bluuebunny/crossref_metadata_2025 huggingface dataset:
Device simultaneous determination of the source and cavity parameters of a microcavity light-emitting diode
Authors: Ochoa D. , Houdré R. , Stanley R. P. , Dill C. , Oesterle U. , Ilegems M.
Published: July 2002

Detuning between the emission line and the Fabry-Pérot wavelength is a critical parameter for microcavity light-emitting diode (MCLED) design with regard to the efficiency and emission directionality. We present here a method to measure simultaneously the detuning and the linewidth of the source emitter on the device itself. This method uses numerical simulations and a fitting procedure with the angular emission pattern measured on the MCLED. It is accurate, nondestructive and easy to implement.
Ion beam etching induced structural and electronic modification of InAs and InSb surfaces studied by Raman spectroscopy
Authors: Frost F. , Lippold G. , Schindler A. , Bigl F.
Published: July 2002

The modification of the structural and electronic properties of InAs and InSb surfaces induced by low-energy N2 and Ar ion beam etching (IBE) were investigated as a function ion energy (⩽500 eV) using Raman spectroscopy. A drastic enhancement of the electron concentration in the near surface region of both materials independent of the ion energy and the process gas was observed. From Raman measurements in different polarization configurations it can be concluded that the electron accumulation observed after IBE is inherently related to the process-induced structural defects. The degree of structural damage and the carrier concentration in the near surface region increase for higher ion energies. By controlled, subsequent removal of the damaged surface layer using wet etching, the depth profile of the structural and electronic damage in InAs was determined. This procedure reveals that the structural and electronic damage extends about 100 nm into the material. Nevertheless, it can be recognized that the utilization of N2 as the etching gas is associated with a lower degree of damage and also a lower electron accumulation at the surface of both InAs and InSb.
Structural, magnetic, transport, and magneto-optical properties of single crystal La2/3Sr1/3MnO3 thin films
Authors: Bobo J. F. , Magnoux D. , Porres R. , Raquet B. , Ousset J. C. , Fert A. R. , Roucau Ch. , Baulès P. , Casanove M. J. , Snoeck E.
Published: July 2002

We have grown La0.7Sr0.3MnO3 thin films by reactive facing target radio frequency sputtering on SrTiO3 (001) substrates with the nominal stoichiometry and the perovskite structure down to nanometer-scale film thickness. The films are found to be perfectly expitaxied on SrTiO3, without misfit dislocations. The surfaces of the layers were found to be highly flat with terraces of ≈4 Å height corresponding to the manganite unit cell. Magnetic measurements evidence a decrease of the Curie temperature for decreasing thickness. Low temperature magnetoresistance is very small, confirming the absence of grain boundaries in the films but it reaches ≈−50% at room temperature. Finally, magneto-optical Kerr effect measurements between 20 and 400 K reveal the onset of ferromagnetic transition via the coercive field increase and the Kerr rotation and ellipticity measurements.
Neutron scattering on magnetic thin films: Pushing the limits (invited)
Authors: Schreyer A. , Schmitte T. , Siebrecht R. , Bödeker P. , Zabel H. , Lee S. H. , Erwin R. W. , Majkrzak C. F. , Kwo J. , Hong M.
Published: July 2002

Neutron scattering has been the scattering technique of choice for the analysis of magnetic structures and their dynamics for many decades. The advent of magnetic thin film systems has posed new challenges since such samples have inherently small scattering volumes. By way of examples, recent progress in the application of neutron scattering for the study of both magnetic structure and dynamics in magnetic thin film systems will be presented. First, a combined high angle neutron scattering and polarized neutron reflectivity investigation of the magnetic order of Cr and its influence on the exchange coupling between the Fe layers in Fe/Cr superlattices is discussed. It is shown that in the whole thickness range up to 3000 Å, the magnetic structure is governed by frustration effects at the Fe/Cr interfaces. Second, it is demonstrated that it is now possible to investigate the dynamic properties of magnetic thin films with neutron scattering. Unlike, e.g., Brillouin light scattering, inelastic neutron scattering provides access to large portions of the Brillouin zone. First results on spin wave excitations in a Dy/Y superlattice are presented.
Trap-assisted tunneling in high permittivity gate dielectric stacks
Authors: Houssa M. , Tuominen M. , Naili M. , Afanas’ev V. , Stesmans A. , Haukka S. , Heyns M. M.
Published: July 2002

The electrical characteristics of SiOx/ZrO2 and SiOx/Ta2O5 gate dielectric stacks are investigated. The current–density JG in these dielectric stacks is shown to be strongly temperature dependent at low voltage (below about 2 V), the more so in the ZrO2 stack. On the other hand, JG is much less temperature dependent at higher voltage. These results are consistent with a model which takes into account the direct tunneling of electrons across the SiOx layer and the trap-assisted tunneling of electrons through traps with energy levels below the conduction band of the high permittivity dielectric layer. The energy levels and densities of these electron trapping centers are estimated by fitting this trap-assisted tunneling model to the experimental results.
Isotopic thermal diffusion of carbon disulfide in the liquid phase
Authors: Rutherford W. M.
Published: July 2002

