bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron transport in o- and m-xylene under high pressure
Authors: Itoh Kengo , Nishikawa Masaru , Holroyd Richard A.
Published: July 2002

The electron drift mobility (μ) was measured by a time-of-flight method in pure liquid o- and m-xylene under high pressures up to 300 MPa, and in the temperature ranges from 15 to 120 °C and 0 to 100 °C, respectively. In both liquids μ increases in the lower pressure region at lower temperatures. At higher pressures μ decreases gradually with pressure at all temperatures studied. The pressure dependence of μ was interpreted in terms of a two-state model and a hopping model. When μ increases with pressure this interpretation leads to a positive volume change upon introduction of electrons into the liquid, showing electrons reside in cavities of radius 0.31 to 0.32 nm, whereas in the high pressure region electron attachment to xylene molecules occurs, accompanied by hopping of electrons between molecules.
Note on the nonequilibrium partition function and generalized potentials
Authors: Eu Byung Chan
Published: July 2002

In this paper, we present a nonperturbative method of calculating the nonequilibrium partition function appearing in the nonequilibrium ensemble method for dilute gases. A set of first-order partial differential equations is derived for the nonequilibrium partition function. The set can be put into an exact differential form and its integral gives the nonequilibrium partition function with which various macroscopic variables can be expressed. In particular, on solving the aforementioned partial differential equations for the derivatives, the nonequilibrium constitutive relations are directly obtained for the generalized potentials of the substance which are conjugate to the nonconserved variables such as stresses, heat fluxes, diffusion fluxes, and so on. The results for a couple of special cases are presented without proof.
Dynamics of high-n Rydberg states employed in zero kinetic energy-pulsed field ionization spectroscopy via the F 1Δ2, D 1Π1, and f 3Δ2 Rydberg states of HCl
Authors: Wales N. P. L. , Buma W. J. , de Lange C. A. , Lefebvre-Brion H.
Published: July 2002

The intensity anomalies in the spin–orbit and rotational branching ratios in the zero kinetic energy pulsed-field ionization (ZEKE-PFI) spectra via the F 1Δ2, D 1Π1, and f 3Δ2 Rydberg states of HCl have been studied. In general, the branching ratios are observed to depend on three parameters employed in the pulsed field ionization experiment: (i) the delay time between excitation and ionization; (ii) the magnitude of the bias electric field; and (iii) the magnitude of the applied pulsed electric field. The results can be rationalized on the basis of the increasing number of autoionization decay channels that become available to the high-n Rydberg states as each ionization threshold is surpassed. The delay dependence of the ZEKE-PFI spectra via the F 1Δ2 state has been analyzed in more detail by ab initio calculations. These calculations show that the observed spin–orbit branching ratios can be reproduced thereby giving evidence for a nonexponential decay of the high-n Rydberg states (n≊100).
Conductive Cu(2,5-dimethyl-dicyanoquinonediimine)2 radical ion salts: Systems with none, one, or two phase transitions
Authors: von Schütz J. U. , Gómez D. , Wachtel H. , Wolf H. C.
Published: July 2002

Phase transitions in the radical ion salts of Cu(2,5-dimethyl-dicyanoquinonediimine)2 (Cu(DCNQI)2) can be achieved either by ‘‘external’’ pressure or by ‘‘internal’’ pressure. The latter is introduced by chemical modifications at the DCNQI-molecules (e.g., deuteration) or by alloying deuterated Cu(2,5-(CD3)2-DCNQI)2 (d6) with undeuterated Cu(2,5-(CH3)2-DCNQI)2 (h8), giving the mixture (h8/d6) in different ratios. In this work we present simultaneous conductivity (σ) and electron spin resonance (ESR) experiments on differently deuterated Cu(DCNQI)2-systems and on alloys (h8/d6) under external pressure. The anticoincidence of σ and ESR allows the determination of phase transition temperatures even in the absence of electrical contacts. For each system an individual phase diagram is established. Introducing an effective pressure peff=p0+p with p0 being the ‘‘internal’’ (chemical) pressure, a general phase diagram could be constructed by determining the individual p0 value for all systems. For the alloys (h8/d6) the relation ∂p0/∂q=5.0 bar/% (q is the percentage of d6) could be evaluated. The occurrence of a phase transition is associated with a change in the lattice parameters. For that, a structural model of a v-shaped temperature dependence of the unit cell volume V is suggested. For thermodynamic considerations, V is used as order parameter. If this order parameter V crosses critical values Vcbig and Vcsmall from above (cooling) or below (heating), phase transitions from conducting to insulating phases or vice versa are induced. This model explains the phase transition temperatures, the re-entry and the hysteresis effects of all systems qualitatively.
Relativistic calculation of the electronic structure of the IF molecule
Authors: Cabrol O. , Girard B. , Spiegelmann F. , Teichteil C.
Published: July 2002

The electronic structure of IF is investigated using ab initio pseudopotentials and a variational/perturbative MRCI scheme (CIPSI/CIPSO). All the valence states dissociating into neutral asymptotes I(2P1/2,2P3/2)+F(2P1/2,2P3/2) and ionic asymptotes I+(3P2,3P1,3P0,1D2,1S0) +F−(1S0) are determined up to the region where they undergo avoided crossing with the neutral excited states dissociating into I*+F. The spectroscopic properties are derived and discussed in comparison with the available experimental results, with emphasis on the X, B, E, A, β, A′, and D′ states.
Giant lifetimes of optically excited states and the elusive structure of sodiumnitroprusside
Authors: Delley B. , Schefer J. , Woike Th.
Published: July 2002

