bluuebunny/crossref_metadata_2025 huggingface dataset:
Reflectivity study of hexagonal GaN films grown on GaAs: Surface roughness, interface layer, and refractive index
Authors: Shokhovets S. , Goldhahn R. , Cimalla V. , Cheng T. S. , Foxon C. T.
Published: July 2002

Detailed reflectivity studies of hexagonal GaN films grown by molecular beam epitaxy on GaAs substrates have been carried out in the energy range from 1.4 to 3.8 eV at room temperature. Measurements using ambient media with different refractive indexes verify that the reflectivity is strongly influenced by a surface roughness. Furthermore, the optical data give a clear evidence for the formation of an interface layer (mixture of GaAs with voids) between the film and the substrate which agrees well with transmission electron microscopy observations. A quantitative analysis has been performed by correcting the reflectivity spectra for the surface roughness and making use of a two-layer model to take into account the interface layer. This procedure yields for all samples the same energy dependent refractive index of GaN despite the differing surface and interface layer properties. The obtained values of the root mean squared roughness are close to the atomic force microscopy data. Growth on (001) substrates has been found to be accompanied by the formation of a thicker interface layer than that for (111)B orientation.
Stress relaxation and precursor decay in laser shock-loaded iron
Authors: de Rességuier T. , Hallouin M.
Published: July 2002

Iron samples of different thicknesses have been submitted to laser shocks of short duration. Transmitted wave measurements have been performed by means of polymeric piezoelectric transducers. The records indicate a decrease of the elastic precursor amplitude with increasing propagation distance, as well as a stress relaxation behind the elastic wave front. These features were already observed in shock-loaded iron, but this paper extends the experimental data to a previously unexplored range of thin targets and high strain rates. Postshock examinations of the recovered specimens reveal significant twin formation. Computer simulations of the experiments have been made using a constitutive model from the literature, including a theoretical description of rate-dependent twinning. The ability of that model to predict the experimental results has been tested. A correct overall agreement has been obtained, except for some discrepancies which have been discussed.
Micro-Raman investigation of stress variations in lead titanate films on sapphire
Authors: Dobal P. S. , Bhaskar S. , Majumder S. B. , Katiyar R. S.
Published: July 2002

Using the sol–gel method, PbTiO3 films of 21, 64, 128, 210, 310, and 420 nm thicknesses were obtained on (0001) sapphire. Raman scattering and x-ray diffraction techniques were used to study the correlation between the film thickness and the structural changes on these films. The Raman and x-ray intensities in 21 nm film were too weak to reveal any structural information, while all other films showed tetragonal structure. At room temperature, the variation of lowest E(1TO) phonon mode frequency with film thickness was observed due to compressive stresses in the films. The lattice parameters and the degree of a axis orientation values of tetragonal PbTiO3 have been evaluated as a function of film thickness. The changes in lattice parameters thus obtained were used to estimate the stress at each thickness. An excellent agreement was found between the stress values obtained using Raman and x-ray results. An exponential decrease in stress with increasing film thickness was observed because of the structural changes in the lattice. Accordingly, a downshift in the tetragonal–cubic transition temperature with decreasing film thickness has been interpreted using the conventional Landau–Devonshire approach.
Analyses of small facets imaged with scanning-probe microscopy
Authors: Hegeman J. B. J. W. , Kooi B. J. , Groen H. B. , De Hosson J. Th. M.
Published: July 2002

Two tools for the analysis of facets as detected by scanning-probe microscopy (SPM) images are proposed. One tool is an adaptation of the radial-histogram transform proposed by D. Schleef et al. in Phys. Rev. B. 55, 2535 (1997). In this article the local slopes in the SPM image are in the present version determined by Savitsky–Golay filters with variable lengths [A. Savitsky and M. J. E. Golay, Anal. Chem. 36, 1627 (1964)]. These variable length filters turn out to be important to suppress the influence of noise obscuring the possibility to detect facets and to analyze corrugations with different length scales in SPM images, e.g., surface reconstructions. The other tool allows the direct quantitative determination of the orientation (with a standard deviation) of user-specified parts of facets. It makes use of a Savitsky–Golay filter as well. Both tools were applied to an artificially constructed SPM image and several experimental SFM images showing (ionic) MnO precipitates protruding out of a (metallic) Cu surface. It is shown that the Miller indices of the facets can be derived experimentally.
Time-dependent evolution of thin TiN films prepared by ion beam assisted deposition
Authors: Kovac J. , Scarel G. , Sancrotti M. , Beghi M. G. , Bottani C. E. , Ossi P. M. , Calliari L. , Bonelli M. , Miotello A.
Published: July 2002

Thin titanium nitride (TiN) films have been prepared by simultaneous Ti evaporation and 30 keV N2+ implantation onto Si(100) wafers, in the presence of a N2 partial atmosphere. A systematic chemical and compositional evaluation of the films has been performed by Auger electron spectroscopy and x-ray photoemission spectroscopy checking the homogeneity level, the degree of gaseous contamination, the different compounds formed by titanium and nitrogen, and their depth distribution. The Rayleigh velocity at various angles of incidence has been determined via Brillouin scattering measurements. The evidence of a Sezawa mode confined in the overlayer was also found. The films, which consist of a homogeneous outer layer above a relatively wide interface region with the substrate, show aging effects. On a mesoscopic scale, the film structure appears porous.
Ab initio relativistic effective potentials with spin-orbit operators. I. Li through Ar
Authors: Fernandez Pacios Luis , Christiansen P. A.
Published: July 2002

