bluuebunny/crossref_metadata_2025 huggingface dataset:
Alkali effect on Co(II) ions structuring in borate glasses
Authors: Ancora B. , Magini M. , Sedda A. F.
Published: July 2002

The color change of cobalt borate glasses obtained by varying the alkali content in the glass has been investigated by x-ray diffractometry. A good number of spectroscopic studies attributed the change of color to a structural rearrangement of the cobalt ion in the host glass but no direct structural information has been presented to date. We examined three cobalt borate glasses containing about 5% in weight of CoO and different K2O contents (about 15%, 22%, and 29% in weight). Their radial distribution functions were compared with those of suitable cobalt-free potassium borate glass matrices and difference distribution curves obtained. Analysis of these curves allowed a quantitative description of the cobalt environment in terms of tetrahedral–octahedral structural rearrangements.
Structure and thermodynamics of the screened Coulomb liquid
Authors: Vlachy V. , Pohar C. , Haymet A. D. J.
Published: July 2002

The structure and thermodynamics of the screened Coulomb liquid have been studied by computer simulations and two approximate integral equations. The results indicate that the hypernetted-chain (HNC) approximation yields reliable predictions only for moderately structured fluids, and it cannot predict accurately thermodynamics variables or pair structure close to the freezing transition. In contrast, the modified HNC equation, which includes an approximation for the bridge function, agrees with grand canonical Monte Carlo (GCMC) data very well. It leads to thermodynamically consistent predictions for the pressure and seems to be reliable over the full range of liquid densities and pressures. Finally, the MHNC predictions are compared directly with experimental measurements by Ottewill and Richardson [Colloid Polym. Sci. 260, 708 (1982)] for the long-wavelength limit of the structure factor S(0).
Characteristics of power spectra for regular and chaotic systems
Authors: Dumont Randall S. , Brumer Paul
Published: July 2002

Power spectra for chaotic and quasiperiodic systems are analyzed in detail. Differences in convergence with increasing T are carefully examined and a number of important results are emphasized. First, we demonstrate that a finite time power spectrum is incapable, in principle, of providing an unambiguous distinction between a grassy power spectrum associated with a chaotic system and a singular power spectrum associated with quasiperiodic motion. By contrast the statistics of power spectra at fixed energy are shown to provide an adequate method for making such a distinction. Second we show two routes to linking the power spectrum to the physically meaningful spectral density. One route results from a correct analysis of the Wiener–Khinchin theorem wherein the power spectrum and spectral density are shown to be related through a weak limit over frequencies. The second route emerges from a study of the asymptotic behavior of averages over chaotic power spectrum leading to a method for extracting, via extrapolation to the long time limit, the smooth spectral density from the grassy power spectra associated with chaotic behavior. Computations on the cat map, and Henon–Heiles and NaClK Hamiltonians are provided throughout.
Diffusion controlled reactions: Fluorescence quenching of cationic dyes by charged quenchers
Authors: Periasamy N. , Doraiswamy S. , Maiya G. B. , Venkataraman B.
Published: July 2002

Rate coefficients of diffusion-limited fast reactions of charged reactants are predicted to be time dependent or time independent in different theoretical approaches to solve the Debye–Smoluchowski equation. Different rate coefficient expressions are also predicted in different time domains. Fluorescence quenching experiments using two cationic dyes (rhodamine B and cresyl violet) and four efficient ionic quenchers (iron complexes) in aqueous solutions were carried out to verify the theoretical prediction. A detailed data analysis of the quenched fluorescence decay over a range of concentration of the quenchers supports the prediction that the ‘‘long’’ time rate coefficient is k(t)∼a+bt−1/2. Exact agreement with the theoretical predictions is not obtained in any of the fluorophore–quencher systems described in this work. Cresyl violet–ferrocyanide and cresyl violet–ferricyanide are two systems for which the experimental values of diffusion coefficient D and distance parameter RHN are justifiably close to theoretical expectation.
PH2 internal energy distribution produced by the 193 nm photodissociation of PH3
Authors: Baugh D. , Koplitz B. , Xu Z. , Wittig C.
Published: July 2002

Experimental results involving 193 nm PH3 photodissociation are reported. Detection of the PH2 fragment using laser induced fluorescence suggests that PH2 is formed with appreciable internal excitation, but no quantitative results concerning nascent PH2 could be obtained using this direct method. In related kinetics studies, the reaction of thermalized (300 K) PH2 with O2 yields a rate coefficient of 2.7×10−13 cm3 molecule−1 s−1, while PH2+NO is rather unreactive (k<10−14 cm3 molecule−1 s−1). In separate experiments, sub-Doppler spectroscopy on the H-atom fragment at Lyman-α (121.6 nm) allowed the center-of-mass kinetic energy distribution to be extracted; the PH2 internal energy distribution was obtained using energy conservation. Most of the available energy (E° ≂hν−D0=22 000 cm−1) appears as PH2 internal excitation; the mean internal energy is 14 000 cm−1 and the distribution peaks at ∼19 000 cm−1. The experimental distribution compares favorably with a simple statistical (prior) calculation, and the agreement is discussed in terms of possible photodissociation mechanisms.
Rotary echo nutation NMR
Authors: Janssen R. , Tijink G. A. H. , Veeman W. S.
Published: July 2002

