bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectral moments of collision-induced absorption of CO2 pairs: The role of the intermolecular potential
Authors: Gruszka Marcin , Borysow Aleksandra
Published: July 2002

In this paper we examine the role of the anisotropy of the intermolecular potential in the rototranslational collision-induced absorption of the CO2 pairs. Using newly developed formulas [Borysow and Moraldi, Phys. Rev. Lett. 68, 3686 (1992); Moraldi, Phys. Rev. A (submitted)] that include the effects of anisotropy of the potential to all orders, we calculate the two lowest spectral moments γ1 and α1 for four different classes of CO2 pair potentials and compare the results with the experimental values. We assumed only multipolar induction in the process of forming the induced dipole, with the second-order contributions included. Using a site–site LJ [Murthy et al., Mol. Phys. 50, 531 (1983)] and a site–site semi-ab initio [Bohm, Mol. Phys. 56, 375 (1985)] intermolecular potentials we were able to reproduce the experimental values of γ1 and α1 moments over entire temperature range from 230 to 330 K. Also, the role of an electrostatic interaction between two CO2 molecules and its impact on the spectral moments is thoroughly investigated. An isotropic core with a point quadrupole centered at each molecule is shown to be an inadequate representation of the CO2–CO2 potential. Additionally, we show the results obtained with the first- and second-order perturbation theory to be more than twice too small.
Two-dimensional vibrational potential energy surface for the ring bending and twisting of 1,3-oxathiolane. Evidence for the anomeric effect resulting from –O–CH2–S– linkages
Authors: Leibowitz Sarah , Laane Jaan
Published: July 2002

The far-infrared spectra of 1,3-oxathiolane and its 2,2-d2 isotopomer have been analyzed in terms of a two-dimensional vibrational potential energy surface expressed as a function of ring-bending and ring-twisting coordinates. The barrier to planarity was determined to be 2289±200 cm−1 while the barrier to pseudorotation, representing the energy difference between the lowest energy twisted form and the highest energy bent form, was found to be 570±20 cm−1. The barrier to planarity is about 500 cm−1 higher than that of cyclopentane, and this is attributed to the anomeric effect resulting from the –O–CH2–S– linkage which tends to twist the molecule out of the planar configuration.
Toward a molecular theory of vapor phase nucleation. IV. Rate theory using the modified liquid drop model
Authors: Weakliem Cheryl Li , Reiss Howard
Published: July 2002

This paper continues the development of the molecular theory of vapor-phase nucleation presented in the preceding papers of this series and focuses on the rate of nucleation. The rate theory is developed within the definition of the modified liquid drop model of paper III, which although macroscopic in nature, is consistent with the molecular level definition of clusters found in papers I and II. In addition to providing the framework for a fully molecular rate theory, the present work provides insight into the mechanism of vapor phase nucleation. The key mechanistic result involves the natural occurrence of a non-ad hoc means of distinguishing density fluctuations that serve as embryos for the formation of liquid drops from those that do not serve in this capacity. In particular, two paths of cluster evolution are found, one leading toward drop formation through a valley on the free energy surface, the other leading toward the decay of clusters back to the supersaturated vapor. The rate of nucleation is taken as the limiting flux through the valley. The calculations for argon agree well with classical theory with respect to the prediction of the dependence of the rate on supersaturation, but differ in the prediction of the temperature dependence. At lower temperatures, the predicted rate is greater than that of classical theory, while at higher temperatures, the reverse is true. This phenomenon has also been observed by other authors using different theories.
Trajectory simulations for unimolecular dissociations with application to the dissociation of NCNO
Authors: Klippenstein Stephen J.
Published: July 2002

A classical trajectory-based procedure for estimating the kinetics of unimolecular dissociations containing no reverse potential barrier is described and implemented for the dissociation of NCNO into NC and NO. The basis of this implementation involves Keck’s procedure of propagating trajectories from the transition state on towards separated fragments and back towards complex. A separation of modes into the ‘‘transitional’’ and ‘‘conserved’’ modes allows for a propagation in only the transitional modes via the implementation of adiabaticity assumptions for the conserved modes. A statistical distribution of initial conditions is obtained via the implementation of Monte Carlo based procedures previously employed in the evaluation of the number of available states. The trajectory results for the rate constants and the product rotational distributions are compared with corresponding statistical results. A recently introduced variable reaction coordinate statistical approach is found to provide an accurate estimate to the rate constants when the effects of two separate transition states are incorporated. Meanwhile, as expected, the product rotational distributions deviate only slightly from phase space theory predictions.
Effect of shear on the desorption of oligomers in nanoscopically confined films
Authors: Manias E. , Hadziioannou G. , ten Brinke G.
Published: July 2002

Bitsanis et al. J. Chem. Phys. 99, 5520 (1993) found that in nanoscopically confined films between strongly physisorbing surfaces chains with many contacts with the walls are irreversibly adsorbed. When shear is imposed to these systems molecular dynamics (MD) simulations show that the majority of the adsorbed oligomers adopts flat conformations on top of the walls. Although these conformations are characterized by high molecular adsorption energies, the same MD simulations show that desorption is strongly promoted by shear. The underlying mechanism is discussed.
Polarization of atomic photofragment fluorescence for excitation along a Fano profile: A quantum-mechanical study
Authors: Siebbeles L. D. A. , Glass-Maujean M.
Published: July 2002

