bluuebunny/crossref_metadata_2025 huggingface dataset:
Grafted polymer brushes in polymeric matrices
Authors: Grest Gary S.
Published: July 2002

A molecular dynamics simulation of polymeric brushes immersed in a melt of mobile polymer chains is presented. The brush height and monomer density profiles are presented for chains of length N grafted at one end to a solid surface immersed in a melt of polymers of the same type. As the chain length P of the free chains increases there is a crossover from a wet to a dry brush in agreement with scaling and self-consistent field theories. Since the interaction between all monomers are identical, this crossover is driven purely by entropic interactions. The simulation results are compared to earlier simulations in which the solvent is treated as a continuum. The explicit introduction of solvent molecules increases the equilibration time of the system by a factor of 40–100 even for small P. The dynamics of the brush chains were also studied. The relaxation time for the radius of gyration and end-to-end distance was found to be independent of P for small P in agreement with the prediction based on a hydrodynamic model by Johner and Joanny [J. Chem. Phys. 98, 1647 (1993)]. For larger P, comparable to an entanglement length, the relaxation time increases as the viscosity of the solvent crosses over from P−1 to P−3.
Polarization effects in time resolved incoherent anti-Stokes Raman spectroscopy
Authors: Hofmann M. , Zürl R. , Graener H.
Published: July 2002

Recently the anti-Stokes spectroscopy after strong vibrational excitation via resonant absorption was rediscovered, for the study of population relaxation dynamics. A closer theoretical inspection of this experimental approach reveals that properly chosen polarization conditions allow the study of molecular reorientational motion. Furthermore the sign of a/γ (the invariants of the Raman polarizability tensor) can be determined. Experimental data for chlorinated alkanes will be presented.
An instantaneous normal mode analysis of solvation: Methyl iodide in high pressure gases
Authors: Kalbfleisch T. S. , Ziegler L. D. , Keyes T.
Published: July 2002

An instantaneous normal mode (INM) analysis of the short-time solvation dynamics of the B-state (200 nm) Rydberg excitation of methyl iodide in high pressures of Ar (ρ*=0.08, 0.3, and 0.8) is presented. Solute–solvent interaction potentials for this system have been determined by previous absorption and resonance scattering studies. The B-state transition energy correlation function (ECF), also known as the solvation correlation function, calculated by the linear coupling INM theory is in good agreement with the ECF given by molecular dynamics simulation at short times (≤150 fs) that are well beyond the so-called inertial regime (≤100 fs). The shape and peak frequency of the solvation spectra are relatively constant over the wide range of bath densities considered here in contrast to the INM total density of states. This is attributed to the relative density independence of the first peak in the solute–solvent pair distribution function. Similarly, the ECFs are also only modestly dependent on solvent density. A cancellation of the density dependence of the solvation spectrum area and the second moment of the absorption spectrum line shape, and the nearly constant solvation spectrum shape, accounts for the relatively weak density dependence of the ECF decay. A computationally fast, semianalytical method for calculating the weighted density of states incorporating both two- and three-body correlations is shown to be in reasonable agreement with the total INM weighted density of states. A participation ratio analysis of the eigenvectors contributing to the solvation spectrum reveals that single solvent–solute interactions are responsible for the solvation response of the ρ*=0.08 and 0.3 solutions. More collective, totally symmetric solvent motions involving just a few solvent particles, in addition to single solvent interactions, contribute to the solvation response at the liquidlike density of ρ*=0.8. The effects of solvent–solvent repulsions on the shape of the solvation spectrum at this density are also evident by this INM analysis and, in part, account for the modest increase in ECF decay rate at the highest density studied here.
A complete active space valence bond (CASVB) method
Authors: Hirao Kimihiko , Nakano Haruyuki , Nakayama Kenichi , Dupuis Michel
Published: July 2002

A complete active space valence bond (CASVB) method is proposed which is particularly adapted to chemical interpretation. A CASVB wave function can be obtained simply by transforming a canonical CASSCF function and readily interpreted in terms of the well known classical VB resonance structures. The method is applied to the ground and excited states of benzene, butadiene, and the ground state of methane. The CASVB affords a clear view of the wave functions for the various states. The electronic excitation is represented in a VB picture as rearrangements of the spin couplings or as charge transfers which involve breaking covalent bonds and forming new ionic bonds. The former gives rise to covalent excited states and the latter to ionic excited states. The physical reasons why it is so difficult to describe the ionic excited states at the CASSCF level with a single active space and why the lowest 1 1B+2 state in cis-butadiene is so stabilized compared to the corresponding 1 1B+u state in the trans isomer are easily identified in view of a VB picture. The CASVB forms a useful bridge from molecular orbital theory to the familiar concepts of chemists.
Low-lying ionization potentials of O3 and NO2 using the multiconfigurational spin tensor electron propagator method
Authors: McKellar Alexander J. , Yeager Danny L. , Nichols Jeffrey A. , Golab Joseph T.
Published: July 2002

