energies and the ionization potential of oxygen
Authors: Sundholm Dage , Olsen Jeppe , Alexander S. A.
Published: July 2002

The 3P, 1D, and 1S states of oxygen and the 4S ground state of the oxygen cation have been studied using large numerical multiconfiguration Hartree–Fock calculations with systematically increased active spaces. The nonrelativistic valence limits for the excitation energies are 1.968 and 4.181 eV for E(3P–1D) and E(3P–1S), respectively. The nonrelativistic valence limit for the ionization potential is 13.593 eV.
Dielectric constant of a hard-sphere fluid with induced dipoles and quadrupoles: Comparison of theory and simulation
Authors: Logan David E. , Madden Paul A.
Published: July 2002

Our previously published virial expansion describing deviations from Clausius–Mossotti behavior in the dielectric constant of a hard-sphere fluid within the quadrupole approximation [Mol. Phys. 46, 715 (1982)] is shown to account well for recent simulation results.
On square-gradient theories of polymer blend interfaces
Authors: Zeng X. C. , Oxtoby David W. , Tang Hai , Freed Karl F.
Published: July 2002

A study is conducted on the square−gradient theories of polymer blend interfaces.Polymer blend exhibits a phase transition at a critical value of the interaction parameter. (AIP)
Raman investigation of rotational and translational excitations in K1−x(NH4)xI mixed crystals
Authors: Berret J.-F. , Sauvajol J.-L. , Haussühl S.
Published: July 2002

Using Raman scattering experiments we have investigated the rotational and translational modes in K1−x(NH4)xI mixed crystals for ammonium concentrations x=0.005, 0.07, 0.16, and x=0.46 and in a fully deuterated isomorph with x(D)=0.42. The A1g+4Eg and T2g Raman responses have been measured at atmospheric pressure in the paraelectric plastic phase (T=295 K) and in the orientational glassy phase (T=7 K). In the low-frequency region ν=5–600 cm−1, seven Raman active components could be identified. These bands are entirely due to the admixture of anisotropic NH+4 to pure KI. Their contributions result either from reorientations of the molecules between equivalent directions and rotational-like excitations around these preferential orientations, or from translational modes of the whole Brillouin zone. In this peculiar case, orientational disorder of the ammonium lifts wave vector conservation of the scattering processes. An interpretation of the different Raman allowed transitions is proposed. The composition and polarization dependences as well as the effect of deuteration for these modes are discussed with respect to earlier reports.
Molecular dynamics simulations of nonequilibrium interactions between two model thermally activated reactions
Authors: Gorecki J. , Kawczyński A. L.
Published: July 2002

The results of molecular dynamics simulations of nonequilibrium interactions between two thermally activated reactions which use the same reactant are presented. It is shown that a fast reaction with low activation energy may significantly reduce the rate of reaction with higher activation energy. Our results indicate that the nonequilibrium interactions between reactions may be important for modeling the behavior of complex systems.
Time-dependent self-diffusion in a semidilute suspension of Brownian particles
Authors: Cichocki B. , Felderhof B. U.
Published: July 2002

We study the time-dependent self-diffusion coefficient for a semidilute suspension of interacting, spherical Brownian particles. It is shown that the parameters needed for a simple approximation to the memory function, which we have proposed in earlier work, may be obtained from the steady-state perturbed pair distribution function. We test the approximation for an exactly solvable model in which the particles interact with a square step or well potential.
Electronic structure of tetraphenyldithiapyranylidene: A valence effective Hamiltonian theoretical investigation
Authors: Viruela-Martín R. , Viruela-Martín P. M. , Ortí E.
Published: July 2002

We present a theoretical investigation of the electronic structure of tetraphenyldithiapyranylidene (DIPSΦ4) using the nonempirical valence effective Hamiltonian (VEH) method. Molecular geometries are optimized at the semiempirical PM3 level which predicts an alternating nonaromatic structure for the dithiapyranylidene (DIPS) framework. The VEH one-electron energy level distribution calculated for DIPSΦ4 is presented as a theoretical XPS simulation and is analyzed by comparison to the electronic structure of its molecular components DIPS and benzene. The theoretical VEH spectrum is found to be fully consistent with the experimental solid-state x-ray photoelectron spectroscopy (XPS) spectrum and an excellent quantitative agreement between theory and experiment is achieved when comparing the energies of the main peaks. A detailed interpretation of all the experimental photoemission bands is reported in the light of the VEH results.
How to grow large clusters from SixD+y ions in silane or disilane: Water them!
Authors: Mandich M. L. , Reents W. D.
Published: July 2002

Growth of large cationic clusters is observed in real time for subsilane and subdisilane cations in the presence of silane/disilane–water mixtures. SiD+0–3 and Si2D+0–6 are created by electron impact in the trapped ion cell of a Fourier transform mass spectrometer and their sequential clustering reactions with 5% water: 95% silane/disilane are monitored for up to 80 s at total pressures of 10−7–10−5 Torr. Formation of SixDyO+z clusters out to at least 450 amu in silane and 650 amu in disilane can be seen on the available experimental time scales. The early portion of the sequence leading to large clusters has been elucidated for silane. Amazingly, of the possible subsilane cations, only SiD+ reacts with silane and water to form increasingly larger cluster sizes. Reactions of the other subsilane cations, SiD+0,2–3, do not continue without apparent limit. Initial growth of SiD+ proceeds in a highly specific fashion involving the formation of two critical doorway ions, Si4D+7 followed by Si4D7O+.

