bluuebunny/crossref_metadata_2025 huggingface dataset:
Terahertz radiation imaging of supercurrent distribution in vortex-penetrated YBa2Cu3O7−δ thin film strips
Authors: Tonouchi Masayoshi , Yamashita Masatsugu , Hangyo Masanori
Published: July 2002

We have developed a supercurrent distribution imaging system for high Tc superconductive thin films and demonstrated the visualization of the supercurrent distribution in the vortex-penetrated YBa2Cu3O7−δ thin film strips. The terahertz (THz) radiation and detection system with a scanning femtosecond laser was employed to visualize the distribution. The imaging system utilizes the principle that the femtosecond optical pulses excite THz radiation into the free space by optical supercurrent modulation, and the radiation amplitude is proportional to the local supercurrent density at the optically excited area. Prior to the observation of the supercurrent distribution, we studied optical excitation effects on the vortices trapped in the strips, calibration of the current density from the THz radiation amplitude, temperature dependence of the THz radiation properties, etc. The laser power dependence of the THz radiation in the remanent state revealed that the excitation with powers larger than the relatively weak finite value (about 10 mW in the present case) strongly affects the vortices trapped in the films. We attributed this behavior to the optically excited depinning effect. We derived a calibration function from the THz radiation images into the supercurrent density distributions by observing the bias-current dependence of the THz radiation, and applied it for the diagnosis of the distributions in the vortex-penetrated strips. The THz radiation images were successfully transferred into the supercurrent density distributions with quantitative agreement. The minimum magnetic flux resolution at the optically excited area was roughly estimated to be 3 φ0 where φ0 is a single flux quantum. The measurement of the laser beam profile indicated that the spatial resolution of the THz radiation images is limited by the laser beam diameter: 25 μm in our case. The observed distributions revealed that the vortices easily penetrate into the strip under an external magnetic field BEX of 0.9 mT, and the persistent supercurrent exists only near the strip edges in the remanent state after removal of the field. The calculations of the convolution between the observed laser pattern and the trial functions suggested that the supercurrent distribution width in the remanent state after removal of the field of 0.9 mT is estimated to be less than 1 μm. The temperature dependence of the supercurrent distributions revealed that, below 60 K, the thermal activation produces no significant effects on the penetrated vortices at BEX=0.9 mT, whereas, the vortices in the remanent state after removal of the field of 15 mT were strongly affected by the thermal activation. The decreasing rate of the supercurrent density at the edge with increasing temperature was larger than that inside the strip. This suggested that the vortices trapped near the edges exhibit rather different behavior from the ones that penetrated into the inner part of the strip.
Fabrication of regularly arranged nanocolumns on diamond(100) using micellar masks
Authors: Koslowski B. , Strobel S. , Herzog Th. , Heinz B. , Boyen H. G. , Notz R. , Ziemann P. , Spatz J. P. , Möller M.
Published: July 2002

Cylindrical diamond columns of 15 nm height and 10 nm diameter have been prepared on a diamond(100) substrate by exploiting the self-organization of metal-loaded diblock copolymers. For this purpose, chemomechanically polished substrates of synthetic (high pressure high temperature) diamond exhibiting extremely low roughness (typical root-mean-square roughness 60 pm) were covered by a monolayer of gold-loaded inverse polystyrene-block-poly(2-vinylpyridine) micelles which self-organize in an hexagonal order on the substrate. After burning off the blockcopolymer in an oxygen plasma, the resulting Au nanoparticles act as a mask during further etching the diamond substrate in this plasma. As a result, Au-capped diamond columns are formed with an approximate size of the former gold particles. After removing the Au caps by evaporation at 1100 °C in ultrahigh vacuum, an array of diamond nanocolumns is obtained. As a consequence of this preparation process, the columns form an hexagonal lattice with a separation of 85 nm reflecting the arrangement of the self-organized micelles. The successive states of sample preparation were characterized by atomic force microscopy, Rutherford backscattering, and in situ scanning tunneling microscopy.
Magnetization reversal and interlayer coupling in magnetic tunneling junctions
Authors: Schrag B. D. , Anguelouch A. , Xiao Gang , Trouilloud P. , Lu Yu , Gallagher W. J. , Parkin S. S. P.
Published: July 2002

We have studied the switching properties of micron-scale magnetic tunnel junctions in two-dimensional magnetic fields. We present data on interlayer magnetic coupling for multiple samples. We interpret these data as the sum of a magnetostatic and a Néel coupling contribution. The data are presented as functions of layer structure. In addition, we have extracted information about interface roughness. We have also studied the area of switching critical curves as a function of device geometry.
The effect of pinning and nucleation field distributions on reversible magnetization behavior
Authors: Crew D. C. , Lewis L. H.
Published: July 2002

A model has been developed to examine the effect that different distributions of pinning and nucleation fields have upon the reversible magnetization behavior. The model allows each grain to be in one of three states, saturated in the positive and negative directions or containing a domain wall. The nucleation and pinning field distributions determine the rate of change of the number of grains in the three states with field. The results indicate that reversible magnetization curves display a well-defined minimum where the domain wall pinning fields exceed the reverse domain nucleation fields. Where the domain wall nucleation fields exceed the domain wall pinning fields no minimum is seen. The exact shape of the reversible magnetization curve and the position of the minimum are indicative of the dynamics of the reversal process. This model is able to reproduce the form of reversible magnetization curves measured previously in a sintered isotropic NdFeB based magnet and a Sm2(Co, Fe, Cu, Zr)17 type magnet.
Magnetoresistance of interleaved and separated Co/Cu multilayers in the CPP mode
Authors: Bozec Didier , Howson M. A. , Hickey B. J. , Shatz Smadar , Wiser Nathan
Published: July 2002