The multiconfigurational spin tensor electron propagator method (MCSTEP) is a single particle Green’s function (or electron propagator) method for determining the low-lying principal vertical ionization potentials (IPs) and electron affinities (EAs) of atoms and molecules. It was specifically designed to handle cases where the initial state has nondynamical correlation and/or is open shell. We have applied MCSTEP for the first time to triatomic molecules composed entirely of second row atoms. The two cases we present are O3 and NO2—for the former nondynamical correlation is present in the ground (initial) state and for the latter the ground (initial) state is open shell. MCSTEP results are accurate compared to experiment and other forefront theoretical techniques.
Microwave spectroscopy of mixed alkali halide dimers: LiNaF2
Authors: Biermann S. , Hoeft J. , Törring T. , Mawhorter R. , Lovas F. J. , Suenram R. D. , Kawashima Y. , Hirota E.
Published: July 2002

Rotational spectra of 6LiNaF2 and 7LiNaF2 were measured by microwave absorption and Fourier-transform spectroscopy at different low rotational temperatures obtained by an adiabatic expansion in a beam and by collisional cooling in a cold absorption cell. Precise rotational constants, quartic centrifugal constants, quadrupole coupling constants, and the electric dipole moment were determined from least-squares fits of the experimental transition frequencies. Bond lengths and angles and the dipole moment are found to be in good agreement with recent ab initio calculations. As in homogeneous dimers, monomer to dimer bond expansions calculated from an ionic model are too large. The electric dipole moment from the ionic model is in excellent agreement with the ab initio result when experimental bond distances and angles are used and a 100% ionic character of the dimer bonds is assumed.
Translational spectroscopy studies of the photodissociation dynamics of O−4
Authors: Sherwood C. R. , Hanold K. A. , Garner M. C. , Strong K. M. , Continetti R. E.
Published: July 2002

An investigation of the photodissociation dynamics of the dimer anion O−4 at 523.6, 349.0, and 261.8 nm is reported. Product translational energy and angular distributions have been obtained using photofragment translational spectroscopy in a fast ion beam. At all wavelengths photodissociation (O−4+hν→O2+O−2) is observed to proceed via a rapid parallel electronic transition, with the photofragment angular distribution strongly peaked along the laser electric vector. The lowest energy photodissociation channel produces O2(a1Δg) and ground state O−2(X2Πg), indicating that O−4 is a doublet anion. The partitioning of energy in the dissociation reveals a complicated wavelength dependence.
Mean-field theory of the interface between a homopolymer and a binary-polymer mixture
Authors: Genzer Jan , Faldi Alessandro , Composto Russell J.
Published: July 2002

In this paper we investigate the properties of the interface between a homopolymer (A) and a binary polymer mixture (B:C). We have extended the self-consistent field (SCF) model of Helfand [Macromolecules 25, 1676 (1992)] by including the effects of the numbers of segments of polymers on the polymer volume fractions, the interfacial excess of B, zB*, the interfacial tension, γABC, the interfacial width, wABC, and the A–B interfacial overlap, wAB. Other parameters include the polymer interaction parameters, χAB, χAC, and χBC, and the B volume fraction in the B:C mixture, φB∞. As expected, the B component segregates to the A/B:C interface when A–B interactions are more favorable than the A–C ones (χAB<χAC). This interfacial adsorption lowers the interfacial tension and width compared to the pure A/C case, demonstrating the compatibilizing effect of B. As χBC varies from favorable to unfavorable, B segregation is found to increase. Similarly, at constant interaction parameters, B segregation increases as the number of the B segments increases. Furthermore, the addition of a small amount of high molecular weight B to an immiscible A/C blend is found to reduce greatly γABC and thus stabilize the system. Calculations for the case of an athermal mixture B:C with χAB=χAC reveals that the shorter chains are ‘‘entropically driven’’ to the A/B:C interface.
Weighted-density approximations for homogeneous binary fluids
Authors: Jo Sang Gyu , Kim Soon-Chul
Published: July 2002

Weighted-density approximations (WDAs) for the binary mixtures, which are based on the position-independent densities, are considered. Homogeneous properties of these approximations are investigated. We show that the WDA’s based on the position-independent densities lead to the hypernetted chain (HNC) equation for the binary mixtures, as do the WDAs based on the global average densities for the single-component systems. From these results, we obtain the required conditions for the WDAs to satisfy the HNC equation for the binary mixtures.
Layering in a two-component liquid system undergoing a phase separation
Authors: Hamraoui A. , Privat M. , Sellami H.
Published: July 2002

The phase change possibilities in the surface phase formed in a binary liquid system (water 2,5dimethylpyridine) in contact with a solid (silica) have been examined on the basis of adsorption isotherms determined at ten different temperatures. The liquid mixture undergoes a liquid–liquid demixing but also, some 20° below the critical point, a solid phase separation. On the l.h.s. of a liquid–liquid coexistence curve 2,5dimethylpyridine, diluted in water, adsorbs step by step, and on the r.h.s. the relative adsorption is wave shaped. Both behaviors can be explained by a layering process, i.e., a solid–liquid surface demixing, which unexpectedly does not exclude a separate liquid–liquid surface demixing, which could constitute the prewetting process. This analysis is compared with known theories and its consistency is carefully checked on a thermodynamical basis.
A study of HeI photoelectron spectroscopy on the electronic structure of the nitrate free radical NO3
Authors: Wang Dianxun , Jiang Peng , Qian Ximei , Hong Gongyi
Published: July 2002

