bluuebunny/crossref_metadata_2025 huggingface dataset:
Solvent mobility in polystyrene/Aroclor solutions by depolarized Rayleigh scattering
Authors: Fytas G. , Rizos A. , Floudas G. , Lodge T. P.
Published: July 2002

We report evidence of modification of solvent reorientation induced by the presence of polymer in polymer/solvent systems. Depolarized Rayleigh spectroscopy (DRS) has been used to probe the rotational motion of neat Aroclor and polystyrene (PS) solutions with polymer concentration 0.05, 0.086, and 0.146 g/cm3. The depolarized (IVH(ω)) spectra were recorded in the temperature range 40 to 140 °C using Fabry-Perot interferometry. Two Lorentzian lines were found to fit well the experimental IVH(ω) of PS/Aroclor above 60 °C revealing two populations of solvent. The fast relaxation time τf is virtually insensitive to variations of PS content in contrast to a considerable slowing down effect on the slow reorientation time τs obtained more precisely from photon correlation and oscillatory electric birefrigence measurements at low temperatures (<0 °C). The intensity Is of the narrow Lorentzian component, which was found to increase rapidly below a characteristic temperature at given PS concentration, is associated with the slow orientation of the solvent in the PS environment. Surprisingly, the onset of PS induced modification occurs at rather high temperatures (∼Tg+110 K). The values of the intensity ratio Is/If are rationalized in the framework of the restricted rotational diffusion model. Alternatively, the study of the single Lorentzian IVH(ω) spectra of the neat solvent suggests a significant amount of orientational pair correlations and is consistent with clustering in liquid Aroclor.
Random sequential adsorption: Series and virial expansions
Authors: Dickman Ronald , Wang Jian-Sheng , Jensen Iwan
Published: July 2002

We introduce an operator formalism for random sequential adsorption on lattices and in continuous space. This provides a convenient framework for deriving series expansions for the deposition rate dθ/dt in powers of t. Several specific examples—the square lattice with nearest-neighbor exclusion, and with exclusion extended to next-nearest neighbors, and disks and oriented squares on the plane—are considered in detail. Precise estimates for θ(t) and the jamming coverage are obtained via Padé approximant analysis. These are found to be in excellent agreement with simulation results. A diagrammatic expansion for dθ/dt is derived, and its relation to the equilibrium Mayer series is elucidated.
Observation of an electronic state of C2H near 9 eV by resonance ionization spectroscopy
Authors: Cool Terrill A. , Goodwin Peter M.
Published: July 2002

A new electronic state of C2H, tentatively assigned as the 3pσ 2Π Rydberg state, has been observed by resonance-enhanced multiphoton spectroscopy (REMPI). The observed absorptions originate from vibrationally excited C2H and C2D formed in the photodissociation of C2H2, C2D2, and C2HD at 193 nm in a molecular beam. Two-laser experiments and REMPI spectroscopy of photofragments of the dissociation of C2HD were used to assign the carriers of the observed spectra to C2H and C2D. Two-photon resonant, three-photon ionization (2+1 REMPI) of C2H and C2D is accompanied by substantial fragmentation to C+2 ions caused by multiple photon absorption by resonantly excited C2H and C2D. The identity of the lower C2H electronic state(s) (Ã 2Π or X̃ 2Σ+ ) was not established.
Hydrogen tunneling in condensed media
Authors: Suárez Alberto , Silbey Robert
Published: July 2002

A microscopic description for reactions in condensed media involving hydrogen tunneling, valid over a large temperature range, is presented. The tunneling system, represented by a pseudospin (S=1/2), reaches equilibrium when coupled to its environment, modeled by a collection of harmonic oscillators that behave like a heat bath. The environment includes both modes of the lattice (or solvent) and those molecular vibrations which play an active role in the tunneling process. Analytical expressions for the reaction rate are given in various regimes.
Experimental deuteron magnetic resonance study of orientational order in the nematic phase of a binary mixture of biaxial molecules
Authors: Hoatson Gina L. , Goetz Jon M.
Published: July 2002

Deuteron magnetic resonance (2H-NMR) spectra have been obtained from a binary mixture of 2-fluorenyl-4′-tetradecyloxy benzoate-d9 (FLOC14) and para-xylene-d10 (p-xy) as a function of temperature [Hoatson et al. Mol. Cryst. Liq. Cryst. (to be published)]. The liquid crystal FLOC14 is selectively deuterated on the rigid fluorene moiety, while p-xy is perdeuterated. Nonlinear least-squares fits to the quadrupolar splittings νkQ allow unambiguous determination of the temperature dependence of the orientational order parameters Sizz and (Sixx−Siyy) of both species i=1,2. These results are the first to provide information on the degree and asymmetry of orientational order of both biaxial molecules in the uniaxial nematic phase of a binary mixture. The new approach introduced to analyze the experimental data allows for an arbitary orientation of the rigid core relative to the molecular axis. The results demonstrate that the splittings and derived order parameters are insensitive to this twist angle. The experimental results are interpreted using a new mean field theory of binary mixtures of biaxial molecules [Palffy-Muhoray and Hoatson (to be published)]. There is good agreement between the predictions of theory and the component order parameters determined from 2H-NMR experiments, over the entire temperature range studied. Thus this description is capable of predicting the temperature dependence and interrelation of the nematic orientational order parameters for both biaxial molecules in a binary mixture. The derived theoretical parameters are interpreted as the ratios of intermolecular interaction strengths.
Surface diffusion and desorption kinetics for perfluoro-n-butane on Ru(001)
Authors: Arena M. V. , Westre E. D. , George S. M.
Published: July 2002