A refined version of the ‘‘shape consistent’’ effective potential procedure of Christiansen, Lee, and Pitzer was used to compute averaged relativistic effective potentials (AREP) and spin-orbit operators for the atoms Li through Ar. These are tabulated in analytic form. Small optimized Gaussian basis sets with expansion coefficients for the lowest energy state for each atom are given and the reliability of the potentials relative to all electric calculations is discussed. Finally a procedure for computing molecular moments and Breit corrections is suggested.
The Fourier transform infrared spectrum of methyleneimine in the 10 μm region
Authors: Halonen L. , Duxbury G.
Published: July 2002

The infrared spectrum of methyleneimine, CH2NH, has been observed in the gas phase with a resolution of 0.0048 cm−1 using the National Optical Observatory solar Fourier transform spectrometer. The CH2NH was produced by the pyrolysis of CH3NH2 at ∼1000 °C. Vibrational band origins and rotational constants have been determined for the ν7, ν8, and ν9 bands. These were analyzed as a set of three states strongly coupled by Coriolis interaction. More than 2500 transitions were assigned, of which many are ‘‘forbidden’’ transitions, such as qQ branches, induced in the ν9 band by the strong Coriolis interaction between ν7 and ν9. Owing to the high precision of the data, and the high J values of many of the observed transitions, it was necessary to include many of the high order interaction constants in order to achieve a quantitative fit.
Kinetic oscillations in the catalytic CO oxidation on Pt(100): Experiments
Authors: Imbihl R. , Cox M. P. , Ertl G.
Published: July 2002

Under properly chosen conditions the rate of catalytic CO oxidation at a clean and well-defined Pt(100) surface exhibits sustained temporal oscillations which are also reflected by periodic changes of the work function (Δφ). These oscillations are associated with periodic changes of the surface structure between the CO-covered (θCO =0.5) 1×1–c(2×2) phase and an oxygen adlayer, respectively the reconstructed hex phase, as demonstrated by LEED. Laterally resolved experiments revealed that the surface phase transformations propagate in the form of trigger waves across the whole surface area. On the basis of detailed experiments and of literature data on the adsorptive properties of the two surface modifications of Pt(100) a microscopic picture on the mechanism of the observed phenomena is developed which also underlies a recently performed theoretical treatment.
The onset of nonrigid dynamics and the melting transition in Ar7
Authors: Amar François G. , Berry R. Stephen
Published: July 2002

We have carried out a molecular dynamics (MD) simulation study of melting of Ar7. By periodically quenching trajectories, we are also able to follow the path of the cluster through configuration space. This procedure yields information about isomerization rates, isomerization dynamics, and the connectivity of the phase space as a function of energy. New criteria for melting and the coexistence of phases in small clusters are compared with the traditional T(E) curves and rms bond fluctuations available from time averages in MD simulations.
Quantitative photoabsorption and fluorescence spectroscopy of H2S and D2S at 49–240 nm
Authors: Lee L. C. , Wang Xiuyan , Suto Masako
Published: July 2002

Photoabsorption and fluorescence cross sections of H2S and D2S were measured in the 49–240 nm region using synchrotron radiation as a light source. Fluorescence from photoexcitation of H2S appears at 49–97 nm but not in the longer wavelength region. Fluorescence spectra were dispersed, and used to identify the emitters to be H2S+(Ã), SH+(A), and H(n>2). The fluorescence quantum yield is about 6%. Photoexcitation of D2S at 49–96 nm produces fluorescence with a quantum yield of about 5%. The emitters are identified from the fluorescence spectra to be D2S+(Ã), SD+(A), and D(n>2). The Franck–Condon factors for the SH+ and SD+ (A–X) transitions were determined. The SD(A–X) fluorescence was observed from photoexcitation of D2S at 100–151 nm, for which the fluorescence cross section and quantum yield were measured.
Structuring in liquid alkanes between solid surfaces: Force measurements and mean-field theory
Authors: Christenson H. K. , Gruen D. W. R. , Horn R. G. , Israelachvili J. N.
Published: July 2002

Measurements have been made of the solvation forces between mica surfaces in the even-numbered n-alkanes from hexane to hexadecane. In all cases the force law is qualitatively very similar, characterized by a decaying oscillatory function of distance, as occurs for simple isotropic liquids. The spacing between successive minima in the force does not increase with carbon number, and is comparable to the width of a linear alkane molecule rather than its length or any average diameter. This suggests that the alkanes have some tendency towards a parallel orientation near the mica surfaces. The measurements give no indication of any strong repulsive component expected from mean-field theories of higher alkanes or polymers. The results of one such theory are presented, and the reasons for its failure to match the experimental data are discussed.
Two-point cluster function for continuum percolation
Authors: Torquato S. , Beasley J. D. , Chiew Y. C.
Published: July 2002

