bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotational spectra in the ν2 vibrationally excited states of MgNC
Authors: Kagi E. , Kawaguchi K. , Takano S. , Hirano T.
Published: July 2002

The pure rotational spectra of MgNC in the ν2 (bending) vibrationally excited states were observed in the 310–380 GHz region to study the linearity of the molecule. The observed 90 spectral lines were assigned to the transitions in the v2=1–5 states and analyzed to determine a set of molecular constants in each state. The bending vibrational frequency was estimated to be 86 cm−1 from the l-type doubling constant of the v2=1 state. The interval of the Φ and Π states in v2=3 was determined to be 29.2280(24) cm−1, giving the anharmonicity constant xll=3.8611(9) cm−1 with one standard deviation in parentheses, which indicates that the molecule has a linear form. However, somewhat peculiar properties were recognized in dependence of the observed l-type resonance and vibration-rotation constants on the v2 vibrational quantum number, suggesting an effect of anharmonicity.
The behavior of grafted polymers in restricted geometries under poor solvent conditions
Authors: Gross Nicholas A. , Zhulina Ekaterina , Balazs Anna C.
Published: July 2002

We determine the behavior of chains that are grafted in restricted geometries, such as a planar stripe of finite width and around a circular pore. Immersed in a poor solvent, the chains associate into a variety of complexes. By calculating the free energy of these complexes, we determine how their sizes scale with chain length, solvent quality, grafting density, and the dimensions of the substrate. We also construct a ‘‘phase diagram’’ for the possible morphologies that can occur in these systems. In the case of circular pores, we use Monte Carlo simulations to visualize the structure of the associated complexes. The results reveal how edge effects can be exploited to design polymer layers that provide protective coatings or regulate the flow of molecules through the substrate.
Pure vibrational spectroscopy of S0 formaldehyde by dispersed fluorescence
Authors: Bouwens Rychard J. , Hammerschmidt Jon A. , Grzeskowiak Martha M. , Stegink Tineke A. , Yorba Patrick M. , Polik William F.
Published: July 2002

Dispersed fluorescence spectra from the 000 rotational level of 40, 41, 51, and 3141 S1 formaldehyde (H2CO) have been recorded. From these spectra, 198 new vibrational states have been assigned with energies up to 12 500 cm−1, and their positions have been determined to within an uncertainty of 1 cm−1. The assignment of vibrational lines to specific vibrational states becomes increasingly difficult at the higher energy regions of the spectra (≳9000 cm−1) due to extensive state mixing. Harmonic and first-order anharmonic vibrational constants were extracted from fits to these vibrational states. For states with highest zero-order coefficient squared greater than 35%, the standard deviation of the spectroscopic fit is 6.9 cm−1. For states which are lower energy (<9500 cm−1) and relatively pure (zero-order coefficient squared greater than 0.75 or largest in a given normal mode combination), the standard deviation is 1.7 cm−1. Good agreement with ab initio vibrational constants calculated by Martin et al. [J. Mol. Spectrosc. 160, 105 (1993)] is achieved, except in cases where all observed states contributing to the determination of a particular constant are significantly mixed. These deviations are readily explained by a consideration of anharmonic vibrational interactions that occur among specific combinations of normal modes. The average mean deviation between all experimentally determined energies and a recent theoretical calculation by Burleigh et al. [J. Chem. Phys. 104, 480 (1996)] is 2.6 cm−1.
Short-range approach to an A+BC collision complex with a contribution of an ionic state A++BC−: Application to A=H
Authors: Dubov Valery , Rabitz Herschel
Published: July 2002

An effective quasi-one-electron asymptotic technique is developed for treating the electronic part of A+BC molecular interaction. The theory proceeds from a model of the motion of an electron in the field of A+ and BC. The interaction between the electron and the ion A+ is handled by a Green function technique while the interaction of the electron with BC is modeled by a short-range approach. The theory provides the electronic wave functions, potential surfaces, and transition dipole moments as functions of three nuclear coordinates specifying the geometrical configuration of the A+BC system. Calculations of potential surfaces are carried out for the case in which A is a hydrogen atom. The main advantage of the theory resides in the fact that the full structure of the potential surfaces and that of transition dipole moments are expressed in terms of a few physical parameters. This feature, along with the ease of computation, turns the theory into a convenient tool for solving the electronic part of molecular dynamics problems for many laser assisted collisions and reactions. The theory significantly extends the method proposed earlier [V. S. Dubov, J. Chem. Phys. 97, 7342 (1992)].
Canonical collective coordinates of electron transfer based on time-dependent variational principle
Authors: Ohta Katsuhisa
Published: July 2002

Electron transfer reactions have been studied as the time development of many-electron wave functions based on the time-dependent variational principle (TDVP). The equation of motion (EOM) of the pseudoclassical mechanics, which is described with canonical coordinates, has been derived by introducing a new trial wave function of the TDVP in the polar form. Based on the EOM, it has been shown that the transition state of the electron transfer corresponds to the saddle point at which an unstable broken-symmetry wave function occurs in the variational method for stationary states. By the maximal decoupling condition, canonical collective coordinates of the electron transfer have been separated in the TDVP–EOM with canonical variables. A simple example has been given for a symmetric electron transfer reaction in H3 by using the time-dependent cluster expansion of wave function.
A selected ion flow tube study of the reactions of NO+ and O+2 ions with some organic molecules: The potential for trace gas analysis of air
Authors: Španěl Patrik , Smith David
Published: July 2002

