bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of ferromagnetic layer structure on the magnetization process in NiFe/Co/Al–O/NiFe and Co/NiFe/Al–O/NiFe junctions
Authors: Yu C. C. , Petford-Long A. K.
Published: July 2002

The effect of ferromagnetic layer structure on the magnetization reversal processes in (NiFe/Co)/Al–O/NiFe (S1) and (Co/NiFe)/Al–O/NiFe (S2) films was investigated. The films were fabricated by sputter deposition, and the Al–O layer was prepared by oxidizing an Al layer in air. Two distinct magnetization processes were observed by transmission Lorentz microscopy (TLM) with increasing in situ applied field. Reversal of the NiFe/Co bilayer in S1 occurs via moment rotation, while reversal of the Co/NiFe bilayer in S2 occurs by domain wall motion, in both cases at higher field than the top NiFe layer. The difference can be ascribed to the difference in the deposition order of the ferromagnetic bilayers (FMBs). High resolution electron microscopy shows that the grains in the top NiFe layer are randomly oriented in both films. In S1, the NiFe grains in the FMB are randomly oriented, with columnar grains present in the Co. In S2, a columnar grain structure of NiFe in the FMB is induced by the Co underlayer. The rough FMB/Al–O interface in S2 leads to weak biquadratic interlayer coupling, which will slightly reduce the field range of the antiparallel magnetization configuration. Hysteresis loops of S1 and S2 show two stage magnetization reversals in each sample, which are consistent with TLM results.
Mössbauer study of the magnetic properties of nanocrystalline Fe80.5Nb7B12.5 alloy
Authors: Kopcewicz M. , Grabias A. , Škorvánek I. , Marcin J. , Idzikowski B.
Published: July 2002

The nanocrystalline body-centered-cubic (bcc)-Fe phase was formed by controlled 1 h annealing of the amorphous Fe80.5Nb7B12.5 alloy at temperatures ranging from 490 to 650 °C. The microstructure and magnetic properties of the nanocrystalline alloy were investigated by Mössbauer spectroscopy, differential scanning calorimetry, and quasistatic hysteresis loop measurements. Conventional Mössbauer spectroscopy allowed identification of phases and the determination of their relative content. The specialized radio frequency (rf)-Mössbauer technique, which employs the effects induced by the rf magnetic field (rf collapse and rf sideband effects) allowed us to distinguish the magnetically soft amorphous and nanocrystalline phase from the magnetically harder microcrystalline Fe. The rf-Mössbauer experiments performed as a function of the rf field intensity allowed determination of the anisotropy fields in each phase of the nanocrystalline alloy (amorphous matrix, nanoscale bcc-Fe grains). The measurements of the hysteresis loops versus annealing temperature revealed minimum coercivity (0.15 Oe) at 610 °C.
Comparison of one- and two-photon optical beam-induced current imaging
Authors: Xu Chris , Denk Winfried
Published: July 2002

Optical beam induced current (OBIC) imaging through the backside of integrated circuits was investigated in the wavelength λ region from 1.15 to 1.26 μm. With a subpicosecond excitation source and approximately 1 mW at the sample, the two-photon contribution to the generated photocurrent dominates at λ=1.25 μm but becomes negligible for λ<1.18 μm. One-photon- (1P-) and two-photon- (2P-) OBIC images are very different. In the 1P case a strong contribution by scattered light to the carrier generation leads to an edge enhancement effect that is entirely missing when 2P excitation dominates. 2P-OBIC images often show supply-voltage dependent intensity steps that are much sharper than the optical resolution permits. The advantages of 2P-OBIC lie in the spatial confinement of the free carrier generation, a more relevant contrast mechanism, and the promise of a substantial increase in spatial resolution because of the quadratic intensity dependence and the possibility of using silicon solid immersion lenses, which could eventually provide resolution sufficient for circuits made by deep UV lithography.
Comment on “Monte Carlo simulation of the laser-induced plasma plume expansion under vacuum: Comparison with experiments” [J. Appl. Phys. 83, 5075 (1998)]
Authors: Itina T. E.
Published: July 2002

An attempt to use direct simulation Monte Carlo (DSMC) method for pulsed laser ablation at laser fluence as high as 17 J/cm2 is analyzed. Under these conditions, however, the evaporation flux is very high which results in a very high laser plume density. Therefore, both collision rate and collisional zone near the target are too large to effectively use a conventional DSMC technique whose possibilities are always limited by finite computer storage and available computation time. Nevertheless, the authors of the considered paper claim to propose a new approach to overcome the restrictions of DSMC. We found that the main idea of the approach is the plume reduction in space. However, according to the dimension theory, under this transformation one should preserve the Knudsen number, that defines the degree of rarefaction, and, hence, the number of cells to be used in DSMC. Therefore, the number density of the plume should be increased and particle collisions should be calculated with higher frequency than in the case without space reduction. In our opinion, this important point was neglected by the authors, the collision frequency was not increased, and the number of cells was not properly chosen. As a result, the proposed approach does not break the DSMC restrictions, and the results obtained do not correspond to the experiments with high evaporation flux.
Transport and magnetic properties of laser ablated La0.7Ce0.3MnO3 films on LaAlO3
Authors: Raychaudhuri P. , Mukherjee S. , Nigam A. K. , John J. , Vaisnav U. D. , Pinto R. , Mandal P.
Published: July 2002