We have measured the field dependence of the magnetoresistance MR(H) in the CPP mode for two types of magnetic multilayers that differ only in the ordering of the magnetic layers: [Co (10 Å)/Cu/Co (60 Å)/Cu]N and [Co (10 Å)/Cu]N [Co (60 Å)/Cu]N, with the Cu layers always being 200 Å. The series resistor model predicts that in the CPP mode, MR(H) is independent of the ordering of the layers. Nevertheless, the MR(H) curves measured were found to be completely different for the two cases. We suggest that the reason for this difference is that the electron mean free path is long enough that the potential that scatters the electrons is due to pairs of neighboring magnetic layers, which differ for the two types of magnetic multilayers.
A simple measurement of the absolute internal quantum efficiency of thin organic films
Authors: Nollau A. , Hoffmann M. , Floreck K. , Fritz T. , Leo K.
Published: July 2002

In this article, we present a simple method for the determination of the absolute internal quantum efficiency of thin organic dye layers. The basic idea is the comparison of the luminescence of the film with the reflection of a white diffusive reflectance standard measured at one angle with a simple spectrofluorometer. The method is compared to the procedure of de Mello [J. C. de Mello, H. F. Wittmann and R. H. Friend, Adv. Mater. 9, 230 (1997)], which uses an integrating sphere. As examples, the quantum yields of films of the two perylene derivatives N,N′-dimethylperylene-3,4:9,10-dicarboximide and 3,4,9,10-perylenetetracarboxylic dianhydride, are determined.
Magnetic switching in submicron-scale periodic magnetic arrays
Authors: Vavassori P. , Donzelli O. , Metlushko V. , Grimsditch M. , Ilic B. , Neuzil P. , Kumar R.
Published: July 2002

The influence of the element shape on the switching of the magnetization in dot arrays has been investigated using a joint in-field magnetic force microscopy analysis and magneto-optical Kerr measurements. The single- or multiple-domain nature of the magnetization at remanence is shown to depend strongly on the aspect ratio of the elements and on how rounded or pointed each element is.
Electrical behavior of ultra-low energy implanted boron in silicon
Authors: Privitera V. , Schroer E. , Priolo F. , Napolitani E. , Carnera A.
Published: July 2002

In this paper an extensive characterization of the electrical activation of ultra-low energy implanted boron in silicon is reported. The Spreading Resistance Profiling technique has been used, in a suitable configuration, for measuring doped layers shallower than 100 nm, in order to extract the carrier concentration profiles. The dependence on the implant energy, dose, and annealing temperature allowed us to gain more insight into the mechanisms responsible for the electrical activation at implant energies below 1 keV. By measuring the electrical activation as a function of time for several annealing temperatures, the thermal activation energy for the electrical activation of the dopant was achieved. It slightly depends on the implant dose and it is in the range of 2–3 eV. In particular, for an implant dose of 1×1014/cm2 it is 2.0 eV, close therefore to the 1.7 eV activation energy found [Napolitani et al., Appl. Phys. Lett. 75, 1869 (1999)] for the enhanced diffusion of ultra-low energy implanted boron. The best conditions to maximize electrical activation, while minimizing diffusion, are identified and junction depths of ∼50 nm with sheet resistance below 500 Ω reported. These data are reported and their implication for the fabrication of future generation devices is discussed.
Chemical vapor deposition of undoped and in-situ boron- and arsenic-doped epitaxial and polycrystalline silicon films grown using silane at reduced pressure
Authors: Pejnefors J. , Zhang S.-L. , Radamson H. H. , Grahn J. V. , Östling M.
Published: July 2002

A nonselective epitaxial growth process for heterojunction bipolar transistors has been studied. The difference in growth rates for epitaxial and polycrystalline films could be used to monitor the thickness of the intrinsic and extrinsic base layers. The films were grown using chemical vapor deposition on Si 〈100〉 (epitaxy) and on silicon dioxide (polycrystalline) at reduced pressure (20–80 Torr) for undoped and in situ B or As doping. The depositions were carried out using silane diluted in hydrogen. Diborane and arsine were used as the source gas for dopants. For the undoped Si films, the deposition of polycrystalline films had a substantially higher rate than that of epitaxial ones. The growth rate of both epitaxial and polycrystalline depositions decreased with increasing total pressure. It was, however, linearly proportional to the silane partial pressure, pSiH4. The dependence of the growth rate on the hydrogen partial pressure was proportional to pH2−0.82 for epitaxial and to pH2−0.60 for polycrystalline depositions. The apparent activation energy was 2.1 and 1.6 eV for the epitaxial and polycrystalline depositions, respectively. A growth mechanism assuming the dissociative adsorption of silane on the Si surface, in combination with first-order hydrogen desorption kinetics, was employed to describe the experimental observations, including the differences in deposition rates, dependency on the hydrogen partial pressure as well as apparent activation energy. In situ B doping influenced neither the epitaxial nor polycrystalline depositions. In situ As doping, on the other hand, largely reduced the growth rate compared to the undoped films to such an extent that there was no appreciable difference in growth rate between the epitaxial and polycrystalline Si. The doping concentration in the epitaxial B and As films were of the order of 1018 cm−3, identical deposition conditions yielded a 5 and 20 times larger dopant incorporation in the B and As doped polycrystalline films, respectively.
Microwave and infrared characterization of several weakly bound NH3 complexesa)
Authors: Fraser G. T. , Nelson D. D. , Charo A. , Klemperer W.
Published: July 2002