A study has been carried out using our selected ion flow tube apparatus of the reactions of NO+ and O+2 ions in their vibronic ground states with ten organic species: the hydrocarbons, benzene, toluene, isoprene, cyclopropane, and n-pentane; the oxygen-containing organics, methanol, ethanol, acetaldehyde, acetone, and diethyl ether. The major objectives of this work are, on the one hand, to fully understand the processes involved in these reactions and, on the other hand, to explore the potential of NO+ and O+2 as chemical ionization agents for the analysis of trace gases in air and on human breath. Amongst the NO+ reactions, charge transfer, hydride-ion transfer, and termolecular association occur, and the measured rate coefficients, k, for the reactions vary from immeasurably small to the maximum value, collisional rate coefficient, kc. The O+2 reactions are all fast, in each case the k being equal to or an appreciable fraction of kc, and charge transfer producing the parent organic ion or dissociative charge transfer resulting in two or three fragments of the parent ion are the reaction processes that occur. We conclude from these studies, and from previous studies, that NO+ ions and O+2 ions can be used to great effect as chemical ionization agents for trace gas analysis, especially in combination with H3O+ ions which we now routinely use for this purpose.
Many-body similarity transformations generated by normal ordered exponential excitation operators
Authors: Nooijen Marcel
Published: July 2002

Normal ordered exponential operators have been used extensively in open-shell formulations of coupled cluster theory. The inverse of such an operator is known to exist, but a closed form explicit expression for the inverse is not available. We will address the evaluation of many-body similarity transformations generated by normal ordered exponential transformation operators without explicit use of the inverse. The similarity transform can be obtained as the solution of a linear system of equations that can be solved trivially using backward substitution. In addition a closed form diagrammatic expression for the similarity transformed operator is presented. Using the many-body similarity transformation strategy a simple and more general formulation of Fock space coupled cluster theory is presented which is akin in spirit to the formulation by Stolarczyk and Monkhorst [Phys. Rev. A 32, 725, 743 (1985); 37, 1908, 1926 (1988)], but which on the other hand is completely equivalent to the conventional wave operator formulation of Fock space coupled cluster theory (under suitable conditions). Other possible applications of the many-body similarity transformation will be briefly discussed.
An accurate quartic force field for formaldehyde
Authors: Burleigh Darin C. , McCoy Anne B. , Sibert Edwin L.
Published: July 2002

An accurate quartic force field for formaldehyde is obtained by refining the ab initio quartic force field of Martin, Lee, and Taylor [J. Mol. Spectrosc. 160, 105 (1993)]. The refinement was achieved by iteratively fitting a subset of the coefficients of a Taylor-series expansion of the potential-energy surface to 138 of the observed transition frequencies, many of which were obtained by Bouwens et al. [J. Chem. Phys. 104, 460 (1994)] using dispersed fluorescence spectroscopy. We fit the vibrational energies (≤7600 cm−1) for 138 states with an absolute mean deviation of 1.5 cm−1.
Organization in pure and alternate deuterated cadmium arachidate monolayers on solid substrates and at the air/water interface studied by conventional and differential Fourier transform infrared spectroscopies
Authors: Blaudez D. , Buffeteau T. , Castaings N. , Desbat B. , Turlet J.-M.
Published: July 2002

The molecular organization in monolayers of deuterated cadmium arachidate deposited on solid metallic and dielectric substrates or spread at the air/water interface is investigated using different techniques of Fourier transform infrared spectroscopy. For metals, the polarization modulated reflectivity shows that the molecules have a high molecular order and are oriented quasi perpendicularly with respect to the surface. The in-plane organization is determined by complementary transmission experiments on CaF2 plates. The CD2 bending vibration in mono- and multilayer Langmuir–Blodgett stacks reveals the hexagonal packing of a single monolayer irrespective of the hydrophilic or hydrophobic character of the substrate. Probing the stack organization using selective monolayer deuteration in hydrogenated Langmuir–Blodgett films shows that the first deposited monolayer of an odd-numbered stack keeps its hexagonal structure while additional bilayers adopt an orthorhombic packing induced by the strong polar head/head interactions. However a monolayer may have an orthorhombic arrangement either if it is deposited on an orthorhombic commensurable substrate or if it is at a low temperature in which case intermolecular interactions are strengthened. Finally, differential reflectivity allows a single monolayer spread at the air/water interface to be studied. It is shown that such a Langmuir film is arranged in a two-dimensional hexagonal packing with its alkyl chains oriented perpendicularly with respect to the surface and its COO− polar groups symmetrically anchored at the interface.
Hydrodynamic boundary conditions for confined fluids via a nonequilibrium molecular dynamics simulation
Authors: Mundy Christopher J. , Balasubramanian Sundaram , Klein Michael L.
Published: July 2002