The properties of sodiumnitroprusside (Na2[Fe(CN)5NO]*2H2O) are investigated by density functional theory. The calculated results both for the free anions as well as for the solid show that the ground-state Born–Oppenheimer surface has local minima for sidewards bonded and inverted NO. The calculated properties for the local minima: very long lifetime because of large barrier, diamagnetism, optical excitation energies, vibrational, and Mössbauer properties are in essential agreement with experiment. The present findings elucidate the possibility of population transfers by illumination with light of different wavelengths.
CO
2
laser spectroscopy of ammonia molecules and complexes adsorbed on large argon host clusters
Authors: Rohmund Frank , Huisken Friedrich
Published: July 2002

The pick-up technique has been employed to deposit single ammonia molecules as chromophores on the surface of large ArN clusters and to create small ammonia complexes attached to host clusters. Mass spectrometric investigations have been carried out to characterize the pressure dependence of the guest cluster formation. The technique is then combined with molecular beam depletion spectroscopy employing the infrared radiation of a pulsed CO2 laser. In the region between 920 and 1060 cm−1 in which the ν2 bending vibration (umbrella mode) can be excited, absorption spectra have been measured for the ammonia monomer as well as for the ammonia dimer, trimer, and tetramer adsorbed on large ArN host clusters. For the monomer a significant blue shift of 14.74 cm−1 is observed. Furthermore, it is found that the inversion motion associated with the umbrella mode is appreciably perturbed, resulting in a reduction of the inversion splitting by a factor of 0.3. As in the argon matrix, the rotation is only slightly hindered. The absorption spectra measured for ammonia dimers, trimers, and tetramers resemble very much the gas phase spectra of these complexes, except that they are blue shifted by 17 (dimer) and 6 cm−1 (trimer and tetramer). From this close resemblance it is concluded that the structures of the small ammonia complexes are not affected by the argon host cluster.
Lithium bonding interaction in H2CY⋯LiF (Y=O,S) complexes: A theoretical probe
Authors: Ammal S. Salai Cheettu , Venuvanalingam P. , Pal Sourav
Published: July 2002

Ab initio calculations at 6-31++G(d,p) level have been done on H2CY⋯LiF (Y=O,S) complexes choosing ten possible orientations in each complex. The effect of correlation on complex binding energies has been studied via single point MP2 (full) calculations done on 6-31++G(d,p) geometry. Binding energies have been corrected for basis set superposition error. Frequency calculations confirm that H2CO⋯LiF and H2CS⋯LiF complexes have three and two stable forms, respectively. The most stable form in each complex has been found to have a strong lithium bonding interaction and a secondary hydrogen bonding interaction. NBO analysis has revealed that in this form oxygen donates nσ lone pair while sulfur donates its nπ lone pair. In yet another stable form of these complexes, mixed donation of π and nσ electrons have been observed.
The influence of high-frequency modes on two pulse spectroscopy
Authors: Kobrak Mark N. , Rice Stuart A.
Published: July 2002

Theoretical analyses of molecular response to ultrafast pulse excitation often do not account for the influence of high frequency modes on the spectrum of the molecule. In a previous article [M. N. Kobrak, E. M. Hiller, and S. A. Rice, J. Chem. Phys. 105, 9403 (1996)], we have shown that the high frequency modes of the molecule can dramatically change the distribution of an observable in processes initiated by a single ultrashort pulse. In this article we extend our treatment to allow for two-pulse processes, and apply the results to two models which have a qualitative resemblence to bacteriorhodopsin.
Nonergodic quasiequilibrium in nuclear magnetic resonance of infinite linear spin-1/2 chains
Authors: Fel’dman Edward B. , Lacelle Serge
Published: July 2002

The long-term behavior of longitudinal spin order is investigated analytically in infinite one-dimensional (1-D) chains of nuclear spins 1/2 driven by a multiple pulse sequence of nonresonant rf pulses in the presence of a strong magnetic field. We consider a chain of spins with only nearest-neighbor dipolar interactions, initially at thermal equilibrium in the rotating frame, and evolving under the Hamiltonian, H=ΔIZ+H+2+H−2, (where Δ is a resonance offset of the carrying frequency of the pulses from the Larmor frequency of the spins, and H+2+H−2 is a two-spin/two-quantum nonsecular average dipolar Hamiltonian). The corresponding spin density operator and longitudinal magnetization are calculated exactly with fermion field operator representations of spin-1/2 operators. The spin density operator is determined by an additional integral of motion, beyond the energy, reflecting inherent nonergodic behavior. The establishment of the spin thermodynamic equilibrium between the Zeeman and the nonsecular dipolar reservoirs is examined. It is shown that such equilibrium is only possible in an external magnetic field H0≫Hloc (where Hloc is the local dipolar field). A discussion of ergodicity in 1-D spin chains with the full dipolar Hamiltonian, and a suggestion of possible NMR measurements of longitudinal magnetization to reveal broken ergodicity in the spin dynamics of materials with quasi-1-D distributions of spins are presented.
Cu
++
and Li+ interaction with polyethylene oxide by ab initio molecular dynamics
Authors: Palma Amedeo , Pasquarello Alfredo , Ciccotti Giovanni , Car Roberto
Published: July 2002