A two-dimensional solid state NMR experiment which combines rotary echoes and nutation NMR is investigated and used to study different sodium sites in zeolite NaA. It is shown that with this technique sodium ions with different relaxation rates in the rotating frame can be distinguished.
Spectroscopy of the 3p 2Π Rydberg state of HCO by resonance-enhanced multiphoton ionization
Authors: Tjossem Paul J. H. , Cool Terrill A. , Webb Douglas A. , Grant Edward R.
Published: July 2002

High resolution two-photon resonant, three-photon ionization spectra of HCO and DCO are presented for wavelengths from 373 to 460 nm. Rotational analysis of the (K′,K″)=(0,2) and (0,1) subbands and measurements of spin–rotation splitting on the (0,2) subband provide a definitive assignment of the (070)←(000) band of the bent-to-linear 3p 2Π(A″)←X̃ 2Π(A′) Rydberg absorption in HCO. Bands from nine vibrational states in HCO and ten in DCO, attributable to a (0n0)←(000) bending mode progression in the 3p Rydberg state, are observed and assigned. Additional bands in HCO and DCO are assignable to (0n1)←(000) 3p progressions. Values for the 3p 2Π fundamentals ν2=822.1(0.7) cm−1, ν3=2177(3) cm−1 for HCO and ν2=657(2) cm−1, ν3=1900(5) cm−1 for DCO were measured. Additional unassigned partial progressions are provisionally attributed to 3s 2∑+←X̃ 2Π(A′) transitions.
Theory of π-electron states in polydiacetylene molecules
Authors: Kollmar C. , Sixl H.
Published: July 2002

We report on π-electron states in polydiacetylene chains using Hückel and self-consistent-field methods (SCF). The Hückel band-structure calculation for the infinitely long chain yields two valence and two conduction bands. The Peierls gap between the upper valence and the lower conduction band is dependent on the bond alternation within the conjugated π system described by a bond alternation parameter −1<q<1. It turns out that the acetylene structure (q>0) is energetically more favorable than the butatriene structure (q<0). Performing Hückel calculations for short chain polydiacetylene intermediates and including the σ electrons of the polymer backbone by a harmonic potential we obtain the chain length dependency of the bond alternation parameter q. The intermediates with two reactive chain ends (diradicals and dicarbenes) show a transition from the butatriene to the acetylene structure. To determine the spin multiplicity of the ground state of these intermediates we perform a SCF calculation for the singlet and the triplet state of the conjugated π system. According to the experiments we find a singlet ground state for chain lengths of less than six monomer units (diradicals) and a triplet ground state for longer chain lengths (dicarbenes). Optical excitation energies of the short-chain intermediates are determined using the method of Pariser, Parr, and Pople. The qualitative tendencies of the chain length dependency agree well with the experimental results of optical absorption measurements, the absolute values are too high as compared to the experiments.
An investigation of the HBCl+–BClH+ system by Mo/ller–Plesset perturbation theory
Authors: Peterson Kirk A. , Woods R. Claude
Published: July 2002

A b initio potential energy functions have been calculated for HBCl+ and BClH+. An absolute minimum was found for HBCl+ in its linear configuration, while only a saddle point, lying 94.2 kcal/mol above the HBCl+ minimum, was found for the BClH+ isomer. MP4SDQ calculations are used to predict B0=18 890±80 MHz for HBCl+. The band origins of the fundamentals of H11B35Cl+ are calculated by MP4SDQ to be 2788, 716, and 1123 cm−1. A CI-SD dipole moment of 3.27 D and quadrupole coupling contants are also reported for HBCl+. The proton affinity of BCl at T=0 K is calculated to be 202 kcal/mol.
Thermodynamics on one-forms
Authors: Story Troy L.
Published: July 2002

Thermodynamics and Hamiltonian dynamics are shown to be different mathematical representations of the same unifying principle, namely, that the use of a single one-form defined on a representative extended phase space leads, by differential geometric analysis, to a set of characteristic differential equations which describe irreversible transformations of physical systems.
Dipole moment of IF and other interhalogen molecules
Authors: Kucharski Stanislaw A. , Noga Jozef , Bartlett Rodney J.
Published: July 2002

The dipole moment of IF and the remaining five interhalogen halides are computed at the MBPT(4) level employing effective potentials. A comparison between the finite-field and the Hellman–Feynman expectation value approach is presented. The theoretical values are in agreement with the experimental ones to within 10%. In the case of IF, whose dipole moment is unknown, we predict a value of 1.87±0.2 D. The correlation corrections constitute about 20% to 30% of the total dipole moment value, the dominant part of which is introduced at the second order. Only slight differences occur between the Hellman–Feynman expectation value formula and the finite-field procedure for dipole moments at the level of a second-order MBPT wave function.
The supersymmetric diffusional influence functional: General theory
Authors: Morgan John D. , Wolynes Peter G.
Published: July 2002

The diffusional influence functional is derived for a general heatbath, using the supersymmetric form of the diffusional path integral. This form has an action which is linear in the potential for the system, rather than quadratic as in the usual diffusional path integral, which allows integration over the heatbath degrees of freedom to form an influence functional. The derivation of the supersymmetric form is reviewed to make the paper more nearly self-contained. The connection between the influence functional and the statistical properties of the heatbath is examined, and some possible applications of the formalism are discussed.
A closed-form, free-energy functional for a binary polymer mixture
Authors: Akcasu A. Ziya , Sanchez Isaac C.
Published: July 2002

