bluuebunny/crossref_metadata_2025 huggingface dataset:
Monte Carlo simulations of electronic energy transfer in three-dimensional systems: A comparison with analytical theories
Authors: Engström Sven , Lindberg Maria , Johansson Lennart B.-Å.
Published: July 2002

Monte Carlo simulations are applied to imitate the incoherent electronic energy transfer among identical luminescent molecules in three-dimensional isotropic systems. The interacting molecules are then either rapidly or slowly rotating compared to the time scale of emission and we refer to these as the fast and slow cases, respectively. The time dependence of the excitation probability of the initially excited molecule [Gs(t)] and the mean square displacement [〈R2(t)〉] of the initial excitation are simulated for the slow and fast cases. The results are compared to those obtained from the analytical model of Gochanour, Andersen, and Fayer (the GAF theory) and to some extent to the computer method of Riehl. In the fast case and at a reduced concentration being less than two both Gs(t) and 〈R2(t)〉 are in very good agreement with the so-called three-body GAF theory. The deviations found at higher concentrations are likely due to the omission of higher orders of coupling in the GAF theory. For the slow case the GAF theory is developed exactly only to two-body order, and the agreement between theory and simulations is not so encouraging. However, if an approximative three-body term is included for Gs(t) the agreement becomes excellent for a large range of concentrations. Finally, the emission anisotropy [r(t)] of a donor–donor system is simulated for the slow case. The emission from any donor in the system, i.e., not necessarily the one excited initially, is thereby considered. The simulated r(t) agrees very well with the approximation of r(t)=(2/5)⋅Gs(t) which justifies the so-called Galanin model.
The rotational spectra and centrifugal distortion parameters for the NNO–DF and ONN–DF complexes
Authors: Kukolich S. G. , Pauley D. J.
Published: July 2002

Microwave measurements of rotational transitions for the linear ONN–DF and bent NNO–DF complexes were made using a pulsed-beam, Fourier transform spectrometer. For ONN–DF, B=1808.959(2) MHz and DJ=2.79(6) kHz. Structure parameters are obtained and compared with ONN–HF parameters. For the bent NNO–DF isomer, seven new transitions and previous data were fit to obtain (A+ΔK)=25 988.4(3) MHz, B=2701.1(2) MHz, C=2422.3(2) MHz, ΔJ=0.052(3) MHz, ΔJK=−2.57(1) MHz, δJ=0.010(5) MHz, and δK=0.24(10) MHz. The centrifugal distortion constants for NNO–DF are used to obtain force field parameters  fRR and  fΘΘ describing motion of the DF center of mass relative to the NNO center of mass. The excellent agreement between these parameters and previous data on NNO–HF supports the simplified model used to describe the centrifugal distortion of this complex.
Polarized infrared spectroscopy of photo-oriented 2-fluoroethanol in solid argon: Mechanism of the mode-specific isomerization
Authors: Kafafi Zakya H. , Marquardt Charles L. , Shirk James S.
Published: July 2002

Single photon infrared (IR) photoisomerization of 2-fluoroethanol isolated in solid argon creates selectively photooriented reactant and product distributions. The IR dichroism resulting from polarized excitation of each of six different bands in the CH stretching region, the OH stretching band, and the ν(OH)+τ(OH) combination band is reported. Each irradiation imparts a dichroism to the spectrum of both the G′g reactant and the Tt product isomers. Analysis of this dichroism shows that during isomerization the CH2F end of the molecule remains essentially stationary in the solid; the photoisomerization occurs primarily by internal rotation of the CH2O and OH groups. New vibrational mode assignments in the CH stretching region and a confirmation of a C2 pseudosymmetry in the Gg isomer are also derived from this analysis. These results demonstrate the general efficacy of this novel application of polarized IR spectroscopy.
Relativistic effect on total energies for determination of correlation energies of atoms from their experimental total energies
Authors: Anno Tosinobu , Teruya Hirohide
Published: July 2002

Relativistic effect Erel upon the total electronic energy of an atom is discussed with particular reference to obtaining the nonrelativistic total energy Eexact from the experimental total energy. Numerical values of this effect obtained by various authors by different nonempirical methods are compared for neutral atoms of rare-gas elements. It is shown that methods either of a Hartree–Fock-type or of a Dirac–Hartree–Fock-type give much the same Erel value for He through Ar. It is pointed out that Erel calculated with Hartree–Fock wave functions is not adequate for use in obtaining Eexact from the experimental total energy and that the Erel value calculated with wave functions including electron correlation should work well, although an actual demonstration can be done only for two-electron systems for lack of data. A semiempirical formula is therefore proposed, which is useful for least-squares fit of experimental total energies of isoelectronic series of atoms to extract nonrelativistic total energies along with the relativistic effect. From nonrelativistic energies thus derived, semiempirical values of correlation energies of atoms are obtained. The results thus obtained are in reasonable agreement with correlation energies derived by Clementi along somewhat different lines. The power series expansion in Z of the fitted formula for the He series shows that numerical values of expansion coefficients agree reasonably well with the corresponding values obtained by accurate relativistic and nonrelativistic Z expansion-type calculations.
Formation and lifetime of metastable complexes in collisions oftrans-glyoxal with helium
Authors: Kroes G-J. , Rettschnick R. P. H.
Published: July 2002