We introduce a two-point cluster function C2(r1,r2) which reflects information about clustering in general continuum–percolation models. Specifically, for any two-phase disordered medium, C2(r1,r2) gives the probability of finding both points r1 and r2 in the same cluster of one of the phases. For distributions of identical inclusions whose coordiantes are fully specified by center-of-mass positions (e.g., disks, spheres, oriented squares, cubes, ellipses, or ellipsoids, etc.), we obtain a series representation of C2 which enables one to compute the two-point cluster function. Some general asymptotic properties of C2 for such models are discussed. The two-point cluster function is then computed for the adhesive-sphere model of Baxter. The two-point cluster function for arbitrary media provides a better signature of the microstructure than does a commonly employed two-point correlation function defined in the text.
Geminate recombination in proton-transfer reactions. II. Comparison of diffusional and kinetic schemes
Authors: Agmon Noam , Pines Ehud , Huppert Dan
Published: July 2002

The diffusional and kinetic approaches are compared for geminate dissociation–recombination reactions. When steady-state rate coefficients to and from a distance defined as a ‘‘complex cage’’ are evaluated from the diffusion equation, one obtains encouraging agreement between the transient analytic solution of the rate equations and the exact numerical solution for diffusion with backreaction over a finite time regime. However, the rate equations cannot accurately describe the decay of the dissociating molecule for very long times, since as we prove below, the asymptotic decay according to the diffusional scheme is t−3/2, while for the rate equations it is exponential. New experiments, over an extended time regime confirm these conclusions.
A systematic analysis of the spectra of the lanthanides doped into single crystal LaF3
Authors: Carnall W. T. , Goodman G. L. , Rajnak K. , Rana R. S.
Published: July 2002

The optical spectra of the lanthanides doped into single crystal LaF3 have been interpreted in terms of transitions within 4f N configurations. Energy matrices combining free-ion terms with a crystal field for an approximate model which assumes C2v instead of the actual C2 site symmetry were diagonalized. Excellent correlations were obtained between experimental transition energies and the computed level structures. We also report the results of previously unpublished experimental spectroscopic investigations of Nd3+ and Sm3+:LaF3, as well as predicted energy levels for Pm3+:LaF3. The spectroscopic data for each ion were independently interpreted using an effective-operator model, then the model parameters were intercompared. Systematic trends have been identified, and a comprehensive energy level diagram is presented.
Size-inconsistency effects in molecular properties for states with valence-Rydberg mixing: The low-lying π→π* states of ethylene and butadiene
Authors: Cave Robert J.
Published: July 2002

A b initio results for the low-lying 1B1u states of ethylene are used to discuss size-inconsistency effects on calculated molecular properties for states where valence-Rydberg mixing is important. Results for the expectation value of x2, x being the coordinate perpendicular to the plane of the molecule, are presented from quasidegenerate variational perturbation theory and multireference configuration-interaction calculations. These results are compared with values from previous studies. It is argued that size inconsistency in configuration-interaction studies can have a significant effect on estimated molecular properties. Calculations on several low-lying states of butadiene are also reported where similar size-inconsistency effects are found.
Coupled cluster response functions
Authors: Koch Henrik , Jo/rgensen Poul
Published: July 2002

The linear and quadratic response functions have been determined for a coupled cluster reference state. From the response functions, computationally tractable expressions have been derived for excitation energies, first- and second-order matrix transition elements, transition matrix elements between excited states, and second- and third-order frequency-dependent molecular properties.
Effects of surface roughness on adsorbed polymers
Authors: Baumgärtner A. , Muthukumar M.
Published: July 2002

Polymer chains adsorbed on rough impenetrable surfaces have been investigated analytically and by simulations. The cases of physical and chemical roughness of surfaces are identified and their distinctive effects on the adsorption characteristics are studied. For the chemically rough surface the adsorption temperature is depressed by an amount proportional to the concentration of the impurities. A polymer adsorbed on a physically rough surface can be interpreted using the analogy to a one-dimensional electron in a periodic potential. It is shown that at low temperatures the chain is ‘‘localized’’ in one of the wells. Between the localized regime and the unbinding transition point, there exists a ‘‘diffusive’’ regime, where the chain is diffusing by being shared by several potential wells. This regime is equivalent to the conduction band in the electron analogy. In contrast to the case of a flat surface, the unbinding transition of a chain from a periodically rough surface is markedly sharp due to an effective anchoring of the chain in the wells.
The generation of O(1S) from the dissociative recombination of O+2
Authors: Guberman Steven L. , Giusti-Suzor Annick
Published: July 2002

The multichannel quantum defect theory (MQDT) method and large scale wave functions are applied to the calculation of the cross sections and rates for dissociative recombination of O+2 along the 1Σ+u dissociative potential. Indirect dissociative recombination is accounted for by simultaneously including both the vibronic and electronic coupling to the intermediate Rydberg resonances. An enhanced MQDT approach involving a second order K matrix is described. Cross sections and rates for the lowest three vibrational levels of the ion are reported. The shapes of the cross sections are discussed in terms of Fano’s profile index. We find that for each of the three ion vibrational levels, the intermediate Rydberg resonances reduce the dissociative recombination rate below the direct recombination rate. Just above threshold, resonances with centers below threshold play an important role.
Atomic resolution scanning tunneling microscopy images of Au(111) surfaces in air and polar organic solvents
Authors: Haiss W. , Lackey D. , Sass J. K. , Besocke K. H.
Published: July 2002

