bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical properties of (AlxGa1−x)0.52In0.48P at the crossover from a direct-gap to an indirect-gap semiconductor
Authors: Dörr U. , Schwarz W. , Wörner A. , Westphäling R. , Dinger A. , Kalt H. , Mowbray D. J. , Hopkinson M. , Langbein W.
Published: July 2002

The optical properties and the dynamics of excitons and the electron-hole plasma have been studied in disordered (AlxGa1−x)0.52In0.48P near to the direct-to-indirect band gap crossover. In particular we have investigated three epitaxial layers grown by solid-source molecular beam epitaxy with varying Al content x. Two of them have compositions in the immediate vicinity of the crossover point, the other is assigned to the indirect-gap regime. Both direct and indirect recombination processes contribute to the photon emission from the material. Since the relative importance of the different recombination processes depends strongly on temperature, excitation intensity, and excitation pulse duration, the processes can be identified by changing these parameters. As a result, we can determine the relative alignment of the conduction band minima and the distribution of the electrons among them. At high excitation levels the two crossover samples show stimulated emission at a photon energy of ∼2.29 eV, i.e., in the green spectral range. Using the variable stripe length method, we find an optical gain of up to ∼600 cm−1 at excitation levels of ∼350 kW/cm2. Stimulated emission involves direct recombination. This conclusion is reached from the experiments and from line-shape modeling, including a self-consistent treatment of populations and renormalization of the conduction band minima.
Meissner state properties of a superconducting disk in a non-uniform magnetic field
Authors: Badı́a A. , Freyhardt H. C.
Published: July 2002

We describe a formalism for studying a superconducting disk shielding an arbitrary magnetic field. The theory is developed for problems with rotational symmetry, and the results are applied to investigating various experimental situations. In particular, we have considered a cylindrical permanent magnet which is brought over an ideal superconductor. The levitation force, F, and the induced supercurrent density, j, have been calculated and the results put into closed forms. Both magnitudes are analyzed in terms of suitable dimensionless parameters related to the relevant length scales: the distance between the magnet and the superconductor, the disk radius and the magnet transversal dimensions. We demonstrate that both F and j provide a very precise description in the whole range of parameters by means of simple expressions which strictly correspond to the limits. In the specific range of magnetic force microscopy, we show how to approximate F to the desired accuracy in terms of the reduced penetration depth λ/a, a being the distance between the given magnetic tip and the superconductor. Finally, the theory is used to obtain a simple criterion which determines whether effective medium theories are suitable or not for highly textured granular superconductors.
Nondestructive imaging and characterization of ferroelectric domains in periodically poled crystals
Authors: Eng L. M. , Güntherodt H.-J. , Rosenman G. , Skliar A. , Oron M. , Katz M. , Eger D.
Published: July 2002

We report the nondestructive investigation and visualization of periodically poled domains in ferroelectric potassium titanyl phosphate (KTP) crystals using polarization sensitive scanning force microscopy (SFM). Applying an alternating voltage technique to SFM allows ferroelectric domain wall resolution beyond 100 nm. Image contrast between KTP and Rb doped KTP, i.e., rubidium titanyl phosphate (RTP) regions arises from the differential piezoelectric response. We find the polarization vectors in both KTP and RTP to be aligned parallel to the negative z axes as deduced (a) when comparing our data with a ferroelectric reference sample, i.e., tri-glycine sulfate (b) from comparison of nanoscale hysteresis loops recorded on KTP and RTP, and (c) from direct domain switching in KTP applying very high electric fields between tip and counter electrode. The latter experiments show that nanoscale ferroelectric domains in KTP switch from the negative to the positive z-axes alignment for electric fields stronger than 750 V/cm. Nevertheless, spontaneous backswitching is observed after a couple of minutes.
Fatigue in sol-gel derived barium titanate films
Authors: Sharma H. Basantakumar , Sarma H. N. K. , Mansingh Abhai
Published: July 2002

The effect of fatigue on the electrical properties of sol-gel derived ferroelectric barium titanate (BaTiO3) thin films have been investigated. At room temperature, the resistivity, breakdown field, dielectric constant (ε), and loss tangent (tan δ) decreases after fatigue while the coercive field increases. Fatigue also influences the dielectric properties of the film at high temperatures. The sharpness and magnitude of the peak in the dielectric constant at the transition temperature decreases after fatigue. The loss tangent of the virgin film showed a peak at the transition temperature while it is absent in the case of the fatigue film. The phenomena are explained in terms of (i) the accumulation of point charge defect, (ii) the pinning of domain wall mobility, and (iii) the space charge layer models of fatigue.
Negative magnetoresistance and impurity band conduction in an In0.53Ga0.47As/InP heterostructure
Authors: Dziuba Z. , Przesławski T. , Dybko K. , Górska M. , Marczewski J. , Regiński K.
Published: July 2002

The electrical conduction in an n-type In0.53Ga0.47As/InP sample grown by molecular beam epitaxy has been analyzed in the magnetic field up to 1.5 T, at temperatures from 15 to 295 K. The electrical conduction has been ascribed to the impurity band (IB), located in the interface between the epilayer InGaAs and the substrate InP. The contribution of the conduction band electrons in bulk InGaAs layer to the electrical conduction was negligible. The IB conduction was almost metallic. We observed within the IB two conducting channels which give positive and negative contribution to the Hall voltage. The magnetoresistance in the IB at low temperatures was negative, and at high temperatures was positive. The negative magnetoresistance in the IB has been phenomenologicaly described by carriers with the constant concentration and the mobility increasing proportionally to the square of the applied magnetic field.
Nanoindentation on AlGaN thin films
Authors: Cáceres D. , Vergara I. , González R. , Monroy E. , Calle F. , Muñoz E. , Omnès F.
Published: July 2002