Using the Monte Carlo classical trajectory method, we have calculated cross sections for complex formation and complex lifetimes for glyoxal–helium collisions at low relative translational energies (<40 cm−1). The cross section for complex formation depends exponentially on the initial translational energy, and is independent of the initial glyoxal rotational state over a wide range of rotational energies. The lifetime of the glyoxal–helium complex depends strongly on the total angular momentum of the complex. The dissociation of the complex (rotational predissociation) is not statistical. Some insight in the dynamics of complex formation is obtained.
A theoretical analysis of third-order nonlinear optical properties of linear polyenes and benzene
Authors: Pierce Brian M.
Published: July 2002

An accurate and computationally tractable theoretical procedure for the calculation of the nonresonant, electronic components of the third-order molecular polarizabilities, γ(0;0,0,0), γ(−3ω;ω,ω,ω), and γ(−2ω;ω,ω,0), can be constructed. This procedure partitions γ into a σ-electron component (γσ) and a π-electron component (γπ). The γσ term is evaluated using the bond-additivity approximation; the γπ term is calculated using the semiempirical INDO all-valence-electron molecular orbital method combined with full single- and double-excitation configuration interaction (SDCI) of singlet π-electron configurations, and Orr and Ward’s sum-over-states expression for γ. The INDO-SDCI method is also used to calculate one- and two-photon spectroscopic properties of the 1ππ* states salient to γπ for the molecules of interest. It is shown that single-excitation CI alone is not sufficient for the calculation of γπ for linear polyenes and benzene. Calculations of the effect of chain length and conformation on the values of γ for ethylene, cis and trans linear polyenes, and benzene indicate that γ is strongly influenced by conjugation chain length. A simple relationship can be established between the calculated value of γπ(0;0,0,0) for the trans linear polyenes investigated and that for ethylene, the molecule with the solitary π-electron C–C bond: γπ(0;0,0,0)≂γπ(0;0,0,0)ethylene NC–C3, where NC–C=1,3,5,7, and NC–C is the total number of C=C and C–C bonds in the given polyene, i.e., the length of the π-bonding network. As γ increases with chain length, so does the ratio γπ/γ. Virtual electronic transitions involving excited π-electron states with extensive charge separation and double excited configurational character are important contributors to γπ for the linear polyenes and benzene. An approximation of γπ(0;0,0,0) for the linear polyenes can be written in terms of the linear π-electron polarizabilities for the ground state and 1 1Bu π-electron excited state. Although this

approximation is strictly applicable to the centrosymmetric linear polyenes, it does suggest a very interesting criterion for the selection of organic molecules with large third-order polarizabilities. Namely, the change in polarizability between the ground state and a strongly one-photon absorbing excited state is an important factor to consider when selecting candidate molecules.
Gauge theory and quasidiabatic states in molecular physics
Authors: Pacher T. , Mead C. A. , Cederbaum L. S. , Köppel H.
Published: July 2002

Following previous works [Moody et al., Phys. Rev. Lett. 56, 893 (1986) and Zygelman, Phys. Lett. A 125, 476 (1987)], we review in some detail the gauge theoretical form of the Born–Oppenheimer description of molecules. The derivative couplings which describe nonadiabatic effects play the role of gauge potentials. Applying the gauge theoretical formalism (well known in particle physics), we rederive a theorem concerning the nonexistence of strictly diabatic states. Further we propose the Lorentz gauge condition to define quasidiabatic states. We show that the recently proposed block diagonalization method to obtain quasidiabatic states leads automatically to the Lorentz gauge locally.
Statistical thermodynamics of monolayers of flexible-chain amphiphiles: Adding nearest-neighbor correlations to a Scheutjens–Fleer approach
Authors: Cantor Robert S. , McIlroy Peter M.
Published: July 2002

In recent work, Wang and Rice have developed a lattice statistical thermodynamic model for amphiphile monolayers, based in part on the work of Scheutjens and Fleer. As in Flory–Huggins theory, their approach relies on a Bragg–Williams approximation to the probability of successful placement of a chain segment into a lattice site. This may be a poor approximation for typical monolayers, which contain relatively short hydrocarbon chains and are characterized by orientational anisotropy, particularly at high surface densities. A model is developed that incorporates intramolecular correlations in nearest-neighbor (bond) approximation and used to predict osmotic pressure and segment and bond distributions as a function of surface density. These results are compared with surface equations of state and segment distributions obtained in Scheutjens–Fleer approximation, i.e., in the absence of bond correlations. The two levels of approximation yield considerably different isotherms, although only minor differences in the segment distributions are predicted. In particular, inclusion of nearest-neighbor correlations is found to reduce greatly the magnitude of both entropic and energetic contributions to the lateral pressure.
High temperature deviations from Rice–Ramsberger–Kassel–Marcus unimolecular rate coefficients expressions
Authors: Oref I. , Tardy D. C.
Published: July 2002