A quantum-mechanical description of the polarization of the fluorescence from photofragments, obtained by excitation along a Fano profile, is presented. For excitation on a Fano profile, dissociation can occur via two indistinguishable pathways, i.e., dissociation via the resonance and via direct excitation to the continuum. This gives rise to quantum interference effects, which result in an asymmetric profile in the photon absorption spectrum. The effect of the interference on the polarization of the photofragments fluorescence is investigated. The polarization as a function of the excitation energy varies asymmetrically around the resonant excitation energy. However, in contrast to intuition, the polarization changes in a symmetric way around an energy which is shifted from the resonant excitation energy. Simple formulas to calculate the polarization for different rotational and electronic transitions are provided.
Vibrational polarizability of polyacetylene chains
Authors: Champagne Benoît , Perpète Eric A. , André Jean-Marie
Published: July 2002

Within the double harmonic oscillator approximation, ab initio vibrational contributions to the static electric dipole polarizability have been computed for the all-trans polyacetylene chains homologous series. Atomic basis set and electron correlation effects have been studied on ethylene and trans-butadiene as well as the use of semiempirical Hamiltonians. By using the 6-31G* atomic basis set within the Hartree–Fock and the second order Mo/ller–Plesset procedures as well as by using semiempirical Austin model 1 Hamiltonians, the evolution with chain length of the vibrational polarizability per structural unit has been investigated and compared to the electronic contribution. Although smaller than the electronic contributions (αvibration≊10% αelectronic), the longitudinal component to the polarizability presents a similar exaltation as chain length grows, but a slower saturation to an asymptotic value per unit cell. Inclusion of electron correlation via the second order Mo/ller–Plesset technique turns out to reduce the longitudinal component calculated at the Hartree–Fock level, but to increase the transversal and perpendicular components. Whereas it reproduces correctly the evolution with chain length of the vibrational polarizability tensor components, the Austin model 1 technique underestimates the longitudinal term and overestimates the perpendicular term. The major contribution to the vibrational polarizability results from large charge fluxes associated with asymmetric stretching motions of the carbon backbone and with the torsion motions presenting very low vibrational frequencies.
Structure and self-diffusion in a model two-dimensional Brownian liquid
Authors: Aranda-Espinoza H. , Carbajal-Tinoco M. , Urrutia-Bañuelos E. , Arauz-Lara J. L. , Medina-Noyola M. , Alejandre J.
Published: July 2002

The static structure and the time-dependent self-diffusion motion of interacting Brownian particles in a model two-dimensional suspension are discussed. For the static structure we report Brownian dynamics results assuming a hard disk plus Yukawa pair potential. The self-diffusion properties of this model system are calculated from two independent theoretical approaches. In order to assess the accuracy of the predictions of these two theories, we also performed Brownian dynamics calculations of the time-dependent self-diffusion coefficient for a wide range of values of both the particle concentration and the pair potential coupling constant. We find that both theories reproduce very well the main features exhibited by the Brownian dynamics data. Quantitatively, there are some discrepancies between both theoretical predictions and the Brownian dynamics results, which are negligible at moderate couplings, but become larger for strongly coupled systems and long times.
Single molecule lines and spectral hole burning of terrylene in different matrices
Authors: Kozankiewicz B. , Bernard J. , Orrit M.
Published: July 2002

We observe fluorescence excitation lines of single terrylene molecules in three new polymer matrices (polyvinylbutyral, polymethylmethacrylate, and polystyrene) and in two crystals, n-hexadecane (polycrystalline Shpol’skii matrix) and anthracene single crystal. We also burn persistent spectral holes in bulk samples of these solutions for comparison to single molecule lines. In all matrices where hole burning is efficient enough, we find good agreement between the average width determined from the distribution of single molecules’ linewidths and the homogeneous width deduced from spectral holes, which demonstrates the consistency and complementarity of the two techniques.
Photocurrent excitation spectra of methyl iodide/methane fluid mixtures
Authors: Krasinsky A. , Shechter R. , Steinberger I. T. , Gürtler P. , Meyer J.
Published: July 2002

Impurity photocurrents of fluid CH4:CH3I were studied in the region of the (2E1/2)nd Rydberg series of CH3I and also beyond the 2E1/2 ionization potential up to the LiF cutoff. In the gas, a step-like rise of the photocurrent was observed at a photon energy somewhat lower (by about 20 meV) than the 2E1/2 ionization potential, indicating that high members of the (2E1/2)nd Rydberg series of methyl iodide can be ionized by further processes besides the well-known autoionization into the 2E3/2 continuum. At the high-energy side of the 2E1/2 limit further peaks appear, corresponding to simultaneous excitation of (2E1/2)nd Rydberg states and one or two quanta of the ν2(a1) vibration of CH3I. In the photon energy region of 10.8–11.8 eV a series of at least eight peaks were observed, due to excitation of vibronic levels of the Jahn–Teller split 4s Rydberg state of methane. The value of the adiabatic ionization potential 2E3/2 of CH3I in liquid CH4 near the critical point was found to be 8.30 eV, while in neat dilute CH3I gas it is 9.48 eV. The overall variation of 2E3/2 in the liquid phase (from 190 to 132 K, 6.4×1021 to 1.5×1022 cm−3) is 70 meV. In the densest (and coolest) liquid samples studied a series of peaks, with intervals of ≊60 meV appeared in the photon energy range 8.37<hν<8.5 eV on top of the continuous background.
Optical–optical double resonance spectroscopy of the 4 1Σ+g ‘‘shelf’’ state of Na2 using an ultrasensitive ionization detector
Authors: Tsai Chin-Chun , Bahns John T. , Wang He , Whang Thou-Jen , Stwalley William C.
Published: July 2002

