bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement of orientational order and mobility of a nematic liquid crystal in random nanometer confinement
Authors: Cramer Ch. , Cramer Th. , Kremer F. , Stannarius R.
Published: July 2002

Confinement of a nematogen in submicrometer cavities can produce substantial effects on the nematic-isotropic phase transition and on order and orientation of the nematic phase. We combine the methods of C13- and H-NMR1 and broadband dielectric spectroscopy to probe liquid crystalline order and molecular mobility. We find that the dynamics of mesogenic molecules (5CB) in the free volume of nanoporous sol-gel glass (5 nm pore diameter) is as fast as in the free bulk phase. In addition, a boundary layer of molecules with reduced mobility covers the pore walls. Isotropic-nematic transition as well as crystallization of the confined nematogen are considerably suppressed. In order to extract the temperature dependence of the nematic order parameter from NMR data, a mathematical model is applied which describes the combined influences of orientational order and diffusional averaging in randomly oriented pores on NMR spectra of the nematogens.
A theoretical study of the HCHO+CCl2 reaction: Cycloaddition or ylide formation?
Authors: Pliego, Jr. Josefredo R. , De Almeida Wagner B.
Published: July 2002

The chemical reaction between HCHO and CCl2 was studied using a high level of theory. Geometry optimizations were performed at the complete active space self-consistent field (CASSCF) level with the 6-31G* basis set, and single point calculations were performed using the coupled-cluster with single, double, and perturbative inclusion of triple excitation [CCSD(T)] method. Additivity approximation of the correlation energy was used, so we have obtained an effective CCSD(T)/6-311G(2d1f,2p) reaction and activation energies. Our results show that the ylide exist as a stable species, having a transient character. Its lifetime in relation to ring closure was estimated to be 1 ms at 298.15 K, and it can react backward forming the HCHO and CCl2 fragments. The cycloaddition reaction is highly competitive with ylide formation, and that is the dominant process. The reaction rate of cycloaddition is about two and a half times higher than the ylide formation at 298.15 K.
Nucleation in the presence of an amphiphile: A density functional approach
Authors: Talanquer V. , Oxtoby David W.
Published: July 2002

Density functional theory is applied to the study of gas-to-liquid nucleation in binary and ternary mixtures with one amphiphilic component. The theory allows the evaluation of density profiles and molecular orientation distributions in the critical nucleus. The predicted cluster compositions for binary and ternary systems resemble qualitatively the experimental behavior for water–n-alcohol and water–n-butanol–n-nonane mixtures, respectively.
Diatomics-in-molecules description of the Rg–Hal2 rare gas–halogen van der Waals complexes with applications to He–Cl2
Authors: Grigorenko B. L. , Nemukhin A. V. , Buchachenko A. A. , Stepanov N. F. , Umanskii S. Ya.
Published: July 2002

The diatomics-in-molecules (DIM) technique is applied for a description of the low-lying states of the Rg–Hal2 van der Waals complexes correlating with the lowest states of constituent atoms Rg(1S)+Hal(2Pj)+Hal(2Pj). The important feature of this approach is the construction of polyatomic basis functions as products of the Hal2 diatomic eigenstates classified within the Hund “c” scheme and the atomic rare gas wave function. Necessary transformations to the other basis set representations are described, and finally all the matrix elements are expressed in terms of nonrelativistic adiabatic energies of Hal2 and Rg Hal fragments and spin-orbit splitting constant of the halogen atom. Our main concern is to test the DIM-based approximations of different levels taking the He–Cl2 system as an example. Namely, we have compared the results obtained within a hierarchy of approaches: (1) the simplest pairwise potential scheme as a far extreme of the DIM model, (2) the same as (1) but with the different components (Σ and Π) for He–Cl interaction, (3) the accurate DIM technique without spin-orbit terms, and (4) the highest level which takes into account all these contributions. The results have been compared to the other DIM like models as well. The shapes of two-dimensional potential surfaces for the ground (X) and excited (B) states of HeCl2, binding energies De with respect to He+Cl2, stretching and bending vibrational frequencies of the complex, binding energies D0, and spectral shifts for the B←X transition are discussed.
Analysis of salt effects on solubility of noble gases in water using the reference interaction site model theory
Authors: Kinoshita Masahiro , Hirata Fumio
Published: July 2002

We have developed robust and very efficient algorithms for solving the reference interaction site model (RISM) equations for salt solutions in the bulk and near a solute atom of noble gases. The theory of dielectric consistency recently developed for solutions at finite salt concentrations is employed in the formalism. The change in water structure in the bulk caused by addition of salts have been examined for model 1–1 salt solutions (LiCl, NaCl, KCl, KF, KBr, KI, and CsI). The density and orientational structures of each salt solution near a solute atom have been analyzed. The water model employed is the extended simple point charge (SPC/E) model. Ions characterized by positive hydration (F−, Li+, and Na+) are strongly hydrated in the bulk and stay significantly far from the atom. Those of negative hydration (Cl− and Br−) or hydrophobic hydration (Cs+ and I−) are excluded from the bulk to the atom. Due to a specific orientational order of water molecules adjacent to the solute atom, there is a trend that cations stay less closer to the atom than anions. Overall, cations indirectly affect the solubility of noble gases via the change in water structure induced by addition of those ions. On the other hand, anions affect the solubility not only indirectly but also directly by interacting with solute atoms. The agreement between the calculated and experimental values for the salting coefficient is excellent for He. However, the discrepancy becomes larger as the number of electrons of the solute atom increases (the calculated value is larger), which implies that the ion-induced dipole interaction neglected has significantly large effects.
Photoabsorption and ionization energies of nonstoichiometric CsI clusters: Metallization of a salt
Authors: Frank S. , Malinowski N. , Tast F. , Heinebrodt M. , Billas I. M. L. , Martin T. P.
Published: July 2002