Hardness and Young’s modulus were measured in AlGaN thin films with different Al content, using a nanoindentation technique. Hardness slightly decreases with increasing Al content, ranging from 20.2 to 19.5 GPa for Al content from 0.09 to 0.27, respectively. No significant variations of Young’s modulus were observed. The resulting value of Young’s modulus is 375 GPa. Discontinuities in load–displacement curves were found, which are associated with dislocation nucleation. The threshold load for this discontinuity depends on the conditions of the nanoindentation test. Below the threshold load, the sample surface flexes elastically in response to the indenter contact and the displacements recover completely when the sample is unloaded.
The temperature dependence of exchange anisotropy in ferromagnetic/PdPtMn bilayers
Authors: Nagasaka K. , Varga L. , Shimizu Y. , Eguchi S. , Tanaka A.
Published: July 2002

We have studied the differences in the temperature dependence of exchange anisotropy of PdPtMn/CoFeB, NiFe, and NiFeTa bilayers to understand the role of ordering temperatures. The three different ferromagnetic layers [Curie temperatures: TCNiFeTa(400 °C)<TCNiFe(570 °C)<TCCoFeB(1040 °C)] exchange biased by the same antiferromagnet [Néel temperature: TNPdPtMn(600 °C)] showed significantly different behavior: the exchange bias field was monotonically decreasing with temperature for the CoFeB and showed distinct, broad peak for NiFe and NiFeTa at a lower temperature before it decreased above 200 °C. The temperature dependence of exchange anisotropy, calculated from the measured exchange bias field and saturation magnetization, was also different for the three bilayers as a result of the differences in Hua and MFM. The results could be understood by the modified thermal fluctuation aftereffect model, which includes the temperature dependence of the ferromagnetic magnetization and the assumption that the ordering temperature of the ferromagnetic film is different at the interface with antiferromagnet from the rest of the ferromagnetic film due to the exchange coupling with a material with high Néel temperature.
Magnetization and FMR studies of crystal-ion-sliced narrow linewidth gallium-doped yttrium iron garnet
Authors: Rachford F. J. , Levy M. , Osgood R. M. , Kumar A. , Bakhru H.
Published: July 2002

Recent interest in employing single-crystal yttrium iron garnet (YIG) films for miniaturized microwave integrated circuit applications has prompted us to study detaching liquid phase epitaxy (LPE) grown YIG films from their gadolinium gallium garnet (GGG) substrates via crystal ion slicing (CIS). We report studies of magnetization and ferromagnetic resonance (FMR) of narrow linewidth gallium-doped YIG (GaYIG) films in various stages of separation from their 〈111〉 oriented GGG substrates. All samples were diced from the same three-inch wafer of Y3Fe4.6Ga0.4O12 with 4πMeff of 1070 G and 9.45 GHz FMR linewidth of 0.4±0.1 Oe. The CIS separation process involved (1) implantation with helium ions, (2) flash annealing to remove surface damage, and (3) chemical etching to detach the YIG at the implantation damaged layer. The starting films were 10.8-μm thick. Separation occurred 7 to 8 μm from the front surface in the YIG. The implantation (5×1016 cm−2 dosage, 3.8 MeV bias) increased the uniaxial anisotropy of these films. A small cubic anisotropy (∼5 G 60° in-plane periodicity) persisted in both implanted and unimplanted samples. Upon implantation the FMR linewidth increased from 0.4 to 3±0.5 Oe. The detached samples have linewidths ranging from 1.7 to 2.5 Oe. The temperature dependence of the linewidths is roughly exponential decreasing as the temperature is lowered for the unprocessed sample, increasing with reduced temperature for all processed samples. Post-detachment annealing restores the FMR linewidth to 0.55±0.5 Oe.
Molecular images and vibrational spectroscopy of sorbic acid with the scanning tunneling microscope
Authors: Smith Douglas P. E. , Kirk Michael D. , Quate Calvin F.
Published: July 2002

Images of sorbic acid molecules absorbed onto graphite have been taken with a scanning tunneling microscope (STM) operating in liquid helium. Molecular clusters were clearly observed, as was the atomic structure of the graphite substrate. The molecules were seen to diffuse across the substrate at a rate of about 1 Å/min. When dI/dV vs V was measured with the STM probe directly over a sorbic acid molecule, a well-defined spectrum of peaks was obtained whose energies corresponded to the vibrational resonances of the molecule. Large changes in the spectra occurred if the tip was moved a lateral distance of 5 Å.
Extraction of kinetic parameters in temperature programmed desorption: A comparison of methods
Authors: Miller J. B. , Siddiqui H. R. , Gates S. M. , Russell J. N. , Yates J. T. , Tully J. C. , Cardillo M. J.
Published: July 2002

An investigation of the temperature programmed desorption (TPD) of CO and D2 from Ni(111) has been carried out. It has been shown that a differential method for the extraction of the kinetic parameters, threshold temperature programmed desorption (TTPD), can be applied with accuracy near the limit of zero coverage. In this limit, agreement is found between integral and differential methods for kinetic parameter evaluation. The factors which limit the applicability of TTPD are explored and a method to verify its proper application is presented.
Polarized Raman spectra of thin films. III. Conformational analysis of Langmuir–Blodgett multilayers (1 to 49) of barium stearate
Authors: Harrand Monique , Masson Michèle
Published: July 2002