Equilibrium positions on the Li+–PEO and on the Cu++–PEO ground state potential energy surfaces have been determined by ab initio molecular dynamics. Our results confirm the previously proposed jump mechanism for ion diffusion in polymer electrolytes. The energy barriers for Li+ and Cu++ ionic diffusion along the PEO chain have been estimated.
Microscopic measurements of correlation functions in colloid dispersions
Authors: Bongers J. , Manteufel H. , Versmold H. , Vondermaßen K.
Published: July 2002

This paper is concerned with video microscopy, an analytical method of growing importance for the studies of colloid suspensions. Particular emphasis of this work will be laid on the derivation of simple recipes for the computation of static and dynamic pair correlation functions from the microscopic images. The strengths and also the weaknesses of the method will be discussed in detail. We will also present a selection of results gained by this method to demonstrate, that video microscopy is a valuable addition to the well established analytical methods like neutron and laser light scattering in colloid science.
Nuclear magnetic and quadrupole resonance study of molecular dynamics in a model bilayer compound C6H13NH3Cl
Authors: Seliger J. , Žagar V.
Published: July 2002

Molecular dynamics in a model bilayer compound C6H13NH3Cl is studied by H1 spin–lattice relaxation in the laboratory and in the dipolar system as well as by N14 nuclear quadrupole resonance. In the low temperature monoclinic phase, the NMR data show the presence of a slow motion which is presumably the 120° reorientation of a C–NH3 group around the C–N bond. In the high temperature tetragonal phase, the nuclear magnetic and quadrupole resonance data show the presence of two motions in addition to the reorientation of the C–NH3 groups: a fast motion causing a strong temperature variation of the N14 NQR frequencies, and a slow motion dominating the proton spin–lattice relaxation in low external magnetic field and in the dipolar system. Various models of molecular dynamics in the high temperature tetragonal phase are examined in relation to the experimental data.
Spin-polarization in HBr: Comparison between theory and experiment
Authors: Irrgang R. , Drescher M. , Spieweck M. , Heinzmann U. , Cherepkov N. A. , Lefebvre-Brion H.
Published: July 2002

Measurements of the electron spin polarization parameter A have been performed for HBr in the energy region between the two ionization thresholds, Π3/22 and Π1/22. Using the multichannel quantum defect theory and formulas derived for the spin parameters in intermediate coupling between Hund’s cases (a) and (e), calculations for both the cross section and the A parameter have been performed. It is shown that the values of A for each Rydberg resonance change with n, following the change from Hund’s case (a) to case (e). In the approximations used in this paper, no simple relation between the sign of A and the value of J+, the rotational angular momentum of the ion core, was found. Values for the calculated angular asymmetry parameter β and the spin polarization parameter ξ are also presented. Since our calculations reproduced only part of the observed resonances, and since the measurements of the A parameter have been performed in relatively narrow energy regions, one could not perform an extensive comparison between theory and experiment. However, some of the peculiarities of the behavior of the A parameter observed experimentally have been qualitatively reproduced by our calculations.
A comparative electron spin echo envelope modulation study of the primary electron acceptor quinone in Zn-substituted and cyanide-treated preparations of photosystem II
Authors: Astashkin Andrei V. , Hara Hideyuki , Kuroiwa Shigeki , Kawamori Asako , Akabori Kozo
Published: July 2002

The electron spin echo envelope modulation spectra of the reduced primary acceptor quinone, QA, in two preparations of plant photosystem II, have been studied. In one of these preparations the Fe2+ ion in the quinone–iron complex has been substituted by diamagnetic Zn2+. In the other preparation this iron ion has been converted into the diamagnetic state using a potassium cyanide treatment. A comparative analysis of two-dimensional three-pulse electron spin echo envelope modulation spectra has shown similar structure of the binding site of QA in both preparations. Two nitrogen nuclei have been found to contribute to the spectra in both preparations. One of these nitrogens is, most probably, an amino nitrogen in the imidazole ring of histidine 215 of the D2 protein. The other nitrogen has been assigned to the peptide group of alanine 261 of the D2 protein. The numerical simulations of the electron spin echo envelope modulation spectra have shown that both nitrogens are simultaneously bound to QA.
Interfacial properties of water in an extended van der Waals theory
Authors: Mühlbacher Lothar
Published: July 2002

An extended van der Waals model is presented which describes a hydrogen-bonded system with an inhomogeneous density distribution. In the framework of this model, the interfacial properties of a coexistence system of gaseous and liquid water are calculated, and it is shown that anomalous behavior can occur in the supercooled region. The predicted surface tension is compared to experimental data.
Improving protein circular dichroism calculations in the far-ultraviolet through reparametrizing the amide chromophore
Authors: Hirst Jonathan D.
Published: July 2002

