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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1480-1486 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structure of a model colloidal suspension in the vicinity of a charged wall is studied in the framework of the Derjaguin–Landau–Verwey–Overbeek interaction potential and the hypernetted-chain approximation. Here we consider the case of dilute suspension of highly charged macroparticles interacting with weakly repulsive, neutral, and attractive walls. As the wall–particle electrostatic interactions become successively less repulsive, the formation of a monolayer of colloidal particles, strongly adsorbed onto the surface, is predicted by our results. This monolayer of electrostatically confined particles mimic the effect of an effective surface charge distribution adjacent to the wall which, together with the bare wall surface charge, induces on the other, nonconfined, colloidal particles the same local-concentration profile as that in front of a highly charged and repulsive wall.
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  • 2
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1474-1479 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The small-angle x-ray scattering from fully and partially derivatized porous silicas has been studied. Power-law-scattering exponents of magnitude greater than 4 have been found in all cases. The magnitudes of the exponents increased with the alkyl chain length and with the degree of surface derivatization. In a preliminary model to explain these observations, a power-law-scattering exponent with magnitude greater than 4 is related to a "fuzzy'' pore boundary, in which the density varies continuously at the pore boundary instead of changing discontinuously from a value of zero in the empty pore to the essentially constant density characteristic of the bulk silica, as is usually assumed in analyses of the small-angle scattering from porous silicas.
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  • 3
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 7656-7661 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Canonical Monte Carlo results are presented for mixtures of primitive model electrolytes with a common ion. Both symmetric mixtures, where the ions differ only in size, and charge asymmetric mixtures were studied for ionic strengths ranging from 0.003 to 1.0 M. The hypernetted chain (HNC) equation theory and the simple "exponential'' (EXP) approximation are both applied to the same electrolyte models. The electrolyte mixing coefficients w0 and w1 were calculated from osmotic coefficients. Comparison with Monte Carlo data indicates that the HNC equation yields accurate predictions for the zeroth mixing coefficient w0, while the simple EXP approximation yields qualitatively correct results. The Monte Carlo results for the first mixing coefficient, w1, are not precise enough to allow a quantitative comparison with other theories. However, a strong concentration dependence of w1 for dilute solutions, found previously for the nonprimitive models in the HNC approximation, is confirmed by the Monte Carlo results.
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  • 4
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 7662-7671 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new model electron–ammonia pseudopotential parameterized to ab initio quantum chemistry calculations on small lithium ammonia clusters, Li(NH3)n(n=1,4), is studied in a variety of environments. For Li(NH3)n clusters, n=16,32,64,128 the valence electron of the lithium is found to exist in a surface state far from the cation which is localized near the center of mass of the cluster. No bulk states were stabilized. Cluster anions (NH3)−n were also studied and the electron centroid-cluster center of mass probability distribution for (NH3)−64 calculated using umbrella sampling. In the present model, there is apparently no barrier to the dissociation of the surface states. No bulk states were found. This set of results appears to disagree with experiments which have been interpreted to indicate bulk states for cluster anions and the clusters containing lithium. Bulk properties of both a single excess electron and the lithium atom in solution are also reported. The solvation energy agrees well with experiment but the spectrum of the excess electron remains somewhat blue shifted as in earlier calculations. However, the valence electron of the lithium atom is found to spontaneously dissociate; a property not reproduced in previous work.
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  • 5
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 7683-7695 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The geometrical structures of M+(Ar)n ions, with n=1–14, have been studied by the minimization of a many-body potential surface with a simulated annealing procedure. The minimization method is justified for finite systems through the use of an information theory approach. It is carried out for eight potential-energy surfaces constructed with two- and three-body terms parametrized from experimental data and ab initio results. The potentials should be representative of clusters of argon atoms with first-row transition-metal monocations of varying size. The calculated geometries for M+=Co+ and V+ possess radial shells with small (ca. 4–8) first-shell coordination number. The inclusion of an ion-induced-dipole–ion-induced-dipole interaction between argon atoms raises the energy and generally lowers the symmetry of the cluster by promoting incomplete shell closure. Rotational constants as well as electric dipole and quadrupole moments are quoted for the Co+(Ar)n and V+(Ar)n predicted structures.
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  • 6
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 3011-3017 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Quantum chemical calculations are presented which predict that in the ground state of di-π-cyclooctatetraene cerium (cerocene) the Ce ion is almost entirely in a 4f1 configuration corresponding to Ce3⊕(C8H1.5(large-closed-square)8)2. The 4f electron forms with an electron of the ligand e2u highest-occupied molecular orbital a 4f1e32u singlet in close analogy to a Kondo ion in a metal. Due to coupling of the 4f1e32u with the 4f0e42u configuration, the latter corresponding to Ce4⊕(C8H2≤8)2, the splitting between the ground state singlet and the first excited triplet is of the order 0.5 eV. The self-consistent-field and multiconfiguration self-consistent-field parts of the calculations are done by employing recently developed pseudopotentials for cerium using basis sets of up to 626 basis functions. The correlation energy is accounted for by means of various correlation-energy density functionals and also by limited coupled electron-pair approximation calculations. Similar results are found in both cases.
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  • 7
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 7672-7682 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Dynamic light scattering (depolarized Rayleigh and polarized Rayleigh–Brillouin) and quasielastic neutron scattering are employed to study the dynamics of the glass-forming liquid di-2-ethylhexyl phthalate (DOP) (Tg=184 K). The depolarized Rayleigh scattering measurements were made in the temperature range from 303 to 433 K, the polarized Rayleigh–Brillouin measurements in the range from 263 to 433 K, and the quasielastic neutron-scattering measurements in the range from 37 to 312 K and in the Q range from 0.33 to 1.84 A(ring)−1. The orientation times for DOP, obtained from a single Lorentzian fit to the experimental depolarized spectra at high T, are in good agreement with recent dielectric data for the primary (α) relaxation. However, at lower T, the viscosity increases more strongly than the orientation times and the Stokes–Einstein–Debye equation which can adequately describe the dynamics in the high-T range is insufficient at temperatures close to Tg. The relaxation time obtained from the Rayleigh–Brillouin experiment is about 1 order of magnitude faster than the orientation times. In the neutron-scattering experiment we find a strong decrease of the elastic intensity and a corresponding increase of the quasielastic intensity around Tg.The data analysis with respect to the dynamics (from a two Lorentzian fit) revealed the existence of three processes affecting the high-frequency range: (i) a "fast'' (τ2∼10 ps) Q-independent motion with weak T dependence (E2=1.54 kcal/mol), (ii) a "slow'' Q-dependent motion, and (iii) a flat background increasing with T and Q. The fast process is discussed in terms of a very localized motion of the phenyl group (β relaxation) and, as such, as a precursor of the the primary (α) relaxation. The relaxation time of this process (τ2) was found to compare nicely with the time τmax from the Rayleigh–Brillouin (RB) experiment suggesting that the latter is caused by fast localized motions. The slow process is discussed in terms of the jump-diffusion model. The activation energy associated with the jump-diffusion times is 6.1 kcal/mol and it is associated with large-scale diffusional motion of the DOP molecule. The relaxation times obtained from this process are compared with the relaxation times obtained from the depolarized and dielectric techniques for the primary relaxation. Finally, the background can be identified with fast local motions and/or low-frequency excitations relaxing outside the energy window of our experiment.
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  • 8
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 7710-7727 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We consider the effect of shear velocity gradients on the size (L) of rodlike micelles in dilute and semidilute solution. A kinetic equation is introduced for the time-dependent concentration of aggregates of length L, consisting of "bimolecular'' combination processes L+L' →(L+L') and "unimolecular'' fragmentations L→L'+(L−L'). The former are described by a generalization (from spheres to rods) of the Smoluchowski mechanism for shear-induced coalesence of emulsions, and the latter by incorporating the tension-deformation effects due to flow. Steady-state solutions to the kinetic equation are obtained, with the corresponding mean micellar size (L¯) evaluated as a function of the Peclet number P, i.e., the dimensionless ratio of flow rate γ(overdot) and rotational diffusion coefficient Dr. For sufficiently dilute solutions, we find only a weak dependence of L¯ on P. In the semidilute regime, however, an apparent divergence in L¯ at P(approximately-equal-to)1 suggests a flow-induced first-order gelation phenomenon.
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  • 9
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 4219-4229 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An efficient implementation of microcanonical, classical variational transition-state theory based on the use of the efficient microcanonical sampling (EMS) procedure is applied to simple bond fissions in SiH2 and Si2H6 using recently constructed global potential-energy surfaces. Comparison is made with results of trajectory calculations performed on the same potential-energy surfaces. The predictions of the statistical theory agree well with and provide an upper bound to the trajectory derived rate constants for SiH2→SiH+H. In the case of Si2H6, agreement between the statistical theory and trajectory results for Si–Si and Si–H bond fission is poor with differences as large as a factor of 72. Moreover, at the lower energies studied, the statistical calculations predict considerably slower rates of bond fission than those calculated from trajectories. These results indicate that the statistical assumptions inherent in the transition-state theory method are not valid for disilane in spite of the fact that many of the mode-to-mode rate constants for intramolecular energy transfer in this molecule are large relative to the Si–Si and Si–H bond fission rates. There are indications that such behavior may be widespread among large, polyatomic molecules.
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  • 10
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 4205-4218 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present differential cross section (DCS) measurements for scattering of HF by Ar. These crossed-beam experiments employ rotational state sensitivity, allowing determination of the DCS as a function of the scattered HF rotational state. The initial HF rotational distribution is generated by nozzle expansion, without further state selection. Its composition is mostly J=0 and J=1, with small admixtures for J〉1. The DCS for each final state J' is measured using a stabilized cw HF chemical laser, in conjunction with a rotatable liquid He-cooled bolometer. Measurable signals are obtained for scattering into 0≤J'≤5, where J'=6 is the thermodynamic limit for our collision energy of 120 meV. The measured DCS's show a strong forward peak, largely from elastic scattering. In addition, the DCS's evolve from a broad shoulder in the θ≈25°–40° region for J'=0—through a flattening of the wide-angle scattering for J'=2 and J'=3—to an increase in the scattering beyond ∼40° for J'=4. The DCS for scattering into J'=5 also shows increased intensity at wide scattering angles, but its onset is delayed until ∼70°. These features are shown to be independent of the laboratory → center-of-mass kinematic transformation. The wide-angle scattering into J'=4 and J'=5 corresponds to transferring up to 40% and 60%, respectively, of the available kinetic energy into HF rotation. Since the center-of-mass scattering angles are up to ∼110°, we interpret the observed features for J'=4–5 in terms of rotational rainbow scattering from the hard core of the HF+Ar potential energy surface. The origin of the shoulder for J'=0 scattering is less clear, but it may arise from the strongly anisotropic nature of the HF+Ar van der Waals attraction.
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  • 11
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 7758-7770 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The methods developed in our earlier papers, dealing with Raman scattering from small isolated, spherical particles, uniformly coated with radially uniaxial molecules, are extended to include scattering driven by applied multipolar fields. Such fields are generated by the action of the incident electromagnetic radiation on extraneous material, such as other particles belonging to the same high-density dispersion. In this paper we treat inelastic scattering associated with the Raman dipole matrix elements of the adsorbed molecules ignoring other contributions. Analytical expressions are developed for calculating the primary Raman–Stokes moments given the amplitudes of the applied multipolar fields. As a test of the practicability of the procedure, a full calculation for parallel polarization has been performed for two identical contacting, CO-coated Ag particles oriented with their line of centers parallel to the direction of polarization of the incident radiation. For this case, the effect of interparticle coupling is to broaden and intensify the excitation spectrum, extending the region of intense Raman scattering to much lower frequencies. Intensity enhancements approaching 106 are calculated for frequencies as low as 80% of the single-particle surface-plasmon resonance frequency.