Atomic features of a close-packed metal surface have been observed for the first time by scanning tunneling microscopy in organic polar solvents. Evaporated gold films, exhibiting large reconstructed (111) terraces, have been imaged with a resolution far superior to previous results in aqueous environments.
Intersystem crossing dynamics in Fe(II) coordination compounds
Authors: Hauser Andreas , Vef Andreas , Adler Peter
Published: July 2002

The 5T2(HS)→1A1(LS) intersystem crossing rates have been determined for a number of Fe(II) coordination compounds between 10 and 270 K using time-dependent optical spectroscopy. Strong deviations from Arrhenius kinetics with nearly temperature independent tunneling at low temperatures and a thermally activated behavior at elevated temperatures with apparent activation energies smaller than the classical energy barrier were found. The tunneling rates range from ∼10−6 s−1 for the doped spin crossover system [Zn1−xFex(ptz)6](BF4)2 to ∼106 s−1 for the doped low-spin (LS) system [Zn1−xFex(bipy)3](PF6)2. The large range of 12 orders of magnitude in the low temperature tunneling rates as well as the activated region can be understood in terms of nonadiabatic multiphonon relaxation. Values for the Huang–Rhys parameter S of 40–50 and for the reduced energy gap p of 1–12 are estimated for the present series of compounds. The validity of an inverse energy gap law in the strong vibronic coupling limit with S≫p is borne out by experiment.
The structure of NiN and PdN clusters: 4≤N≤23
Authors: Stave Mark S. , DePristo Andrew E.
Published: July 2002

Stable geometrical structures of NiN and PdN clusters (N=4–23) are identified using a corrected effective medium (CEM) theory. Structural optimization is accomplished by simulated annealing using analytic derivatives to determine the interatomic forces. Unique structural features of these metal clusters are noted, especially in relation to the bulk and surface phases of these metals and to structures commonly associated with rare gas clusters. Elucidation of the general features of cluster growth leads to the principle that transition metal clusters generally maximize the minimum coordination of any atom. By contrast, rare gas clusters maximize the number of interatomic distances close to the optimal distance for the pairwise interaction between rare gas atoms. The latter can be interpreted as the packing of hard balls. Structural transformations between isomers of similar energy are also examined for selected sizes.
CdSe nanocrystals with a dipole moment in the first excited state
Authors: Colvin V. L. , Alivisatos A. P.
Published: July 2002

Stark effect modulation of the optical absorption spectrum of 40 Å diam CdSe nanocrystals show the first excited state of these clusters has a dipole moment of 32±10 D.
Spectroscopic properties of conducting Langmuir–Blodgett films obtained by the ‘‘homodoping strategy’’
Authors: Yartsev V. M. , Bourgoin J. P. , Delhaes P. , Vandevyver M. , Barraud A.
Published: July 2002

Experimental IR properties of conducting (ODS-TCNQ)ξ(TCNQC18)1−ξ Langmuir–Blodgett (LB) films are presented and discussed within the frame of the electron-vibration coupling model of Rice, extended to take into account the modifications induced by grafting an alkyl chain to the TCNQ molecule. The quite good theoretical fit of experimental spectra (including unusual fine structure of agν5 vibronic band) indicates that (i) optical as well as electrical properties of the films can be accounted for by the existence of dimers the type of which depends on ξ; and (ii) inside the layer, an energy transfer takes place between the electroactive part and the alkyl chain via a two step coupling process.
Response to ‘‘Comment on ‘The exactness of the extended Koopmans’ theorem: A numerical study’’ [J. Chem. Phys. 99, 6221 (1993)]
Authors: Sundholm Dage , Olsen Jeppe
Published: July 2002

The exactness of the extended Koopmans’ theorem (EKT) has been studied by performing extensive multiconfiguration Hartree–Fock and configuration interaction calculations on excited states of Be and Be+. The results show that the EKT ionization potential for the Be(1S) to Be+(2P) ionization is not equal to the energy difference between the cation and the neutral atom. For Be(3P), two of the EKT ionization potential values approach, in the limit of a complete basis, the values of the ionization potentials obtained as the appropriate energy difference.
Ground state dynamics of NO3: Multimode vibronic borrowing including thermal effects
Authors: Mayer M. , Cederbaum L. S. , Köppel H.
Published: July 2002

The X 2A2′ band of the photoelectron spectrum of NO−3 is theoretically calculated in the framework of a multimode vibronic coupling model. Linear coupling between and within the X 2A2′ and the B 2E′ electronic states of NO3 is found to be the important mechanism governing the dynamics. The necessary coupling constants are obtained from the ionization potentials of NO−3 calculated at different geometries using ab initio Green’s functions. Comparison of the theoretical with experimental results [A. Weaver, D. W. Arnold, S. E. Bradforth, and D. M. Neumark, J. Chem. Phys. 94(3), 1740 (1991)] for the PE spectrum shows good agreement if a temperature of the NO−3 anion of 455 K in the experiment is assumed. A general theoretical treatment of thermal effects in vibronically coupled electronic states is presented. The ground electronic state potential surface of NO3 is discussed in the framework of the vibronic coupling model. A shallow minimum at a C2v geometry with a barrier height of 0.006 eV relative to the minimum energy D3h configuration is found. It is too weak to deform the effective geometry of the molecule from D3h to C2v. Mode-mode coupling is found to play a relevant role in the ground electronic state of NO3, in general.
Ab initio relativistic effective potentials with spin–orbit operators. V. Ce through Lu
Authors: Ross Richard B. , Gayen Sanjukta , Ermler Walter C.
Published: July 2002