The two most prominent peaks in the far-ultraviolet (UV) circular dichroism (CD) spectra of proteins occur at 190 nm (associated with the ππ* transition) and 220 nm (associated with the nπ* transition). The mean residue ellipticity at 220 nm, [θ]220, is commonly used to estimate the helix content of a protein. We have assessed first principles CD calculations in the far-UV using 23 different proteins, whose x-ray crystal structures and CD spectra are known. Using the standard parameters, derived from semiempirical calculations, for the ground state, nπ* and ππ* electronic states of models of the peptide group to describe the relevant charge distributions of the backbone chromophore, we find weak correlation between the computed and measured mean residue ellipticity at 220 nm (Spearman rank correlation coefficient, r=0.41) and no correlation at 190 nm. Replacing the standard parameters with ones derived from modern quantum chemistry methods (multireference configuration interaction calculations) gives a significant correlation (r=0.62) between the computed and measured [θ]220, but still no correlation at 190 nm. We suggest that further improvements might be expected from a parametrization of the higher energy states of the backbone chromophore, and from a more modern parametrization of side-chain chromophoric groups.
Relaxation in random pores
Authors: Lee J. C.
Published: July 2002

To study the dynamics of a binary liquid mixture in glass pores we have performed a molecular dynamics simulation and measured the concentration fluctuation autocorrelation function Fm(q,t) and several other dynamic functions. The relaxation pattern of Fm may be described with an exponentially decaying diffusion term and a logarithmically decaying activation term. Although the system is at a very high temperature in the one-phase region, the results for the velocity autocorrelation functions and the mean-square displacement functions are deceptively similar to those of the bulk when the bulk is in its two-phase region. We discuss what this may mean when the pore size is very large.
Orientational order and rotational dynamics of the amide ions in potassium amide. II. Quasielastic neutron scattering
Authors: Müller M. , Asmussen B. , Press W. , Senker J. , Jacobs H. , Büttner H. , Schober H.
Published: July 2002

The ionic molecular crystal potassium amide (KNH2) was investigated by means of quasielastic incoherent neutron scattering (QNS). In the orientationally disordered cubic and tetragonal high–temperature phases as well as in the ordered monoclinic phase reorientational motions of the amide ions (NH2−) are resolved in a frequency range of three orders of magnitude. The QNS spectra are well explained by reorientational models for each phase. Furthermore, the results are compared to structural data and findings of other spectroscopic methods. Finally, a consistent picture of structure and dynamics of all three phases of potassium amide is obtained.
Longitudinal rotational resonance echoes in solid state nuclear magnetic resonance: Investigation of zero quantum spin dynamics
Authors: Karlsson Torgny , Levitt Malcolm H.
Published: July 2002

In the magic angle spinning NMR of rotating solids, it is possible to drive nuclear spin transitions by the macroscopic sample rotation. This rotational resonance phenomenon arises when the chemical shift frequency difference between two neighboring spin sites matches a small integer multiple of the sample rotation frequency. The mechanically driven spin evolution may be reversed by applying a radio-frequency pulse cycle, leading to the formation of rotational resonance echoes. A detailed theory based on a Liouville space formalism is developed. Experimental demonstrations are given for a polycrystalline sample of zinc C213 acetate. The decay of zero quantum coherence is examined at two different magnetic fields.
Spin populations in one-dimensional alternant spin systems: A theoretical approach
Authors: Srinivasan Bhargavi , Kahn Oliver , Ramasesha S.
Published: July 2002

Spin-density maps, deduced from polarized neutron diffraction experiments, for both the pair and chain compounds of the system Mn2+Cu2+ have been reported recently. These results have motivated us to investigate theoretically the spin populations in such alternant mixed-spin systems. In this paper, we report our studies on the one-dimensional ferrimagnetic systems (SA,SB)N where N is the number of AB pairs. We have considered all cases in which the spin SA takes on allowed values in the range 1 to 72 while the spin SB is held fixed at 12. The theoretical studies have been carried out on the isotropic Heisenberg model, using the density matrix renormalization group method. The effect of the magnitude of the larger spin SA on the quantum fluctuations in both A and B sublattices has been studied as a function of the system size N. We have investigated systems with both periodic and open boundary conditions, the latter with a view to understanding end-of-chain effects. The spin populations have been followed as a function of temperature as well as an applied magnetic field. High-magnetic fields are found to lead to interesting re-entrant behavior. The ratio of spin populations |PA/PB| is not sensitive to temperature at low temperatures.
Fourier transform emission spectroscopy of the A 2Π–X2Σ+ system of BeH
Authors: Focsa C. , Firth S. , Bernath P. F. , Colin R.
Published: July 2002

The A 2Π–X 2Σ+ transition of BeH was observed by Fourier transform emission spectroscopy using a hollow cathode discharge lamp. The 0–0 to 6–6 bands were rotationally analyzed and molecular constants extracted. The equilibrium rotational constants Be and bond lengths were found to be 10.331 21(50) cm−1 and 1.341 68(3) Å for the ground state and 10.466 31(27) cm−1 and 1.332 99(2) Å in the excited state. In order to link the diagonal bands together and to determine the vibrational constants, the 0–1 to 6–7 bands in an archival arc emission spectrum were also rotationally analyzed. In the X 2Σ+ and A 2Π states, the spectroscopic constants are nearly identical so the Δv=−1 bands were too weak to be seen in our Fourier transform spectra. Franck–Condon factors were calculated for the A 2Π–X 2Σ+ transition from Rydberg–Klein–Rees potential curves. These new rotational analyses now link up with the previous work on the 0–7, 0–8, 0–9, 1–9 and 1–10 bands of the C 2Σ+–X 2Σ+ system [R. Colin, C. Drèze, and M. Steinhauer, Can. J. Phys. 61, 641 (1983)]. Spectroscopic data are thus available for all bound ground state vibrational levels, v″=0–10, and a set of Dunham Y constants were determined. BeH joins the small group of chemically bound molecules for which a nearly complete set of ground state rovibronic energy levels are known experimentally.
Stochastic resonance in liquid membrane oscillator
Authors: Zuo Xiaobing , Hou Zhonghuai , Xin Houwen
Published: July 2002