The growth pattern then fans out to include numerous alternating and parallel reactions with both SiD4 and D2O. Several general features of the growth reactions are seen. Reactions with SiD4 are noticeably slower than reactions with D2O. Cluster growth by bimolecular reaction with SiD4 and D2O occurs by addition of SiD2 and addition of an oxygen atom, respectively, accompanied by elimination of D2. Loss of additional molecules of D2 sometimes occurs, particularly as clustering proceeds to large sizes. Cluster growth by termolecular attachment of SiD4 or D2O is also seen. This process results in the formation of SixDyO+z complexes with SiD4 and D2O that appear to serve as important intermediates which enhance cluster growth rates as the total pressure is increased. Sequential clustering without apparent limit is only observed for subsilane and subdisilane cations with silane and disilane when water is present. On this basis, it is proposed that low levels of water contamination can provide a key ingredient for the chemistry which leads to the formation of the hydrogenated silicon particles found ubiquitously in silane plasmas.
Effects of product occupancy on self-organization in heterogeneous catalysis. II. Spatial pattern formation
Authors: Li Ru-Sheng , Horsthemke W.
Published: July 2002

We study spatial pattern formation in surface-catalyzed reaction systems where products block active sites. We formulate a reaction-diffusion model that is based on Langmuir–Hinshelwood surface-reaction kinetics and on surface diffusion of the adsorbed species. Product occupancy is found to be a key element in the mechanism of spatial pattern formation. Numerical simulations show that such systems can exhibit multiplicity of stationary patterns, including the coexistence of a spatially homogeneous steady state and inhomogeneous stationary patterns.
Cavities in the hard-disk crystal: A Monte Carlo simulation study
Authors: Sturgeon Kathy S. , Stillinger Frank H.
Published: July 2002

The equilibrium pressure in a crystal composed of hard spheres or disks may be determined from the statistical geometry of a single isolated monocavity (one large enough for addition of only one particle). Additional knowledge of the equilibrium concentration of monocavities permits evaluation of the chemical potential and absolute entropy. We have used Monte Carlo simulation to examine isolated monocavities in the rigid-disk crystal near close packing. The resulting pressures compare very favorably with those from previous simulations, and with asymptotic high-compression equations of state. The monocavities exhibit fluctuating polygonal shapes; the number of sides varies from three to six, with four the most probable occurrence.
A comparative study of time dependent quantum mechanical wave packet evolution methods
Authors: Truong Thanh N. , Tanner John J. , Bala Piotr , McCammon J. Andrew , Kouri Donald J. , Lesyng Bogdan , Hoffman David K.
Published: July 2002

We present a detailed comparison of the efficiency and accuracy of the second- and third-order split operator methods, a time dependent modified Cayley method, and the Chebychev polynomial expansion method for solving the time dependent Schrodinger equation in the one-dimensional double well potential energy function. We also examine the efficiency and accuracy of the split operator and modified Cayley methods for the imaginary time propagation.
Generation of optimal excitation pulses for two energy level systems using an inverse Fourier transform method
Authors: Guan Shenheng
Published: July 2002

The response of a two energy level system to low amplitude excitation is investigated. The flip angle of magnetization or polarization is found to be linearly related to the magnitude spectrum of the excitation pulse under the approximation of low amplitude excitation. This property can be used for synthesis of optimal excitation pulses. A comprehensive algorithm for generating optimal excitation pulses is proposed based on the response theory and the stored wave form inverse Fourier transform (SWIFT) algorithm developed previously. An example of the synthesis of a notch excitation pulse is given to demonstrate the flexibility of this newly developed method.
Phase behavior of binary polymer blends with composition dependent χ
Authors: Tang Hai
Published: July 2002

The introduction of a quadratic composition dependent χ into Flory–Huggins theory of binary blends is shown to create three new types of phase behavior in which two pairs of coexisting phases may be present in the two-phase region. This leads, for instance, to an unstable A-rich mixture separating into two A-rich phases instead of A- and B-rich phases.
Complex dynamical phenomena in systems with multiple stationary states at equilibrium and linear irreversible thermodynamics
Authors: Li Ru-Sheng
Published: July 2002

It is shown that the Onsager reciprocal relations are satisfied in systems with multiple stationary states at equilibrium and the occurrence of complex dynamical phenomena such as chemical oscillation in nonideal systems close to equilibrium is not contradictory to linear irreversible thermodynamics.
Vibronic spectroscopy of amorphous materials using higher order, multiresonant coherent anti-Stokes Raman spectroscopya)
Authors: Hurst Gregory B. , Wright John C.
Published: July 2002