The 4 1Σ+g ‘‘shelf’’ state of Na2 was extensively investigated using optical–optical double resonance spectroscopy and an ultrasensitive ionization detector. A total of 179 vibrational levels-(0≤v≤127 are from previous work by using filter-photomultiplier tube fluorescence detection) were observed in this state, which covered more than 99.8% of the well depth. The Rydberg–Klein–Rees (RKR) potential energy curve was constructed with maximum outer turning point R178+= 17.60 Å. The comparison of the long range portion of the RKR potential energy curve and the potential curve calculated from V(R)=ΣnCn/Rn is discussed. Avoided crossings around the shelf with the 3 1Σ+g state and near dissociation with the 5 1Σ+g state are also discussed.
Interatomic potentials of triplet s-Rydberg series of HgNe and HgAr van der Waals dimers
Authors: Onda Ken , Yamanouchi Kaoru , Okunishi Misaki , Tsuchiya Soji
Published: July 2002

The optical–optical double resonance (OODR) spectra of Rydberg 3Σ+ states of Hg(n3S1)Ne (n=8–10) and Hg(83S1)Ar were measured by using A and B states as intermediate states in the OODR process. The interatomic potentials of three states of HgNe and one state of HgAr were determined over a wide range of interatomic distance, R=3–7 Å, by the analysis of the vibrational structure of their OODR spectra. It was found that the potential shape varies sensitively with n and converges to that of the ion core, HgNe+. Dissociation energies (De) of the Rydberg states for the n=8, 9, and 10 were derived to be 209(2), 284(2), and 309(2) cm−1, respectively. Using the quantum defect orbital [G. Simons, J. Chem. Phys. 60, 645 (1974)], which represents a hydrogenic radial wave function for a Rydberg state with a given quantum defect, was introduced to interpret the characteristic n dependence of the interatomic potential. It was shown that the interatomic potential for the Rydberg states can be expressed by the sum of the ion core potential, Vion(R), and the repulsive potential, Vex(R), which originates mainly from the exchange repulsion between the Rydberg electron and the attached rare gas atom. The interatomic potential for Hg(83S1)Ar, whose dissociation energy [De=1602(4) cm−1] is much deeper than that of Hg(83S1)Ne, was also interpreted consistently by expressing the potential as Vion(R)+Vex(R).
Low temperature dynamics in amorphous solids: A photon echo study
Authors: Meijers Hans C. , Wiersma Douwe A.
Published: July 2002

The long-lived stimulated photon echo is put forward as a powerful technique to probe structural dynamics in glasses and other amorphous solids. We present results of optical dephasing measurements on several doped organic glasses (deuterated ethanol, toluene, and triethylamine) and polymers (polystyrene, polyvinylbutyral, and polymethylmethacrylate). Using a stochastic analysis within the so-called two-level-system model, we can extract from these measurements the distribution function of fluctuation rates over a huge range of time scales, varying from picoseconds to hundreds of milliseconds. We show that the dynamics, which lies at the origin of spectral diffusion, is very dependent on the glass structure and can be described by two separate hyperbolic distribution functions of fluctuation rates. From longitudinal photon echo measurements in the population dimension, we find evidence for structural dynamics which is not accounted for in the standard two-level-system model and which is related to a spatially inhomogeneous distribution of fluctuation centers.
Supersonic jet electronic spectroscopy of benzylidenecyclopentane and benzylidenecyclohexane
Authors: Cable J. R. , Westrick N. R.
Published: July 2002

The resonance-enhanced ionization spectra of the S1←S0 transitions in jet-cooled benzylidenecyclopentane and benzylidenecyclohexane are presented and analyzed to characterize the torsional conformation of these two compounds in their ground and excited electronic states. Both spectra display considerable vibronic activity in low-frequency modes which are associated with torsional motion. In the cyclopentane derivative, a single conformation is found in the ground electronic state with a torsional angle of 23° which is consistent with a symmetric double-well torsional potential in which the torsional coordinate also involves inversion of the cyclopentane ring. A planar excited-state conformation is suggested by a hindered rotor analysis of the torsional energy levels. The cyclohexane compound is found in two ground-state conformations in a He expansion but only a single conformation in an Ar expansion, suggesting that conformer interconversion can occur over a relatively low barrier. Much larger steric interactions are found in this compound which are consistent with a chair conformation of the cyclohexane ring as suggested by semiempirical molecular orbital calculations. Both electronic states of benzylidenecyclohexane appear to have nonzero torsional angles which are considerably larger in magnitude than in the cyclopentane derivative.
The role of surface corrugation in direct translationally activated dissociative adsorption
Authors: Xia Li-Qun , Engstrom J. R.
Published: July 2002

Recent experimental results concerning reactive scattering in the hyperthermal kinetic energy regime can be described by energy scaling relationships Ei cosn θi, where n<2, and Ei and θi are the incident kinetic energy and incident angle, respectively. Such power law scaling arguments are empirical, the results of which cannot easily be related to the fundamental parameters that describe the gas–surface interaction. We present a detailed and thorough analysis where the role of surface corrugation in determining the coupling between incident kinetic energy and incident angle in these translationally activated systems is considered explicitly. The key features of the analysis involve the assumption that the kinetic energy directed along the local surface normal (E⊥) controls the reaction probability (SR), and that by averaging this quantity over the unit cell, one obtains the appropriate energy scaling relationship. The major advantage associated with the proposed analysis is that one need not assume a functional form concerning how the reaction probability depends on kinetic energy, i.e., SR(E⊥).