We present the results of microwave and infrared spectroscopic studies of several van der Waals complexes of NH3. These results were obtained with a molecular beam electric resonance spectrometer. The microwave spectroscopy of the complexes (NH3)2 and Ar–NH3 show that both systems are nonrigid. The observed dipole moments for (NH3)2[0.74(2) D] and (ND3)2[0.57(1) D] are not compatible with the presently accepted theoretical structure. Ar–NH3, which has a complicated and currently unassigned microwave spectrum, exhibits Q branch inversion transitions near 19 GHz which indicate that the NH3 subunit is likely to be a near-free rotor. Infrared studies of the complexes NH3–HCCH, NH3–CO2, (NH3)2, Ar–NH3, NH3–OCS, NH3–N2O, and NH3–HCN have been carried out with a line tunable CO2 laser. Only for NH3–HCN were no infrared resonances discovered. Photodissociative transitions are observed in all of the other systems. Band origins for the photodissociative infrared transitions involving the ν2 umbrella motion of NH3 were determined for NH3–HCCH [984.4(9) cm−1], NH3–CO2 [987.1(2) cm−1]. NH3–OCS [981.5(15) cm−1], and NH3–N2O [980(2) cm−1]. The observation of an infrared transition for Ar–NH3 at 938.69 cm−1, which is 40 cm−1 lower than the band origins in the other NH3 complexes, lends support to the model of Ar–NH3 mentioned above. NH3–HCCH, NH3–CO2, (NH3)2, and Ar–NH3 were studied in microwave-infrared double resonance experiments in order to eliminate much of the inhomogeneous broadening present in their infrared spectra and to aid in the rotational assignment of the infrared spectra. Linewidths were determined for NH3–HCCH (0.15 GHz) and for NH3–CO2 [14(6) GHz]. An important result of this study is that the dissociation energies of all the complexes studied, except for NH3–HCN, are established to be less than 990 cm−1, i.e., 2.8 kcal/mol.
Model calculations for alkali halide clusters
Authors: Diefenbach J. , Martin T. P.
Published: July 2002

Model calculations of the total energy of alkali halide clusters using a rigid-ion potential and a polarizable-ion potential are presented. The calculations were carried out for four different materials for clusters having the composition (MX)n and M(MX)+n, n=1–18. It turns out that for a cluster of a given size the configuration of lowest energy is usually model and material dependent. Certain configurations of especially low total energy are discussed in detail.
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o calculations of radiative transition probabilities in SH, SH+, and SH−
Authors: Senekowitsch Jörg , Werner Hans-Joachim , Rosmus Pavel , Reinsch Ernst-Albrecht , ONeil S. V.
Published: July 2002

Potential energy and dipole moment functions for the ground states of SH, SH+, and SH− have been calculated from highly correlated electronic wave functions. The electric dipole moments in the vibrational ground states of 32SH, 32SH+, and 32SH− are calculated to be 0.74, 1.29, and 0.27 D, and the rotationless rates of spontaneous emission  A10 to be 1,  52, and 75 s−1, respectively. The predicted transition probabilities between the low lying vibrational states of the electronic ground state of SH and SD are among the smallest so far known for dipole allowed rotation-vibration transitions. The calculated A–X transition probabilities in SH confirm recent indirect determinations of the radiative lifetimes and absorption oscillator strengths in the predissociating v′=0 level of the A state. The 4Σ− state is calculated to intersect the A 2Σ+ state at R=3.1 a.u., between the classical turning points of v′=0 and 1 in the A state.
The nonmonotonic behavior of and the presence of slow and fast maxima in the momentum densities of atoms and ions
Authors: Westgate William M. , Simas Alfredo M. , Smith Vedene H.
Published: July 2002

The spherically averaged momentum densities Π̄(p) for the ground states of the 92 neutral atoms from hydrogen to uranium, and of the 73 singly positive ions from helium to barium and from lutetium to radium, constructed from nonrelativistic SCF wave functions, are examined. From these investigations, it is noted that the general shape of Π̄(p) of an atomic system is clearly determined by the orbitals of the two outermost shells. Two distinct types of maxima that make Π̄(p) nonmonotonic have been investigated: (i) a slow type, which typically occurs for pε(0.0,0.6]ℏa−10, and (ii) a fast type, not previously reported in the literature, which either is very small and barely noticeable, or is huge (as compared to those of the slow type), and which occurs for pε[0.7, 1.6]ℏa−10 . The results indicate that while maxima of the slow type result from occupied p orbitals of the outermost shell when that shell possesses no occupied d orbitals, maxima of the fast type are the result of both d and p occupied orbitals.
Solvent influence on atomic spectra: The effect of finite size
Authors: Hall Randall W. , Wolynes Peter G.
Published: July 2002