Polarized Raman spectra of Langmuir–Blodgett multilayers of barium stearate have been studied in the 2800–3000 cm−1 region (C–H stretching vibrations). The samples have been excited by evanescent waves produced by the plasmon illumination method. Only TM excitation can be used for very thin films. Therefore the polarized Raman spectra are complex, but, due to the smallness of some nondiagonal terms of the Raman tensor, these spectra are similar to the pure polarized spectra given by the TE excitation. When the film thickness decreases from 122.5 to 22.5 nm, the spectra are only slightly modified. The study of the ν(C–C) lines in the 1000–1200 cm−1 region have shown that the chains are still all-trans plane. With ultrathin films (<20 nm) changes are observed in the shape and in the polarization of the 2800–3000 cm−1 band. The first modification (appearance and disappearance of shoulders or peaks) is attributed to conformational change of the acyl chains. The second modification (unpolarization) is related to the deformation of the monolayer deposited on the prism surface which is rough relative to the molecular dimensions. It seems that only the first three layers are concerned by this deformation.
Unoccupied molecular orbitals of aromatic hydrocarbons adsorbed on Ag(111)
Authors: Frank K. H. , Yannoulis P. , Dudde R. , Koch E. E.
Published: July 2002

The unoccupied π* bands of epitaxial overlayers of benzene, naphthalene, anthracene, tetracene, perylene, and coronene on a Ag(111) surface have been studied by angle resolved inverse photoemission spectroscopy. A comparison with HAM/3 MO calculations and electron transmission gas phase measurements enables the assignment of the π* affinity levels of these organic systems. In conjunction with previous angle resolved photoemission, optical absorption, and near edge x-ray absorption studies a complete picture of their valence band structure and morphology evolves. Optical and x-ray absorption excitation energies are discussed in the light of the experimentally determined one-electron correlation diagram. Gas phase–solid relaxation energies of the affinity levels are found to decrease in the series of benzene to tetracene (1.7→0.5 eV) attributed to the increasing size of the involved molecular orbitals. For benzene a low lying σ*-shape resonance has been identified in the isochromat inverse photoemission spectrum.
The microwave spectrum and structure of the H2O–SO2 complex
Authors: Matsumura K. , Lovas F. J. , Suenram R. D.
Published: July 2002

The microwave spectrum of H2O–SO2 has been observed with a pulsed beam, Fabry–Perot cavity, Fourier-transform microwave spectrometer. In addition to the normal isotopic form, we have observed the spectra of H2O–34SO2, HDO–SO2, and D2O–SO2. For the normal and dideuterated forms we observe two states with a- and c-type spectra which are split by internal rotation of the water unit, while for the 34 S and HDO species two states are observed but only the a-type spectrum was assigned. Rotational analysis of each spectrum for H2O–SO2 provides the constants A=8763.071(3) MHz, B=3819.655(2) MHz, and C=2900.899(2) MHz for the I=0 state and A=8734.268(4) MHz, B=3821.281(2) MHz, and C=2900.837(2) MHz for the I=l state, where I is the resultant proton nuclear spin. Stark effect measurements give electric dipole components μa =1.984(2) D and μc =0.488(4) D for H2O–SO2. The geometry obtained from fitting the derived moments of inertia has the planes of the two monomer units tilted approximately 45° from the parallel orientation with the oxygen atom of the water closest to the S atom of SO2, giving an S–O distance of 2.824(16) Å and a center-of-mass distance Rc.m. =2.962(5) Å. The (SO2)2 species was also produced with the same nozzle expansion conditions as used for H2O–SO2. New measurements on (SO2)2 are reported and fitted with the measurements from Nelson, Fraser and Klemperer [J. Chem. Phys. 83, 945 (1985)], providing improved rotational and centrifugal distortion constants.
Kinetic theory of transport in a hard sphere crystal
Authors: Kirkpatrick T. R. , Das Shankar P. , Ernst M. H. , Piasecki J.
Published: July 2002

The revised Enskog kinetic theory (RET) is used to describe transport in a hard sphere crystal. The connection between the RET and the exact density functional theory (DFT) description of the solid state is established. The RET is used to derive the dissipative linear equations of elasticity. The elastic moduli in these equations are identical to those obtained from equilibrium like DFT. The expressions for the solid state transport coefficients (determining sound absorption and heat conduction in the hard sphere crystal) are new. As for the analogous calculation in the liquid state, the transport coefficients are determined by the (solid state) equilibrium two-particle distribution function at contact.
Potential energy surfaces for the (ArCO)+ system
Authors: Parlant Gérard , Archirel Pierre , Gislason Eric A.
Published: July 2002

The five lowest doublet potential energy surfaces for the (ArCO)+ system have been determined using the procedure of Archirel and Levy. These states correlate at infinite separation, in order of increasing energy, to Ar+CO+(X 2Σ+), Ar+(2P3/2)+CO, Ar+(2P1/2)+CO, and Ar+CO+(A 2Π). The potential energy curves are shown at several values of the orientation angle. In addition, contour maps of the two lowest surfaces are presented. Both of these surfaces are quite anisotropic, and each has a deep potential well. Adiabatic vibronic potential energy surfaces have also been computed. These give insight into the charge transfer process at low collision energies. For the purpose of comparison the five lowest surfaces for the isoelectronic system (ArN2)+ are also shown.
Development and optimization of multipulse propagators: Applications to homonuclear spin decoupling in solids
Authors: Liu H. , Glaser Steffen J. , Drobny Gary P.
Published: July 2002