The closed form of the solution to the master equation for unimolecular decomposition is investigated. Exact numerical values of the Rice–Ramsberger–Kassel–Marcus unimolecular rate coefficient are calculated for the decomposition of cyclobutene and the isomerization of quadricyclane. A comparison between the approximate strong collision low temperature expression now in general use and the high temperature strong collision exact expression shows that in some cases there are large deviations (∼40%) between the values calculated from the two expressions. The usage of the exact expression causes a shift of the falloff curves toward the high pressure limit. The larger the molecule, the larger the deviation; the higher the value of E0 the smaller the deviation. For E0/RT>20 the approximate expression is sufficient. These deviations may be quite important in practical systems such as combustion and atmospheric chemistry and shock tubes.
Dynamics of latex aggregation. Modes of cluster growth
Authors: Pefferkorn E. , Varoqui R.
Published: July 2002

The kinetics of aggregation of polystyrene latices occuring in the presence of an electrolyte or poly(4-vinylpyridine) in aqueous media are examined. The aggregate size distribution was recorded at different times by counting the flocs containing a given number of elementary colloids. The moment of order zero ( the total number of clusters per unit volume of the suspension), as well as the second moment of the size distribution are computed. In the presence of an excess of electrolyte or an optimum amount of polymer, a dynamic scaling function of the form cg (t)=t−2 F[g/t] and a narrow time invariant polydispersity is found. Under slow flocculation conditions, a large time dependent polydispersity sets in. Large aggregates then react preferentially with equally large ones. This behavior is discussed in term of a sticking probability , using the formalism of the reaction limited cluster–cluster aggregation.
Interference effects in surface enhanced Raman scattering by thin adsorbed layers
Authors: Blue Douglas , Helwig Kate , Moskovits Martin , Wolkow Robert
Published: July 2002

The intensity of the surface enhanced Raman scattering signal from an organic molecule (usually pyrazine) adsorbed on a rough, coldly deposited Ag surface was monitored as a function of dose. The interference maxima observed in the SERS signal as a function of film thickness are not of constant intensity. Model calculations were performed assuming that the organic film adsorbed on the Ag surface is a stratified three phase medium and the modulation in SERS intensity to be the result of interference effects both in the incident and Raman scattered light. Simulations of experimental results using this model give semiquantitative agreement between the experimental and calculated SERS intensity modulation obtained with pyrazine films as a function of increasing film thickness. Experiments and model calculations were also performed for benzene, iodobenzene, and ethylene films and a method for determining the (nonbulk) values of the densities of these films is described. The implications of this interference effect on the photodecomposition of pyrazine at and near rough Ag surfaces is discussed. We reinterpret the increase in photochemical reaction rate observed with increasing pyrazine doses as being due to interference effects in the intensity incident at the Ag surface and not due to a tuning of the surface plasmon as suggested in a previous study [Wolkow and Moskovits, J. Chem. Phys. 87, 5858 (1987)].
The effect of salt identity and concentration on liquid–liquid phase separation in aqueous micellar solutions of C8-lecithin
Authors: Huang Yao-Xiong , Thurston George M. , Blankschtein Daniel , Benedek George B.
Published: July 2002

We report measurements of the location and shape of the coexistence curve for binary liquid–phase separation in aqueous micellar solutions of C8 -lecithin, as a function of the concentrations of KI, NaI, LiI, KCl, NaCl, and LiCl. The salts containing iodide dramatically lower the critical temperature for phase separation Tc as the salt concentration is increased. The chloride salts, however, first lower and then raise Tc as the salt concentration is increased. The identity of the added cations used has a comparatively minor effect on Tc . Potassium ions are more effective in changing Tc than sodium ions, which in turn are more effective than lithium ions. We have used our data to deduce the dependence on salt identity and concentration of the parameters C and Δμ, which reflect the strength of intermicellar interactions and the extent of micellar growth, respectively, within a Gibbs free energy model for micellar solutions. The theoretical coexistence curves calculated using the deduced parameters agree well with our experimental findings.
Swelling equilibria for ionized temperature-sensitive gels in water and in aqueous salt solutions
Authors: Beltran Sagrario , Hooper Herbert H. , Blanch Harvey W. , Prausnitz John M.
Published: July 2002

Swelling equilibrium data in water and in aqueous NaCl solutions are presented for thermally sensitive N-isopropylacrylamide (NIPA) hydrogels containing 0–4 mol % quaternized amine (positively ionizable) comonomer. We report the effect of gel charge and solution ionic strength on the temperature-induced collapse of NIPA gels. Experimental swelling equilibria are compared with predictions based on a recently proposed oriented-quasichemical model. This model has been shown previously to describe lower critical solution behavior in uncharged aqueous polymer solutions and gels (i.e., aqueous NIPA gel). We apply the model here to ionized NIPA gel. Semiquantative predictions are obtained for the effects of gel charge and solution ionic strength on temperature-dependent swelling behavior.
Measurement of Xe desorption rates from Pt(111): Rates for an ideal surface and in the defect-dominated regime
Authors: Rettner C. T. , Bethune D. S. , Schweizer E. K.
Published: July 2002

