bluuebunny/crossref_metadata_2025 huggingface dataset:
A molecular dynamics study of impurity desorption from solid clusters of rigid C60 molecules
Authors: Abramo M. C. , Caccamo C.
Published: July 2002

We investigate through molecular dynamics (MD) simulation the desorption at high temperature of Kr atoms from solidlike clusters of rigid C60 molecules. The inert gas particles are initially allocated in the interstices of a solid C60 matrix, placed at the center of the simulation box with empty space around; desorption is then monitored as a function of the average temperature achieved at the end of different MD run. The assumed sphericalized two-body potential practically excludes endohedral absorption of Kr inside the fullerene cage and is potentially suitable to model impurity species other than the one here considered. The obtained results indicate that in the range of temperatures 1200⩽T⩽1500 K the solid matrix of C60 particles is stable, and that only a part of the ‘doping’ Kr atoms is able to escape from the C60 ‘environment’, the rest remaining trapped inside the fullerene cluster in a configuration in which the rare gas atoms occupy on the average the octahedral sites of the host matrix. The implications of such results as far as the role of impurity species in determining the experimentally observed high temperature instability of fullerites accompanied by carbon amorphization, are discussed.
Normal mode analysis for a comparative study of relaxation processes of charge transfer and photoexcitation in C60
Authors: Zhang G. P. , Sun X. , George Thomas F. , Pandey Lakshmi N.
Published: July 2002

In this article we employ a normal mode analysis to the relaxation processes of charge transfer and photoexcitation in C60. It is found that only a few Hg modes, the so-called dominant modes, play a critical role in the relaxation, which may imply that only a few Hg modes can have a strong electron–phonon coupling constant. This is consistent with recent experimental results, although previous calculations reported that almost all the Hg modes have roughly the same coupling constant. Those dominant modes control mainly the early and the later behaviors and determine the relaxation times of the processes. The difference between the relaxation times of charge transfer and photoexcitation can also be understood from the dominant modes. We discover that the eigenvectors of the dominant modes are very unique, i.e., if the bond lengths are altered along the eigenvectors, one can observe a change in the lattice by a typical laminar distortion structure.
Anharmonic force fields and spectroscopic properties of BF3 and CF3+ using the coupled cluster method
Authors: Pak Youngshang , Woods R. Claude
Published: July 2002

Anharmonic force fields and ro-vibrational spectroscopic properties of BF3 and CF3+ have been investigated, using the coupled cluster method with single and double substitutions augmented by a perturbative estimate of triple excitations [CCSD(T)] with a basis set of 120 contracted Gaussian-type orbitals (cGTOs). A complete set of ro-vibrational spectroscopic constants for each species has been calculated using second-order perturbation theory. For BF3 the CCSD(T) equilibrium bond distance is larger than the experimental value by 0.0058 Å, and the fundamental frequencies are within 5 cm−1 of experiment, except for ν3, which shows a deviation of 16 cm−1. The CCSD(T) method yields vibration-rotation interaction constants αe in excellent agreement with available experimental values (within 3 MHz). The CCSD(T) bond distance of CF3+ is 1.2305 Å with a basis set of 220 cGTOs, which is corrected to 1.2272 Å by applying a correction derived from the identical level of calculation on BF3. The fundamental frequencies ν1, ν2, ν3, and ν4 for CF3+ are predicted to be 1044 cm−1, 813 cm−1, 1683 cm−1, and 593 cm−1, respectively.
High pressure nucleation in water/nitrogen systems
Authors: Luijten C. C. M. , Bosschaart K. J. , van Dongen M. E. H.
Published: July 2002

Nucleation rate measurements of water in the presence of nitrogen as a carrier gas are reported at total pressures near 10, 25, and 40 bar, and temperatures of 230 and 250 K. The results were obtained using our pulse-expansion wave tube, particularly suited for high pressure nucleation research. Enhanced fugacity of water vapor in the mixture, due to the presence of nitrogen, was quantitatively taken into account. Values of the enhancement factors as a function of pressure and temperature were correlated from accurate gravimetric measurements available in literature. The results demonstrate a strong influence of nitrogen pressure on the nucleation behavior of water, when temperature and supersaturation are kept fixed. The effect is associated with a decrease of the surface tension of water, due to the adsorption of nitrogen onto the liquid surface. A tentative model is presented that qualitatively describes this decreasing surface tension with pressure. The competition between the opposing effects of enhanced fugacity and decreasing surface tension is identified as a complicating factor in detecting pressure effects on nucleation. This conclusion is expected to hold for other vapor/carrier gas systems as well.
Ab initio calculations of the polarizability and the hyperpolarizability of C60
Authors: Norman Patrick , Luo Yi , Jonsson Dan , Ågren Hans
Published: July 2002

The linear polarizability, α, and the second hyperpolarizability, γ, of C60 in gas phase have been computed by ab initio cubic response theory in the random phase approximation and with an efficient parallel implementation. With a tailored, well-tested, basis set, containing more than 1000 contracted basis functions the average values of α and γ are predicted to be 8.58×10−23 cm3 and 5.73×10−35 esu, respectively, which are about 8 and 9 times larger than the corresponding values for benzene calculated at the same level of accuracy.
The pressure dependence of the elastic constants of ice III and ice VI
Authors: Tulk C. A. , Gagnon R. E. , Kiefte H. , Clouter M. J.
Published: July 2002