Cs and CsI vapors were mixed to produce clusters having compositions varying from pure metal to the ionic salt. Ionization potential measurements on these clusters were performed using photoionization and a time-of-flight mass spectrometer. Results are reported for two metallization sequences, Cs14In for n=1–13 and CsmIn for m+n=26, 27 and m>n. The ionization potentials show a good qualitative agreement with recent computations and experiments carried out on nonstoichiometric NaF. Photoabsorption spectra are presented for CsmIn+ clusters for m+n=26, 27. The spectra were obtained by heating mass selected clusters in a beam by means of photoabsorption to induce evaporation of atoms. The resulting mass loss was observed in a time-of-flight mass spectrometer. The spectra of metal rich clusters with m−n>8 are very similar, showing one broad absorption peak around 1.42 eV, the energy of the plasmon resonance of pure Cs clusters of this size. CsmIn+ clusters with less than 7 excess valence electrons show a clear peak splitting. The shape and width of the peaks is not consistent with the picture of a homogeneous metallic cluster. In this range, a “demetallization” seems to take place.
A theoretical investigation of the spin–orbit-induced predissociation of Bar C2Δ
Authors: Sohlberg Karl , Yarkony David R.
Published: July 2002

The predissociation of the C2Δ state of Bar deduced from the laser excitation and fluorescence depletion experiments in the accompanying paper [X. Yang and P. J. Dagdigian, J. Chem. Phys. 106, 6596 (1997)] is considered theoretically. Rates for the spin–orbit-induced predissociation of Bar(C2Δ, ν) levels by the repulsive 1 4Π state are reported, based on ab initio electronic structure data. The predissociation rates reflect the heavy atom effect on the spin–orbit coupling. The rates for the higher vibrational levels are quite sensitive to the position of the “wall” of the 1 4Π state.
Refinement of the HeH2 potential surface through inversion of nuclear spin relaxation data
Authors: Lazarides A. A. , Rabitz H.
Published: July 2002

Proton spin relaxation times, T1/ρ (the slope of the longitudinal relaxation time, T1, vs density, ρ) in the infinite dilution limit as measured by Lemaire and Armstrong [J. Chem. Phys. 81, 5275 (1984)] at temperatures between 86 and 298 K are used to refine the anisotropic term of the Rodwell–Scoles HeH2 potential [J. Phys. Chem. 86, 1053 (1982)]. Prior to the refinement, a sensitivity study is performed which indicates that the data are primarily sensitive to the relative anisotropy of the repulsive wall. The inversion problem is posed as a first-order Fredholm integral equation with a kernel composed of functional sensitivities that relate potential perturbations to spin relaxation rate perturbations. The inversion scheme consists of a least-squares regularization procedure with singular system analysis and stabilization as used previously to refine atom–atom potentials [T.-S. Ho and H. Rabitz, J. Chem. Phys. 89, 5614 (1988); 90, 1519 (1989); 91, 7590 (1989)] with modifications which incorporate a priori information about the quality of the experimental data and the starting potential. The inversion yields a refined anisotropic term which in the repulsive region increases the anisotropic radial function V2(R) by approximately 10% and eliminates more than 80% of the discrepancy between the values of T1/ρ predicted by the potential and the values determined by experiment. The relative anisotropy, V2/V0, of the refined repulsive wall closely matches that of two potentials recently derived from ab initio calculation.
Universal scaling for diffusion-controlled reactions among traps
Authors: Torquato S. , Yeong C. L. Y.
Published: July 2002

The determination of the mean survival time τ (i.e., inverse reaction rate) associated with diffusion-controlled reactions among static traps is a problem of long-standing interest, dating back to the classical work of Smoluchkowski. For the broad class of model particulate- and digitized-based models considered here, we find a universal curve for the mean survival time τ for a wide range of porosities. The functional form of this universal scaling relation is motivated by rigorous bounds on τ and is expressible as a simple function of porosity, specific surface, and mean pore size.
Angular distribution of HD produced in the abstraction reaction by incident D atoms on the monohydrided Si(100)
Authors: Takamine Y. , Namiki A.
Published: July 2002

The abstraction of chemisorbed hydrogen on the Si(100) surface by atomic deuterium has been studied by direct observation of the reaction products HD. The angular distribution of the HD yield is found to be shifted towards the specular direction of the incident D atoms by about 20° with respect to the surface normal. The strong forward HD desorption is compared with the thermally associative desorption of D2 peaking towards the surface normal. A direct Eley–Rideal mechanism is suggested for the D abstraction reaction of the chemisorbed H atoms on the Si(100) surfaces.
Differential cross sections for rotationally inelastic collisions of NO(2Π1/2, j′⩽2.5) with NO(2Π1/2, j′⩽2.5) at a kinetic energy of 442 cm−1
Authors: Vonk Matthew T. , Bacon Joel A. , Giese Clayton F. , Gentry W. Ronald
Published: July 2002