Our analysis demonstrates that in the absence of shadowing, a single ‘‘universal’’ scaling function exists EiΘ(θi), which is given by the expression Θ(θi)=(1−Δ)cos2 θi+3Δ sin2 θi, where Δ is a corrugation parameter (0≤Δ≤1) and only in-plane corrugation has been considered. Shadowing plays an important role at sufficiently large corrugation amplitudes and/or sufficiently large angles of incidence. Specifically, it leads to more complex scaling functions, which depend on the shape of the surface corrugation, for which several examples have been considered. Both local minima and local maxima can be observed for Θ(θi) as a function of incident angle. Two factors can introduce errors in the analysis, namely, the presence of nonlinearities, and the effects of nonuniform surface reactivity, and illustrative examples are considered. The model accounts well for recent experimental results concerning the dissociation of silanes on silicon surfaces, and alkanes on a corrugated platinum surface. It is probable that other systems involving reactive scattering in the hyperthermal kinetic energy regime may also be described well employing this analysis.
Fluid alkanes in confined geometries
Authors: Padilla Paz , Toxvaerd So/ren
Published: July 2002

A comparative study of confined fluid films composed of three different alkanes has been carried out using molecular dynamic simulation techniques. The films were confined in thin slit pores, only a few molecular diameters thick, and the substances studied were n-butane, n-decane, and 5-butyl-nonane. The properties of the film were obtained in equilibrium conditions and under shear. All the studied films show a strong layering of the distribution of methylene subunits. Chains at the solid boundaries align with the walls and show a tendency to stretch. The diffusion parallel to the solid walls is found to be higher in the proximity of the walls than in the inner part of the pore. The molecular motion normal to the confining walls can be described as noncorrelated molecular transitions between the contact layer and the inner part of the pore. Shear flow was induced in the film by moving the solid walls. The resulting velocity profiles across the pore were computed as well as the viscosity of the films. The viscosities of the confined fluids in the three cases appear to be the same as those of the bulk, within the uncertainty of the results. No significant influence of the shear flow on the inter- or intramolecular was found.
Ab initio investigation of the Renner–Teller effect in the X 2Πu electronic state of C2H+2
Authors: Perić Miljenko , Peyerimhoff Sigrid D.
Published: July 2002

Results of an ab initio investigation of the vibronic structure of the X 2Πu electronic state of C2H+2 are presented. Calculations are performed using a variational approach for handling the Renner–Teller effect in tetra-atomic molecules [Perić et al., Mol. Phys. 55, 1129 (1985)]. In these computations both the ab initio potential surfaces and those derived on the basis of experimental findings are employed. The results of the calculations strongly support the recent analysis of the C2H+2 spectrum [Pratt et al., J. Chem. Phys. 99, 6233 (1933)] and predict a number of yet unobserved features in the energy range between 0 and 3000 cm−1.
Charge–cavity and charge–dipole effects in ionic fluids
Authors: Ho/ye J. S. , Stell G.
Published: July 2002

The contribution of charge–dipole interactions to the thermodynamics and structure of ionic solutions is considered from the standpoint of two models—a quasicontinuum cavity model of charged spheres of dielectric constant ε0 in a continuum of dielectric constant ε as well as a molecular model consisting of charged-sphere solute particles and dipolar-sphere solvent particles. With suitable scaling of the interaction potentials one expects to obtain the cavity-model description from the molecular model in the limit σd/σi→0, where σd and σi are the ion and dipolar-particle diameters, respectively. We recover for the first time the full r−4 ‘‘cavity-term’’ of the ion–ion cavity-model pair potential in such a limit as well as a corresponding screened r−4 term in the solvent-averaged direct correlation function, which is screened by the factor e−2λr and multiplied by the prefactor (1+λr)2, where λ is the ion–ion inverse screening length. Here, r is the distance between a pair of ions. We also show that if one makes the usual approximation of pairwise additivity of the n-ion potentials of mean force at infinite dilution, one loses both the screening and the prefactor, recovering an earlier result in G. Stell, Phys. Rev. A 45, 7628 (1992). This is thermodynamically significant; if there were no screening of the r−4 term, there could not be a critical point in the ionic fluid if ε0<ε, while if ε0≳ε, the critical point would be mean-field-like if there were no screening, as pointed out in the earlier work.
Chains in the presence of an interacting surface and different boundary conditions
Authors: Stratouras G. K. , Kosmas M. K.
Published: July 2002

The end-to-end distribution function of a linear chain interacting with a penetrable surface with the potential uδ(z) is demonstrated to recover the case of the distribution in the presence of an impenetrable surface with different boundary conditions. The two different boundary conditions of zero probability density and of zero of the gradient of the probability density at the surface correspond to different values of u and the penetrable distribution function can thus be used to describe chains with various degrees of interactions both in the presence of penetrable or impenetrable surfaces. Density profiles of the monomeric units of the chains localized at one or both ends are described, furnishing an insight to the distortion which the interacting surface brings on the shape of a coil. The study includes an extension to the cases of ring and star macromolecules.
Fluid–fluid phase separations in nonadditive hard sphere mixtures
Authors: Jung Jinkyung , Jhon Mu Shik , Ree Francis H.
Published: July 2002