A new, closed-form, free-energy functional is derived for a binary polymer mixture. When the free-energy functional is expanded in series form around the mean concentration, the leading term in the expansion is the usual Flory–Huggins free energy. The Fourier transform of the coefficients of this expansion are approximate vertex functions Γ̄(n). A useful and tractable form for Γ̄(n) is obtained for all n which only depends on the magnitudes of the n wave vectors. It is shown that Γ̄(2) is exact and Γ̄(3) and Γ̄(4) reduce to the correct limiting values in the small and large wave vector limits.
Spectra and structure of organophosphorus compounds. XXXIII. Microwave, infrared, and Raman spectra, structure, conformational stability, and vibrational assignment for ethylphosphonothioic difluoride
Authors: Durig J. R. , Johnson R. D. , Nanaie H. , Hizer T. J.
Published: July 2002

The microwave spectra of ethylphosphonothioic difluoride, CH3CH2P(S)F2, and eight isotopic species have been investigated in the region from 26.5 to 39.5 GHz. Only a-type transitions were observed and R-branch assignments have been made for all the isotopic species in the ground vibrational state for both the gauche and trans (methyl group trans to the P=S bond) conformers from which the rotational constants were determined. From these data the complete r0 structural parameters were determined for the gauche conformer with the values for the heavy atom parameters being: r(C–C)=1.532±0.006 Å, r(C–P)=1.800±0.007 Å, r(P=S)=1.880±0.003 Å, r(P–F)=1.555±0.005 Å, ∢CCP=112.6±0.3°, ∢CPS=119.4±0.4°, ∢CPF=102.0±0.2°, dih ∢FPCS=129.3±0.2°, and dih ∢CCPS=56.9±0.2°. The parameters of the trans conformer which differed significantly from the values for the corresponding ones in the gauche conformer were: r(C–P)=1.814±0.011 Å, r(P=S)=1.861±0.007 Å, and ∢CCP=114.8±0.2°. The infrared (3500 to 40 cm−1) and Raman (3500 to 20 cm−1) spectra of the gaseous and solid CH3CH2P(S)F2 and CD3CD2P(S)F2 as well as the Raman spectrum of the liquid have been recorded. Both trans and gauche conformers have been identified in the vibrational spectra of the fluid phases, but only the trans corformer remains in the solid state and a complete vibrational assignment is proposed for the trans conformer. The barrier to methyl rotation for the trans conformer was determined to be 808 cm−1 (2.31 kcal/mol). The asymmetric torsion for the trans conformer was observed as a series of closely spaced Q branches beginning at 73.25 cm−1 and falling to lower frequency and the gauche transitions begin at 70.82 cm−1. These transitions along with the dihedral angle for the gauche conformer have been used to obtain the potential

function for the asymmetric rotation which indicates that the trans conformer is more stable than the gauche conformer in the gas phase by 63±37 cm−1 (180±106 cal/mol). All of these results are compared with corresponding quantities for several similar organophosphorus compounds.
Memory kernel in liquid phase c
i
s–t
r
a
n
s isomerization
Authors: Zhu S.-B. , Lee J. , Robinson G. W.
Published: July 2002

The memory kernel in liquid phase cis–trans isomerization is examined from the point of view of existing experimental data as well as MD simulations. In agreement with nonlinear system/bath coupling theories, when the generalized Langevin equation is written in the conventional way, we find that the ‘‘effective’’ memory kernel in this equation depends on the properties of the isomerizing particle, including the barrier height. In addition, it is found that the angular velocity distribution for the isomerizing molecule may be strongly influenced by ‘‘shape effects,’’ caused by actual molecular differences between the cis and trans configurations. This distribution may not be Maxwellian when considered over a partial angular range, e.g., the ‘‘barrier region.’’ If this is a general phenomenon, the selection of correct initial conditions in the reactive flux method would create uncertainties in the application of that method to the calculation of barrier crossing rates.
Unimolecular reaction dynamics of dimethylnitramine
Authors: Sumpter Bobby G. , Thompson Donald L.
Published: July 2002

The unimolecular reaction dynamics of dimethylnitramine are studied using classical trajectories on three model potential-energy surfaces. Elimination of NO2 is the predominant reaction. The focus of this study is on the simple, bond-rupture reaction to give NO2. Rate constants are calculated for two of the potentials at two different energies and individual trajectories are examined to determined the nature of the energy flow in the molecule prior to reaction and the product energy distribution. Energy flow into the nitro group increases significantly prior to reaction. Energy becomes trapped in the nitro group as the N–N bond begins to break and rapidly exchanges between the NO2 bending and stretching modes. One of the potentials allows the concerted molecular elimination of HONO. The concerted process which involves hydrogen migration and subsequent HONO elimination accounts for less than 5% of the reactive trajectories. However, the HONO elimination reaction can be enhanced by exciting high CH stretch overtones.
Electron paramagnetic resonance of Mn2+ in FeSiF6⋅6H2O and FeSiF6⋅6D2O above 100 K
Authors: Rubins R. S. , Drumheller John E. , Hutton Stuart L.
Published: July 2002