Multiresonant coherent anti-Stokes Raman spectroscopy is performed with three tunable lasers on perylene doped polymethylmethacrylate (PMMA). Sharp vibronic features can be observed in vibronic scans at constant energy from the parent electronic transition when resonance is established within the inhomogeneously broadened electronic band. These features are attributed to the nonlinear line narrowing predicted by Ouellette and Denariez-Roberge for a higher order saturated coherent anti-Stokes Raman process since line narrowing should be absent for four wave mixing coherent anti-Stokes Raman spectroscopy. It is shown that the features are sharply dependent on the presence of a simultaneous vibrational resonance as is also predicted by the higher order coherent anti-Stokes Raman model. Excited state coherent anti-Stokes Raman spectroscopy with resonance enhancement from higher singlet states does not contribute to the narrow features since such a process would not have vibrational resonances. Conventional two laser coherent anti-Stokes Raman shows only a weak line at the vibronic transition.
An analysis of the methyl rotation and aldehyde wagging dynamics in the S (X̃ 1A′) and T1 (ã 3A″) states of thioacetaldehyde from pyrolysis jet spectra
Authors: Moule D. C. , Bascal H. A. , Smeyers Y. G. , Clouthier D. J. , Karolczak J. , Niño A.
Published: July 2002

Jet-cooled, laser induced phosphorescence (LIP) excitation spectra of thioacetaldehyde (CH3CHS, CH3CDS, CD3CHS, and CD3CDS) have been observed in the 15 800–17 300 cm−1 region in a continuous pyrolysis jet. The responsible electronic transition, T1 ← S0, ã 3A″← X̃ 1A′, results from an n→π* electron promotion and gives rise to a pattern of vibronic bands that can be attributed to activity of the methyl torsion and the aldehyde hydrogen out-of-plane wagging modes. Potential and kinetic energy surfaces were mapped out for the aldehyde wagging (α) and the torsional (Θ) internal coordinates by using 6–31G* Hartree–Fock calculations in which the structural parameters were fully relaxed. The potential and kinetic energy data points were fitted to double Fourier expansions in α and Θ and were incorporated into a two-dimensional Hamiltonian operator. The spectrum was simulated from the transition energies and the Franck–Condon factors and was compared to the observed jet cooled LIP spectra. It was concluded that while the RHF procedure gives a good description to the ground state dynamics, the triplet state surface generated by the UHF method is too bumpy and undulating.
The high-frequency viscosity of dispersions
Authors: de Kruif C. G. , Woutersen A. T. J. M. , Mellema J.
Published: July 2002

Several theories for the high-frequency viscosity of hard sphere dispersions are discussed and compared with experimental results. The theories based on a virial expansion are known to be valid for low volume fractions only. We show that the most satisfactory description is given by Beenakker’s multipole expansion theory.
Rotational spectrum of a short-lived dimer of oxirane and hydrogen chloride: Evidence for a bent hydrogen bond
Authors: Legon A. C. , Rego C. A. , Wallwork A. L.
Published: July 2002

Ground state rotational spectra of the three isotopomers (CH2)2O...H35Cl, (CH2)2O...H37Cl, and (CH2)2O...D35Cl of a short-lived hydrogen-bonded dimer have been detected in the reactive mixture of oxirane and hydrogen chloride by using a fast-mixing nozzle in conjunction with a Balle–Flygare Fourier-transform microwave spectrometer. Rotational constants, centrifugal distortion constants and Cl-nuclear quadrupole coupling constants were determined for each isotopomer. In particular, all four components χaa, χbb, χcc, and χac of the coupling tensor were obtained. A detailed analysis of the rotational constants allows the conclusion that the dimer has Cs symmetry, with a steeply pyramidal arrangement completed at oxygen by the hydrogen bond with HCl. Diagonalization of the complete Cl-nuclear quadrupole coupling tensor leads to the principal axis components χxx, χyy, and χzz (where z is the HCl direction in the dimer). The angle of rotation α is the angle between the HCl (z) direction and the a-axis direction in the equilibrium conformation of the dimer. It is larger by ∼10° than the angle γ between the O...Cl internuclear line and the principal inertial axis a in each case and implies that the hydrogen bond is bent by 180-θ=∼16.5° from the collinear arrangement O...H–Cl (θ=0). The angle 180-θ and the angle φ=76.2° made by the O...Cl internuclear line with the extension of the oxirane local C2 axis are interpreted in terms of a simple model of the hydrogen bond.
Kinetic-energy release in the dissociative double photoionization of OCS
Authors: Masuoka Toshio , Koyano Inosuke , Saito Norio
Published: July 2002