We introduce a new class of semi-windowless pulse sequences for homonuclear decoupling in solids. The development and optimization of the multiple-pulse sequences relies on the definition of quality factors that reflect the proximity of the created effective Hamiltonian to the desired Hamiltonian. A quality factor based on the average Hamiltonian theory gives a global view of the existing solutions. However, the sequences were optimized using a quality factor based on exact numerical solutions of the Liouville von Neumann equation. The new decoupling sequences are stable over a wide range of coupling constants and chemical shifts.
Rotational state dependence of radiationless transitions in S1 propynal
Authors: Bitto H. , Willmott P. R. , Huber J. Robert
Published: July 2002

The rotational state dependence of the radiationless processes of S1 propynal, HC≡CCHO, was investigated in three selected vibronic bands located at an excess energy of about 3000 cm−1 in a molecular beam using molecular quantum beat spectroscopy. The number of quantum beat frequencies counted in single rovibronic fluorescence decays shows a clear dependence on the rotational quantum number N of the excited rovibronic singlet state, reflecting an increase of the number of coupling triplet states nT by a factor of 4, from N=0 to 14. This increase is accompanied by a lengthening of the decay lifetimes by a factor of 3. Given eigenstate resolution, the effect of magnetic interactions in the triplet state is discussed and it is concluded that the N dependence is predominantly due to mixing of K states resulting in a symmetry breakdown. This mixing is proposed to be mainly induced by hyperfine interaction via the dipole–dipole term.
Molecular dynamics simulation of silica liquid and glass
Authors: Della Valle Raffaele Guido , Andersen Hans C.
Published: July 2002

A molecular dynamic computer simulation of the cooling of liquid silica to form an amorphous solid (glassy) phase was performed using the potential of Tsuneyuki, Tsukuda, Aoki, and Matsui, which was found to be consistent with the structure of several crystalline phases of silica. In the simulations, at low temperatures the atoms formed an almost perfect random tetrahedral network, and on subsequent annealing the material formed a perfect network, with each silicon having four oxygen neighbors and each oxygen having two silicon neighbors. The simulated material at low temperatures has a structure that is in close agreement with experimental neutron scattering measurements on amorphous silica. Several properties of the material were calculated and compared with experiment, and the nature of the ‘‘inherent structure’’ of the material as a function of temperature was investigated. The potential of Tsuneyuki et al. provides an excellent model for amorphous silica as well as crystalline silica.
Collision-induced transitions between A 1Σu+ and b 3Πu states of Na2: The ‘‘gateway’’ effect of perturbed levels
Authors: Li Li , Zhu Qingshi , Lyyra A. M. , Whang T.-J. , Stwalley W. C. , Field R. W. , Alexander M. H.
Published: July 2002

We present here the best qualitative and quantitative illustration to date of the perturbation ‘‘gateway’’ effect in collision-induced transitions between two mutually perturbing electronic states. The gateway effect, as described by Gelbart and Freed [Chem. Phys. Lett. 18, 470 (1973)], is a suggestion that all collision-induced transfer of population between two electronic states proceeds through a small number of isolated-molecule eigenstates which are of mixed electronic character, the ‘‘gateway levels,’’ and that the rates for such gateway-mediated processes are related to the mixing fractions in the gateway levels. The gateway levels here are the Na2 A 1Σu+ v′=26∼b 3Π2u v′=28 J′=16e,a-symmetry levels which are significantly mixed owing to an extremely small spin–orbit perturbation matrix element (the neighboring J′=15 and 17e,s-symmetry levels are essentially free of mixing). A cw optical–optical double resonance (OODR) scheme is used to PUMP a single parent level and PROBE single daughter and granddaughter levels.

The oscillator strengths for the PUMP and PROBE transitions are derived, respectively, from the A 1Σu+←X 1Σg+ (26,4) band and the 2 3Π2g←b 3Π2u (28,28) subband. The qualitative observation of the gateway effect is that whenever an a-symmetry A 1Σu+ v′=26 parent level is selected, b 3Π2u v′=28 daughter and granddaughter levels are observably populated, but when an s-symmetry A 1Σu+ v′=26 parent is selected, essentially no population is detected in b 3Π2u v′=28 daughter and granddaughter levels (i.e., no perturbation, no interelectronic state transfer). The quantitative observation of the gateway effect is that when a J′=12 (or 14)e,a parent is selected, the most efficiently populated rotational levels of the other electronic state are granddaughter levels centered about the J′=16e,a gateway daughter level rather than about the J′ value of (or minimum energy gap relative to) the parent level.
Ground and excited states of K2 and K2+ by the open-shell coupled cluster method
Authors: Ilyabaev Ephraim , Kaldor Uzi
Published: July 2002

The Fock space open-shell coupled cluster method is used to calculate the potential functions of the five lowest states of K2 at internuclear separation of 2≤R≤7 Å and the three lowest states of K2+ at 2≤R≤15 Å, with a 112-function contracted Gaussian basis. The usual complete-model-space calculation diverges and a general, incomplete model space is used. Core and core–valence correlation is found to be important, and very good agreement with experimentally known molecular spectroscopic constants is obtained when it is included: equilibrium internuclear separations (except for the very flat x 3Σu+ state of K2) are accurate to 0.07 Å, dissociation and excitation energies to 0.09 eV, and vibrational constants to 6 cm−1.
Spectroscopy of methylene: Einstein coefficients for CH2(b̃ 1B1–ã 1A1) transitions
Authors: García-Moreno I. , Bradley Moore C.
Published: July 2002