In electron paramagnetic resonance measurements on Mn2+ in single crystals of FeSiF6⋅6H2O and FeSiF6⋅6D2O at 9.3 GHz, changes in the resolution of the outer sets of hyperfine lines were used to observe the trigonal to monoclinic structural phase transition, which occurred over the range 223–227 K in both crystals. The magnitude of the axial spin-Hamiltonian parameter D for FeSiF6⋅6D2O was (273±1)×10−4 cm−1 at 300 K and decreased slightly with decreasing temperature in the trigonal phase. In the monoclinic phase ‖D‖ was (263±1)×10−4 cm−1 at 220 K and increased slightly with decreasing temperature. Similar values were obtained for FeSiF6⋅6H2O. An abrupt linewidth increase on warming through the transition is postulated to result from a decrease in the effective Mn2+–Fe2+ exchange interaction due to SiF2−6 reorientation. A gradual decrease in linewidth with temperature at higher temperatures is ascribed to a reduction of the second moment of the Mn2+–Fe2+ dipolar interaction.
A multichannel quantum defect treatment of the Rydberg states of CH3I
Authors: Dagata J. A. , Scott M. A. , McGlynn S. P.
Published: July 2002

An analysis of the observed electronic structure of the high-resolution absorption spectrum of CH3I is presented for the energy regions above and below the first ionization limit. We show that the electronic effects of spin–orbit coupling and l mixing for Rydberg states built on a doublet ion core account for nearly all major features of the spectrum. The present results substantiate a recent ab initio calculation of the electronic multichannel quantum defect theory (MQDT) parameters for HI [H. Lefebvre-Brion, A. Guisti-Suzor, and G. Raseev, J. Chem. Phys. 83, 1557 (1985)]; they also illustrate an important feature of MQDT, namely, the ability to extrapolate channel mixing across threshold from the autoionization region into the discrete region.
An NMR study of the geometry of a CO layer chemisorbed on Pt particles
Authors: Ansermet J. Ph. , Slichter C. P. , Sinfelt J. H.
Published: July 2002

The authors have analyzed the shape of the 13C spin echo envelope of a 13CO layer chemisorbed on supported Pt particles. They show that it contains information about the geometry of the array of molecules on the surface. They find that two mechanisms are responsible for the shape. The coupling of the C spins with the surface Pt spins gives rise to a T2 of 2.9 ms at 77 K. By applying a Carr–Purcell sequence to the 13C spins, the effects of the coupling to the Pt can be eliminated. The dipolar coupling between 13C molecules then remains, producing a ‘‘slow beat’’ in the spin echo envelope. It can be analyzed exactly since mutual spins flips among C spins are not allowed, owing to large field inhomogeneities at the surface of the Pt particles. Analysis of the slow beat yields information about the structure of the array. The authors report evidence for the coalescence of islands of CO at low coverage and demonstrate that the coverage is uneven among the Pt particles if the sample powder is exposed to less than a monolayer of CO.
Prediction of angular distributions for the F+H2 and F+D2 reactions
Authors: Hayes Edward F. , Walker Robert B.
Published: July 2002

The bending corrected rotating linear model is used to predict angular distributions for the reactions F+H2(v=0)→H+HF(v′=2,3) and and F+D2(v=0)→D+DF(v′=3,4). The calculations were performed using the surface (No. 5A) that was reported recently by Steckler, Truhlar, and Garrett. The angular distributions obtained using this new surface differ in several important respects from distributions predicted in earlier quantal scattering studies using the Muckerman-5 surface. More importantly, these new predictions are in much better agreement with the high resolution molecular beam studies of these same reactions. The combination of these predictions with the results of the molecular beam studies provides additional evidence for the role of dynamical resonances in the two title reactions.
Coulomb interaction energy including overlap effects between two one-active electron atoms in various electronic states. Application to the interaction Li(2s)+Li(2s)
Authors: Bussery B. , Aubert-Frécon M.
Published: July 2002

Formulas for the matrix elements of the first- and second-order Coulomb interaction energy between two one-active electron atoms, either in ground or excited initial electronic states, are displayed for the region of small overlap. The appropriate two-center expansions of the electrostatic potential between two spherical charge distributions are explicitly used for overlapping as well as nonoverlapping regions. Test calculations have been performed for the interaction H(1s)+H(1s) at R=6 and R=8 a.u. The results obtained are in very good agreement with the literature reference values. Calculations have been performed for the first-order electrostatic and second-order induction and dispersion energies for the interaction Li(2s)+Li(2s) in the range 5≤R≤24 a.u. using a set of simple semiempirical functions for the Li atom. The results obtained for the dispersion terms are seen to be in an overall reasonable agreement with recent ab initio values. The increasing importance of both the induction terms and the so-called spherical dispersion terms E00iidisp with the decreasing values of R is emphasized. For the largest dispersion term, the relative contribution of the nonoverlapping and overlapping regions is discussed.
Geminate recombination in proton-transfer reactions. III. Kinetics and equilibrium inside a finite sphere
Authors: Agmon Noam
Published: July 2002

The kinetics of reversible dissociation of a single ion pair inside a finite sphere is described by the transient solution of the Debye–Smoluchowski diffusion equation, with backreaction boundary conditions at contact, and a reflective boundary condition at the sphere’s radius. At long times the solution tends to an equilibrium state. A simple analytic expression for the equilibrium coefficient is derived. For an infinitely large sphere (‘‘infinite dilution’’) it reduces to an expression similar to that of Fuoss, while its dependence on the sphere’s radius is qualitatively similar to the concentration dependence of activity coefficients.
The structure and bond energy of ArCl2
Authors: Evard Dwight D. , Cline Joseph I. , Janda Kenneth C.
Published: July 2002