Time dependent Hartree theory is used to determine the solvent effect on atomic spectra for a given solvent configuration. Configuration averaging is performed as in the mean spherical approximation, resulting in an upper bound to the polarizability. Comparisons are made with previous, more approximate theories, including path integral treatments. It is found that deviations from previous theories can be significant in certain regimes.
H2O/Ni(100) and NH3/Ni(100): A computational approach
Authors: Bauschlicher Charles W.
Published: July 2002

The adsorption of NH3 and H2O on the Ni(100) is treated using a cluster model. The adsorption is found to have only a small effect on the HXH angle. Rotation about the principal ligand axis requires virtually no energy. Ligand tilts of 15° are found to require small amounts of energy, leading to the suggestion that the ESDIAD signal is a result excited tilting modes, not a change in the HXH angle. For H2O additional bending modes are considered and all are found to be quite flat in energy.
Triplet state spectroscopy and photophysics of As2 in solid neon
Authors: Heimbrook L. A. , Chestnoy N. , Rasanen M. , Schwartz G. P. , Bondybey V. E.
Published: July 2002

As2 dimer molecules generated by evaporating gallium arsenide were trapped and studied in solid neon. Components of at least three different excited electronic states are populated by direct tunable laser excitation and observed in phosphorescence. Nonradiative vibrational and electronic relaxation processes occurring in the matrix have been investigated. The electronic structure and spectroscopy of As2 is reviewed and summarized.
Evaporative cooling
Authors: Klots Cornelius E.
Published: July 2002

Thermochemical aspects of the cooling of isolated liquid drops by evaporation are considered, with explicit reference to amorphous water. Monte Carlo simulations are used to examine the role of fluctuations in energy dissipation and the effect of local stabilities on the terminal cluster size. Kinetic aspects of the cooling are then considered. We establish the pertinence of our studies to clusters generated in sonic nozzle expansions, and show that they will be metastable. We then find that a given mass-selected cluster will comprise a broad range of internal energies. Severe constraints on the kinetic modeling of evaporation patterns are thus imposed. These are illustrated by reference to data in the literature.
Composition of the first coordination sphere of Ni2+ in concentrated aqueous NiBr2 solutions by x-ray diffraction
Authors: Magini M. , de Moraes M. , Licheri G. , Piccaluga G.
Published: July 2002

Two concentrated solutions of NiBr2 have been examined by x-ray diffraction. The Fourier transformed scattering data indicate inner complex formation between Ni2+ and Br− ions. Average numbers of bonded bromide ions per nickel atom have been determined for each solution and the reliability of the complexation numbers as well as of the other structural parameters has been critically examined.
The role of alternative geometries in alkali–halide clusters
Authors: Dunlap Brett I.
Published: July 2002

The relative importance of the cubic structures that were proposed to explain magic numbers for alkali–halide cluster ions from cluster sources is examined via total-energy calculations on nine-atom cluster ions of various optimized geometries. The relative energies of the planar, tetrahedral, quasioctahedral, lowest energy nonplanar nine-atom clusters for LiF, LiI, NaI, KI, RbI, CsI, NaF, NaCl, NaBr, and NaI are computed using Martin’s Coulomb plus the Born–Mayer potential model. The most stable structure is invariably a slightly puckered plane. The relative energies of these clusters for LiF have also been tested using Hartree–Fock and density functional theory. Other comparisons are made for NaCl clusters and eight-atom LiF clusters. The computationally more tractable Born–Mayer potentials rather accurately predict the relative energies of the clusters in the ab initio calculations. The largest problem is too strong a repulsion between like atoms which overestimates the energy difference between the planar and quasioctahedral structure proposed by Morgan et al. These calculations suggest a greater population of noncubic structures for the larger and more polarizable alkali–halide cluster ions in beams from cluster sources.
Observation and analysis of the ν2 and ν3 fundamental bands of the H2D+ ion
Authors: Foster S. C. , McKellar A. R. W. , Peterkin I. R. , Watson J. K. G. , Pan F. S. , Crofton M. W. , Altman R. S. , Oka T.
Published: July 2002

The observation of the high-resolution absorption spectrum of the H2D+ molecular ion in the region 2010–2610 cm−1 in discharges through mixtures of H2 and D2 gases is reported. Two types of tunable monochromatic sources are employed, either a diode laser (in Ottawa) or a difference-frequency laser system (in Chicago), and the sensitivity is improved by using either discharge modulation or Doppler velocity modulation techniques. A total of 66 new lines of H2D+ have been measured and assigned, mostly on the basis of ground-state combination differences, to specific rotational transitions of the ν2 and ν3 bands. These data, as well as the two known microwave lines, are fitted by means of two theoretical models, either an effective Hamiltonian model including a Padé representation of a conventional A-reduced centrifugal Hamiltonian for each vibrational level together with Coriolis and higher rotational interactions between ν2 and ν3, or a supermatrix model in which the matrix of the untransformed Hamiltonian is set up in a large vibration-rotation basis and diagonalized directly. In the supermatrix model most of the vibrationally off-diagonal matrix elements are constrained to values derived from Carney’s ab initio calculations, while the ν1 parameters are fitted to the observed lines of Amano. Because of the large number of parameters required in the effective Hamiltonian, the less flexible supermatrix model was valuable as a check of the assignments. The results of these fits make it possible to assign seven of the nine lines reported by Shy, Farley, and Wing in 1981. The observed band origins, ν2=2205.87 cm−1 and ν3=2335.45 cm−1, as well as the rotational constants, are in good agreement with ab initio predictions. With the use of one calculated term value to relate the stacks of levels with even and odd values of K″a, a table of observed term values of the lower rotational levels of the ground state and the ν2 and ν3 states is constructed.
Vibronic coupling of short-lived electronic states
Authors: Estrada H. , Cederbaum L. S. , Domcke W.
Published: July 2002