The oil/water liquid membrane oscillator which has been intensively investigated in recent years is generally considered a promising model for excitable biomembranes. The present work deals with the stochastic resonance (SR) phenomena liquid membrane oscillators with the model proposed by Yoshikawa, and the mechanism of SR and some interesting behaviors are also discussed. When SR occurs, the input signal is greatly amplified with the cooperation of noise and the output signal-to-noise ratio is also dramatically enhanced. We consider SR as a possible functional mechanism of some sensory cells.
New aspects of the photodissociation of water in the first absorption band: How strong is excitation of the first triplet state?
Authors: Schröder Thomas , Schinke Reinhard , Ehara Masahiro , Yamashita Koichi
Published: July 2002

We consider the possibility of the X̃ 1A1→3B1 excitation of water at wavelengths near 193 nm, i.e., in the red tail of the first absorption band. The corresponding excited-state potential-energy surface is calculated by quantum mechanical methods and the dynamics calculations are performed in the time-dependent representation. It is shown that an absorption cross section for exciting the triplet state 3B1, that at its maximum is about five hundred times (or more) smaller than the corresponding cross section for excitation of the 1B1 state, might explain the surprisingly small H+OD/D+OH branching ratio in the photodissociation of HOD at 193 nm measured by Plusquellic et al. (the foregoing paper). The singlet–triplet transition dipole moment estimated in this way also explains the unexpectedly small cross section ratio for H2O and D2O measured at 193 nm.
Polyampholyte solutions between charged surfaces: Debye–Huckel theory
Authors: Dobrynin Andrey V. , Rubinstein Michael , Joanny Jean-François
Published: July 2002

We calculate the interaction between two identically charged surfaces immersed in a polyampholyte solution in the framework of the linear response approximation. In the case of the randomly charged polyampholyte with equal fractions of positively f+ and negatively f− charged groups the interaction between charged surfaces is similar to that in electrolyte solutions. The screening of the surface potential in the linear response approximation is due to the charge redistribution on the length scales of the polymer coils without the excess of the polymers. In the solution of asymmetric polyampholytes (f+≠f−) the charge density fluctuations result in additional attractive interactions linearly proportional to the charge asymmetry.
Two dimensional umbrella sampling techniques for the computer simulation study of helical peptides at thermal equilibrium: The 3K(I) peptide in vacuo and solution
Authors: Samuelson S. O. , Martyna G. J.
Published: July 2002

New computational techniques are developed to study conformational equilibrium in helical peptides at finite temperature. Two dimensional umbrella sampling methods involving coordinate systems which incorporate the α-helical hydrogen bonding distances along the peptide backbone as collective variables are presented. The umbrella sampling techniques are combined with new extended system multiple time step molecular dynamics simulation methods to allow peptide equilibria to be examined on a realistic potential energy surface. In particular, this combination of methods is employed to study a sixteen residue alanine-based peptide, Ac−(AAAAK)3A−NH2, at T=300 K in vacuo and solution, under the all-atom CHARMM22 force field. The efficiency of the multiple time step integration methodology permitted a minimum total run length of 30 ns to be employed in each umbrella sampling calculation reported.
Wetting behavior of a Lennard-Jones system
Authors: van Remoortere P. , Mertz J. E. , Scriven L. E. , Davis H. T.
Published: July 2002

The wetting behavior of a sessile cylinder of a Lennard-Jones liquid in equilibrium with its vapor and in contact with a solid substrate was examined. The surface energy of the solid was manipulated by altering the solid–fluid potential interaction relative to the fluid–fluid potential. Temperature was varied between T=68 and T=108 K. The influence of temperature on equilibrium wetting qualitatively followed the predictions of critical wetting theory, i.e., the equilibrium angle became smaller with increasing temperature. Dynamic spreading has been examined for different types of solids. It was found that the displacement of the contact line varied from a linear to a square root dependence on time for respectively a smooth and a textured solid. A differentiation of the spreading behavior with the spreading parameter S was observed. At S≫1 a monolayer preceded the spreading droplet, at S⩾1, a step-function-like pattern of 3 to 4 monolayers emerged while at S⩽1 the spreading droplet was flattened by a multilayer front. Significant collective motion on the free surface of the droplets was observed in connection to the “long-ranged” solid–fluid interaction potential. This behavior hinted to a possible wetting mechanism and was described in a qualitative fashion.
Correlation energies obtained from symmetry-adapted versus symmetry-broken Hartree–Fock solutions in a model delocalized one-dimensional system
Authors: Reinhardt P. , Malrieu J.-P.
Published: July 2002