The geometry and bond energy of ArCl2 are obtained by the technique of parity selected excitation spectroscopy. This is a pump–probe technique in which the probe laser is set to detect specific rotational levels of the B state of Cl2 while the pump laser is tuned through an ArCl2 B–X band. Since the excitation spectrum obtained depends on the parity of the detected Cl2 rotational level, the ArCl2 complex must have a symmetrical, T-shaped geometry on average over the time scale of the experiment, ∼100 ps. Simulation of the observed spectra, which show partially resolved rotational structure, yields an argon to chlorine center of mass distance of 3.7 Å for both the X and B electronic states of the complex. By measuring the threshold for dissociation to the v=6, j=8 level of the Cl2 B state, the Ar–Cl2 bond energy was determined to be 178 cm−1 in the B state and 188 cm−1 in the X state. These results are quite different from those on the isovalent complex ArClF which is a linear molecule with an Ar–Cl bond length of 3.33 Å, and a bond energy of 230 cm−1.
The final state and velocity distribution of the reaction D+H2→HD+H as a function of scattering angle
Authors: Blais Normand C. , Truhlar Donald G.
Published: July 2002

We present quasiclassical trajectory calculations of velocity-angle contour maps and state distributions as a function of scattering angle for the reaction D+H2→HD+H. These are calculated using the Liu–Siegbahn–Truhlar–Horowitz potential energy surface. The results show higher vibrational excitation and lower rotational excitation of the backscattered products as compared to the total product distribution. In addition to their fundamental interest, these calculations should be useful in interpreting experiments in which state-to-state differential cross sections are measured.
Cu+2 vibronic states at 0–1.4 eV from multiphoton ionization–photoelectron spectroscopy
Authors: Sappey Andrew D. , Harrington J. E. , Weisshaar James C.
Published: July 2002

We have used resonant two-photon ionization (R2PI) of cold Cu2 in a pulsed molecular beam in conjunction with time-of-flight photoelectron spectroscopy (TOF-PES) to study the vibronic states of Cu+2 from 0–1.4 eV. The Cu+2 threshold of 7.899±0.005 eV agrees with the ionization energy previously inferred from R2PI spectral intensities. We cleanly resolve a pair of Cu+2 excited state vibrational progressions with origins at 1.143 and 1.256 eV above threshold; both have ω+e=252±17 cm−1. We assign these progressions as a 3d-hole, 2Π spin–orbit pair. In addition, in the range 0–1 eV, we observe a dense set of at least 50 moderately well-resolved bands which can be fit to a single progression with ω+e=187±8 cm−1 and ωex+e=0.7±0.2 cm−1. While this might be the anticipated 2Σ+g ground state, the intensity pattern (multipeaked envelope, with alternation of intensities in some regions) suggests the possibility of multiple low-lying 3d-hole electronic states of Cu+2.
On the solution of Kramers’ equation by Trotter’s formula
Authors: Wio Horacio S.
Published: July 2002

By considering the cases of free Brownian motion, and Brownian motion in a harmonic potential, we show that Trotter’s formula could be applied to solve Fokker–Planck equations with singular diffusion matrices.
Defects in doped polyacetylene: An a
b i
n
i
t
i
o infrared and Raman spectroscopy of solitons
Authors: Villar H. O. , Dupuis M. , Clementi E.
Published: July 2002

We report complete vibrational spectra, including IR and Raman intensities of C22H24, C21H+23, and C12H+14 +, as models for pure and doped polyacetylene. The results provide definitive evidence for charged solitonic rather than bipolaronic defects.
A supertensor formalism for solute–continuum solvent interactions with an arbitrarily shaped cavity. II. Preliminary applications to model systems
Authors: Drummond Malcolm L. J.
Published: July 2002

The mathematical technique of the preceding paper is applied to the glycine zwitterion and to a cyclic hexapeptide. A number of approximations are suggested which would increase the range of applicability of the approach. The greater importance of the solvent effect over the internal self-polarization of the small peptide ring is quantitatively demonstrated.
Rotationally resolved product states of polyatomic photofragmentation by time-resolved FTIR emission: HF elimination from 1,1-CH2CClF at 193 mm
Authors: Fletcher T. Rick , Leone Stephen R.
Published: July 2002

Time-resolved FTIR emission spectroscopy is used to obtain collision-free rotational distributions of the products in a photofragmentation process for the first time. A detailed description of the excimer laser photolysis-FTIR emission method is presented, and results are derived for the internal state distribution of nascent HF eliminated from 1,1-CH2CClF following photoactivation at 193 nm. The HF product is born rotationally excited, with Jmax at least ∼J=15. The vibrational distribution is also highly excited and fits a Boltzmann distribution with a vibrational temperature of 28 000 K. Even though a large barrier to elimination exists, a statistical model for the energy partitioning reproduces the vibrational distribution accurately. In the model used, the full exoergicity is assumed, however, the HF vibrational frequency is chosen to be the value in the transition state, which is one-half of the frequency for free HF. Agreement between the statistical model and the observed rotational distributions is less satisfactory, especially for low J states. The results suggest that some modification of the rotational distribution may occur as the HF and HCCCl products separate during the photofragmentation.
Does reaction path curvature play a role in the diffusion theory of multidimensional activated rate processes?
Authors: Matkowsky Bernard J. , Nitzan Abraham , Schuss Zeev
Published: July 2002