Kinetic-energy release distributions (KERDs) of fragment ions produced in the dissociative double photoionization of OCS have been studied in the photon energy region of 37–100 eV by use of synchrotron radiation and the photoion–photoion coincidence (PIPICO) method. The most probable KERDs are determined for the three major dissociation channels—namely, OC++S+, O++S++C, and C++S++O—of OCS2+ at selected photon energies. This was accomplished by analyzing the spectral profile of the PIPICO peaks measured at the pseudomagic angle (∼55°), which minimizes any effect of the anisotropic angular distributions of the fragment ions. For the OC++S+ channel at hν=37 eV, the KERD ranges from 3.0–6.3 eV with an average kinetic-energy release (AKER) of 4.5 eV, whereas the O++S++C channel at hν=100 eV exhibits a wide KERD (5.0–23.7 eV) with an AKER of 12.8 eV. The nearly continuous KERDs derived for the direct double photoionization from valence orbitals are discussed in relation to the electronic states of OCS2+. From the measurements of the AKERs as a function of the excitation photon energy, it is concluded that the fragment ions are produced in their excited states at the respective dissociation thresholds for most of the dissociation channels examined. This result is in sharp contrast to previous observations.
Two-dimensional nuclear magnetic resonance with sample flip for characterizing orientation distributions, and its analogy to x-ray scattering
Authors: Schmidt-Rohr K. , Hehn M. , Schaefer D. , Spiess H. W.
Published: July 2002

A two-dimensional (2D) exchange NMR (nuclear magnetic resonance) experiment with sample flip during the mixing time (direction exchange with correlation for orientation-distribution evaluation and reconstruction, DECODER) is presented, which allows investigation of molecular order in great detail. For anisotropic NMR interactions with axial symmetry (η=0), the features of the orientation distribution of certain molecular axes are reflected directly in the distribution of the intensity in the 2D spectrum. The orientation distribution in terms of two angles can be largely reconstructed by a simple rearrangement of the spectral intensity. Analogies between this NMR experiment and wide-angle x-ray scattering (WAXS) techniques for pole figure analysis are identified. Practical aspects of the DECODER method are considered in detail, e.g., sample orientations that are particularly suited for elucidating biaxiality with high resolution, or that are convenient in terms of sample preparation. Even with spectral overlap and for η≠0, the degree of order can be read immediately from the spectrum. Simulations and experimental 2H NMR spectra for deuterated polyethylene serve to exemplify the method.
Calculation of nuclear spin–spin couplings. VII. Electron correlation effects on the five coupling mechanisms
Authors: Fukui H. , Miura K. , Matsuda H. , Baba T.
Published: July 2002

Electron correlation effects on the five mechanisms in the indirect nuclear spin–spin coupling tensors are computed for H 19F, H2 17O,14NH3,13CH4, H 35Cl, 33SH2, 31PH3, and 29SiH4. The five coupling mechanisms consist of Fermi-contact (FC), spin-dipole (SD), Fermi-contact and spin-dipole cross term (FC/SD), orbital paramagnetic (OP), and orbital diamagnetic (OD) terms. Electron correlation contributions through the second order are calculated by the finite-field many-body perturbation theory (FF-MBPT). The results show an extremely large contribution of the (FC/SD) cross term to the anisotropic part of the couplings. The FC contribution is dominant in the isotropic part, but the OP term is considerable in HF, HCl, and H2O. Electron correlation effects are large in the FC contribution. They are small, but not negligible, in the other terms except in the OD term.
Sublimation studies of NpF4
Authors: Kleinschmidt P. D. , Lau K. H. , Hildenbrand D. L.
Published: July 2002

The sublimation reaction NpF4(s) = NpF4(g) was measured over the range 820–980 K using Knudsen effusion mass spectrometry. The ionization potential of NpF4(g) is 10.4 +/−0.3 eV. The enthalpy of sublimation from second law analysis is 288.1+/−10.0 kJ mol−1, while the second law entropy of sublimation is 199.1+/−11.0 J K−1 mol−1, both at 298 K. The calculated enthalpy of formation of NpF4(g) is −1585.9+/−17.0 kJ mol−1 and the entropy is 351.8+/−12.0 J K−1 mol−1 at 298 K.
Fragment state correlations in the dissociation of NO⋅HF(v=1)
Authors: Shorter Joanne H. , Casassa Michael P. , King David S.
Published: July 2002

The NO(v,J)-fragment population distributions and recoil energies were measured for the vibrational predissociation of NO⋅HF following excitation of the H–F stretch. Most of the available energy appears in NO vibration and/or HF rotation. There is little recoil momentum. All NO(v,J) fragments are formed with near δ-function recoil kinetic energies, correlated with HF fragments in a single rotational level. Two dissociation channels proceed with similar probability to produce NO(v=0) and NO(v=1). Only two rotational levels of HF are populated. One of the HF rotational states is correlated exclusively with NO(v=0) fragments, the other is paired with NO(v=1) fragments. Constraints on fragment angular momenta as well as energetics appear important for the dissociation dynamics. The presence of significant amounts of vibrationally excited NO fragments, in the absence of observable spectroscopic perturbations, implies that intramolecular vibrational redistribution proceeds as the dimer dissociates. The data support two possibilities for the NO⋅HF dimer bond energy: (1) D0=448±5 cm−1 with coincident pairs of fragments NO(v=0)+HF(J=12) and NO(v=1)+HF(J=8); (2) D0=1769±10 cm−1 with JHF=9 and 2, respectively.
The two-state linear curve crossing problems revisited. I. Analysis of Stokes phenomenon and expressions for scattering matrices
Authors: Zhu Chaoyuan , Nakamura Hiroki , Re Nazzareno , Aquilanti Vincenzo
Published: July 2002

