bluuebunny/crossref_metadata_2025 huggingface dataset:
Simulation of clustering and transient enhanced diffusion of boron in silicon
Authors: Uematsu Masashi
Published: July 2002

We have simulated the postimplantation clustering and transient enhanced diffusion (TED) in boron profiles with peak concentrations below the boron (B) solubility limit. First, we derive an analytical formula for B clustering in terms of the reaction between B atoms and supersaturated self-interstitials. Then, using this formula and taking into account the dissolution of B clusters to emit self-interstitials, a unified simulation is done with just three essential parameters for the B clusters and based on the self-interstitial cluster model and the B diffusion model. We have satisfactorily fitted B TED profiles not only for implanted B layers but also for initially active B layers. Moreover, a comparison with TED induced by P implantation is made in terms of the trapping and diffusivities of self-interstitials.
Crossover behavior in dynamics of frozen ferrofluids
Authors: Duan Xiaodong , Zhang Jinlong , Luo Weili
Published: July 2002

The imaginary part of susceptibility, χ″, in frozen ferrofluids was measured as functions of temperature, frequency, and concentration of magnetic particles. In a very dilute region, where magnetic interaction is negligible, the relaxation time is mainly determined by Neel’s relaxation in isolated particles and the frequency-dependent peak temperature in χ″(T), Tp, obeys the Arrhenius law. In the moderate concentration range, Tp has a Vogel–Fulcher (VF) relaxation with the measuring frequency. However, for concentration larger than a certain value, deviation from VF law occurs. Simulations show that the VF relation could be attributed to Ising spin-glass-like random and frustrated interaction between magnetic moments of particles. We suggest that particle configuration before freezing is critical to determine the distribution of pair exchange parameters. For high concentration, there are more ferromagnetic than antiferromagnetic bonds. The deviation from VF law at high concentration could be due to short range correlation among spins.
Accurate theoretical modeling of NaCl
Authors: Swaminathan P. K. , Laaksonen A. , Corongiu G. , Clementi E.
Published: July 2002

This paper describes an accurate study of the electronic interaction energies of NaCl, from computations with extensive basis set optimizations for the molecule, at the configuration interaction (CI) level. In the analysis of the resultant potential energy surface for predictions of gas-phase spectroscopic properties, the properties demonstrate the accuracy of the present two-body interaction energies and indicate that, unlike the predecessors, they are sufficiently accurate to be combined with future three-body and four-body potential calculations for study of microscopic many-body effects, widely known to be present in the condensed phases of NaCl.
Picosecond photoisomerization and rotational reorientation dynamics in solution
Authors: Lee M. , Bain A. J. , McCarthy P. J. , Han C. H. , Haseltine J. N. , Smith A. B. , Hochstrasser R. M.
Published: July 2002

The trans–cis isomerization rates for stiff-diphenylbutadiene (S-DPB) in n-alkane solvents were measured using single photon counting methods and the rotational reorientation times τR for S-DPB and trans stilbene were obtained by picosecond polarization spectroscopy. In neither case did τR vs viscosity show Stokes–Einstein–Debye (SED) behavior. The values of τR were used to calculate the angular velocity correlation frequencies β using the Hubbard relation. The variation of isomerization rate with β was found to be predicted well by the Kramers equation when barrier frequencies of 154 cm−1 for stilbene and 16 cm−1 for S-DPB were used. This Kramers-Hubbard fit finesses questions regarding the validity of the one dimensional Kramers model and focuses attention on the SED equation. The dynamical relationship between the torsional friction, important in isomerization, and rotational friction, which determines the overall angular motion of the molecules, is discussed.
Transition states and rate constants for ion–molecule association. II. Li++(CH3)2O→Li+[(CH3)2O]
Authors: Vande Linde Scott R. , Mondro Sandra L. , Hase William L.
Published: July 2002

Canonical variational transition state theory is used to study the kinetics of Li++(CH3)2O association. Transition states and rate constants are calculated for a complete analytic potential energy surface which includes all inter- and intramolecular coordinates, and for an ion–dipole/ion–induced-dipole two-body potential. These surfaces have a single transition state at each temperature. Anisotropy in the polarizability is found to have a negligible effect on the association transition states and rate constants. The canonical variational transition state theory rate constants are in good agreement with those calculated by other formalisms.
2H-NMR study of the glass transition in supercooled ortho-terphenyl
Authors: Dries Th. , Fujara F. , Kiebel M. , Rössler E. , Sillescu H.
Published: July 2002

The glass forming molecular liquid ortho-terphenyl has been investigated by 2H-NMR techniques providing spin-relaxation times (T1, T2), and spin-alignment data which yield information on the time scale and geometry of ultra-slow molecular reorientation. The main results are as follows: The primary glass transition (α process) is characterized by rotational molecular jumps with a jump size distribution weighted in favor of large jump angles, and by a distribution of correlation times. In addition intramolecular flip–flop jumps of the lateral phenyl rings are found which do not take part in the α process. Apart from this (secondary) intramolecular dynamics no residual small angle reorientation persists below Tg on the time scale (10−4 to 102 s) of the spin-alignment experiment.
Interaction potentials for alkali ion–rare gas and halogen ion–rare gas systems
Authors: Ahlrichs R. , Böhm H. J. , Brode S. , Tang K. T. , Toennies J. Peter
Published: July 2002

