Regions Of Different Electron Density Research Articles

Determination of plasma parameters by spectral line broadening in an electrosurgical argon plasma

An electrosurgical argon plasma with a 5% admixture of molecular hydrogen is studied in order to investigate time averaged plasma parameters by optical emission spectroscopy (OES). Electron densities in the range of 1015–1016 cm−3 are determined from the Stark broadening of the time averaged line profiles of the Balmer-α and -β emission lines of hydrogen. A two-profile fit corresponding to regions of different electron densities is found to provide a better representation of the line broadening than a single profile fit. This is consistent with time resolved ICCD imaging, acquired with 150 ns time resolution, that shows strong radial gradients in the plasma emission and the asymmetry produced by the discharge arrangement. Gas temperatures are determined using two different methods. Firstly, simulated spectra for different rotational temperatures are fitted to the measured N2(C-B, 0-1) emission band originating from ambient air diffusion into the argon/hydrogen gas flow. From the best fit, rotational temperatures between 1500 K and 1800 K are inferred. These measurements are in good agreement with those inferred by the second method, which is based on the collisional broadening of the emission lines of neutral argon at 750 nm and 751 nm. This latter method may be useful for the measurement of gas temperatures when the device is used inside hollow organs during endoscopic or laparoscopic interventions, where air mixing will be limited. Therefore, the results of this study are highly relevant to applications of these devices, e.g. for controlling tissue effects and the avoidance of excessive heating.

On the determination of stopping cross-sections in ion scattering in solids and deviations from standard models

When atomic particles traverse solids they suffer energy losses due to elastic scattering from nuclei and excitation of electron–hole pairs. These are referred to as nuclear and electronic stoppings respectively. In this paper we discuss methods of determining energy losses and ‘stopping’ cross-sections in ion transmission through thin films, as well as in large angle and grazing backscattering from surfaces. This is done on the basis of deterministic simulations of ion scattering by following ion trajectories as they pass through the solid and sample regions of different electron densities depending on the distance from atomic nuclei. The ab initio calculated electron densities in the crystal are used to determine the stopping power, as predicted by the free electron gas model, and including a threshold value for d electron excitation. We discuss some aspects that are not included in standard descriptions based on the use of free electron models and averaged effective electron densities. In this context, we point out the possibility of inelastic processes involving inner-shell excitations, and briefly summarise main findings.

Sperm morphology of the leafhopper Diaphorina citri Kuwayama (Hemiptera: Sternorrhyncha: Psylloidea: Liviidae)

In this study, by using light and electron microscopy, we describe the sperm morphology of the leafhopper Diaphorina citri, a serious pest of citrus throughout the world. In this species the sperm measures 538.49±8.75μm in length, and as observed in psylloids, the sperm, when manipulated, opens into two filaments, one of which is attached to the nuclear base and the other becomes free. Along the flagellum, and only of it, there are lateral projections, about 2μm in length. Furthermore, at the end of the flagellum three appendages, with approximately 7μm in length, are observed. The head region is formed by the nucleus with compact chromatin, and, parallel to it, a structure of median electron density that extends about 25μm in length ahead of the nuclear tip. The flagellum consists of an axoneme with a 9+9+2 microtubule arrangement, two mitochondrial derivatives, and two accessory bodies each with two regions of different electron density. The presence of lateral projections is a characteristic observed in other Sternorrhyncha. As seen previously, the sperm opening in two filaments, when manipulated, was observed only in Psylloidea, and the presence of the three appendices at the end of the flagellum distinguishes D. citri from the other Psylloidea species studied.

Generation of shocks by the Biermann battery

The generation of magnetic field in shock surfaces separating regions of different electron density is a well known phenomenon. We study how this generation will affect the original structure of ionic flow. In a one‐dimensional geometry, it turns out that the leading magnetosonic wavefront produced by the seed field may be compressional, ultimately evolving into a shock in a finite time. The time where this shock occurs depends on few parameters: sound velocity, Alfvén velocity and the variation of the magnetic field at the original surface at time zero. The alternative is that the magnetosonic wave may stabilize or damp out, which always happens if we start from a null magnetic field.