The classic problems of the two-state linear curve crossing both for the same and the opposite sign of slopes of the linear diabatic potentials are analyzed in a unified way by exactly dealing with the Stokes phenomenon for the four transition points of the associated second-order differential equations. First, distributions of the transition points and the Stokes lines are fully clarified for the whole range of the two parameters which effectively represent the coupling strength and the collision energy. Secondly, the so-called reduced scattering matrices are found to be expressed in terms of only one (complex) Stokes constant. This is made possible by finding the relations among the six Stokes constants. Finally, this one Stokes constant is given exactly and analytically by a convergent infinite series.
Fluorescence lifetimes of EF 1Σ+g, GK 1Σ+g, H 1Σ+g, I 1Πg, and J 1Δg states of H2 studied by extreme ultraviolet–visible double resonance excitation
Authors: Tsukiyama Koichi , Ishii Juntarou , Kasuya Takahiro
Published: July 2002

Extreme ultraviolet–visible double resonance excitation has been employed to populate the EF 1Σ+g, GK 1Σ+g, H 1Σ+g, I 1Πg, and J 1Δg states of hydrogen molecules in the range between 111 500 and 114 000 cm−1. Tunable coherent extreme ultraviolet radiation prepared H2 in the B 1Σ+u (v=5, J) state. The visible laser radiation subsequently brought H2 to higher gerade states. Term values were determined for the J′=0–4 levels, which confirmed the systematic error of 8.01±0.03 cm−1 in the previous analysis by Dieke. The single rovibronic fluorescence lifetimes were measured under a collision-free condition. The fluorescence lifetimes of the I 1Πg(v′ = 0) and J 1Δg(v′ = 0) states, ∼20 and 25 ns, respectively, were found to depend little on the rotational levels. The EF 1Σ+g, GK 1Σ+g, and H 1Σ+g states, on the other hand, exhibited significant rotational dependence. The nonadiabatic coupling among the upper gerade manifolds explained the observed rotational dependence successfully.
Polarized laser induced fluorescence of BaO (X 1Σ+) produced in a crossed-beam reaction of Ba+SO2
Authors: Nakajima Atsushi , Munakata Toshiaki , Nagata Takashi , Kondow Tamotsu , Kasuya Takahiro
Published: July 2002

Rotational alignment of the product BaO in the crossed-beam reaction Ba+SO2→BaO+SO was studied as a function of collision energy. The collision energy was varied from 3.0 to 8.0 kcal/mol by changing the stagnation condition of the supersonic SO2 beam. The rotational alignment was probed by means of the polarized laser-induced fluorescence (PLIF) of the product. The distribution of the rotational vector was isotropic at the collision energy of 3.0 kcal/mol, but anisotropic at a higher collision energy, with the most probable alignment being perpendicular to the relative velocity vector. These results indicate that the reaction proceeds via a long-lived complex at a low collision energy, whereas at a higher collision energy, the lifetime of the complex is short enough to couple the rotational angular momentum to the total angular momentum.
Excitation spectra of 2,5-dihydroxy-p-benzoquinone monomer and hydrates
Authors: Redington Richard L. , Redington Theresa E. , Rajaram Bhavani , Field Robert W.
Published: July 2002

The fluorescence excitation spectrum of the lowest allowed singlet–singlet transition of jet-cooled 2,5-dihydroxy-p-benzoquinone is reported. The transition is assigned as S3←S0, 1Bu←1Ag, π*←π and the 0–0 band origin is at 275.55 nm. Twenty vibrational levels, which include half of the Ag fundamentals, are assigned for the S3 state. The observed laser-induced fluorescence transitions are structureless peaks with bandwidths that depend on the intensity of the excitation laser. No spectral multiplets attributable to intramolecular tunneling were observed. Deuterium isotope shifts of the 0–0 transition are +21 cm−1 per internal hydrogen bond. The 0–0 transitions of hydrate isotopomers with hydration shifts of only +11 cm−1 per water molecule are reported.
The role of microscopic solvation in the association reaction I–Ar12+I
Authors: Hu Xiche , Martens Craig C.
Published: July 2002

Molecular dynamics investigation of a prototypical model reaction I–Arn(n=12)+I reveals the role of microscopic solvation in association reactions: (a) enhancement of capture cross section of the incident I atom; (b) mediation of the migration of I to form I2; and (c) stabilization of I2 product by evaporative cooling.
The rotationally resolved 1.5 μm spectrum of the HCN–HF hydrogen-bonded complex
Authors: Kerstel E. R. Th. , Meyer H. , Lehmann K. K. , Scoles G.
Published: July 2002

We have measured the overtone spectrum of the CH stretching mode in HCN–HF. The vibrational predissociation rate is approximately twice that previously determined for fundamental excitation, whereas the complexation induced frequency shift is only marginally larger than that of the fundamental spectrum. These results are discussed in terms of a first-order perturbation theory treatment as set forth by LeRoy, Davies, and Lam [J. Phys. Chem. 95, 2167 (1991)]. We suggest that the frequency shift observed here might not only be due to complexation, but also to a long-range anharmonic interaction.
Collective dynamics of alkali chloride glasses: Molecular dynamics analyses of the dynamic structure factors
Authors: Kinugawa Kenichi
Published: July 2002