The rate of thermal desorption of Xe from a Pt(111) surface has been measured over a range of 7 orders of magnitude using a combination of molecular-beam techniques. Rates up to ∼104 s−1, corresponding to residence times as short as 100 μs, were extracted from the time-of-arrival distributions for atoms leaving the surface after short Xe beam pulses were applied. Rates as low as 10−3 s−1 were measured using a time-delayed flash-desorption technique. For intermediate rates, the transient decay of the desorbing Xe was recorded directly following the closing of a beam shutter. Temperature programmed desorption (TPD) spectra show first-order desorption kinetics and also reveal the presence of ‘‘defect’’ sites with substantially higher Xe binding energy which dominate the desorption kinetics at low coverages (below 0.005 Xe monolayers). These defects can be specifically saturated with CO molecules, permitting the measurement of rates characteristic of an ideal Pt(111) surface. An Arrhenius plot of these desorption rates is found to be linear over the entire range covered (80–160 K), giving an adsorption energy, Δε, of 245±15 meV and a preexponential, ν, of 14+24−8 ×1011 s−1. In contrast, a similar plot for rates which are controlled by the presence of defect sites gives Δε=410±40 meV and ν=9+40−8 ×1015 s−1. A desorption model including the effect of defects is developed which relates the desorption rate to the microscopic behavior of Xe atoms on terraces and at defects. This model leads to an estimate for the preexponential factor for defect-dominated desorption which is quite consistent with the very large measured value, and allows the simulation of TPD spectra and isothermal coverage decay curves, which involve coverages both higher and lower than the defect-site density.
A theoretical prediction of hydrogen molecule dissociation-recombination rates including an accurate treatment of internal state nonequilibrium effects
Authors: Schwenke David W.
Published: July 2002

This paper presents the results of a detailed study of dissociation and recombination of H2 over the temperature range 1000 to 5000 K. The chemical processes are modeled by solving the master equation for the concentrations of the full set of rovibration states. All of the state-to-state energy transfer rate coefficients required by the master equation are evaluated by the quasiclassical trajectory method using a potential energy surface which is a fit to ab initio electronic structure calculations. The analysis of the results of the master equation to obtain the phenomenological rate coefficients was carried out using several techniques. This is required because there is a mixture of third bodies and their concentrations change as the reaction proceeds. The methods based on one-way fluxes are not reliable, while the methods based upon an eigenvalue solution of the linearized master equation are in reasonable agreement with our preferred method, which is based on fitting the concentrations from a two component master equation. The results from various methods of determining the phenomenological rate coefficient for H2 as a third body are in good agreement with experimental estimates. The recombination rate coefficients are most sensitive to the collision induced dissociation rates of initial states with moderate values of vibrational and rotational quantum numbers. A comparison of the orbiting resonance theory to the accurate results shows that this simple model is not valid under the conditions of the present study.
Static and dynamic light scattering from liquid crystalline solutions of rodlike macromolecules
Authors: van der Schoot Paul , Odijk Theo
Published: July 2002

It is argued that concentration fluctuations in lyotropic nematics can cause significant scattering of light. We calculate the static structure factor numerically for a nematic suspension of rods in the second virial approximation. An analytical approximation is also given based on a variational principle applied to an integral equation of the Yvon type. The relation between the one-particle distribution function and the usual correlation functions is discussed. An expression is derived for the polarized dynamical structure factor. This yields the initial decay rate which emphasizes the diffusion perpendicular to the director. We show that the contribution from anisotropic scattering is negligible.
Integral equations and closure relations for the bridge function and for the triplet correlation function
Authors: Attard Phil
Published: July 2002

The first term in a systematic expansion of the two particle potential of mean force yields the hypernetted chain closure approximation. Here is it shown that the second term (a bridge function) consists of a convolution of two ternary correlation functions, and that these can be related by an integral equation analagous to that of Ornstein and Zernike. An expansion for the three particle potential of mean force which also retains the leading two terms completes a closed set of equations. Also discussed are methods for the numerical resolution of the equations, the asymptotic forms of the triplet total and direct correlation functions, and relations between various quaternary functions.
Trapping of finite-sized Brownian particles in porous media
Authors: Torquato S.
Published: July 2002

It is shown that the trapping of finite-sized spherical Brownian particles of radius βR in a system of interpenetrable spherical traps of radius R is isomorphic to the trapping of ‘‘point’’ Brownian particles (β=0) in a particular system of interpenetrable spherical traps of radius (1+β)R. This isomorphism in conjunction with previous trapping-rate data for β=0 is employed to compute the trapping rate for the case β=1/4 in a system of hard spherical traps as a function of trap volume fraction. The effect of increasing the size of the Brownian particles is to increase the trapping rate relative to the instance of point Brownian particles.
Stiff chain model—functional integral approach
Authors: Lagowski Jolanta B. , Noolandi Jaan , Nickel Bernie
Published: July 2002