The Tang–Toennies model [J. Chem. Phys. 80, 3725 (1984)] has been modified to predict the potentials for ion–atom systems. First order SCF energies are used to describe the repulsive potential. The long range second order induction and dispersion potential terms up to R−10 are either taken from ab initio calculations or estimated and each term is appropriately damped. The potentials for Li+, Na+, K+, F−, and Cl− interacting with He, Ne, and Ar are found to agree well with both theoretical and experimental data within the expected errors. For comparison with the model new ab initio calculations have been performed for Na+–Ar and the results are in excellent agreement with the model predictions (<10%).
Phase separation of polymer solutions in elongational flow
Authors: Bhattacharjee Somendra M. , Fredrickson Glenn H. , Helfand Eugene
Published: July 2002

We have studied the phase separation behavior of polymer solutions in two-dimensional elongational flows that are sufficiently weak to avoid the coil–stretch transition. On the basis of a simple model that neglects hydrodynamic interactions, we find no shifts in the infinite molecular weight θ point, or coexistence curve. For finite chain lengths, there are flow-induced shifts of both quantities. These shifts are calculated to leading order in a loop expansion and are found to be order (Sζ)2N7/2, where S is the extension rate, ζ is the monomer friction coefficient, and N is the degree of polymerization. Our results are in qualitative agreement with experimental observations that find demixing promoted by flow.
Quasielastic light scattering studies of swollen and stretched polymer gels
Authors: Takebe Tomoaki , Nawa Kazunari , Suehiro Shoji , Hashimoto Takeji
Published: July 2002

Cooperative diffusion D of polyacrylamide gels in water was investigated by quasielastic light scattering both under the isotropically swollen state and under the uniaxially stretched and swollen state. The concentration (Ce) dependence of D for the isotropic gels having equilibrium degrees of swelling was measured by systematically changing crosslinking density of gels. The results yielded D=(3.4±0.5)×10−6 C0.76±0.03e cm2/s, in accord with a scaling prediction. For each of the gels prepared and having a given Ce, the uniaxial stretching was applied, and the values D were investigated as a function of extension ratio α and direction β with respect to the stretching direction. The results gave D (α,Ce)=(3.4±0.5)×10−6 αx Cye cm2/s, where x=2/3 for β=0° and −1/4 for β=90°, and y=0.76, independent of α and β. The results clearly indicate that upon increasing α, D increases and decreases, respectively, in the direction parallel and perpendicular to the stretching direction, implying suppression and enhancement of the concentration fluctuations as a result of the network being stiffened and softened in the respective directions.
A
b i
n
i
t
i
o study of transition state spectroscopy: ClHCl− photodetachment spectrum
Authors: Yamashita Koichi , Morokuma Keiji
Published: July 2002

A b initio CASSCF-MRCI calculations with the [7s6p3d1f/4s3p1d] contracted Gaussian basis sets are performed to investigate the photodetachment spectrum of ClCHl− observed experimentally by Neumark et al. [J. Chem. Phys. 88, 1463 (1988); J. Phys. Chem. 94, 1377 (1990)]. The theoretical assignments for the ClHCl− and ClDCl− spectra based on one-dimensional Franck–Condon calculations are qualitatively in good agreement with the experiment. It is found by calculations that some peaks in the experimental spectra are assignable to transitions to the vibrational states of the first electronically excited 2Π state in addition to those of the ground 2Σ+ state in the transition state region of the neutral complex.
Fine structure branching ratios and Doppler spectroscopy of chlorine atoms from the photodissociation of alkyl chlorides and chlorofluoromethanes at 157 and 193 nm
Authors: Matsumi Yutaka , Tonokura Kenichi , Kawasaki Masahiro , Inoue Gen , Satyapal Sunita , Bersohn Richard
Published: July 2002

Alkyl chlorides (R=CH3, C2H5, C3H7, and C4H9), chloromethanes (CHnCl4−n), and chlorofluoromethanes (CFnCl4–n) are photodissociated at 157 and 193 nm. The chlorine atom photofragments are detected by a resonance enhanced multiphoton ionization technique. The branching ratios of the Cl photofragments [Cl*(3p2P1/2)]/[Cl(3p2P3/2)] are almost identical (0.23±0.03) for alkyl monochlorides at 157 and 193 nm and for CH2Cl2 and CHCl3 at 157 nm, while the ratios are rather small (0.10±0.02) for CH2Cl2 and CHCl3 at 193 nm. No discernible isotope effects on the branching ratios were observed when D atoms were substituted for H atoms in chloromethanes. For CCl4, CF3Cl, CF2Cl2, CFCl3 at 157 and 193 nm, the ratios are small (<0.05) irrespective of the number of Cl atoms in the parent molecules. The Doppler profiles of the chlorine photofragments have been obtained and it is found that (a) CF3Cl undergoes a perpendicular optical transition at 157 nm and (b) for halomethanes containing more than two Cl atoms, the Cl photofragment has a Boltzmann distribution in translational energy and an isotropic angular distribution.
Stability and structure of benzene dimer cation (C6H6)+2 in the gas phase
Authors: Hiraoka Kenzo , Fujimaki Susumu , Aruga Kazuo , Yamabe Shinichi
Published: July 2002

The gas-phase equilibria of the clustering reaction (C6H6)+n−1+C6H6 =(C6H6)+n were studied with a pulsed electron-beam high-pressure mass spectrometer. The enthalpy changes −ΔH0n−1,n for the reaction with n=2–4 were determined to be 20.6±1.0, 7.8±0.5, and ∼7 kcal/mol, respectively. The structure of dimer cation (C6H6)+2 was determined by ab initio spin-restricted open-shell Hartree–Fock molecular orbital (ROHF MO) calculations. Two isomers, ‘‘axial C2 ’’ and ‘‘parallel C2v, ’’ are calculated to be located at the potential-energy minima. The axial C2 is 3.0 kcal/mol more stable than the parallel C2v by ROHF/6-31G**//ROHF/3-21G.
Molecular beam optical Stark spectroscopy of calcium monocyanide
Authors: Steimle Timothy C. , Fletcher David A. , Jung Kook Y. , Scurlock Christopher T.
Published: July 2002