A nonequilibrium molecular dynamics algorithm is employed to study a fluid confined between two immobile walls. The linear response theory of Bocquet and Barrat [Phys. Rev. E 49, 3079 (1994)] is used to relate phenomenological parameters, which determine the boundary conditions of the interface, to averages over microscopic quantities that can be obtained directly from the simulation. The parameters, which characterize ‘‘stick’’ or ‘‘slip’’ behavior and the thickness of the immobile fluid layer at the boundaries, have been determined for a model system: namely, a soft-sphere fluid interacting with corrugated walls.
A comparison of the dynamics of CO oxidation by oxygen atoms and molecules on Pt and Pd surfaces
Authors: Wei Cong , Haller Gary L.
Published: July 2002

The rovibrational excitation of desorbed CO2 produced from CO oxidation by adsorbed oxygen atoms and nascent oxygen atoms on a 600 K Pd foil and a 803 Pt foil under steady-state and quasi collision free conditions was probed by infrared emission using a Fourier transform infrared (FTIR) spectrometer operating at 0.012 cm−1 resolution. For CO2 produced from CO oxidation by adsorbed oxygen atoms on the 600 K Pd foil, the apparent temperatures associated with each vibrational mode are Tantisymmetric(Tanti)=2200±200 K, Tbend=2200±200 K, and Tsymmetric(Tsym)=2500±200 K. Whereas, Tanti=4000±1000 K, Tbend=3000±1000 K, and Tsym=8000±2000 K are found for CO2 produced from CO oxidation by nascent oxygen atoms on the 600 K Pd foil. For CO2 produced on the 803 K Pt foil, in the case of CO oxidation by adsorbed oxygen atoms, the apparent vibrational temperatures of CO2 are Tanti=1500±50 K, Tbend=2100±100 K, and Tsym=2000±100 K, while Tanti=3400±600 K, Tbend=6000±2000 K, and Tsym=3700±700 K were obtained for CO2 produced from CO oxidation by nascent oxygen atoms. Thus, it is clearly shown that, on both Pd and Pt, CO2 molecules produced from CO oxidation by nascent oxygen atoms are more excited than CO2 produced from CO oxidation by adsorbed oxygen atoms. The above result indicates that more energy flows into the system of CO oxidation by nascent oxygen atoms and suggests that the oxygen atoms sticking to the surface are not fully accommodated before they react with adsorbed CO. The fact that more energy flows into the system of CO oxidation by nascent oxygen atoms demonstrates that a different reaction mechanism is involved in CO oxidation by nascent oxygen atoms relative to CO oxidation by adsorbed oxygen atoms.
Dynamics of dissipation processes in the Ag–Xe complex
Authors: Kometer R. , Schwentner N.
Published: July 2002

2ω–2ω bleaching measurements of the ground state absorption and 2ω–1ω fluorescence dip measurements were carried out on the s–p transitions of Ag atoms in Xe matrices by ps laser pulses with 2ω corresponding to about 320 nm and 1ω to 640 nm. The absorption spectrum is analyzed in terms of a dynamic Jahn–Teller (JT) effect with a depth of the bound −JT state of about 30 meV and a pseudorotation frequency of about 20 ps. The −JT state seems to be rather long lived with a depopulation time of 3(±1) ps and a small energy dissipation rate of about 1 phonon per ps. Fluorescence occurs after a significant static lattice deformation accompanied by an energy relaxation of 0.4 eV. This static deformation and the energy relaxation to the emitting states proceed on a similar time scale of 3 ps which is very fast if the involved energies and the dissipation rate of about 30 phonons per ps are considered. The different rates are related to the different coupling to the lattice in the pseudorotating and the statically deformed geometry. Transient absorption is only observed in the relaxed state with σ=1.6(±0.5)⋅10−16 cm−2.
Diazasiline (SiNN): Is there a conflict between experiment and theory?
Authors: Ornellas Fernando R. , Ueno Leonardo T. , Iwata Suehiro
Published: July 2002

The molecule SiNN was investigated with the highly correlated coupled cluster method with single and double excitations and corrections for the connected triples [CCSD(T)], and with the multireference single and double excitations configuration interaction approach (MRCI). A series of basis sets ranging from the simple 6-31G*, 6-311G*, and Dunning’s double-zeta (DZ) plus polarization basis sets to the more extended correlated consistent cc-pVTZ and cc-pVQZ ones were employed to check the dependence of the geometry optimization and of the frequency evaluation on the basis sets. Our highest level result of 1859 cm−1 for the harmonic stretching frequency of the NN bond differs considerably from a previous CCSD(T)/DZ prediction of 1726 cm−1. In fact, most of the results analyzed in this study point to a frequency value greater than 1810 cm−1. Reexamining the scant experimental evidence, we estimate the harmonic frequency to be very close to 1830 cm−1. Surprisingly, an extended internally contracted MRCI calculation with the cc-pVTZ basis predicts a frequency 144 cm−1 higher than the corresponding CCSD(T)/cc-pVTZ result. Comparisons with existing density functional studies and with a previous MRCI calculation are also carried out. At the CCSD(T)/cc-pVTZ level, other isomers of SiN2 are further investigated for the first time. One symmetric linear (∑g−3), and one symmetric bent (1A1) structure are found to lie relatively high in energy: 84.60 and 102.23 kcal/mol, respectively, relative to SiNN (3Σ−). However, one cyclic 1A1 and the singlet asymmetric isomer SiNN (1Σ+) are only 6.09 and 15.81 kcal/mol above the global minimum. Although the higher frequencies of the former isomers do not fall in the region of relevance to the experimental assignment discussed in this work, that of the lowest lying 1A1 state (1850 cm−1) is practically identical to the CCSD(T)/cc-pVTZ frequency computed for the 3Σ− state.
Sampling protein conformations using segment libraries and a genetic algorithm
Authors: Gunn John R.
Published: July 2002