The b̃ 1B1→ã 1A1 emission spectrum of the CH2 radical has been studied in the 13 330–18 180 cm−1 region. Methylene (ã 1A1) is produced by photolysis of ketene at 308 nm and excited to single rovibrational levels of the b̃ 1B1 state by a tunable pulsed dye laser. The emission from the b̃ 1B1 state is spectrally dispersed with a resolution of 0.25 Å. Term values and transitions are reported for new levels of the b̃ 1B1(0,v2,0) (v2=14 and 15) and ã 1A1(0,0,0) states with J≥6 and 0≤Ka≤6. Unusually intense ΔKa=3 transitions and several ΔKa=5 transitions are observed. Analysis of the emission spectra gives the relative intensities of transitions from a selected rovibrational level in the upper b̃ 1B1 state to rovibrational levels in the ã 1A1 state. These relative intensities together with the fluorescence lifetimes of the b̃ 1B1 levels determine the absolute Einstein emission coefficients for rovibronic transitions in the visible spectrum of methylene. These data permit accurate measurement of rovibrational state populations for CH2(ã 1A1) by laser-induced fluorescence spectroscopy.
A Monte Carlo simulation of localized corrosion
Authors: Reigada R. , Sagués F. , Costa J. M.
Published: July 2002

A two-dimensional Monte Carlo simulation algorithm is formulated following the tunneling mechanism for localized (pitting) corrosion. Depending on the values of a minimum set of control parameters, easily interpreted in terms of physicochemical variables, the two limiting regimes of stable and unstable corrosion are reproduced. Pits grown under such distinctive conditions are analyzed from the point of view of both their electrochemical response (current vs time behavior) and their morphology. In this last respect, fractal concepts are advantageously used to describe their exposed profiles as well as their distributions of local currents.
Molecular softness, hypersoftness, infrared absorption, and vibrational Raman scattering: New relations derived from nonlocal polarizability densities
Authors: Liu P.-H. , Hunt K. L. C.
Published: July 2002

This work rigorously relates electromagnetic properties and characteristics of molecular potential energy surfaces to the empirical concept of ‘‘softness,’’ used to categorize Lewis acids and bases, and to summarize observed patterns of reactivity. New equations are derived that connect infrared absorption intensities, vibrational force constants, intermolecular forces at first order, and linear electric-field shielding tensors to softness kernels as defined in density functional theory. A generalization to nonlinear response—by introduction of the hypersoftness—leads to new equations in density-functional terms for vibrational Raman band intensities, the cubic anharmonicities in molecular potential energy surfaces, intermolecular forces at second order, and nonlinear electric-field shielding tensors. The analysis employs relations of the softness and hypersoftness to nonlocal polarizability and hyperpolarizability densities that represent the intramolecular distribution of response to inhomogeneous electric fields.
New strategies to incorporate the solvent polarization in self-consistent reaction field and free-energy perturbation simulations
Authors: Luque F. J. , Bofill Josep Maria , Orozco Modesto
Published: July 2002

A first-order perturbation treatment of the polarization contribution to the free energy of hydration is presented. Very simple expressions for the computation of the total electrostatic free energy of solvation, the polarization contribution, and its components (distortion and stabilization) are derived. These equations can be used with either continuum (quantum and classical) or discrete approaches. The reliability of these equations is examined by comparison with rigorous expressions derived previously within the framework of the self-consistent reaction field theory. Indeed, the suitability of the classical expressions for the distortion and stabilization terms used in the context of discrete strategies has been explored by comparison between self-consistent reaction field and molecular dynamics–free-energy perturbation results. The excellent agreement found between the two techniques allows us to envisage a procedure to account for the polarization in force field-derived methods.
Structure and vibrations of phenol(H2O)2
Authors: Gerhards M. , Kleinermanns K.
Published: July 2002

Extensive ab initio calculations at the Hartree–Fock (HF) level using different basis sets have been performed in order to obtain the minimum energy structure of the phenol(H2O)2-cluster. Several hydrogen bonding arrangements and a van der Waals structure are discussed. The most stable structure turns out to be cyclic with nonlinear hydrogen bonds. This structure is similar to the one calculated for the water trimer. In contrast with the water trimer the average binding energy of a hydrogen bond decreases with increasing cluster size of Ph(H2O)n (n=1,2). This is a result of non equal hydrogen bonds. A normal coordinate analysis has been carried out for the fully optimized minimum energy structure of phenol(H2O)2 and its deuterated isotopomer d-phenol(D2O)2. The calculated harmonic intramolecular vibrational modes are compared with experimental values and the intermolecular stretching vibrations are assigned.
Optimal control of population transfer in an optically dense medium
Authors: Wang Ningjun , Rabitz Herschel
Published: July 2002

We apply the optimal control technique (OCT) to design an optical pulse pair that controls the population transfer in a medium of three-level atoms. The absorption and reshaping of the controlling pulses by the medium are taken into account. The efficiency of the population transfer is improved significantly compared with designs that do not incorporate pulse absorption and reshaping.
High-resolution laser spectroscopy of the X1Σ+ and (1)3Σ+ states of 23Na85Rb molecule
Authors: Kasahara Shunji , Ebi Tsuyoshi , Tanimura Mari , Ikoma Heiji , Matsubara Kensuke , Baba Masaaki , Katô Hajime
Published: July 2002