Thermal diffusion factors were measured for several isotopically substituted pairs of carbon disulfide molecules. The effect of a unit mass increment at the carbon position in the molecule was found to be substantially less than the effect of a unit mass substitution at one of the sulfur positions. The results were represented within experimental accuracy by αTij=2.20 (mi−mj)/(mi+mj ) +1.67(Ii−Ij)/ (Ii+Ij), where αTij is the isotopic thermal diffusion factor, mi and mj are the molecular masses, and Ii and Ij are the second moments.
MPI photoelectron spectroscopy of u
n
g
e
r
a
d
e excited states of acetylene: Intermediate state mixing and ion state selection
Authors: Orlando Thomas M. , Anderson Scott L. , Appling Jeffrey R. , White Michael G.
Published: July 2002

Three photon resonant, four photon (3+1) ionization spectroscopy and photoelectron spectroscopy have been used to study the ungerade excited states of acetylene in the energy range from 74 500 to 90 000 cm−1. Sharp bands from the nR (π3u nsσg) and 1Φu (π3u ndδg) Rydberg series dominate the MPI spectrum. A large number of Rydberg and valence states which are prominent in VUV absorption spectra are absent or weak in MPI studies. These weak bands are only observable under high power conditions, which suggests that nonradiative decay is rapid enough to depopulate these states before ionization occurs. The photoelectron results provide further insight into the nature of the excited states. Ionization through the sharp bands occurs via Δν=0 Franck–Condon transitions, resulting in ions in a single vibrational state. Ionization through bands which are mixed results in complicated ion vibrational distributions including excitation of both cis and trans bends.
Measurement of the perpendicular rotation-tunneling spectrum of the water dimer by tunable far infrared laser spectroscopy in a planar supersonic jet
Authors: Busarow Kerry L. , Cohen R. C. , Blake Geoffrey A. , Laughlin K. B. , Lee Y. T. , Saykally R. J.
Published: July 2002

Fifty-six transitions from the K=1 lower→K=2 lower tunneling–rotation band of water dimer have been measured and assigned at 22 cm−1 by direct absorption spectroscopy in a cw planar supersonic jet expansion using a tunable far infrared laser spectrometer. Two different models were used to fit the data and several spectroscopic constants were determined for the upper and lower states. This work supports the local IAM model recently proposed by Coudert and Hougen for the hydrogen bond tunneling dynamics of the water dimer. This model includes four different tunneling motions, all of which contribute to the observed tunneling splittings. This is the most complicated hydrogen bonded system considered to be well understood at this time, at least in the lowest few K states.
The multiphoton dissociation of acetylene. I. Long-lived intermediates, sequential and concerted processes of dissociation
Authors: Hsu Yen-Chu , Lin M.-S. , Hsu Chao-Ping
Published: July 2002

The multiphoton dissociation processes of acetylene via a two-photon resonant predissociative state, v=0 of 1Σ+g, have been studied by three techniques: time-resolved photofragment excitation spectroscopy (TRPFES), laser-induced fluorescence (LIF) of the C2 fragments, and dispersed emission. We found that the major dissociation products are H atoms, H2 molecules, and C2 molecules in the X 1Σ+g, a 3Πu and A 1Πu states; among the latter, C2 X 1Σ+g molecules are formed by a sequential bond–rupture mechanism whereas some C2 in a 3Πu is formed by a concerted two-bond fission process. Other, minor dissociation channels due to three-photon processes, such as C2(d 3Πg)+2H(2S(1)/(2)), C2(d 3Πg)+H2(X 1Σ+g), C2(C 1Πg)+H2(X), C2(e 3Πg)+H2(X), and C2(D 1Σ+u)+H2(X), were also detected. In the 2+1 concerted dissociation yielding C2(C 1Πg)+H2(X), a long-lived intermediate C2H2, likely a cis isomer or other conformer in which the hydrogen atoms are relatively close to each other, was revealed by TRPFES; its zero-pressure lifetime was estimated to be (8±1) μs. A long-lived intermediate C2H was also found in the 2+1 sequential dissociation by TRPFES.
Chemistry and kinetics of size-selected cobalt cluster cations at thermal energies. I. Reactions with CO
Authors: Guo B. C. , Kerns K. P. , Castleman A. W.
Published: July 2002

The chemistry and kinetics of size-selected Co+n cluster-ion (n=2–8) reactions with CO are studied using a selected ion drift tube affixed with a laser vaporization source operated under well-defined thermal conditions. All reactions studied in the present work are found to be association reactions. Their absolute rate constants, which are determined quantitatively, are found to have a strong dependence on cluster size. Similar to the cases of reactions with many other reactants such as H2 and CH4, Co+4 and Co+5 display a higher reactivity toward the CO molecule than do clusters of neighboring size. The multiple-collision conditions employed in the present work have enabled a determination of the maximum coordination number of CO molecules bound onto each Co+n cluster. It is found that the tetramer tends to bond 12 CO molecules, the pentamer 14 CO, hexamer 16 CO, and so on. The results are interpreted in terms of Lauher’s calculation and the polyhedral skeletal electron pair theory. All the measured maximum coordination numbers correlate extremely well with the predictions of these theories, except for the trimer where the measured number is one CO less than the predicted value. The good agreement between experiment and theory enables one to gain some insight into the geometric structure of the clusters. Based on the present findings, the cobalt tetramer cation is interpreted to have a tetrahedral structure, the pentamer a trigonal bipyramid, and the hexamer an octahedral structure. Other cluster structures are also discussed.
Pressure-induced widths and shifts for the ν3 band of methane
Authors: Neshyba S. P. , Lynch R. , Gamache R. , Gabard T. , Champion J.-P.
Published: July 2002