The dependence of single-crystal Brillouin spectra on both crystal orientation and pressure (P) has been measured for phases III and VI of ice. The conditions, with P in kbar, were T=−20 °C, 2.2⩽P⩽3.0 for ice III, and T=−2 °C, 6.2⩽P⩽8.2 for ice VI. The pressure dependencies of the elastic constants and the polycrystalline elastic properties were determined.
Theory of radiative recombination with strong laser pulses and the formation of ultracold molecules via stimulated photo-recombination of cold atoms
Authors: Vardi A. , Abrashkevich D. , Frishman E. , Shapiro M.
Published: July 2002

A time dependent theory for radiative recombination induced by strong pulses is presented. Analytic solutions in the adiabatic limit are derived and found to be in excellent agreement with exact numerical solutions. Both the pump-before-dump “intuitive” and dump-before-pump “counter-intuitive” schemes are considered. Resonantly enhanced two-photon recombination of ultracold atoms is shown to be an efficient mechanism for the production of ultracold molecules. We have performed detailed calculations on the radiative recombination of cold Na atoms by short laser pulses. Our calculations show that, per pulse, it is possible for up to 97% of all head-on Na-Na colliding pairs to end up as v=0, J=0 translationally cold Na2 molecules. We show that these findings, translated to thermally cooled ensemble conditions, mean that the fraction of Na atoms at μKelvin which can be recombined by a pulse of 20 ns duration and 108 W/cm2 peak intensity, to form J=0 molecules is 6×10−6 per pulse. With the above parameters, a laser operating at 100 Hz can convert half of an ensemble of cold atoms to cold molecules in ∼25 min. The efficiency of the process can be increased by going to longer pulses of lower intensity, by going to lower temperatures or by increasing the density of the ensemble. In particular, the “counter-intuitive” scheme which allows for use of longer pulses of lower intensities, with no spontaneous emission losses, considerably increases the yield.
Comment on “Long-time behavior of the angular velocity autocorrelation function” [J. Chem. Phys. 105, 9695 (1996)]
Authors: Cichocki B. , Felderhof B. U.
Published: July 2002

It is pointed out that Masters, in his analysis of the long-time tail of the angular velocity autocorrelation function of a Brownian particle, incorrectly incorporates a nonlinear effect in the calculation of the linear frequency-dependent friction matrix.
Kinetics of two-dimensional phase transition of amphiphilic monolayers at the air/water interface
Authors: Fainerman V. B. , Vollhardt D. , Melzer V.
Published: July 2002

A theoretical model is developed which describes the adsorption kinetics of the two-dimensional first-order phase transition in an adsorption layer. The theory includes a kinetic model of the phase transition in Langmuir and Gibbs monolayers, the adsorption from bulk solution, and the dissociation kinetics of bulk micelles. The theory describes well the experimental results of the adsorption kinetics of the amphiphile N-dodecyl-hydroxy-butyric acidamide dissolved in the aqueous bulk phase. The inflection point of dynamic surface pressure curves corresponds to the first-order phase transition in the adsorption layer. The rate constants for the 2D aggregation in the adsorption layer and the bulk micelle formation are calculated. The relaxation time of the 2D aggregation is approximately ten times lower than that of the association/dissociation of bulk micelles.
Absolute emission cross sections of the Paschen-α line and production dynamics of the 4F state of the excited hydrogen atom in e-H2 collisions
Authors: Yonekura Nobuaki , Furuya Kenji , Nakashima Keiji , Ogawa Teiichiro
Published: July 2002

Absolute emission cross sections of the Paschen-α line from H (4S,4P,4D,4F) produced in e-H2 collisions have been determined to be 4.31–10.1×10−20 cm2 at 20–100 eV by utilizing the Balmer-α emission cross sections as the standard; these values are about half those of the Balmer-β emission. From these values, the formation cross section of the 4F state has been first determined, together with the angular momentum distributions, which are the fraction of the formation cross sections of the 4S, 4P, 4D, and 4F state. The 4F state had appreciable cross sections at electron energies below 24 eV and was assigned to be produced from the singly-excited states of H2 . In the doubly-excited state channel opening above 27–32 eV, the 4P and 4D states are preferentially produced, and the 4F state is suppressed.
Probability distribution of the chemical states of a closed system and thermodynamic law of mass action from kinetics: The RNA example
Authors: Jacob Christine , Breton Nicolas , Daegelen Patrick , Peccoud Jean
Published: July 2002

We show some thermodynamic results concerning chemical kinetics, either from statistical physics, under the strong assumption (EXS) of the statistical exponential distribution of the physical states, or from kinetics. In that frame, the reaction rates are modeled according to the stochastic activation collision theory, under the weaker assumption (EXW) of the statistical exponential distribution not on the whole space but only within each structure; moreover a symmetry condition (SY) concerning the local properties of the rate constants is assumed. The Boltzmann distribution and an extension of the more general result of the thermodynamic law of mass action are proved. Moreover, we show that the assumptions (EXS) and (EXW) are equivalent, under (SY). At last, we deal with the stochastic example of the RNA structure for which we calculate the temporal probabilities of the structures by means of a jump process.
On the excess enthalpy and volume of two-component Lennard-Jones fluids in supercritical region: Is the mixture of simple liquids simple?
Authors: Fujihara Ichiro , Nakanishi Koichiro
Published: July 2002