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  • 12
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 7771-7787 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have simulated the photodissociation of CH3Br adsorbed at a variety of surfaces. We have considered photodissociation at a smooth LiF (001) substrate and at three rough LiF surfaces which were constructed by removing atoms from the smooth surface. We have also considered photodissociation from several surfaces which have the same structure as the β phase of solid CH3Br to simulate dissociation from high coverages of the adsorbate where CH3Br ice is formed. The simulations were performed using the stochastic classical trajectory method. The asymptotic photofragment kinetic energy and angular distributions were determined and compared with the experimental results of Harrison et al. [J. Chem Phys. 89, 1475 (1988)] and Tabares et al. [J. Chem. Phys. 86, 738 (1987)]. When CH3Br is oriented with CH3 toward a surface, the CH3 kinetic energy distributions are shifted to much lower energies due to energy loss from multiple collisions with Br and the surface; the angular distributions are also significantly broadened. Much of the energy loss in these collisions goes into the translational mode of the Br fragments, causing the Br kinetic energy distributions to have a high-energy tail. When the molecule is in this orientation in a restricted geometry, collisions from the CH3 fragment lead to more effective energy transfer causing the peak of the Br kinetic energy distributions to be shifted to much higher energies and the corresponding angular distributions to become narrower. The main features of the experimental results from photodissociation of CH3Br adsorbed on LiF can be qualitatively explained using the results of the classical trajectory simulations.
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  • 13
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 7788-7796 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The microscopic properties of the inner surface of the cavities of Anopore and Nuclepore membranes are investigated with scanning electron microscopy (SEM), nitrogen adsorption isotherms, and deuterium nuclear magnetic resonance (2H-NMR). Useful information about the cavity orientation and morphology, and internal surface area is obtained. Analysis of SEM photographs yields estimates of the surface area and the porosity of these membranes which complements the adsorption results. The orientation of liquid crystals permeated in the cavities of Anopore and Nuclepore membranes is probed with the 2H-NMR technique. It is found that the orientation of the liquid crystal molecules is governed by the confining volume and surface conditions. The 2H-NMR spectral line shape of the confined liquid crystal also provides information on the substrate morphology and roughness that is consistent with SEM and adsorption experiments.
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  • 14
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    The Journal of Chemical Physics 96 (1992), S. 7797-7813 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We develop a simple but general three-variable model skeleton to describe complex nonlinear behaviors in electrochemical processes taking place at either a hanging mercury drop electrode (HMDE) or a rotating-disk electrode (RDE). We apply our formalism to the reduction of indium(III) at a HMDE in the presence of thiocyanate, a reaction known to exhibit complex mixed-mode and chaotic oscillations. Besides the role of the negative Faradaic impedance in destabilizing the electrochemical system, mass transport appears to be crucial as the model explicitly takes into account, in a truncated fashion, the time-dependent relaxation of the concentration profile. We study in detail the nonlinear dynamic behavior of our model of the indium/thiocyanate system and a RDE model. The models support mixed-mode sequences that appear either as incomplete Farey sequences or as periodic-chaotic sequences, which we discuss in terms of an incomplete homoclinic scenario whose definition and properties are worked out here. Our results compare very well to the experimental observations in the indium/thiocyanate system and the electrodissolution of a rotating copper disk in phosphoric acid. This satisfactory agreement strongly suggests that diffusion relaxation is an important phenomenon in electrochemical oscillations and could be the essential third variable in many dynamical electrochemical processes.
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  • 15
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    The Journal of Chemical Physics 95 (1991), S. 5348-5354 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics trajectories have been computed and analyzed for linear chains, with sizes ranging from C10H22 to C100H202, and for cyclic C100H200. All hydrogen atoms are included discretely. All bond lengths, bond angles, and torsion angles are variable. Hazard plots show a tendency, at very short times, for correlations between rotational isomeric transitions at bond i and i±2, in much the same manner as in the Brownian dynamics simulations reported by Helfand and co-workers. This correlation of next nearest neighbor bonds in isolated polyethylene chains is much weaker than the correlation found for next nearest neighbor CH–CH2 bonds in poly(1,4-trans-butadiene) confined to the channel formed by crystalline perhydrotriphenylene [Dodge and Mattice, Macromolecules 24, 2709 (1991)]. Less than half of the rotational isomeric transitions observed in the entire trajectory for C50H102 can be described as strongly coupled next nearest neighbor transitions. If correlated motions are identified with successive transitions, which occur within a time interval of Δt≤1 ps, only 18% of the transitions occur through cooperative motion of bonds i and i±2. An analysis of the entire data set of 2482 rotational isomeric state transitions, observed in a 3.7 ns trajectory for C50H102 at 400 K, was performed using a formalism that treats the transitions at different bonds as being independent. On time scales of 0.1 ns or longer, the analysis based on independent bonds accounts reasonably well for the results from the molecular dynamics simulations. At shorter times the molecular dynamics simulation reveals a higher mobility than implied by the analysis assuming independent bonds, presumably due to the influence of correlations that are important at shorter times.
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  • 16
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    The Journal of Chemical Physics 94 (1991), S. 3114-3131 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Presented are the results of testing the method for estimating chemical potentials which was described in paper I. The method, which is based on scaled particle theory, provides accurate chemical potentials in mixtures of softly repulsive particles when used with the Rogers–Young integral equation. Calculated excess Gibbs energies agreed with simulations to an average of −0.67% for 2:1 diameter ratio mixtures. The method provides approximate results in Lennard-Jones mixtures when used with the hybrid mean spherical approximation integral equation theory. Results for supercritical isotherms reproduce simulation data to an average of −3.0%. For subcritical isotherms, vapor results are exact while liquid results are qualitatively correct. The method used with the integral equation theory correctly predicts the effect of energy ratio on the Henry's Law constant. The predicted effect of size ratio on the constant has an incorrect slope at subcritical temperatures when the solvent density is near the value for a saturated liquid. The incorrect slope results from inaccuracies in the predicted correlation functions for the fluid surrounding the test particle. The method allows estimates to be made of the work of cavity formation and of the strength of solvent–solute binding in near-critical mixtures.
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  • 17
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    The Journal of Chemical Physics 94 (1991), S. 3132-3140 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Transport properties of moderately dense fluids reflect several effects not present at the Boltzmann level: excluded volumes, finite collision times, and bound states. The description and relative importance of these effects is illustrated here by a discussion of the self-diffusion coefficient for the square well fluid. At high temperatures the particles behave as hard spheres and the Enskog kinetic theory describing excluded volume effects is adequate. Generalizations of the Enskog theory for application at lower temperatures are discussed critically. One approach assumes partial (uncorrelated) collisions for initial conditions modified to account for excluded volume effects. Another describes completed binary collisions at the core and edges of the well for the same modified initial conditions. Although both reduce to the Enskog theory at high temperatures, neither is adequate outside this limit for prediction of either the temperature or the density dependence. The proper treatment of both temperature and density effects is found to require a more detailed description of interrupted binary collisions and bound states. Such a treatment is described and shown to be in good agreement with computer simulation results. We conclude that generalization of the hard-sphere Enskog theory to the square well, and probably other potential models, is more subtle than might be expected and requires dynamical as well as static many-body effects.
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  • 18
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    The Journal of Chemical Physics 94 (1991), S. 3141-3149 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The sticky electrolyte mode for a weak unsymmetrical electrolyte is solved in the mean spherical approximation (MSA) when there are adhesive interactions between oppositely charged ions. The distribution functions at contact and the thermodynamic properties in this approximation are derived; the solutions reduce to those of corresponding symmetrical adhesive electrolyte studied by Rasaiah and Lee [J. Chem. Phys. 83, 6396 (1985)] when the sizes of the ions and the magnitudes of the charges are made the same and to those of adhesive nonelectrolytes when the charges are removed. When the stickiness is turned off the solutions of the primitive model electrolyte in the MSA are recovered.
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  • 19
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    The Journal of Chemical Physics 94 (1991), S. 3150-3168 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A comprehensive theoretical analysis is presented which accounts for nuclear spin relaxation due to molecular surface diffusion on spheroidal aggregates undergoing restricted rotational diffusion in a uniaxial potential of mean torque. The model is formulated primarily with surfactant systems in mind, and should provide a realistic description of spin relaxation in nematic and smectic liquid crystals as well as in isotropic micellar solutions. Two efficient numerical schemes are developed to compute the joint time correlation functions and spectral density functions. In addition, a variety of useful analytical results are derived for special cases. Numerical calculations suggest that nuclear spin relaxation may be a unique method for monitoring small deviations from spherical aggregate shape.
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  • 20
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    The Journal of Chemical Physics 94 (1991), S. 3177-3182 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamics of solvation of newly created charged species in dense dipolar liquids can proceed at a high speed with time constants often in the subpicosecond domain. The motion of the solvent molecules can be in the inertial limit at such short times. In this paper we present a microscopic study of the effects of inertial motion of solvent molecules on the solvation dynamics of a newly created ion in a model dipolar liquid. Interesting dynamical behavior emerges when the relative contribution of the translational modes in the wave-vector-dependent longitudinal relaxation time is significant. Especially, the theory predicts that the time correlation function of the solvation energy can become oscillatory in some limiting situations. In general, the dynamics becomes faster in the presence of the inertial contribution. We discuss the experimental situations where the inertial effects can be noticeable.
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  • 21
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    The Journal of Chemical Physics 94 (1991), S. 3169-3176 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Direct measurements have been made on the average single-particle kinetic energies, 〈Ek〉, of a series of solid and liquid specimens of heavy noble gases near melting. To do this, the distributions of energy losses of incident eV neutrons, scattered at angles ranging from 87 to 136 deg, were analyzed on the assumptions that atoms in these specimens have a Gaussian momentum distribution n(p) and that for these experimental conditions the atoms scatter independently of one another. It is known that the solids, Ar, Kr, and Xe show considerable differences in such characteristics as (a) their respective ratios of triple point to Debye temperatures, (b) the extent of multibody force contributions to cohesion, and (c) phonon anharmonicity. Regardless, it is found for all three solids near their respective triple points that 〈Ek〉 is equal to the equipartition value (3/2)kT well within the various estimated experimental uncertainties of 3% to 7%. Moreover, liquid Kr near its triple point yields the same result, as does liquid Xe at temperature up to 1.8 times its triple point temperature. Because all these systems are expected to be classical at these temperatures, these results demonstrate applicability of this neutron recoil method to determine 〈Ek〉 even of heavy atoms in suitable cases where a value may not be known in advance.
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  • 22
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    The Journal of Chemical Physics 94 (1991), S. 3183-3191 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A density functional theory for polymer fluids is derived. The form of the theory is identical to a continuum self-consistent-field theory but we show that with the correct "mean field,'' the theory is exact. We apply the theory to a polymer fluid made up of tangential hard spheres and borrow from a successful density functional theory for simple hard spheres to obtain a nonlocal functional expression for this mean field. The latter depends only upon the monomer density. This approximation is used to treat the problem of a mixture of such polymers and simple hard spheres, contained in slitlike pores. Comparison with simulations show that the theory, though simple in structure, and containing a single adjustable parameter, is surprisingly accurate.