The collective dynamical properties of two glassy alkali chlorides [pure LiCl and the mixture 50(mol %) LiCl–40 KCl–10 CsCl] have been investigated by using molecular dynamics simulations. On the basis of van Hove’s description of the microscopic density fluctuations, the dynamic structure factors S(k,ω) have been calculated for 0.3≤k≤5.9 Å−1 to reveal phonon excitation and motional correlations between each ionic species. Examining the phonon dispersion curves, it is found that there are one longitudinal acoustic and two longitudinal optic branches in both glasses. In the LiCl glass, collective vibrations of the Li+ and Cl− ions are spatially delocalized for k≤1.6 Å−1 and are strongly correlated with each other. On the other hand, in the mixture glass, collective motions of the Li+ ions are almost independent of those of the Cl− ions. In this glass, there are the intense longitudinal optic (LO) vibrations localized on single Li+ ions, similar to an Einstein oscillation. The LO mode frequency for k≥1.5 Å−1 is constant at ∼280 cm−1 which corresponds to the Einstein frequency. In addition to the standard method to calculate the dynamic structure factors by means of van Hove’s description, we have successfully calculated the dynamic structure factors of the glasses in terms of the normal modes for Stillinger and Weber’s inherent structures. In this calculation, we have utilized Zemach and Glauber’s normal mode description which was originally formulated to give the dynamic structure factors for polyatomic molecules in gas phase. In this scheme, the effects of various physical factors on the k and ω-dependence of the dynamic structure factors are analyzed. Moreover, it is revealed that the mass-effect rather than the interionic forces dominates the differences in the phonon density of states between the two systems.
Real-time dynamics of clusters. III. I2Nen (n=2–4), picosecond fragmentation, and evaporation
Authors: Gutmann M. , Willberg D. M. , Zewail A. H.
Published: July 2002

In this paper (III) we report real-time studies of the picosecond dynamics of iodine in Ne clusters I*2Nen(n = 2–4) → I*2 + nNe. The results are discussed in relation to vibrational predissociation (VP), basic to the I2X systems, and to the onset of intramolecular vibrational-energy redistribution (IVR). The latter process, which is a precursor for the evaporation of the host atoms or for further fragmentation, is found to be increasingly effective as the cluster size increases; low-energy van der Waals modes act as the accepting (bath) modes. The reaction dynamics for I2Ne2 are examined and quantitatively compared to a simple model which describes the dynamics as consecutive bond breaking. On this basis, it is concluded that the onset of energy redistribution is observed in I2Ne2. Comparison of I2Ne and I2Ne2 to larger clusters (n=3,4) is accomplished by introducing an overall effective reaction rate. From measurements of the rates and their dependence on v′i, the initial quantum number of the I2 stretch, we are able to examine the dynamics of direct fragmentation and evaporation, and compare with theory.
Effects of lattice morphology upon reaction dynamics in matrix-isolated systems
Authors: Raff Lionel M.
Published: July 2002

The dynamics of the cis-d2-ethylene+F2 addition reaction and the subsequent reaction dynamics of the products isolated in vapor-deposited Ar matrices at 12 K are investigated using trajectory methods that incorporate nonstatistical sampling to enhance the reaction probabilities. The matrix-isolated cis-d2-ethylene+F2 system is generated using a combination of Monte Carlo, damped trajectory, and volume contraction methods. Transport effects of the bulk are simulated using the velocity reset procedure developed by Riley et al. [J. Chem. Phys. 88, 5934 (1988)]. The potential-energy hypersurface is the same as that employed in our previous investigations of the matrix-isolated, decomposition dynamics of 1,2-difluoroethane-d4 and the bimolecular cis-d2-ethylene+F2 system in face-centered-cubic (fcc) matrices [J. Chem. Phys. 93, 3160 (1990); 95, 8901 (1991)]. It is found that matrices generated by these methods are amorphous with numerous vacancies and other imperfections. On the average, there are approximately three vacancies about each lattice atom compared to the fcc crystal.

The calculated lattice density is about 82% that for a bulk fcc Ar solid. Computed radial distribution functions resemble those expected for a liquid which exhibits some short-range order. The imperfections of the lattice remain even after substantial annealing at 50 K. The calculated energy relaxation rate to the lattice phonon modes in these amorphous matrices is about a factor of 4 less than that for a close-packed fcc lattice. The 1,2-difluoroethane product is formed primarily via an αβ-addition process, as is the case for fcc matrices. However, the prominence of this pathway is greatly reduced. The major process leading to a fluoroethylene elimination product in amorphous matrices involves an atomic addition mechanism. Such a reaction path accounts for 94% of the elimination reactions. The probability of internal rotation about the C■C double bond in the fluoroethylene product is increased fivefold over that for fcc lattices. The calculated stabilization/elimination product ratio, the cis/trans ratios of fluoroethylene products, and the HF/DF elimination ratio are all found to be in fair to good accord with the reported experimental data. It is concluded that accurate simulation of matrix-isolation experiments requires a matrix model that properly represents the lattice structure present in the experiments.
Spontaneous curvature of surfactant films
Authors: Ennis J.
Published: July 2002