La 0.7 Ce 0.3 MnO 3 is a relatively new addition to the family of colossal magnetoresistive manganites, in which the cerium ion is believed to be in the Ce4+ state. In this article, we report the magnetotransport properties of laser ablated La0.7Ce0.3MnO3 films on LaAlO3, and the effect of varying the ambient oxygen pressure during growth and the film thickness. We observe that the transport and magnetic properties of the film depend on the oxygen pressure, surface morphology, film thickness, and epitaxial strain. The films were characterized by x-ray diffraction using a four-circle goniometer. We observe an increase in the metal-insulator transition temperature with decreasing oxygen pressure. This is in direct contrast to the oxygen pressure dependence of La0.7Ca0.3MnO3 films and suggests the electron doped nature of the La0.7Ce0.3MnO3 system. With decreasing film thickness we observe an increase in the metal-insulator transition temperature. This is associated with a compression of the unit cell in the a-b plane due to epitaxial strain. On codoping with 50% Ca at the Ce site, the system (La0.7Ca0.15Ce0.15MnO3) is driven into an insulating state suggesting that the electrons generated by Ce4+ are compensated by the holes generated by Ca2+, thus making the average valence at the rare-earth site 3+ as in the parent material LaMnO3.
Dependence of the coherence spike on the material dephasing time in pump–probe experiments
Authors: Balk Michael W. , Fleming Graham R.
Published: July 2002

The coherence spike observed in the transmitted probe intensity decay curve in pump–probe experiments is analyzed with a model which depends explicitly upon the population decay and dephasing rate constants. Using this model and simple pulse shape functions a series of decay curves are calculated throughout the range of wide to narrow pulse widths relative to the dephasing time. These calculations suggest that the coherence spike may be used to obtain information about the phase coherence in the electronic or vibrational transition being investigated, when the pulse width is comparable to or smaller than the dephasing time. Methods of extracting T2 from the coherence spike are also discussed.
Nonlinear rupture of thin free liquid films
Authors: Prévost Martine , Gallez Dominique
Published: July 2002

We study nonlinear effects on film rupture by investigating the stability of a viscous free film to finite amplitude disturbances. Until now, theoretical results were obtained through a linear stability analysis for infinitesimal perturbations, which is only valid for a short time since disturbances grow to finite size at rupture. The liquid film considered is uncharged, nondraining, and laterally unbounded. The dynamics of the film is described by the Navier–Stokes equations where attractive van der Waals forces are operative, with suitable boundary conditions at the two surfaces (in the particular case of tangentially immobile surfaces). A nonlinear evolution equation describing the variation of the film thickness along the lateral space dimension, is derived for the squeezing mode which is characterized by symmetrical surface waves. This highly nonlinear partial differential equation is solved numerically and rupture characteristics are predicted and compared to experiments. It is shown that the nonlinearities significantly accelerate the rupture process.
Optimization of Brownian dynamics methods for diffusion-influenced rate constant calculations
Authors: Northrup Scott H. , Curvin Marc S. , Allison Stuart A. , McCammon J. Andrew
Published: July 2002

One-dimensional Brownian dynamics algorithms for reaction and reflection recently developed by Lamm and Schulten are adapted into the special boundary topology necessary to compute diffusion-influenced rate constants of arbitrarily complicated bimolecular reactions in three dimensions. Performance of these relative to the primitive free diffusion algorithm commonly employed is discussed. Remaining sources of error arising from boundaries are (1) boundary curvature effects and (2) reactive discontinuity effects in cases where orientational criteria for reaction exist. The magnitudes of these errors are calculated as a function of simulation time step size. In addition, a special statistical sampling procedure is developed which allows the simultaneous treatment of a large class of reactive boundary problems in one simulation. This procedure is illustrated by the treatment of reactive patch size effects on the rate constant in the model of Solc and Stockmayer.
Numerical coupled Hartree–Fock study of the total (electronic and nuclear) parallel polarizability and hyperpolarizability for the FH, H+2, HD+, and D+2 molecules
Authors: Adamowicz Ludwik , Bartlett Rodney J.
Published: July 2002

Polarizabilities and hyperpolarizabilities of molecules have both electronic and vibrational contributions. The latter can be important in experimental determinations involving static electric fields, as in the Kerr effect. This paper reports purely numerical coupled Hartree–Fock calculations for the electronic states of FH. Electronic energy surfaces obtained for different electric field strengths are then used to solve, numerically, the nuclear equation for several vibrational states. The vibrational contributions to βzzz exceed the electronic component and change the sign of the total hyperpolarizability. γzzzz is similarly increased by an order of magnitude. It is also shown that a Born–Oppenheimer treatment of H+2 for γzzzz is in perfect agreement with a recent fully nonadiabatic three particle solution.
Study of underpotentially deposited copper on gold by fluorescence detected surface EXAFS
Authors: Blum L. , Abruña H. D. , White J. , Gordon J. G. , Borges G. L. , Samant M. G. , Melroy O. R.
Published: July 2002