Widths and shifts of methane lines perturbed by nitrogen are calculated using a complex-valued implementation of Robert–Bonamy (RB) theory. The static intermolecular potential is described as a sum of electrostatic forces and Lennard-Jones (6-12) atom–atom terms, using literature values for all physical parameters. Vibrational dependence of the isotropic potential is obtained from the polarizability of methane assuming a dispersion interaction. The repulsive part of the Lennard-Jones accounts for the greatest part of widths, while dispersion interactions are largely responsible for shifts. Although the average error between calculated and observed linewidths (up to J=8) is less than 6%, their distribution suggests the influence of interactions not described in the present theory.
Collision induced dissociation of titanium–carbon cluster cations
Authors: Kerns K. P. , Guo B. C. , Deng H. T. , Castleman A. W.
Published: July 2002

Titanium–carbon clusters are investigated by collision induced dissociation (CID) using our newly designed triple quadrupole mass spectrometer coupled with a laser vaporization source. Their fragmentation patterns are determined under various collision conditions. It is observed that the Met–Car Ti8C+12 mainly loses a neutral metal atom in the primary dissociation step and several metal atoms in sequential dissociation processes. The dissociation threshold of the Met–Car Ti8C+12 is estimated to be about 9 eV. In addition, several of the larger TixC+y cluster ions, including those containing nine or ten metal atoms, are found to fragment directly to Ti8C+12 during single step dissociation. Product channels for dissociation of Ti9C+12 establishes the ionization potential for Ti8C12 to be equal to, or less than the IP of the titanium atom.
MP2/6-311++G(d,p) study of ten ionic hydrogen-bonded binary systems: Structures, normal modes, thermodynamics, and counterpoise energies
Authors: Pudzianowski Andrew T.
Published: July 2002

Fully optimized structures, normal vibrational modes, and thermodynamic functions were obtained at the MP2/6-311++G(d,p) level for the following ionic hydrogen-bonded systems: H3O+/H2O, NH+4/H2O, NH+4/NH3, CH3NH+3/H2O, CH3NH+3/NH3, OH−/H2O, CH3O−/H2O, cyanide/H2O, HCC−/H2O, and formate/H2O. The calculated ΔH fell within error limits of experimental values for all but OH−/H2O and the two NH3 complexes. In the latter cases expansion of the polarization basis to (2d,2p) satisfactorily improved the model, but for OH−/H2O increasing the level of correlation to MP4 was more important. Where comparison was possible, optimized structures were quite similar to those obtained at MP2/6-31+G(d,p) by other workers. All the complexes examined here have a hydrogen bond stretching mode in the 200–350 cm−1 range which may contribute to proton transfer, and whose frequency correlates qualitatively with the acid strength of the H-bond donor. For the cyanide/H2O system, it is not possible to choose between CN−/H2O and NC−/H2O as the dominant complex in the gas phase: the thermodynamic results suggest a mixture. In formate/H2O a C2V ‘‘cyclic’’ complex is thermodynamically favored but a ‘‘linear’’ complex is apparently observed at ∼500 K. This might well be an entropy effect, but the C2V complex was found here to have a vibrational mode at 92 cm−1 which could facilitate conversion to the linear complex at all but very low temperatures. Counterpoise calculations with ghost functions were done at MP2 for all complexes, and HF/6-311++G(d,p) for three of the more strongly bound cases. Comparison of the MP2 and self-consistent-field results, along with a close examination of the virtual orbitals obtained with and without ghost functions for the components of NH+4/NH3, supports a recent contention that counterpoise calculations with correlation are highly suspect.
Formation of C119 by thermal decomposition of C60O
Authors: Beck Rainer D. , Bräuchle Götz , Stoermer Carsten , Kappes Manfred M.
Published: July 2002

Laser desorption mass spectra of thin films of pure C60O were recorded in a range of equilibrium sample temperatures from 300 to 1100 K. On heating of the sample above 650 K a new peak corresponding to C−119 is detected in the anion mass spectrum which persists even after the sample is cooled to 300 K. Possible solid state reactions leading to the formation of C119 are discussed based on the mass spectrometric observations as well as temperature programmed decomposition measurements which were carried out for C60 and C60O samples. The results are discussed in connection with recently reported polymerization reactions of fullerene materials.
Infrared spectroscopy of model electrochemical interfaces in ultrahigh vacuum: Surface–cation solvation in the Pt(111)/K+–methanol system
Authors: Villegas Ignacio , Weaver Michael J.
Published: July 2002

Infrared reflection–absorption spectroscopic (IRAS) measurements are reported for methanol dosed onto Pt(111) in ultrahigh vacuum (UHV) in both the presence and absence of adsorbed potassium atoms at 90 K with the objective of elucidating the nature of sequential cation solvation at this model electrochemical interface. Corresponding variations in the metal-UHV work-function (Φ), evaluated with a Kelvin probe, yield additional insight into the interfacial electrostatic environment as a function of the alkali and methanol dosages. Methanol forms a particularly suitable solvent for such a ‘‘double-layer modeling’’ study since both the O–H stretching (νOH) and C–OH stretching (νC–OH) vibrations are sensitive to the local coordination environment. In addition, comparisons are made with the detailed infrared spectral data available for progressive methanol solvation of gas-phase alkali cations [(a) A. J. Draves, Z. Luthey-Schulten, W.-L. Liu, and J. M. Lisy, J. Chem. Phys. 93, 4589 (1990); (b) T. J. Selegue, N. Moe, J. A. Draves, and J. M. Lisy, ibid. 96, 7268 (1992)], allowing unprecedented insight into the manner and extent to which cation solvation is affected by the metal surface.

