bluuebunny/crossref_metadata_2025_split huggingface dataset:
Role of adsorbate interactions in surface dynamics and phase transformations: Mean-field and quasichemical approximation approaches
Authors: Datar Asha , Prasad S. D. 
Published: July 2002

The significant influence of adsorbate interactions in surface dynamics is quantified using mean-field approximation (MFA) and quasichemical approximation (QCA) approaches and two typical situations (i) T≳Tc (critical temperature for surface phase transformation) and (ii) T&lt;Tc are analyzed. The formulation involves transition state theory (TST) and the key parameters involved are: (1) the sign and magnitude of the pairwise adsorbate interaction energy (w≳0, w&lt;0 meaning repulsive and attractive interactions, respectively) (2) wA♯, the interaction energy between a molecule in the ground state and the activated complex. wA♯A is in turn related to w by a coupling parameter σ. σ=0, σ=1 are shown to result in extreme divergence of the rate behavior for both repulsive and attractive interactions. First T≳Tc is considered. For σ=0, attractive interactions retard and repulsive interactions enhance the surface rates. The rates display nonmonotonic behavior for attractive interactions and steady increase with surface coverage for repulsive interactions. However, when σ=1, the rates monotonically increase for both types of forces. In addition the attractive forces show an instability of the slope due to a cooperative catalytic effect. Both attractive and repulsive forces display maxima when plotted against temperature, the maxima being sharper for the former case.
The case T&lt;Tc is more interesting, as a discontinuous phase separation can occur for attractive interactions. The density and internal energy differences between the coexisting phases are computed proceeding from closed-form expressions of the canonical ensemble partition functions and employing standard methods of statistical mechanics. Since repulsive forces can only show continuous order–disorder transitions, they are not considered for T&lt;Tc. The surface rate expressions (both corrected and uncorrected for ground-state internal energy differences between the phases) display a symmetric rate curve (symmetric about θ=0.5) vs surface coverage with a maximum at θ=0.5. A certain type of hole-particle symmetry is present in the rate expression as the rate expression is invariant with respect to the exchange of an occupied and vacant site. This conclusion is valid for both σ=0, σ=1. The appearance of symmetry in the rate curve is suggestive of the phase separation. The qualitative differences between the rate predictions of MFA and QCA are significant enough to warrant refinement in the analysis of surface dynamics.
Excess electron states on the microsurfaces of Ne and H2 clusters
Authors: Rosenblit Michael , Jortner Joshua 
Published: July 2002

In this paper we report on the ground and excited electronic states of localized excess electron surface states of (Ne)−N (N=1.1×104–6×1023) and (H2)−N (N=4.6×103–6×1023) clusters. We used an electron-cluster model potential, which consists of a short-range repulsive interaction with a strength V̄0 [with a lower limit V̄0 (≳0) corresponding to the energy of the quasifree electron in the macroscopic condensed material], and a long-range attractive polarization potential, to explore cluster size effects on the energetics and on the charge distribution of these excess electron clusters. The onset of the cluster size for excess electron localization in the ground (n=1, l=0) electronic state was inferred from a near-threshold scaling analysis, being characterized by a ‘‘critical’’ cluster radius R(1,0)c≂2(1−Q)a0/Q, where Q=(ε−1)/4(ε+1) is the effective cluster charge (for the cluster dielectric constant ε), R(1,0)c=39 Å for Ne(s), R(1,0)c=46 Å for Ne(l), R(1,0)c=35 Å for H2(s) and R(1,0)c=41 Å for H2(l), where (s) and (l) denote rigid and nonrigid cluster structures, respectively. With a further increase in the cluster radius R≳R(1,0)c, higher nl electronic states become localized.
Moving up in the cluster size above the localization threshold, the energy levels Enl can be expressed (for low values of ε≤1.5) in terms of a ‘‘universal’’ scaling relation Enl/Ef=Φnl(rf/R), where Ef=(e2/2a0)Q2, rf=a0/Q and the scaling function Φnl is independent of ε. This scaling relation allows for the determination of isotope effects and the state of aggregation of the cluster on the energetics of electron localization. In order to make contact with experiment, we have studied electric field-induced ionization and the electronic spectroscopy of these excess electron clusters. The threshold dc electric field F(nl)c for field-induced ionization from the n,l state (over a broad range of R, i.e., R&lt;320 Å for the 1s and 1p states and R&lt;900 Å for the 2p state) is of the form F(nl)c∝‖Enl‖5/4 (ε−1)−1/4R−3. Information on electronic spectroscopy was inferred from the cluster size dependence of the transition energies and oscillator strengths for the 1s(n=1,l=0)→n′p(n′=1,2,...,l=1) transitions. The cluster size dependence of the spectroscopic data for the 1s→1p transition reveals that both the transition energy ΔE(1s→1p) and the oscillator strength f(1s→1p) are proportional to (1/R)2, with ΔE(1s→1p)→0 and f(1s→1p)→0 for R→∞, exhibiting the l degeneracy of the flat surface. On the other hand, for the 1s→2p transition, the energy ΔE(1s→2p) and the oscillator strength f(1s→2p) increase with increasing R, reaching the flat macrosurface value for R→∞.
Single-particle analysis of the free-electron laser sideband instability for primary electromagnetic wave with constant phase and slowly varying phase
Authors: Davidson Ronald C. , Wurtele Jonathan S. 
Published: July 2002