The 617.7 and 614.7 nm bands of calcium monocyanide, CaNC/CaCN, have been recorded at high resolution by laser-induced fluorescence using a supersonic molecular beam. The two bands consist of twelve branches that are assigned to a case a (0,0,0)A 2Πr–(0,0,0)X 2Σ+ transition. A reduction of the data to an effective Hamiltonian model produced the spectroscopic parameters (cm−1): T00= 16229.5417(26), B″=0.13499(14), γ″=6.1837(33)×10−4, A′=77.6451(40), B′=0.15027(23), AD′=2.69(11) × 10−3, D′= −3.50(25) × 10−5, p′=0.0754(18), q′=−0.04808(87), qD′′= 2.64(65) × 10−5. It is proposed that the anomalous values of the excited state parameters arise because of Renner–Teller interactions. The magnitude of the permanent electric dipoles, |μ|, were also determined and are 5.949(1) D[A 2Π1/2(0,0,0),J= 0.5] and 6.895(9) D[X 2Σ+(0,0,0),J= 1.5]. The large value of |μ| is consistent with an isocyanide structure, CaNC. A comparison with theoretical predictions is presented.
Analysis of CF3I quasicontinuum states. III
Authors: Angelie C.
Published: July 2002

CF3I quasicontinuum (QC) states are analyzed in the framework used previously for SF6 in papers I and II of this series. The existence of a hierarchy of intramolecular couplings Vk, decreasing vs the number k of vibrational quanta exchanged, with an effective density of coupled states ρk, is pointed out. The oscillator strength redistribution is described taking into account isolated resonances and perturbative redistribution associated to the lowest orders k, dissipative redistribution associated to higher orders, multiple transitions, and mode inhomogeneous spreading. Published homogeneous spectra of 12CF3I and 13CF3I at E=19 000 cm−1, near the dissociation threshold, are reinvestigated in this framework and a new assignment of the resonances is proposed. Using fluorescence data at 〈E〉=3200 cm−1, an interpolation of the position and weight of the main resonances ν1, ν4, ν2+ν3, 2ν5, in the frequency range 900–1200 cm−1, becomes possible. This interpolation allows to extract from IR multiphoton absorption data, at the laser frequency νL=1072 cm−1, an average Lorentzian half-width γ̄ (〈n〉) vs the average number of photons absorbed 〈n〉. γ̄ increases from 1 to 5 cm−1 for 〈n〉 varying from 5 to 10, and varies slowly around 4–5 cm−1 for 10≤〈n〉≤18. Finally, the onset of the QC, near E∼6000 cm−1, is explained using a modelization of the hierarchy Vk, ρk, without arbitrary parameters, and an intramolecular relaxation time ∼20 ps is predicted. This analysis leads to a very good agreement between the different sets of data concerning the CF3I molecule.
High resolution S1←S fluorescence excitation spectra of hydroquinone. Distinguishing the cis and trans rotamers by their nuclear spin statistical weights
Authors: Humphrey Susan J. , Pratt David W.
Published: July 2002

Hydroquinone (HYQ) has two rotational isomers, differing in the orientation of the two O–H bonds with respect to the benzene ring. In the S1←S0 fluorescence excitation spectrum of HYQ, two electronic origins, one for each rotamer, are present with a separation of 34.7 cm−1. Rotationally resolved spectra of both origins have been obtained and analyzed. This analysis reveals that the rovibronic lines in the two origins exhibit different alternating intensity patterns due to nuclear spin statistical weights, thereby providing a means for distinguishing the two rotamers. On this basis, we assign the origin at 33 500 cm−1 to the trans, C2h rotamer and the origin at 33 535 cm−1 to the cis, C2v rotamer of HYQ. Coordinate determinations of the hydroxy–hydrogen atoms of both rotamers, in both electronic states confirm this assignment.
Observation of the 4 3Σ+g, 3 3Πg, 2 3Δg, and b 3Πu states of 39K2 by perturbation facilitated optical–optical double resonance spectroscopy
Authors: Kim J. T. , Wang H. , Tsai C. C. , Bahns J. T. , Stwalley W. C. , Jong G. , Lyyra A. M.
Published: July 2002

The 4 3Σ+g, 3 3Πg, and 2 3Δg states of K2 have been studied by perturbation facilitated optical–optical double resonance spectroscopy for the first time. Molecular constants of those states are derived and compared with theoretical calculations. Varying the trial vibrational quantum numbers of these upper states, absolute vibrational quantum number assignments are obtained by comparison between the observed resolved fluorescence intensities and the calculated Franck–Condon factors. The Rydberg–Klein–Rees potential curves are constructed and compared with theoretical calculations. Finally, the observation of the lowest ‘‘dark’’ vibrational levels of the b 3Πu state by resolved fluorescence from the 2 3Δg state confirmed the absolute vibrational numbering of this state and allowed refinement of the b 3Πu molecular constants.
The structure of alkali halide dimers: A critical test of ionic models and new ab initio results
Authors: Törring T. , Biermann S. , Hoeft J. , Mawhorter R. , Cave R. J. , Szemenyei C.
Published: July 2002