Monte Carlo (MC) simulations have been carried out for supercritical mixtures of Lennard-Jones (LJ) fluids to investigate the unique behavior of excess thermodynamic functions in supercritical region obtained experimentally. It is reported that constant pressure excess molar enthalpies HpE and excess molar volumes VpE take large absolute values and show complex concentration dependencies in such regions. In this paper, HpE and VpE have been calculated for a total of 12 kinds of mixtures, each having different combinations of energy and size parameters and the combining rule for unlike interactions. It is found that such a unique thermodynamic behavior of real systems in their supercritical region can be reproduced, at least qualitatively, by simple model systems. It is also found that the unique thermodynamic behavior is closely related to the equation-of-state of pure components in their supercritical region. In order to estimate the contribution to the constant pressure excess thermodynamic functions from the equation-of-states of pure components, we have developed a data conversion method by which we can calculate a new function, excess internal energy at constant volume from HpE, VpE, and equation-of-states of pure components. The converted excess internal energies at constant volume for the present model systems become quite simple; that is, their magnitudes are reduced to be small and the concentration dependencies become symmetrical. Further analysis indicates that the large differences of molar volume and internal energy between pure components and large VpE are responsible for the unique behavior of excess functions at constant pressure. The data conversion method is also applied to the experimental data for the CO2+C2H6 system in the supercritical region. The obtained excess internal energies at constant volume also show similar simple behavior as those for the model systems. Furthermore, it is found that the complex dependence of experimental HpE on pressure is also closely related to equation-of-states of pure components.
Low-lying electronic states of lanthanocenes and actinocenes M(C8H8)2 (M=Nd, Tb, Yb, U)
Authors: Liu Wenjian , Dolg Michael , Fulde Peter
Published: July 2002

Large-scale state-averaged multiconfiguration self-consistent field, multireference configuration interaction, and averaged coupled-pair functional calculations, including relativistic effects by means of energy-consistent quasirelativistic pseudopotentials, have been carried out for the ground and low-lying excited states of the di-π-cyclooctatetraene (or bis[8]annulene) metal sandwich complexes M(C8H8)2 (M=Nd, Tb, Yb, U). It is found that the ground state configurations for the lanthanocenes are 4fnπ3, while for the actinocenes they are 5fn−1π4. The ground states for the lighter and heavier lanthanocenes are, respectively, the lower and higher multiplicity states resulting from the coupling between the highest possible spin–multiplicity of the central metal 4fn subshell and the unpaired ligand π electron, whereas they always have the highest possible multiplicity of 5fn−1π4 for the actinocenes. The metal–ring distances and symmetric metal–ring stretching frequencies are reported. The special characteristics of the uranium 5f orbitals in uranocene are described. Extensive spin–orbit configuration interaction calculations were performed for uranocene and confirm the assignment of the ground state and first excited state of uranocene made previously by other authors. However, a different ordering is obtained for the higher states. The calculated term energy for the first excited state of uranocene is in excellent agreement with the experimental value.
H
(n=4)/
D
(n=4)
branching ratio in the electron-impact dissociation of HOD
Authors: Furuya Kenji , Koba Fumihiro , Ogawa Teiichiro
Published: July 2002

The Balmer-β emission from the H and the D atoms has been observed by electron impact on the mixtures of H2O and D2O with several mixing ratios at selected incident electron energies over the range of 22–100 eV. Intensity ratios of the emission have been estimated from computer simulation of the emission spectra on each combination of the mixing ratios and the electron energies. Analyzing the intensity ratios, the H(n=4)/D(n=4) branching ratio in the dissociation of pure HOD has been estimated for the first time to be about 1.8 below the second threshold (∼26 eV) of the Balmer-β emission, where it is about 1.5 over 40–100 eV. It has also been found that the ratios of the cross section of the H Balmer emission from H2O to the D Balmer emission from D2O are equal to those of HOD, and the ratios of the D Balmer emission from D2O to that from HOD are constant and about two over 22–100 eV.
Dissociative resonance activation of cyclopropane monolayers on copper: Evidence for CH and CC bond scission
Authors: Martel Richard , McBreen Peter H.
Published: July 2002

Energy selective electron beam modification of cyclopropane on Cu(110) and Cu(111) single crystal surfaces was performed using the electron beam intrinsic to a high resolution electron energy loss (HREELS) spectrometer. Vibrational spectra of the resulting adsorbed dissociation fragments were recorded using the same HREELS spectrometer. Electron irradiation in the range 10–18 eV of weakly adsorbed cyclopropane was found to lead to the formation of chemisorbed dissociation fragments. HREELS and complementary thermal desorption data were used to identify the chemisorbed products as metallacyclobutane and cyclopropyl groups. The formation of these two surface species implies the existence of two dissociation channels involving CC and CH bond scission, respectively. Variations in the cross section for electron induced modification indicates the presence of a dissociative attachment resonance centered at approximately 10 eV. Cyclopropyl formation and ring opening of cyclopropane at 10 eV are attributed to competition between two decay channels of a core-excited resonance. The cross section for metallacyclobutane formation, at 10 eV, was evaluated to be roughly 0.8×10−17 cm2. The ratio of cyclopropyl to metallacyclobutane surface species, formed at 10 eV, was found to be 4:1.
Equation of state and electrical conductivity of “synthetic Uranus,” a mixture of water, ammonia, and isopropanol, at shock pressure up to 200 GPa (2 Mbar)
Authors: Nellis W. J. , Holmes N. C. , Mitchell A. C. , Hamilton D. C. , Nicol M.
Published: July 2002