Ab initio averaged relativistic effective core potentials, spin–orbit operators, and valence basis sets are reported for the fourteen lanthanide elements Ce through Lu in the form of expansions in Gaussian-type functions. Gaussian basis sets with atomic orbital coefficients for low energy states of each atom are also tabulated. Hartree–Fock orbital energies calculated in conjunction with the relativistic effective core potentials using the j-j coupling scheme agree to within 10% of those due to numerical all-electron Dirac–Fock calculations. Excitation energies, ionization potentials, and spin–orbit splittings are compared to all electron, energy-adjusted effective core potential, and experimental results.
Hysteretic behavior and irreversibility of polymer gels by pH change
Authors: Suzuki A. , Suzuki H.
Published: July 2002

Swelling equilibrium has been studied on N-isopropylacrylamide gel which is randomly but slightly incorporated both by trisodium salt of copper chlorophyllin and by sodium acrylate. An ionic gel with both hydrophobic interaction and hydrogen bonding was found here to display hysteretic behavior in swelling degree with an interesting nature when the pH is cycled between 7 and 12. In the shrinking transition to the collapsed state observed during pH decrease, there are two different processes characterized by the first order phase transitions with different pHs (7.3 and 7.6). These could be selected whether or not the pH had been increased beyond the threshold (pH=9). Namely, in the larger pH region beyond the threshold, the gel can reswell larger than the first swollen state which appeared directly from the collapsed phase. The hysteretic behavior as well as the irreversibility observed in higher pH region than the threshold was attributed to the screening effect on the ionic groups in the gels because of excess Na+ ions from NaOH that is introduced for increasing the solvent pH. A simple theoretical model is presented to qualitatively explain the phenomena on the basis of the Landau theory.
Time-of-flight neutron powder diffraction study on the third row transition metal hexafluorides WF6, OsF6, and PtF6
Authors: Marx R. , Seppelt K. , Ibberson R. M.
Published: July 2002

A neutron diffraction study on the third-row transition metal hexafluorides MF6 (M≡W, Os, Pt) has been performed using the high resolution neutron powder diffractometer (HRPD) at the spallation source ISIS, England. The previously unknown structures of the low-temperature phases of OsF6 and PtF6 are reported. WF6, OsF6, and PtF6, which exhibit a (5dt2g)0, (5dt2g)2, and (5dt2g)4 electronic configuration, respectively, are found to be isostructural and crystallize in the UF6 structure, space group Pmnb, (No. 62). The geometry of the MF6 molecules is to good approximation octahedral for each compound, the mean M–F bond length increasing only slightly from 182.5 (W) to 185.0 (Pt). For WF6 deviations from ideal octahedral geometry are only marginally significant [181.8(2) to 183.2(2) pm] and may be interpreted on the basis of packing effects. Deviations for the d2 complex OsF6 are somewhat larger [181.5(2) to 184.4(3) pm] and may be assumed to be caused by packing effects essentially the same as for WF6, in addition to a first-order Jahn–Teller effect arising from the (5dt2g)2 electronic configuration. While eliminating the effects of packing by a comparison of individual M–F bond lengths for WF6 and OsF6, the OsF6 molecule shows to have D4h symmetry with two apical M–F bonds about 1.8 pm longer than the four equatorial bonds as a result of the Jahn–Teller distortion. Only small deviations from ideal octahedral geometry [184.4(3) to 185.8(3) pm] are found for the d4 complex PtF6. Within the series W to Pt a substantial shortening of the F...F van der Waals contact distances is observed. This shortening more than compensates for the increase in the M–F bond lengths and leads to unit cell volumes and cell parameters decreasing continuously from W to Pt. The variation of F...F contact distances and M–F bond lengths may be rationalized in terms of polarization of the F-ligands in the field of the highly charged nuclei of the central atoms which are only incompletely shielded by the 5d electrons.
Formulation and implementation of a relativistic unrestricted coupled-cluster method including noniterative connected triples
Authors: Visscher Lucas , Lee Timothy J. , Dyall Kenneth G.
Published: July 2002

The formalism for a relativistic open-shell CCSD(T) method is presented and implemented in a computer program, RELCCSD. The code can be used for calculations with 2- or 4-component relativistic reference wave functions and allows a full inclusion of the spin–orbit coupling. The code is interfaced to the MOLFDIR program system. We illustrate its use with ab initio calculations of the fine structure splittings of Cl, FO, ClO, O+2, and O−2. The triples correction is found to make a large contribution to the Cl atom splitting, which is within 23 cm−1, of the experimental value. The molecular results are within 4 cm−1 of the experimental values where these are available. The value for FO is predicted to be −195±4 cm−1, in good agreement with experiment.
C
v
extrema on isotherms for simple fluids and why they exist
Authors: Freasier Ben C. , Woodward Cliff E. , Bearman Richard J.
Published: July 2002