In semiempirical ionic models a number of adjustable parameters have to be fitted to experimental data of either monomer molecules or crystals. This leads to strong correlations between these constants and prevents a unique test and a clear physical interpretation of the fit parameters. Moreover, it is not clear whether these constants remain unchanged when the model is applied to dimers or larger clusters. It is shown that these correlations can be substantially reduced when reliable information about dimers is available from experiments or ab initio calculations. Starting with Dunham coefficients of the monomer potential determined from microwave measurements, we have calculated the monomer to dimer bond expansion and the bond angle without any additional adjustable parameter. Assuming that the overlap repulsion between nearest neighbors remains unchanged, the bond expansion is mainly determined by the simple Coulomb repulsion between equally charged ions and depends only very little on the effective ion polarizabilities. Deviation of the bond angle from 90° sensitively tests the difference of effective polarizabilities of the two ions. A comparison with previously available data and new ab initio MP2 results presented here for the heavy-atom containing dimers shows that bond angles can be modeled reasonably well with Seitz–Ruffa corrected Pauling polarizabilities while calculated bond expansions are much too long. This shows that changes of the overlap repulsion term must be considered for reliable predictions of the structure of dimers and larger clusters.
A collision induced reaction: CO2 production on O2 and CO covered Pt(111)
Authors: Åkerlund C. , Zorić I. , Kasemo B.
Published: July 2002

Impingement of energetic Xe atoms on a coadsorbed CO+O2 monolayer on Pt(111) induces two concurrent collision-induced reaction channels: (a) formation of CO2 and (b) desorption of O2 below the surface temperature where any thermal reaction occurs. We report measurements of the initial reaction probability, its energy dependence and the reaction kinetics. We also discuss the possible reaction mechanism for the collision-induced CO2 oxidation reaction.
Electron transfer through interfacial water layer studied by scanning tunneling microscopy
Authors: Hong Y. A. , Hahn J. R. , Kang H.
Published: July 2002

The effective barrier height for electron tunneling is measured across the aqueous capacitor junction composed of a scanning tunneling microscopy (STM) tip and a surface. At a junction distance of a water monolayer, the barrier height strongly varies with the polarity and the magnitude of the applied junction voltage. This monolayer barrier height increases with an increasing positive value of the sample bias, while it does not for the negative bias. Such asymmetric variation of the tunneling barrier height manifests the influence of water molecular geometry on electron tunneling.
Dehydrogenation and physisorption of saturated hydrocarbons (n-butane and isobutane) on Nbx+
Authors: Wu Qifei , Lu Wenyun , Yang Shihe
Published: July 2002

Niobium cluster cations, Nbx+, are produced from a laser vaporization/supersonic expansion cluster ion source, and their reactivities are studied using a fast-flow reactor. The reaction patterns of n-butane and isobutane are found to be quite similar. In general, the relative reactivity of the cluster cations shows a decreasing trend with increasing cluster size (x=1–9). Dehydrogenation products from the butane molecules are observed within the small cluster size range (x=1–6). The degree of dehydrogenation increases with cluster size (x=1–6); but right after x=7, no dehydrogenation is observed and only molecular adsorption takes place. The product distribution for isobutane is found to be more scattered to channels of different extent of dehydrogenation than that for n-butane. The results are explained on the basis of the prompt insertion and the lowering of the activation barrier by the charge-induced polarization.
Dynamics and structure of solid hexafluoroethane
Authors: Zeng S. X. , Simmons R. O. , Timms D. N. , Evans A. C.
Published: July 2002

Neutron Compton profiles were measured of atomic momentum distributions in solid hexafluoroethane (C2F6) at saturated vapor pressure from 20 to 170 K, and of the liquid phase at 180 K. There are conflicting reports in the literature, from different kinds of measurements, about the nature and extent of solid phases of C2F6. Present neutron diffraction and thermal analysis shows that there is a clear crystallographic transformation near 103 K from a higher-temperature body-centered-cubic (bcc) phase which has considerable orientational disorder to a lower-temperature phase of different symmetry, which has relatively little disorder. The C and F atomic momentum profiles are found to have only small dependencies upon temperature and upon condensed phase. They are analyzed in terms of a dominant contribution from intramolecular vibrations plus contributions from translational and rotational motions of molecules. The nature of rotational contributions is of special interest, because of questions about the extent of static versus dynamic disorder in this kind of crystal. Brief comparisons to related solids such as SF6 are made.
Real time quantum correlation functions. II. Maximum entropy numerical analytic continuation of path integral Monte Carlo and centroid molecular dynamics data
Authors: Krilov Goran , Berne B. J.
Published: July 2002

We propose a method which uses centroid molecular dynamics (CMD) [J. Cao and G. A. Voth, J. Chem. Phys. 100, 5106 (1994)] real-time data in conjunction with the imaginary-time data generated using path integral Monte Carlo simulations in a numerical analytic continuation scheme based on the maximum entropy approach. We show that significant improvement is achieved by including short-time CMD data with the imaginary-time data. In particular, for a particle bilinearly coupled to a harmonic bath, these methods lead to significant improvements over previous calculations and even allow accurate determination of transport coefficients such as the diffusion coefficient and mobility for this system. In addition we show how maximum entropy method can be used to extract accurate dynamic information from short-time CMD data, and that this approach is superior to the direct Fourier transform of long-time data for systems characterized by broad, featureless spectral distributions.
Triality principle and G2 group in spinor language
Authors: Hasiewicz Z. , Kwasniewski A. K.
Published: July 2002

The presented paper is aimed at providing a systematic study of a relation between octonions and spinors corresponding to S7 7-sphere, starting from a natural point of view, enabling us to endow spinor space S+(8,0) with octonion algebra structures. As a result we arrive at formulations of triality principle in its finite form in terms of vector and fundamental representations of Spin (8) group—both for spinors and vectors. The group of automorphisms of octonion algebra, as well as its Lie algebra, gains clear interpretation in the context. The method proposed is purely algebraic and could be applied as well to 𝒞(4,4) Clifford algebra corresponding to 4S7 indefinite 7-sphere geometry.
Comment on a paper by Z-Z. Zhong [J. Math. Phys. 2
6, 404 (1985)]
Authors: Demys K.
Published: July 2002