Scattering of F atoms and anions on a TiO 2(1 1 0) surface

Results of a study of energy losses and electron transfer processes for grazing scattering of fluorine atoms and anions scattering along different azimuthal orientations of the TiO 2 crystal are presented. We observe strong variations in the overall intensity of scattered particles which are due to channelling effects. The energy losses do not show strong variations as a function of crystal azimuth except for the case of scattering along the (0 0 1) direction between the bridging oxygen atom rows, where we also observe differences in the energy losses of scattered ions and neutrals. We attribute this to the fact that larger F − survival occurs for trajectories staying farther from the surface, when also the energy losses remain small. The overall characteristics of energy losses are attributed mainly to trajectory effects due to scattering in regions of different electron density. Measurements of the ratio of scattered ions to the total scattered flux, i.e. the ion fractions which reflect electron capture and loss processes, show that these are not the same for incident anions and atoms. A strong difference for scattering along the (0 0 1) direction is observed, where at low incident energies a strong survival of incident ions occurs. These results are tentatively discussed in terms of non resonant electron capture at lattice O − sites and electron loss into the conduction band or by collisional detachment with bridging O atoms.

Complex melting of semi-crystalline chicory ( Cichorium intybus L.) root inulin

When concentrated solutions (30–45% by weight) of inulin (degree of polymerization 2–66, number average degree of polymerization 12) are cooled at 1 °C/min or 0.25 °C/min from 96 °C to 20 °C, suspensions of semi-crystalline material in water are formed. A thermal nucleation process was detected by optical microscopy: the 8-like shaped crystallites resulting from primary nucleation at higher temperature are larger than those resulting from secondary nucleation at lower temperature. Differential scanning calorimetry (DSC) thermograms display melting profiles with three to four partly overlapping endotherms that vary as a function of concentration, cooling rate during crystallization and storage time at 25 °C of the crystallite suspension. Recrystallization during melting was observed. The wide-angle X-ray scattering patterns of the samples at 25 °C correspond to those of the hydrated crystal polymorph. The structural changes during melting indicated the existence of a single crystal polymorph throughout melting. A periodicity of 95 Å, arising from alternating regions of different electron density, is detected in the small angle X-ray scattering patterns at 25 °C. The stepwise increase of the long period upon heating is related to the existence of two types of lamellar stacks: one with a higher long period, resulting from the primary nucleation and thus crystallized at high temperature, and a second one with a smaller long period, formed by crystallization at lower temperature. The lamellae formed at low temperature melt at a lower temperature than those formed at high temperature, explaining the existence of the two DSC-endotherms.

Chromatin condensation and acrosome development during spermiogenesis of Ensis ensis (Mollusca, Bivalvia).

We describe chromatin condensation and acrosome development during spermiogenesis of Ensis ensis. The overall shape of the mature spermatozoon corresponds to the primitive type. The nucleus is oval and on its superior pole there is an elongated acrosome; the middle piece contains four mitochondria around the centriolar complex. The condensation of the nuclei seems to occur in three steps: first the diameter of chromatin fibers increases slightly from 17 to 20 nm; second, in midspermatids fiber pairs coalesce; and third, the coalescence continues by addition of other fibers until the nuclei become highly compacted. Chromatin changes are related with nuclear protein composition. Small proacrosomal vesicles show two regions of different electron density. At a later stage they fuse to give a single, spherical vesicle in round spermatids, which migrates to the upper pole and transforms into a tapered acrosome (18 microns long) with a central channel filled with finely fibrous material.

Ballistic electron propagation in macroscopic four-terminal structures and its gate controlled transmission and reflection

Ballistic electron transport in macroscopic four-terminal devices with a high electron mobility is studied. The bend resistance characteristics are well analyzed using the Landauer-Büttiker formula although the leads are too wide to form well-defined one-dimensional subbands. The ballistic length determined from this analysis agrees well with the mean free path deduced from a mobility and electron density. Transmission and reflection of ballistic electrons are manipulated using a strip gate placed so as to bisect the device. The transmission and reflection probabilities are determined as a function of gate voltage, and related to the change in the refraction of ballistic electron propagation across the regions of different electron densities.