The surface diffusion and desorption kinetics for perfluoro-n-butane on Ru(001) were examined using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. The surface diffusion displayed Arrhenius behavior and was coverage independent. The surface diffusion parameters for perfluoro-n-butane on Ru(001) were Edif=2.9±0.3 kcal/mol and D0=5.9×10−2±0.2 cm2/s. The desorption parameters for perfluoro-n-butane on Ru(001) were Edes=13.8±0.6 kcal/mol and νdes=2.8×1021±0.1 s−1. In comparison, the surface diffusion parameters for n-butane on Ru(001) were Edif=3.5±0.2 kcal/mol and D0=1.4×10−1±0.2 cm2/s. The desorption parameters for n-butane on Ru(001) were Edes=11.9±0.5 kcal/mol and νdes=3.6×1015±0.1 s−1. The corrugation ratio, defined as Ω≡Edif/Edes, was determined to be Ω=0.21 for perfluoro-n-butane on Ru(001). This corrugation ratio was substantially different than the corrugation ratio of Ω≊0.30 measured for n-butane and various other n-alkanes, cycloalkanes and branched alkanes on Ru(001). The comparison between perfluoro-n-butane and the other alkanes indicates that fluorination lowers the surface corrugation ratio on Ru(001). Likewise, fluorination significantly increases the preexponential for desorption from Ru(001). This study illustrates the magnitude of substituent effects on surface diffusion and desorption kinetics for a physisorbed molecule on a single-crystal metal surface.
Density-functional perturbation theory of inhomogeneous simple fluids
Authors: Tang Zixiang , Scriven L. E. , Davis H. T.
Published: July 2002

The mean-field approximation, widely used in the nonlocal-density-functional theories of inhomogeneous simple fluids, is reexamined. Comparing its predictions of the density profiles of a Lennard-Jones fluid near a hard wall with those of Monte Carlo simulations reveals that the approximation is qualitatively incorrect at low densities and quantitatively inaccurate at intermediate and high densities. A density-functional perturbation theory is proposed. It combines the nonlocal-density-functional model of an inhomogeneous hard-sphere system with the Barker–Henderson second-order perturbation theory of uniform simple fluids. It also takes into account the softness of the short-range repulsive potential. The new theory is shown to be qualitatively correct and quantitatively more accurate over the whole range of liquid densities. The effects of the pair potential truncation and the self-consistency of the nonlocal-density-functional theories are discussed.
Path-integral calculation of the tunnel splitting in aqueous ferrous–ferric electron transfer
Authors: Marchi Massimo , Chandler David
Published: July 2002

We examine path-integral methods for computing electronic coupling matrix elements relevant to long-ranged electron transfer. Formulas are derived that generalize those already found in the literature. These extensions allow for efficient computation, especially for complex systems where there is either no inherent symmetry, or that symmetry is difficult to ascertain a priori. The usefulness of the computational methods based on these formulas is demonstrated by carrying out calculations for the ferrous–ferric exchange.
Dimensional interpolation of hard sphere virial coefficients
Authors: Loeser John G. , Zhen Zheng , Kais Sabre , Herschbach Dudley R.
Published: July 2002

We examine the dependence on spatial dimension D of the Mayer cluster integrals that determine the virial coefficients Bn for a fluid of rigid hyperspheres. The integrals vary smoothly with D, and can be characterized analytically in both the low-D and high-D limits. Dimensional interpolation (DI) allows one to evaluate individual Mayer cluster integrals at D=2 and D=3 to within about 1%. The resulting low-order virial coefficients have an accuracy intermediate between those of the Percus–Yevick and hypernetted chain approximations. Much higher accuracy can be achieved by combining the DI and HNC approximations, using DI to evaluate those integrals omitted by HNC. The resulting low-order virial coefficients are more accurate than those given by any existing integral equation approximation. At higher order, the accuracy of the individual cluster integrals is insufficient to compute reliable virial coefficients from the Mayer expansion. Reasonably accurate values can still be computed, however, by taking partial sums of the Ree–Hoover reformulation of the Mayer expansion. We report hard disk virial coefficients through B15 and hard sphere values through B10; the maximum errors with respect to known values are about 1.2 and 4.3%, respectively. The new coefficients are in good agreement with those obtained by expanding certain equations of state which fail to diverge until unphysical densities (those beyond closest packing), and so help to explain the surprising accuracy of some of these equations. We discuss the possibility that the exact virial expansion has a radius of convergence which corresponds to an unphysical density. Several new equations of state with desirable analytic or representational characteristics are also reported.
A 1H and 2H nuclear magnetic resonance study of amorphous ices at 77 K
Authors: Ripmeester J. A. , Ratcliffe C. I. , Klug D. D.
Published: July 2002