Singlet instabilities of the Hartree–Fock (HF) solutions, leading to bond-centered or atom-centered charge-density waves, occur in extended one-dimensional systems, such as a regular cyclic H4n+2 ring. These solutions are much less delocalized than the symmetry-adapted one, as appears when comparing the equivalent localized molecular orbitals. The symmetry-broken solutions exhibit a finite gap at the Fermi level, whereas for the symmetry-adapted solution the gap can be extrapolated with a 1/N law towards a zero asymptotic value. If one performs post-Hartree–Fock calculations, of Mo/ller–Plesset nth order or linearized coupled cluster type, one obtains a higher final energy from the symmetry-broken HF solutions than from the symmetry-adapted HF wave function. A procedure is proposed to obtain directly symmetry-adapted localized self-consistent field orbitals in systems presenting HF instabilities.
Solid-state nuclear magnetic resonance spectra of dipolar-coupled multi-spin systems under fast magic angle spinning
Authors: Filip Claudiu , Hafner Siegfried , Schnell Ingo , Demco Dan E. , Spiess Hans W.
Published: July 2002

A general treatment of nuclear magnetic resonance (NMR) spectra under magic-angle spinning (MAS) conditions is provided that is applicable both to homogeneously and inhomogeneously broadened lines. It is based on a combination of Floquet theory and perturbation theory, and allows the factorization of the spin system response into three factors that describe different aspects of the resulting MAS spectrum. The first factor directly reflects the Floquet theorem and describes the appearance of sidebands. The other two terms give the integral intensities of the resulting sidebands and their line shapes and depend on the specific features of the considered interaction. The analytical form of these two factors is derived for multi-spin dipolar interactions under fast MAS. The leading term in the expansion of the integral intensities involves products of only two spin operators whereas the linewidths, which are found to be different for the different sideband orders, are determined predominantly by three-spin terms. The higher-spin contributions in both cases scale with increasing powers of the inverse rotor frequency and thus becomes less and less important when approaching the limit of fast spinning. From numerical simulations and the analysis of experimental MAS NMR spectra it was found that for typical spin systems, spinning frequencies of the order of the strongest couplings are sufficient to allow the analysis of the sideband intensities within the approximation of two-spin terms. This scaling of the different contributions together with the strong distance dependence of the dipolar interaction thus leads to a considerable simplification in the fast spinning limit and provides the basis for using the dipolar interaction in high-resolution MAS spectra to obtain local structural information.
Simulation of swelling of model polymeric gels by subcritical and supercritical solvents
Authors: Escobedo Fernando A. , de Pablo Juan J.
Published: July 2002

Monte Carlo simulations have been performed to examine the volumetric behavior of model nonionic gel systems where the solvent is at subcritical and supercritical conditions. It is found that the degree of swelling correlates well with the density changes of the solvent: swelling decreases as the solvent becomes less dense. As the temperature is increased at constant pressure, gel swelling exhibits a monotonic decreasing trend whose onset approximately agrees with the solvent’s boiling point for subcritical pressures and, for supercritical pressure, with the location of the lower critical solution temperature (LCST) of the solvent–uncrosslinked polymer system. Evidence of an incipient first-order volume phase transition has been detected for polymeric networks with long strand lengths in contact with a supercritical solvent.
Diffusion in random pores
Authors: Lee J. C.
Published: July 2002

We recently reported the results of molecular dynamics simulations for a binary liquid mixture in porous glass. The results for the concentration-fluctuation autocorrelation function showed that the mutual diffusion and the self-diffusion are different. Unfortunately, the linear size of the three-dimensional system used for this simulation was rather small. We carry out the same simulations here using a relatively large two-dimensional system. The results are the same as those for the small three-dimensional system. The self-diffusion departs from the q2 Fick’s pattern while the mutual diffusion follows it more closely.
Avoidance model for soft particles. I. Charged spheres and rods beyond the dilute limit
Authors: Kramer Eric M. , Herzfeld Judith
Published: July 2002

The avoidance model introduced by Han and Herzfeld [Mater. Res. Soc. Symp. Proc. 463, 135 (1997)] for parallel, charged spherocylinders is extended to the case of charged spherocylinders with orientational freedom. The accuracy of the theory in the dilute solution limit is checked by comparing the predicted second virial coefficient to exact values. For dilute charged spheres, the avoidance model predictions are accurate to within 17% for all values of the charge and Debye–Huckel decay length. For dilute charged spherocylinders in the long rod limit, the theory is less accurate. The second virial coefficient is overestimated by 35%–90%. However, qualitative trends in the data are captured and smaller errors are expected for shorter rods or more concentrated solutions. Isotropic-nematic phase diagrams are presented for a range of ionic strengths, rod lengths, and charges. Specific comparison is made to experimental data for colloidal suspensions of tobacco-mosaic virus (TMV) particles and the bacteriophage fd. The concentration dependence of the short range order is also discussed.
Adsorption of O2 on TiO2(110): A theoretical study
Authors: Shu Chutian , Sukumar N. , Ursenbach C. P.
Published: July 2002

First-principles calculations are carried out with the objective of assigning a chemical identity to the variety of forms of molecular oxygen which are known to exist on a TiO2(110) surface. Six different geometries and spin configurations of O2 are shown to be stable in the bridging oxygen defect site. The relationship of these results to experimental observations is discussed.
Reaction of state-selected ammonia ions with methane
Authors: Everest Michael A. , Poutsma John C. , Flad Jonathan E. , Zare Richard N.
Published: July 2002

