Formation of Discharges between Glass Insulated Electrodes in Ne + 1% N2 Mixtures. II. Frequency Dependence and Discharge Modes

Discharges between glazed electrodes for p = 2.67 kPa Ne + 1% N₂ and pd = 1.3 kPa cm (p = gas pressure, d = electrode distance) are investigated. They are sustained by a sine wave voltage of variable frequency f. Measurements of the space time structure of the radiation emission provide information of the build up and quenching of discharges and of the structure of the fully developed ac-discharge. It is shown that the discharge types for the different values of frequency and amplitude of the sine wave sustaining voltage are dynamically developed Townsend and glow discharges. For f ≦ 4 kHz the optical structure and its temporal behaviour largely depends on the increment of the sustaining voltage. For f ≧ 20 kHz the axial structure of radiation emission is modified by the influence of consecutive discharges due to residual ions (f ≌ 20 kHz) and metastable Ne(³P₂)-atoms (f > 20 kHz) existing for several periods of sine wave. The Penning effect of metastable Ne-atoms with N₂ molecules then produces start electrons for each discharge in a sequence which improve the initial conditions for the breakdown. Relations between space time structure and breakdown voltage as a function of frequency f are discussed.     

Polymer chain in a quenched random medium: slow dynamics and ergodicity breaking

The Langevin dynamics of a self-interacting chain embedded in a quenched random medium is investigated by making use of the generating functional method and one-loop (Hartree) approximation. We have shown how this intrinsic disorder causes different dynamical regimes. Namely, within the Rouse characteristic time interval the anomalous diffusion shows up. The corresponding subdiffusional dynamical exponents have been explicitly calculated and thoroughly discussed. For the larger time interval the disorder drives the center of mass of the chain to a trap or frozen state provided that the Harris parameter, (Δ/b ^d)N ^{2 - νd}≥1, where Δ is a disorder strength, b is a Kuhnian segment length, N is a chain length and ν is the Flory exponent. We have derived the general equation for the non-ergodicity function f (p) which characterizes the amplitude of frozen Rouse modes with an index p = 2πj/N. The numerical solution of this equation has been implemented and shown that the different Rouse modes freeze up at the same critical disorder strength Δ _c ∼ N ^{- γ} where the exponent γ ≈ 0.25 and does not depend from the solvent quality. Received 17 December 2002 Published online 23 May 2003 RID="a" ID="a"e-mail: vilgis@mpip-mainz.mpg.de     

Floating double probe in non‐Maxwellian plasmas: Determination of the electron density and mean electron energy

A theoretical study of the floating double probe based on the Druyvesteyn theory is developed in the case of non‐Maxwellian electron energy distribution functions (EEDFs). It is used to calculate the EEDF in the electron energy range larger than –e(V_f ? V_p) from the I–V double probe characteristics. V_f and V_p are the floating and plasma potential, respectively. The analytical distribution function corresponding to the best fit of EEDF in the energy range larger than e(V_f ? V_p) allows the determination of the total electron density (n_e) and the mean electron energy (<?_e>). The method is detailed and tested in the case of a theoretical Maxwell–Boltzmann distribution function. It is applied for experiments that are performed in expanding microwave plasmas sustained in argon. Analytical EEDFs determined by this method are compared with those measured by means of single probes under the same experimental conditions. A good agreement is observed between single and double probe measurements. Results obtained under different experimental conditions are used to define the best conditions to obtain reliable results by means of the double probe technique.     

Effect of Configuration Mixing and Core Excitation on Predominantly (f 7/2)n States in the Lower fp Shell Nuclei

Shell model calculations in the lower fp shell region, (for ^44,45Ca and ⁴⁷Sc nuclei) have been performed, with different model spaces, to probe the effect of configuration mixing from the p^3/2, p^1/2 and f_5/2 orbitals on the predominantly (f_7/2)ⁿ states and the contribution arising from the excitation of the N = Z = 16 core. Our calculations indicate that excitation of nucleons across the N = Z = 16 magic shell closure do contribute significantly towards the wavefunctions of the observed level structures of ^44,45Ca nuclei. However the inclusion of these configurations did not result in a better agreement for the observed level structure of ⁴⁷Sc nulcei. A plaussible explaination for this phenomena could be attributed to the two-body matrix elements used and calls for a detailed micro-scopic calculations involving fundamental interactions substantiated by additional spectroscopic data such as lifetime measurements to have an unambigious understanding of the intrinsic configurations of nuclei in this region.     