We investigated the phase stability of a system of nonadditive hard sphere (NAHS) mixtures with equal diameters, d, between like species and an unequal collision diameter, d(1+α), between unlike species. It is based on an analytic equation of state (EOS) which refines an earlier expression [J. Chem. Phys. 100, 9064 (1994)] within the mixed fluid phase range. The new EOS gives a reliable representation of Monte Carlo EOS data over a wide range of density, composition, and nonadditivity parameters (α). Comparisons with available computer simulations show that the new EOS predicts satisfactory phase boundaries and the critical density line. It is superior to results derived from integral equations (the Percus–Yevick, the Martynov–Sarkisov, and the modified Martynov–Sarkisov) and analytic theories (the MIX1 model, the van der Waals one-fluid model, and the scaled particle theory). The present study shows that, unless α exceeds 0.026, the fluid phase will remain fully miscible up to the freezing point of pure hard spheres. We have also investigated structural aspects of the phase stability by Monte Carlo computations. The radial distribution functions, the local mole fraction, and coordination numbers for like and unlike pairs of hard spheres exhibit significant number dependencies close to the fluid phase boundary. They provide precursory signals to an impending phase change. Finite systems used in the Monte Carlo sampling limit fluctuations in sizes and shapes of heterogeneous clusters. The observed number dependence simply reflects this fact.
The generation of CH(X 2Π,v″,N″) fragments in the photolysis of CH2(1 3B1) radicals
Authors: Bohn B. , Stuhl F.
Published: July 2002

Ground state methylene, CH2(1 3B1), was shown to dissociate into CH(X 2Π, v″, N″)+H fragments by excitation via the first triplet absorption continuum around 200 nm. Triplet CH2 was generated in the 351 nm photolysis of ketene. CH(X) fragments were formed in the 193 nm photolysis of these radicals and were detected by laser induced fluorescence at around 430 nm. The relative populations of the v″=0 and 1 vibrational states, and the rotational and Λ-doublet levels of these states were characterized. These results are compared with those given in the literature by ab initio and dynamics calculations for the absorption and dissociation of triplet CH2. Further comparisons are made with the photofragmentations H2O→OH(X 2Π)+H and NH2→NH(A 3Π)+H.
Theoretical study of the bonding of ammonia, carbon monoxide, and ethylene, to copper atom, dimer, and trimer
Authors: Fournier René
Published: July 2002

Kohn–Sham density functional theory (KS-DFT) calculations were performed for the association complexes Cun–L, with n=1, 2, 3 and L=NH3, CO, and C2H4. Two geometries for Cu2–L are considered; with the ligand bonded to a single copper atom (‘‘atop,’’ or A), and with the ligand bonded to both atoms (‘‘bridge,’’ or B). In addition to A and B, a third geometry was considered for Cu3–L, with the ligand bonded to all three copper atoms; in each case, no minimum was found for that third geometry. I report fully optimized equilibrium geometries and harmonic frequencies calculated within the local spin density (LSD) approximation for all the bound complexes and estimates of their binding energies obtained with a gradient-corrected exchange-correlation functional. Structure A is the most stable in all cases but, for Cu3CO and Cu3C2H4, structure B is only a few kcal/mol higher in energy. The energetic contribution from the geometrical relaxation of Cu3 ranges from essentially zero (Cu3NH3 B) to 3.4 kcal/mol (Cu3CO B). In agreement with previous calculations on Cun–C2H2 and with experiments, the calculated Cun–L binding energy is found to increase with n for all ligands. Although the bonding mechanism differs among the three ligands, repulsion of a filled ligand orbital with the half-filled 4s orbital of copper (or 4s-derived molecular orbitals of Cu2 and Cu3) always plays an important role and is responsible for the smaller binding energies in the CuL complexes. This repulsion decreases from Cu to Cu2 because of charge accumulation in Cu–Cu midbond region and of the greater polarizability of Cu2. The Cu3L binding energies are larger than those of Cu2L mostly because of the greater involvement of copper 4p orbitals in bonding to the ligand. The ligand vibrational frequency shifts relative to the free molecules are compared to experiment and discussed in relation to the nature of the metal–ligand interaction. In particular, an interesting correlation, between the frequency of the NH3 umbrella mode and the metal–NH3 binding energy, is likely due to the electrostatic nature of the bond.
Comment on ‘‘Thermodynamics of fluids in quenched disordered matrices’’ [J. Chem. Phys. 100, 5172 (1994)]
Authors: Madden William G.
Published: July 2002

A recent contention by Rosinberg et al. [J. Chem. Phys. 100, 5172 (1994)] that the Replica Ornstein–Zernicke equations are limited to host matrices arising from a thermal quench is disputed. A clarification of the meaning of the required direct correlation functions of the matrix is provided.
Spin-polarized electronic structure of cobalt cluster anions studied by photoelectron spectroscopy
Authors: Yoshida Hiroyuki , Terasaki Akira , Kobayashi Katsuyoshi , Tsukada Masaru , Kondow Tamotsu
Published: July 2002