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  • 23
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    The Journal of Chemical Physics 94 (1991), S. 1528-1542 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Angle-resolved supersonic molecular beam scattering and time-of-flight techniques (TOF) are used to probe the dynamics of the Cl2/GaAs{110} thermal etching reaction. TOF spectra are recorded for the unreacted Cl2 and the GaCl3 reaction product as a function of incident translational energy and surface temperature at various final scattering angles. Our results show that the reaction mechanism is precursor mediated. The weakly bound molecular (Cl2)ads species is a key reaction intermediate through which subsequent reaction steps occur. The reaction probability, determined by angle integration of the unreacted Cl2 flux, increases with surface temperature (Ts) reaching a plateau and decreasing slightly between Ts∼425 K and 525 K before increasing sharply again beyond 525 K. This trend is nearly independent of incident kinetic energy. Detailed analysis of the TOF spectra reveal that the dynamical origin of this effect is due in part to a change in the reaction kinetics in which Cl2 desorption competes with product formation. The insights gained in this study are used to formulate plausible explanations for several discrepancies existing in the literature concerning the temperature dependence of the etch rate and of the high temperature product distribution.
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  • 24
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    The Journal of Chemical Physics 94 (1991), S. 1555-1563 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation rate of phosgene (Cl2CO) adsorbed on Pt(111) has been measured as a function of photon energy at normal incidence and as a function of incident angle using p-polarized light. Compared to the gas phase, the wavelength dependence of the initial photolysis cross section on the surface is redshifted. The angular response to p-polarized light is wavelength dependent. Above 315 nm, the angular dependence correlates with calculated metal absorption. At 280 nm, the angular dependence is much too strong to be accounted for solely by substrate excitation. A combination of substrate and direct excitation is adequate. This is the first direct evidence, for monolayers on metals, that both direct and substrate excitation contribute to surface photochemistry but dominate at different wavelengths.
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  • 25
    ISSN: 1089-7690
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    Topics: Physics , Chemistry and Pharmacology
    Notes: We have investigated the influence of two-photon absorption on the third-order nonlinear optical properties of model organic molecules using the technique of degenerate four-wave mixing (DFWM). A theoretical formulation developed here shows that the presence of two-photon absorption, which is related to the imaginary part of the third-order susceptibility χ(3), leads to an enhancement of the effective third-order nonlinearity and to the appearance of effects caused by the formation of two-photon generated excited states. The dynamic behavior of the nonlinearity is then governed by the properties of excited molecules. The nonlinear effects also involve contributions which depend on the fifth power of the electric field. We have performed a systematic study of third-order nonlinear optical properties of alkoxy (–C10H21OCH) substituted p-polyphenyl oligomers using the technique of time-resolved degenerate four-wave mixing with subpicosecond pulses at 602 nm. Experimentally determined values of the second-order hyperpolarizability γ for the oligomers increase smoothly from the monomer to the trimer, with a more rapid increase to the pentamer and to the heptamer. In addition, the hyperpolarizabilities for the pentamer and the heptamer appear to be complex. A smooth increase of the γ value is expected from an increase of the π conjugation from a shorter chain oligomer to a longer chain oligomer. The more rapid increase of the γ value in the pentamer, and especially in the heptamer, however, cannot be explained satisfactorily by only taking into account the π-conjugation length. Two-photon absorption for the pentamer and the heptamer at the measurement wavelength of 602 nm is suggested to be important as the observed dynamic behavior is satisfactorily explained by the predictions of the theoretical model presented here. It is shown that the effective γ value for a two-photon absorbing material is a function of optical intensity, pulse width, and sample length if one uses the conventional degenerate four-wave mixing description.
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  • 26
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    The Journal of Chemical Physics 95 (1991), S. 4040-4043 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Gaussian-2 (G2) theoretical procedure, based on ab initio molecular orbital theory, is used to calculate the energies of CH2OH, CH3O, and their cations. The resulting G2 adiabatic ionization potential for CH3O of 10.78 eV supports the new value of 10.726±0.008 eV reported by Ruscic and Berkowitz [J. Chem. Phys. 95, xxxx (1991)] from a photoionization study. A previous photoelectron result of 7.37±0.03 eV is probably an incorrect assignment. The G2 ionization potential for CH2OH is 7.45 eV, consistent with the experimental value of 7.55 eV. The calculated O–H and C–H bond dissociation energies of CH3OH are 105.0 and 96.2 kcal/mol, respectively. The results suggest that the D0(H–CH2OH) from kinetics measurements may be too low.
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  • 27
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    The Journal of Chemical Physics 94 (1991), S. 4352-4355 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The collinear reaction of Fe+ (4F) and Fe+ (6D) with H2 leading to FeH+ and H has been studied at the configuration interaction (CI) level of theory, using a relativistic pseudopotential approach. The 6D electronic state of Fe+ reacts with H2 through a very large energy barrier that slightly exceeds the endothermicity of the reaction. The 4F electronic state appears to be much more reactive than the 6D state, in good agreement with the experimental results.
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  • 28
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    The Journal of Chemical Physics 95 (1991), S. 4081-4093 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Intramolecular carbon atom exchange in highly vibrationally excited ketene was studied by monitoring the carbon monoxide fragments (12CO and 13CO ) from the photodissociation of 12CH213CO and 13CH212CO. Two experimental techniques were employed. In one set of experiments the IR transient absorptions of 12CO and 13CO were measured following pulsed excimer excitation of ketene 13C isotopomers, giving carbon atom exchange yields at 351 and 308 nm in a low pressure gas cell. In the other set of experiments, jet-cooled ketene 13C isotopomers were excited with tunable near-UV radiation, and the CO products were detected by monitoring their VUV laser-induced fluorescence. Carbon atom exchange yields were measured for energies extending from below the triplet decomposition threshold (CH2CO→CH2(X 3B1)+CO(X 1Σ+)) to about 4000 cm−1 above the singlet threshold (CH2CO→CH2(a 1A1)+CO(X 1Σ+)).The exchange yields range from 4 to 19%, and the energy dependence of the yield exhibits pronounced structure, with maxima at the triplet and singlet decomposition thresholds. Kinetic measurements of the appearance of the CO products were also performed. The time constant for the appearance of the exchanged CO (e.g., 13CO from 13CH212CO ) is significantly longer than that for the direct CO fragment (e.g., 12CO from 13CH212CO ). All the experimental observations are consistent with a simple reaction mechanism involving ketene isomerization, 13CH212CO(arrow-right-and-left)12CH213CO, and dissociation, 13CH212CO→13CH2+12CO and 12CH213CO→12CH2+13CO. The isomerization rate constant was determined by analyzing the CO kinetics and the carbon atom exchange yields in terms of the simple isomerization mechanism. A fit of the energy dependence of the isomerization rate constant to the results of tunneling-corrected Rice–Ramsberger–Kassel–Marcus (RRKM) calculations gave the threshold (28360±60 cm−1 ) for the isomerization process.
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  • 29
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    The Journal of Chemical Physics 94 (1991), S. 4356-4368 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The quadrupole moment of acetylene has been studied at the multiconfiguration self-consistent field (MCSCF) and multireference single and double configuration interaction (MRSDCI) level of theory. At the MCSCF level the π-CI complete active space SCF (CASSCF) and the valence-CI CASSCF were employed. The subsequent MRSDCI calculations were continued until the reference space included all configuration state functions (CSFs) of the MCSCF wave function with a coefficient larger than 0.01 [MRSDCI(0.01)]. The higher level basis sets in this study were all based on van Duijneveldt's C(13s 8p) and H(6s) and extensions of that basis set. The study shows in a consistent way that both the one- and n-particle spaces are saturated at the highest level of theory. The study has revealed that in addition to the well known increase of the quadrupole moment due to the inclusion of polarizing functions in the basis (typically 0.20 a.u.), the inclusion of electronic correlation in the model wave function as well as vibrational corrections will decrease the quadrupole moment significantly more, −0.66, −0.49, and −0.36 a.u., for the correlation correction and zero-point correction for HCCH and DCCD, respectively. The most accurate computations predict the quadrupole moment of HCCH, including zero-point correction, to be 4.29±0.12 a.u., which discriminates the experimental estimates of 4.03±0.30, 4.28±0.30, and 4.57±0.30 a.u. (the first being the favored value). The quadrupole moment of DCCD is computed to 4.42±0.10 a.u. In the study it was observed that in contradiction to previous experiences the use of the model equilibrium geometries rather than the experimental geometry gives a smoother convergence as the level of theory is increased. The effects of basis set quality and electron correlation on the quadrupole moment are studied in detail. These effects are analyzed with reference to the redistribution of the electronic charge.
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  • 30
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    The Journal of Chemical Physics 94 (1991), S. 4369-4383 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Potential energy curves are calculated for the ten lowest states in HeH which correlate with the hydrogen asymptote in the n=1, 2, 3 occupation; these are X, A, C, D, 5 2Σ+, 6 2Σ+, and B, E, 3 2Π as well as the 1 2Δ states. Multireference configuration interaction calculations are employed thereby in an atomic orbital (AO) basis of contracted Gaussians. Extensive calculations of the ∂/∂R, ∂2/∂R2, Lx, and L2 matrix elements are carried out to account explicitly for the effects beyond the Born–Oppenheimer approximation. The positions of rovibrational levels are thereby determined in pairwise close-coupling calculations for the X/A and C/D states of 2Σ+ symmetry for the four isotopomers 4HeH, 3HeH, 4HeD, and 3HeD. Radial, angular, and mass polarization corrections affect the A and C states differently, so that the A–C energy gap increases by 39 cm−1 in 3HeD and by 53 cm−1 in HeH upon introduction of these terms, e.g., whereby the contribution of the mass polarization is by far the smallest. By employing a two-parameter correction function to the calculated electronic potential energy and making use of the calculated non-Born–Oppenheimer terms, a large number of levels for the A, C, and D states as a function of (v,J) quantum numbers are computed which agree with those, which are experimentally available for the C–A and D–A transitions within wave number accuracy.
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  • 31
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    The Journal of Chemical Physics 94 (1991), S. 4384-4390 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The theorem proposed by Pathak and Gadre [J. Chem. Phys. 93, 1770 (1990)], that the electrostatic potential (ESP) of negative ions must exhibit a directional negative valued minimum along any arbitrary direction has been verified for some small negative molecular ions, viz., OH−, CN−, N−3, NO−3, and NH−2. Also, as predicted by Gadre and Pathak [Proc. Ind. Acad. Sci. (Chem. Sci.) 102, 18 (1989)], the molecular ESP (MESP) maps are found to be devoid of local maxima. As a consequence, these maps reveal rich topographical details in the form of several saddle points as well as point minima. From the location of these critical points, estimates of the sizes and shapes of the negatively charged molecular ions are obtained. For anions, there exists a surface on which (large-closed-square)V⋅dS=0 and which passes through all the negative valued critical points (∇V=0). The ionic size estimates from the location of the critical points of the MESP are found to be in good agreement with the corresponding (spherically averaged) literature values.
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  • 32
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    The Journal of Chemical Physics 94 (1991), S. 4391-4404 
    ISSN: 1089-7690
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Two nondegenerate quantum levels coupled off-diagonally and linearly to a bath of quantum-mechanical harmonic oscillators are considered. In the weak-coupling limit one finds that the equations of motion for the reduced density-matrix elements separate naturally into two uncoupled pairs of linear equations for the diagonal and off-diagonal elements, which are known as the Bloch equations. The equations for the populations form the simplest two-component master equation, and the rate constant for the relaxation of nonequilibrium population distributions is 1/T1, defined as the sum of the "up'' and "down'' rate constants in the master equation. Detailed balance is satisfied for this master equation in that the ratio of these rate constants is equal to the ratio of the equilibrium populations. The relaxation rate constant for the off-diagonal density-matrix elements is known as 1/T2. One finds that this satisfies the well-known relation 1/T2=1/2T1. In this paper the weak-coupling limit is transcended by deriving the Bloch equations to fourth order in the coupling. The equations have the same form as in the weak-coupling limit, but the rate constants are calculated to fourth order. For the population-relaxation rate constants this results in an extension to fourth order of Fermi's golden rule. We find that these higher-order rate constants do indeed satisfy detailed balance. Comparing the dephasing and population-relaxation rate constants, we find that in fourth order 1/T2≠1/2T1.