Reflection and refraction of ballistic electrons through different carrier concentration regions

Transmission and reflection characteristics of ballistic electrons are studied in a macroscopic four-terminal square device with a strip Schottky gate placed so as to bisect the device. Since the gate is negatively biased, a bend resistance peak at zero magnetic field, which is induced by ballistic electron injection into an opposite voltage probe, varies from positive to negative, and a magnetic focusing peak shifts to a lower field. These phenomena arise from the reflection of ballistic electrons crossing regions of different electron densities. The transmission and reflection probabilities are determined as a function of gate voltage, and are related to the change in the refractive index of ballistic electrons.

Determination of size and structure of lipid IVA vesicles by quasi-elastic light scattering and small-angle X-ray scattering

Heavy sonication of lipid IVA in different buffer solutions at pH values around 7.4 produces mostly unilamellar vesicles, which are stable over several weeks. For short sonication times these vesicles are very large, about 250 nanometers in terms of an average hydrodynamic radius with a slight polydispersity, as determined by quasi-elastic light scattering. Therefore small-angle X-ray scattering curves obtained with a conventional instrument do not contain enough information to determine the overall size of the lipid IVA aggregates. However, the pair distance distribution function (PDDF) of the scattering curve is in agreement with the assumption of large vesicular particles. The thickness of the double layer is very small compared to the overall dimension. It can be evaluated assuming a lamellar particle. The corresponding PDDF provides a thickness of about 5 nm. The electron density distribution within the double layer can be calculated directly from the PDDF by a convolution square root operation. Head groups and hydrocarbon chains are represented by two regions of different electron densities.

Tertiary and quaternary structural differences between two genetic variants of bovine casein by small-angle X-ray scattering

The casein complexes of bovine milk consist of four major protein fractions, α s1, α s2, β, and κ. Colloidal particles of casein (termed micelles) contain inorganic calcium and phosphate; they are very roughly spherical with an average radius of 650 Å. Removal of Ca 2+ leads to the formation of smaller protein aggregates (submicelles) with an average radius of 94 Å. Two genetic variants, A and B, of the predominant fraction, α s1 -casein, result in milks with markedly different physical properties, such as solubility and heat stability. To investigate the molecular basis for these differences, small-angle X-ray scattering was performed on the respective colloidal micelles and submicelles. Scattering curves for submicelles of both variants showed multiple Gaussian character; data for the B variant were previously interpreted in terms of two concentric regions of different electron density, i.e., a “compact” core and a relatively “loose” shell. For the submicelle of A, there was a third Gaussian, reflecting a negative contribution due to interparticle interference. Molecular parameters for submicelles of both A and B are in agreement with hydrodynamic data in the literature. Data for the micelles, for which scattering yields cross-sectional information, were fitted by a sum of three Gaussians for both variants; for these, the corresponding two lower radii of gyration represent the two concentric regions of the submicelles, while the third reflects the average packing of submicelles within the micellar cross section. Most of the molecular parameters obtained showed small but consistent differences between A and B, but for submicelles within the micelle several differences were particularly notable: A has a greater molecular weight for the “compact” region of the constituent submicelle (82,000 vs 60,000) and a much greater submicellar packing number (6:1 vs 3:1). Reasons for these and other differences are to be sought in sequence differences and in differences in calcium-binding sites and charge distribution.