We present a new simulation algorithm for minimizing empirical contact potentials for a simplified model of protein structure. The model consists of backbone atoms only (including Cβ) with the φ and ψ dihedral angles as the only degrees of freedom. In addition, φ and ψ are restricted to a finite set of 532 discrete pairs of values, and the secondary structural elements are held fixed in ideal geometries. The potential function consists of a look-up table based on discretized inter-residue atomic distances. The minimization consists of two principal elements: the use of preselected lists of trial moves and the use of a genetic algorithm. The trial moves consist of substitutions of one or two complete loop regions, and the lists are in turn built up using preselected lists of randomly-generated three-residue segments. The genetic algorithm consists of mutation steps (namely, the loop replacements), as well as a hybridization step in which new structures are created by combining parts of two “parents’’ and a selection step in which hybrid structures are introduced into the population. These methods are combined into a Monte Carlo simulated annealing algorithm which has the overall structure of a random walk on a restricted set of preselected conformations. The algorithm is tested using two types of simple model potential. The first uses global information derived from the radius of gyration and the rms deviation to drive the folding, whereas the second is based exclusively on distance-geometry constraints. The hierarchical algorithm significantly outperforms conventional Monte Carlo simulation for a set of test proteins in both cases, with the greatest advantage being for the largest molecule having 193 residues. When tested on a realistic potential function, the method consistently generates structures ranked lower than the crystal structure. The results also show that the improved efficiency of the hierarchical algorithm exceeds that which would be anticipated from tests on either of the two main elements used independently.
On the relation between electronic structure and molecular dynamics. II. Sensitivity of collision induced rotational excitation of H2 by He to the electronic wave function
Authors: Lazarides A. A. , Rabitz H.
Published: July 2002

A formalism for relating dynamic observables of collision processes to the electronic structure of the colliding species is illustrated for the case of He collision-induced rotational excitation of H2. Expressions are presented for the functional derivative of transition cross sections and rate constants with respect to the electronic wave function. Regions of electronic coordinate space in which the electron probability densities have particularly strong influence on one or more inelastic transitions are identified. The possible role of functional derivatives of this sort for guiding electronic structure calculations to produce intermolecular potentials is discussed.
Response to “Comment on ‘New strategies to incorporate the solvent polarization in self-consistent reaction field and free-energy perturbation simulations’ ” [J. Chem. Phys. 107, 1291 (1997)]
Authors: Luque F. J. , Bofill J. M. , Orozco M.
Published: July 2002

The main features of the first-order perturbative treatment to the solvent polarization effects reported by us [J. Chem. Phys. 103, 10183 (1995)] are presented. The final expressions for the electrostatic and polarization components of the free energy of solvation are compared with the corresponding equations derived upon application of the Rayleigh–Schrödinger perturbation theory to the SCRF equations, which has been recently reported by Angyan (preceding Comment). The discussion emphasizes the similarities and differences of the basic equations developed in the framework of the two perturbation approaches, which in the limit lead to the same perturbative expressions. The analysis shows that the two procedures are useful to gain insight into the contribution of polarization effects to solvation.
Pump–probe spectroscopy and photophysical properties of single di-benzanthanthrene molecules in a naphthalene crystal
Authors: Jelezko F. , Lounis B. , Orrit M.
Published: July 2002

We report on a new crystalline host–guest system for single molecule spectroscopy; dibenzanthanthrene (DBATT) in a naphthalene crystal. As compared to DBATT in an n-hexadecane Shpol’skii matrix (a combination studied earlier), this system is shifted to the red (618.7 nm), has a lower triplet yield and shorter triplet lifetimes. Thanks to the narrow lines of this system (20 MHz), to its high photostability and to the absence of spectral diffusion, even in strained crystals, we could measure line shapes and widths with great accuracy. As an illustration of the potential of this new system for quantum optical experiments, we present pump–probe experiments with strong laser fields. The experiments are in very good agreement with calculations based on Bloch’s equations, demonstrating the validity of these equations for single molecules in solids under intense resonant fields.
Theory of quasielastic light scattering of a ternary solution consisting of two homopolymers with arbitrary optical, hydrodynamic, and thermodynamic properties
Authors: Wang C. H.
Published: July 2002