The initial stage of methanol solvation of interfacial K+ is signaled by substantially downshifted and relatively sharp νOH and νC–OH bands at ∼3100 and 1010 cm−1, respectively, which are not observed in the absence of K+. This spectral behavior is consistent with the formation of a primary solvation shell featuring methanol–cation coordination via the oxygen along with –OH hydrogen bonding to the metal surface. The significant (∼0.5–1 eV) Φ increases observed under these conditions support the presence of primary solvation methanol with a negative-outward δ−O–Hδ+ dipole orientation. The second solvation stage, referring to K+–methanol stoichiometries above ∼3, is accompanied by the appearance of markedly upshifted νOH and νC–OH bands, at ∼3300 and 1050 cm−1, respectively, suggesting the occurrence of extensive first–second shell H-bonding.

Marked Φ decreases are observed in this dosage regime, more closely akin to the behavior observed in the absence of adsorbed alkali. The methanol dosage-dependent interfacial νC–OH behavior is markedly different to that observed in the gas phase, highlighting the role of the metal in modifying the nature of both the primary and second-shell solvation structure. The structure of methanol on uncharged (i.e., K+-free) Pt(111) is also addressed by combined IRAS and work-function measurements. The H-bonded structures even within multilayer methanol films differ significantly from the analogous bulk phases. The effects of competitive CO chemisorption on K+ solvation are also considered.
Transition states of charge-transfer reactions: Femtosecond dynamics and the concept of harpooning in the bimolecular reaction of benzene with iodine
Authors: Cheng P. Y. , Zhong D. , Zewail Ahmed H.
Published: July 2002

We describe an approach for real-time studies of the transition-state dynamics of charge-transfer reactions. An application to the bimolecular reaction of benzene with iodine is reported. The measured 750±50 fs transient growth of the free I atom product elucidates the nature of the transition state and the mechanism for the dissociative charge-transfer reaction. The mechanism is formulated in relation to the impact geometry and the dative bonding, which are crucial to condense-phase and surface reactions, and is supported by molecular dynamics.
The solubility of rare gases in fused silica: A numerical evaluation
Authors: Guillot Bertrand , Guissani Yves
Published: July 2002

The solubility of rare gases in a simulated model of fused silica is evaluated by the test particle method. It is shown that the order of magnitude of the rare gas solubility in liquid silica is mainly governed by the entropy of cavity formation and only marginally by the solvation energy of the solute in the melt. Hence, the hierarchy of solubilities is dictated by the size of the noble gases: The smaller the atom the higher the solubility. Moreover, the solubility exhibits only a moderate temperature dependence at fixed density. In silica glass the same solubility hierarchy is found (in agreement with the experimental data) although the energetic contribution to the absorption process may become significant, particularly when the temperature decreases. These results in silica are in strong contrast with those obtained in liquid water where the energetic contribution to the rare gas solubility is large enough to overcompensate the entropic loss of cavity formation, the net result being that light noble gases (e.g., He) are less soluble than heavier ones (e.g., Xe). These contrasting behaviors are explained by pointing out that the liquid phase occurs in very different temperature ranges for these two systems (SiO2 and H2O) while the rare gas–solvent interaction energy is essentially of the same order of magnitude. Finally, the structure of silica around helium and neon atoms is found to be interstitial (cristobalitelike) while in the case of heavier rare gases, the structure of cavities is more reminiscent of the polyhedral arrangement encountered with clathrasils.
Linewidths and shifts of very low temperature CO in He: A challenge for theory or experiment?
Authors: Thachuk Mark , Chuaqui Claudio E. , Le Roy Robert J.
Published: July 2002

The pressure broadening and shifting coefficients for pure rotational transitions of CO in a He bath gas at very low temperatures are calculated from the best available potential energy surfaces, and compared with very recent measurements by M. M. Beaky, T. M. Goyette, and F. C. De Lucia [J. Chem. Phys. 105, 3994 (1996)]. The results obtained for two recent empirical potentials determined from fits to Van der Waals spectra, and for a recent high quality purely ab initio surface, are consistent with one another. The best of the spectroscopic potentials also yields good agreement with high temperature virial coefficients and transport properties. Predictions from all three of these potentials agree with linebroadening and shifting measurements at temperatures above ∼20 K, but are in substantial disagreement with the measurements at temperatures below 4 K. At present, the source of this discrepancy is not known.
Semiempirical methods with conjugate gradient density matrix search to replace diagonalization for molecular systems containing thousands of atoms
Authors: Daniels Andrew D. , Millam John M. , Scuseria Gustavo E.
Published: July 2002