The 2H quadrupole coupling constants (QCC) of deuteriated high-density amorphous (hda) ice and low-density amorphous (lda) ice were measured to be 226.6 and 218.6 kHz with asymmetry parameters (η) 0.106 and 0.104, respectively, at 77 K. Correlations of the QCC’s with near-neighbor O–O distances in the crystalline phases of ice imply near-neighbor O–O distances of 2.84 and 2.79 Å in high- and low-density amorphous ice, respectively, compared with values of 2.79 and 2.77 Å for cubic ice (Ic) and hexagonal ice (Ih). The 1H second moment for hda ice of 37.5 G2 can be understood in terms of a short water interproton distance of 1.565 Å and the greater density of hda ice with respect to ice Ih. The 1H spin-lattice relaxation times at 77 K for hda ice and lda ice at 124 and 773 s are much shorter than the value found for the crystalline ices. It is suggested that the T1 parameter may be useful in characterizing different amorphous ice phases.
Fluorescence anisotropy of rotating molecules in the presence of energy migration
Authors: Engström Sven , Lindberg Maria , Johansson Lennart B.-Å.
Published: July 2002

Energy migration between fluorescent molecules undergoing Brownian rotational motion has been studied by numerically solving the master equation of a many-body system. The molecules interact according to Förster’s mechanism of dipole–dipole coupling. An algorithm for solving the master equation, and for calculating the fluorescence anisotropy and the mean-square displacement of the excitation is presented. In this study the molecules rotate like spherical particles in a liquid solvent at a reduced concentration of unity. The time-dependent fluorescence anisotropy was determined and compared to an analytical theory [G. H. Fredrickson, J. Chem. Phys. 88, 5291 (1988)]. At rotational rates less than the rate of fluorescence, the agreement between our results and those predicted by the theory is not satisfactory. The decay of the fluorescence anisotropy predicted by Fredrickson’s theory is generally too fast. A modification of his model using parameters obtained in our simulations gives a better agreement.
Ionic conductivity in poly(propylene glycol) complexed with lithium and sodium triflate
Authors: Albinsson I. , Mellander B.-E. , Stevens J. R.
Published: July 2002

Conductivity and viscosity measurements have been made for poly(propylene glycol)-MCF3SO3 (M=Li, Na) complexes in order to examine more closely the Vogel–Tammann–Fulcher (VTF) empirical relationship which has been found in previous reports to provide a good fit to the experimental data. Further, a dynamic bond percolation model of ion conduction in polymer electrolytes has predicted VTF behavior and an inverse relationship between molar conductivity and viscosity or Walden ‘‘rule’’ behavior. We find that deviations occur from both the VTF and Walden empirical relationships and propose a modest alteration in the form of the dynamic percolation model for ions moving in polyether systems.
Molecular electrostatic potentials: A topographical study
Authors: Gadre Shridhar R. , Kulkarni Sudhir A. , Shrivastava Indira H.
Published: July 2002

The topography of the molecular electrostatic potential (MESP) is studied for some small neutral molecules and OH− ion. Different kinds of critical points (CP’s) of rank 3 are identified and their occurrences are discussed. The correlation of these CP’s with the molecular structure is brought out. Bond ellipticities are determined in terms of curvatures of bond CP’s. These ellipticities show trends similar to those reported by Bader et al. [J. Am. Chem. Soc. 105, 5061 (1983)]. A typical example of OH− illustrates the existence of a degenerate nonisolated ring of CP’s, which is a rather unique feature of the topology of the MESP of linear molecules. Some suggestive arguments with suitable examples, regarding indeterminacy of the nonisolated degenerate CP’s, have been made.
Intramolecular vibrational relaxation and forbidden transitions in the SEP spectrum of acetylene
Authors: Jonas D. M. , Solina S. A. B. , Rajaram B. , Silbey R. J. , Field R. W. , Yamanouchi K. , Tsuchiya S.
Published: July 2002

Ã 1Au→X̃ 1Σg+ SEP spectra of acetylene near EVIB=7000 cm−1 show that Darling–Dennison resonance between the cis- and trans-bending vibrations is the first step in a series of anharmonic resonances which can transfer nearly all the vibrational energy out of the Franck–Condon bright states at higher energy. In addition to allowed ‖ΔK‖≡‖K′−ℓ″‖=1 rotational transitions, nominally forbidden ‖ΔK‖=0,2,3 rotational transitions have also been observed due to axis-switching and rotational-ℓ-resonance. Although the range of detectable fluorescence dips is only about 30, the range of detectable SEP intensities in these spectra is probably about 500.
The interaction between H and CO coadsorbed on Ni{110}
Authors: Love J. G. , Haq S. , King D. A.
Published: July 2002

CO adsorption on Ni{110} and on hydrogen precovered Ni{110} surfaces at 180 K has been studied by FT-RAIRS. On a clean Ni{110} surface CO adsorbs at on-top sites upto a coverage of 0.7 ML. At higher coverages the on-top CO band at 2060 cm−1 decreases in intensity with a concomitant increase in a band at 1960 cm−1 which shifts up in frequency. The (2×1)p2mg structure at 1.0 ML CO coverage shows a single band at 1998 cm−1 attributed to CO tilted at bridged sites. FT-RAIRS of CO adsorption on H precovered Ni{110} surfaces shows complex CO site switching within different CO and H coverage regimes. A model is presented involving compression of H into islands of high coverage and both mixed H and CO and unmixed CO islands are formed. CO adsorbs in the unmixed CO islands the same as that on clean Ni{110}. Within the mixed H/CO islands the first layer nickel atoms reconstruct and a new hydrogen desorption state is observed. The CO site switching observed by FT-RAIRS within the various H and CO coverage regimes is discussed in terms of a dominant indirect interaction.
Transient hole-burning and time-resolved fluorescence spectra of dye molecules in solution: Evidence for ground-state relaxation and hole-filling effect
Authors: Murakami H. , Kinoshita S. , Hirata Y. , Okada T. , Mataga N.
Published: July 2002