The interaction of short-lived electronic states through the nuclear motion is investigated. Particular attention is paid to the impact of this interaction on differential and integrated cross sections for resonant electron–molecule scattering. A vibronic coupling model which has been successfully applied to study vibronic interactions in bound electronic states is extended to resonance states. Calculations of the nuclear dynamics in the nonlocal potential of the coupled resonance states are presented and discussed for several examples including up to two vibrational modes.
The temperature dependence of ion–molecule association rate coefficients in the low pressure limit
Authors: Viggiano A. A.
Published: July 2002

A modification to the theories of Herbst and Bates for ion–molecule association rate coefficients is presented that gives a simple formula for calculating the temperature dependence of these reactions. The present theory is compared to phase space theory and yields similar results. The modified theory is applied to several experimental systems and excellent agreement is found between experiment and theory. It is shown that a temperature dependent stabilization step is necessary to explain the experimental results in He buffers.
Diffusion and trapping at arbitrary trap size and concentration
Authors: Richards Peter M.
Published: July 2002

The average trapping rate and effective steady-state diffusion coefficient are derived for a particle hopping in a medium containing a random distribution of static traps. The traps may be of arbitrary size and concentration as long as they are allowed to overlap. Results are based on the author’s previous calculation of the spatially averaged survival probability. The finite-density corrections are much smaller than found by previous workers but in good agreement with simulations. Validity of a diffusion-reaction equation is established for steady-state conditions and the distribution of filled traps. (Only ideal sinks with no detrapping are considered.) Cases where the macroscopic trap concentration varies with position and there is a distribution of trap sizes are also considered. Transient motion of the nontrapped particles is shown to be nondiffusive, both theoretically and by simulations.
Infrared spectrum of the fundamental vibration–rotation band of OD−
Authors: Rehfuss B. D. , Crofton M. W. , Oka T.
Published: July 2002

The fundamental v=1←0 vibration–rotation band of OD− has been observed using the tunable infrared radiation from a difference frequency laser system and the velocity modulation technique for detection. The band origin is determined to be 2625.332(3) cm−1. The rotational constant B and the centrifugal distortion constant D have been determined for both the ground state and the first excited state. A remarkable similarity between molecular constants of OD− and OD has been noticed and utilized to estimate equilibrium vibration–rotation constants. These vibration–rotation constants were used to estimate the equilibrium bond length and the quadratic, cubic, and quartic force constants.
Thermal desorption mass spectrometry of alkali metal atoms from transition metal surfaces. The influence of coadsorbed oxygen
Authors: Albano Ezequiel V.
Published: July 2002

The coverage dependence of the activation energy of desorption for a planar array of electrical dipoles has been calculated in order to understand thermal desorption traces of alkali metal atoms from transition metal surfaces. The successful comparison between the computed spectra and available experimental data for K/Fe(110), K/Fe(100), K/Fe(polycrystalline), and K/Pt(111) allows us to demonstrate that the mutual dipolar repulsion within the adlayer is responsible for the coverage dependence of the thermal desorption spectra (TDS) characteristic of the above mentioned systems. Shifts of the TDS peaks up to about 450 K due to the lowering of ∼50% in the energy of desorption are well described by the proposed model. Based on these results a simple and accurate method to assess the coverage dependence of the desorption energy is presented. The coadsorption of electropositive and electronegative atoms has been modeled assuming a planar array of electrical dipoles of opposite directions. It is demonstrated that oxygen atoms substantially reduce the repulsion energy on the alkali adlayer which becomes thermally stabilized in agreement with published experimental results. In fact, for the systems K+O/Fe(polycrystalline) and K+O/Pt(111) the onset of K desorption is found to be shifted by about 200 K due to the presence of coadsorbed oxygen atoms.
Flow birefringence and dichroism of polymers. I. General theory and application to the dilute case
Authors: Onuki Akira , Doi Masao
Published: July 2002

Birefringence of polymer solutions has two origins, the orientation of polymer segments (the intrinsic birefringence), and the anisotropy of the segment density correlation function (the form birefringence). Rigorous formulas are given for the two contributions. Also special attention is focused on the form dichroism, which seems to have potential applicability in the study of polymer dynamics. As the simplest application we examine the birefringence and dichroism of dilute solutions in weak oscillatory shear for arbitrary light wavelength.
Are membrane microwave effects related to a critical phase transition?
Authors: Bond James D. , Wyeth N. Convers
Published: July 2002

We present a model that can explain reported nonthermal effects in biological membranes that occur when they are exposed to very low intensity electromagnetic fields in the microwave portion of the spectrum. One argument is that the external field drives the membrane closer to a critical point, and that critical fluctuations begin to dominate the behavior of the system. In an alternative argument, the membrane is initially close enough to its critical point for the system to be highly susceptible to specific external perturbations. Thus continuous, i.e., critical, phase transitions serve to mediate the field–membrane coupling via the greatly enhanced susceptibility of the system when it is near its critical point. We also discuss a specific example related to the experimentally observed anomaly in passive sodium ion permeability at the phase transition in the erythrocyte membrane when exposed to microwaves, and we propose additional experiments that will serve to test our hypotheses.
Dynamics of the activated dissociative chemisorption of N2 on W(110): A molecular beam study
Authors: Pfnür H. E. , Rettner C. T. , Lee J. , Madix R. J. , Auerbach D. J.
Published: July 2002