We report crossed molecular beam measurements of state-resolved differential cross sections for NO+NO collisions at relative kinetic energy of 442 cm−1. The initial state (NO 2Π1/2v=0,j⩽2.5) was prepared in both beams by pulsed supersonic expansion of pure NO gas. Scattered products were detected by resonance enhanced two-photon ionization. NO product distributions were measured in both scattering angle and final rotational state. Intensity maxima were found in both types of scans, and comparable populations were found in both of the spin-orbit manifolds (2Π1/2 and 2Π3/2). The results obtained here are compared to previous NO+Ar scattering experiments and theory, NO dimer studies, and NO+NO bulb kinetics experiments.
Charge penetration in dielectric models of solvation
Authors: Chipman Daniel M.
Published: July 2002

Dielectric continuum models are widely used for treating solvent effects in quantum chemical calculations of solute electronic structure. These invoke a reaction field wherein solute-solvent electrostatic interactions are explicitly or implicitly described by means of certain apparent polarization charges. Most implementations represent this polarization through an apparent surface charge distribution spread on the boundary of the cavity that nominally encloses the solute. However, quantum chemical calculations usually lead to a tail of the wave function penetrating outside the cavity, thereby causing an additional volume polarization contribution to the reaction field that is rarely recognized or treated. In principle the volume polarization should be represented by a certain apparent volume charge distribution spread throughout the entire dielectric medium. It is shown here that this effect can be closely simulated by means of a certain additional apparent surface charge distribution. This provides a convenient and efficient route to treat volume polarization in practice. A very simple approximation to this correction can be obtained from knowledge only of the amount of penetrating solute charge. This supplies a theoretical context as well as justification for the concept of surface charge renormalization that some workers have advocated. The analysis also points to a new prescription for properly making this renormalization in practice, improving on various ad hoc procedures that have been previously suggested for this purpose.
Crossed-beam reaction of carbon atoms with hydrocarbon molecules. III: Chemical dynamics of propynylidyne (l-C3H;X 2Πj) and cyclopropynylidyne (c-C3H;X 2B2) formation from reaction of C(3Pj) with acetylene, C2H2(X 1Σg+)
Authors: Kaiser R. I. , Ochsenfeld C. , Head-Gordon M. , Lee Y. T. , Suits A. G.
Published: July 2002

The reaction between ground state carbon atoms, C(3Pj), and acetylene, C2H2(X 1Σg+), is studied at three collision energies between 8.8 and 45.0 kJ mol−1 using the crossed molecular beams technique. Product angular distributions and time-of-flight spectra of C3H at m/e=37 are recorded. Forward-convolution fitting of the data yields weakly polarized center-of-mass angular flux distributions decreasingly forward scattered with respect to the carbon beam as the collision energy rises from 8.8 to 28.0 kJ mol−1, and isotropic at 45.0 kJ mol−1. Reaction dynamics inferred from the experimental data and ab initio calculations on the triplet C3H2 and doublet C3H potential energy surface suggest two microchannels initiated by addition of C(3Pj) either to one acetylenic carbon to form s-trans propenediylidene or to two carbon atoms to yield triplet cyclopropenylidene via loose transition states located at their centrifugal barriers. Propenediylidene rotates around its B/C axis and undergoes [2,3]-H-migration to propargylene, followed by C–H bond cleavage via a symmetric exit transition state to l-C3H(X 2Πj) and H. Direct stripping dynamics contribute to the forward-scattered second microchannel to form c-C3H(X 2B2) and H. This contribution is quenched with rising collision energy. The explicit identification of l-C3H(X 2Πj) and c-C3H(X 2B2) under single collision conditions represents a one-encounter mechanism to build up hydrocarbon radicals in the interstellar medium and resembles a more realistic synthetic route to interstellar C3H isomers than hitherto postulated ion–molecule reactions. Relative reaction cross sections to the linear versus cyclic isomer correlate with actual astronomical observations and explain a higher [c-C3H]/[l-C3H] ratio in the molecular cloud TMC-1 (≈1) as compared to the circumstellar envelope surrounding the carbon star IRC+10216 (≈0.2) via the atom-neutral reaction C(3Pj)+C2H2(X 1Σg+).
Local parabolic reference approximation of thermal Feynman path integrals in quantum Monte Carlo simulations
Authors: Chao Cecilia E. , Andersen Hans C.
Published: July 2002

We have developed a new propagator, called the local parabolic reference (LPR), for use in the numerical evaluation of discretized Feynman path integrals by Metropolis Monte Carlo simulations. The form of the propagator is motivated by fitting a local quadratic reference potential (with positive, negative or zero curvature) to the potential energy surface of interest, and constructing the exact propagator for this reference potential. The final form of the propagator contains adjustments designed to eliminate artifacts that can develop at very low temperatures. In the low temperature regime, the approximation accommodates tunneling and zero-point motion with a small number of discretization points in the path integral. In the limit of high temperature, the LPR propagator approaches the form of the standard high temperature propagator. Both the single- and multi-dimensional formulations are discussed in this paper. The accuracy of the Monte Carlo path integrals is demonstrated in the calculation of the equilibrium average potential energies for a set of model systems with one degree of freedom, and for a system of ten coupled double-well oscillators. Also, for a one-dimensional quartic oscillator system, the LPR approximation results are compared with those of the approximations of Messina, Garrett and Schenter [J. Chem. Phys. 100, 6570 (1994)], Mak and Andersen [J. Chem. Phys. 92, 2953 (1990)], and Zhang, Levy and Freisner [Chem. Phys. Lett. 144, 236 (1988)]. It is anticipated that this approach to constructing propagators will be useful for multi-dimensional barrier-crossing problems.
Scattering functions for multicomponent mixtures of charged hard spheres, including the polydisperse limit: Analytic expressions in the mean spherical approximation
Authors: Gazzillo Domenico , Giacometti Achille , Carsughi Flavio
Published: July 2002