We present a modified probability distribution for a single stiff chain. This probability distribution includes the usual Wiener measure that accounts for the bond length distribution and a term that associates an explicit energy with the bending of the chain. The modification includes additional factors that suppress the end fluctuations to make the chain homogeneous. Our result for the mean squared end-to-end distance is that for the Kratky–Porod chain but the analysis clarifies a number of points in the literature.
Comment on: Rectilinear diameters and extended corresponding states theory
Authors: de Bruyn John R. , Goldstein Raymond E.
Published: July 2002

In a recent paper [R. R. Singh and K. S. Pitzer, J. Chem. Phys. 92, 3096 (1990)] it is shown that the slope of the coexistence curve diameter and the critical compressibility can be well described by linear functions of Pitzer’s acentric factor. Singh and Pitzer conclude that variations in these quantities are therefore due to changes in the two-body intermolecular potential. We argue that they are primarily due to changes in the relative importance of three-body interactions, as shown by Pestak et al. [Phys. Rev. B 36, 599 (1987)].
Monte Carlo simulations of adsorption on heterogeneous surfaces with a random topography of surfaces sites
Authors: Cortes Joaquin , Araya Paulo
Published: July 2002

Making use of Monte Carlo experiments, a simulation has been carried out of the adsorption of a gas on heterogeneous solids characterized by energy distribution and a random topography of the superficial sites. A good interpretation of the results is achieved by means of the theoretical models introduced by Hill, and later by Rudzinsky, for these types of systems.
A mean-field theory of grain boundary segregation
Authors: Alba William L. , Whaley K. Birgitta
Published: July 2002

This paper presents a mean-field solution for a one-dimensional spin Hamiltonian in the presence of spatially varying interactions and external field. In a binary alloy, such inhomogeneous interactions appear in the presence of a grain boundary. We derive the model and place it in the context of previous theories. We show how our theory is a natural extension of traditional segregation isotherm models, with the advantages that much finer detail can be observed and that no assumption is required about the grain boundary binding energy. Solving the model requires finding the global minimum of a function of several hundred variables and yields detailed concentration profiles in the presence of spatially inhomogeneous and long-range interactions. We apply the theory to the system of copper with bismuth impurities and observe on an atomic scale how the extent of segregation varies with temperature. The results predict that with lower temperature the impurity concentration in a given layer increases, the segregant peak broadens, and ordering can occur within the boundary. The results also indicate that the presence of segregation at the grain boundary can serve as a nucleus for order–disorder phase transitions in the bulk.
Molecular dynamics simulations of ordered alkane chains physisorbed on graphite
Authors: Hentschke Reinhard , Schürmann Britta L. , Rabe Jürgen P.
Published: July 2002

Scanning tunneling microscopy (STM) studies at the interface between the basal plane of graphite and organic solutions or melts of long chain alkanes and alkyl derivatives reveal that the molecules order in lamellae with the main molecular axes oriented parallel to the substrate. Here we employ molecular dynamics (MD) simulations to obtain more details on the molecular order and dynamics within the alkane lamellae as a function of density. We find that the orientation of the molecular carbon zigzag planes relative to the graphite is governed by a subtle interplay of packing and entropic effects. In addition, we consider multiple layer adsorption and investigate the rapid loss of order with increasing distance from the interface. Finally, we study the diffusive behavior of an isolated long chain alkane, C350H702, on graphite, which is of interest in the context of STM imaging of isolated macromolecules at interfaces. The sensitive dependence on atomic parameters renders MD simulations a valuable complementary tool for the detailed interpretation of STM data.
Jet spectroscopy of benzyl and benzyl-α-d2
Authors: Fukushima Masaru , Obi Kinichi
Published: July 2002

Benzyl and benzyl-α-d2 radicals are produced by the ArF laser (193 nm) photolysis of benzylchloride and benzylchloride-α-d2, respectively, in a supersonic free jet. The spectroscopy of the D1 1 2A2–D0 1 2B1 transition of these radicals is studied by means of the laser induced fluorescence (LIF) method. LIF excitation spectra show well resolved but unusual vibrational structure. The assignments of vibronic bands have been carried out on the basis of dispersed spectra from the single vibronic level (SVL) and transition band types derived from rotational analysis of high resolution LIF excitation spectra. The intensity anomaly of the vibronic bands in the excitation spectra is interpreted as the breakdown of the accidental forbidden character of the D1–D0 and D2–D0 electronic transitions, whose mechanism will be discussed in terms of vibronic coupling.
The charge structure factor of a 1-1 electrolyte in solution
Authors: Kunz W. , Calmettes P. , Jannink G. , Belloni L. , Cartailler T. , Turq P.
Published: July 2002

The charge structure factor of a 1-1 electrolyte solution has been measured by means of small angle neutron scattering. To this end the system was chosen so as to achieve a zero average contrast for the salt. The data are used to test the Stillinger and Lovett sum rules, and also to test the modelization of the solution structure including short range as well as Coulombic interactions. This modelization is based on the hypernetted chain integral equation and on suitable solvent-averaged pair potentials. At small values of wave number transfer, the experimental results are compared to a new approximation for the charge structure factor.
A site–site theory for finite concentration saline solutions
Authors: Perkyns John , Pettitt B. Montgomery
Published: July 2002