Using grazing incidence geometry and fluorescence detection, surface EXAFS of a monolayer of underpotentially deposited copper on epitaxially deposited gold (111) on mica was observed. Both Cu–Au and Cu–O scattering are observed. The results are interpreted in terms of models in which the Cu–O distance is 2.08±0.03 Å and the Cu–Au distance is 2.5±0.06 Å. The copper and three gold atoms at the metallic surface form an elongated tetrahedron, with the oxygen on top of the copper. Two plausible models for the oxygen arrangement are proposed, one involving only one oxygen, the other with a sulfate ion adsorbed. This second model yields a slightly better fit of the data.
Cluster formation of hydrogen cyanide in solid and liquid CCl4 matrices
Authors: Knözinger Erich , Kollhoff Helmut , Langel Walter
Published: July 2002

The cluster formation of hydrogen cyanide (HCN) in carbon tetrachloride (CCl4) matrix was studied by IR and far IR spectroscopy. This included three different phases of the matrix material: (a) The liquid (conventional solution studies), where only the formation of linear HCN dimers occurs. (b) The plastic crystalline solid (matrix isolation studies in the high temperature modification with randomly oriented rotating CCl4 molecules) where linear dimers and trimers of HCN appear to be stable. (c) The crystalline solid (matrix isolation studies in the low temperature modification with orientationally ordered CCl4 molecules) where long linear HCN chains of unspecified degree of aggregation are observed. For dilute solutions the phase transition from (a) to (b) induces a thermodynamically unstable state which relaxes towards equilibrium on a time scale of minutes depending on the temperature applied. This relaxation process is accessible to time-resolved Fourier transform IR and far IR spectroscopy.
Implementation of a phase and amplitude modulated π pulse for coherent optical spectroscopy
Authors: Lin C. P. , Bates J. , Mayer J. T. , Warren W. S.
Published: July 2002

We have implemented an amplitude and phase modulated laser pulse of the form [sech(αt)]1+3i. Such a pulse shape creates uniform population inversion within a selected bandwidth, compensates for Rabi frequency inhomogeneity, and leaves transitions outside the bandwidth unaffected. The unique features of this pulse shape are verified by measuring total fluorescence in iodine vapor as a function of laser power.
Polarized absorption spectroscopy of Λ-doublet molecules: Transition moment vs electron density distribution
Authors: Bigio Laurence , Grant Edward R.
Published: July 2002

Polarized two-photon photodissociation of NO2 in the region of 480 nm yields NO(v″=0) with an anisotropic distribution of J. Measured polarization ratios are compared to quantum mechanical calculations for a range of expected ratios for the various isolated and mixed branches of the NO X̃2Π1/2→Ã 2Σ+ transition. Theoretical results show that main branches and their respective satellites (e.g., R11 and R21 branches) have the same transition moment directionally, though their intensities are in general different, implying that care is needed in interpreting polarization data from the mixed branches, such as (Q21+R11) or (Q11+P21), which measure the Π+ and Π− Λ doublet, respectively. Recognition of this fact is particularly important for properly separating the consideration of electron density distributions of Λ doublets from transition moment directionalities, as this has been a source of confusion in the literature. The measured results indicate that the principal two-photon photoexcitation pathway in NO2 photolysis is 2A1→1 2B2→2 2B2, with moderate A″ state mixing in the intermediate.
Dynamics of ionic solvation
Authors: Rips Ilya , Klafter Joseph , Jortner Joshua
Published: July 2002

Dynamics of solvation in simple polar liquids is studied within the framework of the mean spherical approximation. Exact results are derived for the Born solvation energy and for the correlation function for the solvation time of an instantaneously formed ion (or dipole) in a polar solvent. The results are in qualitative agreement with the recent approximate treatment by P. Wolynes [J. Chem. Phys. 86, 5133 (1987)]. Implications of the results for the solvation dynamics of dipoles and of excess electrons in polar solvents are considered.
Dynamics of energy flow from CH overtone excitations: Theoretical and experimental studies of CH3C≡CH
Authors: Hofmann P. , Gerber R. B. , Ratner M. A. , Baylor L. C. , Weitz E.
Published: July 2002

A study has been made of the vibrational energy flow mechanisms and time scales pertaining to the overtone stretch excitations of methyl and acetylenic CH stretches in propyne. Classical trajectories are used to interpret the experimental data for the overtone linewidths, as well as to analyze the role that individual modes play in determining energy flow. The full anharmonic potential surface for these calculations, including all modes, has been developed from spectroscopic and structural information, including the linewidth data. The principal results are: (1) The trajectory calculations show a localization transition, corresponding to a switch over from normal-mode behavior for CH3 excitations up to v≅3 to a local-mode CH excitation within the CH3 moiety for excitations of v≳6, with transition behavior for v=4,5. (2) The acetylenic CH shows local-mode behavior from v=1. Extremely long lifetimes are found for the excitations of this mode, and the trajectories indicate that the experimental width is predominantly rotational. (3) The rocking and deformation modes are dominant receiving modes in the relaxation of the methyl stretch. (4) A shorter lifetime is calculated for the v=6 vs the v=5 or v=7 overtones of the methyl C–H stretch. Experimental results are qualitatively consistent with this prediction. The origin of this shorter lifetime is a band of resonances between the stretch excitation and combinations of rocking, deformation, and pseudorotation modes. (5) CH3 internal rotation figures importantly in the relaxation of some levels (v=5, 8 of CH3) where it ‘‘closes the energy gap’’ for achieving resonant energy transfer. (6) For v=8 of the methyl CH, some direct energy transfer to both C–C≡C stretching modes is seen. The switching on of the stretches as receiving modes is a consequence of sufficiently strong interactions between the excited H and the C–C≡C chain, which take place at these high vibrational energies. (7) Evidence is found for long distance ‘‘through-space’’ energy transfer due to long-range dipole–dipole forces. This transfer occurs from the acetylenic to the methyl CH stretches. This result is illustrated for the v=2 excitation of the acetylenichydrogen, and constitutes a direct demonstration of intramolecular long-distance, through-space v–v energy transfer. These results demonstrate the potential importance of large amplitude modes such as rocking and deformation as initial receiving modes for vibrational energy from excited CH overtones. On the time scale probed here (∼1 ps), despite the availability of many degrees of freedom, the transfer process is dominated by specific energy transfer channels and by the specific behavior of individual modes, rather than by statistical considerations, which will certainly prevail on longer time scales.
Transient microwave conductivity of electrons in helium and argon
Authors: Viehland L. A. , Ranganathan S. , Shizgal B.
Published: July 2002