The temporal dependence of the time correlation function detected by quasielastic light (QEL) scattering in solution consisting of two homopolymers with arbitrary hydrodynamic and thermodynamic properties is investigated. New results appropriate for the ternary polymer solution in various concentrations are obtained. These new results should facilitate the analysis of the QEL scattering data of a ternary solution in any concentrations. Information about tracer diffusion coefficients of the constitutive polymers and their excluded volume interaction parameter is shown to be obtainable from the QEL experiment of the ternary polymer solution. Applications to ternary polymer solutions with one polymer isorefractive with the solvent, solutions with two polymers differing only in molecular weight, and solutions with two polymers having arbitrary refractive index increments are discussed. Clarification in regard to the ternary solution under the zero average contrast (ZAC) condition is also made. It is shown that under the ZAC condition, the autocorrelation function of the composition fluctuation in general decays with two exponentials, even at 1:1 symmetric composition.
Three-dimensional infinite order sudden quantum theory for indirect photodissociation processes. Application to the photofragment yield spectrum of NOCl in the region of the T1(13A″) ←S(11A′) transition. Fragment rotational distributions and thermal averages
Authors: Grinberg Horacio , Freed Karl F. , Williams Carl J.
Published: July 2002

The analytical infinite order sudden (IOS) quantum theory of triatomic photodissociation, developed in paper I, is applied to study the indirect photodissociation of NOCl through a real or virtual intermediate state. The theory uses the IOS approximation for the dynamics in the final dissociative channels and an Airy function approximation for the continuum functions. The transition is taken as polarized in the plane of the molecule; symmetric top wave functions are used for both the initial and intermediate bound states; and simple semiempirical model potentials are employed for each state. The theory provides analytical expressions for the photofragment yield spectrum for producing particular final fragment ro-vibrational states as a function of the photon excitation energy. Computations are made of the photofragment excitation spectrum of NOCl in the region of the T1(13A″) ←S0(11A′) transition for producing the NO fragment in the vibrational states nNO=0, 1, and 2. The computed spectra for the unexcited nNO==0 and excited nNO=2 states are in reasonable agreement with experiment. However, some discrepancies are observed for the singly excited nNO=1 vibrational state, indicating deficiencies in the semiempirical potential energy surface. Computations for two different orientations of the in-plane transition dipole moment produce very similar excitation spectra. Calculations of fragment rotational distributions are performed for high values of the total angular momentum J, a feature that would be very difficult to perform with close-coupled methods. Computations are also made of the thermally averaged rotational energy distributions to simulate the conditions in actual supersonic jet experiments.
Electronic properties of Cs+CO coadsorbed on the Ru(0001) surface
Authors: Fichtner-Endruschat S. , De Renzi V. , Morgante A. , Schwegmann S. , Bludau H. , Schuster R. , Böttcher A. , Over H.
Published: July 2002

The variation of the Cs 6s and the Cs 5p emission in He* and Ne* metastable deexcitation spectroscopy (MDS) as a function of the CO exposure indicates a demetallization of the Ru(0001)–(2×2)-Cs and the Ru(0001)–(√3×√3)R30°-Cs surfaces upon CO coadsorption. This observation corroborates a (substrate-mediated) charge transfer from the Cs atom to the 2π* orbital of CO. With the Ru(0001)–(2×2)-Cs system even at CO saturation, MD spectra show emission associated with the Cs 6s state, indicating that the Cs atoms are not completely ionized. Exposing the (√3×√3)R30°-Cs-pre-covered Ru(0001) to CO, surplus Cs of the first layer is displaced into a second layer. In this way, CO molecules are able to be accommodated into the first layer. Desorbing this second layer Cs by heating the sample to 600 K produces a (2×2) structure with one Cs and CO in the unit cell as evidenced by MDS and low energy electron diffraction.
Vibration-torsion-rotation polarizability tensor operator for a molecule with an internal rotor
Authors: Sun Zhen-Dong , Duan Yun-Bo , Xu Jun
Published: July 2002

The sequential contact transformation technique is applied to the vibration-torsion-rotation polarizability tensor operator for a molecule containing a threefold symmetric internal rotor. Detailed discussions about various types of polarizability tensor operators and corresponding vibration-torsion-rotation transitions are presented. The formalism of the transformed polarizability tensor operator makes it possible to analyze various high-order vibration-torsion-rotation interactions and their effects as well as to determine the intensities of transitions between vibration-torsion-rotation states within the Raman spectrum of the molecule. As an example, the transition matrix elements of the transformed polarizability tensor operators are discussed for CH3OH-like molecules.
Origin of polarization in polar nanocrystals
Authors: Huong Nguyen Que , Birman Joseph L.
Published: July 2002

A simple theoretical model for the origin of spontaneous polarization in nanocrystals is developed. We propose that the origin of the spontaneous polarization is in the boundary layer between “cap” and the nanocrystal, and the internal electric field in the dot is due to the piezoelectric effect caused by the strain existing in the interface region of materials with different lattice constants. The model, based on spherical rotation symmetry without inversion [SO(3)], employs a distribution of polarization with symmetry which is a subgroup of SO(3) consistent with the hexagonal structure of wurtzite structure. We predict the internal electric field and compare with experimental data.
Contribution of the center-of-mass fluctuation of a liquid cluster to the free energy: A Monte Carlo simulation study
Authors: Oh K. J. , Zeng X. C.
Published: July 2002