The paper of Zhong [J. Math. Phys. 26, 404 (1985)], though very interesting, did contain errors of both scientific and historical fact that should be corrected for the record.
Scattering and complete integrability in conformally invariant nonlinear theories
Authors: Baez John C.
Published: July 2002

Conformally invariant nonlinear wave equations in four dimensions, corresponding to multicomponent massless scalar fields with a quartic interaction, are studied. It is proved that the scattering operator S on the space H of finite-Einstein-energy Cauchy data has infinitely many fixed points, as well as periodic points of all orders. There are also Φ∈H such that SnΦ is almost periodic, but not periodic and Φ∈H such that SnΦ is not almost periodic. It is also proved that H admits no conformally invariant Kähler metrics, but infinitely many distinct Kähler metrics invariant under the Poincaré group and scale transformations. Moreover, it is proved that time evolution for these nonlinear wave equations is completely integrable on the space H.
Comment on ‘‘Effects of neutral beam injection on poloidal rotation and energy transport in tokamaks’’ [Phys. Fluids B5, 3012 (1993)]
Authors: Stacey Weston M.
Published: July 2002

A comment on effects of neutral beam injection on poloidal rotation and energy transport in tokamaks plasma is presented. The enhanced neoclassical effects such as poloidal mass flow, viscous heating and the convective flow of main plasma ions which is inward for coinjection should be taken into account in the interpretation of experimental data. (AIP)
Electrostatic ion–ion two-streaming instability in a thermal-barrier cell
Authors: Thomas V. A. , Nevins William McCay
Published: July 2002

Some designs for tandem mirror devices rely on a thermal-barrier cell, a region of depressed potential located between the central cell and the end plugs. This region of depressed potential relies on, to some extent, an ion distribution function that has two peaks in vz (ion velocity parallel to the magnetic axis). Therefore there is a possibility that some ion–ion two-stream modes can be unstable. Using one-dimensional models, the stability of model equilibria to electrostatic ion two-stream modes is considered. The results suggest that for parameters of greatest interest (i.e., for planned experiments), these modes could be stable. Particle simulation techniques are used to examine the nonlinear consequences for those conditions where instability is present.
Effects of beam plasma instability on current drive via injection of an electron beam into a torus
Authors: Okuda H. , Horton R. , Ono M. , Wong K. L.
Published: July 2002

One- and two-dimensional particle simulations of beam–plasma interaction have been carried out in order to understand current drive experiments that use an electron beam injected into the Advanced Concepts Torus (ACT)-1 device [Rev. Sci. Instrum. 53, 409 (1982)]. Typically, the initial beam velocity along the magnetic field is V0=109 cm/sec, while the thermal velocity of the background electrons is vt=108 cm/sec. The ratio of the beam density to the background density is about 10% so a strong beam–plasma instability develops, causing rapid diffusion of beam particles. For both one- and two-dimensional simulations, it is found that a significant amount of beam and background electrons are accelerated considerably beyond the initial beam velocity when the beam density is more than a few percent of the background plasma density. In addition, the electron distribution along the magnetic field has a smooth negative slope, f′(v∥)<0, for v∥>0 extending to v∥=1.5 V0∼2 V0, which is in sharp contrast to the predictions from quasilinear theory. An estimate of the mean-free path for beam electrons caused by Coulomb collisions reveals that the beam electrons can propagate a much longer distance than is predicted from a quasilinear theory because of the presence of a high-energy tail. These simulation results agree well with the experimental observations from the ACT-1 device.
Separation of isotopes by oscillatory flow
Authors: Howell Gary W.
Published: July 2002

In 1978, Paiva [Nature 271, 434 (1978)] accomplished separation of gases by use of the ‘‘Taylor diffusion’’ which occurs in slow steady pipe flow. Results of Chatwin [J. Fluid Mech. 71, 513 (1975)], Watson [J. Fluid Mech. 133, 233 (1983)], and Kurzweg, Howell, and Jaeger [Phys. Fluids 27, 1046 (1984)] demonstrate that Taylor diffusion occurs also in the case of oscillatory pipe flow. A generalization of Paiva’s separation process to oscillatory flow is presented and possible advantages of such an implementation are explored.
Slow wave wigglers for compact free electron lasers
Authors: Riyopoulos Spilios
Published: July 2002

The use of slow waves excited in magnetron-type cavities as wigglers for compact free electron lasers (FEL) is analyzed. Because of group velocities much below the speed of light, slow waves offer an order of magnitude increase in FEL gain under given circulating power in the wiggler resonator, compared to fast wave wigglers of a similar period. In addition, the radiation emission involves the relativistic upshifting of the spatial harmonics of the wiggler period. That offers the versatility of operation, either at high beam energy, via upshifting of the fundamental wavelength, or at low beam energy, benefiting from the submillimeter wiggler harmonics. Because the main electron undulation is in the transverse direction for all spatial harmonics, the radiated power is increased by a factor γ2 relative to the Smith–Purcell approach that relies on axial electron undulation. Technical advantages offered by magnetron-type wiggles are the generation of the wiggler microwaves and the FEL interaction take place inside the same cavity, avoiding the issue of high-power coupling between cavities; the excitation of wiggler microwaves relies on distributed electron emission from the cavity wall and does not require separate beam injection.
High current ion source
Authors: Brown Ian G. , Galvin James E. , MacGill Robert A.
Published: July 2002