Use is made of the single-particle orbit equations together with Maxwell’s equations and appropriate statistical averages to investigate detailed properties of the sideband instability for a helical-wiggler free-electron laser with wiggler wavelength λ0=2π/k0=const and normalized wiggler amplitude aw=eB̂w/mc2k0 =const. The model describes the nonlinear evolution of a right-circularly polarized primary electromagnetic wave with frequency ωs, wavenumber ks, and slowly varying amplitude âs(z,t) and phase δs(z,t) (eikonal approximation). The orbit and wave equations are analyzed in the ponderomotive frame (‘‘primed’’ variables) moving with velocity vp=ωs/(ks+k0) relative to the laboratory. Detailed properties of the sideband instability are investigated for small-amplitude perturbations about a quasisteady equilibrium state characterized by â0s =const (independent of z′ and t′). Two cases are treated. The first case assumes constant equilibrium wave phase δ0s =const, which requires (for self-consistency) both untrapped- and trapped-electron populations satisfying 〈∑j exp[ik′pzj0(t′) +iδ0s]/γ′j〉=0.
Here kp=(ks+k0)/γp is the wavenumber of the ponderomotive potential, z′j0(t′) is the equilibrium orbit, and γjmc2 is the electron energy. The second case assumes that all of the electrons are deeply trapped, which requires a slow spatial variation of the equilibrium wave phase, ∂δ0s/∂z′=2Γ0(Γ0ck0/ΩB)2 k′p≠0. The resulting dispersion relations and detailed stability properties are found to be quite different in the two cases. Both the weak-pump and strong-pump regimes are considered.
The evolution of a decaying spheromak
Authors: Sgro A. G. , Mirin A. A. , Marklin G. 
Published: July 2002

The evolution of a low beta spheromak initially in a two-dimensional stable equilibrium having a constant J/B (the so-called minimum energy state) is calculated within the context of resistive magnetohydrodynamics. Since the initial equilibrium is stable, the spheromak at first resistively evolves through a sequence of stable quasiequilibria. This phase of the evolution is calculated with a transport code in which the resistivity is assumed to be largest at the wall and lowest at the magnetic axis. Resistive diffusion causes the safety factor q to decrease everywhere while decreasing fastest near the wall. The quasiequilibria through which the spheromak evolves are tested for stability with both an ideal linear stability code and a resistive one. The results of both stability codes are in basic agreement and show that when q drops to below (1)/(2) everywhere the spheromak becomes unstable to an n=2 mode. The agreement of the stability codes implies that the unstable mode is a resistively modified ideal mode. The unstable equilibrium is used as the initial
condition in a 3-D nonlinear magnetohydrodynamic simulation. This simulation shows that after the unstable mode saturates, the spheromak resistively evolves through a sequence of three-dimensional quasiequilibria until it reaches another unstable configuration, after which it approaches the 2-D minimum energy state again. This evolutionary cycle can conceivably start again, unless the cycle time becomes comparable to the configuration decay time, which happens at high S. One consequence of the evolutionary cycle is that as the 3-D spheromak approaches the minimum energy state, the magnetic axis and hot plasma near it approach the wall and a new magnetic axis is formed. At high enough S, when the cycle time is comparable to the configuration lifetime, this convective heat loss mechanism is minimized. The 3-D code predicts that only the n=0 and n=2 modes are active. Simulations in which the n=1, 3, and 4 modes are present show that these modes decay while the n=2 grows and saturates. Closed flux surfaces are present over most of the domain so that heat loss along stochastic field lines should not be large. When the Hall term is included a toroidal rotation of the magnetic configuration is induced which most likely corresponds to an experimentally measured rotation. The results of this calculation are in qualitative agreement with various experimental data.
Sentence processing strategies in children with expressive and expressive‐receptive specific language impairments
Authors: Evans Julia L. , Whinney Brian Mac 
Published: July 2002