Molecular beam techniques have been used to study the dissociative chemisorption of nitrogen on W(110). Chemisorption probabilities have been measured as a function of incidence angle θi and kinetic energy Ei surface coverage and temperature. In addition, angular scattering distributions have been measured for a range of conditions and LEED has been used to examine surface structure. The initial (zero coverage limit) sticking probability is found to depend strongly on the incidence energy, scaling approximately with Ei, rather than with the velocity component normal to the surface. This probability is ≤3×10−3 for Ei≤30 kJ mol−1, and rises by more than a factor of 100 by ∼100 kJ mol−1, where it levels off at ∼0.35. It is argued that this behavior arises due to a strong chemical interaction prior to the barrier to dissociation. Angular scattering distributions revealed predominately quasispecular scattering with evidence as well for a diffuse component at low energies. The sticking probability falls steadily with increasing surface coverage and a saturation coverage of ∼0.25 atomic ML is observed for Ei∼10 kJ mol−1. At higher incidence kinetic energies, this saturation coverage increases to ∼0.5 ML at 200 kJ mol−1. LEED structures are also reported, corresponding to coverages of 0.25, 0.3, 0.5, and 0.52 ML. The 0.25 and 0.5 ML structures are identified as p(2×2) and c(4×2), respectively, for which structure models are proposed.
Observation of circular dichroism in photoelectron angular distributions
Authors: Appling Jeffrey R. , White Michael G. , Orlando Thomas M. , Anderson Scott L.
Published: July 2002

The first observations of dichroic effects in photoelectron angular distributions are reported for photoionization of aligned molecular excited states with circularly polarized light. Photoelectron angular distributions resulting from the two-color, (2+1) REMPI of NO via the A 2∑+, v=0, J=3/2,5/2 excited states exhibit significant left–right asymmetry. The experimental CD angular distributions are found to be well described by the general theoretical framework recently developed by Dubs, Dixit, and McKoy and are in good qualitative agreement with their calculated REMPI–CD distributions.
Observation of doubly charged diatomic cluster ions of a metal
Authors: Tsong T. T.
Published: July 2002

Field evaporation of Mo under pulsed-laser heating produces Mo2+2 instead of Mo+. The observation of this ion species has interesting implications to Coulomb explosion of doubly charged ions, and also to the current theory of field evaporation.
Electron attachment by chloro and bromomethanes
Authors: Olthoff James K. , Moore John H. , Tossell John A.
Published: July 2002

Electron transmission spectra and mass spectra of negative ions from dissociative electron attachment have been obtained for the series CCl4, CBrCl3, CBr2Cl2, and CBr4. Systematic changes are observed in the attachment energies of the t2 unoccupied orbitals in the end members and their split components in the mixed compounds. Cl− and Br− ion-production maxima are observed to correspond to attachment energies measured by ETS. Multiple scattering-Xα continuum calculations give elastic-electron-scattering cross sections for CCl4 and CBr4 which are consistent with experiment in the low energy region, but fail to reproduce the maximum in total cross section observed at higher energy. Calculated and experimental valence orbital I.P.’s and UV excitation energies for CCl4 and CBr4 are shown to be consistent with our electron transmission results.
Reflection and transmission of light by dye monolayers
Authors: Orrit Michel , Möbius Dietmar , Lehmann Urs , Meyer Horst
Published: July 2002

Using a point-dipole model, we discuss short- and long-range interactions in a 2D array representing a monolayer. Long-range forces may be treated exactly if the film is homogeneous at the wavelength scale and if the effective polarizability including short-range forces is known. Reflection and transmission amplitudes for polarized light are calculated for two monolayer models taking the subphase into account for arbitrary incidence angle and reflection strength. Experimental results are then compared to the theory. The good agreement shows that oblique incidence reflection and transmission spectra are valuable tools for investigating orientation and organization of chromophores in monolayers. Although in most of the studied cases—including J aggregates—the dye chromophore lies flat on the water surface, two instances are presented of molecular aggregates whose transition moments are vertical.
The interphase between jellium and a hard sphere electrolyte: Capacity–charge characteristics and dipole potentials
Authors: Schmickler Wolfgang , Henderson Douglas
Published: July 2002

The jellium/hard sphere electrolyte model for the electric double layer is reconsidered and extended in several ways. The effect of various approximations for the electronic density profile on the calculated capacity values is investigated. On the whole, the new calculations, which are performed with a higher accuracy, confirm our previous results. A heuristic formula for the dielectric response of the ensemble of hard spheres to high electric fields is presented; capacity–charge characteristics calculated from this formula show a capacity maximum close to the potential of zero charge. The dipole potential at the uncharged interface is also calculated; its variation with the metal agrees well with the values derived by Trasatti from experimental data.
Observation and analysis of the ν2 and ν3 fundamental bands of the D2H+ ion
Authors: Foster S. C. , McKellar A. R. W. , Watson J. K. G.
Published: July 2002