We describe a new kind of ion source using a metal vapor vacuum arc as the plasma formation mechanism. The source is simple and reliable and can produce long pulse intense ion beams from any solid electrically conducting material. Using a range of materials from lithium to uranium we have extracted low divergence beams with ion current up to 1 A.
‘‘Ballistic’’ injection devices in semiconductors
Authors: Levi A. F. J. , Hayes J. R. , Bhat R.
Published: July 2002

‘‘Ballistic’’ electron transistors are of considerable interest for high-frequency operation. Regardless of the mechanism of electron injection or collection it is anticipated that device performance will be dominated by base transit dynamics. We address this issue by calculating the scattering rate for hot electrons in selected semiconductor materials holding some common band structure and transport properties. It is shown that the scattering rate is critically dependent on the carrier concentration and that GaAs is not suitable for fabrication of traditional ‘‘ballistic’’ electron transistors. We suggest that semiconductors with small effective electron mass or a two-dimensional system would be more suitable.
Tryptophan zippers: Stable, monomeric β-hairpins
Authors: Cochran Andrea G. , Skelton Nicholas J. , Starovasnik Melissa A.
Published: July 2002

A structural motif, the tryptophan zipper (trpzip), greatly
stabilizes the β-hairpin conformation in short peptides. Peptides (12
or 16 aa in length) with four different turn sequences are monomeric
and fold cooperatively in water, as has been observed previously for
some hairpin peptides. However, the folding free energies of the
trpzips exceed substantially those of all previously reported
β-hairpins and even those of some larger designed proteins. NMR
structures of three of the trpzip peptides reveal exceptionally
well-defined β-hairpin conformations stabilized by cross-strand pairs
of indole rings. The trpzips are the smallest peptides to adopt an
unique tertiary fold without requiring metal binding, unusual amino
acids, or disulfide crosslinks.
Translocation events in the evolution of aminoacyl-tRNA synthetases.
Authors: Brenner S , Corrochano L M
Published: July 2002

We have characterized hisS, the gene encoding the histidyl-tRNA synthetase (HisRS) from the tetraodontoid fish Fugu rubripes. The hisS gene is about 3.5 kbp long and contains 13 exons and 12 introns of 172 bp, on average. The Fugu hisS gene encodes a putative protein of 519 amino acids with the three motifs identified as signatures of class 2 aminoacyl-tRNA synthetases. A model for the shifting of intron 8 between Fugu and hamster is proposed based on the successive appearance of a cryptic splicing site followed by an insertion mutation that created a new acceptor site. In addition, sequence comparisons suggest that the hisS gene has undergone a translocation through the first intron. As a result, the Fugu HisRS has an N-terminal sequence markedly different from that in the human and hamster enzymes. We propose that similar events have been responsible for variations at the N-terminal end of other aminoacyl-tRNA synthetases. Our analysis suggests that this involves exchanges through introns of two exons encoding an ancestral 32-amino acid motif.
p140/c-Abl that binds DNA is preferentially phosphorylated at tyrosine residues.
Authors: Dikstein R , Agami R , Heffetz D , Shaul Y
Published: July 2002

EP is a DNA element found in the enhancer and promoter regions of several cellular and viral genes. Previously, we have identified the DNA binding p140/c-Abl protein that specifically recognizes this element. Here we show that phosphorylation is essential for the p140/c-Abl DNA binding activity and for the formation of DNA-protein complexes. Furthermore, by 32P labeling of cells and protein purification, we demonstrate that in vivo the EP-DNA-associated p140/c-Abl is a tyrosine phosphoprotein. By employing two different c-Abl antibodies, we demonstrate the existence of two distinct c-Abl populations in cellular extracts. p140/c-Abl is quantitatively the minor population, is heavily phosphorylated at both serine and tyrosine residues, and is active in autophosphorylation reactions.
Slow cycling of unphosphorylated myosin is inhibited by calponin, thus keeping smooth muscle relaxed
Authors: Malmqvist Ulf , Trybus Kathleen M. , Yagi Shinobu , Carmichael Jeff , Fay Fredric S.
Published: July 2002

A key unanswered question in smooth muscle biology is whether phosphorylation of the myosin regulatory light chain (RLC) is sufficient for regulation of contraction, or if thin-filament-based regulatory systems also contribute to this process. To address this issue, the endogenous RLC was extracted from single smooth muscle cells and replaced with either a thiophosphorylated RLC or a mutant RLC (T18A/S19A) that cannot be phosphorylated by myosin light chain kinase. The actin-binding protein calponin was also extracted. Following photolysis of caged ATP, cells without calponin that contained a nonphosphorylatable RLC shortened at 30% of the velocity and produced 65% of the isometric force of cells reconstituted with the thiophosphorylated RLC. The contraction of cells reconstituted with nonphosphorylatable RLC was, however, specifically suppressed in cells that contained calponin. These results indicate that calponin is required to maintain cells in a relaxed state, and that in the absence of this inhibition, dephosphorylated cross-bridges can slowly cycle and generate force. These findings thus provide a possible framework for understanding the development of latch contraction, a widely studied but poorly understood feature of smooth muscle.
The increased level of β1,4-galactosyltransferase required for lactose biosynthesis is achieved in part by translational control
Authors: Charron Martin , Shaper Joel H. , Shaper Nancy L.
Published: July 2002