The transient microwave conductivity of electrons in partially ionized helium or argon gas is determined from the solution of the appropriate Fokker–Planck equation. The time dependence of the real and imaginary components of the electron conductivity are calculated for several initial velocity distribution functions. Good agreement is obtained with available experimental measurements for the pressure and frequency dependence of the transient (real) conductivity in helium. For argon, the present results for the transient mobilities are in qualitative agreement with the results obtained with a Monte Carlo simulation. Electron thermalization times in helium and argon are reported for a range of strengths and frequencies of the external microwave electric field.
Electron mobility, free ion yields, and electron thermalization distances in n-alkane liquids: Effect of chain length
Authors: Gee Norman , Senanayake P. Chandani , Freeman Gordon R.
Published: July 2002

The electron mobility μo was measured as a function of temperature in liquid n-hexane, n-heptane, n-octane, n-nonane, and n-undecane, and at 295 K in n-pentane. Combination of these with earlier measurements of ours showed that μ0 at 295 K decreased monotonically with increasing carbon chain length in n-alkane liquids from ethane to n-tetradecane. There was no significant difference between odd and even carbon number compounds. The results were in accord with two-state interpretations of electron transport. Free ion yields were measured in liquid n-Cx H2x+2 (4≤x≤14, except 13) and electron thermalization ranges bGP were estimated using the extended Onsager model. The zero field free ion yield G0fi at 295 K decreased with increasing chain length. The density-normalized thermalization range of electrons was bGPd=(41±1)×10−7 kg/m2 in all n-alkanes from C4 to C14 under the conditions of this study.
Adiabatic processes in monatomic gases
Authors: Carrera-Patiño Martin E.
Published: July 2002

A kinetic model is used to predict the temperature evolution of a monatomic ideal gas undergoing an adiabatic expansion or compression at a constant finite rate, and it is then generalized to treat real gases. The effects of interatomic forces are considered, using as examples the gas with the square-well potential and the van der Waals gas. The model is integrated into a Carnot cycle operating at a finite rate to compare the efficiency’s rate-dependent behavior with the reversible result. Limitations of the model, rate penalties, and their importance are discussed.
Predissociation of O2 in the B state
Authors: Wodtke A. M. , Huwel L. , Schlüter H. , Voges H. , Meijer G. , Andresen P.
Published: July 2002

Dispersed LIF spectra of O2 in the Schumann–Runge band were measured with a modified tunable ArF laser in a flame. Spin-state selective predissociation of the B state was directly observed in fluorescence excitation spectra, revealing the relative coupling matrix elements of the triplet components to the manifold of repulsive states. Such data determines the symmetries of the important predissociating curves for each observed B-state vibrational level and shows that past interpretation of absorption linewidth data is in error. Due to the fast predissociation, quench-free emission spectra arising from laser prepared single rovibronic levels in the B state were observed even in an atmospheric flame. Fluorescence to X-state vibrational levels as high as v″=35 was observed and relative emission probabilities were derived.
Mo/ller–Plesset third order calculations with large basis sets
Authors: Alberts Ian L. , Handy Nicholas C.
Published: July 2002

The value of the Mo/ller–Plesset third order calculations is examined. An efficient method for the evaluation of the gradient of the MP3 energy is reported, and it has been programmed for both restricted and unrestricted Hartree–Fock wave functions. Large basis set calculations (TZ2P or better) are reported for the optimization of geometries and the determination of harmonic frequencies (which are obtained by finite differences of analytic gradients). The molecules selected are NH2, PH2, AsH2, H2O, NH3, H2CO, HCN, and C2H2. For the closed shell systems, the RMP3 predictions for bond lengths are inferior (≈0.006 Å) to RMP2 predictions (≈0.003 Å) for single bonds, and for multiple bonds the RMP3 bond lengths are too short by approximately the same amount (≈0.01 Å) that RMP2 are too long. For the open shell systems, the UMP3 geometrical parameters show only a marginal improvement over UMP2, except for PH2 where the bond length error is reduced to 0.003 Å. The results for harmonic frequencies show a similar comparison between MP2 and MP3. On the basis of this experience, it appears that large scale calculations at the MP3 level are not recommended; MP2 calculations with a large basis set are much cheaper and provide results with a similar, if not superior, accuracy.
Born–Green hierarchy for continuum percolation
Authors: Given James A. , Klein W.
Published: July 2002