A liquid state theory based on site–site integral equations is constructed to have the asymptotics given by angular expansion theory. This results in a theory which shows dielectric consistency, e.g., the dielectric constant as viewed from the solvent is the same as that viewed by the ions. Such consistency is lacking in other extended reference interaction site model (XRISM)-based theories and leads to unrealistic structural predictions. The Kirkwood–Buff route to thermodynamics is used and allows a physical partitioning of the terms responsible for the solvation process. Sample results for a 1–1 salt are given.
Resonances and the effects of interchannel coupling in the photoionization of CS2
Authors: Stratmann R. E. , Lucchese Robert R.
Published: July 2002

The photoionization cross sections of CS2 leading to the X 2Πg, A 2Πu, B 2Σu+, and C 2Σg+ states of CS2+ have been computed including the effects of interchannel coupling in the multichannel frozen core Hartree–Fock approximation. The results were obtained using the iterative Schwinger variational method using a partial wave expansion up to l=120. The photoionization cross sections are obtained in the mixed dipole representation which ensures that the Thomas–Reiche–Kuhn sum rule is satisfied. We have found shape resonances in the kπg and kπu continuum channels. In particular, the kπg resonance is seen to affect dramatically both total cross sections and photoelectron asymmetry parameters in all other channels when interchannel coupling is included. The form of the resonant wave function indicates that this resonance is caused by low lying virtual d orbitals on the sulfur atoms. Using a Schwinger-type iterative method we have found the position of the corresponding poles of the single channel T matrix in the complex plane. We have determined that the position of the pole is relatively insensitive to the interatomic bond lengths. This is indicative of the atomic nature of the resonant state. We have also determined cross sections and asymmetry parameters in the region of autoionization in the channel leading to the X 2Πg state of CS2+.
Close coupling calculations on rotational excitation and inversion of NH3 by collisions with Ar
Authors: van der Sanden G. C. M. , Wormer P. E. S. , van der Avoird A. , Schleipen J. , ter Meulen J. J.
Published: July 2002

State-to-state total cross sections for rotational excitation and inversion of NH3 by collisions with Ar have been calculated within the accurate close coupling framework. The inversion motion in NH3 was included both via a delta function model and by taking the inversion coordinate explicitly into account. We used an ab initio potential and a potential in which one term in the angular expansion of the ab initio potential is scaled in order to reproduce spectroscopic data. At the energies of these calculations the delta function model is found to be in nearly quantitative agreement with the ‘‘exact’’ inversion results. Comparison with experiment shows the original ab initio potential to be better than the scaled one. The state-to-state cross sections for ortho-NH3 are in general accord with the measurements. For para-NH3 the agreement is good also, but the relative magnitudes of the cross sections for transitions to the ± inversion states of the same rotational level are not reproduced correctly for all levels.
Evaluation of thermal rate constants in the eigenbasis of a Hamiltonian with an optical potential
Authors: Brown David , Light J. C.
Published: July 2002

Miller and co-workers [J. Chem. Phys. 61, 1823 (1974); ibid., 79, 4889 (1983)] have derived an exact quantum mechanical expression for reactive thermal rate constants in terms of the time integral of a flux autocorrelation function. The evaluation of this integral in a finite basis poses the problem that spurious oscillations in the correlation function due to recurrences can occur at long times, corrupting the result. To obviate this difficulty, we add to the Hamiltonian an optical potential in the asymptotic region, and evaluate eigenvalues and eigenvectors using the technique of successive truncation. These operations allow a diagonal (although nonorthogonal) representation of the propagator in which the eigenvalues are exponentially decaying functions of time, which damp the components of the propagated vectors before the spurious reflection back into the interaction region. In this manner, the infinite time limit of the integral may be evaluated properly. Furthermore, the results of a single diagonalization may be used to compute the thermal rate constant over a range of temperatures.
Deexcitation of He(2 1P) in collisions with diatomic molecules
Authors: Morishima Yasumasa , Yoshida Hiroaki , Ukai Masatoshi , Shinsaka Kyoji , Kouchi Noriyuki , Hatano Yoshihiko
Published: July 2002

Deexcitation cross sections of He(2 1P) by N2 and O2 have been measured using a pulse radiolysis method in a region of the mean collisional energy between 18 and 38 meV. Fairly large deexcitation cross sections (≂100 Å2) by N2 and the negative slope of the curve of cross section vs energy are interpreted in terms of the energy transfer cross section based on a dipole–dipole interaction. Validity of theoretical formula based on a semiclassical approach with rectilinear trajectories, i.e., the Watanabe–Katsuura formula, is discussed. The deexcitation cross section of He(2 1P) by O2 and its collisional energy dependence are not very different from that by N2. Optical model calculation of the deexcitation cross section has been also presented for collisions between He(2 1P) and M (M=H2 or N2) with an isotropically averaged complex potential optimized through a fitting procedure to the experimental cross sections. It has been found by the model calculation that an electron exchange interaction is not negligible in the deexcitation process.
The performance of a family of density functional methods
Authors: Johnson Benny G. , Gill Peter M. W. , Pople John A.
Published: July 2002