β1,4-Galactosyltransferase (β4GalT-I) participates in both glycoconjugate biosynthesis (ubiquitous activity) and lactose biosynthesis (mammary gland-specific activity). In somatic tissues, transcription of the mammalian β4GalT-I gene results in a 4.1-kb mRNA and a 3.9-kb mRNA as a consequence of initiation at two start sites separated by ≈200 bp. In the mammary gland, coincident with the increased β4GalT-I enzyme level (≈50-fold) required for lactose biosynthesis, there is a switch from the 4.1-kb start site to the preferential use of the 3.9-kb start site, which is governed by a stronger tissue-restricted promoter. The use of the 3.9-kb start site results in a β4GalT-I transcript in which the 5′- untranslated region (UTR) has been truncated from ≈175 nt to ≈28 nt. The 5′-UTR of the 4.1-kb transcript [UTR(4.1)] is predicted to contain extensive secondary structure, a feature previously shown to reduce translational efficiency of an mRNA. In contrast, the 5′-UTR of the 3.9-kb mRNA [UTR(3.9)] lacks extensive secondary structure; thus, this transcript is predicted to be more efficiently translated relative to the 4.1-kb mRNA. To test this prediction, constructs were assembled in which the respective 5′-UTRs were fused to the luciferase-coding sequence and enzyme levels were determined after translation
in vitro
and
in vivo
. The luciferase mRNA containing the truncated UTR(3.9) was translated more efficiently both
in vitro
(≈14-fold) and
in vivo
(3- to 5-fold) relative to the luciferase mRNA containing the UTR(4.1). Consequently, in addition to control at the transcriptional level, β4GalT-I enzyme levels are further augmented in the lactating mammary gland as a result of translational control.
Stripe phases in high-temperature superconductors
Authors: Emery V. J. , Kivelson S. A. , Tranquada J. M.
Published: July 2002

Stripe phases are predicted and observed to
occur in a class of strongly correlated materials describable as doped
antiferromagnets, of which the copper-oxide superconductors are the
most prominent representatives. The existence of stripe correlations
necessitates the development of new principles for describing charge
transport and especially superconductivity in these materials.
Context-dependent anticodon recognition by class I lysyl-tRNA synthetases
Authors: Söll Dieter , Becker Hubert D. , Plateau Pierre , Blanquet Sylvain , Ibba Michael
Published: July 2002

Lysyl-tRNA synthesis is catalyzed by two unrelated families of
aminoacyl-tRNA synthetases. In most bacteria and all eukarya, the known
lysyl-tRNA synthetases (LysRSs) are subclass IIb-type aminoacyl-tRNA
synthetases, whereas many archaea and a scattering of bacteria contain
an unrelated class I-type LysRS. Examination of the recognition of
partially modified tRNA
Lys
anticodon variants by a
bacterial (from
Borrelia burgdorferi
) and an archaeal
(from
Methanococcus maripaludis
) class I lysyl-tRNA
synthetase revealed differences in the pattern of anticodon recognition
between the two enzymes. U35 and U36 were both important for
recognition by the
B. burgdorferi
enzyme, whereas only
U36 played a role in recognition by
M. maripaludis
LysRS. Examination of the phylogenetic distribution of class I LysRSs
suggested a correlation between recognition of U35 and U36 and the
presence of asparaginyl-tRNA synthetase (AsnRS), which also recognizes
U35 and U36 in the anticodon of tRNA
Asn
. However, the class
II LysRS of
Helicobacter pylori
, an organism that lacks
AsnRS, also recognizes both U35 and U36, indicating that the presence
of AsnRS has solely influenced the phylogenetic distribution of class I
LysRSs. These data suggest that competition between unrelated
aminoacyl-tRNA synthetases for overlapping anticodon sequences is a
determinant of the phylogenetic distribution of extant synthetase
families. Such patterns of competition also provide a basis for the two
separate horizontal gene transfer events hypothesized in the evolution
of the class I lysyl-tRNA synthetases.
An orphaned mammalian β-globin gene of ancient evolutionary origin
Authors: Wheeler David , Hope Rory , Cooper Steven J. B. , Dolman Gaynor , Webb Graham C. , Bottema Cynthia D. K. , Gooley Andrew A. , Goodman Morris , Holland Robert A. B.
Published: July 2002

Mammals possess multiple, closely linked β-globin genes that
differ in the timing of their expression during development. These
genes have been thought to be derived from a single ancestral gene, by
duplication events that occurred after the separation of the mammals
and birds. We report the isolation and characterization of an atypical
β-like globin gene (ω-globin) in marsupials that appears to be more
closely related to avian β-globin genes than to other mammalian
β-globin genes, including those previously identified in marsupials.
Phylogenetic analyses indicate that ω-globin evolved from an ancient
gene duplication event that occurred before the divergence of mammals
and birds. Furthermore, we show that ω-globin is unlinked to the
previously characterized β-globin gene cluster of marsupials, making
this the first report of an orphaned β-like globin gene expressed in
a vertebrate.
Online peer review?
Authors: Wood Dee
Published: July 2002

Journal publishers are moving to electronic methods of marketing, production and distribution but peer review largely remains untouched by these new developments. This paper discusses the reasons for this and examines some possible ways forward.
Oblique projections in atomic spaces
Authors: Aldroubi Akram
Published: July 2002