High-resolution spectra of the B1Π→X1Σ+ transition of 23Na85Rb molecule are measured by the technique of the Doppler-free optical–optical double resonance polarization spectroscopy (OODRPS). The molecular constants of the X1Σ+(v″=5−30) levels are determined, and the potential energy curve is constructed up to v″=30 by the RKR method. The time-resolved fluorescence intensity following the excitation to the B1Π(v′=5,J′= around 20) level is measured, and the lifetime of the B1Π(v′=5) level in collisionless limit is determined to be 17.8 ns. The absolute value of the electric dipole moment of the B1Π−X1Σ+ transition is determined to be 7.0 D in the region of 3.73 Å<R<4.98 Å. Transition lines to the (1)3Σ+ state from the B1Π(v′=8,J′=15) level, which is perturbed by the (1)3Π1(v0,N=J=15) level, are measured by the Doppler-free OODRPS. The energy spacing between the F1 and F3 components of the (1)3Σ+(v=4,N=15) level is observed to be smaller than 0.001 cm−1. The hyperfine splittings, which are described by Hund’s case (bβS), are observed, and the hyperfine constants ANa and ARb of the (1)3Σ+(v=4) level are determined to be 0.0293 and 0.0336 cm−1, respectively. The hyperfine splittings are identified as originating from the Fermi contact interaction. From the analysis, it is concluded that the electron spins in the (1)3Σ+ state are almost equally populated to the 5sRb and 3sNa orbitals.
Diffusion–annihilation dynamics in one spatial dimension
Authors: Santos J. E. , Schütz G. M. , Stinchcombe R. B.
Published: July 2002

We discuss a reaction–diffusion model in one dimension subjected to an external driving force. Each lattice site may be occupied by at most one particle. The particles hop with rates (1±η)/2 to the right or left nearest neighbor site if it is vacant, and annihilate with rate one if it is occupied. The representation of an uncorrelated random initial state in terms of free fermions allows the calculation of multiple time-dependent higher order correlation functions of the local density. We outline the procedure using a field theoretic approach. We also compute the long time behavior of the density profile if the initial density profile is a step function. The fermion representation of the step function state also allows the calculation of multiple time-dependent correlation functions. Experimental applications of these results are pointed out.
Two-photon ionization spectroscopy and all-electron ab initio study of LiCa
Authors: Russon L. M. , Rothschopf G. K. , Morse M. D. , Boldyrev A. I. , Simons Jack
Published: July 2002

Resonant two-photon ionization spectra of LiCa have been obtained from a laser vaporization, supersonic expansion source. The ground state of the molecule is confirmed to be X 2Σ+. Three band systems have been observed near 15 282, 19 310, and 22 250 cm−1 and the upper states have been assigned as 2Σ+, 2Πr, and, tentatively, 2Π. Bond lengths and vibrational frequencies are reported for 7Li40Ca for the various states as X 2Σ+: r0=3.3796(11) Å, ΔG1/2=195.2 cm−1; 2Σ+: r0=3.4275(47) Å, ΔG1/2=283.5 cm−1; 2Πr: re=3.5451(36) Å, ωe=144.5 cm−1; and 2Π: ωe=178.53(5) cm−1. The ionization energy was also measured to be 4.471(1) eV by observing the onset of one-photon ionization. Results of ab initio all-electron calculations on twelve low-lying states [2Σ+(1), 2Σ+(2), 2Σ+(3), 2Σ+(4), 4Σ+(1), 4Σ−(1), 2Π(1), 2Π(2), 2Π(3), 2Π(4), 4Π(1), and 4Π(2)] are also reported, along with results on the X 1Σ+ ground electronic state of LiCa+. The dissociation energies of LiCa(X 2Σ+) into Li(2S)+Ca(1S) and of LiCa+ (X 1Σ+) into Li+(1S)+Ca(1S) are calculated to be 0.24 and 1.20 eV respectively, and the vertical and adiabatic ionization energies of LiCa have been determined to be almost the same, 4.43 eV, at the quadratic configuration interaction, including singles and doubles with approximate triples and all correlated electrons [QCISD(T,FULL)] level of theory.
Electronic absorption spectrum of cold naphthalene cation in the gas phase by photodissociation of its van der Waals complexes
Authors: Pino Thomas , Boudin Nathalie , Bréchignac Philippe
Published: July 2002

The electronic absorption spectrum of the naphthalene cation has been obtained in conditions relevant for comparison with the diffuse interstellar bands in astrophysics, i.e., cold species in the gas phase. The novel technique consisting to photodissociate a selectively R2P2CI-prepared PAH–argon van der Waals complex in a molecular beam [Ph. Bréchignac and T. Pino, Astron. Astrophys. 343, L49 (1999)] has been used. The various aspects of the method are described in detail. The whole visible range has been explored revealing two electronic transitions displaying 28 vibronic bands. Absolute absorption cross sections have also been measured, and found much larger than reported from rare gas matrices studies. The additional information on the matrix-induced or complex-induced shifts and widths, and on the intramolecular and intermolecular processes involved in these species, is discussed. No definite conclusion about the possible presence of the cation in space can be drawn so far.
Nucleation at high pressure. II. Wave tube data and analysis
Authors: Luijten C. C. M. , Peeters P. , van Dongen M. E. H.
Published: July 2002