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    The Journal of Chemical Physics 95 (1991), S. 4327-4330 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Using a technique based on the hypervirial theorem along with a second quantization formalism, recursion relations for matrix elements and Franck–Condon factors of Morse potentials are obtained. These expressions can be used to calculate recursively matrix elements of the operators qk=(r−re)k, xs=exp[−as(r−re)], dl/dr l, or any combination of them, corresponding to vibrational–rotational states belonging to two distinctly separated Morse oscillators of arbitrary De. The proposed equations give accurate numerical results even when the traditional methods (numerical integration, asymptotic expansions, semiclassical, etc.) present serious inconveniences. The results are valid for any value of the power of the operator and of the quantum numbers v and J of both oscillators.
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    The Journal of Chemical Physics 94 (1991), S. 4416-4420 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The evolution of the basins of attraction of coexisting stable (periodic) states in a forced bistable system is considered. One attractor undergoes a period-doubling cascade to chaos. The basin of attraction also becomes fractal (d = 2.07) at sufficiently high forcing amplitude, leading to the loss of integrity of this state−but this is not coincident with the onset of chaos.
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  • 35
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    The Journal of Chemical Physics 94 (1991), S. 4411-4415 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the anisotropy of the proton momentum distribution in potassium hydrogen carbonate, KHCO3. Using the deep inelastic neutron scattering technique, the mean proton momenta along, and across, the O–H–O bond have been determined. While the momentum across the bond corresponds to a proton in a harmonic potential, that along the bond shows this potential is consistent with a previously proposed double well shape.
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  • 36
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    The Journal of Chemical Physics 95 (1991), S. 4397-4406 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An analysis of electronic polarizabilities, interionic potentials, and equilibrium geometries for 20 alkali–halide dimers such as Na2Cl2 is presented. The electronic polarizabilities of ions in dimers have been calculated by taking account of the electrostatic potential due to neighboring ions within the Seitz–Ruffa energy level analysis. The polarizabilities thus obtained for dimers are then used to evaluate the magnitudes of polarization energies and van der Waals dipole–dipole energies. For describing the overlap repulsive potential we have adopted two forms: (i) the revised version of the Born–Mayer potential and (ii) the Harrison form for the overlap repulsive energy. These potentials have been used to obtain the binding energies of monomers as well as dimers, spectroscopic constants of monomers, equilibrium geometries of dimers, and dimerization energies. The results have been discussed and compared with available experimental data.
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  • 37
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    The Journal of Chemical Physics 95 (1991), S. 4439-4443 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The influence of disorder on shear viscosity of undercooled melts is described by the formula η=η0⋅exp(θ/T )a. It is shown that this expression reproduces the existing experimental data with a reasonable accuracy for a broad range of substances of various nature (silicates, borates, germanates, phosphates, Se, glucose, glycerin, As2Se3, etc.). Angell and Sichina [Ann. NY Acad. Sci. 279, 53 (1976)] distinguish between "strong'' and "fragile'' liquids on the basis of their heat capacity. The present model provides a physical ground for such classification. Also the isostructural viscosity of glasses is easily described. The present approach gives a natural explanation to the empirical rule of Bimen and Kauzmann.
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  • 38
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    The Journal of Chemical Physics 94 (1991), S. 1684-1691 
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    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Fluorescence and fluorescence excitation spectra for low temperature n-alkane solutions of α,ω-dithenylpolyenes with two, three, and four polyene double bonds have been measured. The fundamental frequencies in the vibronic development of the fully resolved spectra are either nearly identical to modes seen for unsubstituted polyenes or nearly identical to modes seen for polythiophene oligomers. The degree to which thiophene ring modes contribute to the spectra decreases with increasing polyene chain length. In the tetraene, the 2 1Ag state is 2760 cm−1 lower in energy than the 1 1Bu state, in the triene it is 1570 cm−1 lower, and in the diene these two states are nearly degenerate. The 2 1Ag and 1 1Bu excitation energies are well fit by a simple theoretical model which also gives a description of the π-electron distributions in the 1 1Ag, 2 1Ag, and 1 1Bu states.
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    The Journal of Chemical Physics 94 (1991), S. 1698-1704 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The nondegenerate ν2←0 and 2ν2←ν2 bands of the ammonia cation NH+3 have been observed using tunable diode lasers combined with the velocity modulation technique. The ion was produced in a water-cooled 6 kHz ac glow discharge cell with a gas mixture of [He]@B:[N2]@B:[H2]=9@B:0.06@B:0.3 (Torr). From the observed spectra, the band origins were determined to be ν1←0=903.3898 cm−1 and ν2←1=939.771 cm−1. Vibration–rotation interaction constants and potential constants were derived. The missing of K=0 R-branch transitions with N=odd in the ν2←0 band and N=even in the 2ν2←ν2 band clearly shows the characteristics of a D3h planar structure with a 2A‘2 ground electronic state, just like its isoelectronic case CH3 radical. Spin–rotation doublets were well-resolved in R-branch transitions and spin–rotation interaction constants of the ground, ν2, and 2ν2 states were determined. A potential function consisting of a quadratic term and a quartic term was used for modeling the out-of-plane bending vibration of NH+3. In comparison with the methyl radical, the ammonia cation is more rigid and closer to a harmonic oscillator.
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  • 40
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    The Journal of Chemical Physics 95 (1991), S. 4565-4579 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We investigate the existence of a fluid–fluid phase separation in binary mixtures of equal-size hard spheres with positively nonadditive diameters [i.e., d11=d22≡d, d12=(1+Δ)d with Δ〉0]. An integral-equation approach is used to evaluate both thermodynamics and structure of many symmetric (equal to equimolar) mixtures (with Δ=0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 1) and some asymmetric cases. We present the results obtained via the Percus–Yevick, the Martynov–Sarkisov, and the Ballone–Pastore–Galli–Gazzillo closures; the thermodynamic consistency of these approximations is discussed and some possible ways to get further improvements are proposed too. The integral-equation results are then compared with the available "exact'' simulation data, a first-order perturbation approach, and a scaled particle theory. Our study predicts that there exists a demixing for each considered value of the nonadditivity parameter Δ.
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    The Journal of Chemical Physics 95 (1991), S. 4593-4612 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: As a model for salt and water movement in polyelectrolyte solutions, we present theoretical calculations of small-ion and water transport through regular arrays of parallel, charged circular cylinders. We formulate the problem for steady fluxes of water and univalent co- and counterions in response to gradients of salt concentration, pressure, and electric potential. The fluxes are related to these gradients by effective transport coefficients, such as the flow resistance and the effective ionic diffusion coefficients. These coefficients, and the equilibrium ionic partition coefficients, are calculated numerically in two approximations. First a linear approximation is used, appropriate when the cylinders are weakly charged, and second a nonlinear calculation is performed, in which the geometry is simplified using a cylindrical-cell model. The physical significance of these numerical results is discussed.
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    The Journal of Chemical Physics 94 (1991), S. 1717-1723 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: p-cresol and its complexes with H2O and CH3OH were cooled in a pulsed supersonic free jet and studied by resonant multiphoton ionization with time-of-flight mass analysis. Detailed mass and concentration analysis allowed an unambiguous assignment of cluster size. The electronic origins of p-cresol (H2O)1,2,3 show irregular red- and blueshifts with change of cluster size, which is referred to the bivalent role of p-cresol as proton donor and acceptor. Ab initio and semiempirical quantum chemical calculations support this interpretation and show the spectral shifts to be essentially due to the inductive effect of the solvent molecules Y exerted on X in X-H...Y. While the vibronic bands of p-cresol (H2O)2 are quite broad, those of p-cresol (H2O)3 are sharp again. The ab initio calculations show that this may be attributed to the quite rigid "open cyclic'' structure of p-cresol (H2O)3. Our experimental and theoretical investigations show a completely analogous behavior of phenol (H2O)1,2,3 clusters
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    The Journal of Chemical Physics 94 (1991), S. 1724-1729 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The first characterization of the bending potential of the C7 cluster is reported via the observation of the v=11 and v=20 levels of the ν11(πu) bend as hot bands associated with the ν4(σu) antisymmetric stretch fundamental. The lower state hot band rotational constants are measured to be 1004.4(1.3) and 1123.6(9.0) MHz, constituting a 9.3% and 22% increase over the ground state rotational constant [918.89(41) MHz]. These large increases are strong evidence for extremely large amplitude, anharmonic bending modes in this cluster. In addition, quartic and sextic centrifugal distortion constants determined for the ground and ν4=1 states are found to be anomalously large and negative, evidencing strong perturbations between stretching and bending modes.
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    The Journal of Chemical Physics 94 (1991), S. 1730-1739 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper we present a study of the vibrational energy relaxation processes of chloroform, bromoform, and iodoform in solution after excitation of the C–H stretch vibration. The relaxation is studied with ultrafast infrared saturation spectroscopy using intense infrared pulses with a pulse duration of 19 ps. The experiments were performed in a polar and a nonpolar solvent in order to study the effects of the polarity of the solvent on the relaxation processes. We observe that in both types of solvent the relaxation takes place via two consecutive relaxation processes and that the relaxation leads to ultrafast changes of the absorption band of the C–H stretch vibrations. We discuss the differences in the time constants of the relaxation processes of the haloforms in terms of the energy differences between the vibrational levels and the interactions with the solvent.
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    The Journal of Chemical Physics 94 (1991), S. 1752-1758 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A laser vaporization/photolysis technique for obtaining spectra of jet-cooled organometallic molecules is reported. It makes use of an excimer laser which vaporizes a metal sample and simultaneously photolyzes an organic precursor passing over the surface of the metal. The metal atoms and organic fragments then react to yield the organometallic species. In principle, this approach could be used to prepare a wide variety of gas-phase compounds of any metal for spectroscopic study. To illustrate its capabilities, laser excitation spectra of the monocyclopentadienide and monomethyl derivatives of Ca and Cd are presented. In the case of cadmium cyclopentadienide, this report constitutes the first spectroscopic observation of this molecule.
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    The Journal of Chemical Physics 94 (1991), S. 4495-4508 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The process of thermalization of a hot subexcitation photoelectron in polar medium is studied. The master equation is used as a starting point to derive analytic expression for the thermalization distances distribution function. The resulting expression depends upon the mechanism of energy dissipation via the first two moments of the energy loss probability. Asymptotic decay of the distribution with the distance depends on the character of electron motion (exponential for the ballistic motion; Gaussian for the diffusion). A general scheme was developed for evaluation of the energy loss probability within the framework of the linear response theory. Explicit results are derived for electron thermalization in polar medium whose dissipative properties are characterized by the Debye macroscopic dielectric susceptibility function (the Fröhlich–Platzman model). Comparison of the estimates of thermalization distance in water with the experiment shows that the randomization of the direction of the electron motion is much faster than the thermalization process. The dependence of the most probable (average) thermalization distance on the initial kinetic energy, En, and on the dielectric relaxation time, τD, is found for the ballistic and the diffusive motion of the electron (∝E3/4nτ 1/2D for diffusive motion). An explicit relation is derived between the probability to escape geminate recombination and the excess kinetic energy as well as the polarity of the medium and its relaxation time. Results are used to interpret the recent experimental data on the yield and kinetics of geminate recombination in normal and heavy water.