We present a projection operator technique that yields hierarchies of integral equations satisfied exactly by the n-point connectedness functions in a continuum version of the site-bond percolation problem. The n-point connectedness functions carry the same structural information for a percolation problem as the n-point correlation functions do for a thermal problem. Our method extends the Potts model mapping of Fortuin and Kastelyn to the continuum. We use the projection operator technique to produce an integral equation hierarchy for percolation similar to the Born–Green thermal hierarchy. The Kirkwood superposition approximation (SA) is extended to percolation in order to close this hierarchy and yield a nonlinear integral equation for the two-point connectedness function. We discuss the fact that this function, in the SA, is the analytic continuation to negative density of the two-point correlation function in a corresponding thermal problem. The Born–Green–Yvon (BGY) equation for percolation is solved numerically, both by an expansion in powers of the density, and iteratively, using the modified Picard method. We argue, both analytically and numerically, that the BGY equation for percolation, unlike its thermal counterpart, shows nonclassical critical behavior, with η=1 and γ=2.2±0.2. Finally, we develop a sequence of refinements to the superposition approximation that can be used to give increasingly accurate calculations of the two-point connectedness function.
Dielectric study on dynamics of water in polymer matrix using a frequency range 106–1010 Hz
Authors: Shinyashiki Naoki , Asaka Nobuyuki , Mashimo Satoru , Yagihara Shin
Published: July 2002

Dielectric measurements were made on polyethylene glycol (PEG) and polyvinyl pyrolidone (PVP)–water systems over a frequency range 106 –1010 Hz by a time domain reflectometry. Two relaxation peaks were observed in the PVP system. The high frequency process is caused by rotational diffusion of water clusters and concentration dependence of the relaxation time is well explained by the free volume theory. The low frequency process is attributed to water molecules bound to the polymer and its relaxation time is reasonably irrespective of the concentration. On the other hand, the PEG system shows a single relaxation process which is caused by the rotational diffusion of water clusters. A sign of the segmental motion was recognized barely for highly concentrated system of PEG.
Application of a coupled-surface time-dependent Hartree grid method to excited state optical spectroscopy
Authors: Waldeck Janet R. , Campos-Martínez José , Coalson Rob D.
Published: July 2002

The dynamics of multidimensional wave packet motion on nonadiabatically coupled electronic potential surfaces is explored by numerically exact time-dependent quantum mechanics and by the time-dependent Hartree grid (TDHG) approximation. Excellent agreement is found between the TDHG and the exact evolution of the wave packet; in particular, the approximation yields accurate total cross sections for electronic absorption, excitation profiles for resonance Raman scattering, and partial cross sections for photofragmentation in a two degree of freedom model of direct dissociation (which is qualitatively related to experimentally observed processes in methyl halides and ICN).
The cubic susceptibility dispersion of Alkoxy-nitro-stilbene (MONS) and Di-alkyl-amino-nitro-stilbene (DANS) side chain substituted polymers: Comparison with the two-level model
Authors: Torruellas W. E. , Zanoni R. , Stegeman G. I. , Möhlmann G. R. , Erdhuisen E. W. P. , Horsthuis W. H. G.
Published: July 2002

The complex cubic susceptibility dispersion of side chain polymers containing 4-dialkylamino-4’-nitro-stilbene, 4-alkoxy-4’-nitro-stilbene as their side groups has been measured in the range of 0.953 μm to 1.904 μm with third harmonic generation. A two level model is compared to the experimental data in the three-photon-resonance spectral region. Microscopic local field cascading was found to give an important contribution to the third order susceptibility. Nonlinear susceptibilities comparable to those of fully conjugated backbone polymers are obtained.
Pressure-induced phase transitions in clathrate hydrates
Authors: Handa Y. P. , Tse J. S. , Klug D. D. , Whalley E.
Published: July 2002

Ice I transforms to a high-density amorphous phase when pressed to 10 kbar at 77 K. Similar transformations in structure I and structure II clathrate hydrates have been studied by pressing samples in a piston-cylinder apparatus and by molecular dynamics simulations. The simulations were also carried out on structure I and II empty lattices. The hydrates and the empty lattices were found to transform to high-density phases under pressure at 77 K. The high-density phases of the empty lattices could be recovered at zero pressure, as is possible in the case of high-density amorphous phase of ice. However, it was not possible to recover the high-density phases of the hydrates at zero pressure. Instead, they reverted back to their original crystalline structures when the pressure was released. The molecular dynamics results suggest that under pressure the water molecules in the hydrates collapse around the guest molecules, and the repulsive forces between the guest and the water molecules are mainly responsible for the reversible transition to the original structure when the pressure is released.
Molecular dynamics simulations of chain relaxation and crack propagation in polymer liquid crystals
Authors: Bo/ński Sa/womir , Brostow Witold
Published: July 2002