Nucleation rate data, obtained from expansion wave tube experiments, are reported for several vapor–gas mixtures at high pressure. Results are given for water–vapor in the presence of helium and nitrogen gas, and for n-nonane in helium and methane. For all these mixtures, carrier gas pressures of 10, 25, and 40 bar have been applied, with temperatures ranging from 230 to 250 K. An extended form of the nucleation theorem (in terms of the derivative of the nucleation rate with respect to carrier gas pressure) is derived, which appears to be very helpful in the interpretation of high pressure data. It can be used to obtain the carrier gas content of the critical nucleus directly from the pressure dependence of experimental nucleation rates. Combining this method with the theoretical considerations of part I of this paper [J. Chem. Phys. 111, 8524 (1999), preceding paper]: the nucleation behavior of water at high pressures of both helium and nitrogen can quantitatively be understood. For n-nonane in helium our “pressure perturbation approach” is also valid. For n-nonane in methane, however, this approach fails because of the high methane solubility in the liquid phase.
Rapid and stable determination of rotation matrices between spherical harmonics by direct recursion
Authors: Choi Cheol Ho , Ivanic Joseph , Gordon Mark S. , Ruedenberg Klaus
Published: July 2002

Recurrence relations are derived for constructing rotation matrices between complex spherical harmonics directly as polynomials of the elements of the generating 3×3 rotation matrix, bypassing the intermediary of any parameters such as Euler angles. The connection to the rotation matrices for real spherical harmonics is made explicit. The recurrence formulas furnish a simple, efficient, and numerically stable evaluation procedure for the real and complex representations of the rotation group. The advantages over the Wigner formulas are documented. The results are relevant for directing atomic orbitals as well as multipoles.
Two-dimensional inverse scattering: Compactness of the generalized Marchenko operator
Authors: Cheney Margaret
Published: July 2002

Recent work on the two-dimensional inverse scattering problem for the Schrödinger equation has resulted in a generalized Marchenko integral equation. The main result of this paper is that the integral operator appearing in this Marchenko equation is of Hilbert–Schmidt type. A result of Newton’s shows that its spectrum therefore consists of point eigenvalues whose moduli are at most 1.
Asymptotic eigenvalue degeneracy for a class of one-dimensional Fokker–Planck operators
Authors: Angeletti Angelo , Castagnari Cinzia , Zirilli Francesco
Published: July 2002

Let f(x), x∈R, be a fourth-degree polynomial with lim‖x‖→∞ f(x) =+∞ with two minima, and let Lε( ⋅ )=ε2/2 ∂2( ⋅ )/∂x2+(∂/∂x) ×((∂f/∂x/( ⋅ ))) be the corresponding Fokker–Planck operator. We study the spectrum of Lε in the limit ε→0. We show that in the limit ε→0 the spectrum of Lε degenerates in the spectrum of three decoupled harmonic oscillators.
Maxwell’s equations and the bundle of null directions on Minkowski space
Authors: Kent Steven L. , Kozameh Carlos N. , Newman E. T.
Published: July 2002

We reexpress the Maxwell field as a cross section of a line bundle over M×S2, the six-dimensional space of null directions on Minkowski space. Maxwell’s equations then become a pair of linear equations for a Herz-like scalar on M×S2. We obtain a deeper understanding of the simple, yet nontrivial relationship between the self-dual and the anti-self-dual parts of a real Maxwell field. Our results are then applied to study solutions which are globally regular (on M×S2) namely, the pure radiation solutions, as well as solutions associated with discrete sources (the Lienard–Wiechert fields).
Representations of the current algebra of a charged massless Dirac field
Authors: Borek R.
Published: July 2002

It is shown that the current algebra A0 of a charged, massless Dirac particle has representations with positive energy of all types I∞, II∞, and III in the classification of Araki–Woods.
Coulomb potential envelopes for a relativistic fermion in a central field
Authors: Hall Richard L.
Published: July 2002

We consider a hydrogenlike system in which the Coulomb potential is replaced by the more general central potential V(r)=vf(r/b)=vg(−b/r), where g is monotone increasing and convex. The method of potential envelopes is applied to this problem and approximations are obtained for the energy trajectories based on the expression εnj=minuε(0,1)[Dnj(u) −uD′nj(u) +vf{−1/bDnj(u)}], where Dnj(u) is the known exact trajectory function for the hydrogenic atom. General formulas are given for linear combinations of power-law potentials and the log potential. Some graphical results are presented in the case of the Coulomb-plus-linear potential f(r)=−α/r+βr.
Quasiprimary composite fields and null vectors in critical Ising-type models
Authors: Furlan P. , Sotkov G. M. , Todorov I. T.
Published: July 2002

Operator product expansions (OPE’s) are studied in unitary ‘‘minimal conformal models’’ on the circle. The presence of null vectors at a certain level (signaled by the the vanishing of Kac’s determinant) leads to linear relations among SU (1,1)-covariant (‘‘quasiprimary’’) fields at that level. In the Ising model this is shown to imply the proportionality of two composite tensor currents of dimension 6 and the vanishing of 4 among the first five composite fields expected to appear in the OPE of the stress energy tensor and the canonical Fermi field (of weight 1/2 ). For the supersymmetric tricritical model it only implies the vanishing of the quasiprimary Fermi current of dimension (9)/(2) .
Extensions of Wigner’s distribution to particles with spin 1/2
Authors: Landau Basil V.
Published: July 2002

For particles of spin 1/2 a class of distributions closely related to Wigner’s is introduced on dynamical grounds. It is found that they may be refined to give correct expectation values of higher powers of spin components, but depart somewhat from a criterion that has been used in characterizing the Wigner distribution. For a special choice amongst this class, a more subtle refinement is possible satisfying this criterion exactly. This requires, however, a dubious distinction at every point between positive spin about a direction and negative spin about the reverse direction.
Quadratic zeros of Racah 6j coefficients: A geometrical approach
Authors: Labarthe J. J.
Published: July 2002