Monte Carlo simulation method is used to calculate the Helmholtz free energies of liquid cluster with boundary fixed in laboratory frame and a fluctuating center-of-mass (CM) and with a fixed boundary centered on a fixed CM. The former type of cluster is called drop conventionally and the latter type is the so-called Lee–Barker–Abraham (LBA) cluster. The free energy difference between the drop and the LBA cluster characterizes the free energy contribution due to the CM fluctuation. The simulation result is close to the theoretical estimate of Reiss, Katz, and Cohen than that of Abraham and co-workers.
Classical vibrational predissociation dynamics: The effects of phase-space bifurcations
Authors: Granovsky A. A. , Medvedev A. V. , Buchachenko A. A. , Stepanov N. F.
Published: July 2002

Extensive classical investigation of the vibrational predissociation dynamics of the model He⋯Br2 and Rg⋯I2 (Rg=He, Ne, and Ar) van der Waals complexes is performed. Classical trajectory calculations of the fragmentation rates are accomplished with the numerical analysis of the phase-space structure within the two-dimensional T-shaped model. Various bifurcations of the phase portrait with increasing excitation energy are found to produce a remarkable effect on the fragmentation dynamics causing irregular variations of the decay rate. This effect is proven to be quite persistent and pertinent to the more realistic three-dimensional dynamics as well. The implications of the results for studying quantum-classical correspondence for metastable states are indicated.
A method for measuring the diffusivity of a liquid into a porous matrix
Authors: Choi Changho , Peternelj J. , Pintar M. M.
Published: July 2002

A method for measuring diffusivity of a liquid into a saturated porous matrix is introduced. The experiment involves monitoring, by the “whole sample” NMR, the diffusion of D2O water into a H2O-saturated porous cylinder indirectly by detecting the proton NMR signal as a function of immersion time. An H2O-saturated hardened cement cylinder was immersed into a D2O bath, and the diminishing proton magnetization was monitored over 400 h of immersion. The average radial diffusivity which characterizes this porous medium was found to be 1.9×10−7 cm2/s (±10%), which is two orders of magnitude smaller than the water diffusion constant. The proton spin-spin relaxation time and its stretched exponential parameter α of water were found to be independent of immersion time. This means that, throughout the porous medium, the pores of different size are uniformly distributed. The movement of a radial advection front of D2O towards the center of the porous cylinder describes the phenomenon only for short immersion times. If this process is monitored by magnetic resonance imaging, a distinct dividing line between the D2O advection “front” and H2O is observed only during the beginning of the process. As the diffusion proceeds, the line between H2O and D2O domains becomes progressively more blurred. This problem is avoided if the signal of the whole cylinder is recorded, as in the whole-sample NMR.
Anisotropy of the charge-carrier mobility in polydiacetylene crystals
Authors: Hoofman Romano J. O. M. , Siebbeles Laurens D. A. , de Haas Matthijs P. , Hummel Andries , Bloor David
Published: July 2002

The anisotropic mobility of excess charge carriers in pure and mixed crystals of two polydiacetylene derivatives was determined. The charge carriers were produced by pulsed irradiation and detected by time-resolved measurement of the microwave conductivity. The charge-carrier mobility was measured as a function of the orientation of the polymer backbone in the crystal with respect to the probing electric microwave field. A lower limit in the intrinsic anisotropy in the charge-carrier mobility was found to be 15 in favor of charge transport in the direction of the polymer backbone as compared to the transverse direction for polydiacetylene-(bis p-fluorobenzene sulfonate) (pFBS), while a value of 90 was found for polydiacetylene-(bis p-toluene sulfonate) (pTS) and the 50:50 pTS/FBS copolymer. A lower limit of the charge-carrier mobility along the backbone of 3 cm2/V s was found for both pTS and pFBS. The charge-carrier mobility in the copolymer was found to be one order of magnitude lower than in the pure polymers.
Velocity map imaging studies of the Lyman α photodissociation mechanism for H atom production from hydrocarbons
Authors: Jackson William M. , Price Roosevelt J. , Xu D. D. , Wrobel Jacek D. , Ahmed Musahid , Peterka Darcy S. , Suits Arthur G.
Published: July 2002

H atoms produced in Lyman α photolysis of ethane, propane, and ethylene have been studied using velocity map imaging techniques. Two types of H atoms are identified, one formed along with an alkyl radical in the Rydberg state and the other by the subsequent decomposition of this Rydberg radical.
On the origin of the thermodynamic stability of lyotropic lamellar liquid crystals
Authors: Ruckenstein Eli
Published: July 2002

A thermodynamic formalism is developed for lamellar liquid crystals, in which the free energy is written as the sum of two terms: one which treats the two kinds of lamellae (water and oil) as bulk media and a second which (i) accounts for the thinness of the lamellae and (ii) includes the interactions among them. By “thin” we mean thicknesses small compared to the range of interaction forces between the molecules of each of the lamellae. On the basis of this free energy expression, equations are derived which can provide information regarding the maximum dilution of the liquid crystal with water or hydrocarbons. Considering planar layers and only van der Waals type interactions between molecules, we find that there are conditions under which the lamellar liquid crystals display the unlimited swelling observed experimentally. The effect of the undulation disorder of the interfaces between the lamellae is also discussed.
Quadrupolar spin relaxation of N14 in NNO in collisions with various molecules
Authors: Jameson Cynthia J. , ter Horst Marc A. , Jameson A. Keith
Published: July 2002