Quark mass and the masses of Goldstone bosons

Based on the Dyson-Schwinger Equations (DSEs) of QCD in the “rainbow” approximation, the fully dressed quark propagator S _f (p) is investigated, and then an algebraic parametrization form of the propagator is obtained as a solution of the equations. The dressed quark amplitudes A _f and B _f which built up the fully dressed quark propagator, and the dynamical running masses M _f , which is defined by A _f and B _f for light quarks u, d and s, are calculated, respectively. Using the predicted current masses m _f , quark local vacuum condensates, and our predicted value of pion decay constant, the masses of Goldstone bosons K, π and η and their in-medium values are also evaluated. Our predictions fit to data and to many other different calculations quite well. The numerical results show that the mass of quark is dependent of its momentum p ². The fully dressed quark amplitudes A _f and B _f have correct behaviors and can be used for many purposes in our future researches on non-perturbative QCD.      

Longitudinal-transverse interference structure function in the reaction2H(e,e′ p)

The longitudinal-transverse interference structure functionf ₀₁ has been measured in deuterium electrodisintegration experiments at various values of the four-momentum transferQ ². Data taken at different laboratories and at roughly the same value ofQ ² are shown to be consistent. When compared to non-relativistic impulse-approximation calculations mostf ₀₁ data show an enhancement. A relativistic treatment of the reaction²H(e, e p) removes the discrepancy. More definite evidence for relativistic effects in deuterium electrodisintegration can be obtained at higher values of the recoil momentum. The experimental implications for such²H(e, e p) experiments are discussed as well.Dedicated to Prof. Werner Sandhas on the occasion of his 60th birthday     

Additional symmetry for the electronic shell in its ground state and many-electron effects

The additional symmetry for the properties related to the ground state of the atom is considered taking into account many-electron effects. Calculations of the I _4f, I _3d,I _2p,I _3p binding energies, 4f ^N-15d - 4f^N system differences and 2p, 3p electron affinities in the second order of perturbation theory and in the configuration interaction approximation have been performed for the ground configurations with one open shell. The analysis of separate many-electron corrections for these quantities and their variation along the sequences of atoms and ions shows that the main corrections maintain the considered symmetry. Received 18 January 1999 and Received in final form 17 July 1999     

On Different Approaches to Estimate the Mass Fractal Dimension of Coal Aggregates

Several methods to measure the structures of coal aggregates are compared. Loose and compact coal aggregates were generated through flocculation of ultrafine coal particles (mean volume diameter of 12 μm) under specific shearing conditions. Aggregate structure in terms of mass fractal dimension, D_f, was determined using various methods; namely 2D and 3D image analysis, interpretation of intensity patterns from small angle light scattering, changes in aggregation state through light obscuration, and settling behavior. In this study, the measured values of D_f ranged from 1.84–2.19 for coal aggregates with more open structures, and around 2.27–2.66 for the compact ones. All of these approaches could distinguish structural differences between aggregates, albeit with variation in D_f values estimated by the different techniques. The discrepancy in the absolute values for fractal dimension is due to the different physical properties measured by each approach, depending on the assumptions used to infer D_f from measurable parameters. In addition, image analysis and settling techniques are based on the examination of individual aggregates, such that a large number of data points are required to yield statistically representative estimations. Light scattering and obscuration measure the aggregates collectively to give average D_f values of the particulate systems; consequently ignoring any structural variation between the aggregates, and leaving possible small contaminations undetected (e.g. by dust particles or air bubbles). Appropriate utilization of a particular method is thus largely determined by system properties and required data quality.     

Sharp Estimates in Ruelle Theorems for Matrix Transfer Operators

A matrix coefficient transfer operator , on the space of -sections of an m-dimensional vector bundle over n-dimensional compact manifold is considered. The spectral radius of is estimated by\para; and the essential spectral radius by

Phase shift analysis of pion-4He scattering by optimal polynomial expansion

A novel method of phase shift analysis for the scattering of charged pion from⁴He nuclei is proposed. The nuclear amplitudef _N (ϑ) has been parametrized in terms of a polynomial expansion in a conformally mapped variable, which is obtained by optimally exploiting the analytic property off _N . The method exhibits a significant reduction in the number of free parameters required for the fixed energy phase shift analysis of the differential cross-section data. The nuclear amplitude thus constructed is then used to obtain phase shifts and inelastic parameters of all possible orders. Reliable values of the real and imaginary parts of the forward amplitude are also obtained.     