Molecular dynamics simulations of polymer liquid crystals (PLC’s) under tensile deformation were made. Systems of fully flexible chains were simulated for comparison. Stress-strain behavior and fracture mechanics have been studied. The fracture stress has been defined as the stress level at which the fraction of broken bonds increases dramatically. The results show that strength of PLC’s is affected by the concentration of rigid sequences, their distribution throughout the material, and by the temperature of the specimen. Transition from ductile to brittle behavior has been observed for random copolymers with different fractions of rigid sequences. PLC block copolymers exhibit molecular reinforcement.
Laser spectroscopic studies of the pure rotational U(0) and W(0) transitions of solid parahydrogen
Authors: Chan Man-Chor , Lee Szetsen S. , Okumura Mitchio , Oka Takeshi
Published: July 2002

High resolution spectrum of multipole-induced transitions of solid parahydrogen was recorded using diode and difference frequency laser spectroscopy. The J=4←0 pure rotational U0(0) transition observed in the diode spectrum agrees well in frequency with the value reported by Balasubramanian et al. [Phys. Rev. Lett. 47, 1277 (1981)] but we observed a spectral width smaller by about a factor of 4. The J=6←0 W0(0) transition was observed to be exceedingly sharp, with a width of ∼70 MHz, using a difference frequency spectrometer with tone-burst modulation. This transition is composed of three components with varying relative intensity depending upon the direction of polarization of laser radiation. These components were interpreted as the splitting of the M levels in the J=6 state due to crystal field interactions. In addition, a new broad feature was found at 2452.4 cm−1 in the low resolution Fourier-transform infrared (FTIR) spectrum of solid hydrogen and was assigned to be the phonon branch WR(0) transition of the W0(0) line. The selection rules, crystal field splitting of J=4 and J=6 rotons, and the measured linewidth based on these observations are discussed.
Tight rigorous bounds to atomic information entropies
Authors: Angulo J. C. , Dehesa J. S.
Published: July 2002

The position-space entropy Sρ and the momentum-space entropy Sγ are two increasingly important quantities in the study of the structure and scattering phenomena of atomic and molecular systems. Here, an information-theoretic method which makes use of the Bialynicki–Birula and Mycielski’s inequality is described to find rigorous upper and lower bounds to these two entropies in a compact, simple and transparent form. The upper bounds to Sρ are given in terms of radial expectation values <rα≳ and/or the mean logarithmic radii <ln r≳ and <(ln r)2≳, whereas the lower bounds depend on the momentum expectation values <pα≳ and/or the mean logarithmic momenta <ln p≳ and <(ln p)2≳. Similar bounds to Sγ are also shown in a parallel way. A near Hartree–Fock numerical analysis for all atoms with Z≤54 shows that some of these bounds are so tight that they may be used as computational values for the corresponding quantities. The role of the mean logarithmic radius <ln r≳ and the mean logarithmic momentum <ln p≳ in the improvement of accuracy of the aforementioned bounds is certainly striking.
Accurate potential energy surface for Xe/Pt(111): A benchmark gas/surface interaction potential
Authors: Barker J. A. , Rettner C. T.
Published: July 2002

We have determined an empirical potential energy function for the interaction of xenon with the Pt(111) surface which is consistent with a wide range of dynamical and equilibrium experimental data. These include scattering measurements, with detailed angular distributions and energy transfer data, at incidence energies from 0.5 to 14.3 eV. Also used are thermal desorption rates and trapping probabilities, as well as thermodynamic properties of monolayer phases including the ‘‘energy jump’’ at the transition from the commensurate to the uniaxially compressed incommensurate phase. The potential also agrees with an experimental value for the frequency of vibration normal to the surface, and has the correct asymptotic behavior at large distances from the surface (V=−c3/z3, with an experimental estimate of c3). The equilibrium position for a single Xe atom lies directly above a surface platinum atom, and the calculated height above this atom is 3.35 Å.
Structure and properties of disilyne
Authors: M. Hühn Markus , Amos Roger D. , Kobayashi Rika , Handy Nicholas C.
Published: July 2002

The ground state of disilyne, Si2H2, has been studied using state of the art methodology. In particular a quartic force field has been determined at the self-consistent field level, and the geometry has been optimized with large basis sets at the Mo/ller–Plesset second order level, at the Brueckner doubles coupled cluster level and with Kohn–Sham density functional theory. Our best calculations give excellent agreement with observed data for the rotational constants. The very intense ν3 mode is predicted to lie near 1090 cm−1.
Theory and simulation of polar and nonpolar polarizable fluids
Authors: Cao Jianshu , Berne B. J.
Published: July 2002

The theory of polarizable fluids is developed on the basis of a model fluid consisting of anisotropic Drude oscillators with embedded permanent dipoles. A matrix Monte Carlo method is used to simulate these fluids and to compare their dielectric properties with the predictions of the mean spherical approximation (MSA) theory. Simple identities are used in conjunction with the matrix expressions to derive mean spherical approximations to the screening functions for charges and permanent multipoles in polarizable fluids. This is useful in the theory of the solvated electron.
Molecular beam optical Stark spectroscopy of MoN
Authors: Fletcher D. A. , Jung K. Y. , Steimle T. C.
Published: July 2002