We review experimental results for the locus in the temperature–density plane of isothermal Cv (constant volume heat capacity) extrema of Ar, Kr and Xe, and published Percus–Yevick and simulation equations of state. It is likely that the locus of Cv maxima terminates at the critical point. We report new long (960 million Monte Carlo steps), 864 particle simulations of the heat capacity for the truncated Lennard-Jones potential (cutoff=2.5σ) near the liquid coexistence line, and establish directly that the locus of Cv minima intersects the coexistence line. On the basis of calculations and simulations for model systems, we induce that previously reported Cv extrema are caused by the interplay of three physical effects that we term, “aggregation,” “caging,” and “soft-core penetration.” We test our hypothesis by carrying out calculations for a one-dimensional, nearest-neighbor, infinite-chain, truncated interaction model with the following potentials: Lennard-Jones, Lennard-Jones with hard core, Lennard-Jones with hard core and no soft repulsion, square well, and the inverse twelfth. Using our physical understanding, we successfully explain the qualitative changes in the behavior of the Cv extrema as the interaction potential changes.
Limited range fractality of randomly adsorbed rods
Authors: Lidar (Hamburger) Daniel A. , Biham Ofer , Avnir David
Published: July 2002

Multiple resolution analysis of two dimensional structures composed of randomly adsorbed penetrable rods, for densities below the percolation threshold, has been carried out using box-counting functions. It is found that at relevant resolutions, for box sizes, r, between cutoffs given by the average rod length 〈l〉 and the average inter-rod distance r1, these systems exhibit apparent fractal behavior. It is shown that unlike the case of randomly distributed isotropic objects, the upper cutoff r1 is not only a function of the coverage but also depends on the excluded volume, averaged over the orientational distribution. Moreover, the apparent fractal dimension also depends on the orientational distributions of the rods and decreases as it becomes more anisotropic. For box sizes smaller than 〈l〉 the box counting function is determined by the internal structure of the rods, whether simple or itself fractal. Two examples are considered—one of regular rods of one dimensional structure and rods which are trimmed into a Cantor set structure which are fractals themselves. The models examined are relevant to adsorption of linear molecules and fibers, liquid crystals, stress induced fractures, and edge imperfections in metal catalysts. We thus obtain a distinction between two ranges of length scales: r<〈l〉, where the internal structure of the adsorbed objects is probed and 〈l〉<r<r1, where their distribution is probed, both of which may exhibit fractal behavior. This distinction is relevant to the large class of systems which exhibit aggregation of a finite density of fractal-like clusters, which includes surface growth in molecular beam epitaxy and diffusion-limited-cluster-cluster-aggregation models.
The reliability of the determination of tensor parameters by solid-state nuclear magnetic resonance
Authors: Hodgkinson Paul , Emsley Lyndon
Published: July 2002

The accuracy of determination of tensor parameters measured using solid-state nuclear magnetic resonance is investigated. In particular, the reliability of the determination of the anisotropy and the asymmetry parameter of the chemical shift is calculated using the Cramér–Rao lower bounds. Minimizing this measure of the error as a function of an experimental parameter (in this case the spinning speed of the sample) enables the optimization of any given experiment. Hence, an optimum number of sidebands is found for which the determination of the anisotropy is most reliable. Comparison to the static limit shows that the reliability of the determination of the anisotropy is always greater in spinning experiments than in static experiments. An analogous analysis for the asymmetry parameter shows it to be consistently more reliably determined from a static spectrum. The sensitivity of the fitting to the simulation algorithm is found to become pronounced with slower speeds and static spectra, and a discussion of such systematic errors is provided. The reliability of the determination of dipolar or first-order quadrupolar tensors is also calculated, and we find that this presents a surprisingly different situation to that of the chemical shift.
The low frequency vibrations in clathrate hydrates
Authors: Tse J. S. , Shpakov V. P. , Murashov V. V. , Belosludov V. R.
Published: July 2002

The mechanism for the unexpectedly strong interactions between the guest and lattice vibrations in clathrate hydrates is revealed from molecular dynamics and lattice dynamics calculations on model systems. It is shown that the vibrational couplings are the result of avoided crossings of the enclathrated guest localized rattling modes with the host lattice acoustic phonon branches of the same symmetry. Through these couplings, energy can be exchanged between the host lattice and the guest vibrations and lead to the anomalous glasslike behavior in the thermal conductivity of gas hydrates.
Kinetic theory of bimolecular reactions in liquid. III. Reversible association–dissociation: A+B⇄C
Authors: Yang Mino , Lee Sangyoub , Shin Kook Joe
Published: July 2002