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  • 47
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    The Journal of Chemical Physics 94 (1991), S. 4532-4545 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We examine the validity of an assumption commonly made when carrying out free energy perturbation (FEP) calculations: The contributions to the free energy difference from intraperturbed group interactions are negligible. We show: (A) that these contributions are not, in fact, necessarily negligible; and (B) that, in particular, the bond-stretching contribution resulting from changes in equilibrium bond lengths is important, but can be extremely difficult to calculate using standard procedures. We show how this contribution can be determined using an alternative "pseudopotential of mean force'' correction. We also examine the behavior of the calculated free energy with the length of the simulation, and conclude that simulation times significantly longer than those typically used at present (e.g., ≥200 ps) may be required to yield reliable and accurate results.
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  • 48
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    The Journal of Chemical Physics 94 (1991), S. 4521-4531 
    ISSN: 1089-7690
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Small-angle neutron scattering experiments at various contrasts were performed on concentrated mixtures of colloidal spheres differing in size. Both colloidal components consisted of fairly monodisperse silica cores coated with a layer of octadecyl chains. Cyclohexane was used as dispersing medium; variation of the contrast was achieved by using mixtures of 1H -cyclohexane and 2H -cyclohexane. Scattered intensities were measured at three volume fractions up to 0.4, at equal partial volume fractions. The different contrast dependence of the scattering amplitudes of both colloids allowed us to calculate partial structure factors. This was done using a method which has not been reported previously. Describing the intraparticle structures with layered-sphere models, and using a decoupling approximation, three partials were obtained from a system of linear equations. The scattering curves at the various contrasts constitute a consistent data set, at all volume fractions. Although the separate components interact like hard spheres, the partial structure factors in the mixture reflect marked deviations from hard-sphere behavior. Their nature can be qualitatively explained with an attractive interaction between unequal particles. This is confirmed with simple model calculations.
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  • 49
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    The Journal of Chemical Physics 94 (1991), S. 4546-4556 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Dense fluids of biaxial hard convex bodies exhibit a phase transition from an isotropic to a nematic (I–N) liquid–crystalline state. In the Onsager theory, the I–N transition is attributed to the tendency of pairs of molecules to minimize their excluded volume (or second virial coefficient, B2 ). In the present work, the mutual orientation dependence of B2 is calculated for hard biaxial ellipsoids and is expressed in a Wigner function expansion. Orientational distribution functions are determined for the Onsager model and for the Lee model, which incorporates some density correlations beyond those in the Onsager theory. In the uniaxial phase of biaxial molecules, the two nonzero order parameters calculated on the basis of hard-body models are in approximate accord with the findings of Maier–Saupe based theories. Introduction of biaxiality into a body has a pronounced effect: both the 〈P2〉 order parameter and the first orderness of the I–N transition are greatly reduced from that of comparable uniaxial bodies.
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  • 50
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    The Journal of Chemical Physics 94 (1991), S. 4557-4567 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An efficient computational scheme has been developed that includes many-body hydrodynamics for viscosity in a suspension of spheres following the approach of Jones and Muthukumar [J. Chem. Phys. 89, 6406 (1988)] for friction coefficients. The method combines the hydrodynamic scattering formalism of Freed and Muthukumar [J. Chem. Phys. 76, 6186 (1982)] with angular momentum diagrammatic techniques from quantum mechanics. The concentration dependence of the viscosity is expressed as a series of finite integrals labeled by the equivalent of angular momentum quantum numbers. This series is smoothly convergent in contrast to the case of the alternate formalism of a series of terms of inverse powers of the sphere separation. A comparison between different methods and the slowness of convergence of the series are addressed in detail. At the two-body level, the virial expansion for the shear viscosity η of a suspension of hard spheres of volume fraction Φ is determined to be η/η0=1+2.5Φ+4.8292Φ2+O(Φ3), where η0 is the viscosity of the fluid and the correction for the Brownian motion [G. K. Batchelor, J. Fluid Mech. 83, 97 (1977)] has been ignored. Our value for the two-body coefficient differs in 3.4% from the extrapolated value 5.0019 obtained by Cichocki and Felderhof [J. Chem. Phys. 89, 1049 (1988)].
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    The Journal of Chemical Physics 94 (1991), S. 4581-4586 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Dynamic storage and loss moduli of dilute solutions of linear poly(2-vinylpyridine) with molecular weights ranging from 3.0×104 to 1.0×106 have been measured under conditions where the polymer can be charged by protonation. A pronounced maximum is exhibited in the reduced moduli occurring at a concentration which is independent of molecular weight and measurement frequency. No change in the polyion hydrodynamic interaction upon dilution is observed, as determined from Zimm model fits of the reduced modulus vs frequency at finite polymer concentrations. The origin of the viscoelasticity maximum is explained in terms of an electrostatically driven polyion coil expansion and contraction resulting in the scaling of the relaxation spectrum by the longest relaxation time τ1.
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  • 52
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    The Journal of Chemical Physics 94 (1991), S. 1803-1811 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: 7Li nuclear magnetic resonance (NMR) linewidths and spin–lattice relaxation times for poly(propylene-glycol) complexed with a range of concentrations of LiCF3SO3 are reported over the temperature region from 205 to 405 K. Calculations suggest that the spin–lattice relaxation mechanism is caused by the interaction between the 7Li (I=3/2) quadrupole moment and fluctuations in the surrounding electric field gradients, whereas the line shapes are influenced by both the dipolar and quadrupolar interactions. The motional parameters reported indicate that ion–polymer or ion–ion interactions are important in determining the Li+ cation mobilities. This is reflected in the lengthening of the correlation time with increase in Li+ ion concentration which suggests a decreased mobility for the cations resulting from a transient coordination of the cation to the polymer matrix or ion aggregation. Also, the activation energies in this study (∼0.24 eV) are in agreement with values obtained from recent pulsed field gradient studies suggesting that the NMR techniques employed in this study are approriate methods for probing the dynamics of ion transport on a macroscopic scale in these materials.
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    The Journal of Chemical Physics 94 (1991), S. 1812-1819 
    ISSN: 1089-7690
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A tunable far-infrared difference frequency spectrometer has been used to examine the fully protonated form of malonaldehyde in the region near the ground-state tunneling frequency (21 cm−1). An extremely dense and complex spectrum is observed in which the strongest features have been assigned as pure rotational lines involving high values of J and K−1. These transitions, which occur within the individual rotational manifolds of the two halves of the ground-state tunneling doublet, have been analyzed simultaneously with existing microwave data for this species. The value of the ground-state tunneling splitting, determinable indirectly from analysis of vibration–rotation interactions, is 21.584 76(17) cm−1, and is in close agreement with that similarly obtained in previous microwave work. A thorough treatment of the centrifugal distortion in this system significantly extends the range of rotational states whose energies may be reliably calculated, and should therefore be valuable in the future direct measurement of the tunneling frequency. Aspects of the far-infrared spectrum of this species, and of the indirect method of determining the tunneling splitting, are discussed.
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    The Journal of Chemical Physics 94 (1991), S. 1820-1824 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The influence of light alkane perturbers (up to densities 5×1020 cm−3 for methane and ethane and 2×1020 cm−3 for propane, respectively) on the autoionization spectrum of CH3I nd'(2E1/2) Rydberg series was investigated by photoconductivity excitation spectroscopy. In the experiments, we recorded photocurrents due to electronic transitions from the absorber molecule to the conduction state of each perturber. The absorption maxima of the perturbed autoionization Rydberg states appeared as maxima in the photocurrent spectrum. The asymptotic pressure shift of the Rydberg lines equals the shift of the second vertical ionization potential I(2E1/2) of the perturbed absorber. The energy shift also provides the electron scattering length A in methane, ethane, and propane. The data obtained are compared with results in the literature on photoinjection, electron beam, and swarm experiments in pure alkanes.
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  • 55
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    The Journal of Chemical Physics 95 (1991), S. 3399-3403 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Relaxational processes close to a hysteresis limit in the bistable iodate–arsenous chemical system are analyzed under the effect of external fluctuations affecting the flow rate parameter. A dynamical characterization is proposed based on the use of the first-passage–time-distribution (FPTD) technique. Exact numerical results are presented showing the different relaxational behavior above and below marginality. Supplementary, we analytically obtain characteristic scaling laws for the first two moments of the FPTD in terms of a unified description valid on both sides of the hysteresis limit.
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    The Journal of Chemical Physics 95 (1991), S. 3476-3487 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Canonical Van Vleck perturbation theory is used to transform curvilinear and rectilinear normal coordinate vibrational Hamiltonians of HCN, C2H2, and CO2 to block-diagonal effective Hamiltonians. Accurate energies as high as 11 000 cm−1 above the zero point are reported for all three molecules. In the absence of off-diagonal coupling terms in the effective Hamiltonians, these two coordinate systems yield identical perturbative expansions for the vibrational energies. Only when coupling terms are introduced do differences between the calculated energies in the two representations become apparent. In CO2, where there is pronounced configuration interaction between nearly degenerate states, we find that the perturbative energies obtained from the curvilinear normal coordinate Hamiltonian are converging significantly faster than those obtained in the rectilinear normal coordinate representation.
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  • 57
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    The Journal of Chemical Physics 95 (1991), S. 3521-3527 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The changes in harmonic vibrational frequencies and line intensities with respect to different strengths of an applied electric field are computed using analytically calculated energy derivatives. The ground states of several small molecules are studied at the ab initio restricted Hartree–Fock self-consistent-field (RHF SCF) level with a double ζ plus polarization basis set. The dependence of vibrational frequencies and line intensities upon the applied field strength is discussed.
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    The Journal of Chemical Physics 94 (1991), S. 7602-7618 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The theory of Van der Waals complexes formed from atoms and open-shell (Σ and Π) diatomic molecules is developed, paying particular attention to the quantum numbers that are conserved in the complex and the angular momentum coupling cases that may be observed. Complexes formed from diatoms in multiplet Σ states may exhibit several different coupling schemes closely analogous to Hund's coupling cases for diatomic molecules. Complexes formed from diatoms in Π states usually exhibit a coupling scheme in which the (signed) projection P of the diatom angular momentum j onto the intermolecular axis is nearly conserved. Correlation diagrams showing the bending energy levels as a function of potential anisotropy are given for complexes containing diatomic molecules in both Σ and Π states. The transition from free internal rotor quantum numbers to near-rigid bender quantum numbers with increasing anisotropy is investigated. The cases of Ar–OH and Ne–OH are considered as examples.
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    The Journal of Chemical Physics 95 (1991), S. 3596-3606 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Cu+ and Mn2+ impurities in NaF are studied within the perturbed cluster approximation. The MTXα method is used to compute potential energy curves for the a1g vibrational modes of CuF5−6 and MnF4−6 clusters embedded in the host. The clusters are stabilized by an electrostatic lattice potential evaluated analytically from Slater orbital (SO) approximations to the true lattice ion wave functions. This approach is superior to the common Watson Sphere model for the cluster environment, but gives unsatisfactory muffin-tin total energy curves. To remedy this, the implementation and evaluation of non-muffin-tin corrected total energies for embedded clusters is described and shown to give improved results. Further refinements to the model are discussed.
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    The Journal of Chemical Physics 95 (1991), S. 3607-3613 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: We develop a variational method for constructing the effective Hamiltonian that best propagates the state of a system, based on the minimization of a temporal error functional. As an illustrative example the procedure is used to derive the effective Hamiltonian in the time-dependent self-consistent-field method. The present variational approach is particularly useful to build the best effective Hamiltonian that is a linear combination of the generators of a Lie algebra. We explicitly consider systems having classical analogs with many degrees of freedom and calculate transition probabilities for a simple semiclassical model of the collinear collision between an atom and a diatomic molecule. We show that our variational approach is more general and accurate than the widely used local harmonic approximation.