The photoelectron spectra of Co−n (3≤n≤70) were measured at the photon energy of 4.025 eV by use of a XeCl excimer laser. For Co−n with n=3, 4, and 6, the geometric and electronic structures were obtained from the spectra in comparison with the calculated spectra by the spin-polarized DV-Xα method. The spectra observed are reproduced reasonably well by the calculation with postulating the most probable geometrical structures. It is revealed that the 3d band with the majority spin is separated by 1.0–2.8 eV from that with the minority spin; the former is completely filled while the latter is partly filled and extends above Fermi level. The magnetic moments and the average exchange energies of these cluster anions were estimated. For Co−n with n≥7, the observed electron affinity depends linearly on the reciprocal of the cluster radius and approach the work function of a cobalt metal, as n increases. Below n=6, the electron affinity deviates from the linear dependence. This finding indicates that a size-dependent transition in the electronic structure occurs at n≂7. The spherical conducting drop model suggests the presence of mobile electrons in Co−n with n≥7.
Concentration fluctuations and coupling to stress relaxation in viscoelastic binary solutions
Authors: Wang C. H.
Published: July 2002

Dynamics and coupling of concentration fluctuations to longitudinal stress relaxation in a viscoelastic binary polymer solution have been analyzed using nonequilibrium thermodynamics. The calculation supplements the result previously obtained by this author using the Bearman–Kirkwood hydrodynamic equation for a multicomponent system. We have shown that the dynamics of concentration fluctuations is affected by the stress modulus of the solution only when there is a difference in the specific volume between polymer and solvent components. The present work also extends Bearman–Kirkwood’s result by further relating the partial longitudinal stress modulus of the polymer to the total longitudinal stress modulus of the polymer solution.
Desorption kinetics of copper fluorides in the reaction of fluorine with copper surfaces
Authors: Sugawara K. , Wach Th. , Wanner J. , Jakob P.
Published: July 2002

The surface reaction of molecular or atomic fluorine beams with single crystal Cu(111) and polycrystalline copper surfaces has been studied at surface temperatures between 760 and 1050 K using laser-induced fluorescence spectroscopy (LIF), Auger electron spectroscopy (AES), and mass spectrometry. Thereby the fluorination of a hot copper sample was found to be an efficient mode of preparing a clean surface as proven by AES. At low surface temperatures CuF and CuF2, as well as Cu3F3 and Cu4F4 clusters are products of the dry etching reaction, desorbing into the gas phase. At high temperatures copper monofluoride, CuF in the X 1Σ+ electronic ground state is the dominant product. The kinetics of CuF desorption has been studied under steady-state conditions as a function of fluorine flux and surface temperature using the LIF approach. These parametric measurements can be fitted to an apparent activation energy, Ea=157±7 kJ mol−1. Further, it was possible to measure the time-resolved CuF desorption. The LIF signal is comprised of a first- and zeroth-order contribution. The observation of the zeroth-order component can be seen as an indication of the formation of fluorinated copper layers which are responsible for the (CuF)n (n=3,4) cluster formation. From the first-order decay the Arrhenius parameters Ea=167±6 kJ mol−1 and A=1011.0±0.4 s−1 are derived. Both values for Ea derived in this study are in reasonable agreement though the latter value is regarded to be more significant. The Arrhenius parameters reflect themselves in a long residual lifetime of CuF molecules once they are formed at the surface in the order of milliseconds at surface temperatures of 1000 K.
Comparison of the quantum dynamics and sensitivity analysis for different isotopomers of the H+H2 reaction
Authors: Chang Johnny , Brown Nancy J.
Published: July 2002

A new formalism for quantum functional sensitivity analysis (QFSA) of atom–diatom reactions in the gas phase is developed within a version of Manolopoulos et al.’s [J. Chem. Phys. 93, 403 (1990)] log-derivative Kohn variational method containing contracted translational basis functions. A reference energy, Emid, is introduced to define boundary translational functions which completely remove all scattering energy dependence from the basis functions. This greatly facilitates scattering calculations for a range of energies about Emid without having to recalculate any of the so-called ‘‘stiffness’’ matrix elements. Our new approach to QFSA is applied to study the sensitivity of the H+H2, D+H2, and H+D2 reaction probabilities to the Boothroyd–Keogh–Martin–Peterson (BKMP) potential energy surface. The transition probability sensitivities of both D+H2 and H+D2 are very similar to those of H+H2 at low energies, but at higher energies, the sensitivities of the H+D2 reaction differ from those of the other two isotopomers. Isotopomers that have very similar reaction probability profiles also have very similar sensitivities to the potential. All three isotopomers exhibit a large region of positive sensitivity at the top of the barrier for an approximately 0.1 to 0.2 eV energy range above threshold. For these energies (∼0.3 to 0.5 eV above the barrier), it is possible to increase the reaction probability with slight increases in barrier height. Sensitivity results from our new code have also provided a wealth of information about (i) how small, localized changes in the potential affect product state distributions, resonance features, and reactivity; and (ii) where the dynamics is most sensitive to inaccuracies in the potential. Comparisons are also made of transition probabilities and sensitivities on the BKMP surface to those of other potential energy surfaces.
Electronic absorption spectra of linear carbon chains in neon matrices. I. C−6, C6, and C6H
Authors: Forney Daniel , Fulara Jan , Freivogel Patrick , Jakobi Michael , Lessen Daniel , Maier John P.
Published: July 2002