We employ a mean-field model for monolayers of short-chain surfactants at an oil–water interface, and include a quantitative consideration of the head-group interactions (both steric and electrostatic) and the interfacial geometry. The behavior is then analyzed numerically, giving values for the spontaneous curvature, the bending moduli, and the neutral plane position, as the molecular parameters are varied. The spontaneous curvature is found to have a ‘‘universal’’ behavior for different type of head groups, which leads to a confirmation of the geometric theory of the surfactant parameter. The values for the bending moduli are consistent with experiments, and for the case of electrostatic head group interactions we predict that kc is only weakly dependent on electrolyte concentration. The local continuum analysis of the bending energy of monolayers, on which other theories depend, is shown to be only valid close to the spontaneous curvature.
Exact versus truncated spectrally resolved exchange in a
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o calculations
Authors: Pendás A. Martín , Francisco E. , Recio J. M.
Published: July 2002

The truncated spectral resolution of the exact exchange operator for closed-shells systems, first proposed by Andzelm et al. [J. Chem. Phys. 83, 4565 (1985)], and currently used in atomic, molecular, and solid-state ab initio calculations is revisited. We show how stating the exchange matrix elements in terms of projected functions over the subspace subtended by the exact exchange operator provides a means to extract valuable information about the quality of the approximation. Special interest is paid on two-center exchange interactions with truncated monocentric spectral resolution (TMSR) of the operator. We have undertaken numerical studies of the exact and TMSR exchange for the Li+–Li+, Li+–Cl−, Cl−–Li+, Cl−–Cl−, and He–He closed-shell pairs with an special attention to the fictitious asymmetry introduced by the approximation. Our main conclusions are: (a) the TMSR approximation for two-center exchange interactions deviates monotonically from the exact exchange with increasing internuclear distance; (b) the TMSR matrix elements are more accurate as more diffuse the primitive functions are; (c) for a particular pair of dislike ions, the approximation works better when the expansion center contains the most contracted primitive functions.
Ion solvation in clusters
Authors: Rips Ilya , Jortner Joshua
Published: July 2002

Solvent size effects on ion solvation in clusters of polar molecules are explored. Expressions for the adiabatic and the vertical solvation energies of an ion imbedded in a cluster are derived within the framework of the mean-spherical approximation (MSA). The short-range order in the finite system is assumed to be similar to that in the bulk liquid, allowing to use the bulk MSA pair correlation function. The solvent shell structure is manifested in the size dependence of the solvation energy for several (4–5) solvent shells. The hydration energies and the vertical electron photodetachment threshold energies of halide anions in water clusters were evaluated. The bulk values of these physical parameters for large anions (I− and Br− ) are in good agreement with experimental data. The predicted size dependence of the vertical photodetachment threshold, Iv(n), is in good agreement with experimental data for the small I(H2O)−n clusters (1≤n≤15). For intermediate sized clusters (12≤n≤50) the theory predicts plateaus and local minima in the absolute values of solvation energy and Iv(n) due to the parallel solvent dipoles alignment in the distant solvation shells. For large clusters (n>125) the finite size correction to the solvation energy scales inversely with the cluster size (∝n−1/3). This behavior, as well as the value of the slope, are in agreement with the prediction of the continuum dielectric (CD) model. The absolute values of the solvation energy and Iv(n) predicted by MSA are significantly lower than within the CD model.
High frequency dielectric dispersion of polyelectrolyte solutions and its relation to counterion condensation
Authors: Penafiel L. Miguel , Litovitz Theodore A.
Published: July 2002

The dielectric properties of polyelectrolyte solutions are studied in terms of counterion condensation by measurements of the dielectric response of pH buffered Na polyacrylate solutions. pH values are selected to allow variation of the charge density parameter ξ in the range between 0.5–2.8, that is, across ξ=1, the theoretical critical level for counterion condensation. The dielectric increment of the high frequency dispersion, Δε2, is found to have nonzero values only above the counterion condensation threshold and is therefore linked to the occurrence of counterion condensation. Above the condensation threshold Δε2 (≊6) and its corresponding polarizability α∥2 (≊6×10−16 cm3) are found to be approximately constant with increasing ξ. This result is predicted by Manning’s polarizability model for condensed counterions which results in a good fit to the experimental data when the average length of the polyion segments parallel to the external field, Ls, is set to 284 Å. This value of Ls is also shown to be in relatively close agreement with the value calculated for the length of a rigid subunit in Mandel’s polyion model, obtained using the relaxation time of the high frequency dispersion. The length Ls, which is larger than the persistence length, is estimated to be of the order of magnitude of the correlation length between segments.
An electron–nuclear double resonance study of the lowest triplet state of pyrazine
Authors: Donckers M. C. J. M. , Schwencke A. M. , Groenen E. J. J. , Schmidt J.
Published: July 2002