The two-dimensional Kramers’ barrier crossing problem in the overdamped (diffusion) limit is investigated with particular attention given to possible effects of the geometry of the potential surface on the rate. Previous work ascribes corrections to the two-dimensional Kramers’ formula to curvature of the reaction path. In contrast, we find that these corrections are due to the anharmonicity of the potential surface at the saddle and may become appreciable for small window frequency, i.e., flat potential surface at the saddle in the direction perpendicular to the reaction path. A general formalism to calculate such corrections is described.
Externally driven fluctuation of concentration and dynamics of second order chemical reaction
Authors: Morita Akio
Published: July 2002

The dynamics of ordinary second order reversible reaction: A+B⇄C is considered by assuming that concentration of A fluctuates around a certain value like the random white noise by external means, whereas that of B does not fluctuate explicitly. This problem is treated in order to elucidate how externally driven fluctuation may affect the reaction, and it is reduced to solving a nonlinear stochastic differential equation which has been studied recently. It is shown that unlike a catalytic reaction, the fluctuation shifts the most probable equilibrium concentration so as to induce the formation of C. The fluctuation also affects the dynamics. It encourages the formation of C at the initial stage and discourages it later in comparison with the deterministic case where no fluctuation is taken into account. These effects are absent in the linear first order reaction. In the special case of irreversible reaction, we obtain an analytical asymptotic expression for the probability density valid at long time.
Solution reaction path Hamiltonian for reactions in polar solvents. II. Applications
Authors: Lee Sangyoub , Hynes James T.
Published: July 2002

The solution reaction path Hamiltonian (SRPH) developed in the previous paper is applied to model SN2 and ionic dissociation reactions in water solvent. The solution reaction paths are determined and show marked deviations from a standard equilibrium solvation picture. The impact of potential anharmonicities, reaction path curvature, and varying solvent mass on the rate constant is calculated via the variational transition state theory approach of I, and the deviations from harmonic van der Zwan–Hynes (ZH) theory are calculated. Typically only minor deviations from ZH theory are found. The reasons for this are discussed.
Molecular dynamics simulation of ice at constant volume
Authors: Kerr J. A. , Kiefer J.
Published: July 2002

The melting of ice, as modeled by the RSL2 potential, is investigated in a series of molecular dynamics simulations at constant volume. According to the behaviors of the total energy and structure factor with temperature, melting of the model system occurs between 260 and 330 K. Estimates of the constant volume specific heats are made which indicate that the molecules ought to be treated as rigid bodies. The necessity of running these simulations for many picoseconds, rather than only two or three, is illustrated.
Solution of the Percus–Yevick equation for hard spherocylinders. I. The entire pair correlation function
Authors: Lago S. , Sevilla P.
Published: July 2002

The Percus–Yevick equation for hard spherocylinders has been numerically solved using a recent algorithm to calculate the shortest distances between rods proposed by ourselves. The equation is solved for four different reduced densities to η=0.3879 and length-to-breadth ratio L*=1.0. Comparison with available Monte Carlo results for some selected orientations shows moderate agreement. Dependence of the pair correlation function and direct correlation function on mutual orientations and density are analyzed for these orientations. We found that none of the current approximations used in perturbation theories or in simplified integral equations can give even a qualitative description of the behavior of correlation functions for the above elongation L*.
Simple model for ternary mixtures with nonionic surfactants
Authors: Carneiro G. M. , Schick M.
Published: July 2002

A simple three-component lattice model employed previously to describe ternary mixtures with ionic surfactants is extended in order to describe mixtures with nonionic surfactants. This entails replacing the orientation-dependent hydrogen-bonding interaction between water and surfactant with an effective interaction which is isotropic but temperature dependent. We obtain the progression from two-phase to three-phase to two-phase coexistence with temperature which is typical of these systems. The behavior of characteristic temperatures with varying system parameters, such as the alkane number, is qualitatively reproduced.
Spectral hole burning in polysilanes
Authors: Trommsdorff H. P. , Zeigler J. M. , Hochstrasser R. M.
Published: July 2002

The first observation of hole burning in the optical spectra of delocalized polymer backbone states is reported. The polymer chosen for this study is poly−(di−n−hexyl)−silane, consisting of chains of roughly 5100 silicon atoms. Absorption spectra of polysilane in solution at 1.6K and 77K are reported.(AIP)
Vibrational relaxation in fluids: A critical analysis of the independent binary collision theory
Authors: Dardi Peter S. , Cukier R. I.
Published: July 2002

We analyze intermolecular vibrational energy transfer in fluids from a statistical perspective in order to obtain a qualitative understanding of the role of many-body effects. Our focus is on an explicit derivation of the independent binary collision (IBC) theory starting from a many-body scattering formalism which we previously developed. The derivation exposes an inherent contradiction in the IBC theory as applied to fluids. We conclude that many body effects cannot be correctly included through the collision frequency, and therefore, that the radial distribution function cannot be used to describe the nonlinearity of the rate constant as a function of the density. We also question the validity of neglecting many-body effects in the dynamics. These are neglected to obtain a two-body transition probability in the IBC theory. Numerical calculations are proposed (which are in progress) to test the importance of the dynamical many-body effects.
On the dynamical viscosity of dilute polymer solutions in theta solvents
Authors: Stepanow S. , Helmis G.
Published: July 2002