A picosecond transient hole-burning (THB) spectroscopy has been performed for organic dyes in solution. The THB spectra of rhodamine 640 have been found to show a time-dependent spectral change. This phenomenon corresponds to the solvent relaxation effect observed in the time-resolved fluorescence (TRF) spectrum. Although TRF spectrum is related only to the excited-state relaxation, THB spectrum is affected by both ground- and excited-state relaxations. Comparing with the TRF spectrum measured under the same exciting energy, we have clarified the presence of the ground-state relaxation. Further, the THB spectrum of styryl-8 gives an antihole between well-separated two holes corresponding to ground- and excited-state contributions. This may originate from the hole-filling effect due to the nonadiabatic relaxation from the nonequilibrium excited state to the ground state. The analysis based on a configuration coordinate model gives an overall understanding for these phenomena.
Hyperfine structure of the NaK a 3Σ+ state: Interaction of an electron spin with the sodium and potassium nuclear spins
Authors: Ishikawa Kiyoshi
Published: July 2002

The hyperfine structures of the NaK a 3Σ+ state are observed for the first time by using a Doppler-free spectroscopy named perturbation facilitated polarization (PFP) spectroscopy. The hyperfine constants ANa and AK of the a 3Σ+ (v=4, 5, 9, and 12) levels are determined. The observed hyperfine splittings are found to be well explained by the Fermi contact interactions of an electron spin with the sodium and potassium nuclear spins. The intensities and signs of the PFP spectra are calculated by using the determined hyperfine constants, and the results also agree with the observed ones. From the width of the observed spectra, energies of the spin–spin and spin–rotation interactions are obtained to be <5×10−3 and 5×10−5 cm−1, respectively.
Limits of stability of the liquid phase in a lattice model with water-like properties
Authors: Sastry Srikanth , Sciortino Francesco , Stanley H. Eugene
Published: July 2002

Explicit study of the hydrogen bond network in water offers a microscopic approach to understanding the anomalous properties of water, while an alternate, thermodynamic approach is offered by the reentrant limit of stability (spinodal) conjecture. To relate the two approaches, we develop a lattice model based on microscopic considerations. We show that the model displays anomalous thermodynamic behavior that is in qualitative agreement with the behavior of water. We study the model in the mean field approximation and by numerical simulations. We explicitly demonstrate the interrelation between density maxima and the reentrance of the spinodal: both originate from the contribution of orientational degrees of freedom to the thermodynamics of the system. The metastable liquid state is bounded by a spinodal at positive pressures as well as negative pressures, where the positive pressure spinodal is the limit of stability with respect to the solid state. The liquid–gas and liquid–solid spinodals form a continuous locus, but the ‘‘critical’’ properties of these two spinodals are quite different. While the response functions (specific heat, compressibility) diverge at liquid–gas spinodal, at the liquid–solid spinodal they do not—even though the response functions tend to higher values in the same fashion as occurs near the liquid–gas spinodal.
Kinetics of sol–gel transition in thermoreversible gelation of gelatin
Authors: Bohidar Himadri B. , Jena Sidhartha S.
Published: July 2002

The sol–gel transition in dilute gels of gelatin are studied by differential scanning calorimetry (DSC) and static light scattering techniques. The sol–gel phase diagram clearly shows the existence of an upper critical solution temperature in this system. In addition, the DSC data conclusively exhibits two more phase curves in the sol state—one pertaining to the coexistence of monomers and aggregates and the second one separating the random coil and helix domains in the solution phase. The Ferry–Eldridge equation has been used to determine the enthalpy of the melting of the gel structure (ΔHg) which is equal to (30±2.0) kcal mol−1. The gelation temperature Tg is correlated to Flory’s statistical model of gelation, which gives the enthalpy of melting of pure gelatin crystallites (ΔHv) as ΔHv=(35±2.0) kcal cm−3 and crystallite melting temperature (Tm) as Tm=(588±20.0) K. The Flory–Huggins interaction parameter (χ) has been determined as χ=0.49±0.05. Interesting scaling behavior has been observed through light scattering measurements. The intensity of scattered light, Is∼ε−γ and Is∼ε0−β; ε=(T/Tg−1); T≥Tg and ε0=(t/tg−1); t≥tg, where t is the time and tg is the gelation time for ε and ε0≤0.5 with γ≂β≂0.02±0.005. The initial phase of cooling of the sol has been analyzed within the framework of Smoluchowski aggregation kinetics. On the basis of the above results it is being proposed that the sol–gel transition path in this polymer has three distinct steps—monomer aggregation, coil–single-helix transition, and single-helix–triple-helix transition followed by gelation.
Photodissociation dynamics of the N3 radical
Authors: Continetti R. E. , Cyr D. R. , Osborn D. L. , Leahy D. J. , Neumark D. M.
Published: July 2002