The results of a systematic study of molecular properties by density functional theory (DFT) are presented and discussed. Equilibrium geometries, dipole moments, harmonic vibrational frequencies, and atomization energies were calculated for a set of 32 small neutral molecules by six different local and gradient-corrected DFT methods, and also by the ab initio methods Hartree–Fock, second-order Mo/ller–Plesset, and quadratic configuration interaction with single and double substitutions (QCISD). The standard 6-31G* basis set was used for orbital expansion, and self-consistent Kohn–Sham orbitals were obtained by all DFT methods, without employing any auxiliary fitting techniques. Comparison with experimental results shows the density functional geometries and dipole moments to be generally no better than or inferior to those predicted by the conventional ab initio methods with this particular basis set. The density functional vibrational frequencies compare favorably with the ab initio results, while for atomization energies, two of the DFT methods give excellent agreement with experiment and are clearly superior to all other methods considered.
Second hyperpolarizability of Hückel rings: Analytical results for size and alternation dependencies
Authors: Spano Frank C. , Soos Z. G.
Published: July 2002

We analyze the size and alternation dependence of the second static hyperpolarizability γ(0;δ,N) of N-site Hückel rings with transfer integrals t(1±δ). Regular (δ=0) rings lead to a simple γ(0;0,N)̃ N5 dependence, while alternating (δ≳0) chains show a δ−6 divergence in the band limit. Analytical results for noninteracting electrons clarify the evolution from molecules to polymers, show strong enhancement of γ(0;δ,N) in the region Nδ≊1, and connect previous oligomer and polymer calculations.
Fluxionality and low-lying transition structures of the water trimer
Authors: Schütz Martin , Bürgi Thomas , Leutwyler Samuel , Bürgi Hans Beat
Published: July 2002

The minimum energy structure of the cyclic water trimer, its stationary points, and rearrangement processes at energies <1 kcal/mol above the global minimum are examined by ab initio molecular orbital theory. Structures corresponding to stationary points are fully optimized at the Hartree–Fock and second-order Mo/ller–Plesset levels, using the 6-311++G(d,p) basis; each stationary point is characterized by harmonic vibrational analyses. The lowest energy conformation has two free O–H bonds on one and the third O–H bond on the other side of an approximately equilateral hydrogen-bonded O...O...O (O3) triangle. The lowest energy rearrangement pathway corresponds to the flipping of one of the two free O–H bonds which are on the same side of the plane across this plane via a transition structure with this O–H bond almost within the O3 plane. Six distinguishable, but isometric transition structures of this type connect six isometric minimum energy structures along a cyclic vibrational-tunneling path; neighboring minima correspond to enantiomers. The potential energy along this path has C6 symmetry and a very low barrier V6=0.1±0.1 kcal/mol. This implies nearly free pseudorotational interconversion of the six equilibrium structures. The corresponding anharmonic level structure was modeled using an internal rotation Hamiltonian. Two further low-energy saddle points on the surface are of second and third order; they correspond to crown-type and planar geometries with C3 and C3h symmetries, respectively. Interconversion tunneling vibrations via these stationary points are also important for the water trimer dynamics. A unified and symmetry-adapted description of the intermolecular potential energy surface is given in terms of the three flipping coordinates of the O–H bonds. Implications of these results for the interpretation of spectroscopic data are discussed.
Exact solution for the correlation times of dielectric relaxation of a single axis rotator with two equivalent sites
Authors: Coffey W. T. , Kalmykov Yu P. , Massawe E. S. , Waldron J. T.
Published: July 2002

It is shown how exact formulas for the longitudinal and transverse dielectric correlation times and complex polarizability tensor, of a single axis rotator with two equivalent sites may be found. This is accomplished by writing the Laplace transforms of the dipole autocorrelation functions as three term recurrence relations and solving them in terms of continued fractions. The solution of these recurrence relations, in the zero frequency limit, yields the correlation times in terms of modified Bessel functions of the first kind. The previous result of Lauritzen and Zwanzig for the longitudinal relaxation time, based on an asymptotic expansion of the Sturm–Liouville equation, is regained in the limit of high potential barriers.
Theory of activated rate processes in the weak and intermediate friction cases: New analytical results for one and many degrees of freedom
Authors: Shushin A. I.
Published: July 2002

Simple analytical expressions for the reaction rate of activated rate processes are derived in the weak/intermediate friction limit for one and many degrees of freedom and for finite microcanonical reaction rates. The expressions are obtained by analytical solution of the steady-state integral master equations (in energy variables). The microcanonical reaction rate is taken to be independent of energy (higher than the activation energy). Irreversible transitions from one state and reversible transitions between many states are discussed in detail. A simple interpolation formula for the reaction rate is derived which describes a turnover from the weak friction regime to a strong friction one. The formula takes into account an interplay between activation and reaction at energies close to the activation energy. When applied to unimolecular gas phase reactions this interpolation formula bridges between the weak and strong collision limits. The formulas obtained are generalized to multidimensional activated rate processes.
An interpretation of the vibrational spectra of insulating and electrically conducting poly(3-methylthiophene) aided by a theoretical dynamical model
Authors: Hernandez V. , Ramirez F. J. , Otero T. F. , Lopez Navarrete J. T.
Published: July 2002