The path integral formulation of the Kirkwood diffusion equation is used to derive the perturbation expansion of the intrinsic viscosity in powers of the hydrodynamic interaction. Within the preaveraging approximation the perturbation expansion yields the Zimm formula for the intrinsic viscosity. The renormalization of the dynamical viscosity [η(ω)] has been explicitly carried out for both zero and large frequencies. At intermediate frequencies the renormalization of [η(ω)] has been performed in an approximate manner. A simple analytical expression for the dynamical viscosity is given. For low ω the real part of [η(ω)] has a plateau, while for large ω the Zimm behavior ([η(ω)]∼ω−1/3) is obtained.
Relaxation processes in molecular crystals. Temperature dependence of the bandwidth of Raman active lattice phonons of crystalline CO2
Authors: Ranson P. , Ouillon R. , Califano S.
Published: July 2002

We have measured the bandwidth of the three Raman active lattice phonons of crystalline CO2 as a function of temperature in the range 6–130 K. For bandwidths narrower than 1 cm−1 we used a high resolution tandem interferometer–spectrometer instrument and for broader bands a standard Raman instrument. We found that our measurements, made at constant pressure, differ considerably from those obtained previously by other authors at constant volume. These differences are explained as due to the deconvolution procedure utilized for the constant volume measurements. The evolution with temperature of the phonon bandwidth shows that fourth-order processes play a significant role in the phonon relaxation.
Nonuniversal field dependence of the critical behavior of the interface between fluid phases in two dimensions
Authors: Knackstedt M. , Robert M.
Published: July 2002

The equilibrium structure of the interface between two-dimensional fluid phases in the presence of an external field is investigated. Unlike in three or more dimensions of space, it is found that in two dimensions the interfacial thickness is very sensitive to the external field. In particular, the critical exponent that describes the divergence of the interfacial thickness as the critical point is approached is found to be strongly nonuniversal, i.e., field dependent, in two dimensions, whereas it is universal, i.e., field independent, in three or more dimensions.
Use of the Fixman–Alben distribution function in the analysis of non-Gaussian rubber-like elasticity
Authors: Erman B. , Mark J. E.
Published: July 2002

A probability distribution of the end-to-end distance of a polymer chain obtained by Fixman and Alben and recommended particularly for short chains is employed here in the calculation of stress–strain isotherms for a polymer network in elongation. The specific systems investigated are bimodal elastomeric networks containing a large enough number of very short chains to give markedly non-Gaussian behavior. The general shape of the experimental isotherms and its dependence on the relative numbers of very short and long chains are satisfactorily reproduced by the calculated results.
An infrared study of the symmetric P–F stretch of PF3 chemisorbed on the Pt(111) surface
Authors: Liang Shoudeng , Trenary Michael
Published: July 2002

We have used the technique of Fourier transform infrared reflection absorption spectroscopy (FT-IRAS) to characterize the symmetric P–F stretch of PF3 chemisorbed on the Pt(111) surface over the temperature range of 300–82 K. At 300 K and very low coverage we observe the P–F stretch at 899 cm−1. At intermediate coverages the P–F stretch shows two components of comparable intensity while at saturation coverage the spectrum is characterized by an intense band at 950 cm−1. The temperature dependence of the spectrum is distinctly different at different coverages. One of the two components of the broad band seen at 300 K after 0.8 L exposure becomes much sharper as the temperature is lowered while the other component becomes broader and the relative intensities of the two bands are reversed by 82 K. After an exposure of 0.5 L a symmetric band with a FWHM of 6.6 cm−1 at 300 K broadens upon cooling to 82 K into a band with an overall FWHM of 11.1 cm−1 which is asymmetric due to an unresolved high frequency shoulder. We attribute the two components of the P–F stretch to molecules within ordered ((3)1/2×(3)1/2)R 30° islands and to molecules in a lower density disordered lattice gas. Because of lateral interactions the higher frequency band is associated with molecules within the islands and the lower frequency band to the lattice gas. The relative intensity of the two components changes due to a shift in the equilibrium toward larger islands at lower temperatures and/or higher coverages.
Nonperturbative calculation of energies and widths of predissociative states of diatomic molecules
Authors: Gilbert Richard D. , Porter Richard N.
Published: July 2002

A nonperturbative theory of predissociation based upon an effective Hamiltonian is presented. For a model diatomic system in which a (bound-state) Morse potential-energy curve is crossed by an (unbound-state) exponential potential-energy curve, the exact effective Hamiltonian may be obtained in high precision for any well behaved interaction potential. The real and imaginary parts of the eigenvalues of the effective Hamiltonian give the (shifted) energies and widths, respectively, of the predissociative states. Numerical results are obtained and compared with both a semiclassical approximation and those derived from solutions of coupled Schrödinger equations. In the weak-coupling regime the agreement between the present method and semiclassical results is good, and better in general than the coupled-equations results, which apparently suffer from instability problems. For couplings of intermediate strength, the semiclassical method breaks down, as do fully quantum-mechanical first-order perturbation approximations, as is demonstrated by comparison with the converged nonperturbative results. Fixed points of the spectrum appear at ‘‘resonance’’ values of the coupling strength which bring the exact energy eigenvalue into resonance with one of the adiabatic levels so that the imaginary part (level width) of the former vanishes. Thus beyond a critical coupling strength, stronger coupling enhances the stability of a resonance.
Electronic states and nature of bonding of the molecule NiGe by all electron a
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o Hartree–Fock (HF) and configuration interaction (CI) calculations and mass spectrometric equilibrium experiments
Authors: Shim Irene , Kingcade Joseph E. , Gingerich Karl A.
Published: July 2002