Measuring the optical parameters of thin films by p-polarized laser beams

The refractive index n_f, extinction coefficient k_f and thickness d_f of a dip coated film are measured by using p-polarized laser beams. A sample is oblique illuminated with a p-polarized laser beam, and then two reflected beams, from the front and back surfaces, are received with a detector. After measuring their intensity ratio versus the angle of incidence, it is convenient to obtain the parameters of the film by means of data fitting. The films of polymethyltriethoxy silane (PMTES), which were made on a BK-7 glass substrate by dip coating, were measured. The method is non-contact, non-destructive and has the advantages of simplicity of both equipment and understanding. It is also shown that the values measured by this method are coincident with those measured by ellipsometry.     

Block persistence

We define a block persistence probability p _l (t) as the probability that the order parameter integrated on a block of linear size l has never changed sign since the initial time in a phase-ordering process at finite temperature T<T _c . We argue that in the scaling limit of large blocks, where z is the growth exponent (), is the global (magnetization) persistence exponent and f(x) decays with the local (single spin) exponent for large x. This scaling is demonstrated at zero temperature for the diffusion equation and the large-n model, and generically it can be used to determine easily from simulations of coarsening models. We also argue that and the scaling function do not depend on temperature, leading to a definition of at finite temperature, whereas the local persistence probability decays exponentially due to thermal fluctuations. These ideas are applied to the study of persistence for conserved models. We illustrate our discussions by extensive numerical results. We also comment on the relation between this method and an alternative definition of at finite temperature recently introduced by Derrida [Phys. Rev. E 55, 3705 (1997)]. Received: 25 February 1998 / Revised: 24 July 1998 / Accepted: 27 July 1998     

Experimental investigations of fast-proton production in a picosecond laser plasma

Results of experimental investigations of fast-proton production in a laser plasma are presented for the case where the intensity of laser radiation at the targets is 2 × 10¹⁸ W/cm². Three processes of fast-proton acceleration in laser plasma are investigated: (1) the acceleration of protons from the front surface toward the laser pulse, (ii) the acceleration of protons from the front surface of the target toward its interior, and (iii) the acceleration of protons from the rear foil surface in the outward direction. The activation procedure and CR-39 tracker detectors featuring a set of various-thickness aluminum filters were used to record fast protons. It turned out that the proton-acceleration process is the most efficient in the case of proton acceleration from the rear foil surface in the outward direction. Experimental results revealed that about N _p = 10⁷ protons of energy in the region E _p > 1.9 MeV that are accelerated from the target surface toward a laser ray, N _p = 4× 10⁷ protons of energy in the region E _p > 1.9 MeV that are accelerated fromthe front surface of the target toward its interior, and N _p = 4×10⁸ protons of energy in the region E _p > 1.9 MeV that are accelerated from the rear foil surface in the outward direction are generated at a laser-radiation intensity of 2 × 10¹⁸ W/cm² at the surface of aluminum, copper, and titanium targets. Experimental investigations aimed at optimizing the process of proton acceleration from the rear surface of aluminum foils were performed by varying the foil thickness over the range between 1 and 100 μm. The results of these experiments showed that there is an optimum foil thickness of 10 μm, in which case protons of maximum energy 5 MeV are generated.     

Directed spiral site percolation on the square lattice

A new site percolation model, directed spiral percolation (DSP), under both directional and rotational (spiral) constraints is studied numerically on the square lattice. The critical percolation threshold p _c ≈ 0.655 is found between the directed and spiral percolation thresholds. Infinite percolation clusters are fractals of dimension d _f ≈ 1.733. The clusters generated are anisotropic. Due to the rotational constraint, the cluster growth is deviated from that expected due to the directional constraint. Connectivity lengths, one along the elongation of the cluster and the other perpendicular to it, diverge as p → p _c with different critical exponents. The clusters are less anisotropic than the directed percolation clusters. Different moments of the cluster size distribution P _s(p) show power law behaviour with | p - p _c| in the critical regime with appropriate critical exponents. The values of the critical exponents are estimated and found to be very different from those obtained in other percolation models. The proposed DSP model thus belongs to a new universality class. A scaling theory has been developed for the cluster related quantities. The critical exponents satisfy the scaling relations including the hyperscaling which is violated in directed percolation. A reasonable data collapse is observed in favour of the assumed scaling function form of P _s(p). The results obtained are in good agreement with other model calculations. Received 10 November 2002 / Received in final form 20 February 2003 Published online 23 May 2003 RID="a" ID="a"e-mail: santra@iitg.ernet.in     