A continuous nitrate free radical (NO3) beam is produced in situ by the pyrolysis of pure gas phase N2O5 at 280 °C (±0.5 °C) in a double-heater inlet system on a double-chamber UPS Machine-II which was built specifically to detect transient species. The HeI photoelectron spectroscopy (PES) of the NO3 radical is recorded for the first time. Five obvious bands emerge on the PE spectrum of the NO3 radical. A very sharp peak with the lowest ionization energy at 12.55±0.01 eV originates from electron ionization of the HOMO for the NO3 radical, corresponding to the NO3+(1A1′)←NO3(2A2′) transition. To assign the bands of the PE spectrum of the NO3 radical, the improved density functional theory (DFT) calculation based on the Amsterdam density functional (ADF) program package has been carried out according to D3h symmetry for the neutral ground state of the NO3 radical and the equilibrium geometries of several ionic states of the cationic species. The ionization energies computed are in reasonable agreement with the PES experimental ionization energies. Both PES experimental and DFT calculation provide evidence on the lowest triplet and singlet states of the NO3+ cation.
Density functional calculations for Mgn+ clusters
Authors: Davidson Ernest R. , Frey Regina F.
Published: July 2002

Calculations using an extended basis set and the Becke 3-parameter exchange functional and Perdew 86 correlation functional are used to predict the geometry and isotropic hyperfine coupling constants for Mgn+ clusters (n⩽6). The Mg3+ results agree with our previous MRSDCI results and disagree with another recent DFT calculation. For n=4 and 5, our results agree with previous work. For n=6, we obtain a trapezoid-based bipyramid that was not considered in any previous papers. Based on these calculations, we are able to rationalize the experimental hyperfine couplings.
Fluctuations in the random sequential adsorption of disks and parallel squares: Finite size effects at low coverages
Authors: Román F. L. , White J. A. , Velasco S.
Published: July 2002

This work is focused on explicit finite size corrections in the calculation of the fluctuation in the number of hard disks and parallel (aligned) hard squares deposited on a finite flat surface through a random sequential adsorption process. Explicit size effects are made evident by using a finite-system pair correlation function for calculating the fluctuation. The method is based on the relation between this pair correlation function and its infinite-system counterpart. A diagrammatic density (coverage) expansion of the corresponding infinite-system pair correlation function is used to calculate the low-coverage behavior of the fluctuation. Results also include border effects due to consider a finite size region for evaluating the fluctuation. A comparison with Monte Carlo computer simulations shows an excellent agreement between theoretical and simulation results.
Reversible diffusion-controlled reactions between two species in a fluid
Authors: Felderhof B. U. , Jones R. B.
Published: July 2002

A model suspension of diffusing spherical particles with an internal degree of freedom taking two discrete values, or equivalently a two-component suspension with reactions between species, is studied. The internal degrees of freedom of neighboring particles are coupled by an Ising type interaction of short range. The dynamics of the Ising variables is described by a master operator of Glauber form. On a macroscopic level small deviations of the two densities from their equilibrium averages obey transport equations with memory character. The memory kernel is derived from the Mori formalism. The two-body contribution to the cluster expansion of the memory function is obtained analytically at zero wavevector. The spectrum of the memory function shows a marked dependence on the presence or absence of an external field, the ratio of time scales for transitions between states and for translational diffusion, and on the antiferromagnetic or ferromagnetic character of the Ising interaction.
Photoinduced desorption of potassium atoms from a two dimensional overlayer on graphite
Authors: Hellsing B. , Chakarov D. V. , Österlund L. , Zhdanov V. P. , Kasemo B.
Published: July 2002

We present an experimental and theoretical investigation of K atom desorption from the basal plane of graphite at 83 K induced by low energy photons (3–6 eV). The 2D potassium overlayer is characterized by low energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and work function measurements. At monolayer coverage (5.2×1014 atoms cm−2), the dependence of the cross section on photon energy has a threshold at ℏω≈3.0 eV and rises up to a maximum of 1.8±0.4×10−20 cm2 at 4.8 eV. The coverage dependence of the photoyield reflects the existence of two phases of adsorbed K, dilute ionized photo-active and close-packed photo-neutral, respectively. The observed photodesorption is a single-photon, nonthermal event, consistent with a substrate-mediated mechanism. The desorption results from attachment of optically excited hot electrons to the empty 4s state of ionized potassium. The theory predicts in this case a Gaussian line shape of the photoyield vs photon energy. Fitting the model parameters to the experimental data, we determine (i) the energy and slope of the excited state potential energy curve, and (ii) the position and width of the potassium-induced 4s resonance. The present findings combined with other available data for potassium on graphite are used to construct 1D potential energy curves along the surface normal for K+ and K0. The calculated cross sections for s- and p-polarized light are in qualitative agreement with the measurements.
Threading dynamics of a polymer through parallel pores: Potential applications to DNA size separation
Authors: Åkerman Björn
Published: July 2002