The hyperfine and quadrupole tensors of the two nitrogen atoms and the hyperfine tensors of the four hydrogen atoms of pyrazine in the lowest triplet state T0 are obtained from optically-detected electron–nuclear double resonance (ENDOR) experiments on pyrazine-h4 in a single crystal of benzene-d6 at 1.2 K. Analysis of these tensors shows that pyrazine is in good approximation a planar molecule of D2h symmetry in the lowest triplet state. The in-plane structure changes significantly upon excitation into T0. An increase of the C–N–C angles is observed and a reduction of the C–N bond lengths. From the hyperfine tensors the distribution of the electron-spin density is derived. The nitrogen nonbonding orbitals together carry 36% of the total spin density and it is shown that the lowest triplet state of pyrazine is appropriately described in terms of a 3nπ* excitation. The results of the optically-detected ENDOR experiments give no indication of vibronic coupling of the lowest 3nπ* state with nearby 3ππ* states.
Theoretical interpretation of acetone–HF infrared spectrum in the gas phase
Authors: Bouteiller Y. , Latajka Z.
Published: July 2002

The substructure of the infrared spectrum for acetone-HF hydrogen-bonded complex has been reconstructed for the first time using ab initio calculations at the self-consistent-field approximation. Vibrational calculations of the νFH and νF⋅⋅⋅O transitions taking into account the mechanical anharmonicity effects have been made using a variational method. The main shoulders of the infrared spectrum are interpreted in terms of combination transitions and hot combination transitions.
Surface phase transitions and hydration forces
Authors: Kornyshev A. A. , Kossakowski D. A. , Leikin S.
Published: July 2002

The interplay between surface phase transitions and hydration forces (considered to be important in multilamellar lipid systems, DNA assemblies, and proteins) is studied using a mean-field Landau theory with two coupled order parameters. The latter represent water ordering at two surfaces separated by a thin water film. A complete analytical solution gives a phase diagram in the temperature/surface-separation plane, phase trajectories, and force–distance curves. The phase diagram shows nontrivial effects on surface phase transitions from hydration interaction between the surfaces. For example, these are second- and first-order transitions at finite separation, which are absent at isolated surfaces (infinite separation). The force–distance diagram displays correspondingly drastic effects of the phase transitions on hydration forces. This is a spike in the force at the second-order transition and a jump crossover from repulsion to attraction at the first-order transition.
Dielectric relaxation spectroscopy of reaction-controlled slowing of molecular diffusion in liquids
Authors: Tombari E. , Johari G. P.
Published: July 2002

The dielectric relaxation spectra of a liquid in which chemical reactions lead to the growth of a macromolecule have been measured at different instants of reaction and in the kHz and MHz frequency range. The change in the complex permittivity measured for a fixed frequency, as the reaction proceeds, is phenomenologically similar to that observed on varying the frequency for a time-invariant system and obeys a stretched exponential relaxation function. The increase in the relaxation time of the liquid with reaction time is similar to the corresponding increase in the extent of reaction. As the reaction proceeds, a second relaxation, namely the α-process, separates and shifts towards the lower frequency side of the spectrum. Concomitantly, its strength decreases as does the strength of the remaining high-frequency process. The manifestation of the negative feedback between chemical reactions and molecular diffusion is remarkably similar to that observed on supercooling a molecular or polymeric liquid. A formalism given here illustrates the general aspects of this similarity.
Molecular-dynamics simulations for charged colloidal dispersions
Authors: Fushiki Makoto
Published: July 2002

Salt-free charged colloidal dispersions are studied by molecular-dynamics simulations based on the primitive model. The system consists of macro ions and counter ions. The counter ions are treated with the continuous density distribution obtained by the Poisson–Boltzmann equation for a given configuration of the macro ions. The macro ions are evolved in time through interaction with the counter ion distribution. Simulations are done for a micellar solution and a latex dispersion. For the latter system, the radius and the volume fraction are set to 160 Å and 0.04, respectively, and the colloidal charge Z0 is varied from 50 to 400. Then the height of the first peak of the structure factor, S(kmax), is plotted as a function of colloidal charge. When this is extrapolated, it becomes much larger than the experimental result from small-angle neutron scattering at Z0=850. The S(kmax) of the simulation is also compared with that arising from the one component system with the Derjaguin–Landau–Verwey–Overweek (DLVO) potential. The results of the one component system are larger than that of the two component system. However, when the charge and another parameters in the DLVO potential are renormalized, the results of the one-component system become comparable to that of the two-component system.
Calculations of Einstein A coefficients for transitions between vibrational levels in NO(a 4Π)
Authors: Cosby P. C. , Huestis D. L. , Slanger T. G.
Published: July 2002

The dipole moment function calculated by Langhoff et al. [J. Mol. Spectrosc. 138, 123 (1989)] for NO(a  4Π) has been used in conjunction with an RKR potential to obtain the vibrational transition probabilities of this state, for levels v=1–19. The probabilities increase with v, from 5 s−1 for v=1 to a maximum of 34 s−1 for v=13. Spectral intensities have been calculated for the situation that a 4Π molecules are created at the top of the potential, and cascade down without collisional quenching. Inclusion of a radiative loss in the a–X system results in the accentuation of overtone 1100–2000 cm−1 emission in the spectrum. These infrared bands may be significant in an N+O recombination environment, such as afterglows and airglow.