Equation-of-state, temperature, and electrical-conductivity data were measured for a solution of water, ammonia, and isopropanol at shock pressures up to 200 GPa. The chemical composition is similar to that of the fluid mixture thought to be the major constituent of the giant planets Uranus and Neptune.
Embedded cluster model for chemisorption using density functional calculations: Oxygen adsorption on the Al(100) surface.
Authors: Duarte Hélio A. , Salahub Dennis R.
Published: July 2002

An embedded cluster model to study chemisorption on metal surfaces is presented. The approach is based on a method proposed by Whitten and Pakkanen [Phys. Rev. B 21, 4357 (1980)], which constructs an embedding potential using a set of molecular orbitals (MO) obtained for a large cluster considered appropriate to describe the adsorption site correctly. The embedded cluster approach combined with the Kohn–Sham density functional theory (KS-DFT) formalism has special features that are discussed in this work. There are several possible approximations that can be explored in practical implementations: the localization procedure used to localize the KS MOs on the surface, the choice of the cluster, the way the embedding potential is constructed and the possibility to use fractional MO occupancies and even fractional charges in order to set up the embedded cluster, are some of the points that are discussed. We have implemented this approach in a modified version of deMon-KS. Results are presented for oxygen adsorption on the Al(100) surface. The embedding potential is constructed from the MOs obtained for an Al70 cluster. The atop, bridge and four-fold sites are described by the Al10, Al20 and Al21 clusters, respectively. The bare cluster results are also presented and compared to those for the embedded cluster. The embedded cluster calculations are in better agreement with the available experimental results. The four-fold site is the most favored. The oxygen atom is found to be in the surface plane, while for the bare cluster model the oxygen lies 0.5 Å above the surface. The local density of states of the oxygen at the four-fold site of Al70, and the bare and embedded clusters have been evaluated and compared with the available UPS data.
Molecular beam study of gateway-coupling N2(C 3Πu/a′ 1Σu−) and chemical quenching of the metastable N2(a′) state
Authors: Ottinger Ch. , Shen G.
Published: July 2002

Collisions of metastable N2 molecules in a beam with a variety of gases result in nitrogen “second positive” emission from a single, isolated level N2 (C 3Πu1, v=0, J=14). The highly selective collisional excitation was explained by a gateway-type coupling with the accidentally nearly degenerate level N2 (a′ 1Σu−, v=16, J=14). This identifies even the Λ component of the emitting level, namely “f”. Some additional, minor gateway emissions could in part be related to known N2(C) perturbations. Unlike gateway emissions reported earlier from this laboratory, the nature of the collision partner was important here. He, Ne, Ar, Kr, N2, CO, and CO2, all gave about equally strong emission, while almost none was observed with NH3, O2, and Xe. This is ascribed to the collisional quenching of N2(a′) by Penning ionization (for NH3) or by a harpooning mechanism (for Xe and O2).
The dissociative recombination of hydrocarbon ions. II. Alkene and alkyne derived species
Authors: Rebrion-Rowe C. , Lehfaoui L. , Rowe B. R. , Mitchell J. B. A.
Published: July 2002

Dissociative recombination reactions of a number of cyclic ions, derived from ion-molecule reactions involving acetylene as parent molecule and of protonated butane derived from reactions involving ethylene have been studied. It is found that the recombination rate coefficient does not seem to be much affected by the substitution of functional groups for hydrogen atoms in the cyclic cases. Also, protonated butane has a rate coefficient very similar to that for protonated methane.
Efficient construction of self-avoiding walks for protein folding simulations on a torus
Authors: Greenwood Garrison W.
Published: July 2002

Monte Carlo simulations of protein folding often have large running times because many generated conformations are not self-avoiding walks. This paper shows how a greater number of valid self-avoiding walks can be produced by first embedding a linear chain (representing a sequence of amino acids) into a binary n-cube and then embedding a torus into the n-cube to find the conformation. The described technique will permit analysis of both secondary and tertiary protein structures.
Efficient scalar spin relaxation in the rotating frame for matched radio-frequency fields
Authors: Skrynnikov N. R. , Lienin S. F. , Brüschweiler R. , Ernst R. R.
Published: July 2002

A theoretical analysis is presented for liquid-state T1ρ relaxation in a coupled two-spin system I=1/2, S=1 in the presence of two radio-frequency fields applied to each of the spins individually. It is demonstrated that the relaxation rate constant T1ρ−1 of the spin I due to scalar relaxation sharply increases when the two radio-frequency fields are matched according to the Hartmann–Hahn condition. Relaxation measurements on the amino-protons of 3-nitroaniline show good agreement with theory.
Photodesorption of physisorbed molecules from a Ag(111) surface: The low photon energy threshold and the low translational temperature of desorbed molecules
Authors: Howe Pui-Teng , Dai Hai-Lung
Published: July 2002