AbstractThe purpose of this study was to investigate the sentence comprehension strategies used by children with expressive and expressive‐receptive specific language impairments (SLI) within a language processing framework. Fourteen children with SLI (ages 6;10‐7;11) meeting strict selection criteria were compared to seven age‐matched and seven younger normal controls. Children were asked to determine the agent in sentences composed of two nouns and a verb (NVN, NNV, VNN) with animacy of the noun as a second factor. Results of group comparisons revealed that children with E‐SLI and ER‐SLI differed from each other in the comprehension strategies they employed as well as differing from both age‐matched and younger normal language control groups. Children with E‐SLI relied exclusively on a first noun as agent strategy across all conditions, whereas children with ER‐SLI used animacy cues when available. Additionally, maximum likelihood estimates were calculated to investigate individual patterns of performance under different cue conditions. Results revealed a significant correlation between severity of receptive language abilities and the type of strategy used, with better receptive language skills being highly correlated with children's use of word order cues.
How Mira Variables Change Visual Light by a Thousandfold
Authors: Reid M. J. , Peek J. E. G. 
Published: July 2002

Mira variables change visual light by up to 8 mag over their roughly yearly cycle. Here we present a simple explanation for the extremely large amplitudes of light curves of oxygen-rich Mira variables. Metallic oxides, such as TiO, form throughout the stellar atmosphere as the star cools when approaching minimum light. When this happens, the visual light can be almost completely absorbed at large radii, extending the visual photosphere to nearly twice its nominal size. At these large radii, temperatures can fall to ≈1400 K and essentially all of the star's radiation emerges in the infrared. Since almost no optical light is emitted at these low temperatures, Mira variables can decrease their visual light by more than a thousandfold and almost "disappear" to the human eye.
The measurement of the elastic constants of four alkali halides
Authors: S Hart  
Published: July 2002

Discrepancies of up to 10% exist in the published values of the elastic constants of alkali halides and these are too large to be accounted for by individual experimental errors. Static, resonance and ultrasonic techniques have been applied to the determination of the elastic constants of LiF, NaF, NaCl and KCl at room temperature. All the specimens of each material were taken from the same parent single crystal which was grown by the Stockbarger method.
Close agreement was obtained with the three methods, indicating that the discrepancies which do exist in the published data may be due to differences in the single crystals used by the various investigators.
Triggering of a pressurized spark gap by a laser beam
Authors: F Deutsch  
Published: July 2002

A delay line was discharged into a terminating resistor by a spark gap of coaxial design. The spark gap was triggered by a focused laser beam, introduced along the axis; a Q-switched ruby laser giving pulses of 20 ns duration and up to 50 MW power was used.
The range of operation of the gap, formative time of the breakdown and jitter were investigated for different gases at pressures above atmospheric, gap widths of 4-10 mm and voltages of up to 120 kv. Mixtures of argon and nitrogen were found to have certain advantages, such as a low threshold for ionization by the laser beam, sufficient dielectric strength, low values of the formative-time jitter and chemical inertness.
Formative times of down to about 1 ns and jitters below 1 ns were found. The laser power can be relatively low (0·5-5 MW). An explanation for the breakdown mechanism is proposed.
Precious metal alloys for high-temperature resistance strain gauges
Authors: R Bertodo  
Published: July 2002