We have investigated the reaction of ammonia ions with methane molecules (CD4) over the collision energy range of 0.5–10.0 eV and for ammonia ion vibrational states ranging from ν2=1–10. Under these conditions, the two main product channels are NH3D+ and CD3+. The cross section for formation of NH3D+ is enhanced with increasing internal energy at collision energies below 6.0 eV, and independent of internal energy at higher collision energies. The enhancement is greater for forward-scattered products indicating that ammonia-ion vibrational energy enhances reactivity at large impact parameters. The mechanism for formation of CD3+ involves collision-induced dissociation of CD4 (or NH3+) which leads to the formation of a short-lived [NH3CD3]+ ([NH2CD4]+) complex, which then decays to products. This reaction is found not to be vibrationally mode selective, which is consistent with the hypothesis that mode selectivity in reactions of ammonia ions is driven by the Franck–Condon overlap whenever charge transfer is involved.
A spline approach to trial wave functions for variational and diffusion Monte Carlo
Authors: Bressanini Dario , Fabbri Giordano , Mella Massimo , Morosi Gabriele
Published: July 2002

We describe how to combine the variational Monte Carlo method with a spline description of the wave function to obtain a powerful and flexible method to optimize electronic and nuclear wave functions. A property of this method is that the optimization is performed “locally”: During the optimization, the attention is focused on a region of the wave function at a certain time, with little or no perturbation in far away regions. This allows a fine tuning of the wave function even in cases where there is no experience on how to choose a good functional form and a good basis set. After the optimization, the splines were fitted using more familiar analytical global functions. The flexibility of the method is shown by calculating the electronic wave function for some two and three electron systems, and the nuclear wave function for the helium trimer. For 4He3, using a two-body helium–helium potential, we obtained the best variational function to date, which allows us to estimate the exact energy with a very small variance by a diffusion Monte Carlo simulation.
Quantum Monte Carlo study of rovibrational states utilizing rotating wavefunctions: Application to H2O
Authors: Acioli Paulo H. , Costa L. S. , Prudente Frederico V.
Published: July 2002

We applied the procedure developed by Prudente et al. [Chem. Phys. Lett. 302, 249 (1999)] to compute the rovibrational energy levels of the water molecule. The procedure utilizes rotating wavefunctions as the trial basis in the correlation-function quantum Monte Carlo method. The procedure originally tested for a rotating harmonic oscillator and rotating Morse potential, has been extended for triatomic systems, replacing the spherical harmonics by the Wigner functions. We computed the rovibrational levels of the water molecule and compared the results with the experiment, and they are shown to be accurate.
Molecular dynamics studies of semifluorinated hydrocarbon monolayers
Authors: Kim Namshin , Shin Seokmin
Published: July 2002

We present the results of molecular dynamics simulations of Langmuir monolayers of a semifluorinated hydrocarbon molecule F(CF2)12(CH2)18H. Our simulations show that highly ordered structures can be formed at low temperature after quenching from a random structure at high temperature. The structural formation process of the monolayer is characterized by a decrease in the gauche defects of the hydrocarbon block and an increase in the global bond-orientational order throughout the chain molecules. The behavior of the order parameter exhibits a sudden change, signaling a structural phase transition. Structural formation of the semifluorinated monolayer is found to depend on temperature, suggesting that the time scale for the ordering process is determined by the rate of conformational change. It is also found that the monolayer structure consists of clusters of different chain orientations relative to the interface for the head-group-free F(CF2)12(CH2)18H molecule, with a slightly larger fraction for a hydrocarbon-down, fluorocarbon-up configuration.
Experimental simulation of macromolecules in trehalose aqueous solutions: A photon correlation spectroscopy study
Authors: Magazù S. , Maisano G. , Migliardo P. , Villari V.
Published: July 2002

The protective effect of trehalose on biological membranes against freezing or dehydration has been the subject of many studies aimed to understand the reasons why some lower organisms, under stress conditions, synthesize trehalose. In this work we report the results of a study on Poly(Ethylene Oxide)/trehalose/water mixtures performed by Photon Correlation Spectroscopy. The chemical structure of the polymer, simpler than that of proteins and its helical conformation in water, constitute a useful starting point for understanding the more complex protein/trehalose/water interactions. In order to distinguish the different dynamics, trehalose and PEO have been studied separately in water, at different concentration and temperature values; then the ternary PEO/trehalose/water system has been investigated at different sugar amounts. The obtained findings support the “water-replacement” hypothesis, indicating that a direct polymer–trehalose interaction occurs. Furthermore, trehalose is shown to affect the swelling properties of the polymer with temperature, stabilizing its conformation.
Trapping sites of hydrogen atoms in solid HD and D2: An electron spin echo study
Authors: Kumada Takayuki , Noda Tomoyoshi , Kumagai Jun , Aratono Yasuyuki , Miyazaki Tetsuo
Published: July 2002

Trapping sites of H and D atoms in solid HD and D2 have been determined using electron spin echo (ESE) spectroscopy. It was found that all the H and D atoms are trapped in substitutional sites and that the H atoms push back surrounding HD(D2) molecules to produce local lattice distortion around the atoms, whereas the D atoms do not. It is expected that the local lattice distortion is produced by zero-point motion of the H atoms whose amplitude is larger than that of host HD(D2) molecules and that the isotope effect is due to difference in the amplitude between the H and D atoms. The lattice distortion around the H atoms may induce the increase in rate constant for the tunneling reaction D+DH→D2+H with the increase in temperature in solid HD reported in Chem. Phys. Lett. 261, 463 (1996).
Structural assembly of Cd-arachidate molecules in multilayers
Authors: Vitta Satish , Metzger T. H. , Major S. S.
Published: July 2002