Conventional semiempirical methods using diagonalization are not practical for calculations on molecular systems containing more than a few hundred atoms because of O(N3) time and O(N2) memory requirements, where N is the number of atoms. Currently, the time dominating step is diagonalization of the Fock matrix. This paper demonstrates how O(N3) diagonalization and O(N2) memory requirements are eliminated by using a conjugate gradient search for the density matrix with sparse matrix techniques. Our method makes high accuracy energy calculations on molecules containing thousands of atoms possible on the typical workstation. Benchmark examples are presented on polyglycine chains (20000 atoms), water clusters (up to 1800 atoms), and nucleic acids (up to 6304 atoms).
Hydrogen bonding described through diatomics-in-ionic-systems: The HF dimer
Authors: Grigorenko B. L. , Nemukhin A. V. , Apkarian V. A.
Published: July 2002

With the proper inclusion of ion-pair configurations, the diatomics-in-molecules formalism can be used to accurately describe hydrogen bonding. This is demonstrated for the well characterized prototype, the HF dimer, the structure and entire potential energy surface of which is reproduced within its known accuracy: At the stationary points (potential minimum and saddle points) energies and bond lengths are reproduced with an accuracy of ∼1%, and the soft hydrogen bond angles are determined to within ∼5%. This is accomplished through a minimal basis Hamiltonian—19-dimensional matrix to describe the planar complex—constructed with analytic fits to accurately known or determined pair potentials. The construct includes the H+F− ion-pair states of the HF monomer units. The three-body nature of the inductive ion-pair interactions with neutrals is preserved, in the spirit of diatomic-in-ionic-systems. Based on ab initio estimates, in the limited range of interest, a Gaussian function describes the mixing between ionic and neutral states. The amplitude of this function is the only adjustable parameter in the model. The ionicity anisotropy and nonadditivity of interactions, responsible for the structure of the HF dimer, result naturally from mixing between ionic and neutral surfaces.
A theory for compressible binary lattice polymers: Influence of chain conformational properties
Authors: Wang Suxin , Nies Erik , Cifra Peter
Published: July 2002

The influence of long-range chain connectivity on the thermodynamic properties of athermal compressible single component and binary polymer mixtures is studied for the lattice model both theoretically and by Monte Carlo simulations. Theoretical expressions for the thermodynamic properties are derived based on the chain insertion probabilities. The chain conformations enter the theoretical insertion probabilities by the number of intramolecular contacts. The distribution of the number of intramolecular contacts of a single athermal chain is taken as input, of which the dependence on density is predicted by the theory. The theory successfully predicts the Monte Carlo simulation data for the equation of state of pure components and mixtures. Also microscopic details on the different types of self-contacts and cross contacts in the mixtures are accurately predicted.
Structure of concentrated aqueous NaCl solution: A Monte Carlo study
Authors: Degrève Léo , da Silva Fernando Luı́s B.
Published: July 2002

The structure of aqueous NaCl 1 M solution was studied by Monte Carlo simulation data obtained with 2117 rigid water molecules 40 Na+ and 40 Cl− at 293 K. The structural features were determined from the usual radial distribution functions, angular-radial distribution functions, angular correlations and the mean force potential. The ion hydration structure was obtained as such as the structure of ions associations. The first hydration shell structure is almost not affected by the finite concentration. The second shell is concentration dependent mainly with respect to the tilt angle. The cation hydration is less affected than the anion hydrated shell. Anion–cation, anion–anion and cation–cation pairs were detected. A fraction 0.46 of the cations and 0.32 of the anions are associated in pairs formed by two ions sharing, at least, two water molecules in their solvation shells. The idealized structures of all pairs include two to four water molecules located in such a way that the attractive energies are increased and the repulsive ones decreased. The structure of 1 M NaCl is also determined by recovering the water tetrahedral structure around the solute ions and by the stabilizing influence of water molecules that are shared by the ions. The last conclusion is that about 20% of the water molecules lose some degree of freedom because they solvate directly the solute. Almost all the water molecules are involved to some degree in the ion solvation.
Rydberg transitions in X-ray absorption spectroscopy of alkanes: The importance of matrix effects
Authors: Weiss K. , Bagus P. S. , Wöll Ch.
Published: July 2002

Precise ab initio electronic structure calculations on isolated alkanes (propane, butane) and on propane embedded in a matrix of several propane molecules were used in order to characterize the near-edge x-ray absorption fine structure (NEXAFS) resonances of condensed saturated hydrocarbons. The results demonstrate that upon condensation the dominating NEXAFS spectral features, i.e., the Rydberg resonances located between 287.4 and 288.1 eV, have a significant blue shift from the corresponding transition energies of an isolated alkane molecule. Furthermore, the theoretical results confirm the excitonic character of the final state. Additional calculations carried out for different conformations (trans/gauche) of isolated butane demonstrate that the transition energies of the corresponding NEXAFS resonances do not differ significantly. Previously observed changes in the electronic structure of disordered thin organic films of calciumarachidate are thus attributed to changes in the intermolecular spacing of the molecules rather than to changes in the electronic structure of the different, trans and gauche, conformations of the alkane chain.
Multiple spin echo generation by gradients of the radio frequency amplitude: Two-dimensional nutation spectroscopy and multiple rotary echoes
Authors: Kimmich Rainer , Ardelean Ioan , Lin Yung-Ya , Ahn Sangdoo , Warren Warren S.
Published: July 2002