The high-resolution absorption spectrum of the D2H+ molecular ion in the 1800–2300 cm−1 region has been measured in a discharge through a mixture of H2 and D2 using a tunable infrared diode laser source and a cooled hollow-cathode absorption cell. A total of 72 new lines of D2H+ have been observed, as well as five previously measured in ion-beam experiments by Wing and Shy, and these have been assigned to specific rotational transitions of the ν2 and ν3 fundamental bands. Two different and complementary theoretical models are used to fit these data: one is an A-reduced asymmetric rotor effective Hamiltonian including the Coriolis and higher-order rotational interactions between ν2 and ν3; and the other is a supermatrix model in which the matrix of the untransformed Hamiltonian is set up and diagonalized directly, using a large vibration–rotation basis that diagonalizes the vibrational energy. The former approach is less expensive and provides a better fit, but because of the large number of parameters varied may be more tolerant of incorrect assignments, whereas the latter is more expensive but may be more reliable because a smaller number of parameters is varied, most of the vibrationally off-diagonal parameters being constrained at values from ab initio calculations. These analyses have made it possible to assign 11 other lines observed by Wing and Shy, giving a total of 88 assignments. The observed band origins, ν2=1968.17 cm−1 and ν3=2078.42 cm−1, and rotational constants are in good agreement with recent ab initio calculations.
Photodissociation of carbon cluster cations
Authors: Geusic M. E. , Jarrold M. F. , McIlrath T. J. , Freeman R. R. , Brown W. L.
Published: July 2002

Mass resolved carbon cluster cations from C+3 to C+20 have been photofragmented using 248 and 351 nm light. For an initial cluster C+n , the dominant fragment observed is C+n−3 . Fluence dependence measurements of photofragmentation yield bracketed dissociation thresholds, photofragmentation cross sections, and product branching ratios. Photodissociation with 248 nm light is found to be primarily linear with laser fluence for n>5, but there are indications that C+3 and C+5 fragment only with the absorption of two or more photons. For 351 nm light, clusters with six and more atoms all show a linear dependence. The photofragmentation cross sections for both 351 and 248 nm light show a significant change as a function of cluster size.
Collision induced dissociation of metal cluster ions: Bare aluminum clusters, Al+n (n=3–26)
Authors: Jarrold Martin F. , Bower J. Eric , Kraus J. S.
Published: July 2002

The collision-induced dissociation of aluminum clusters, Al+n (n=3–26), by argon, at a center of mass collision energy of 5.25 eV, has been studied using a low energy ion beam apparatus. Product branching ratios and collision induced dissociation cross sections are presented and discussed. The main product is Al+ for the smaller clusters and Al+n−1 for the larger ones. The cross sections rise to a peak at Al+6−Al+9 and then decrease with increasing cluster size. Cross sections for Al+7, Al+13, Al+14, and Al+23 are significantly smaller than their neighbors. A crude kinetic model is used to derive approximate cluster ionization potentials from the product branching ratios. The IPs initially rise with cluster size, peak at Al6 and then decrease. The IP of Al7 is particularly low and there is a sharp drop in IP at Al14 where the IP falls below that of the atom. The results suggest that the dissociation energies increase for the larger clusters and there is evidence that Al+7, Al13, Al+13, Al+14, and Al+23 have enhanced stability. The results are compared to the predictions of the electronic shell model which can account for some of the results but predicts additional features which are not observed.
The Hückel model for small metal clusters. II. Orbital energies, shell structures, ionization potentials, and extrapolation to the bulk limit
Authors: Lindsay D. M. , Wang Youqi , George Thomas F.
Published: July 2002

We analyze the results of HMO calculations (this paper and its preceding companion) for the neutral and cation alkali-like metal clusters, M2–M14. The filled HMO energy levels develop as well separated energy shells 1s, 1p, 1d, ... where s, p, and d denote the global nodal character of the Hückel orbitals. The HMO energy shells decrease in energy with increasing cluster size. By smoothly fitting the HMO orbital energies, we obtain trends in (a) atomization energies, (b) relative cluster stabilities, and (c) ionization potentials which are highly reminiscent of those derived from jellium calculations. The HMO atomization energies are best described by a classical drop model. When extrapolated to infinity, the HMO cohesive energies are within 15% of the experimental results for bulk Li→Cs. Thus we are able to unify within the single framework of HMO theory the quantum, jellium, and droplet models for alkali metal clusters. For the neutral clusters, HMO and local-spin-density atomization energies agree to within ∼4%. The HMO cation energies, modified by addition of a classical charge-correlation term, differ from the local-spin-density results by about 2%. Except for an exaggerated drop at the n=8 shell closing, the HMO ionization potentials are in good agreement with the experimental data for Na and K clusters.
Reactions of metastable argon with photodissociation aligned carbon disulfide: A study of the steric dependence of two competing reaction channels
Authors: de Vries Mattanjah S. , Tyndall George W. , Cobb Cathy L. , Martin Richard M.
Published: July 2002

CS2 molecules in a beam were aligned using laser photodissociation at 193 nm. Two competing reactions with metastable Ar were studied as a function of CS2 alignment. The Penning ionization reaction cross section for formation of CS+2 (X 2Πg) was found to be largest when the CS2 bond is parallel to the relative velocity vector, while the cross section for dissociative excitation giving CS*(A 1Π) is largest when the CS2 bond is perpendicular to the relative velocity vector.
Excitation cross sections in collisions of He+, Ne+, Ar+, N+, N+2, O+2, H+2, and H+3 ions with CF4
Authors: Sasaki Jun , Kuen Ingrid , Howorka Franz
Published: July 2002