Photodesorption of HFCO, H2CO, CH2CO, and CH3Cl from Ag(111) has been studied using mass spectrometry flight-time measurements and temperature programmed desorption. All these molecules are molecularly adsorbed on Ag(111). Under nanosecond pulsed laser irradiation, desorption from Ag(111) in the experimental photon energy range of 1.17⩽hν⩽4.67 eV has been detected. No threshold behavior has been established within this energy range. The translational energy distribution of the desorbing molecules is characterized by a Maxwell–Boltzmann temperature in the range 110–150 K. From the photodesorption cross section dependence on photon energy and intensity, a nonthermal, substrate mediated excitation mechanism is inferred. The common observation of low translational temperatures and low photon energy thresholds is in striking contrast to the high average translational temperatures and threshold behavior reported for photodesorption of chemisorbed molecules from metal surfaces. The unique observations can be understood through the substrate–electron mediated excitation model. However, it requires either that the substrate electrons attach to the molecules with positive electron affinities or that electron scattering excites the high frequency, v=1 molecular vibrational levels, possibly through dipole interactions.
Scanning near-field optical microscopy of cholesteric liquid crystals
Authors: Huser Thomas , Lacoste Thilo , Heinzelmann Harry , Kitzerow Heinz-Siegfried
Published: July 2002

We have studied glasslike cholesteric liquid crystals by means of scanning near-field optical microscopy. A periodic modulation of the optical polarization was used in order to achieve an optical contrast due to the birefringence of the liquid crystal. Variation of the cholesteric pitch indicates an optical resolution of ≈200 nm for a wavelength of 633 nm.
An infrared study of H8Si8O12 cluster adsorption on Si(100) surfaces
Authors: Eng Joseph , Raghavachari Krishnan , Struck Lisa M. , Chabal Yves J. , Bent Brian E. , Banaszak-Holl Mark M. , McFeely F. R. , Michaels Amy M. , Flynn George W. , Christman Stan B. , Chaban Ed E. , Williams Gwyn P. , Radermacher Klaus , Mantl Siegfried
Published: July 2002

Motivated by a controversy about the proper interpretation of x-ray photoelectron spectra of Si/SiO2 interfaces derived from the adsorption of H8Si8O12 spherosiloxane clusters on Si(100) surfaces, we have studied the adsorption geometry of the H8Si8O12 clusters on deuterium-passivated and clean Si(100) surfaces by using external reflection infrared spectroscopy. Access to frequencies below 1450 cm−1 was made possible through the use of specially prepared Si(100) samples which have a buried metallic CoSi2 layer that acts as an internal mirror. A comparison of the infrared spectrum of the clusters on a deuterium-passivated Si(100) surface at 130 K with an infrared spectrum of the clusters in a carbon tetrachloride solution reveals that the clusters are only weakly physisorbed on the D/Si(100) surface and also provides evidence for the purity of the cluster source. We also present infrared spectra of clusters directly chemisorbed on a clean Si(100) surface and show evidence that the clusters are adsorbed on the Si(100) via attachment by one vertex. A complete assignment of the observed vibrational features, for both physisorbed and chemisorbed clusters, has been made based upon comparisons with the results obtained in ab initio calculations using gradient-corrected density functional methods.
Numerical transfer-matrix simulations of S=1 molecular magnetic chains
Authors: D’Auria Alvaro Caramico , Esposito Ugo , Esposito Filippo , Gatteschi Dante , Kamieniarz Grzegorz , Wałcerz Stanisław
Published: July 2002

A numerical quantum transfer-matrix approach to S=1 macroscopic chains with single-site anisotropy and alternating bonds is worked out in the framework of statistical mechanics, using a real-space decomposition scheme and the Trotter formula for the noncommuting operators. A fit of the experimental susceptibility data for a number of the uniform and bond-alternating quasi-one-dimensional molecular magnets is performed down to the low-temperature region. New microscopic parameters for the nonuniform systems are established, the characteristic temperature locating the maximum of the zero-field uniform susceptibility is estimated and the temperature behavior of the zero-field susceptibility for different ferro—antiferro coupling ratios is presented.
Molecular model for the simultaneous orientational and translational ordering in a two-dimensional liquid
Authors: Kaganer V. M. , Osipov M. A.
Published: July 2002

We present a molecular–statistical theory of the phase transition from the two-dimensional isotropic liquid to the phase which has the elements of the orientational and translational order combined in a single order parameter. This phase possesses the glide symmetry, similar to the herringbone order in crystals, and is related to the mesophases observed in Langmuir monolayers. The microscopic definition of the herringbone order parameter is presented and the transition temperatures from the isotropic to the nematic, smectic, and the herringbone phases are expressed in terms of the direct correlation function of the two-dimensional isotropic fluid. The relative stability of these phases is discussed. The transition temperature into the herringbone phase is estimated using the simple model of hard discs interacting via the quadrupole–quadrupole potential that promotes the herringbone order.
Lifetime lengthening of molecular Rydberg states in the condensed phase
Authors: Vigliotti F. , Zerza G. , Chergui M. , Rubayo-Soneira J.
Published: July 2002

We report on fluorescence lifetime lengthenings of molecular Rydberg states in condensed media in the case of the NO molecule trapped in inert gas matrices. In rare gas matrices, the fluorescence of the A 2Σ+ state originates from two types of sites, hereafter called red and main. The red site is considered to be a loose site with more than one vacancy in Ar, Kr, and Xe and an h.c.p. site in Ne matrices. It exhibits a lifetime lengthening with respect to the gas phase of 25% in Kr matrices and 100% in Xe matrices. The main site fluorescence stems from monosubstitutional sites. It exhibits lifetime enhancements of up to 100% when going from Ne to Xe matrices. When, however, the fluorescence quantum yields are taken into account, the lifetime increases from the gas phase value to up to two orders of magnitude in the sequence H2–Ne(D2)–Ar–Kr–Xe. Furthermore, this change in transition moment is not observed in the absorption spectrum. These results stress the influence of the solvent and its microscopic structure on molecular Rydberg lifetimes. Different mechanisms are discussed in relation with the observations.
Band-gap closure of H2–He mixtures under pressure
Authors: Chacham H. , Reis S. P. , Koiller Belita
Published: July 2002