DNA orientation measurements by linear dichroism (LD) spectroscopy and single molecule imaging by fluorescence microscopy are used to investigate the effect of DNA size (71–740 kilo base pairs) and field strength E (1–5.9 V/cm) on the conformation dynamics during the field-driven threading of DNA molecules through a set of parallel pores in agarose gels, with average pore radii between 380 Å and 1400 Å. Locally relaxed but globally oriented DNA molecules are subjected to a perpendicular field, and the observed LD time profile is compared with a recent theory for the threading [D. Long and J.-L. Viovy, Phys. Rev. E 53, 803 (1996)] which assumes the same initial state. As predicted the DNA is driven by the ends into a U-form, leading to an overshoot in the LD. The overshoot-time scales as E−(1.2−1.4) as predicted, but grows more slowly with DNA size than the predicted linear dependence. For long molecules loops form initially in the threading process but are finally consumed by the ends, and the process of transfer of DNA segments, from the loops to the arms of the U, leads to a shoulder in the LD as predicted. The critical size below which loops do not form (as indicated by the LD shoulder being absent) is between 71 and 105 kbp (0.5% agarose, 5.9 V/cm), and considerably larger than predicted because in the initial state the DNA molecules are housed in gel cavities with effective pore sizes about four times larger than the average pore size. From the data, the separation of DNA by exploiting the threading dynamics in pulsed fields [D. Long et al., CR Acad. Sci. Paris, Ser. IIb 321, 239 (1995)] is shown to be feasible in principle in an agarose-based system.
Clusterization in supercooled states of glycerol-like liquids and its manifestations in different phenomena
Authors: Lishchuk Sergey V. , Malomuzh Nikolay P.
Published: July 2002

The present work is devoted to investigation of the temperature and frequency dependencies of the different relaxing values in highly-viscous glycerol-like liquids. The consideration is based on microinhomogeneous structure conception, allowing us to give the consecutive explanation to both the thermodynamic and kinetic properties. The main attention is paid to analysis of heat capacity, dielectrical permeability and longitudinal sound velocity. The influence of measuring time and preparation one of highly-viscous states on observed values are discussed.
Near ultraviolet photolysis of HFCO: The H+FCO channel
Authors: Reed Claire L. , Kono Mitsuhiko , Langford Stephen R. , Hancock Thomas W. R. , Dixon Richard N. , Ashfold Michael N. R.
Published: July 2002

The technique of H (Rydberg) atom photofragment translational spectroscopy has been used to study the process HFCO(S1)→H+FCO(X̃), near its appearance threshold, at excitation wavelengths ca. 247 nm. Analyses of the resulting total kinetic energy release spectra lead to an accurate determination of the C–H bond strength: D0(H–FCO)=34950±20 cm−1. The resulting FCO fragments are observed to be formed with little internal energy, distributed mainly in the form of a-axis rotation. Fragmentation is shown to involve S1–T1 intersystem crossing, followed by rapid passage along the minimum energy path to the eventual H+FCO products. This minimum energy path involves passage over (or H atom tunnelling through) a saddle point, the height of which is ⩾4740 cm−1 above the dissociation asymptote. The observed propensity for a-axis rotation in the FCO product reflects changes in the parent geometry as it evolves along the C–H dissociation coordinate on the T1 surface; past the saddle point, the barrier energy is released largely in the form of product recoil.
Dielectric study of phase transitions in 3-nitro-4-chloro-aniline
Authors: Wróbel Stanislaw , Gowda B. Thimme , Haase Wolfgang
Published: July 2002

3-nitro-4-chloro-aniline (NCA) exhibiting a phase transition between two solid phases has been studied by frequency domain dielectric spectroscopy. Dielectric studies show that molecular dynamics of NCA changes distinctly at the freezing point (370 K) and at the transition (242 K) from the high-temperature solid phase I(Ph I) to the low-temperature solid phase II(Ph II). In the Ph I just below the freezing point the molecules most probably perform a uniaxial fast reorientation around the pseudohexagonal symmetry axis of the benzene ring. On the other hand upon decreasing temperature in Ph I there are strong and temperature-dependent antiparallel correlations of molecular dipole moments. The correlations are of electrostatic origin and they cause a strong temperature dependence of the Kirkwood correlation factor leading to freezing out intermolecular reorientation. However, in the lower temperature range of the Ph I there is still an enhanced contribution to the static dielectric permittivity originating most probably from anharmonic and anisotropic librations of molecules and/or from intramolecular motion of the NO2 group observed also by x-ray studies. The relaxation process observed in the Ph I-Ph II pretransition region behaves like a soft mode. The relaxation time shows non-Arrhenius behavior in the vicinity of the Ph I-Ph II transition. Below this transition the intramolecular reorientation becomes frozen out leading to antiferroelectric order in the low-temperature Ph II. The phase transition observed is discussed qualitatively in terms of the Fröhlich model of the order-disorder transition.
Influence of association on the liquid–vapor phase coexistence of simple systems
Authors: Bresme Fernando , Lomb Enrique , Abascal José L. F.
Published: July 2002