Much work has been carried out over the years in an attempt to develop a gauge for the measurement of steady strains up to 1000°C. As a result satisfactory elements exist for use up to 300°C; over the range 300-1000°C alloys of Pt and W were shown to be suitable in 1963. Since that date a number of discrepancies have come to light.The present paper reviews earlier work on precious metals and their alloys and examines in detail the behaviour of some platinum metals and alloys exposed to oxidizing atmospheres for extended periods of time. Preliminary work aimed at obtaining more satisfactory alloys for use at extreme temperatures is outlined.It is concluded that the apparently anomalous behaviour of some samples of Pt-W is associated with the metallurgical state of the alloy. On the basis of preliminary test results an alloy of Pt, Pd and Mo is shown to be superior to Pt-W for steady strain element manufacture.
Condensation pumping of hydrogen and deuterium on to liquid-helium-cooled surfaces
Authors: J N Chubb  , L Gowland  , I E Pollard  
Published: July 2002

The condensation of hydrogen and deuterium on to liquid-helium-cooled surfaces is being studied for high-efficiency contamination-free pumping applications in plasma physics experiments.
The present experimental studies show that the sticking coefficient of hydrogen and deuterium at room temperature increases during initial coverage of a liquid-helium-cooled condensing surface from around 0·5 at about 1013 molecules cm−2 to between 0·7 and 0·95 at 1016 molecules cm−2, and then remains essentially independent of further coverage. The sticking coefficient tends to increase with increasing gas incidence rate, but is only susceptible to surface temperature at the higher end of the range 2·1-3·7°K for very low gas inflow rates.
During initial coverage of the condensing surface the desorption rate of gas molecules increases with surface coverage and reaches a steady value for coverages of about 3 × 1016 molecules cm−2. This desorption rate does not decrease as rapidly with surface temperature as expected from published hydrogen and deuterium vapour-pressure curves. Experiments show that this is due to thermal-radiation desorption effects. The application of condensation pumping in practice to maintain gas densities below about 5 × 108 molecules cm−3 (about 10 ntorr) thus requires some restriction of the thermal-radiation loading at the condensing surface.
The entry of aerosols into sampling tubes and heads
Authors: C N Davies  
Published: July 2002

No general solution exists of the very complicated problem of aerosol sampling. In this paper the problem is broken down into sampling with a thin-walled tube and sampling with a bulky head. Tube sampling can be classified into `small tubes', which give the same results independent of their orientation, and `large tubes' where there is a change of sampling efficiency when the direction of the tube is altered.
The limits of radius of a `small tube' sampling with an efficiency near to 100% are laid down in terms of particle size and suction speed; tables are given for evaluating the sampling efficiency in a wind. Calm air is defined by the wind speed being less than a value which is dependent on the sampling speed and particle size.
The sampling efficiency of a `large tube' may depend on its orientation owing to the fall of particles due to gravity or to a wind blowing past the tube. Isokinetic sampling is necessary to obtain correct samples in strong winds; the effect on efficiency of sampling due to departure from the isokinetic conditions is discussed.
The conditions are deduced for accurate sampling with a bulky suction head containing a small orifice: if the orifice faces downwards a critical suction speed exists below which no sample is obtained; at higher speeds sampling is selective and depends on the falling speed of the particles.
When a bulky head is used in a cross wind there is no effect due to wind when the orifice is in the top. For a downwards facing orifice the effect of wind is to cause sampling to start at a lower rate of suction and to be more efficient at lower rates of suction. At high rates of suction the wind has no effect.
The d.c. properties of an alkali plasma diode
Authors: M T C Fang  , D A Fraser  , J E Allen  
Published: July 2002

The theoretical d.c. current-voltage characteristics of a `collisionless' alkali plasma diode are derived. It is assumed that electrons and ions are created only at the two incandescent plane electrodes, the latter being at the same temperature. The electrons and ions are assumed to leave the electrodes with half-Maxwellian distributions of velocities. Some ions are trapped in the potential well between the electrodes.
It is found that the potential distribution is associated with a double sheath in front of the negative electrode, a plasma region of constant potential and an electron sheath at the positive electrode. The theoretical (I, V) characteristics are compared with experimental results obtained with a caesium plasma. At low applied voltages good agreement is obtained between theory and experiment; at higher voltages (where oscillations occur) the d.c. current falls to a lower value. Possible mechanisms for this drop are described.
Effect of coolant channel diameter on homogeneous and heterogeneous methods of reactor analysis in reflected subcritical assembly
Authors: M R Haroon  
Published: July 2002