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    The Journal of Chemical Physics 95 (1991), S. 3679-3689 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A three-dimensional lattice model of water molecules against a charged wall is presented, both in the absence and in the presence of nonpolar monomeric solute molecules. To account for water–water interactions, the water molecules are represented as spheres embodying three partial charges. The statistical-mechanical treatment of this model, which does not embody adjustable parameters, is carried out on the basis of Barker's self-consistent field theory [J. A. Barker, J. Chem. Phys. 44, 4212, (1966)]. In the absence of solute molecules, the model provides the interfacial potential difference, the differential capacity, and the entropy surface excess as a function of the charge density σ on the wall; the qualitative behavior of these quantities compares favorably with that of the experimental, Gouy–Chapman corrected quantities at the interface between sp metals and aqueous solutions of nonspecifically adsorbed 1–1 valent electrolytes. Generalizing the model so as to include the presence of nonpolar monomeric solute molecules, yields calculated adsorption isotherms, both at constant charge and at constant potential, which compare favorably with the experimental adsorption behavior of simple, monofunctional aliphatic compounds on mercury from aqueous solutions.
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    The Journal of Chemical Physics 95 (1991), S. 3657-3665 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A recently developed method of analytical solution of the Smoluchowski equation is applied to the investigation of the kinetics of diffusional escape from a potential well for different space dimensionalities n. The kinetics is described by the time dependence of the well occupation probability Nn(t). The formulas derived are valid for times longer than the time t0=a 2/D of relaxation within the well U(r) [a is the Onsager's radius determined by U(a)=kT ]. In the absence of absorption (or reaction) within the well the kinetics for low n=1,2 is found to be nonexponential at any time from the very beginning and for any depth of the well. The characteristic escaping time, however, significantly depends on the well depth. At longer times we obtain long time tails Nn(t)∼t−n/2 (n=1,2), typical for corresponding free diffusion processes. A simple general analytical expression for N1(t) is derived. In the limit of high reactivity within the well N1(t) appears to be exponential at moderately short t, but at long times it changes to the asymptotic function ∼t−3/2 (unlike the weak reactivity tail ∼t−1/2 ). In the two-dimensional (2D) case in the high reactivity limit the kinetics is also exponential at short t. The long time asymptotics somewhat differs from the weak reactivity one (∼t−1)@B:N2(t)∼1/t ln2 t. It is shown that the three-dimensional (3D) escaping process is described by the same expression as the one-dimensional (1D) one in the high reactivity limit, i.e., at moderately short t, N3(t)∼exp(−Wt), while at long times N3(t)∼t−3/2. Some possible extensions of the method are discussed.
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    The Journal of Chemical Physics 95 (1991), S. 3719-3727 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Computer simulations on water and aqueous solutions of noble gases have been carried out in order to study the structures of water around a solute. The hydration energy and free energy evaluated for neon (Ne) and xenon (Xe) solutions in the present study were in good agreement with those by experiments. The detailed hydration structures were investigated by means of the so called inherent structures and normal-mode analyses. It was found that the positive excess free energy in the hydration of Xe arises from a decrease in the number of distinct potential-energy minima in configuration space and that the free energy increase in the Ne solution is due partly to the decrease in the number of the potential minima and partly to the anharmonic modes which are harder than those in pure water. The soft anharmonic modes in the Xe solution were almost equivalent to those in pure water. The introduction of a Xe solute gives rise to a change in water structure to a clathrate-like structure and yields an increase in population of the cyclic pentamer connected by hydrogen bonds, which leads to the exothermic hydration.
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    The Journal of Chemical Physics 95 (1991), S. 3749-3755 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The adsorption of hard chains in slitlike pores is studied via an integral equation theory. The theory uses the growing adsorbent model in conjunction with the polymer reference interaction site model (polymer-RISM) theory to predict the wall–fluid correlation functions; wall–fluid and fluid–fluid correlations are treated using the Percus–Yevick closure relations. The theory is compared to Monte Carlo simulation data for the density profiles of 4-mers and 8-mers in slitlike pores; the theory is fairly accurate, though it tends to overestimate the density at the wall at low densities and underestimate the density at the wall at high densities. The theory also underestimates the amplitude of the oscillations in the density profile. The theory is very accurate, however, for the adsorption isotherm of 4-mers and 8-mers.
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    The Journal of Chemical Physics 95 (1991), S. 3843-3846 
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    The Journal of Chemical Physics 95 (1991), S. 3859-3860 
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    The Journal of Chemical Physics 95 (1991), S. 3861-3862 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: We calculate binding energies for the molecular mechanics for clusters (MMC) geometries obtained by Dykstra and Andrews [J. Chem. Phys. 92, 6043 (1990)] for neutral ammonia clusters and compare these energies with those obtained from previous calculations by Sagarik, Ahlrichs, and Brode [Mol. Phys. 57, 1247 (1986)] and Greer, Ahlrichs, and Hertel [Chem. Phys. 133, 191 (1989)]. All geometries are compared energetically near the Hartree–Fock limit; for the dimer, correlation effects are included by means of the coupled pair functional (CPF) method.
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    The Journal of Chemical Physics 95 (1991), S. 2259-2264 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The radiative lifetimes and reactivity of metastable NO+(a 3Σ+) and O+2(a 4Πu) have been investigated using a triple cell ion cyclotron resonance (ICR) spectrometer. Radiative lifetimes of (100±20) ms and (135±25) ms have been found for NO+(a 3Σ+) v≥1 and v≥0 respectively, with Ar and CO2 as monitor gases. The overall reaction rate of NO+(a 3Σ+,v≥1) with Ar (6.5×10−10 cm3 s−1 ) is 1 order of magnitude larger than the rate of v=0 determined in flowing afterglows. CO2 reacts at collision rate. For O+2 the overall lifetime of v≥0, determined with Ar as monitor, is (30±6) ms in between the two values (7 and 130 ms) found by Bustamente et al. using the photodissociation technique. The possible reasons for the longer lifetimes determined in previous ICR experiments is discussed. In the absence of any ab initio calculation, the experimental results on NO+(a 3Σ+) may only be compared to values calculated using an approximated perturbation model. Unfortunately, the reliability of this model, also used in the previous studies, is very limited because of the large uncertainty on the coupling between the metastable a 3Σ+ and the perturbing radiative A 1Π states.
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    The Journal of Chemical Physics 95 (1991), S. 2336-2351 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The optical spectrum of N2O in the region from 80 000 to 87 000 cm−1 has been investigated by two- and three-photon resonant multiphoton ionization mass spectrometry. State symmetry identification has been performed by the use of linearly and circularly polarized light in both two- and three-photon absorption, conclusively confirming the assignment of the spectrum as that of the 3pσ 1Π Rydberg state of N2O. The new spectra and polarization experiments also permit observation and analysis of previously unreported vibronic coupling effects in this state. Mass spectral data reveal significant variation in the degrees of fragmentation into NO+ and N+2 as a function of vibrational and rotational excitation in the intermediate state. An apparent progression of a previously unreported second electronic band is also observed and for which a possible assignment is discussed.
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    Notes: A method for quantitative analysis of piezomodulation spectra of a thin film on a substrate, including the internal reflection in the system, is presented. This method is applied to Langmuir–Blodgett (LB) films of copper phthalocyanine (CuPc) derivatives on glass substrates. A uniaxial stress was applied along the symmetry axis of the film. The spectra were measured in the region from 1.5 to 2.4 eV in three configurations; transmission, reflection from the film side, and reflection from the substrate side. The optimum Lorentzian parameters and their changes due to the stress are determined by the least-mean-square fit. The microstructures and electronic states of the LB film are discussed from the stress effects. The very small feature, which is hardly identified in the absorption spectra because of the large Frenkel exciton band, is attributed to a charge transfer (CT) excitation.
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    The Journal of Chemical Physics 95 (1991), S. 2407-2415 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The adiabatic I.P. of Si2H6 obtained by a photoionization mass spectrometric study at two temperatures is 9.74±0.02 eV. The first fragment, Si2H+4, initially appears with a shallow slope at ≤10.04±0.02 eV, and with a much steeper slope at ≤10.81±0.02 eV. It is argued that the initial onset corresponds to formation of H2SiSiH+2, while the steeper onset is attributed to formation of H3SiSiH+. The second fragment, Si2H5, has an appearance potential of ≤11.59±0.02 eV (11.41±0.03 is a probable value). Successive decomposition leads to Si2H+2 (from Si2H+4 ) and Si2H+3 (from Si2H+5 ). The photoion yield curve for Si2H+3 also displays shallow and steep onsets. Upper limits for the appearance potentials can be readily extracted, but the true thermochemical onsets are less well defined. Heats of formation (or upper limits) are presented for each of these species. For Si2H+6, Si2H+5, and Si2H+4, the experimental values are in good agreement with recent ab initio calculations. For the daughter species, the experimental values exceed the calculated ones, as expected.
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    The Journal of Chemical Physics 95 (1991), S. 2458-2464 
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    Notes: The possible connection between the equilibrium structure of a solution and the chemical reaction dynamics that occur in that solution has been discussed by Adelman and co-workers. In this work, we present a computational demonstration of this connection using molecular dynamics simulations and the generalized Langevin equation (GLE). A favorable example of a reaction loosely based on thermally activated Cl+Cl2→Cl2+Cl in argon solvent is used for this demonstration by (1) computing equilibrium solution structural information in terms of the Ar–Ar and Ar–Cl radial distribution functions, both from integral equations and from molecular dynamics; (2) deriving a memory function for Cl in argon solvent from the radial distribution functions and the Ar–Cl potential; and (3) using this memory function in a simple GLE to compute the dynamics of the reaction. Energy flow results both for climbing and descending the barrier are in gratifying agreement with the dynamics of the same reaction as computed by full deterministic molecular dynamics.
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    The Journal of Chemical Physics 95 (1991), S. 2491-2496 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation of the OH radical was studied at 157 nm via the detection of the product H atoms with the resonance fluorescence technique. OH radicals were produced in a fast flow cell from the reaction between H and NO2 and subsequently photodissociated by an excimer laser operating on the F2 emission. The quantum yield for photodissociation of OH was measured to be 1.10±0.28. The photodissociation cross section was calculated to be 6.6×10−18 cm2 (±25%) which is in good agreement with the work of Nee and Lee (Ref. 8).
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    The Journal of Chemical Physics 94 (1991), S. 3332-3346 
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    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Femtosecond optical Kerr dynamics in various transparent liquids were measured using incoherent light with a 60 fs autocorrelation width. From the measurement of the optical Kerr effect (OKE) of binary mixtures of CS2 and various liquids, the contribution of the intermolecular interaction-induced polarizability change to the OKE was found to be affected remarkably by the femtosecond molecular dynamics of CS2. Especially, data from diluted solutions of CS2 in nonviscous solvents composed of molecules with a low molecular weight are consistent with the binary collision model in free space. An oscillatory feature, which was attributed to an intermolecular vibrational mode, was found in the OKE dynamics of neat benzene and several benzene derivatives. A theoretical expression for the delay-time dependence of the signal intensity was also derived with no restriction on the statistical properties of the incoherent light. It is expressed in terms of the autocorrelation function of the intensity fluctuation of the incident incoherent light which was utilized as both the pump and the probe.