The liquid–vapor phase diagram of an associating fluid interacting via a central force model potential is computed by means of the Gibbs ensemble Monte Carlo simulations. The Hamiltonian contains two components, a harmonic oscillator potential which allows for chemical association of particles and a Lennard-Jones interaction. The bonding potential depends on three parameters, bonding distance L, potential depth De, and force constant ke. We have studied the influence of L on the phase coexistence properties of the system. For small L the liquid phase shrinks and the results suggest that for short enough L, the stable liquid phase disappears. In addition to this, the coexistence curves exhibit a large change in the coexistence densities as bonding distance is shortened. The fitting of the coexistence data to scaling laws shows that a classical value for the critical exponent β may be adequate to describe the phase boundaries of a system with short bonding distance whereas both classical and Ising values would be suitable to describe the coexistence densities for a large L. Finally, the effect of association on the asymmetry of the liquid–vapor coexistence curve is discussed.
Thermal conductivity of an organic clathrate: Possible generality of glasslike thermal conductivity in crystalline molecular solids
Authors: Michalski Darek , White Mary Anne
Published: July 2002

The present investigation of an organic clathrate, hexakis(phenylthio)benzene with carbon tetrabromide guests, shows its thermal conductivity (determined from 39 to 146 K) to be similar to the “universal” thermal conductivity of glasses, despite its crystalline structure. We attribute the low thermal conductivity and its positive temperature derivative to efficient scattering of the heat-carrying acoustic phonons by localized optic modes. This behavior may well be common for molecular solids.
The infrared spectrum of pyrazine in its metastable triplet state. A free-electron-laser study
Authors: Kok P. , Groenen E. J. J. , van Amersfoort P. W. , van der Meer A. F. G.
Published: July 2002

Vibrational transitions of pyrazine in the lowest triplet state have been observed as transient changes in the intensity of the phosphorescence induced by the free-electron laser FELIX. All seven fundamental infrared-active modes in the range of 250 to 1600 cm−1 have been detected and all vibrational frequencies are found to be considerably lower than the corresponding ones in the ground state.
Exchange energy of H2 calculated by the surface integral method with the Coulson–Fischer wave function
Authors: Johann Ch. , Tang K. T. , Toennies J. P.
Published: July 2002

The exchange energy of the H2 molecule in the chemical bond region is calculated by the surface integral method with the well known Coulson–Fischer wave function. Compared with the triplet–singlet energy difference from separate electronic structure calculations, the surface integral method is shown to be simple and accurate.
Fluctuations around nonequilibrium steady states of electrochemical reaction systems and concentration polarization: A formalism of stochastic thermodynamics of concentration polarization
Authors: Luo Jiuli
Published: July 2002

At mesostatistical level a stochastic thermodynamics of the concentration polarization in nonequilibrium steady state electrode processes is presented. In this formalism a simplified stochastic model of irreversible electrode process is suggested. Based on this model the fluctuations around the nonequilibrium steady states of electrochemical reaction systems are discussed. It turns out that the nonequilibrium departure of electrode potential of the steady electrode process with hesitative diffusion step consists of two pasts; the concentration polarization following the Nernst formula of order V0(V, volume of the electrochemical reaction system) and an additional contribution (of order V−1 in the Gaussian regime of the distribution) due to the nonequilibrium fluctuations. Furthermore, the nonequilibrium nonideality of the electrode solution is analyzed. An explicit relation between the coefficient of the electrolytic ions at nonequilibrium steady state and corresponding nonequilibrium correlation has been derived.
Instantaneous normal mode spectra of quantum clusters
Authors: Chakravarty Charusita , Ramaswamy Ramakrishna
Published: July 2002

The spectrum of instantaneous normal mode (INM) frequencies of finite Lennard-Jones clusters is studied as a function of the extent of quantum delocalization. Configurations are sampled from the equilibrium distribution by a Fourier path integral Monte Carlo procedure. The INM spectra, average force constants and Einstein frequencies are shown to be interesting dynamical markers for the quantum delocalization-induced cluster solid–liquid transition. Comparisons are made with INM spectra of quantum and classical Lennard-Jones liquids. The methodology used here suggests a general strategy to obtain quantal analogs of various classical dynamical quantities.
Non-additive intermolecular forces from the spectroscopy of Van der Waals trimers: A comparison of Ar2–HF and Ar2–HCl, including H/D isotope effects
Authors: Ernesti Andreas , Hutson Jeremy M.
Published: July 2002

Variational calculations of vibrational energies, rotational constants and angular expectation values are carried out for the trimers Ar2–HCl, Ar2–DCl, Ar2–HF and Ar2–DF. Calculations are performed on pairwise additive potential energy surfaces and on surfaces including a variety of non-additive contributions. Attention is focused on the bending levels corresponding to hindered internal rotation of the HX molecule in the complex, several of which have been observed by high-resolution spectroscopy. The results confirm that it is crucial to include dispersion, induction and short-range effects when modelling the non-additive forces in molecular systems. It is found that the model of non-additive forces previously proposed by Ernesti and Hutson [Phys. Rev. A 51, 239 (1995)] works well for the bending bands of Ar2–HCl, Ar2–DCl and Ar2–DF as well as Ar2–HF. In addition, a new distributed model of the non-additive dispersion energy is proposed, in which the triple-dipole energy is partitioned between two anisotropic sites in the HX molecule.
Gas–liquid nucleation of nonideal binary mixtures. II. Examination of classical predictions
Authors: Laaksonen Ari
Published: July 2002