We present a closed analytical formula for the scattering intensity from charged hard sphere fluids with any arbitrary number of components. Our result is an extension to ionic systems of Vrij’s analogous expression for uncharged hard sphere mixtures. Use is made of Baxter’s factor correlation functions within the mean spherical approximation (MSA). The polydisperse case of an infinite number of species with a continuous distribution of hard sphere diameters and charges is also considered. As an important by-product of our investigation, we present some properties of a particular kind of matrices (sum of the identity matrix with a dyadic matrix) appearing in the solution of the MSA integral equations for both uncharged and charged hard sphere mixtures. This analysis provides a general framework to deal with a wide class of MSA solutions having dyadic structure and allows an easy extension of our formula for the scattering intensity to different potential models. Finally, the relevance of our results for the interpretation of small angle neutron scattering experimental data is briefly discussed.
Conical intersections between the two lowest A′1 potential energy surfaces of HCN, and the role of three-body effects
Authors: Varandas A. J. C. , Voronin A. I. , Jimeno P.
Published: July 2002

We report detailed evidence on the crossing and avoided crossing between the X̃ 1A′ and 2 1A′ potential energy surfaces of HCN from ab initio full valence complete active space calculations. The role of three-body effects on explaining the crossing seam and the implications of the latter on reaction dynamics are also briefly discussed.
Photodissociation dynamics of OClO: O(3PJ) state and energy distributions
Authors: Roth Melanie , Maul Christof , Gericke Karl-Heinz
Published: July 2002

The photodissociation dynamics of OClO in the near ultraviolet (UV) was investigated in a state specific and energy selective manner. At a dissociation wavelength of 308 nm, OClO(X̃ 2B1) was excited to the OClO(Ã 2A2(18,0,0)) state, from which it decays into ClO(X 2Π3/2,1/2) and O(3P2,1,0). The nascent oxygen fragments were detected spin selectively by resonant enhanced multiphoton ionization and time of flight measurements (REMPI-TOF). Based on the measurements and the conservation of energy and linear momentum, the internal energy of the ClO partner fragment was obtained. On average, more than 60% of the available energy is transferred into internal excitation of the ClO radical. Nearly the whole internal energy is vibrational energy with vibrational levels populated up to the energetic limit. Besides, the internal energy depends on the oxygen spin-orbit state because the fraction of highly excited ClO fragments increases with increasing total angular momentum J. The bimodal behavior of the fragment energy distribution indicates two different dissociation pathways, in which one leads to ClO radicals excited up to v=10 and the other one up to v=15. Furthermore, the decay is anisotropic, which was proved by polarization experiments. This is a hint for a short decay time estimated to be in the order of a few hundred femtoseconds.
Comment on “Heterogeneous two-dimensional nucleation and growth kinetics” [J. Chem. Phys. 106, 1870 (1997)]
Authors: Nelson Jon
Published: July 2002

Some comments concerning heterogeneous two-dimensional nucleation on crystal surfaces are made: the similarity to three-dimensional heterogeneous nucleation, the fate of the nuclei, a possible condition for kinetic roughening on dirty crystal surfaces, and recurring nucleation at favored nuclei.
Effect of shear on order–disorder and order–order transitions in block copolymers
Authors: Huang Chien-Yueh , Muthukumar M.
Published: July 2002

We have studied theoretically the role of an external constant shear on the extent of suppression of composition fluctuations and stabilization of different ordered morphologies in block copolymer systems. The dependence of steady state structure factor on constant shear is calculated self-consistently in the weak segregation limit. The effect of shear rate on order–order and order–disorder transitions involving lamellar and cylindrical morphologies is computed. Previous theoretical work calculated effects to the lowest order in shear rate. This paper performs a numerical calculation that is not limited to small shear rates. This more general approach is required in order to explain experimental results. Our results are in good agreement with known experimental data and make new quantitative predictions. The controversy involving an apparent discrepancy between experimental data and previous predictions from perturbation analysis is resolved.
Atomistic studies of O2 dissociation on Pt(111) induced by photons, electrons, and by heating
Authors: Stipe B. C. , Rezaei M. A. , Ho W.
Published: July 2002

The adsorption and subsequent dissociation of O2 on Pt(111) was studied by variable temperature scanning tunneling microscopy in the temperature range of 40 to 215 K. Tight clustering of bridge site molecules is observed on terraces between 40 and 70 K, indicating a highly mobile precursor to chemisorption. Coexistence of bridge and fcc hollow site molecules in fractal-shaped islands is observed after dosing between 70 and 95 K. Dissociation of these species was induced by uv radiation, inelastic tunneling electrons, and heating. In all three cases, two O atoms are found within two lattice constants of the original molecule and one to three lattice constants apart.
Control of chemical reaction pathways by femtosecond ponderomotive forces: Time-resolved multiphoton ionization spectroscopic study of OCIO photodissociation
Authors: Blackwell M. , Ludowise P. , Chen Y.
Published: July 2002