Charge Radii of Argon Isotopes in the f7/2 Shell and Radii Systematics in the Ca-Region

Changes in mean square (ms) nuclear charge radii of Ar isotopes across the 1f_7/2 shell are studied by fast-beam collinear laser spectroscopy using an ultra-sensitive detection method based on optical pumping and state-selective collisional ionization. The new data set on Ar, in combination with the known charge radii of K, Ca and Ti in the ν1f_7/2 shell, offers an opportunity to obtain a more complete overview of nuclear radii trends around the proton shell closure Z = 20 and between the neutron shell closures N = 20 and N = 28.     

P -wave excited baryons from pion- and photo-induced hyperon production

We report evidence for N(1710)P ₁₁ , N(1875)P ₁₁ , N(1900)P ₁₃ , D \Delta(1600)P ₃₃ , D \Delta(1910)P ₃₁ , and D \Delta(1920)P ₃₃ , and find indications that N(1900)P ₁₃ might have a companion state at 1970MeV. The controversial D \Delta(1750)P ₃₁ is not seen. The evidence is derived from a study of data on pion- and photo-induced hyperon production, but other data are included as well. Most of the resonances reported here were found in the Karlsruhe-Helsinki (KH84) and the Carnegie-Mellon (CM) analyses but were challenged recently by the Data Analysis Center at GWU. Our analysis is constrained by the energy-independent p \pi N scattering amplitudes from either KH84 or GWU. The two p \pi N amplitudes from KH84 or GWU, respectively, lead to slightly different p \pi N branching ratios of contributing resonances but the debated resonances are required in both series of fits.     

Stilbene-Like Molecules as Fluorescent Probes Applied for Monitoring of Polymerization Process

2-(p-N,N-dimethylaminostyryl)benzoxazole (OS), 2-(p-N,N-dimethylaminostyryl)-benzothiazole (SS) and 2-(p-N,N-dimethylaminostyryl)naphtiazole (PS) were prepared and their absorption and fluorescence spectra were measured in various solvents at room temperature. On the basis of the solvatochromic behavior the ground state (μ_g) and excited state (μ_e) dipole moments of these pN,N-dimethylaminostyryl derivatives were evaluated. The dipole moments (μ_g and μ_e) were estimated from solvatochromic shifts of absorption and fluorescence spectra as function of the dielectric constant (ɛ) and refractive index (n) of applied solvents. The absorption spectra only slightly are affected by the solvent polarity in contrast to the fluorescence spectra that are highly solvatochromic and display a large Stokes shift. The analysis of the solvatochromic behavior of the fluorescence spectra as function of Δf (ɛ, n) revealed that the emission occurs from a high polarity excited state. The large dipole moment change along with the strongly red-shifted fluorescence, as the solvent polarity is increased, demonstrate the formation of an intramolecular charge transfer state (ICT). Compounds under the study were used as fluorescence probes for monitoring the kinetics of polymerization. The study on the changes in fluorescence intensity and spectroscopic shifts of studied compounds were carried out during thermally initiated polymerization of methyl methacrylate (MMA) and during photoinitiated polymerization of 2-ethyl-2-(hydroxymethyl)propane-1,3-diol triacrylate (TMPTA).     

空间网络上的随机游走

本文以一维均匀环为基础, 通过添加有限数量的长程连接构造出了一维有限能量约束下的空间网络, 环上任意节点i与j之间存在一条长程连接的概率满足p_ijα d_ij^-α (α≥ 0),其中d_ij为节点i与j之间的网格距离, 并且所有长程连接长度总和受到总能量∧=cN(c≥ 0)的约束, N为网络节点总数.通过研究该空间网络上的随机游走过程,存在最优幂指数α₀ 使得陷阱问题的平均首达时间最短.进一步研究发现,平均首达时间与网络规模N之间存在着幂律关系, 随着网络规模N和总能量∧的增加,最优幂指数α₀单调增加,并趋近最优值1.5.