NMR (nuclear magnetic resonance) nutation is treated with respect to demagnetizing-field effects on the evolution of spin coherences. A two-dimensional NMR nutation spectroscopy scheme is suggested consisting of a single radio frequency (RF) pulse and a free-evolution period. The RF pulse amplitude as well as the external magnetic field are assumed to be subject to gradients in the same but otherwise arbitrary direction. Cross peaks are predicted as frequency domain counterparts of multiple echoes. It is suggested to analyze the cross peak shape in terms of distributions of internal gradients arising from magnetic susceptibility inhomogeneities in heterogeneous samples. Furthermore, a pulse scheme solely based on gradients of the RF amplitude is treated resulting in the prediction of multiple rotary echoes as counterparts to the conventional rotary echo. The origin again is evolution in the presence of spatially modulated longitudinal magnetization in the tilted rotating frame.
Equilibria and kinetics of polydisperse mixture adsorption
Authors: Olson Carolyn B. , Talbot Julian
Published: July 2002

We study the equilibrium and kinetic properties of a model for polydisperse mixture adsorption. The system consists of a bulk phase of hard disks with a given size distribution and overall concentration that adsorb and desorb on a continuous planar surface. The disks adsorb at a rate proportional to their bulk concentration and desorb at a rate that may depend on the particle size. The model is characterized by α, the dimensionless binding energy of a solute per unit area, and K which is proportional to the total bulk concentration. The properties of the model are determined with scaled particle theory (SPT) and with numerical simulation. If the desorption rate is independent of particle size, an equilibrium is rapidly established between the bulk and adsorbed phases. The resulting adsorption isotherms predicted by SPT agree well with the numerical simulations. If the desorption rate depends exponentially on the binding energy of the adsorbed particle, the approach to equilibrium is dramatically slowed. At high bulk concentrations and low values of α the adsorbed density increases monotonically with time, while the coverage displays an overshoot. At low K and high α, it is the coverage that increases monotonically, while the density passes through a maximim. For a given bulk phase distribution, one can construct an (α,K) kinetic phase diagram delineating this behavior.
A dynamic model for exciton self-trapping in conjugated polymers. I. Theory
Authors: Kobrak Mark N. , Bittner Eric R.
Published: July 2002

In this article we present a time-dependent quantum/classical model for the dynamics of excitons in photoexcited conjugated polymer systems. Within this model, the excitation is treated quantum mechanically as a fully correlated electron/hole pair that interacts self-consistently with the vibrational motions of the polymer lattice. Spin and spatial symmetry considerations allow us to segregate singlet and triplet components into odd and even parity manifolds upon exchange of coordinates. We adapt the parameters used in various semiempirical models to produce a Hamiltonian that is continuous in the two-dimensional space and integrate the coupled equations of motion for the exciton wave function and the lattice. Ths approach includes the electronic correlations necessary to reproduce excitonic behavior and allows the study of both singlet and triplet exciton states. In this article, we use the approach to study the structure and formation of a self-trapped exciton at T=0 K starting from an initially free state. Within our model, the net stabilization of the singlet exciton upon localization is 238 cm−1 indicating that self-trapped exciton states in these systems are weakly bound relative to a free exciton.
Generalized quantum Fokker–Planck theory and its application to laser driven intramolecular hydrogen transfer reactions in condensed phases
Authors: Kühn Oliver , Zhao Yi , Shuang Feng , Yan YiJing
Published: July 2002

A generalized quantum Fokker–Planck theory is proposed to treat the correlated dynamics of coherent driving and Markovian dissipation. The resulting formulation is applicable to arbitrary external time-dependent driving fields and satisfies the detailed balance condition at arbitrary temperatures. Analyzed are also the formal relations among the Caldeira–Leggett quantum Fokker–Planck equation, the Bloch–Redfield theory, and the present formulation. The approach is numerically implemented to study the intramolecular hydrogen transfer reaction dynamics in a one-dimensional model system. Different forms of external pulsed driving fields are exploited and their ability to compete with concurring relaxation processes is investigated. Energy relaxation and pure dephasing are shown to have rather different influences on the reaction yield.
Thermodynamic integration of the free energy along a reaction coordinate in Cartesian coordinates
Authors: den Otter W. K.
Published: July 2002

A generalized formulation of the thermodynamic integration (TI) method for calculating the free energy along a reaction coordinate is derived. Molecular dynamics simulations with a constrained reaction coordinate are used to sample conformations. These are then projected onto conformations with a higher value of the reaction coordinate by means of a vector field. The accompanying change in potential energy plus the divergence of the vector field constitute the derivative of the free energy. Any vector field meeting some simple requirements can be used as the basis of this TI expression. Two classes of vector fields are of particular interest here. The first recovers the conventional TI expression, with its cumbersome dependence on a full set of generalized coordinates. As the free energy is a function of the reaction coordinate only, it should in principle be possible to derive an expression depending exclusively on the definition of the reaction coordinate. This objective is met by the second class of vector fields to be discussed. The potential of mean constraint force (PMCF) method, after averaging over the unconstrained momenta, falls in this second class. The new method is illustrated by calculations on the isomerization of n-butane, and is compared with existing methods.
Gas phase study of silicon–C60 complexes: Surface coating and polymerization
Authors: Pellarin M. , Ray C. , Lermé J. , Vialle J. L. , Broyer M. , Mélinon P.
Published: July 2002