It is shown that the projective symmetries of the polynomial Φ of quadratic 6j coefficients form the symmetrical group S6. Nonlinear rational symmetries of Φ are found. Partial parametrizations of the zeros of Φ are presented.
The generalized Weyl correspondence and time-dependent stochastic processes
Authors: Gadella M. , Nieto L. M. , Noriega J. M. , Santos E.
Published: July 2002

The definition and some general properties of the generalized Weyl correspondence between stochastic processes and operator-valued real functions on a Hilbert space plus a trace class operator are given. The relation between the derivatives of an operator-valued function and the derivative of the corresponding stochastic process are studied. When the operator-valued function is the position (or momentum) in the Heisenberg picture, a condition for the positivity of the joint distribution functions of the corresponding process is given, provided that the evolution Hamiltonian be quadratic in the position and momentum. Finally, the case of an arbitrary Hamiltonian evolution for the position operator is studied and the two-dimensional density functions of the process is related to the Wigner function associated to some state ρ̂, and a necessary condition for the positivity of the densities is given.
Symplectic geometry and integrable m-body problems on the line
Authors: Francoise J. P.
Published: July 2002

A conjecture of Gallavotti and Marchioro [J. Math. Anal. Appl. 44, 661 (1973)] is proved by using symplectic techniques.
Stochastic field theory and finite-temperature supersymmetry
Authors: Ghosh Pradip , Bandyopadhyay Pratul
Published: July 2002

The finite-temperature behavior of supersymmetry is considered from the viewpoint of stochastic field theory. To this end, it is considered that Nelson’s stochastic mechanics may be generalized to the quantization of a Fermi field when the classical analog of such a field is taken to be a scalar nonlocal field where the internal space is anisotropic in nature such that when quantized this gives rise to two internal helicities corresponding to fermion and antifermion. Stochastic field theory at finite temperature is then formulated from stochastic mechanics which incorporates Brownian motion in the external space as well as in the internal space of a particle. It is shown that when the anisotropy of the internal space is suppressed so that the internal time ξ0 vanishes and the internal space variables are integrated out one has supersymmetry at finite temperature. This result is true for T=0, also. However, at this phase equilibrium will be destroyed. Thus for a random process van Hove’s result involving quantum mechanical operators, i.e., that when supersymmetry remains unbroken at T=0 it will also remain unbroken at T≠0, occurs. However, this formalism indicates that when at T=0 broken supersymmetry results, supersymmetry may be restored at a critical temperature Tc.
N=4 irreducible scalar superfields
Authors: Ketov S. V. , Lokhvitsky B. B.
Published: July 2002

The N=4 extended supersymmetry in four-dimensional space-time is considered from the viewpoint of the two included N=2 extended supersymmetries. Using the results for the structure of the irreducible superfields in N=2 superspace obtained earlier the task of explicit construction of N=4 irreducible superfields with SU(2)×SU(2) internal symmetry in N=2×2 superspace is solved. The decomposition of the N=4 scalar superfield into N=4 irreducible ones is described and the rules for collecting the irreducible scalar N=2×2 superfields with SU(2)×SU(2) internal symmetry into N=4 irreducible scalar superfields with SU(4) internal symmetry are set. To illustrate this method a simple consistent way of constructing the linearized off-shell N=4 conformal supergravity in N=4 superspace is shown.
Anomalous congruences
Authors: Braden H. W.
Published: July 2002

Congruences between different Chern classes and hence anomaly coefficients are observed. These are determined in general for SU(n) vector bundles over spheres. The results are of use in calculating global anomaly freedom for gauge theories as well as other index theory calculations.
Clifford associativity and the Pfaffian expansion
Authors: Wilmot G. P.
Published: July 2002

With the help of the Pfaffian, an operator on the tensor algebra which generates the explicit coset structure of the Clifford quotient of the tensor algebra is defined. The associativity property of this tensor representation of Clifford algebra leads to a Pfaffian expansion analogous to the Laplace expansion of a determinant. Further, the explicit statement of the algebra norm provides a proof for the Hadamard theorem.
R structures, Yang–Baxter equations, and related involution theorems
Authors: Oevel Walter
Published: July 2002

There are several ways to construct functions in involution on a Lie bi-algebra, a Lie algebra equipped with a second Lie bracket. For the solvable systems associated to the Casimir functions a second Hamiltonian formulation can be constructed and a class of bi-Hamiltonian Korteweg–de Vries-like evolution equations with explicit space dependence is derived. Translating the Casimir functions with the flow of a special vector field yields another set of functions in involution. Lenard relations are found for the corresponding Hamiltonian systems. Finally, solutions of the classical Yang–Baxter equation lead to an analog of compatible Hamiltonian pairs. The invariants of the resulting hereditary operators are in involution.
Fundamental representations of the m-principal realization of g
l∞
Authors: Dodd R. K.
Published: July 2002

The m-principal realization of gl∞ is introduced and its fundamental representations are defined on the representation space associated with the homogeneous Heisenberg subalgebra of a(1)m introduced by Frenkel and Kac. The Hirota equations associated with the representations are briefly reviewed. The main results concern the reduction of the algebras to the finite rank affine Lie algebras gl(1)n and a(1)n−1 where it is shown that a complete nonredundant set of realizations can be obtained. This means that the associated Hirota equations can be obtained directly from the reduction of the Hirota equations for the fundamental m-principal representations of gl∞ without requiring the complete nonredundant set of fundamental representations of the Lie algebra a(1)n−1 itself.