LetH\mathcal {H}be a Hilbert space,O\mathbf {O}a unitary operator onH\mathcal {H}, and{ϕi}i=1,…,r.\{\phi ^i\}_{i=1,\dots ,r.}rrvectors inH\mathcal {H}. We construct anatomic subspaceU⊂HU \subset \mathcal {H}:<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="upper U equals left-brace sigma-summation Underscript i equals 1 comma ellipsis comma r Endscripts sigma-summation Underscript k element-of bold upper Z Endscripts c Superscript i Baseline left-parenthesis k right-parenthesis bold upper O Superscript k Baseline phi Superscript i Baseline colon c Superscript i Baseline element-of l squared comma for-all i equals 1 comma ellipsis comma r right-brace period">U={∑i=1,…,r∑k∈Zci(k)Okϕi:ci∈l2,∀i=1,…,r}.\begin{equation*} U=\left \{ { \sum \limits _{i=1,\dots ,r} {\sum \limits _{k\in \mathbf {Z}} {c^i(k)\mathbf {O}^k\phi ^i}:\;c^i\in l^2,\forall i=1,\dots ,r}} \right \}. \end{equation*}We give the necessary and sufficient conditions forUUto be a well-defined, closed subspace ofH\mathcal {H}with{Okϕi}i=1,…,r,k∈Z\left \{ {\mathbf {O}^k\phi ^i} \right \}_{i=1,\dots ,r, \;k\in \mathbf {Z}}as its Riesz basis. We then consider the oblique projectionPU⊥V\mathbf {P}_{{\scriptscriptstyle U\bot V}}on the spaceU(O,{ϕUi}i=1,…,r)U(\mathbf {O},\{\phi ^i_{\scriptscriptstyle U}\}_{i=1,\dots ,r})in a direction orthogonal toV(O,{ϕVi}i=1,…,r)V(\mathbf {O},\{\phi ^i_{\scriptscriptstyle V}\}_{i=1,\dots ,r}). We give the necessary and sufficient conditions onO,{ϕUi}i=1,…,r\mathbf {O},\{\phi ^i_{\scriptscriptstyle U}\}_{i=1,\dots ,r}, and{ϕVi}i=1,…,r\{\phi ^i_{\scriptscriptstyle V}\}_{i=1,\dots ,r}forPU⊥V\mathbf {P}_{{\scriptscriptstyle U\bot V}}to be well defined. The results can be used to construct biorthogonal multiwavelets in various spaces. They can also be used to generalize the Shannon-Whittaker theory on uniform sampling.
Global existence for a quasi-linear evolution equation with a non-convex energy
Authors: Feireisl Eduard , Petzeltová Hana
Published: July 2002

We establish the existence of global in time weak solutions to the initial-boundary value problem related to the dynamics of coherent solid-solid phase transitions in viscoelasticity. The class of the stored energy functionals includes the double well potential, and a general convolution damping term is considered.
Calculation of x‐ray transmission through a multileaf collimator
Authors: Chen Yan , Boyer Arthur L. , Ma C.‐M.
Published: July 2002

A ray tracing based method has been developed to calculate the x‐ray transmission through a multileaf collimator (MLC) for beam delivery verification and dose calculation in intensity modulated radiotherapy (IMRT). The path length of a ray line in the MLC is accurately calculated using the exact geometry of the MLC leaves. The fluence distribution of an IMRT field is calculated first using a point source. The fluence distribution for a realistic beam model is obtained, as an approximation, by convolving the point source fluence distribution with the distribution of source strength. Full ray tracing calculations are performed using analytic and Monte Carlo simulated beam models to verify the accuracy of the convolution method. The calculation is in better agreement with measurements using either film or a beam imaging system (BIS) than previous calculations for MLC transmission using a simplified model. This ray tracing calculation can be applied to the problem of verifying dynamic MLC leaf sequences as part of a patient‐specific quality assurance process for IMRT.
A patient‐specific Monte Carlo dose‐calculation method for photon beams
Authors: Wang Lu , Chui Chen‐Shou , Lovelock Michael
Published: July 2002

A patient‐specific, CT‐based, Monte Carlo dose‐calculation method for photon beams has been developed to correctly account for inhomogeneity in the patient. The method employs theEGS4system to sample the interaction of radiation in the medium. CT images are used to describe the patient geometry and to determine the density and atomic number in each voxel. The user code (MCPAT) provides the data describing the incident beams, and performs geometry checking and energy scoring in patient CT images. Several variance reduction techniques have been implemented to improve the computation efficiency. The method was verified with measured data and other calculations, both in homogeneous and inhomogeneous media. The method was also applied to a lung treatment, where significant differences in dose distributions, especially in the low‐density region, were observed when compared with the results using an equivalent pathlength method. Comparison of the DVHs showed that the Monte Carlo calculated plan predicted an underdose of nearly 20% to the target, while the maximum doses to the cord and the heart were increased by 25% and 33%, respectively. These results suggested that the Monte Carlo method may have an impact on treatment designs, and also that it can be used as a benchmark to assess the accuracy of other dose calculation algorithms. The computation time for the lung case employing five 15‐MV wedged beams, with an approximate field size ofand the dose grid size of 0.375 cm, was less than 14 h on a 175‐MHz computer with a standard deviation of 1.5% in the high‐dose region.
Voltaic cells for physicists: Two surface pumps and an internal resistance
Authors: Saslow Wayne M.
Published: July 2002

A proper discussion of voltaic cells—the basic elements of any battery—must invoke the fact that chemical reactions occur at each of the two electrode–electrolyte interfaces. The corresponding physical picture is that voltaic cells have two chemical-reaction-driven “ion pumps” and an internal resistance within the electrolyte. (These ion pumps can move electric charge; hence the historical terminology of “electromotive force,” or emf.) A correct physical picture for the “seat of emf” of a voltaic cell gives, not a “volume pump” (represented by a linear rise in voltage, as in many introductory physics textbooks), but two “surface pumps” (represented by two steplike jumps in voltage). Within this framework, some basic properties of voltaic cells are considered: their emf, maximum current, internal resistance, and energy storage. We also discuss some diffusion effects within voltaic cells, whose manifestations—but not origins—are well known when batteries are charged and discharged. Because of these diffusion effects, treating the electrolyte as an internal resistance is an oversimplification.