Excitation processes in the collisions of He+, Ne+, Ar+, N+, N+2, O+2, H+2, and H+3 with tetraflouromethane have been studied in the energy range 1–1800 eV laboratory frame and the wavelength region 2000–8000 Å. Absolute cross sections dependent on energy are measured. Several excitation processes have been observed: Excitation of a continuous emission band and discrete emission bands, and excitation of atomic lines; neutral and ionic carbon and fluorine lines, X i and X ii lines (X+ being the primary ion), and excitation of the Balmer series in collisions of H+2 and H+3 with CF4. The recombination energy of the incident ion seems to play a dominant role for the excitation of molecular emissions, whereas the kinetic energy seems to be responsible for the excitation of atomic lines.
Basis set and correlation effects on computed hydrogen bond energies of the dimers (AHn)2: AHn=NH3, OH2, and FH
Authors: Del Bene Janet E.
Published: July 2002

Basis set expansion and correlation effects on computed hydrogen bond energies of the dimers (AHn)2, AHn=NH3, OH2, and FH, have been evaluated. The addition of diffuse functions is the single most important enhancement of split-valence plus polarization basis sets, leading to a significant lowering of hydrogen bond energies at all levels of theory. In general, basis set enhancement effects do not appear to be additive. The correlation energy contribution increases the stabilization energies of these neutral hydrogen bonded complexes, with the second order Mo/ller–Plesset term being the dominant term.
Absolute photoionization cross sections for Kr(5s) and Kr2 5s 3Σ+u (1u, 0−u) excited states at 248 nm
Authors: Geohegan D. B. , McCown A. W. , Eden J. G.
Published: July 2002

Absolute cross sections for the photoionization of Kr (5s) and Kr2 5s 3Σ+u (1u, 0−u ) at 248 nm have been measured to be (5.7±1.1)×10−19 and (1.4±0.3)×10−18 cm2, respectively. In these experiments, absolute photoelectron densities were measured in real time with a microwave absorption apparatus, and the excited atomic and molecular species were produced and subsequently photoionized by excimer lasers. The measured Kr (5s) cross section is consistent with the upper limit experimentally determined by Dunning and Stebbings [Phys. Rev. A 9, 2378 (1974)] and is close to the theoretical value calculated by Hartquist [J. Phys. B 11, 2101 (1978)]. For Kr2 5s 3Σ+u (1u, 0−u) , the cross section reported here is intermediate to those measured previously for the lowest 3Σ+u states of Ar2 and Xe2.
Theoretical study of C2 and C−2 : X 1Σ+g , a 3Πu , X 2Σ+g , and B 2Σ+u potentials
Authors: Nichols Jeffrey A. , Simons Jack
Published: July 2002

We employ multiconfigurational self-consistent field and multiconfigurational electron propagator methods to characterize the X 2Σ+g and B 2Σ+u states of C−2 and the X 1Σ+g and a3Πu states of C2 over a wide range of bond lengths (1.0–1.9 Å). We find a systematic difference of approximately 0.3 eV in the relative positioning of our anion- and neutral-state potentials compared to the anion–neutral spacing in the best curves constructed from experimental data. Once this energy shift is taken into consideration, all four of our computed potential energy curves are in reasonably good agreement with experimental information. However, there remains a substantial difference in the relative positioning of our B 2Σ+u and a 3Πu curves, compared to the best available experimental data, at larger bond lengths. The relevance of this discrepancy and of our other data to the present state of experimental knowledge on C−2 /C2 is discussed.
Avoided crossings: A study of the nonadiabatic transition probabilities
Authors: Desouter-Lecomte M. , Leyh-Nihant B. , Praet M. T. , Lorquet J. C.
Published: July 2002

An approximate solution to the problem of constructing a pair of diabatic states exists only if certain requirements are fulfilled, for example, when the nonadiabatic coupling results from an interaction between two electronic configurations which are doubly excited with respect to one another. It is then possible to build up a model in which the series expansion of the elements of the Hamiltonian matrix is truncated after the first nonzero term. This leads to several conclusions concerning the nonadiabatic transition probability which differentiate conical intersections from avoided crossings. For the latter, the nonadiabatic coupling matrix elements (which are Lorentzians with an area equal to π/2) reach their maximum at the nuclear geometry for which ΔE (the energy gap between adiabatic surfaces) is a minimum. The loci along which the angle θ of the orthogonal transformation which relates adiabatic and diabatic wave functions keeps a constant value are a set of parallel straight lines which coincides with the loci along which ΔE remains constant. This reference direction in the configuration space corresponds to nuclear trajectories which are unable to bring about a nonadiabatic transition. In the case of avoided crossings, there exists only one nuclear degree of freedom which gives rise to surface hopping. Conical intersections, on the other hand, have two such active degrees of freedom. This creates a qualitative difference between the two cases which makes conical intersections more efficient as funnels than avoided crossings. A two-dimensional extension of the Landau–Zener formula is derived for avoided crossings. It contains a factor of anisotropy. It is possible, at least in favorable cases, to extract approximate diabatic quantities from ab initio calculations and to compare them with the predictions of these models. This has been done for two 2A1 electronic states of the CH+2 ion. The results are found to agree with the predictions of the model, at least in a restricted range of internuclear distances.