Femtosecond time-resolved multiphoton ionization spectroscopy is applied to the study of the photodissociation of OClO. The observed ratio of O2+/ClO+ signal increases 12-fold with a 3-fold increase of the pump laser intensity. They are attributed to the change in the branching ratio between the two independent reaction channels leading to Cl+O2 and ClO+O, respectively. We believe this is the first experimental demonstration of laser controlled chemical reactions by femtosecond ponderomotive forces. At low pump power, the photodissociation dynamics at 386 nm is shown to be a two-step process, with the OClO slowly approaching (time constant 4.6 ps) a transition state that falls apart rapidly (time constant 250 fs).
Hydrophobic interaction of tetrapropylammonium ions in water: A neutron diffraction and reverse Monte Carlo study
Authors: Polydorou N. G. , Wicks J. D. , Turner J. Z.
Published: July 2002

Neutron diffraction measurements from 2.4 mole% solutions of tetrapropylammonium bromide (TPrABr), (C3H7)4NBr, in water, using hydrogen/deuterium isotope substitution on the alkyl groups of the cation, is used to investigate the distribution of cations in the solution. The intermolecular partial pair correlation function for the alkyl group hydrogens is obtained, which shows a broad peak with a maximum at about 11 Å, with structure in the 2.5–4 Å region, suggesting that some of the alkyl chains of neighboring ions may not be separated by water molecules. The three-dimensional structure is modeled using the reverse Monte Carlo method to fit the HH partial structure factor. The simulation results show that at least half of the cations are not separated by water from one other cation and that a few clusters of three or four cations also occur. The close contact found between the TPrA+ ions is discussed in relation to previous results on the structure of water in TPrABr solution at the same concentration.
The Cotton-Mouton effect of liquid water. Part I: The dielectric continuum model
Authors: Ruud Kenneth , Helgaker Trygve , Rizzo Antonio , Coriani Sonia , Mikkelsen Kurt V.
Published: July 2002

We present a gauge-origin independent method for calculating the electric-field dependence of the molecular magnetizability—that is, the hypermagnetizability, related to the Cotton–Mouton Effect (CME)—of solvated molecules. In our approach, the solvated molecule is placed in a spherical cavity surrounded by a linear, homogeneous, and polarizable dielectric medium. We apply the model to investigate the dielectric-medium effects on the CME of liquid water. The effects of electron correlation, molecular geometry, and the surrounding dielectric continuum on the hypermagnetizability and the CME are investigated. The change induced in the hypermagnetizability anisotropy by the dielectric medium is the dominating effect, being almost twice as large as the correlation contribution. The combined effect of electron correlation and the dielectric continuum leads to a doubling of the hypermagnetizability anisotropy when going from the SCF gas phase value (Δη=17.89 a.u.) to the value obtained for the MCSCF wave function in the dielectric medium (Δη=39.74 a.u.). The effects of change in geometry are shown to be small. Our result for the static Cotton–Mouton constant averaged in the temperature range 283.15 K to 293.15 K, C=15.2×10−201  G−2 cm3 mol−1, differs from experiment still by the sign and by a factor of almost 8. The major reason for this discrepancy is the neglect of short-range interactions such as hydrogen bonding and van der Waals interactions not accounted for by the continuum model.
Optical–optical double resonance spectroscopy: The G̃ 2Π state of CaOH and CaOD
Authors: Hailey R. A. , Jarman C. , Bernath P. F.
Published: July 2002

The optical–optical double resonance spectra of CaOH and CaOD have been recorded with continuous wave dye lasers using a Broida oven. A new Rydberg electronic state, the G̃ 2Π state, has been located.
Effect of the Donnan osmotic pressure on the volume phase transition of hydrated gels
Authors: Sasaki Shigeo , Maeda Hiroshi
Published: July 2002

A theory to describe the effect of the Donnan osmotic pressure on the volume phase transition of hydrated gels is presented by extending the previous theory. The theory takes into consideration a coupling of the volume change affected by the Donnan osmotic pressure and the dehydration of the gel chain. The theory reveals that the swelling force suppresses the dehydration and diminishes the volume change at the transition. When applied to a model accounting for the volume phase transition of N-isopropylacrylamide (NIPA) gel, the theory predicts that the transition disappears as the Donnan osmotic pressure increases.
Nonlinear optical response of cofacial phthalocyanine dimers and trimers
Authors: Manas Eric S. , Spano Frank C. , Chen Lin X.
Published: July 2002