A theoretical formalism based on the fully renormalized kinetic theory is applied to a diffusion-influenced pseudo-first order reaction kinetics of reversible association–dissociation A+B⇄C including unimolecular decay processes. Linear response of the system, initially at equilibrium, to a thermal perturbation is examined and a rate kernel equation for the reactant concentrations is derived. The rate kernel has a hierarchical structure and the propagator appeared in the kernel expression is truncated by a disconnected approximation. When the unimolecular reactions are turned off, the response of the system not only shows the long-time power law of t−3/2 but also displays the proper behavior over the whole time region in accordance with previous computer simulation results. Moreover, it is shown that the amplitude of the long-time behavior predicted by previous workers is modified by a certain correction factor P which contains dynamical correlation effects. In this way, many-body complication inherent to the history of reactive pair creation is properly implemented in the description of the reversible kinetics. We compare the present theory with the other existing theories such as the rate equation, the superposition approximation, and the convolution approaches. In some limiting cases, results obtained from the present theory can be reduced to those from the existing theories.
Size-consistent wave functions for nondynamical correlation energy: The valence active space optimized orbital coupled-cluster doubles model
Authors: Krylov Anna I. , Sherrill C. David , Byrd Edward F. C. , Head-Gordon Martin
Published: July 2002

The nondynamical correlation energy may be defined as the difference between full configuration interaction within the space of all valence orbitals and a single determinant of molecular orbitals (Hartree–Fock theory). In order to describe bond breaking, diradicals, and other electronic structure problems where Hartree–Fock theory fails, a reliable description of nondynamical correlation is essential as a starting point. Unfortunately, the exact calculation of nondynamical correlation energy, as defined above, involves computational complexity that grows exponentially with molecular size and is thus unfeasible beyond systems of just two or three heavy atoms. We introduce a new hierarchy of feasible approximations to the nondynamical correlation energy based on coupled-cluster theory with variationally optimized orbitals. The simplest member of this hierarchy involves connected double excitations within the variationally optimized valence active space and may be denoted as VOO-CCD, or VOD. VOO-CCD is size-consistent, has computational complexity proportional to the sixth power of molecule size, and is expected to accurately approximate the nondynamical correlation energy in such cases as single bond dissociation, diradicals, and anti-ferromagnetic coupling. We report details of our implementation of VOO-CCD and illustrate that it does indeed accurately recover the nondynamical correlation energy for challenging multireference problems such as the torsion of ethylene and chemical bond breaking.
Ewald artifacts in computer simulations of ionic solvation and ion–ion interaction: A continuum electrostatics study
Authors: Hünenberger Philippe H. , McCammon J. Andrew
Published: July 2002

The use of Ewald and related methods to handle electrostatic interactions in explicit-solvent simulations of solutions imposes an artificial periodicity on systems which are inherently nonperiodic. The consequences of this approximation should be assessed, since they may crucially affect the reliability of those computer simulations. In the present study, we propose a general method based on continuum electrostatics to investigate the nature and magnitude of periodicity-induced artifacts. As a first example, this scheme is applied to the solvation free-energy of a spherical ion. It is found that artificial periodicity reduces the magnitude of the ionic solvation free-energy, because the solvent in the periodic copies of the central unit cell is perturbed by the periodic copies of the ion, thus less available to solvate the central ion. In the limit of zero ionic radius and infinite solvent permittivity, this undersolvation can be corrected by adding the Wigner self-energy term to the solvation free-energy. For ions of a finite size or a solvent of finite permittivity, a further correction is needed. An analytical expression for this correction is derived using continuum electrostatics. As a second example, the effect of artificial periodicity on the potential of mean force for the interaction between two spherical ions is investigated. It is found that artificial periodicity results in an attractive force between ions of like charges, and a repulsive force between ions of opposite charges. The analysis of these two simple test cases reveals that two individually large terms, the periodicity-induced perturbations of the Coulomb and solvation contributions, often cancel each other significantly, resulting in an overall small perturbation. Three factors may prevent this cancellation to occur and enhance the magnitude of periodicity-induced artifacts: (i) a solvent of low dielectric permittivity, (ii) a solute cavity of non-negligible size compared to the unit cell size, and (iii) a solute bearing a large overall charge.
Theoretical and experimental studies of chemically induced dynamic nuclear polarization kinetics in recombination of radical pairs by the method of switched external magnetic field. II. C13 CIDNP of micellized radical pairs
Authors: Fedin M. V. , Bagryanskaya E. G. , Purtov P. A.
Published: July 2002

The method of C13 chemically induced dynamic nuclear polarization in a switched external magnetic field (SEMF CIDNP) has been applied for the first time in an experimental investigation of micellized radical pairs (RP). Using the examples of three photochemical reactions it has been shown, that SEMF CIDNP allows the investigation of the kinetics of short-lived micellized RPs with high time-resolution in low and intermediate magnetic fields. The experimental kinetics have been analyzed and simulated on the basis of a previously developed theory [Parnachev et al., J. Chem. Phys. 107, 9942 (1997)]. It has been demonstrated that such an analysis provides information on the rates of radical escape from the micelle, on electron relaxation and on the rate of S–T− transitions. The analysis of the estimated rates of S–T− transitions showed that the exchange interaction is essentially anisotropic in the RPs studied.
Asymmetric block copolymers confined in a thin film
Authors: Huinink H. P. , Brokken-Zijp J. C. M. , van Dijk M. A. , Sevink G. J. A.
Published: July 2002