A high resolution optical spectroscopic study of a molybdenum mononitride, MoN, supersonic molecular beam sample has been performed. The Ree (0.5) branch feature of the (0,0) A 4Π3/2–X 4 Σ1/2− band system for 98MoN was recorded as a function of static electric field strengths of up to 3.5 kV/cm. The splittings and shifts were analyzed to produce values of the permanent electric dipole moment of 4.56(9) and 3.38(7)D for the A 4Π3/2 and X 4Σ1/2− states, respectively. The field free splittings for the 95MoN and 97MoN isotopic forms were interpreted as arising from X 4Σ1/2 − magnetic hyperfine effects and the resulting spin density of 5.09(8) a.u.−3 was determined. Comparisons with predictions from theoretical models are given.
The 1/Z expansion and renormalization of the large-dimension limit for many-electron atoms
Authors: Kais Sabre , Herschbach Dudley R.
Published: July 2002

Analytic expressions for the large-dimension limit, when renormalized by introducing a suitable effective nuclear charge ζ yield accurate D=3 nonrelativistic energies for ground states of many-electron atoms. Using Hartree–Fock data to estimate ζ, which typically differs from the actual charge Z by ∼1% or less, we find this dimensional renormalization method (denoted DR-0) gives results substantially better than the HF input. Comparison of the 1/Z expansion for the large-D limit with that for D=3 atoms provides expressions for the leading error terms in the renormalized total energy and correlation energy. When configuration mixing occurs in the Z→∞ limit (as for Be and many other atoms), we find the renormalization procedure is markedly improved by including the zeroth-order mixing (denoted DR-1); this contributes a term linear in Z. Including the Z-independent term (DR-2) also improves the accuracy when zeroth-order mixing is absent (e.g., ground-state atoms with N=2, 3, and 7–11) but not otherwise. Correlation energies for atoms and cations with N=2–18 electrons and Z=2–28 are obtained with a mean error of 26% using just the large-D limit or HF input (DR-0); the mean error improves to only 5% when the leading 1/Z term is included (either DR-1 or DR-2). Results much better than the HF approximation are likewise obtained for the ionization potentials and electron affinities of neutral atoms.
Preparation and decay of alignment in N2 (v=1)
Authors: Sitz G. O. , Farrow R. L.
Published: July 2002

We report measurements of the collisional decay of the angular momentum alignment for N2 (v=1) at 298 K. Stimulated Raman pumping of S-branch (v=1←0) transitions is used to prepare a selected rotational state of N2 with an anisotropic spatial J distribution in the v=1 state. After allowing an appropriate time interval for collisions to occur, 2+2 resonance-enhanced multiphoton ionization is used (through the a 1Πg←X 1Σ+g transition) to detect the relative population and alignment of the pumped level and other levels to which rotational energy transfer has occurred. We have performed a series of measurements in which a selected even rotational level (Ji=0–14) is excited and the time-dependent level population and alignment are measured at several delay times. We find the decay of alignment to be no faster than the decay of the population of the pumped level, indicating that pure m-state changing collisions are slower that J-state changing collisions. We have also observed substantial alignment of molecules which have undergone a J-state changing collision. The alignment is this case is found to be inconsistent with a Δm=0 selection rule often used in models of rotational energy transfer rates.
Extension of Gaussian-2 (G2) theory to bromine- and iodine-containing molecules: Use of effective core potentials
Authors: Glukhovtsev Mikhail N. , Pross Addy , McGrath Mark P. , Radom Leo
Published: July 2002

Basis sets have been developed for carrying out G2 calculations on bromine- and iodine-containing molecules using all-electron (AE) calculations and quasirelativistic energy-adjusted spin–orbit-averaged seven-valence–electron effective core potentials (ECPs). Our recommended procedure for calculating G2[ECP] energies for such systems involves the standard G2 steps introduced by Pople and co-workers, together with the following modifications: (i) second-order Mo/ller–Plesset (MP2) geometry optimizations use polarized split-valence [31,31,1] basis sets for bromine and iodine together with 6-31G(d) for first- and second-row atoms; (ii) single-point higher-level energies are calculated for these geometries using our new supplemented bromine and iodine valence basis sets along with supplemented 6-311G and McLean–Chandler 6-311G bases for first- and second-row atoms, respectively; and (iii) first-order spin–orbit corrections are explicitly taken into account. An assessment of the results obtained using such a procedure is presented. The results are also compared with corresponding all-electron calculations. We find that the G2[ECP] calculations give results which are generally comparable in accuracy to those of the G2[AE] calculations but which involve considerably lower computational cost. They are therefore potentially useful for larger bromine- and iodine-containing molecules for which G2[AE] calculations would not be feasible.
Thin liquid film structure and stability: The role of depletion and surface-induced structural forces
Authors: Chu X. L. , Nikolov A. D. , Wasan D. T.
Published: July 2002

Film stability and structure formation inside a liquid film containing colloidal particles are investigated by Monte Carlo (MC) numerical simulations and by analytical methods. The effective pair interaction between particles is calculated from the Ornstein–Zernike theory with Percus–Yevick closure. Consistent with the recent experimental observations and theoretical studies, these MC simulations reveal the phenomena of internal particle layering as well as inlayer structure formation. In particular, an ordered two-dimensional hexagonal structure is observed at a particle concentration of 37 vol% (instead of 43 vol% for the hard-sphere potential) when the effective pair interaction between particles is taken into account. Furthermore, the particles inside a layer ‘‘condense’’ due to the attractive depletion force which leads to the formation of voids. The formation of such void structures results in the formation of ‘‘dark spots’’ which have been observed in film thinning experiments. The calculated film structural disjoining pressure, created by the particles inside the film, is found to be in agreement with the experimentally measured force-distance curves using the surface force apparatus.
Density functional theory of crystal growth: Lennard-Jones fluids
Authors: Shen Yu Chen , Oxtoby David W.
Published: July 2002