The effects of intermacrocycle interactions on the second hyperpolarizabilities 〈γ(−ω;ω,−ω,ω)〉 of cofacial phthalocyanine dimers and trimers are studied. A theoretical analysis is presented based on the Frenkel exciton model for a chain of three level molecules. Using a simplified analysis in the static and near-resonant regimes we identify two mechanisms which lead to enhancements in the dimer or trimer value of 〈γ(−ω;ω,−ω,ω)〉 over that of the monomer. The first mechanism is a disruption of the balance between type I and type II terms in the sum over states expression for the second hyperpolarizability tensor γkjih(−ω;ω,−ω,ω), caused by weak intermacrocycle interactions. The second is a near-resonance enhancement of the type II terms due to an intermacrocycle interaction induced shift in the monomer derived two-photon allowed states towards twice the laser photon energy. This analysis is in good agreement with recent degenerate four wave mixing experiments [SPIE Proc. 2527, 61 (1995)] which showed a strong enhancement of 〈γ(−ω;ω,−ω,ω)〉 for SiPcO oligomers as a function of the number of macrocycles. Our calculations suggest that the first mechanism is responsible for the 25-fold monomer to dimer enhancement measured in this system, and that the additional 4-fold enhancement found in going from the dimer to the trimer is primarily the result of the second mechanism.
Steric interactions between two grafted polymer brushes
Authors: Ruckenstein Eli , Li Buqiang
Published: July 2002

A lattice model and a generator-matrix method are employed to calculate the interaction force profile between two grafted polymer brushes. The correlation between neighboring bonds and the interdigitation between the two brushes are taken into account. The calculations show that the effect of incorporating the bond correlations is equivalent to an increase in the value of the polymer–solvent interaction parameter when the bond correlations are ignored. The interdigitation between the two brushes decreases the free energy of the system and consequently results in a smaller steric repulsion. A complete interdigitation occurs at a separation close to half the separation between the two plates for which the interaction force is zero. The model is compared with the experimental interaction force profiles for ten systems which involve poly(2-vinylpyridine)-polyisoprene (PVP-PI), poly(2-vinylpyridine)-polystyrene (PVP-PS) block copolymers as well as end-functionalized polystyrenes (PS-X). For most of the systems, the theoretical predictions are in good agreement with experiment. In addition, the present results are compared with the equations proposed by de Gennes, based on the assumption of a step distribution function for the segment density, and by Milner et al., based on the parabolic distribution of the segment density. Both equations neglected interdigitation. It is shown in this paper that the interdigitation is not negligible and that it can decrease by an order of magnitude the repulsive force.
Low-energy excitations in noncrystalline arsenic trioxide
Authors: Yannopoulos S. N. , Papatheodorou G. N. , Fytas G.
Published: July 2002

The liquid to glass transition for arsenic trioxide has been studied by performing low-frequency Raman scattering in the temperature range 300–770 K. For this glass forming system the light scattering intensity ratio of the relaxational (ω<15 cm−1) to the vibrational (Boson peak) contribution is much higher than that expected for low fragility systems near the glass transition temperature. A possible explanation for this unexpected feature is given by employing a structural model that is also able to interpret the hypersound propagation and absorption peculiarities found previously in this system. The depolarization ratio, in the quasi-elastic frequency region for different oxide glasses, is associated with local microstructural transformations causing fast relaxations in these systems. The Boson peak region is discussed in the context of different models and current theoretical approaches for the glass transition.
Density dependence of the conduction-band minimum in fluid krypton and xenon from field ionization of (CH3)2S
Authors: Altmann K. N. , Reininger R.
Published: July 2002

The ionization potential distribution of (CH3)2S doped in fluid krypton and xenon has been determined by means of field ionization in wide density ranges. These distributions are simulated by convoluting the field spectrum of pure (CH3)2S with the calculated polarization energy distribution of the dopant ion in the media. The density dependence of the conduction-band energy of excess electrons (V0) in the fluids is obtained from the energy shift between the experimental and simulated spectra. The resulting V0 values are compared with previous experimental results and several theoretical calculations.
The NO(a 4Π) state: Collisional removal of v=11 and a 4Π–B 2Π interactions
Authors: Copeland Richard A. , Dyer Mark J. , Bloemink Hannelore I. , Slanger Tom G.
Published: July 2002

Vibrationally excited levels of the a 4Π state, the lowest electronically excited state of the NO molecule, can be studied through their collisional energy transfer to the radiating B 2Π state. Following one-photon excitation in the 11–0 band of the forbidden a 4Π–X 2Π transition with the output of a tunable dye laser, we monitor the temporal evolution of the emission from the v=2 level of the B 2Π state. From pressure-dependence analysis, we determine the removal rate coefficients for a 4Π(v=11) with NO, O2, N2O, N2, CO2, He, and Ar. In addition, using direct B 2Π excitation of the v=2, 3, 4 levels, we obtain removal rate coefficients for the same colliders. These vibrational levels span the region around a 4Π(v=11). In general, a 4Π removal is quite fast, ranging from rate coefficients of 1.0×10−12 cm3 s−1 to more than 1.0×10−10 cm3 s−1, and is substantially faster than B 2Π removal for N2, N2O, CO2, and Ar. NO(B 2Π, v=3) removal is generally much faster than that for the v=2 and v=4 levels, because of a localized perturbation between the v=12 level of the a 4Π state and v=3 of the B 2Π state. Preliminary measurements on transfer between the b 4Σ− and B 2Π states are reported.
Exact quantum stereodynamics: The steric effect for the Li+HF→LiF+H reaction
Authors: Alvariño J. M. , Aquilanti V. , Cavalli S. , Crocchianti S. , Laganà A. , Martı́nez T.
Published: July 2002