Electronic absorption spectra of linear C−6, C6, and C6H have been identified in neon matrices at 5 K. The species were produced by codepositing mass selected cations and anions with neon. The ions were generated in a hot cathode discharge source using diacetylene. The spectra of C−6 and C6 could also be observed using a pure carbon anion source or laser vaporization of graphite. The assignment is based on the mass selection, experimental and spectroscopic evidence, leading to the location of the 000 transitions of C−6: 2Πg←X 2Πu, C6: 3Σ−u←X 3Σ−g, and C6H: 2Π←X 2Π at 16 458, 19 558, and 18 854 cm−1, respectively. The frequencies of the symmetric carbon stretching vibrations have been obtained for these species in their excited electronic states.
Ab initio model potential embedded-cluster study of V2+-doped fluoroperovskites: Effects of different hosts on the local distortion and electronic structure of 4T2g–4A2g laser levels
Authors: López-Moraza Sara , Pascual José Luis , Barandiarán Zoila
Published: July 2002

In this paper we present the results of ab initio model potential (AIMP) embedded-cluster calculations on the ground 4A2g and excited 4T2g state levels of V2+-doped KMgF3, KZnF3, KCdF3, and CsCaF3. Complete active space SCF (CASSCF) and averaged coupled-pair functional (ACPF) calculations are performed on the (VF6)4− embedded cluster. The AIMP embedding potentials represent both static and relaxed/polarized lattice effects which are found to be an important refinement due to the large local distortions produced by the V2+ impurity. The calculated local distortions are found to be considerably large, but much smaller than expected in terms of the mismatch of ionic radius of the impurity and the substituted cation. The host dependency of the crystal field splitting, which was found to violate the simple ligand field R−5-law in a wide family of V2+-doped halide crystals, if R is the metal–ligand distance in the host, is also examined and is found to be quite close to the simple ligand field theory prediction, as long as the calculated impurity-ligand distances are considered. The results of the 4A2g→4T2g absorptions are in close agreement with available experimental data. The comparatively high values of the calculated 4T2g→4A2g fluorescence indicate the need of the inclusion of intracluster Jahn–Teller coupling of the excited state. However, the host variation of fluorescence is, again, well reproduced.
Molecular dynamics simulation of the zero-field splitting fluctuations in aqueous Ni(II)
Authors: Odelius Michael , Ribbing Carl , Kowalewski Jozef
Published: July 2002

The fluctuations in the zero-field splitting (ZFS) of the electronic ground state of the Ni(II) ion in aqueous solution have been studied through a combination of ab initio quantum chemistry calculations, including spin–orbit coupling, and molecular dynamics (MD) simulations. The ab initio calculations for the hexa-aquo Ni(II) complex have been used to generate an expression for the ZFS as a function of the distortions of the idealized Th symmetry of the complex along the normal modes of Eg and T2g symmetries. The MD simulations provide a 200 ps trajectory of motions in the system consisting of a Ni(II) ion and 255 water molecules, which is analyzed in detail in terms of both the structure and the dynamics in the solvation sphere around the ion. The time correlation function (TCF) for the ZFS interaction has been computed and analyzed. It is found that the mean square amplitude of the ZFS is about 5.2 cm−1, which is about twice the estimates based on the model-dependent analysis of the proton spin relaxation in the aqueous Ni(II) solution. The decay of the ZFS TCF is found to occur on a subpicosecond time scale, which is much faster than earlier proposals. It is also interesting to note, for comparison with theoretical models, that the ZFS tensor is far from cylindrical and that the normal modes of Eg och T2g symmetry both contribute to its fluctuations.
First principles calculation of prepairing mechanism for H2 desorption from Si(100)−2×1
Authors: Pai Sharmila , Doren Douglas
Published: July 2002

Density functional calculations show that H2 desorption from Si(100)−2×1 via a ‘‘prepaired’’ state is consistent with energetic and dynamic measurements. The corresponding adsorption process is discussed and comparisons are made to earlier theoretical studies.
Solvent effects on first-order molecular hyperpolarizability: A study based on vibrational observables
Authors: Zuliani P. , Del Zoppo M. , Castiglioni C. , Zerbi G. , Marder S. R. , Perry J. W.
Published: July 2002

Using a recently developed method, we show here that solvent effects on the first molecular hyperpolarizabilities of push-pull polyenes can be predicted by evaluation of the relaxation contribution βr to the hyperpolarizability from vibrational infrared and Raman intensities and vibrational frequencies. The molecules studied in this work are a few organic polymethine dyes with different donor and acceptor groups. The analysis of their vibrational spectra confirms the key role played by the structural parameters of the polyene chain in determining the nonlinear optical response.
Monomer-excimer kinetics in solution. III. Generalized Smoluchowski approach
Authors: Molski Andrzej , Naumann Wolfgang
Published: July 2002

A generalized Smoluchowski approach developed in A. Molski, Chem. Phys. 182, 203 (1994) is employed to study reversible excimer formation in solution. For contact excimer formation, relations among the rate coefficients are analyzed for three modes of excitation; initially pulsed, steady-state, and periodic. A new kinetic Laplace transform relation for the frequency domain is demonstrated in the linear harmonic regime. The Laplace transform relations between the time domain and steady states, derived in W. Naumann and A. Molski, J. Chem. Phys. 100, 1511, 1520 (1994) for interaction-free models, are shown to be also valid when the interaction forces are included.
Partitioning of interaction energy in van der Waals complexes involving excited state species: The He(1S)+Cl2(B 3Πu) interaction
Authors: Cybulski Sl/awomir M. , Burcl Rudolf , Chal/asiński Grzegorz , Szczȩśniak M. M.
Published: July 2002