The dissociation dynamics of the B̃ 2Σu+ state of N3 were investigated using fast radical beam photodissociation coupled with a new coincidence wedge-and-strip-anode particle detector. With this detector, detailed photofragment kinetic energy and angular distributions can be measured as a function of excitation energy. Calibration of the detector by studies of the predissociation of the O2 B 3Σu− state is discussed. Methods of extracting the center-of-mass kinetic energy and angular distributions are presented. The photodissociation results for N3 show that from the vibrationless level of the B̃ 2Σu+ state in N3, both spin-allowed [N3→N(2D)+N2(1Σg+)] and spin-forbidden [N3→N(4S)+N2(1Σg+)] dissociation processes occur. Bend excitation in the B̃ 2Σu+ state, however, enhances the spin-allowed dissociation process considerably. The kinetic energy distributions reveal partially resolved vibrational structure in the N2 fragment, and indicate substantial rotational excitation of the N2. This implies that bent geometries play a major role in the dissociation of the N3 B̃ state. Possible mechanisms for the spin-forbidden and spin-allowed dissociation channels are discussed.
The reaction of argon ions with hydrogen and deuterium molecules by crossed beams: Low energy resonances and role of vibronic levels of the intermediate complex
Authors: Tosi Paolo , Dmitrijev Oleg , Soldo Yvonne , Bassi Davide , Cappelletti David , Pirani Fernando , Aquilanti Vincenzo
Published: July 2002

In a crossed beam experiment, cross sections have been measured for the ion–molecule reactions Ar++H2→ArH++H and Ar++D2→ArD++D. Low collision energies (0.025≤E≤ 1 eV) and high resolution (ΔE∼10 meV, half-width at half-maximum) have been obtained using the method of guiding the ion beam by an octopole field and the technique of supersonic beams for H2 or D2. A structure in the energy dependence of cross sections has been found and attributed to a manifestation of vibronic resonances. Calculations are presented and compared to experimental findings to illustrate this effect, which arises because of the successive population of vibronic levels of the charge transfer complex Ar–H2+ or Ar–D2+, which are the intermediates for these reactions. Empirical potential energy surfaces for the entrance channels have been constructed accounting explicitly for the open shell nature and spin–orbit effects in Ar+(2PJ); symmetry considerations have also been used to establish the sequence of pertinent vibronic surfaces of the charge transfer intermediate complex—the role of configuration interaction in the latter is also discussed. The reaction dynamics has been treated as a sequence of nonadiabatic transitions at crossings of potential energy surfaces—quantum mechanical tunneling has been found crucial for the proper description of the observed energy dependence of the cross sections and the vibronic resonance structure. A higher frequency structure, borne out by the calculations and due to a manifold of metastable states supported by the vibronic levels of the intermediate charge transfer complex, appears to be washed out by the finite experimental resolution. It is also shown that finite experimental resolution had been the reason for the failure of detecting vibronic resonances in previous experiments and that the present ones are in general agreement with them when resolution is artificially lowered. Finally, it is pointed out that the present approach, when applied to charge transfer processes, provides a model which appears consistent with existing measurements. It also accounts for the observed selective reactivity of the fine structure components of argon ions.
Fast beam photodissociation of the CH2NO2 radical
Authors: Cyr D. R. , Leahy D. J. , Osborn D. L. , Continetti R. E. , Neumark D. M.
Published: July 2002

The photodissociation of the nitromethyl radical, CH2NO2, has been studied using a fast beam photofragment translational spectrometer. In these experiments, a fast beam of mass selected, internally cold nitromethyl radicals is formed via negative ion photodetachment of CH2NO−2 and subsequently dissociated. The recoiling photofragments are detected in coincidence using a microchannel plate detector equipped with a time- and position-sensing anode. Two dissociation product channels are observed at each of three dissociation wavelengths investigated in the range 240–270 nm and are identified as (I) CH2NO2→CH2NO+O and (II) CH2NO2→H2CO+NO. In marked contrast to the ultraviolet photodissociation of CH3NO2, no evidence is found for simple C–N bond fission to give (III) CH2NO2→CH2+NO2. Translational energy and angular distributions were obtained for the two observed channels. The translational energy distribution of channel (I) peaks at only 5–8 kcal/mol, while the distribution for channel (II) peaks at ∼60 kcal/mol. The angular distributions for both channels are largely isotropic. The nature of the electronic excitation and dissociation dynamics are considered at length. The upper state in the electronic transition is assigned to the 1 2B1 state. Results of attempts to model various aspects of the dissociation dynamics as statistical processes on the ground state surface indicate this mechanism is very unlikely. Instead, both dissociation channels are believed to occur primarily on excited state surfaces, and mechanisms for these processes are proposed.
Adsorption of polymeric fluids in microporous materials. I. Ideal freely jointed chains
Authors: Thompson Aidan P. , Glandt Eduardo D.
Published: July 2002

The behavior of molecular and polymeric fluids adsorbed within microporous solids plays an important role in many technologies. We apply a recently proposed statistical mechanical model which combines ideas found in two different formalisms: The polymer RISM theory of Curro and Schweizer, and the integral equations of Madden and Glandt for a simple fluid adsorbed within a random porous solid. These approaches yield a detailed systematic microscopic model for the structure of flexible polymers in microporous materials. We have obtained numerical results for the structural properties of ideal polymer melts confined within a matrix of hard-sphere obstacles.
Basis-set-free local density-functional calculations of geometries of polyatomic molecules
Authors: Dickson Ross M. , Becke Axel D.
Published: July 2002