To study the stereodynamics of atom diatom reactions, we apply the formalism developed by Aquilanti et al. [J. Phys. Chem. 95, 8184 (1991)]. As a case study the prototype Li+HF→LiF+H reaction at zero total angular momentum J is considered. For this reaction we calculated the scattering SJ-matrix in the standard |lj〉 representation and transformed it into a stereodirected representation. In this way it is possible to investigate the effect on the reaction probability of the orientation of the target HF molecule with respect to Li attack. In the investigated collision energy range (0.45–0.54 eV) propensity is found for Li attack on the side of H atom.
Four mathematical dimensional quantum mechanical studies of tetra-atom systems: State-to-state J=0 probabilities for the H2+OH→H2O+H reaction
Authors: Szichman Henrik , Baer Michael , Nakamura Hiroki
Published: July 2002

This paper presents reactive state-to-state J=0 probabilities for the title system as obtained in a four-mathematical-dimensional quantum mechanical treatment. The present treatment differs from our previous one by the fact that in addition to the three Jacobi radial coordinates also the angular coordinate related to the H2 axis is treated as coordinates. As a result only the Jacobi angle related to the OH axis is treated as a parameter and the final probabilities follow from an integration over this angle (the out-of-plane angle, β, is eliminated by using a β-averaged potential). The calculations yielded final rotational and vibrational distributions that were analyzed and discussed with respect to more accurate (i.e., five- and six-mathematical-dimensional) results.
Inverse perturbation via singular value decomposition: Application to correction of potential surface for HCN
Authors: Wu Qian , Zhang John Z. H. , Bowman Joel M.
Published: July 2002

We report a new potential energy surface (PES) for HCN which is able to exactly reproduce experimentally measured eigenvalues for total angular momentum of J=0 and 1. The new PES is obtained by adding a correction ΔV to a previous PES of Bowman et al., and the correction ΔV is determined through a direct inversion technique based on first-order inverse perturbation via singular value decomposition as described in a previous publication. Since the PES of Bowman et al. is a good starting PES, the inversion calculation is relatively straightforward. A total of 51 and 20 experimental eigenvalues, respectively, for J=0 and J=K=1, including both the H-CN and H-NC configurations, are used in the inversion calculation. Absolutely accurate eigenvalues (maximum deviation of less than 0.01 cm−1 from experimental values) on the new PES are obtained. Since the potential correction ΔV is expressed in a basis set, the corrected new PES is an analytical function of the coordinates. The contours of the new PES and of the potential correction ΔV are plotted to show locations of the correction to the PES of Bowman et al.
Size-effects on energy relaxation and excited-species desorption in krypton clusters: Fluorescence lifetime measurements with 10 eV laser excitation
Authors: Kanaev A. V. , Museur L. , Castex M. C.
Published: July 2002

Fluorescence lifetime measurements of KrN clusters (N̄=2–2000) have been carried out using intense 10 eV laser excitation near 3P2 metastable atomic energy level. Two principal groups of electronically excited dimers Kr2* have been found in desorption: dimers, loosely bound near the (3P2+1S0) dissociation limit, ejected from cooled clusters and dimers undergoing vibrational relaxation from hot clusters. The desorption is principally terminated when N̄⩾50 at./cluster. The relaxation kinetics seems to converge to the properties of a solid state for 102⩽N̄⩽103 at./cluster. A variation of the Kr2*(1u/0u−) radiative lifetime, from 264 ns (in gas phase) to 440 ns (N̄=102), has been found. An equilibrium cluster temperature of 57 K has been calculated from this τ(N) dependence.
The potential surface for the three methyl rotations in the tertiary-butyl cation, (CH3)3C+
Authors: East Allan L. L.
Published: July 2002

The intricate symmetry of the potential energy surface for the three methyl rotation dimensions in the tertiary-butyl cation, (CH3)3C+, is examined with the presentation of analytic three-dimensional potential functions for the first time. The simplest function is a four-parameter model which is reminiscent of of a form commonly used in the studies of two-rotor systems. This function, arising purely from symmetry and proper Fourier series truncation, dictates the existence of four “canonical” stationary-point conformations, whose ab initio energies are sufficient to determine its parameter values. The simplicity of the surface leads to mirror-image relations concerning the normal mode curvatures at the canonical stationary points, and these relations are shown to be approximately valid on 33-dimensional and nonanalytic ab initio surfaces. A 14-parameter function is also developed, and values for its parameters were determined from fitting to ab initio energies and tested against normal mode frequencies at the canonical locations. Kinetic (inertial) terms are presented as well. To consider the neglected coupling of the pure torsional coordinates with other internal coordinates along the internal rotation pathways, total energy distribution (TED) analyses of ab initio harmonic force fields were performed, and contributions of from 5% to 40% from other internal coordinates were observed in the internal rotation normal modes at various stationary points. This suggests that further improvement of three-dimensional functions beyond those presented here may be of limited use without incorporation of bending potential terms.
Solvation dynamics and electronic structure development of coumarin 120 in methanol: A theoretical modeling study
Authors: Ando Koji
Published: July 2002