( C 60 ) n Si m + cationic clusters are produced in a laser vaporization source by quenching the vapors from two independent C60 and silicon targets. They are analyzed in the gas phase by abundance and photofragmentation time-of-flight mass spectroscopy. For complexes containing only one C60 molecule, silicon is unlikely to wet the fullerene surface. Mass spectroscopic studies are rather in favor of a three-dimensional growth of silicon clusters weakly bound to C60. For larger systems, one can distinguish two classes of silicon atoms: most of them group in the form of compact islands (or clusters) and some others are directly involved in the linkage of C60 molecules. Particular geometric structures for the stable polymers (C60Si)n−2(C60)2+, (C60Si)n−1C60+, and (C60Si)n+ are postulated.
Terahertz laser spectroscopy of the water dimer intermolecular vibrations. II. (H2O)2
Authors: Braly L. B. , Liu K. , Brown M. G. , Keutsch F. N. , Fellers R. S. , Saykally R. J.
Published: July 2002

Terahertz VRT laser spectra of four (H2O)2 intermolecular vibrations consisting of 362 transitions have been measured between 87 and 108 cm−1 with ca. 2 MHz precision. The results differ both qualitatively and quantitatively from the predictions of dimer potentials tested. The spectra also reveal an ordering of the intermolecular vibrations which differs dramatically from that predicted by normal mode analysis. Strong coupling is indicated between the low barrier tunneling motions and the intermolecular vibrations as well as among different vibrations. In particular the 102.1 cm−1 (H2O)2 vibration assigned as the acceptor wag (ν8) exhibits two types of perturbations. In one of these a component of Ka=1 coupling with a tunneling component of Ka=0 in the 108 cm−1 acceptor twist (ν11) vibration. There is also an indication that the 103.1 cm−1 (H2O)2 band assigned as the donor in-plane bend (ν6) is coupled to the acceptor wag resulting in a lower of the in-plane bend frequency and a higher acceptor wag frequency. Detailed analysis of the VRT levels confirms the extreme nonrigidity of this complex, indicating that the use of approximate models with reduced dimensionality to calculate its properties are likely to fail.
A heterogeneous picture of α relaxation for fragile supercooled liquids
Authors: Viot Pascal , Tarjus Gilles , Kivelson Daniel
Published: July 2002

We examine some of the consequences, and their connection to experiments on supercooled liquids, of a scaling model of heterogeneous relaxation that is based on the theory of frustration-limited domains. In particular, we focus on what appears to be the two slowest components of structural relaxation, the one usually described by a stretched exponential or a Cole–Davidson function and the somewhat faster, apparently power-law decay known as von-Schweidler relaxation. Based on our model we study the α-relaxation activation free energy, the imaginary part of the dielectric frequency-dependent susceptibility, the susceptibility-mastercurve of Dixon et al. [Phys. Rev. Lett. 65, 1108 (1990)], and the breakdown of the Stokes–Einstein relation for translational diffusion at low temperatures. We also obtain estimates for the characteristic domain sizes as a function of temperature. As with all mesoscopic approaches, a number of assumptions must be introduced, but they all fit the overall scaling picture that motivates this approach. The good agreement with experimental dielectric relaxation data on two representative supercooled liquids, salol and glycerol, though necessarily dependent upon adjustable parameters, gives support to the theory.
Static uniform-density stars must be spherical in general relativity
Authors: Lindblom Lee
Published: July 2002

In this paper the uniqueness of the static solutions of Einstein’s equation that represent isolated uniform-density perfect-fluid stellar models is demonstrated: any static asymptotically flat space-time containing only a uniform-density perfect fluid confined to a spatially compact world tube is necessarily spherically symmetric. This result generalizes to relativistic uniform-density models the well known Newtonian theorem of Carleman and Lichtenstein.
Scattering for step-periodic potentials in one dimension
Authors: Roberts Thomas M.
Published: July 2002

Quantum scattering is developed for impurities in potentials that tend to a periodic function in one direction and a constant in the other. Two new technical results are obtained for Hill’s equation. Analytic, asymptotic, and spectral properties are established for solutions of the Schrödinger equation for step-periodic potentials, with and without impurity. The properties have all been used in Marchenko–Newton inverse scattering. Results apply feasibly to electron, photon, and phonon propagation in layered media.
Integrals, invariant manifolds, and degeneracy for central force problems in R
n
Authors: Lacomba E. A. , Llibre J.
Published: July 2002

Since the notion of angular momentum is defined in any dimension by using the exterior product in Rn, one would guess that central force problems in any dimension are completely integrable, as it is known for n=2 or 3. This is proved explicitly in this paper, by constructing n first integrals independent and involution: the energy and some combinations of the angular momentum components. It is shown that these problems are always reduced to a two-dimensional plane, and the invariant manifolds are topologically, the Cartesian product of those of the reduced problem times n−2 circle factors. It is also proved that for n≳2 these problems are degenerate in the sense that their Hamiltonians do not satisfy one of the hypotheses of the Kolmogorov–Arnold–Moser theorem for persistence of invariant tori, when one considers small perturbations.
High- and low-energy estimates for the Dirac equation
Authors: Blancarte Herminio , Grebert Benoît , Weder Ricardo
Published: July 2002