Geometries of 61 small, neutral, singlet-ground-state molecules have been calculated using the local spin-density approximation (LSDA) density-functional theory. The computational method employed [A. D. Becke, Int. J. Quantum Chem. S 23, 599 (1989)] is free of conventional LCAO basis-set error. Errors due to basis-set truncation in previously published LSDA geometries are thus distinguished from errors purely due to the LSDA. It is found that the LSDA consistently overestimates bond lengths between hydrogens and main-group elements by 0.01–0.04 bohr, and usually underestimates bond lengths between nonhydrogens by less than 0.05 bohr. The tabulated geometries should be useful in calibrating basis sets and in developing beyond-LSDA exchange-correlation functionals.
Joint treatment of ab initio and experimental data in molecular force field calculations with Tikhonov’s method of regularization
Authors: Kuramshina G. M. , Weinhold F. , Kochikov I. V. , Yagola A. G. , Pentin Yu. A.
Published: July 2002

We describe a novel method for employing calculated ab initio potential data together with Tikhonov’s variational procedure to extract fundamental molecular force field parameters from experimental spectral data, the formal ‘‘inverse problem’’ of vibrational spectroscopy. In this approach, the ab initio quantities serve to ‘‘regularize’’ the initially ill-posed problem (in the sense of Tikhonov), leading to variationally stable and unique force field parameters that optimally mimic overall patterns of the (approximate) ab initio quantities, but exactly reproduce the available experimental data within specified experimental precision. In this manner, ab initio and experimental data can be jointly combined to produce more stable and reliable force fields (improvable to any degree through higher level ab initio treatment, additional experimental data, etc.) than could be attained by theoretical or experimental methods alone. The proposed procedure allows use of any system of generalized coordinates, including redundant systems of internal or symmetry coordinates, simplifying the transfer and comparison of force constants in series of related molecules. The procedure is illustrated with numerical application to CHF2Cl and its isotopomers at MP2/3-21G*, MP2/6-31G* levels of theory, demonstrating the stability and consistency of force fields obtained from different levels of theoretical input.
Surface recombination of atoms in a nitrogen afterglow
Authors: Marković V. Lj. , Petrović Z. Lj. , Pejović M. M.
Published: July 2002

The surface recombination of nitrogen atoms in afterglow is studied by the time delay method, accompanied by the macrokinetic diffusive model. The method consists of the measurement of the dependence of the mean value of the breakdown time delay on afterglow period td=f(τ) and fitting of the data by the model that was developed. Excited N2(A 3∑+u) nitrogen molecules formed in the surface-catalyzed recombination on cathode produce secondary electrons. The electrons entering the interelectrode space determine the time delay in electrical breakdown. The time delay method is very efficient in nitrogen atom detection down to a natural radioactivity level. By fitting the calculated curve to the experimental data, we have: (1) shown that the nitrogen atom recombination on the glass container walls is second-order in N while the recombination on the copper electrode is the first order; (2) determined the value of the surface recombination coefficient for molybdenum glass; (3) determined the combined probability of N2(A 3∑+u) metastable formation by recombination at electrode surface and of secondary electron emission. Furthermore, we derive the adsorption isotherm of nitrogen atoms on molybdenum glass, the type of recombination mechanism and the dependence of the activation energy for desorption (or the heat of adsorption) on the fractional coverage.
Translational and rotational expansion of spherical Gaussian wave functions for multicenter molecular integrals
Authors: Chiu Lue-yung Chow , Moharerrzadeh Mohammad
Published: July 2002

The expansion of the regular solid spherical harmonics rlYlm(θφ) [and the irregular solid spherical harmonics r−(l+1)Ylm(θφ)] about a displaced center is shown to be an irreducible tensor coupling of two solid spherical harmonic tensors—one refers to the displaced center and the other is made of the displacement vector. The Gaussian exponentials are expanded at the displaced center through the modified plane wave expansion. Combining these two expansions, the multicenter molecular integrals of the overlap, nuclear attraction, kinetic energy, and two-electron Coulomb repulsion over the homogeneous solid harmonic spherical Gaussians r2n+lYlm(θφ)exp(−αr2) are integrated straightforwardly in spherical coordinates. The overlap integral involving nonhomogeneous solid harmonic spherical Gaussians r(2n+1)+lYlm(θφ)exp(−αr2) has also been integrated. The results obtained are in simple analytical expressions. Within these expressions, all the magnetic quantum numbers appear only in two places—in the Clebsch–Gordan coefficients and in the spherical harmonics of the displacement vector (referring to an arbitrary frame of reference). The general analytical expression for each integral is similar to that obtained through the Talmi transformation. They become identical when explicit expressions for Talmi coefficients and numerical values for quantum numbers n and l are used.
Path-integral Monte Carlo energy and structure of the quantum hard-sphere system using efficient propagators
Authors: Sesé Luis M. , Ledesma Ricardo
Published: July 2002