It is shown that the relevant spectral features which arise in the infrared spectrum of poly(3-methylthiophene) upon chemical doping or photoexcitation (i.e., in the electrically conducting form) can be properly explained by means of the effective conjugation coordinate (ECC) formalism. This theoretical dynamical model accounts for the intramolecular hopping of π electrons in the class of polyconjugated aromatic systems. A complete assignment for the infrared and Raman spectra of the polymer in the pristine state (i.e., in the insulating form) is proposed as the result of a theoretical vibrational potential function derived from semiempirical calculations on short oligomers. The dependence of the bandgap energy on the internal rotation about the inter-ring single bond is analyzed theoretically in dimers as model molecules and the results are compared with experiments.
Two-dimensional model of geometric effects in thin film crystal orientation
Authors: Birnie Dunbar P. , Weinberg Michael C.
Published: July 2002

A simple two-dimensional model is employed to derive the fraction of oriented crystalline area in thin film crystallization processes, and to assess the influence of various physical parameters upon the extent of orientation. The final expressions, which are given as averages over probability distributions, are shown to be precise via comparisons with numerical calculations. We find that for a given film thickness the seed (nucleus) density and nucleus orientation probability distribution have a dramatic influence upon the extent of orientation, while the crystal growth rate anisotropy has a smaller effect.
Polymer chains with excluded volume: Critical exponents from free energy optimization
Authors: Allegra Giuseppe , Colombo Emanuele
Published: July 2002

A self-consistent approach to the polymer excluded-volume problem is proposed, based on the Domb–Gillis–Wilmers (DGW) probability distribution for all atom pairs W(rj)∝ρθjexp(−Dρδj), ρj=rj/〈r2j〉1/2, rj being the interatomic distance between atoms separated by j bonds. As usual, we take 〈r2j〉∝j2ν for j≫1. The DGW distribution represents the simplest realistic assumption beyond the Gaussian distribution and is well supported by both theoretical and numerical evidence. Upon optimization of the chain free energy with respect to the mean-square Fourier amplitudes, the numerical values θ=0.31 and ν=0.604 are obtained, in rough agreement with the expected results θ≂0.7, ν=0.588. The results are shown to be invariant with the type of the polynomial expansion yielding the potential of mean force; the numerical disagreement may be due to the assumption of a single, average value of θ for all atomic pairs, consistent with a configurationally uniform chain model. Unlike with the Gaussian approximation, no factors comprising ln(j), ln[ln(j)]... are obtained within the expression of 〈r2j〉. Constraining to the values required by the DGW distribution the quadratic and quartic averages of the Fourier components constructed with the chain bond vectors, we obtain an approximate distribution function with a quartic polynomial of the chain coordinates within the exponent; this polynomial is proportional to the potential of mean force, useful in chain dynamics.
Direct observation of HSF+6 and the bracketing of the SF6 proton affinity at 5 K
Authors: Latimer Darin R. , Smith Mark A.
Published: July 2002

Formation of stable HSF+6 via proton transfer at 5 K is reported. A bracketing study determines the 298 K proton affinity of SF6 to be 139.4±2.0 kcal mol−1 in agreement with the only previously reported observation of HSF+6. In addition, an upper bound to the appearance energy for SF+5 of 321.9 kcal mol−1 is determined.
Binary nucleation kinetics: A matrix method
Authors: Vehkamäki Hanna , Paatero Pentti , Kulmala Markku , Laaksonen Ari
Published: July 2002

The kinetics of binary nucleation is investigated and an exact method to evaluate the nucleation rate is developed. The exact method accounts for all flows to the stable clusters, and thus the nucleation rate is obtained independently of determining the location of the saddle point in the Gibbs free energy surface. The exact method is based on determination of cluster concentrations, which can be obtained after solving a set of linear equations. The values of the nucleation rate and the direction of flow at the saddle point are calculated for water–sulphuric acid and water–ammonia systems. The results are compared to the results obtained by Stauffer’s saddle point integration method for binary nucleation kinetics. The saddle point integration method is found to give a reasonably accurate estimation of nucleation rate in the water–sulphuric acid system. However, in the water–ammonia system the difference is found to be as high as three orders of magnitude.
A computer simulation study of the chain configurations in poly(ethylene oxide)-homolog melts
Authors: Neyertz Sylvie , Brown David
Published: July 2002

The configurations of a series of diethyl ether chain molecules with the general formula C2H5–O–(CH2–CH2–O)m–C2H5, i.e., homologs of poly(ethylene oxide), PEO, have been determined from NpT molecular dynamics simulations of the pure melts at 400 K. The fraction of trans conformers, the mean square radii of gyration, and the mean square end-to-end distances have been compared to those predicted by a pivot Monte Carlo sampling method based on the assumption that chain configurations in the melt are largely determined by highly localized intramolecular near-neighbor interactions. Discrepancies are significant and point to inadequacies in the latter picture when Coulomb potentials are explicitly taken into account.