We present a theoretical study on the behavior of the energy gap of H2–He mixtures under pressure. The calculations are performed within a large-unit-cell tight-binding formalism parametrized for solid molecular H2. We investigate how the energy gap is affected at a given density by the He content as well as by the orientational and the positional disorder of the hydrogen molecules. We find that the increase of the He content in the mixture systematically increases the energy gap, shifting the insulator-to-metal transition to higher pressures. For small values of the He content x, we also find that the lattice constant at the metal–insulator transition scales as (1−x)1/3. This scaling remains approximately valid to estimate the energy gap in the insulating regime. This provides a simple rule to obtain the energy gap of H2–He mixtures under pressure from the energy gap of pure H2 under pressure.
Correlation dimension as a measure of surface roughness of protein molecules
Authors: Choi Jun-Ho , Kim Hojing , Lee Sangyoub
Published: July 2002

We propose to use the correlation dimension as a measure of surface roughness of protein molecules which varies with the size of guest molecules. Smaller guest molecules can probe the finer details of surface corrugation than the larger ones. Hence the correlation dimension has the larger value for smaller guest molecules. Molecular dynamics simulations of a protein molecule at constant temperature and pressure show that the magnitude of structural fluctuation and the coefficient of self-diffusion vary with the size of solvent molecules and correlate nicely with the correlation dimension.
Size-consistent self-consistent configuration interaction from a complete active space: Excited states
Authors: Amor N. Ben , Maynau D. , Sánchez-Marı́n J. , Nebot-Gil I. , Evangelisti S.
Published: July 2002

The self-consistent size consistent on a complete active space singly and doubly configuration interaction (SC)2CAS-SDCI method is applied to excited states. The (SC)2 correction is performed on a closed shell state, and the excited states are obtained by diagonalization of the dressed matrix. A theoretical justification of the transferability of the improvement concerning the dressing state to all roots of the matrix is presented. The method is tested by three tests on the spectrum of small molecules.
Fourier transform infrared observation of the vibrational spectrum of the H2SiCCH radical in Ar at 10 K
Authors: Han D. S. , Rittby C. M. L. , Graham W. R. M.
Published: July 2002

A Fourier transform infrared study has resulted in the first observation of a vibrational spectrum of the nonplanar H2SiCCH radical produced by trapping the products of the vacuum ultraviolet photolysis of a mixture of SiH4 silane, and C2H2 acetylene, in an Ar matrix at ∼10 K. Two vibrational fundamentals have been observed, the ν3(a′), silicon–carbon stretching mode at 2055.6 cm−1 and the ν4(a′), hydrogen–silicon bending mode at 926.8 cm−1. The vibrational frequencies, relative intensities, and extensive measurements of D and C13 isotopic shifts are all in good agreement with the values predicted by new density functional theory calculations.
The effect of a uniform electric field on homogeneous vapor–liquid nucleation in a dipolar fluid. I. Stockmayer fluid
Authors: Oh K. J. , Gao G. T. , Zeng X. C.
Published: July 2002

The rate of homogeneous vapor–liquid nucleation is controlled mainly by the Gibbs formation free energy of critical clusters. Within the framework of the i/v cluster theory of Reiss and co-workers, the formation free energies of Stockmayer clusters in the presence of a uniform electric field are determined. The Helmholtz free energy of the clusters, an input required in the i/v cluster theory, is calculated via Monte Carlo simulations. The most significant result is that as a uniform electric field is applied the formation free energy at fixed supersaturation of the vapor becomes larger than that in the absence of a field, whereas at fixed chemical potential the formation free energy becomes smaller.
Determination of multiple steady states in a family of allosteric models for glycolysis
Authors: Li Hsing-Ya
Published: July 2002

To predict glycolytic oscillations, Goldbeter and Lefever [Biophys. J. 12, 1302 (1972)] proposed a complex allosteric model, consisting of 14 species and 32 reactions. Under the usual assumption of a quasisteady state for all the enzymatic forms, they simplified it to a two-variable model and ruled out the possibility of multiple steady states. In this work, the original network is determined to admit multiplicity of steady states by a zero eigenvalue analysis. It is shown that the existence of the multiplicity in the original network can be determined by a subnetwork with five species and eight reactions. The fourteen-species network can be treated as containing four such subnetworks. The analysis is extended to a general modified allosteric model, consisting of n active subunits. It can be shown that the general network has no steady-state multiplicity if all the four subnetworks follow the case of n=1; otherwise, multiple steady states can occur.
Molecular dynamics simulation of diffusion-limited catalytic reactions
Authors: Toxvaerd S.
Published: July 2002