Spin-lattice relaxation times were measured for the N14 relaxation of both N14 nuclei in NNO in the pure gas and in mixtures with the following buffer gases; Ar, Kr, Xe, HCl, N2, CO, CO2, CH4, CF4, and SF6. Effective collision cross sections for molecular reorientation of NNO in collisions with these ten molecules are obtained, as a function of temperature, directly from the measured relaxation times of the end N14 nucleus in the NNO molecule.
A theory for dynamic friction on a molecular bond
Authors: Vergeles Maxim , Szamel Grzegorz
Published: July 2002

We present a microscopic theory for dynamic friction on an intramolecular vibrational coordinate of a diatomic molecule dissolved in a simple liquid. Previous theoretical approaches to calculating dynamic friction have either used molecular hydrodynamics or employed a concept of instantaneous normal modes. Both methods have their limitations: molecular hydrodynamics is unable to correctly describe the dynamics on short time or length scales, while the instantaneous normal modes approach can be expected to work at short times only. We apply the theoretical formalism developed by us earlier to describe self-diffusion in liquids [M. Vergeles and G. Szamel, J. Chem. Phys. 110, 3009 (1999)] to the calculation of dynamic friction. We begin by deriving an equation of motion for the phase space probability distribution of the diatomic molecule. From it we obtain an equation for the bond velocity autocorrelation function. This equation has the same form as the one obtained from the generalized Langevin equation, which allows us to identify the dynamic friction kernel. Our predictions quantitatively agree with the results of molecular dynamics (MD) simulations.
Variations in chain compactness and topological complexity uncover folding processes in the relaxation dynamics of unfolded in vacuo lysozyme
Authors: Arteca Gustavo A. , Velázquez I. , Reimann C. T. , Tapia O.
Published: July 2002

Chain collapse and the formation of a near-native tertiary structure are believed to be two key features controlling the progress of a protein folding transition. In this work, we study the interrelation between these two properties along computer-simulated relaxation trajectories of unfolded in vacuo lysozyme. Large-scale molecular shape transitions are monitored within a space defined by two discriminating descriptors of chain compactness and entanglement (or “topological”) complexity. For the system studied here, results indicate that successful refolding into native-like conformers requires a balance between polymer collapse and a topologically “correct” organization of chain loops. Although no single factor dominates the relaxation paths, compactization appears to be a necessary condition for near-native refolding. Whenever initial collapse is limited or absent, we find a “derailed” folding path with high configurational frustration. We also show that disulfide-reduced lysozyme unfolds differently, yet relaxes to the pattern of molecular shapes characteristic of the folded states of disulfide-intact lysozyme.
Calculations of Argon broadening and shift for HCO+ rotational lines at 77 K
Authors: Buffa G. , Tarrini O. , Cazzoli G. , Dore L.
Published: July 2002

Different methods of calculations are compared to experiments at 77 K for the broadening Γ and shift s of some HCO+ rotational transitions by collisions with Argon. Capture method and Anderson, Tsao, and Curnutte (ATC) method with straight line trajectories are both not able to reproduce the trend of Γ with J. A discussion of the origin of such a discrepancy allows to obtain better insights into the different causes of collisional relaxation. Quite good agreement is obtained by both a hybrid method or ATC calculations using a more realistic translational dynamics. However, some discrepancy still exists for high J values.
Physical aging by periodic creep and interrupted creep experiments
Authors: Bernatz Kevin M. , Giri Lily , Simon Sindee L. , Plazek Donald J.
Published: July 2002

A newly developed “interrupted creep” experiment has been used to study the physical aging of a low molecular weight polystyrene, Tg∼69 °C. The results of the new experiment are compared to those obtained from traditional “periodic creep” experiments. The interrupted creep experiment provides information about the viscosity, the recoverable creep compliance and the steady-state compliance, Js, during aging. Low molecular weight polystyrene was chosen because it exhibits a steady-state compliance that is a strong function of temperature. Aging was conducted at three temperatures, 68.2, 65.7 and 61.0 °C, using both down-jump and up-jump experiments. The behavior observed in the new experiments mirrors the behavior observed in the traditional experiments. In addition, the new experiments allow the first ever determination of how Js evolves during aging. The change of Js with aging time was calculated using the relationship between the shift factors, obtained from the recoverable creep compliance data, and the average relaxation times, obtained from the viscosity. The advantage of the new experiment is that it provides both the short-time recoverable creep compliance information and the long-time viscous flow. By combining these contributions to the creep compliance in a simple additive fashion, one can obtain a more complete picture of how the material is behaving during aging.
Comment on “Fine and magnetic hyperfine structure in the A 2Π and X 2Σ+ states of yttrium monoxide” [J. Chem. Phys. 88, 598 (1988)]
Authors: Simard B. , Watson J. K. G. , Merer A. J. , Steimle T. C.
Published: July 2002

It is pointed out that a factor of 2 was omitted from the formula for the γ spin-rotation parameter of a 2Σ electronic state in the above reference. The values of γ in the X 2Σ+ ground states of YO and isoelectronic or isovalent molecules are discussed in terms of contributions from various excited 2Π states.
Assembling alkali–lead solid compounds from clusters
Authors: Molina L. M. , Alonso J. A. , Stott M. J.
Published: July 2002