The predictions of binary classical nucleation theory for a surface active system are compared with results obtained from density functional calculations. It is found that the unphysical predictions produced by the classical theory are connected to a large disparity of vapor and liquid mole fractions in the equilibrium diagram. In this regime, density functional nuclei show, when the vapor mole fraction is held fixed, very different compositions in the center of the nucleus at low and high vapor densities, although the surface layer density profiles are qualitatively very similar. The classical theory, on the other hand, is not able to account for the compositional differences of the nuclei. However, it is found that the classical theory is able to predict the slopes of nucleation rate versus vapor density correctly when the vapor mole fraction is held constant. It is shown that this is equivalent to a correct prediction of the total number, n1+n2, of molecules in the nucleus.
Mobility of atomic hydrogen in solid krypton and xenon
Authors: Eberlein J. , Creuzburg Martin
Published: July 2002

Atomic hydrogen is produced in xenon and krypton matrix by in situ x-ray induced photolysis of the dopants water, butane, acetone, or methane and trapped interstitially forming the “caged hydrogen” with characteristic VUV absorption bands. Their thermal bleaching as investigated between 8 K and 45 K cannot be described by a first-order process. A random walk model is introduced considering the hydrogen atoms to move in a thermally activated diffusive motion to traps where they become “invisible.’’ The temperature dependence of the mean trapping time is Arrhenius-type and characteristic only for the matrix. Infrared experiments have confirmed a partial recovery of the dopant molecule only in krypton. Annealing-induced IR absorption bands are assigned to a new complex which involves two hydrogen atoms.
Spinodal decomposition under shear flow
Authors: Padilla Paz , Toxvaerd So/ren
Published: July 2002

The spinodal decomposition of a two-dimensional model binary fluid undergoing planar Couette flow has been studied by molecular dynamics simulation. The effect of the strength of the shear field on the growth of the domains was analyzed. The main effect of the shear field is the deformation of the domains which results in anisotropic structure developments. We have characterized these anisotropic structures by measuring the domain size in two different directions, the direction of the flow and the direction of the shear. We find that the dependence of the deformation of the domains on the strain applied to the system shows the same behavior as found in experiments. Moreover, we find that the shear flow can enhance the domain growth in the direction of the flow and it can restrain and even suppress this growth in the direction normal to the flow. The influence of the morphology on rheological properties was also analyzed. We find that viscosity depends on the quench time and the shear field, and is caused by the extension and direction of the interfacial area.
Small angle scattering of a polydisperse system of interacting hard spheres: An analytical solution
Authors: Riello Pietro , Benedetti Alvise
Published: July 2002

Recently, general results obtained from the study of the autocorrelation functions of liquids were used for a more accurate analysis of the full small angle scattering pattern from mono and polydisperse systems with interparticle interaction. The effects of polidispersity and density, in close-packed systems of hard spheres (case 1) and hard spheres with permeable shell (case 2) with Schulz distribution of radii, were investigated by Mengoni [J. Chem. Phys. 87, 2560 (1987)] by a numerical integration of the intensity in the framework of the Born–Green kinetic theory for fluids. In this paper we generalize the case 1 reported by Mengoni to a system of spherical particles of radius ri with an interacting hard sphere potential of radius RHSi∝ri and analytically solve the equations for the intensity.
Energy transfer processes in organized media. II. Generalization of the crystal model for dipole–dipole interactions in cubic sites
Authors: Vásquez Sergio O.
Published: July 2002

The study of energy transfer processes between optical centers in crystalline cubic materials is reexamined within the conceptual framework of the crystal model. The initial assumption of modest transfer rates is relaxed, thereby correcting the master equation of the crystal model and making it possible to explain the nonexponential behavior of the radiation decay curves from excited states. This study shows that such experimental trends are the result of a purely statistical effect related to the fluctuation in the population of optically active ions in a crystal lattice. The corrected crystal model does not need to invoke migration of excitation processes to explain the fast exponential decay observed in crystals with a high concentration of optical centers.
Vapor pressure of partially deuterated methanes (CH3D, CH2D2, and CHD3)
Authors: Calado J. C. G. , Lopes J. N. C. , Nunes da Ponte M. , Rebelo L. P. N.
Published: July 2002

The difference between the vapor pressure of completely protiated or deuterated methane (CH4 or CD4) and partially deuterated methanes (CH3D, CH2D2, or CHD3) has been measured over the 96–121 K temperature range. The vapor pressure data obtained were fitted to equations of the type T ln(p/p)=A/T+B, where the prime always refers to the lighter molecule. Within the studied temperature range, the vapor pressure isotope effect was found to be “inverse” (p>p) for all the systems, except in the low-temperature range of the (CH4/CH3D) system. Our data agrees with other results found in the literature, both experimental and theoretical. Differences in the enthalpy of vaporization were calculated from the experimental results. In the case of the (CH4/CHD3) system, our vapor pressure isotope effect (VPIE) results were also compared with liquid–vapor isotope fractionation factor (LVIFF) data from other authors.
Comment on “Relaxation of collective excitations in LJ-13 cluster” [J. Chem. Phys. 105, 3679 (1996)]
Authors: Wales David J. , Doye Jonathan P. K.
Published: July 2002

We comment on the explanation offered by Salian et al. for the abrupt onset of damping in collective excitations of an atomic cluster. We note that this effect cannot be due to the cluster escaping from the global minimum potential well because the critical energy lies below that of the lowest known transition state.
Three-dimensional calculations of collision-induced dissociation in He+H2 using infinite-order-sudden approximation
Authors: Nobusada Katsuyuki , Sakimoto Kazuhiro
Published: July 2002