The kinetics of a bimolecular and locally catalytic-activated reaction in a liquid is investigated by molecular dynamics (MD) and compared with the results of the theoretical model by Oshanin and Blumen [J. Chem. Phys. 108, 1140 (1998)]. The MD confirms the predictions. At late times, when the substrate concentration is low, the kinetics approach a second order kinetics with an effective rate constant proportional to the concentration of active subvolumens (enzymes) and proportional to r04, where r0 is the radius of the (spherically) active subvolumens.
Measurement of high-frequency rotational transitions of H2O+ in its ground state by far-infrared laser magnetic resonance (LMR) spectroscopy
Authors: Mürtz P. , Zink L. R. , Evenson K. M. , Brown J. M.
Published: July 2002

Thirteen new rotational transitions of H2O+ in the (0,0,0) level of the X̃ 2B1 state have been measured in the wavenumber region between 80 and 200 cm−1 (50 and 120 μm) by far-infrared laser magnetic resonance (LMR) spectroscopy. LMR data measured previously between 25 and 90 cm−1 (110 and 400 μm), as well as optical and infrared combination differences, have been combined with the new LMR data in a weighted least-squares analysis using an A-reduced expression of the rotational-fine structure Hamiltonian. Thirty-two molecular constants were simultaneously determined, some sextic centrifugal distortion parameters and some quartic and sextic spin-rotation parameters for the first time. From this improved set of molecular parameters, very accurate calculations of rotational term values and zero-field predictions of the 111–000 transition, including hyperfine structure, have been performed. Moreover, the electronic g-tensors and the hyperfine coupling constants are consistent with ab initio calculations which had been carried out for these constants.
Particle shape reconstruction by small-angle scattering: Integration of group theory and maximum entropy to multipole expansion method
Authors: Spinozzi Francesco , Carsughi Flavio , Mariani Paolo
Published: July 2002

The reconstruction of the shape of biological macromolecules in solution from small-angle scattering data is an important task for understanding the structure-function relationship. In the last few decades, the multipole expansion method was used to attack this problem. Here we present an improvement of the existing techniques by including the advantages of the group theory and maximum entropy. To show the capability of this method, pseudoexperimental scattering curves have been simulated and analyzed. Six different particle shapes have been considered: sphere, cylinder, cone, double cone, ellipsoid and cube. The results obtained by the proposed procedure gave an excellent agreement between the original and reconstructed shapes. The method is quite general, and can be applied to systems of different kinds.
Scaling properties of the ν1 Raman active nitrate mode in doped ammonium and potassium nitrate crystals
Authors: Li Shuling , Ma Shuguo , Wu Guozhen
Published: July 2002

The Raman intensity behavior of the nitrate modes during phase transitions is analyzed via the power law |T−Tc|β for (F−, Cl−, Br−)-doped NH4NO3 and (Na+, Rb+, Cs+, Ag+)-doped KNO3 crystals. The exponent β is shown to be highly dependent on the degree of doping. This observation is based on the resonance behavior of the resident dopant. The spatial extent of the resonance mode is shown to be the characteristic dimension for the crystal domains during phase transition. This extent is demonstrated to be proportional to dc, the characteristic average distance between dopant ions at which β values of the nitrate ν1 and ν4 modes are the same. The scaling property as evidenced by β of the ν1 mode is analyzed via the (mass of the dopant)−1/2 scaled average distance between dopant ions which is an important parameter for characterizing the doping effect. Previous results for the (Na+, K+, Ag+, Pb2+)-doped NH4NO3 crystals are also employed to complete the analysis [S. Ma, G. Wu, and H. Wang, J. Chem. Phys. 108, 7758 (1998)].
Density-functional study of van der Waals forces on rare-gas diatomics: Hartree–Fock exchange
Authors: Pérez-Jordá José M. , San-Fabián Emilio , Pérez-Jiménez Angel J.
Published: July 2002

A density-functional theory study of van der Waals forces on rare-gas diatomics is carried out. Hartree-Fock-Kohn-Sham formalism is used, that is, the exchange-correlation functional is expressed as the combination of Hartree-Fock exchange plus an approximation to the correlation energy functional. Spectroscopic constants (Re,ωe, and De) and potential energy curves for the molecules He2, Ne2, Ar2, HeNe, HeAr, and NeAr are presented. Several approximations to the correlation functional are tested. The best results, in good agreement with reference experimental data, are obtained with the functional proposed by Wilson and Levy [L. C. Wilson and M. Levy, Phys. Rev. B 41, 12930 (1990)].
The rotational spectra of single molecular eigenstates of 2-fluoroethanol: Measurement of the conformational isomerization rate at 2980 cm−1
Authors: McWhorter David A. , Hudspeth Evan , Pate Brooks H.
Published: July 2002

The rotational spectroscopy of single molecular eigenstates has been used to measure the conformational isomerization rate in 2-fluoroethanol. Eigenstates in the asymmetric –CH2(F) stretch spectrum of the Gg′ conformer near 2980 cm−1 are prepared with an infrared laser. These eigenstates are approximately 2000 cm−1 above the barrier to Gg′−Tt conformational isomerization. The rotational spectrum is measured using an infrared-microwave double-resonance technique based on the Autler–Townes splitting of states in a strong microwave field. This technique does not require saturation of the infrared preparation step. Two types of rotational transitions are observed. These are assigned to rotational transitions from vibrational states with Tt conformation (near 15.8 GHz) and to “isomerization states” (near 17.1 GHz) where the torsional wave functions are above the isomerization barrier. The isomerization kinetics are obtained from the linewidth of the ensemble eigenstate rotational spectrum. The lifetime for the Tt conformer is 2.7 ns. The isomerization states relax at approximately twice the rate of the Tt states (1.5 ns lifetime). This result is consistent with a kinetics model where the isomerization proceeds by “over-the-barrier” pathways. Both lifetimes are longer than the bright-state IVR lifetime (275 ps) indicating that the intramolecular dynamics occur on two distinct time scales. The isomerization rate for the Tt states is three orders-of-magnitude slower than predicted by a simple RRKM rate expression.
Crossed-beam reaction of carbon atoms with sulfur containing molecules. I. Chemical dynamics of thioformyl (HCS X2A′) formation from reaction of C(3Pj) with hydrogen sulfide, H2S(X1A1)
Authors: Kaiser R. I. , Ochsenfeld C. , Head-Gordon M. , Lee Y. T.
Published: July 2002