Ab initio total energy calculations are used to simulate the assembling of equiatomic APb alloys (A=Li, Na, K) from A4Pb4 free clusters in order to investigate the remarkable stability of tetrahedral Pb4 units in some of these solid and liquid alloys. Starting from a greatly expanded array of eight Na4Pb4 clusters per unit cell, the NaPb solid alloy was assembled by shrinking the unit cell and allowing full atomic relaxation at each stage. The evolution of cluster properties as the clusters are drawn together into the solid was studied. Charged (Pb4)4− tetrahedral units were found to dominate the structural and electronic properties, being very robust and quite insensitive to their alkali environment. Trends with the choice of alkali were also investigated, showing that the stability of the (Pb4)4− units decreases as we progress from K to Li and leads to their absence in the LiPb alloy in accordance with experiment. The critical factor for the survival of the units proved to be the distance between them, which was determined by the atomic size of the alkali. The effect of pressure on the systems was also considered.
H
2
+
scattered off Al surfaces: The role of the negative ion resonance Σu+2
Authors: Lorente N. , Teillet-Billy D. , Gauyacq J.-P.
Published: July 2002

A theoretical study of the electronic processes occurring in collisions of H2+ molecular ions on an Al surface is presented, with an emphasis on the molecular dissociation induced by electronic transitions. The H2+ neutralization proceeds through two different channels: the H2 (b 3Σu+) dissociative state (resonant process) and the H2 (X 1Σg+) ground state (Auger process). The Auger deexcitation process of the b state, mediated by the H2− (2Σu) resonant state is studied in detail and shown to result in an efficient quenching of the excited state and a reduction in the molecular dissociation probability. Its efficiency depends on the competition between the molecular dissociation and the electronic process. The importance of this process can explain the experimental observation of significant amounts of bound H2 molecules surviving the electronic processes.
Adsorption of ammonia on the rhodium (111), (100), and stepped (100) surfaces: An ab initio and experimental study
Authors: Frechard F. , van Santen R. A. , Siokou A. , Niemantsverdriet J. W. , Hafner J.
Published: July 2002

The adsorption of ammonia on the two low index (111) and (100) surfaces of rhodium has been studied by periodic calculations with density functional theory and compared to experimental results. The geometries of the adsorbates and the surfaces are completely optimized. For both surfaces the top site is found to be the most stable while the adsorption energy of ammonia is 8–10 kJ⋅mol−1 larger on the (100) surface. The presence of steps on the (100) surface has a minor effect on the heat of adsorption. The theoretical predictions of the adsorption energies and the changes in work function by NH3 are in good agreement with experimental data. Moreover the prediction of the ontop adsorption as well as the weak interactions between the adsorbates is confirmed. The broadening of the temperature programmed desorption spectra and the two desorption peaks for the first adlayer are mainly due to an entropy effect which affects the preexponential factor of the desorption rate constant.
Optical dephasing in defect-rich crystals
Authors: Hizhnyakov V. , Reineker P.
Published: July 2002

The optical dephasing, caused by vibrational modulation of interactions of optical centers with defects is studied. This mechanism is essential for crystals with a substantial amount of defects, being more efficient at low temperatures than the Raman mechanism, caused by scattering of acoustic phonons at optical centers. A theory is developed, which allows one to find averaged widths and shapes of spectral holes in these systems dependending on the temperature T and concentration c of defects. The shape of the hole is found to be close to a Lorentzian, and at intermediate temperatures its width is proportional to T3, and strongly depends on c (for charged point defects as c8/3).
Liquid crystal phase transitions in clusters of spherocylinders
Authors: Casey Alison , Harrowell Peter
Published: July 2002

The stability of nematic and smectic order in clusters of spherocylinders is examined via Monte Carlo simulations. A discontinuous isotropic–nematic transition is observed as the cluster size is varied through N=35. A continuous transition to smectic A order takes place from N=55 to N=200. The layered N=200 cluster exhibits a square cross section, an effect attributed to the increased orientational entropy of rods at surfaces of low curvature.
Experimental proof of the case (ab) coupling hypothesis in the first excited triplet state of selenoformaldehyde (H2C=Se)
Authors: Joo Duck-Lae , Clouthier Dennis J. , Judge R. H.
Published: July 2002

A high-resolution laser-induced phosphorescence spectrum of the vibronically induced 401 band of the ã 3A2−X̃ 1A1 system of D2C80Se has been recorded with Doppler-limited resolution. Rotational analysis revealed that transitions to all three spin components occur within the 12 265–12 525 cm−1 region. The rotational structure of the band was fitted to the experimental precision of measurement using a triplet state Hamiltonian that allowed for slight variations in the geometries of the individual spin components. The results show that the triplet state of selenoformaldehyde involves case (ab) coupling, in which two of the spin components are close together and the third is ca. 125 cm−1 higher in energy. This unusual occurrence of large zero-field splittings in the triplet state of an asymmetric top arises because of the close proximity of the A13(π,π*) state which substantially displaces two of the spin components to lower energy.