We have used a dynamic density functional theory (DDFT) for polymeric systems, to simulate the formation of micro phases in a melt of an asymmetric block copolymer, AnBm(fA=1/3), both in the bulk and in a thin film. In the DDFT model a polymer is represented as a chain of springs and beads. A spring mimics the stretching behavior of a chain fragment and the spring constant is calculated with the Gaussian chain approximation. Simulations were always started from a homogeneous system. We have mainly investigated the final morphology, adopted by the system. First, we have studied the bulk behavior. The diblock copolymer forms a hexagonal packed array of A-rich cylinders, embedded in a B-rich matrix. Film calculations have been done by confining a polymer melt in a slit. Both the slit width and surface-polymer interactions were varied. With the outcomes a phase diagram for confined films has been constructed. Various phases are predicted: parallel cylinders (C∥), perpendicular cylinders (C⊥), parallel lamellae (L∥), and parallel perforated lamellae (CL∥). When the film surfaces are preferentially wet by either the A or the B block, parallel oriented microdomains are preferred. A perpendicular cylindrical phase is stable when neither the A nor B block preferentially wets the surfaces. The predicted phase diagram is in accordance with experimental data in the literature and explains the experimentally observed differences between films of asymmetric block copolymers with only two parameters: the film thickness and the energetic preference of the surface for one of the polymer blocks. We have also observed, that confinement speeds up the process of long range ordering of the microdomains.
Bell’s inequalities and quantum field theory. I. General setting
Authors: Summers Stephen J. , Werner Reinhard
Published: July 2002

Bell’s inequalities are briefly presented in the context of order-unit spaces and then studied in some detail in the framework of C*-algebras. The discussion is then specialized to quantum field theory. Maximal Bell correlations β(φ,𝒜(𝒪1), 𝒜(𝒪2)) for two subsystems localized in regions 𝒪1 and 𝒪2 and constituting a system in the state φ are defined, along with the concept of maximal Bell violations. After a study of these ideas in general, properties of these correlations in vacuum states of arbitrary quantum field models are studied. For example, it is shown that in the vacuum state the maximal Bell correlations decay exponentially with the product of the lowest mass and the spacelike separation of 𝒪1 and 𝒪2. This paper is also preparation for the proof in Paper II [S. J. Summers and R. Werner, J. Math. Phys. 28, 2448 (1987)] that Bell’s inequalities are maximally violated in the vacuum state.
The exact solutions of the equation for a composite particle in the external electromagnetic fields
Authors: Lu Jing-fa , Kruglov S. I.
Published: July 2002

Exact analytic solutions are provided for a class of equations describing a composite scalar particle in constant electric and magnetic fields, and also in plane electromagnetic fields. The external fields are taken in both classical and quantum cases. A detailed physical analysis of the results is given.
Double three-wave interaction of four waves: Lax representations and exact solutions
Authors: Romeiras Filipe J.
Published: July 2002

The nonlinear resonant interaction of coherent waves is a fundamental process in the study of wave phenomena which has received a great deal of attention in its many aspects. In the present article a system of four interacting waves which constitute two resonant triplets is considered. The system is described, in a simplified model, by a Hamiltonian system of eight autonomous ordinary differential equations, with time as the independent variable; both positive and negative energy waves are allowed in the interaction. Two distinct Lax representations for this system, two new classes of exact solutions in terms of elliptic functions, a solution in the form of a convergent generic Laurent series expansion around a movable pole in the independent variable, and a stabilization criterion for the explosive instability that may occur when waves of different energy sign interact are obtained herein.
A remark on the Casimir elements of Lie superalgebras and quantized Lie superalgebras
Authors: Leśniewski Andrzej
Published: July 2002

It is shown that the second order Casimir operators C and Cq of the Lie superalgebra osp(1‖2) and the quantized Lie superalgebra ospq(1‖2), respectively, possess natural square roots.
Quantum gravity as topological quantum field theory
Authors: Barrett John W.
Published: July 2002

The physics of quantum gravity is discussed within the framework of topological quantum field theory. Some of the principles are illustrated with examples taken from theories in which space–time is three dimensional.
Electron counting statistics and coherent states of electric current
Authors: Levitov Leonid S. , Lee Hyunwoo , Lesovik Gordey B.
Published: July 2002

A theory of electron counting statistics in quantum transport is presented. It involves an idealized scheme of current measurement using a spin 1/2 coupled to the current so that it precesses at the rate proportional to the current. Within such an approach, counting charge without breaking the circuit is possible. As an application, we derive the counting statistics in a single channel conductor at finite temperature and bias. For a perfectly transmitting channel the counting distribution is Gaussian, both for zero-point fluctuations and at finite temperature. At constant bias and low temperature the distribution is binomial, i.e., it arises from Bernoulli statistics. Another application considered is the noise due to short current pulses that involve few electrons. We find the time-dependence of the driving potential that produces coherent noise-minimizing current pulses, and display analogies of such current states with quantum-mechanical coherent states.
Simultaneous Beltrami conditions in coupled vortex dynamics
Authors: Yoshida Z. , Mahajan S. M.
Published: July 2002

The two-fluid model of a plasma describes the strong coupling between the magnetic and the fluid aspects of the plasma. The Beltrami condition that demands alignment of vortices and flows becomes a system of simultaneous equations in the magnetic field and the flow velocity. Combining these equations yields the double curl Beltrami equation. General solvability of the equation has been proved using the spectral theory of the curl operator. The set of solutions contains field configurations that can be qualitatively different from the conventional constant-α-Beltrami fields (which are naturally included in the set). The larger new set may help us understand a variety of structures generated in plasmas.