The partitioning of interaction energy between a closed-shell and an open-shell system is proposed. This allows us to describe the unrestricted Mo/ller–Plesset interaction energy as a sum of fundamental contributions: electrostatic, exchange, induction and dispersion. The supermolecular energies derived within unrestricted Mo/ller–Plesset perturbation theory are analyzed in terms of perturbation theory of intermolecular forces. The latter has been generalized to allow for the description of monomer wave functions within the unrestricted Hartree–Fock approach. The method is applied to the potential energy surfaces for the first excited triplet states, 3A′ and 3A″, of the He+Cl2(3Πu) complex. The 3A′ and 3A″ potential energy surfaces have different shapes. The lower one, 3A′, has a single minimum for the T-shaped structure. The higher one, 3A″, has the global minimum for the T-shaped structure and the secondary minimum for a linear orientation. The calculated well depth for the 3A′ state is 31.1 cm−1 at the 3.75 Å intersystem separation at the UMP2 level with extended basis set involving bond functions. The 3A″ well depth is approximately 2.3 cm−1 smaller at this level. This order is reversed by higher correlation effects. The angular and radial behaviors of the individual components of the 3A′ and 3A″ interaction energies are compared to reveal the different nature of interaction energies in both states. A comparison with the ground state reveals that the A″ state has a typical van der Waals character similar to that of the ground state. The A′ state, on the other hand, differs considerably from the ground state. The A′ and A″ states differ primarily in different role of the intramonomer correlation effects.
The low-temperature dynamics of rhodamine 640 in polyvinylalcohol
Authors: Baer Bruce J. , Chronister Eric L.
Published: July 2002

The low temperature (1.2–2.1 K) dynamics of rhodamine 640 in polyvinyl alcohol have been investigated by 2-pulse photon echo measurements. The low temperature dephasing results of this study cannot be fit utilizing only the tunneling two-level-system (TLS) model used to describe the higher temperature (10–30 K) data of Nakanishi et al. [J. Chem. Phys. 100, 3442 (1994)] on the same system. In the present study, both the low temperature (1.2–2.1 K) data and the high temperature (10–30 K) data of Nakanishi et al. are fit by a model which includes both a TLS and an electron–phonon contribution to the optical dephasing, yielding a homogeneous dephasing rate (T2*)−1=aTα+b(exp(ΔE/kT)−1)−1, where α=4/3, a=(2915 ps)−1, b=(2.9 ps)−1 and ΔE=16.7 cm−1. The present results indicate that TLS models alone are not sufficient to describe the optical dynamics of Rh640 in PVA at temperatures above 5 K. This is particularly important for studies which attempt to use chemical or physical modification of the matrix to investigate the nature of TLS.
Athermal stiffness blends: A comparison of Monte Carlo simulations and integral equation theory
Authors: Weinhold Jeffrey D. , Kumar Sanat K. , Singh Chandralekha , Schweizer Kenneth S.
Published: July 2002

Off-lattice Monte Carlo computer simulations and numerical polymer reference interaction site model (PRISM) integral equation calculations were performed to quantitatively probe the origins of entropic corrections to Flory–Huggins theory for athermal polymer blends with stiffness disparity. This model system is of interest since it has been recently proposed for describing commercially relevant hydrocarbon polymer mixtures. The novelty of the simulations is that the chemical potential changes on mixing for both components are evaluated. We have considered mixing under constant density conditions, and find surprisingly that the stiffer component is stabilized on blending, while the flexible component is characterized by a positive interaction or χ parameter. The net effective single χ parameter describing these blends, however, is close to zero suggesting that they are completely miscible over a wide range of stiffness disparities and chain lengths. PRISM theory is found to be in good agreement with the simulations for both structural and mixing thermodynamic properties. While purely entropic nonrandom mixing effects could be relevant in determining system thermodynamics, especially for large stiffness disparity, the dominant contribution to the chemical potential changes on mixing arise from equation-of-state (EOS) effects since the two pure components and the mixture are at different pressures when examined at the same density. The EOS contribution to the mixing free energy for small stiffness mismatch is shown to be quantitatively reproduced through an extension of the generalized Flory approach. Through the use of PRISM theory we find that athermal, nonlocal entropy-driven phase separation can occur for long enough chains and high enough stiffness disparity. However, since no phase separation is predicted for stiffness disparities relevant to experimental hydrocarbon systems, regardless of chain length, we suggest that enthalpic effects have to be evoked to explain the limited miscibility of these commercially important mixtures.
Infrared spectroscopy of the siliconium ion, SiH+5
Authors: Boo Doo Wan , Lee Yuan T.
Published: July 2002

The infrared spectrum for the H–H stretching mode of the siliconium ion SiH+5 in the frequency range of 3650–3740 cm−1 is presented. The observed vibration–rotation transitions were fitted with the A-type rotational transitions of an asymmetric top using the Watson S-type asymmetric top rotational Hamiltonian. The results suggested that the siliconium ion SiH+5 can be described as a complex between SiH+3 and a freely internally rotating H2 groups, with a highly localized three-center two-electron bond.