Path-integral Monte Carlo simulations neglecting exchange and involving different propagators (crude, Barker’s, Jacucci-Omerti’s, and Cao-Berne’s) have been performed to study the quantum hard-sphere system at several state points ranging from the fluid to the solid phase (ρ*=0.7834; 0.1<λB*≤0.4). Energies, necklace radii of gyration and quantum pair radial distribution functions (instantaneous, linear response, self-correlation, and necklace center of mass) have been computed and compared where possible with available data. The results indicate remarkably great performances for the efficient propagators as compared with the crude choice. Even though by lowering the temperature the three efficient propagators lead to the formation of a solid phase, quantitative differences between them are significant just from that stage (λB*≥0.3).
Linear dependencies among basis set products and near locality of some nonlocal operators
Authors: Hoch Douglas E. , Harriman John E.
Published: July 2002

Linear dependencies among basis-set products and the decomposition of the matrices of some one-electron operators into local and nonlocal components have been investigated for several common quantum chemical basis sets and for a harmonic oscillator basis. For the first ten atoms and some simple diatomics, the kinetic energy, Fock, and density matrices were investigated. It was found that, for the basis sets used, these operators are all nearly local, in the sense that their matrices could be reproduced as the matrices of simple multiplicative-function operators, even though there are significant numbers of linear dependencies among the products of the basis functions. SCF eigenfunctions for these systems were found to have no linear dependencies among products. Basis sets of one-dimensional harmonic oscillator eigenfunctions were found to give a kinetic energy matrix that is not nearly local.
Polymer and vesicle conformation and scaling in elongational flow fields
Authors: Kroll D. M. , Gompper G.
Published: July 2002

The scaling behavior of polymers and low-bending-rigidity fluid vesicles in elongation flow fields is analyzed in the free draining approximation using transfer-matrix methods, polymer growth algorithms, and Monte Carlo methods. The critical behavior at the coil-stretched transition is characterized and it is shown that both polymers and vesicles exhibit new, unexpected scaling behavior in the stretched phase.
Accurate ab initio total atomization energies of the Cn clusters (n=2–10)
Authors: Martin Jan M. L. , Taylor Peter R.
Published: July 2002

The total atomization energies of the carbon clusters Cn (n=2–10) have been calculated ab initio using augmented coupled-cluster methods and basis sets of spdf and spdfg quality. The values agree to within experimental uncertainty with recent Knudsen effusion measurements for Cn (n=2–7), but suggest actual values on the high side of their experimental range. For the odd-numbered clusters, an additivity approximation holds very well, and extrapolated ∑D0 values are proposed for C11, C13, C15, C17, and C19.
Experimental and theoretical studies of hydrogen bonding in neat, liquid formamide
Authors: Ludwig R. , Weinhold F. , Farrar T. C.
Published: July 2002

Quadrupole coupling parameters (QCPs), χQ, for nitrogen, oxygen, the carbonyl deuteron, and both amide deuterons in neat, liquid formamide were measured via NMR relaxation time experiments as a function of temperature. At room temperature the experimental values obtained for nitrogen, oxygen, carbonyl deuteron, and cis and trans amide deuteron were 2.84, 9.18, 0.170, 0.280, and 0.233 MHz, respectively. These parameters and also the quadrupole coupling asymmetry parameters, ηQ, were calculated using standard ab initio self-consistent field methods at the 6-31G* level for eight different clusters of formamide molecules. The cluster sizes varied from one to six molecules and include linear and cyclic structures. At room temperature the theoretical calculations indicate that the dominant (95%) species is a six-membered ring of formamide molecules involving hydrogen bonding between the trans amide deuteron and the oxygen. This ring shows strong cooperative effects. The ab initio values obtained for the QCPs and the asymmetry parameters of this ring are 2.94 MHz and 0.394 for the nitrogen, 9.28 MHz and 0.402 for the oxygen, 0.182 MHz and 0.038 for the carbonyl deuteron, 0.285 MHz and 0.147 for the cis, and 0.221 MHz and 0.223 for the trans deuteron, respectively. These results are in excellent agreement with the experimental NMR results; both show clearly that primarily the trans amide proton is involved in hydrogen bonding. Other structural data from x-ray, electron, and neutron diffraction, low-frequency Raman and far-IR spectra are consistent with the conclusion that cyclic hexamers are the dominant species of liquid formamide under standard state conditions.
Ab initio model potential embedded cluster calculations including lattice relaxation and polarization: Local distortions on Mn2+-doped CaF2
Authors: Pascual José Luis , Seijo Luis
Published: July 2002

The ab initio model potentials initially developed as effective core potentials, have been proposed as embedding potentials in the field of embedded cluster calculations on impurities in ionic crystals [J. Chem. Phys. 89, 5739 (1988)] and, since then, efficiently used in the theoretical study of bulk and surface problems. These potentials bring into an ab initio cluster calculation, classical and quantum mechanical interactions with a frozen crystalline environment (Madelung, short-range Coulomb, exchange, and orthogonality) at a reasonable cost. In this paper, we extend the ab initio model potential embedding method in order to include the effects of dipole polarization and site relaxation of lattice ions external to the cluster, which are represented by an empirical shell model. We apply the method to the ab initio calculation of local distortions around a Mn2+ impurity in CaF2 lattice in the ground state (6A1g) and two excited states (4T1g,4A1g) of the cubic MnF6−8 embedded cluster. In this material, x-ray-absorption near-edge structure and extended x-ray-absorption fine-structure measurements exist which provide quantitative experimental values of first and second coordination shell distortions around Mn2+ in its ground state; the comparison with these experiments is very satisfactory. The calculated distortions in the excited states are indirectly validated by calculations of absorption/emission transitions.
Cavity ringdown laser absorption spectroscopy and time-of-flight mass spectroscopy of jet-cooled gold silicides
Authors: Scherer J. J. , Paul J. B. , Collier C. P. , O’Keefe A. , Saykally R. J.
Published: July 2002