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  • 75
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    The Journal of Chemical Physics 95 (1991), S. 2549-2559 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: When the charge overlap between interacting molecules or ions A and B is weak or negligible, the first-order interaction energy depends only upon the molecular positions, orientations, and the unperturbed charge distributions of the molecules. In contrast, the first-order force on a nucleus in molecule A as computed from the Hellmann–Feynman theorem depends not only on the unperturbed charge distribution of molecule B, but also on the electronic polarization induced in A by the field from B. At second order, the interaction energy depends on the first-order, linear response of each molecule to its neighbor, while the Hellmann–Feynman force on a nucleus in A depends on second-order and nonlinear responses to B. One purpose of this work is to unify the physical interpretations of interaction energies and Hellmann–Feynman forces at each order, using nonlocal polarizability densities and connections that we have recently established among permanent moments, linear response, and nonlinear response tensors. Our theory also yields new information on the origin of terms in the long-range forces on molecules, through second order in the interaction.One set of terms in the force on molecule A is produced by the field due to the unperturbed charge distribution of B and by the static reaction field from B, acting on the nuclear moments of A. This set originates in the direct interactions between the nuclei in A and the charge distribution of B. A second set of terms results from the permanent field and the reaction field of B acting on the permanent electronic moments of A. This set results from the attraction of nuclei in A to the electronic charge in A itself, polarized by linear response to B. Finally, there are terms in the force on A due to the perturbation of B by the static reaction field from A; these terms stem from the attraction of nuclei in A to the electronic charge in A, hyperpolarized by the field from B. For neutral, dipolar molecules A and B at long range, the forces on individual nuclei vary as R−3 in the intermolecular separation R; but when the forces are summed over all of the nuclei, the vector sum varies as R−4. This result, an analogous conversion at second order (from R−6 forces on individual nuclei to an R−7 force when summed over the nuclei), and the long-range limiting forces on ions are all derived from new sum rules obtained in this work.
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    The Journal of Chemical Physics 95 (1991), S. 2623-2638 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Analytic energy gradients for the coupled-cluster singles and doubles (CCSD) method have been implemented for closed-shell systems using restricted Hartree–Fock (RHF) and open-shell systems using unrestricted Hartree–Fock (UHF) reference functions. To achieve maximum computational efficiency, the basic theory has been reformulated in terms of intermediates, thus reducing the number of required floating-point operations, and all computational steps are given in terms of matrix products in order to exploit the vector capabilities of modern supercomputers. Furthermore, the implementation has been designed to take full advantage of Abelian symmetry operations. To illustrate the computational efficiency of our implementation and in particular to demonstrate the possible savings due to the exploitation of symmetry, computer timings and hardware requirements are given for several representative chemical systems. In addition, the newly developed analytic CCSD gradient methods are applied to calculate the equilibrium geometry and energy splitting of the lowest singlet and triplet states of the C4O2 molecule.
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  • 77
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    The Journal of Chemical Physics 94 (1991), S. 7775-7778 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Temperature dependence of stretching modes are presented for two kinds of Rb2ZnCl4 crystals which exhibit different phase transition sequences. The comparison between both sets of results points out a special behavior of the line which appears below the normal–incommensurate phase transition. Appearance of the incommensurability is observed via the temperature dependence of certain stretching lines.
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  • 78
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    The Journal of Chemical Physics 95 (1991), S. 2669-2682 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Recent theoretical developments have shown how such examples of excitation properties as the electronic band structure and the set of vibrational normal modes of a liquid can be studied by traditional classical-liquid-theory methods. In this paper, we add another example to this collection: the set of polarization modes of a liquid. The basic notion is that in any polarizable but nonpolar fluid, the dynamics of the instantaneous dipoles can be represented as a linear combination of harmonic contributions from independent, microscopically defined, polarization modes. We note first how many of the properties one would like to know about the liquid—its full dielectric behavior, its optical absorption spectrum, its effect on the absorption spectrum of a solute, and even how the net polarization of the liquid fluctuates with time—are available from these polarization modes. We then point out how the requisite information about the modes can be ascertained by the same liquid theory methods used to treat p-orbital-based electronic problems. These considerations allow us to show how the mean spherical approximation can be used to obtain more accurate versions of optical spectra than was possible heretofore. It also suggests how one might begin to look at the dynamics of polarization in polar liquids.
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    The Journal of Chemical Physics 95 (1991), S. 2717-2726 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: This work, which is purely methodological, demonstrates new applications of perturbation methods in computer simulations of simple liquids. Most applications are based on the calculation of bulk and local excess chemical potentials of one or several inserted test particles, using a Widom technique in the canonical ensemble. This gives a powerful tool for obtaining distribution functions, some of which are virtually impossible to determine with other techniques. Results are also presented for single-ion activity coefficients and Donnan potentials. A perturbation approach is used to calculate thermodynamic response functions with respect to particle number, temperature, and volume changes. The applicability is exemplified by studies of hard-sphere fluids, uniform and nonuniform electrolyte solutions within the primitive model, and screened Coulomb systems.
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    The Journal of Chemical Physics 95 (1991), S. 2787-2790 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A study of phase angles of the electric birefringence including inertia is proposed in the case of the simultaneous action of two electric fields, namely a direct current (dc) bias field added to an alternating current (ac) field of angular frequency ω. The solution of the Fokker–Planck–Kramers (FPK) equation applied to the orientational motion of a polar (permanent moment μ) and anisotropically polarizable (induced moment Δα) molecule in two-dimensional (2D) space (disk model) leads to distinct phase angles θ1(ω) and θ2(ω), both of them corresponding to harmonic components in ω and 2ω of the birefringence. The evolution of the plots θj(ω), where j=1,2, for different values of the inertial parameter a2=τI/τ and given a value of P=(Δα/μ2)kT is presented. It is shown that in the high-frequency region, the phase angles may take up to twice the values they would have with zero inertia. τI and τ are the friction time and the Debye relaxation time, respectively, while kT stands for the usual thermal energy. Such variations must therefore be measurable by experiment and allow a new way of research in Kerr effect relaxation when inertia is taken into account.
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  • 81
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    The Journal of Chemical Physics 94 (1991), S. 7810-7818 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Measurements of ion-velocity distributions of CO+ in a He buffer gas are presented as a function of an applied electric field. The distributions are obtained by single frequency, laser-induced fluorescence from various initial rotational states with the laser beam propagating parallel and perpendicular to the drift velocity vector. All distributions are well represented by a Maxwellian for the observed E/N range of 0–13 Td. The reduced mobilities, calculated from the shift of the mean velocity as a function of electric field, increase from 18.7±1.0 cm2 V−1 s−1 at very low fields to 26.4±0.7 cm2 V−1 s−1 at 13 Td. From the width of the Doppler profiles, translational "temperatures'' are calculated, which are compared to simple attractive and repulsive Maxwell models as a function of the field. The measured values disagree with the predictions, which are well established for atomic ion systems. The differences are discussed in terms of rotationally inelastic energy transfer in the collisions, which is predicted by kinetic theory models. This argument is strengthened by the fact that even though the rotational states rapidly equilibrate, measurements on different lines yield higher temperatures for higher rotational levels. Finally, the small influence of selective quenching of the electronically excited CO+ on the Doppler profiles is demonstrated by measuring effective lifetimes as a function of the applied drift field.
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  • 82
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    The Journal of Chemical Physics 94 (1991), S. 3389-3406 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a simple classical model for understanding time-resolved absorption spectra of molecules that are in the process of dissociating. The model applies to absorption spectra that are obtained by measuring the spectral power density of an ultrafast, continuum probe pulse after transmission through the sample. We show that the classical model can yield results in good agreement with quantum-mechanical wave packet propagation calculations. In a close analogy with collisional line broadening, the time-resolved absorption spectra are shown to have an impact region near the separated-atom transition frequency and a far-wing region. The impact region is due to radiation emitted after the molecule has separated into atomic fragments, and the far-wing region is due to radiation emitted during the time of strong molecular interaction. The spectrum in the impact region depends upon an effective phase shift for a "partial'' collision, which begins at the time that the probe pulse sweeps through the molecular transition frequency. For narrow wave packets, this phase shift can be directly measured, and the molecular transition frequency can be recovered as a function of time along the path of dissociation. For very broad wave packets, the time-resolved absorption spectra approach a statistical limit, in which the absorption line shape becomes an image in frequency space of the probability density in configuration space at the time of excitation by the probe pulse. In all cases, the frequency-integrated absorption is proportional to the net population of molecules that are excited by the probe pulse. In principle, this result can be used to obtain the strength of the transition dipole moment as a function of internuclear separation. We also consider fluorescence induced by a short optical probe pulse, as in the experiments of Zewail and co-workers. Fluorescence measurements are shown to be fundamentally different from measurements of the transmitted spectral power density: fluorescence depends upon the net population excited by the probe pulse, whereas the transmitted spectral power density depends upon interference between the incident probe field and the polarization field. Thus these two experimental techniques are sensitive to different aspects of the dissociation process.
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  • 83
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    The Journal of Chemical Physics 95 (1991), S. 2862-2866 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: An investigation of the longitudinal acoustic mode (LAM) in a series perfluoro-n-alkanes under pressure has been carried out using Raman spectroscopy in conjunction with a diamond anvil cell. Perfluoro-n-alkanes are shown to undergo a phase transition from a helical to a planar zigzag structure at 5 kbar. This conformational change with pressure results in an abrupt increase in the LAM frequency. Since the chain axis elastic modulus of a planar zigzag molecule is larger than that of a helical one, such a conformational change leads to stiffening of the backbone giving rise to the observed change in the LAM frequency.
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  • 84
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    The Journal of Chemical Physics 95 (1991), S. 2882-2889 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The oxygen adsorption site on the Ir{110}-c(2×2)-O surface has been studied by time-of-flight scattering and recoiling spectrometry (TOF-SARS) using 4 keV Ne+ for backscattering and Ar+ for recoiling. The oxygen site was analyzed from scans of (i) backscattering intensity versus incident angle, (ii) oxygen recoil intensity versus incident and azimuthal angle, and (iii) oxygen recoil energy versus azimuthal angle. Calibrated shadow cones and trajectory simulations were used to obtain the site coordinates. This TOF-SARS data is contrasted with that of Ni{110}-p(2×1)-O, in which it is well established that the adsorption site is in the long-bridge position along the 〈001〉 rows. Adsorption of oxygen in the short-bridge sites above the 〈11¯0〉 Ir rows is the only model consistent with all of the experimental data and simulations. The O–Ir bond length is estimated to be ≈1.8 A(ring).
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    The Journal of Chemical Physics 95 (1991), S. 2927-2935 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: We have calculated the evaporation rate for every cluster size of Ar+n and Xe+n in the microcanonical ensemble (as a function of its internal energy) up to n=37. This allows us to perform numerical experiments simulating the evolution of the size and energy distributions in a cluster beam up to arbitrary long times. We find that there is a time lag for the onset of magic numbers (especially abundant cluster sizes) of molecular clusters. This time is required for the clusters to cool down by evaporation and become solid-like and it may differ by several orders of magnitude for different species thus explaining apparently contradictory experiments. For longer times we find, in agreement with theoretical predictions but contrary to simple intuition, that the relative abundance of magic/nonmagic clusters decreases with time.
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    The Journal of Chemical Physics 95 (1991), S. 2965-2974 
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    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We explore the usefulness of the delta self-consistent-field (ΔSCF) approximation in connection with high-resolution x-ray photoelectron spectra for component and structural analysis of organic compounds. Results for core electron binding energy shifts for model molecules of the polymethylmethacrylate polymer are presented. A previously devised method for proper self-consistent-field solutions for core hole states in molecules is evaluated. The results indicate that chemical shifts can be obtained within a few tenths of an eV. A discussion is presented on the inherent errors in the ΔSCF approximation, the proper corrections for zero-point vibrational energies, and the role of relaxation of core orbitals.
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    The Journal of Chemical Physics 95 (1991), S. 3001-3004 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the first observation of eliminative cationic polymerization within van der Waals (vdW) clusters following electron impact ionization at pressures of 10−8 Torr. The elimination reactions of C2H3Cl+ within the clusters terminate after three successive steps, each involving elimination of HCl or Cl. The results provide a mechanism for the early stages of gas phase cationic polymerization of vinyl chloride and demonstrate the feasibility of using vdW clusters as a means of studying gas phase cationic polymerization.