The three-dimensional (vertical and lateral) structure of Cd-arachidate multilayers prepared on two different substrates, glass and silicon, is studied simultaneously by grazing incidence x-ray reflection/diffraction. The grazing incidence reflectivity studies indicate the formation of a well-ordered layered structure on both the substrates. The reflectivity simulations show that the bilayer spacing in the case of a glass substrate is 5.54 nm while that on a silicon substrate is only 5.44 nm, indicating a vertical tilt of the Cd-arachidate molecules on a glass substrate by about 9.5°. The interface roughness is found to be 0.3 nm for multilayers on a glass substrate and 0.4 nm on the silicon substrate. The interface topography, determined by studying the nonspecular scattering behavior, is found to be replicated between the various interfaces in the multilayers, independent of the type of substrate. The lateral topography of the interfaces in multilayers on a glass substrate shows a self-affine nature with no lateral length limit for the roughness fluctuations. The interfaces in multilayers on a silicon substrate, however, exhibit a saturation behavior for the roughness fluctuations. In the plane of the multilayers the Cd-arachidate molecules on a glass substrate have a centered rectangular lattice arrangement while they have a distorted hexagonal lattice arrangement on a silicon substrate.
Transfer of a chemical substrate pattern into an island-forming diblock copolymer film
Authors: Heier Jakob , Genzer Jan , Kramer Edward J. , Bates Frank S. , Walheim Stefan , Krausch Georg
Published: July 2002

We investigate the transfer of a chemical pattern on a substrate into a symmetric diblock copolymer thin film of poly(styrene-2-vinylpyridine) (PS-PVP). The substrates have patterns of self-assembled monolayers (SAMs) produced by microcontact printing H3C-terminated (H3C-) SAM stripes alternating with HO-terminated (HO-) SAM stripes. The PS-PVP lamellae over the H3C-SAM have a defect structure that attracts excess PS-PVP that would normally form islands on a uniform HO-SAM stripe. We seek to understand the process that limits our ability to accommodate all excess polymers on top of the H3C-SAM. In the early stages of annealing, waves of thickness develop from the H3C/HO-SAM boundary and propagate into the film over the HO-SAM. For very short annealing times, the wavelength λ of these thickness waves is constant at any given time for all grating periodicities. Large amplitude patterns develop when λ=2d/(2n−1), where d is the width of the HO-SAM stripe and n is an integer ⩾1. Such patterns suggest constructive interference of the thickness waves and indeed much lower amplitudes over the HO-SAM stripes are observed when λ=d/n (destructive interference). This behavior seems close to that seen for surface-directed spinodal decomposition waves in thin films of binary polymer mixtures. We achieve more complete transfer of excess copolymers from the HO-SAM stripe to the H3C-SAM ones if the film is preordered under a confining layer that does not permit the formation of surface features.
Resonance Raman study of the A-band short-time photodissociation dynamics of axial and equatorial conformers of iodocyclopentane
Authors: Zheng Xuming , Lee Cheong Wan , Phillips David Lee
Published: July 2002

We have obtained resonance Raman spectra of iodocyclopentane in cyclohexane solution at three excitation wavelengths resonant with the A-band absorption. The A-band resonance Raman spectral bands can be assigned to fundamentals, overtones, and combination bands of seven axial conformer and eight equatorial conformer Franck–Condon active modes. The resonance Raman and absorption cross sections were simultaneously simulated using wave packet calculations and a simple model. The best fit parameters of the simulations and the normal mode descriptions were used to determine the A-band short-time photodissociation dynamics of the axial and equatorial conformers of iodocyclopentane. The axial and equatorial conformers exhibit noticeably different short-time photodissociation dynamics that suggest that the C–I bond cleavage process is conformation dependent. The axial conformer short-time photodissociation dynamics have larger changes in the carbon–carbon stretch and three carbon atom bending motions as well as the torsional motion about the α and β carbon atom bond. The CCI bending motions for the axial and equatorial conformers of iodocyclopentane as well as previously reported results for the equatorial conformer of iodocyclohexane are significantly smaller than CCI bending motions found for most noncyclic iodoalkanes examined so far. This suggests that the cyclic backbone restricts the initial motion of the C–I bond cleavage along the CCI bend in iodocycloalkanes compared to the noncyclic iodoalkanes.
Theoretical predictions for the mechanical response of a model quartz crystal microbalance to two viscoelastic media: A thin sample layer and surrounding bath medium
Authors: White Christopher C. , Schrag John L.
Published: July 2002

Theoretical predictions are presented for the operating characteristics of a quartz crystal microbalance (QCM) system consisting of the crystal, an attached viscoelastic sample layer, and a surrounding viscoelastic bath medium. Predictions are given for the spatial variation of “particle” velocity and velocity gradient throughout the sample layer, for the characteristic mechanical impedance acting on the crystal surface due to the viscoelastic sample layer plus surrounding bath medium, and for the resultant changes, Δfr, in the resonance frequency of the QCM. Errors introduced by employing the usual simple Sauerbrey-type Δfr/sample-layer–mass relationship, together with a constant frequency offset to approximate the effect of the viscoelastic bath medium, are explored. In general, the viscoelastic properties of both the sample and bath media can substantially affect the apparent thickness (mass) of the sample layer if it is obtained by employing the typical Sauerbrey-type approximation noted above.