The reaction between ground state carbon atoms, C(3Pj), and hydrogen sulfide, H2S(X1A1), was studied at four average collision energies between 16.7 and 42.8 kJ mol−1 using the crossed molecular beam technique. The reaction dynamics were deducted from time-of-flight spectra and from laboratory angular distributions combined with ab initio calculations. These data suggest that the reaction proceeds through an addition of C(3Pj) to the sulfur atom to form a triplet CSH2 van der Waals complex. Successive H atom migration on the triplet or singlet surface forms a thiohydroxycarbene intermediate, HCSH, which decomposes through a tight exit transition state to HCS(X2A′)+H(2S1/2). At lower collision energies, a weak L-L′ coupling leads to isotropic center-of-mass angular distributions. As the collision energy rises, the angular distributions show increasing forward scattering thereby documenting that the reaction goes through an osculating HCSH complex. Identification of the HCS isomer under single collision conditions is a potential one-step pathway by which to form organo-sulfur molecules in interstellar environments during the collision of the comet Shoemaker-Levy 9 with Jupiter, and in combustion flames of sulfur containing fuels.
Determination of orientational distributions from 2H NMR data by a regularization method
Authors: Winterhalter J. , Maier D. , Grabowski D. A. , Honerkamp J. , Müller S. , Schmidt C.
Published: July 2002

A Tikhonov regularization method is used to estimate the orientational distribution function from deuteron NMR line shapes. Two sets of data, obtained for different orientations of a sample with respect to the magnetic field, are used in the regularization procedure to overcome the ambiguity arising from the superposition of the two NMR transitions of the spin I=1 system. A refined model of the NMR signal that takes into account line shape distortions resulting from finite pulse duration is developed. Tests on simulated data show that the results of the regularization procedure obtained with this model and a simple one that does not consider the finite pulse width differ significantly for broad spectra. The regularization method is applied to lyotropic liquid crystals of surfactant/water mixtures. The director distributions estimated for both hexagonal and lamellar lyomesophases give evidence of director undulations in the hexagonal phase.
Mapped Fourier methods for long-range molecules: Application to perturbations in the Rb2(0u+) photoassociation spectrum
Authors: Kokoouline V. , Dulieu O. , Kosloff R. , Masnou-Seeuws F.
Published: July 2002

Numerical calculations of vibrational levels of alkali dimers close to the dissociation limit are developed in the framework of a Fourier Grid Hamiltonian method. The aim is to interpret photoassociation experiments in cold atom samples. In order to avoid the implementation of very large grids we propose a mapping procedure adapted to the asymptotic R−n behavior of the long-range potentials. On a single electronic potential, this allows us to determine vibrational wave functions extending up to 500a0 using a minimal number of grid points. Calculations with two electronic states, A 1Σu+ and b 3Πu states, both correlated to the Rb(5s)+Rb(5p) dissociation limit, coupled by fine structure are presented. We predict strong perturbation effects in the Rb2(0u+) spectrum, manifested under the 5s, 5p 2P1/2 dissociation limit by an oscillatory behavior of the rotational constants.
Definition and properties of the emission anisotropy in the absence of cylindrical symmetry of the emission field: Application to the light quenching experiments
Authors: Kuśba Józef , Lakowicz Joseph R.
Published: July 2002

We considered the properties of the fluorescence anisotropy when the cylindrical symmetry of the fluorescence emission field is absent due to the effects of polarized light quenching. By light quenching we mean stimulated emission by a second longer wavelength pulse following the excitation pulse. In these experiments one observes the excited state population which remains following stimulated emission. When cylindrical symmetry is not present the generally known definition of the emission anisotropy cannot be applied. A generalized theory of anisotropy was described previously by Jabloński. However, we found this formalism to be inadequate for the expected experimental results of light quenching. An extension of this concept, which we call an anisotropy vector, appears capable of describing the expected orientation under all conditions of light quenching. We found that the anisotropy vector can exist within a plane defined by two projections rH and rV. The projection rV is comparable to the classical steady state or time-dependent anisotropy with cylindrical symmetry. The projection rH has no direct analogue in classical anisotropy theory. The interesting behavior of the anisotropy vector is that all possible points (rH,rV) are placed inside a certain triangle, which we call a triangle of anisotropy. For symmetrical molecules, or for molecules which display isotropic depolarizing rotations, the anisotropy vector is expected to decay on the anisotropy triangle along straight lines towards the origin. The concept of the anisotropy vector should allow predictions of the effect of polarized light quenching on the anisotropy decays, and suggests experimental methods to study anisotropy decays in the presence of light quenching. Further work is needed to apply these concepts to anisotropic rotators.