The systematic variation of neutron flux distribution and buckling measurements as a function of fuel loading in a subcritical assembly (graphite, natural uranium) for two different coolant channel diameters have been investigated. The void ratio (volume of voids/lattice volume) in the two cases was 19% and 24%. The experimental results have been compared with theoretical predictions based on diffusion theory. For full assembly cases the results are in good agreement (as might be expected) with the predictions of Syrett's model for graphite moderated reactors. The correlation of 10 partially filled assembly cases was carried out in three ways:Syrett method lattice parameters with reflector savings calculated from one- and two-group theory for an infinite-slab system.Numerical solution of the two-group, two-region diffusion equations.Two-group heterogeneous (source sink) theory for finite systems.The results show that method (i) breaks down for all cases except those with the full assembly, method (ii) breaks down in nearly all cases and method (iii) holds well down to 36 fuel channels. The breakdown in method (i) and (ii) is most probably due to inaccurate streaming corrections to reactor systems of smaller size. In method (ii), excessive computer time is an additional complicating factor. If the experiments are analysed by heterogeneous methods it is possible to obtain accurate reactor para meters with a partially loaded exponential assembly in cases where only a limited amount of fuel is available.
Simultaneous measurements of the brightness and the energy distribution of electrons emitted from a triode gun
Authors: W H J Andersen  , A Mol  
Published: July 2002

The energy distribution of electrons in a part of the electron beam selected by two apertures situated on the axis was measured using a wienfilter-type energy analyser. These measurements gave brightness values for electrons in specific energy intervals of the distribution which fitted the theory extremely well. The influence of the measuring system on the energy distribution was also demonstrated and it indicated that the discrepancy between experiment and theory observed by Nottingham and Hutson was due to a crude approximation in the electron optics of the theory rather than a large electron reflection effect.The anomalous broadening of the energy distribution (Boersch effect) was found to be independent of the measuring system and had no influence on the brightness. Energy gains were also observed in the broadening process.
Magnetic anisotropy in single-crystal nickel films
Authors: E Secemski  , J C Anderson  
Published: July 2002

Single-crystal films of nickel have been deposited in ultra high vacuum under closely controlled conditions, with a resulting high standard of lattice perfection as determined by electron microscopy. Anisotropy measurements made by torque magnetometry yielded the following conclusions:K1has the same value as for bulk nickel (−5·0 × 104erg cm−3at room temperature). The magnetostrictive behaviour is predictable from bulk properties. An estimate of the order ofK3is also obtained (−3 × 103erg cm−3).Kucan be reduced to extremely low levels by taking steps to exclude all known mechanisms.The effect of stress on bothK1andKuhave been investigated and correlated. A constraint temperature (about 110°C), well below the substrate temperature during deposition, has been established.No contribution toKufrom mechanisms involving oxygen has been observed.The effects of lattice imperfections are quite marked, and can be the predominantKusource.
The effect of the neutron energy spectrum on neutron displacement damage in niobium
Authors: S Blow  
Published: July 2002

This paper presents an analysis of displacement damage produced in niobium, which has potential use as the first wall material of a fusion reactor.Lindhard's formalism was used together with cross sections taken from the UK nuclear data file on niobium. Data sets describing damage energy created at various neutron incident energies were generated. It was calculated that:The ratio of damage density (ie mean damage energy × cross section) at 14 MeV to that at 1 MeV is 3·5 to 1·00.The damage energy data sets were integrated over two different neutron spectra in order to compare the total effects. The spectra were taken from: (a) the first wall of a model fusion reactor, with a total flux of 2·8 × 1015neutrons/cm2s; (b) the core centre of the Dounreay Fast Reactor (DFR), total flux 2·5 × 1015neutrons/cm2s.Analysis showed that the number of atomic displacements per atom per year is165 displacements for spectrum (a);78 displacements for spectrum (b).