Electronic structure evolution of an organic dye coumarin 120 coupled to polar solvation dynamics is examined by combining ab initio electronic structure calculations and molecular dynamics (MD) simulations. Sets of nonorthogonal Hartree–Fock molecular orbitals optimized in vacuo and in dielectric continuum are utilized for a quantum mechanical description of the solute electronic polarization coupled to the solvent fluctuation. The adiabatic MD simulation for methanol solution is performed to evaluate the equilibrium and nonequilibrium dynamics of the (S0–S1) energy gap coordinate and the dipole moments. The absorption and fluorescence spectra are computed via the spectral density functions obtained from the simulation analysis. The results for the quantum polarizable (Q-Pol) model of the solute probe are compared with those for a nonpolarizable fixed-charge (Fix-Z) model. It is shown that the solute electronic polarization notably affects the solvent-induced key quantities such as the reorganization energy, the spectral linewidth, and the Stokes shift (which are mutually related): For example, the computed Stokes shifts are ∼2500 and ∼4000 cm−1 for Fix-Z and Q-Pol, respectively. On the other hand, the solute polarization tends to slightly slow down the methanol solvation, which is not necessarily attributed to reduction of the “solvent force constant” because the effective mass of the coordinate is reduced as well.
Investigating intramolecular dynamics: Hierarchical analysis and the topography of energy flow
Authors: Davis Michael J.
Published: July 2002

Methods for understanding the global nature of intramolecular dynamics are developed. A complete picture of the intramolecular dynamics is inferred from the detailed study of a small set of initially localized wave packets chosen from a much larger set. The smaller set is chosen based on a topographic map generated from the spectral decompositions of the wave packets in the larger set. The hierarchical decomposition of the wave packets in the smaller set is used to understand their often complicated dynamics. The combination of the topographic map and the hierarchical analysis, in conjunction with some other standard techniques, allows for a complete picture of the energy flow.
Relationship between spectral intensities and nonlinear optical properties
Authors: Lee Jin Yong , Kim Kwang S.
Published: July 2002

The relationship between hyperpolarizability (β) and spectral intensities has been investigated for polyene (or oligoene) systems. The vibrational polarizability, first hyerpolarizability, and second hyperpolarizability have been derived for the geometrical and electrical potentials with anharmonic terms included. This relationship has been tested by performing ab initio calculations on polyene derivative systems of X-(HC=CH)n−Y, where 1⩽n⩽5 for X/Y=H/NH2, H/NO2, and NH2/NO2. Particular attention has been given to the first hyperpolarizability. The electronic hyperpolarizability shows a linear relationship with the square root of the product of IR and Raman intensities of the concurrent C=C stretching vibrational mode. The spectral intensities represent the vibrational hyperpolarizabilities which are of equivalent magnitude to the electronic hyperpolarizabilities. In the presence of an external electric field, the C=C bond stretches are responsible for the electronic hyperpolarizability, while the effects by other bond stretches are not significant. Thus the hyperpolarizability can be estimated by obtaining the IR and Raman intensities of C=C stretching vibration. This relationship would be valid for poly π-conjugated molecules.
Extracting the dynamics in classically chaotic quantum systems: Spectral analysis of the HO2 molecule
Authors: Main Jörg , Jung Christof , Taylor Howard S.
Published: July 2002

We present a scaling technique to analyze quantum spectra, i.e., to obtain from quantum calculations detailed information about the underlying important classical motions. The method can be applied to a general quantum system without a classical scaling property. A demonstration on the conventionally unassignable vibrational spectrum of the HO2 radical reveals remnants of classical broken tori embedded in the chaotic phase space and leads to a new assignment of spectral patterns in terms of classical Fermi resonances between the local mode motions. The scaling technique also allows to investigate the statistical properties of level spacings at fixed energies. The nearest neighbor spacing distribution of the HO2 molecule undergoes a transition from mixed phase space behavior at low energies to the Wigner distribution characteristic for chaotic systems at energies near the dissociation threshold.
An orbital-based density difference index for the comparison of electron density distributions
Authors: Worsnop S. Kent , Wang Jiahu , Boyd Russell J.
Published: July 2002

A new semiquantitative measure of the difference between two electron density distributions is proposed. Specifically, the natural orbitals of an accurate electron density, such as those given by a configuration interaction method, are used to expand the Kohn–Sham orbitals of an approximate density functional method. The difference between the two density distributions is then reduced to a single number, the orbital-based density difference index (DDI). With the reference densities calculated from quadratic configuration interaction calculations including single and double substitutions, DDIs were obtained for four diatomic molecules using three basis sets and seven approximate functionals. Results are also included for an additional six small molecules with a flexible extended basis set and the same set of functionals. These results show that the DDI leads to conclusions that are consistent with those obtained previously by visual comparison of density difference plots. The orbital-based density difference indices are very dependent on the choice of the functional, but are fairly stable with respect to the choice of the basis set.
Closures for exact relations between mixture pair correlation integrals
Authors: Hamad Esam Z.
Published: July 2002

A new approach for obtaining mixture properties is proposed. It involves combining recently derived exact equations relating integrals of the mixture pair correlation functions with approximate closures. A partial differential equation for the partition function results. This equation is integrated using an appropriately selected initial condition. The procedures are illustrated for additive and nonadditive hard spheres, and new mixture equations are obtained. Comparison with simulation data show that the accuracy of the predicted properties depends on the level of approximation introduced by the closure. Certain mixture equations gave accurate representation of the equation of state and the phase boundaries of nonadditive hard spheres. In applications where the structure of the fluid is not needed, the proposed approach is better than traditional approaches, because it gives accurate analytical expressions for mixture properties and for certain integrals of the pair correlation functions.