The cavity ringdown technique has been employed for the spectroscopic characterization of the AuSi molecule, which is generated in a pulsed supersonic laser vaporization plasma reactor. Fifteen rovibronic bands have been measured between 340 nm–390 nm, 8 of which have been analyzed to yield molecular properties for the X and D 2Σ states of AuSi. This assignment is in disagreement with previous emission studies of AuSi, which had assigned the ground electronic state as a 2Π state. A time-of-flight mass spectrometer simultaneously monitors species produced in the molecular beam and has provided evidence for facile formation of polyatomic gold silicides. Comparison of AuSi with our recent results for CuSi and AgSi indicates regular bonding trends for the three coinage metal silicide diatoms.
Ab initio molecular dynamics of H2O adsorbed on solid MgO
Authors: Langel Walter , Parrinello Michele
Published: July 2002

The Car–Parrinello method has been applied to study the adsorption of water on solid magnesium oxide with surface defects. A step consisting of an (100) and an (010) surface on an (011) base plane allows us to model the experimentally observed microfaceting. In and on this step dissociation of water into a hydroxyl group and a H-atom took place following a complicated pathway only accessible by the simulation of thermal motion. Under comparable conditions physisorption only was observed on a regular (001) plane. This solves an experimental controversy and it is in agreement with the observation, that disordered surfaces are more active in initiating the dissociation of the water molecules. Our work allows us to identify an important active center. We can also account for the experimentally observed broadening and shifting to the red of the stretching mode of hydrogen bonded hydroxyl groups, and we provide a detailed explanation of the origin of this effect. This allows us to verify earlier theories of hydrogen bonding such as that of the adiabatic separation of the proton dynamics.
Expected significance of weakly chaotic vibrational motions in single molecule spectroscopy
Authors: Hashimoto Naoyuki , Takatsuka Kazuo
Published: July 2002

It is known that a new kind of large amplitude motion gives rise to as a very characteristic mode of weak chaos. This is essentially an unpredictable and intermittent motion taking place in a thin quasiseparatrix which wanders among several very clear vibrational modes. In this paper, we study the spectroscopic characterization of the quantum version of this large amplitude motion in terms of the dynamics of a wave packet, which is prepared in a narrow energy-range so that it is localized along a thin quasiseparatrix. In particular, we discuss possible significance of the weak chaos in single molecule spectroscopy, for which the spectra are supposed to be averaged neither in ensemble of molecules nor in time. That this wavepacket state is unusual originates from the extremely long-time behavior and the strong sensitivity to the initial condition at which the wave packet is prepared. The weak chaos combined with the statistical nature of quantum mechanics brings about a notion of unreproducibility in the spectrum. Conversely, it is anticipated that one of the distinguished features inherent to a single molecule spectroscopy manifests itself when weak chaos is observed.
Radiation damping in high resolution liquid NMR: A simulation study
Authors: Vlassenbroek A. , Jeener J. , Broekaert P.
Published: July 2002

Radiation damping generates surprising new features in two-dimensional and spin–echo experiments. The theoretical interpretation of the unexpected results has led to some controversy. The purpose of the present paper is to show, by computer simulations, that the experimental results can be completely understood by describing radiation damping through a classical magnetic field caused by the current induced in the sample coil (resonantly enhanced by the high quality factor of the tuned circuit), and acting back on the spins. We examine a variety of two-dimensional and ‘‘multiple quantum’’ experiments in homogeneous fields, and spin-echo experiments in a fixed field gradient (with and without molecular diffusion).
Laser-induced fluorescence spectroscopy of CCH (X̃ 2Σ+) in vibrationally excited levels up to 4500 cm−1
Authors: Hsu Yen-Chu , Shiu Ying-Jen , Lin Chi-Min
Published: July 2002

Vibrationally excited levels of the CCH radical in the X̃ 2Σ+ state up to 4500 cm−1 above the ground vibrational level have been studied both at high pressure and in a supersonic jet by laser-induced fluorescence (LIF). The CCH radical was produced by photolyzing acetylene with 193 nm laser light. Nineteen UV bands are rotationally analyzed and assigned as transitions of parallel type from X̃(0,υl2,υ3), (υ2=0–10, υ3=0–2, l=0–3) to four K sublevels (K=0–3) of a common vibrational level, T, of an upper electronic state most likely B̃ 2A′. Among them, bands at 37 010 and 36 075 cm−1 were previously reported [J. Chem. Phys. 98, 6690 (1993)]; an improved spin–orbit constant of X̃(0,71,0) was determined in this work. Vibrational energies of the l=0 and 1 levels were determined in a simultaneous analysis of UV and IR bands. Assignment of a vibrational level T as the common upper state of the observed LIF bands permits us to determine spectroscopic parameters of the l=2 and 3 levels of the X̃ state, which are reported for the first time. The geometry of level T at 39 157.41 cm−1 is accordingly deduced and agrees with the ab initio prediction of the B̃ state. Our vibrational assignments of the X̃ state, particularly those of bending progressions, are supported by observations of the l=2 and 3 levels.