Quantum mechanical calculations of three-dimensional dissociative He+H2 collisions are made using an infinite-order-sudden approximation. Vibrational and dissociative motions are treated accurately in the direct numerical method of Sakimoto and Onda [J. Chem. Phys. 100, 1171 (1994)]. The dissociation cross sections for all the initial vibrational states (v=0–14) are presented at the total energy E=8 eV. The dissociation cross section becomes much larger for higher v. The dissociation mechanism is different depending on the collision configuration. Two modes of dissociation are found to be important: The dissociation occurs (1) through the compressive action on the vibrational motion of H2 and (2) through the expansive action on the vibrational motion.
Barrier suppression in high intensity photodissociation of diatomics: Electronic and permanent dipole moment effects
Authors: Conjusteau André , Bandrauk André D. , Corkum Paul B.
Published: July 2002

The infrared multiphoton photodissociation of a molecular ion, HCl+ in intense (I>1013 W/cm2), short (τ⩽1 ps) laser pulses is studied numerically by solving the nonperturbative time-dependent Schrödinger equation for this system. In particular, since molecular ions have large permanent moments upon dissociation, the present calculation examines the relative importance of electronic and permanent dipole moments effects at high intensities. Both long (λ=20.6 μm) and short (λ=1.064 μm) wavelength are compared to previous experiments and barrier suppression models. It is found in general that at long wavelength electronic effects predominate, thus justifying the use of a static field barrier suppression mechanism to predict the onset of dissociation. High intensity low frequency photodissociation also implies considerable redistribution of ponderomotive energies for light particles such as protons by charge transfer effects.
General nucleation theory with inhibition for chemically reacting systems
Authors: Wattis Jonathan A. D. , Coveney Peter V.
Published: July 2002

We present a general theoretical model of nucleation based on the Becker-Döring kinetic scheme, according to which clusters grow and shrink by one-step aggregation and fragmentation processes. Our model includes the catalytic effects of clusters on the rate of formation of other clusters, which we propose as the microscopic mechanism underlying secondary nucleation; the rôle of a precursor, or source chemical species, which spontaneously decays to form the nucleating material; and an inhibitor which hinders the growth of clusters beyond a certain threshold size. A systematic procedure is developed for extracting from the full model, which comprises an infinite set of differential equations, a low dimensional dynamical system containing only a few key equations which determine the experimentally measurable macroscopic behaviour of the system. From these macroscopic equations it is possible to deduce the most important elements of the reaction scheme, and to isolate the rate-determining stages. One realization of this reduced model, motivated by an application to cement hydration, is solved in a particular asymptotic limit, and the results are shown to compare well with experimental data.
Diffusion and clustering of N2O molecules in argon clusters: A theoretical approach by molecular dynamics simulations
Authors: Gaigeot M.-P. , de Pujo P. , Brenner V. , Millié Ph.
Published: July 2002

Results of molecular dynamics simulations of small van der Waals clusters composed of one argon cluster of size 147 or 125 (incomplete outer-layer cluster) and from one to four N2O molecules deposited at thermal relative collision energy on the argon cluster are presented. The potential energy is calculated through the semiempirical Claverie method. We discuss here the necessity and the practical application of fitting some of the potential parameters in order to reproduce the N2O experimental dipole moment value as well as the experimentally observed N2O⋯N2O and N2O⋯Ar equilibrium geometries. We first show that, as in the case of atomic projectiles, a very efficient capture by collision of the N2O molecules by the argon clusters is observed, independently of the initial molecular orientation. Studying trajectories over tens of nanoseconds then gives evidence that the N2O molecules move independently on the surface of the argon clusters, and that the molecules migrate randomly through jump displacements on the surface of the clusters. We observe a very high N2O mobility and we explain the influence of the argon cluster outer-layer structure on mobility. Collisions of the N2O molecules on the surface of the argon clusters result in a sequential and fast clustering. The geometries of the energetically stable (N2O)m microclusters have been characterized. Using the formula of Perrin, we calculate and interpret single N2O diffusion coefficients and (N2O)m microcluster diffusion coefficients, whose values are 1 to 100 times lower than in the liquid state. Finally, we extend our results to larger argon clusters, such as Ar1000, through a random walk model taking place on the surface of a sphere, which enables us to calculate mean encounter times between particles. We thus interpret the fluorescence quenching that occurs in chemical reactions taking place on finite-size argon clusters.
Matrix isolation spectroscopy of Na atoms deposited as Na+ ions
Authors: Silverman Derek C. , Fajardo Mario E.
Published: July 2002

The results of matrix isolation experiments of Na atoms deposited as Na+ ions in Ar matrices are reported. The optical absorption spectra include features which were observed for the first time in Na/Ar matrices prepared using laser ablation of Na metal [S. Tam and M. E. Fajardo, J. Chem. Phys. 99, 854 (1993)]. In that study, the observed novel trapping sites and improved atomic isolation efficiency were attributed to the high kinetic energy of the fast Na atoms produced by laser ablation. These new results challenge that interpretation and demonstrate that, at least for Ar matrices, the same effects can be reproduced by the deposition and subsequent neutralization of Na+ ions.
Direct subsurface absorption of hydrogen on Pd(111): Quantum mechanical calculations on a new two-dimensional potential energy surface
Authors: Olsen R. A. , Philipsen P. H. T. , Baerends E. J. , Kroes G. J. , Lo/vvik O. M.
Published: July 2002