We employ an extension of density functional theory to the dynamics of phase transitions in order to study the velocities of crystal growth and melting at planar undercooled and superheated crystal-melt interfaces. The free energy functional we use has a square-gradient form, with the parameters for a Lennard-Jones interaction potential determined by a modified weighted density approximation (MWDA) applied locally through the liquid–solid interface. We explore the role of the density change on freezing in crystal and melt growth, and discover a significant asymmetry between freezing and melting both close to and far from the equilibrium freezing point. The behavior of the superheated solid is governed by the close proximity of a spinodal, whereas in the undercooled liquid there is no evidence for a spinodal and the growth at large undercoolings is affected instead by the density deficit that appears in front of the growing interface. Comparisons are made with other density functional approaches and with computer simulations.
Velocity dispersion and viscous relaxation in supercooled water
Authors: Cunsolo A. , Nardone M.
Published: July 2002

Brillouin scattering spectra of water from T=30 °C down to supercooling temperatures of about T=−26 °C have been measured, at different scattering angles and for different exciting wavelengths, covering exchanged wave vector values from 0.0137 to 0.0351 nm−1. A fit of the spectra using the hydrodynamic line shape for a simple fluid confirms the existence of a relevant dispersion in the wave vector dependence of the hypersonic sound velocity. The parameters obtained fitting the data with a relaxing hydrodynamic model indicate the ratio between volume and shear viscosity is almost temperature independent also in the supercooled regime and that a single relaxation process, involving almost entirely both shear and volume viscosities, accounts for the observed dispersion. The infinite frequency sound velocity is found to be of about 2150 m/s almost independently on temperature. The temperature dependence of the relaxation time is discussed and compared with that of other relaxation times available in the literature. Predictions are made for the temperature and wave vector values at which the dispersion of sound velocity should be best observed.
Nonadiabatic molecular dynamics simulations of the photofragmentation and geminate recombination dynamics in size-selected I2−⋅Arn cluster ions
Authors: Batista V. S. , Coker D. F.
Published: July 2002

We investigate the photodissociation, geminate recombination and relaxation dynamics in size-selected I2−⋅Arn cluster ions using a coupled quantum-classical molecular dynamics method and a model Hamiltonian gained from diatomics-in-ionic systems. We calculate photofragmentation yields of various charged product clusters of the dissociated I−⋅Arf or recombined I2−⋅Arf′ forms as a function of precursor cluster size and find almost quantitative agreement with experimental results. The trends in photofragmentation are explained in terms of various participating electronically nonadiabatic channels coupled with vibrational relaxation on these different surfaces. We also explore the role of long range electrostatic interactions and underlying precursor cluster geometry on the photofragmentation dynamics.
Vibrational predissociation of the ND3-Ar Van der Waals complex: Comparison with NH3-Ar
Authors: Millan J. , Halberstadt N. , van der Sanden G. , van der Avoird A.
Published: July 2002

We study the vibrational predissociation of ND3-Ar through excitation of the ν2 “umbrella” mode of ND3, with the same (scaled or unscaled) ab initio potential and the same quantum mechanical method as used earlier for NH3-Ar [J. Millan et al., J. Chem. Phys. 103, 4138 (1995)]. Just as in that case, we find that a simultaneous excitation of one of the Van der Waals modes (stretch, Σ or Π bend, of symmetry A1, A2, or E) has a strong effect on the lifetime of the complex and on the product state distribution. In particular, the excitation of the Van der Waals stretch mode leads to shorter lifetimes and colder rotational state distributions of the emerging ND3 fragment. With the original (unscaled) ab initio potential, which produces the most realistic lifetimes (line widths in the infrared spectrum) for NH3-Ar, the spread of the lifetimes for different initial quasi-bound states of ND3-Ar is much larger than for NH3-Ar. This may be related to the more localized nature of the (quasi-)bound states of ND3-Ar and the shorter average distance 〈R〉. The rotation-inversion (jk±) state distributions of the emerging ND3 fragment are highly non-statistical, as in ND3-Ar, but this fragment contains (absolutely and relatively) less rotational energy than the NH3 fragment in the photo-dissociation of NH3-Ar. Most of the (quasi-)bound levels of the ND3-Ar complex show a clear preference for an ND3 inversion-tunneling state of definite + or − parity, but the distribution of the emerging ND3 fragment over its +/− states does not reflect this preference.
Solubility limits of silicate melts
Authors: Corrales L. René , Keefer Keith D.
Published: July 2002

A statistical mechanical model of silica melt is presented in which metal oxides are incorporated into the bonding network. In this approach a Flory-type lattice model for binary silicate melts is coupled with a set of chemical reactions that determine the extent of metal oxide incorporation into the silica network and regulate the distribution of nonbridging oxygens around a central silicon. The theory produces two- and three-phase coexistence curves over a range of parameter space. The three-phase equilibrium regions coalesce to critical end points, where two of the three phases have become critical, and to a nonsymmetric tricritical point, where all three phases have simultaneously become critical. The theory captures the qualitative behavior and essential features of simple binary silicate melts.