All electron ab initio Hartree–Fock (HF) and configuration interaction (CI) calculations have been applied to investigate the low-lying electronic states of the NiGe molecule. The ground state of the NiGe molecule is predicted to be 1Σ+. The chemical bond in the 1Σ+ ground state is a double bond composed of one σ and one π bond. The σ bond is due to a delocalized molecular orbital formed by combining the Ni 4s and the Ge 4pσ orbitals. The π bond is a partly delocalized valence bond, originating from the coupling of the 3dπ hole on Ni with the 4pπ electron on Ge. The low-lying electronic states of the NiGe molecule have all been characterized by the symmetry of the hole in the 3d shell of Ni. The dissociation energy of the NiGe molecule has been determined from our high temperature mass spectrometric equilibrium data in combination with the theoretical results as D○0 =286.8±10.9 kJ mol−1. The standard heat of formation of the NiGe molecule has been obtained as ΔH○f,298 =514±12 kJ mol−1.
The electronic spectrum of the Ar–vinoxy van der Waals complex
Authors: van de Burgt Lambertus J. , Heaven Michael C.
Published: July 2002

The Ar–vinoxy radical van der Waals complex has been observed in a free-jet expansion. The complex was detected by laser excitation of the bands associated with the vinoxy B̃–X̃ transition. A van der Waals vibrational mode, with a frequency of approximately 41 cm−1, was found to be active in this spectrum. Rotational contour analyses showed that the complex is nonplanar, with the Ar atom ‘‘in contact’’ with the CCO frame. The extreme congestion of the rotational contours has been ascribed to a tunneling inversion motion.
Dynamics of stiff polymer chains
Authors: Fixman Marshall
Published: July 2002

The dynamics of worm-like polymer chains is considered for models with constrained bond lengths and elastic or constrained bond angles. Previous work attempted either the representation of the constraints on a complete basis set, which is impractical for chains of more than a few bonds, or the analytical preaveraging of inverse constraint matrices, which is too crude an approximation for local motions. Here the constraint matrices are evaluated from equilibrium simulations. A comparison of the analytical results with dynamical simulations shows good agreement for both global and local motions, indicating that the relevant memory in chains without rotational barriers does not extend past vibrational relaxation times. The dynamical diffusion operator is diagonal on a Rouse basis set to an adequate approximation, indicating that the effects of constraints can be imitated by the inclusion of internal friction in the basic Langevin equation. The internal friction approximation is useful even for extremely stiff chains, and the dependence of the apparent internal friction on mode number, chain stiffness, and chain length is examined. With fluctuating hydrodynamic interaction included, the constraints generate a coupling between local motions and translational diffusion which causes a small decrease in the translational diffusion constant. The effects are similar to those found in a previous phenomenological study of the effects of internal friction on translational diffusion, and do not seem to vanish with increasing chain length.
Kilo-electron-volt Ar–ion-induced neutral atom desorption from Rh{331}: Relation of angular distributions to surface structure
Authors: Reimann C. T. , Walzl K. , El-Maazawi M. , Deaven D. M. , Garrison B. J. , Winograd N.
Published: July 2002

The surface structure of single crystal metals in reflected in the angular distributions of neutral atoms desorbed during keV Ar ion bombardment. Results are presented here of a study of desorption from a stepped surface, Rh{331}. Recently, classical dynamics simulations of the desorption process were shown to give excellent agreement with ejection data from Rh{111}, and the same model potential has been used to simulate desorption from Rh{331}. The agreement with experiment remains excellent despite the different coordination of the surface atoms on Rh{111} and Rh{331}. The azimuthal ejection distributions are strongly affected by the steps.
Cross section and optical theorem for defects on a corrugated surface
Authors: Xu Han , Huber Daniel , Heller Eric J.
Published: July 2002

The definition for the surface scattering cross section of an object (defect, adatom, etc.) on a periodic surface in three dimensions is given. The analog of the Optical Theorem for gas phase scattering is derived. Calculations (made using semiclassical wave packets) are reported and compared to experiments for CO on Pt(111).
Damping and softening of low frequency vibrational modes of long molecules when placed in a viscous solvent
Authors: Sokoloff J. B.
Published: July 2002

The effects of a viscous solvent on the damping and softening of the vibrational modes of a long molecule in solution are studied by considering the solution of the equations of motion for the vibrations of the molecule coupled to the equations of motion of the fluid. These can be transformed to an equation for the vibrations of the molecule with an effective self-energy term, which takes account of the effect of the solvent. The inclusion of this term in the equations used in lattice dynamical calculations is proposed as a simple way of including the effects of the solvent in such calculations. When this method is applied to van Zandt’s cylinder model for acoustic vibrations of DNA, the gigahertz frequency acoustic modes are found to be over damped when the zero frequency value of the viscosity of water is used, although under-damped modes occur if the viscosity is half this value. The method, when applied to the low frequency interchain optic mode, predicts that its frequency should drop to zero at higher wave vectors. This could signify the denaturation of DNA in solution.