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    The Journal of Chemical Physics 94 (1991), S. 7875-7892 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: We present equations for generalized-normal-mode vibrational frequencies in reaction-path calculations based on various sets of coordinates for describing the internal motions of the system in the vicinity of a reaction path. We consider two special cases in detail as examples, in particular three-dimensional atom–diatom collisions with collinear steepest descent paths and reactions of the form CX3+YZ→CX3 Y+Z with reaction paths having C3v symmetry. We then present numerical comparisons of the differences in harmonic reaction-path frequencies for various coordinate choices for three such systems, namely, H+H2→H2+H, O+H2→OH+H, and CH3+H2→CH4+H. We test the importance of the differences in the harmonic frequencies for dynamics calculations by using them to compute thermal rate constants using variational transition state theory with semiclassical ground-state tunneling corrections. We present a new coordinate system for the reaction CH3+H2 that should allow for more accurate calculations than the Cartesian system used for previous reaction-path calculations on this and other polyatomic systems.
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    The Journal of Chemical Physics 94 (1991), S. 7893-7900 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The NO γ emission is observed from the reaction of NO+(a)+NO. The emission provides a new detection method for studying the NO+(a) reaction kinetics at thermal energy without electric field. The NO+(a) is produced by photoionization of NO at 76.5 nm as well as by the reaction of Ar++NO, where Ar+ is also produced by photoionization of Ar. The vibrational population distributions of NO(A) resulted from the ion–molecule reactions are measured and used to discuss the mechanisms for the production of the emission. The reaction rate constants are determined from the decay rates of the emission intensity as a function of time. The reaction rate constants of NO+(a)+NO and Ar++NO at room temperature are measured to be (5.8±0.7)×10−10 and (2.1±1.0)×10−10 cm3/s, respectively. The reaction rate constant of NO+(a)+Ar at thermal energy is estimated to be about 10−12 cm3/s.
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    The Journal of Chemical Physics 94 (1991), S. 7901-7909 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a new complex-scaling Fourier-grid Hamiltonian (CSFGH) method for accurate and efficient determination of laser-induced (multichannel) molecular resonance states without the use of basis set expansions. The method requires neither the computation of potential matrix elements nor the imposition of boundary conditions, and the eigenvectors provide directly the values of the resonance wave functions at the space grid points. The procedure is particularly valuable for excited-state problems where basis set expansion methods face the challenge. The simplicity and usefulness of the CSFGH method is demonstrated by a case study of the intensity-dependent complex quasivibrational energy eigenvalues (ER, −Γ/2) and eigenvectors associated with multiphoton and above-threshold dissociation of H+2 ions in the presence of intense laser fields (I=1012–1014 W/cm2 ).
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    The Journal of Chemical Physics 94 (1991), S. 7910-7928 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Three-dimensional quantum theory of triatomic exchange reactions in strong laser fields is presented. Our theory consists of an exact partitioning technique for treating the effects of optical fields on reactive scattering, based on approximate hindered-rotor adiabatic wave functions describing the pure nonradiative events. The method enables computations to be performed for an arbitrary number of field intensities with very little effort beyond that required for a single-intensity computation. Differential and integral cross sections for the H+H2 exchange reaction, involving the ground and first excited electronic states, in the presence of laser fields, are computed. The dependence of reactive nonlinear optical effects, and especially that of "laser catalysis,'' on laser intensity; the way isolated and overlapping power-broadened resonances affect the optically induced reaction; the role of relative orientation of two incident molecular beams in crossed beams experiments are investigated. The three-dimensional computations confirm our previous expectations, based on a collinear model, that laser catalysis is achievable using only moderately high powered lasers. The above is expected to be true for all reactive systems (of which H+H2 is one) possessing optically allowed stable excited electronic states.
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    The Journal of Chemical Physics 94 (1991), S. 7929-7936 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: We investigate final rotational state distributions following the decay of long-lived resonance states with k*=0, 1, and 2 quanta of internal bending excitation. The calculations are related to the photodissociation of HONO on the S1 electronic state surface, truncated to two degrees of freedom namely the HO–NO dissociation bond and the ONO bending angle. The decay of the k*=0 resonance yields a smooth Gaussian-type distribution, in very good agreement with recent measurements. The distributions following the decay of the excited bending states show a bimodal behavior with the main maxima at high rotational states. The final angular momentum distributions reflect the coordinate-dependence of the dissociation wave function in the region of the transition state, mediated by the dynamics in the exit channel when the wave packet slides down the steep potential slope. A qualitative interpretation of the rotational state distributions is provided by a simple classical model which applies the transition-state wave function as a weighting for trajectories starting on a line that separates the intermediate complex from the product channel.
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  • 93
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    The Journal of Chemical Physics 94 (1991), S. 7951-7957 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The deactivation processes of Cd(5 3P2) by H2 and D2 were studied by employing pulsed laser techniques. The cross sections for the intramultiplet relaxation to produce Cd(5 3P1) and Cd(5 3P0) were determined as follows: Cd(5 3P2)+H2→Cd(5 3P1)+H2; 6.3, Cd(5 3P2)+H2→Cd(5 3P0)+H2; 1.0, Cd(5 3P2)+D2→Cd(5 3P1)+D2; 4.4, Cd(5 3P2)+D2→Cd(5 3P0)+D2; 0.9, in units of 10−16 cm2 at 630 K. The cross sections for the overall deactivation of Cd(5 3P2) by H2 and D2 were determined to be 9.8×10−16 cm2 and 6.9×10−16 cm2, respectively. These values are compared with the calculated results based on a semiclassical curve crossing mechanism. It is suggested that electronic-to-rotational energy transfer without sharp resonances plays an important role in the deactivation of Cd(5 3P2).
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    The Journal of Chemical Physics 94 (1991), S. 2149-2158 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics simulations have been performed on a simple fluid in the non-Newtonian regime in order to study the nature of diffusion in the presence of shear stress. The nonequilibrium molecular dynamics (NEMD) sllod algorithm is used to generate a homogeneous boundary driven Couette flow. The elements of the tensor of diffusion coefficients have been evaluated by three different routes: through Einstein relations with the corresponding elements of the tensor of molecule displacements, through Green–Kubo expressions involving tensor of momentum autocorrelation functions and cross-correlation functions, and by utilizing an NEMD color field method. All three routes to the tensor of diffusion coefficients are shown to yield consistent results. A simple spherically symmetric Lennard-Jones fluid at its triple point was used in the simulation study. We find that in a Couette strain field of sufficiently large strain rate, diffusion in all directions is enhanced substantially. We propose and verify a new asymptotic relationship between the diagonal elements of the tensor of diffusion coefficients and the strain rate.
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    The Journal of Chemical Physics 94 (1991), S. 2172-2175 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The angular distribution of light intensity scattered from a nonionic micellar solution of tetraethylene glycol n-decylether (C10E4) in water has been examined close to its critical point of mixing. Including multiple-scattering corrections to the light-scattering data, we have found the exponent γ=1.25±0.02 for the osmotic susceptibility χT and the exponent ν=0.63±0.01 for the long-range correlation length ξ in the temperature range of 1.03×10−4≤ε≤4.25×10−3, where ε=(Tc−T)/Tc. The values of the critical exponents obtained in this work are in good agreement with the theoretical and experimental ones for three-dimensional Ising systems.
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    The Journal of Chemical Physics 94 (1991), S. 2176-2186 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: We present an analysis of crystal growth of a pure material from its melt, using a phase field model. This model for a first-order phase transition couples nonconserved order parameter kinetics to thermal diffusion. It has been shown to give unique one-dimensional solutions, with velocity selection. We demonstrate that these steady-state solutions cease to exist for the thermal diffusivity above a critical value, which depends on the parameter coupling the kinetics to the temperature. We suggest a scaling which leads to agreement between numerical integration of the time-dependent equations and a perturbation analysis of the coupling. For small velocity, our model reduces to a standard model commonly applied to dendritic growth, which replaces the kinetic equation with one of interfacial equilibrium. Because the velocity vanishes at the critical point, the thermal diffusion length and dendritic wavelength diverge at this point. We relate this critical point to bifurcations in the phase space of a mechanical equivalent to the steady-state growth equations. We conclude that solutions cease because thermal diffusion drives the system to local kinetic equilibrium, and we discuss experimental accessibility and means of avoiding equilibrium.
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    The Journal of Chemical Physics 94 (1991), S. 2207-2218 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the results of molecular dynamics simulations of the liquid–vapor interface of water. Two different water–water interactions were studied. The density profile of the interfacial transition region between liquid and vapor is monotone with a 10%–90% width of 3.45 A(ring), in good agreement with the value determined from x-ray reflectance measurements, namely 3.30 A(ring). The water molecules in the interface tend to lie with the HOH bisector in the plane of the surface and one OH bond pointing out of the surface. As the density falls in the transition region the tetrahedral structure of the bulk liquid breaks up and there is a tendency towards dimerization. The diffusion constant in the interfacial region is 58% larger than that in the bulk region.
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    The Journal of Chemical Physics 93 (1990), S. 5586-5593 
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    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The formation of C2H− is observed in two broad resonance bands when C2H2 is irradiated with vuv light. The higher-energy band has partially resolved structure, approximately linear pressure dependence, and a threshold at 16.335±0.021 eV. It is attributed to photoion-pair formation (C2H−+H+) consequent upon predissociation of one or more Rydberg states. This threshold, together with IP(H) and EA(C2H), gives D0(HCC–H)≤5.706±0.022 eV≡131.6±0.5 kcal/mol, or ΔH0f0 (C2H)≤134.5±0.5 kcal/mol. The lower-energy band has an approximately quadratic pressure dependence and curved step-like structure. It is attributed to photoelectron-induced dissociative attachment mediated by a πg shape resonance. The threshold, at 878.5±2.0 A(ring), corresponds to a photoelectron energy of 2.715±0.032 eV. This threshold combined with EA(C2H)=2.969±0.010 eV, yields D0(HCC–H)≤5.684±0.033 eV≡131.1±0.7 kcal/mol, or ΔH0f0 (C2H)=134.0±0.7 kcal/mol.
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  • 99
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5580-5585 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The time-dependent form of the Lippmann–Schwinger integral equation is used as the basis for a novel wave-packet propagation scheme. The method has the advantage over a previous integral equation treatment in that it does not require extensive matrix inversions involving the potential. This feature will be important when applications are made to systems where in some degrees of freedom the potential is expressed in a basis expansion. As was the case for the previous treatment, noniterated and iterated versions of the equations are given; the iterated equations, which are much simpler in the present new scheme than in the old, eliminate a matrix inversion that is required for solving the earlier noniterated equations. In the present noniterated equations, the matrix to be inverted is a function of the kinetic energy operator and thus is diagonal in a Bessel function basis set (or a sine basis set, if the centrifugal potential operator is incorporated into an effective potential). Transition amplitudes for various orbital angular momentum quantum numbers can be obtained from: (1) Fourier transform of the amplitude density from the time to the energy domain, and (2) direct analysis of the scattered wave packet. The approach is illustrated by an application to a standard potential scattering model problem.
    Type of Medium: Electronic Resource
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  • 100
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 8021-8028 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Hydrogen adsorption on the (0001) beryllium surface is modeled using a Be45 cluster containing seven layers. The ab initio Hartree–Fock calculations employ effective core potentials and full D3h point group symmetry. Six low-lying electron configurations are investigated and the effect of electron spin coupling on the adsorption process is discussed. The adsorption of H on Be is calculated to be stable by 20 kcal/mol and lowers the work function by 1.3 eV.
    Type of Medium: Electronic Resource
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