Small-angle X-ray scattering investigation of the micellar and submicellar forms of bovine casein

SummarySmall-angle X-ray scattering was performed on whole casein under submicellar (Ca2+ removed) and micellar (Ca2+ re-added) conditions. Submicellar scattering curves showed two Gaussian components which were interpreted in terms of a spherical particle with two concentric regions of different electron density, a relatively compact core of higher electron density and a looser shell. Normalized scattering curves and calculated distance distribution functions were consistent with this picture. Micellar scattering data, which can yield only cross-sectional information related to a window of scattered intensities, could be analysed by a sum of three Gaussians with no residual function. The two Gaussians with the lower radii of gyration were again taken to indicate the two concentric regions of different electron density of inhomogeneous spherical particles; the third Gaussian was shown to reflect the packing number of these particles within a cross-sectional portion of the micelle, which was 3:1 for this system. These results are a strong indication that submicellar inhomogeneous particles containing hydrophobically stabilized inner cores exist within the colloidal micelle.

Determination of the quaternary structural states of bovine casein by small-angle X-ray scattering: Submicellar and micellar forms

Whole casein occurs in milk as a spherical colloidal complex of protein and salts called the casein micelle, with approximate average radii of 650 Å as determined by electron microscopy. Removal of Ca 2+ is thought to result in dissociation into smaller noncolloidal protein complexes called submicelles. Hydrodynamic and light scattering studies on whole casein submicelles suggest that they are predominantly spherical particles with a hydrophobic core. To investigate whether the integrity of a hydrophobically stabilized submicellar structure is preserved in the electrostatically stabilized colloidal micellar structure, small-angle X-ray scattering (SAXS) experiments were undertaken on whole casein from bovine milk under submicellar (without Ca 2+) and micellar (with added 10 m m CaCl 2) conditions. All SAXS results showed multiple Gaussian character and could be analyzed best by nonlinear regression in place of the customary Guinier plot. Analysis of the SAXS data for submicellar casein showed two Gaussian components which could be interpreted in terms of a particle with two concentric regions of different electron density, designated as a “compact” (subscript C) core and a “loose” (subscript L) shell, respectively. The submicelle was found to have an average molecular weight of 285,000 ± 14,600 and a mass fraction of higher electron density core, k, of 0.212 ± 0.028. The radius of gyration of the core, R c, was 37.98 ± 0.01 Å with an electron density difference, Δ ρ C, of 0.0148 ± 0.0014 e −/Å 3, while the loose region had values of R L = 88.2 ± 0.8 A ̊ with Δρ L = 0.0091 ± 0.0003 e −/ A ̊ 3 . Calculated distance distribution functions and normalized scattering curves also were consistent with an overall spherical particle with a concentric spherical inner core of higher electron density. These results, and in particular the remarkably low electron densities of the shells, can be interpreted in terms of a loosely packed spherical aggregate stabilized by a hydrophobic inner core and surrounded by an even more loosely packed hydrophilic region, in agreement with the results of other studies. The SAXS data for the colloidal micellar casein, which yield only cross-sectional information related to a window of scattered intensity, were analyzed by a sum of three Gaussians with no residual function. The two Gaussians with the lower values of the radius of gyration were interpreted again as an indication of an inhomogeneous spherical particle of two electron densities with the same centroid. The third Gaussian was shown to reflect the packing number of these particles, which was 3:1 for this system. The molecular weight determined from the two Gaussians of lower radii of gyration was in agreement with the value obtained from SAXS of submicellar casein, as were the k and Δ ρ C values. However, a lower value of Δ ρ L = 0.0065 ± 0.0003 was observed under these micellar conditions. These results are an indication of the existence of submicellar inhomogeneous particles containing a hydrophobically stabilized inner core within the colloidal micellar structure.

Mannan localization with concanavalin A in conjunction with electron microscopy and chemical analysis of differently prepared cell walls of the food protein yeast Candida spec. H

Regions of different electron densities after concanavalin A-peroxidase-reaction have been observed in cell walls of intact cells as well as in isolated cell walls: In the peripheral region of intact cell walls two mannan layers appear. Treating isolated cell wall with pronase, NaOH, ethylendiamine or citrate buffer, respectively, we find on A binding spots in the inner cell wall regions which appear electron transparent in the untreated cell walls. Apparently the mannan component of the inner cell wall regions is masked by proteins. In correspondence with chemical analysis our results cannot confirm the existence of inner cell wall regions which are free of mannan.