The scattering theory for the Dirac equation with radial potential is studied. The leading term at high energy is computed and Parzen’s theorem is proved. In the case of zero mass, the behavior at low energy is analyzed, which turns out to be different from the low-energy behavior for positive mass, and an appropriate version of Levinson’s theorem is proved under the assumption that the potential is integrable over (0,∞).
Quantum homogeneous spaces as quantum quotient spaces
Authors: Brzeziński Tomasz
Published: July 2002

It is shown that certain embeddable homogeneous spaces of a quantum group that do not correspond to a quantum subgroup still have the structure of quantum quotient spaces. A construction of quantum fibre bundles on such spaces is proposed. The quantum plane and the general quantum two-spheres are discussed in detail.
Geometric approach to inverse scattering for the Schrödinger equation with magnetic and electric potentials
Authors: Arians Silke
Published: July 2002

We consider the Hamiltonian H=(p−A(x))2/(2m)+V(x) of a quantum particle in a magnetic field B=rot A and a potential V in space dimensions ν⩾2. If V is of short range, then the high-velocity limit of the scattering operator uniquely determines the magnetic field B and the potential V. If, in addition, long-range potentials Vl are present, some knowledge of (the far out tail of) Vl is needed to define a modified Dollard wave operator and a scattering operator SD. Again its high- velocity limit uniquely determines B and V=Vs+Vl. Moreover, we give explicit error bounds which are inverse proportional to the velocity.
An inverse scattering problem with the Aharonov–Bohm effect
Authors: Nicoleau François
Published: July 2002

A direct and an inverse scattering method is developed for Schrödinger operators with electromagnetic fields in the case of obstacles in order to study the well-known Aharonov–Bohm effect. In dimension greater or equal to three, we show that the electric potential and the magnetic field are uniquely determined by the S-operator. In the two-dimensional case, some obstruction appears based on a quantification of the magnetic flux.
General scheme of splittings of degenerate eigensubspaces and eigenelements of self-adjoint operators in high orders of perturbation theory
Authors: Tsaune A. Ya. , Dyachenko M. P.
Published: July 2002

The possibility of arbitrary alternation of splitting and nonsplitting orders is taken into consideration through the idea of a stage of perturbation theory. In every order an eigenvalue equation is to be solved; its effective operator is obtained by means of a unified scheme from the operator of the previous order. The complete aggregate of subspace splittings leads to the correct set of eigenvectors of zero approximation. Expressions of any order corrections for the correct vectors leading to eigenvectors of a perturbed operator are obtained and discussed.
Low-temperature plasticity of Zn-doped β–Sn crystals
Authors: Diulin A. N. , Kirichenko G. I. , Natsik V. D. , Soldatov V. P.
Published: July 2002

Work hardening curves are obtained for Sn–Zn single crystals with Zn concentration 0.01 and 0.53 at.% in the temperature interval 1.6–300 K. It is found that the Zn impurity significantly affects the shape of the curves as well as the work hardening parameters like the strain-hardening factor, duration of slip stage, temperature dependences of critical shear stress, and strain-rate sensitivity. A comparative analysis is carried out for the effect of Zn on low-temperature creep of Sn, and the effect of the same concentration of Cd impurity (which was studied earlier).
EPR investigation of electronic excitations in rare gas solids (Review Article)
Authors: Zhitnikov R. A. , Dmitriev Yu. A.
Published: July 2002

The methods are described for producing unstable paramagnetic excited states in rare gas cryocrystals Ne, Ar, Kr, and Xe through the trapping, in the cryocrystals growing from the gas phase, the products of the gas discharge taking place in the same or other rare gas. The paper presents a technique and results of an observation and investigation of excited states in rare gas cryocrystals with electron paramagnetic resonance (EPR). The discovered unstable paramagnetic centers are interpreted as being local metastable excited np5(n+1)s atomic-type states in rare gas cryocrystals which are subject to the action of the anisotropic electric field resulted from the crystal surroundings distorted by the center. An account is given of the mechanisms for formation of observed paramagnetic excited states in cryocrystals which arise owing to the excitation energy of the metastable P23 atoms of Ne, Ar, Kr, Xe and He 23S1 and 21S0 atoms that form in the discharge in an appropriate gas and trap in the growing cryocrystal.
Cross-linking of two antiparallel vortex tubes
Authors: Melander M. V. , Hussain F.
Published: July 2002

The detailed mechanisms in vortex cross-linking are unveiled by adequately resolved, direct numerical simulation of two viscous vortex tubes. There are three characteristic phases: (i) inviscid induction followed by core flattening and stretching; (ii) bridging of the two vortices by accumulation of annihilated and then cross-linked vortex lines; and (iii) threading of the remnants of the initial vortex pair in between the two bridges as they pull apart. These phases and the role of threading—along with bridging—in the mixing and the enstrophy cascade are explained, and it is shown that the mechanism is insensitive to asymmetries.