We have calculated a two-dimensional (2D) potential energy surface (PES) for H2 interacting with a Pd(111) surface. The geometry considered is for H2 approaching a bridge site and dissociating into neighboring hollow sites and the subsurface sites directly below these. Density functional calculations were performed using both the local density approximation (LDA) and the generalized gradient approximation (GGA). The LDA PES gives the usual overbinding and shows no barrier (relative to the bottom of the H2 potential) to subsurface absorption, while the GGA PES agrees with the experimental adsorption energies and has a large barrier. We have performed quantum mechanical wave packet calculations on the GGA PES to obtain the direct subsurface absorption probability. We have also calculated the barrier height’s dependence on a coordinate that can be associated with a local surface vibrational mode and the results suggest that this degree of freedom should be taken into account in the dynamical calculations.
Anharmonicity of the O2–Ag(001) chemisorption potential
Authors: Buatier de Mongeot F. , Cupolillo A. , Valbusa U. , Rocca M.
Published: July 2002

The coverage dependence of the vibrational modes of O2 chemisorbed on Ag(001) was studied by electron energy loss spectroscopy. We observe the dipole active modes associated to the intramolecular stretch and to the molecule–surface vibrations. Moreover two further features are present in the EEL spectra, due to the anharmonicity of the molecular potential: the combination-band of the stretching and of the molecule–surface vibration, and the first overtone of the O–O stretching mode. From the EELS intensity of the O–O stretching mode versus coverage the dynamical charge transfer e*=0.61±0.02 e− is determined which corresponds to a dynamical dipole moment μ=0.16±0.01 D. The O2–Ag vibrational frequency is found to decrease with coverage, suggesting that a weakening of the molecule–surface bond takes place. This observation is confirmed by thermal desorption spectroscopy. The anharmonicity parameter xe of the O–O potential increases significantly upon chemisorption, in analogy to O2–Ag(110) and O2–Pt(111). The intensity of the overtone peak is analyzed within the Morse approximation, allowing us to discriminate the contributions arising from the mechanical anharmonicity, and from the nonlinearity of the dynamical charge flow. A correlation is observed between the magnitude of e* and the contribution of the latter term to the overtone intensity.
Benchmark calculations with correlated molecular wave functions. VIII. Bond energies and equilibrium geometries of the CHn and C2Hn (n=1–4) series
Authors: Peterson Kirk A. , Dunning, Jr. Thom H.
Published: July 2002

Using systematic sequences of correlation consistent basis sets, the accuracy of calculated bond energies De(CH) and equilibrium geometries (re, θe) has been investigated for the CHn and C2Hn series (n=1–4). Perturbation theory (MP2, MP3, MP4), coupled cluster [CCSD, CCSD(T)], and single and multireference configuration interaction (HF+1+2, CAS+1+2) methods have been investigated. Except for the vinyl radical, all of the calculated bond energies showed significant basis set dependence with average errors (standard deviations) of 5.6 (±3.0) kcal/mol for the cc-pVDZ set, 1.4 (±0.8) kcal/mol for the cc-pVTZ set, and 0.5 (±0.4) kcal/mol for the cc-pVQZ set with CCSD(T) wave functions. For the vinyl radical the total variation with basis set was just 0.6 kcal/mol. Strong basis set dependence was also observed for the equilibrium geometries, e.g., for re(CH) the average error decreased from 0.020 Å (cc-pVDZ) to 0.003 Å (cc-pVTZ) to 0.002 Å (cc-pVQZ). The effect of including the core electrons in the correlated calculations was also investigated for the two series. Inclusion of core correlation in the CHn series increased De(CH) by 0.13 (CH) to 0.61 kcal/mol (CH2) and decreased the equilibrium CH bond lengths by approximately 0.0015 Å. For the C2Hn series, correlation of the core electrons increased De(CH) by 0.18 (C2H4) to 1.01 (C2H) kcal/mol, but decreased De(CH) in C2H2 by 0.25 kcal/mol. Predictions are also made for the equilibrium geometries of C2H, H2CC, and C2H3, as well as the CH bond strength of vinylidene and the acetylene–vinylidene isomerization energy.
Theory of the kinetic critical nucleus in binary systems
Authors: Li Jin-Song , Nishioka Kazumi , Maksimov Igor L.
Published: July 2002

We extend the theory of kinetic critical nucleus [K. Nishioka and I. L. Maksimov, J. Cryst. Growth 163, 1 (1996)] to binary systems for vapor–liquid transition and derive the equations to determine its size and composition. The kinetic critical nucleus corresponds to the extreme point (or saddle point) on the surface of the kinetic potential. By employing an available formula for the reversible work of forming a cluster, it is shown that the composition of the kinetic critical nucleus is the same as that of the thermodynamic one and the equation to determine the size of the kinetic critical nucleus has the same features as those for single-component systems. Similar to single-component systems, there are two values for the size of the kinetic critical nucleus.