Continuous Energy Distribution Research Articles

Influence ofppπions on pion absorption inH2

The existence of three-body resonances in the molecular ion $\mathrm{pp}\ensuremath{\pi}$ has been shown by means of a variational calculation in Jacobi coordinates. The relativistic and strong-force corrections to the Coulombic binding energies and the decay widths of the resonances have been calculated. We consider two mechanisms that supply the $p\ensuremath{\pi}$ atom with large kinetic energies before the moment of pion absorption by the proton, with significant impact on the interpretation of experiments measuring the mass difference ${m}_{{\ensuremath{\pi}}^{\ensuremath{-}}}\ensuremath{-}{m}_{{\ensuremath{\pi}}^{0}}$ from pion absorption in pionic atoms. These mechanisms are (a) the formation and subsequent Coulomb decay of the metastable $\mathrm{pp}\ensuremath{\pi}$ states and (b) pion absorption from the molecular $\mathrm{pp}\ensuremath{\pi}$ state, giving, respectively, a discrete or a continuous distribution of kinetic energies of the resulting $p\ensuremath{\pi}$ atoms.

Evidence of deep-level defects in an MQW electroabsorption modulator through current-voltage and electrical noise characterization

We present experimental and theoretical results concerning the current-voltage and electrical noise characteristics of an integrated electroabsorption modulator without illumination. Two components were studied: a device whose current-voltage and electrical noise characteristics were representative of the expected behavior and one exhibiting much higher leakage currents and different noise spectra. The first part of the paper contains a detailed description of the structure of the studied components. In the second part, the dark current of modulators is modeled using a thermal or phonon-assisted tunneling current process via electron traps thought to be localized at the multiquantum-well interfaces of the modulator's active layer. This model allows us to deduce the energy levels of the traps involved and demonstrates that the concentrations of the electron traps can vary by several orders of magnitude between the components. In the third part, measurements of electrical noise and the modeling of these measurements allow us to show that the current generation mechanisms involve deep levels of carriers with a continuous energy distribution which is of different width in the two components. In the fourth part, the dynamic chirp of the modulator is analyzed. The characteristic of the Henry modulator parameter (/spl alpha//sub H/) as a function of the applied voltage is given for each of these two components. The difference between the two curves can he attributed to deep levels of carriers.

Trap spectroscopy and tunnelling luminescence in feldspars

Comparative studies of tunnel luminescence, thermally stimulated luminescence (TSL), and trap energy spectra by the fractional glow technique (FGT) have been made both with sanidine and microcline feldspars. After X-irradiation at LNT these feldspars show intense tunnelling afterglow. As the temperature is raised, several TSL glow peaks may be observed, starting at 135K. An intense peak at 250K is common to all four investigated microclines. In samples of sanidine, tunnelling afterglow overcomes TSL up to 250–260K. With sanidine and microcline samples, the distributions of trap activation energies vs temperature obtained by the FGT are continuous and quasi-linear. Maxima in the trap density are observed, indicating the presence of several kinds of stable defect structures. It may be concluded that below 100K the thermally stimulated processes in the samples are frozen. Tunnelling recombination occurs between deeper localized states which are stable up to RT and higher. These results are attributed to the known ‘Al-Si order–disorder’ effect in feldspar crystals. It leads to the formation of wide continuous energy distributions of localized states. Donor–acceptor defects are caused by the association of Al3+ substituted in the centers of SiO4 tetrahedra and alkali ions in interstitial positions.

The Influence of the Temperature on the Parameters of Extended Freundlich's Isotherm

In this paper the expression for adsorption isotherm expressed by an asymptotic development, the extended Freundlich's isotherm, was applied to the study and interpretation of energetic heterogeneity. This expression presents the relative monolayer coverage of the entire surface as a dependence on adsorbate pressure (or concentration) and on parametersmandb0, which were identified with the Freundlich's parameters. The value ofmandb0were obtained by adjusting the cited expression to the experimental data for hydrogen adsorption on tungsten. Themparameter shows a quadratic dependence on temperature. Our analysis shows thatN(Q), the exponential form for the continuous energy distribution, is very dependent onm. A representative parameter is the dispersion ofN(Q), δ =RT/m. We interpret the dependence ofN(Q) on m as an indication of the energetic heterogeneity of this system. The parameterb0was adjusted by a van't Hoff expression, indicating a thermal process on the Langmuir adsorption sites.

Beta particle spectrometer for measuring aggregate beta spectra following fission

The beta spectrometer consists of a gated plastic scintillator suitable for measuring aggregate beta energy spectra following fission. This general utility spectrometer is very insensitive to the gamma rays accompanying the fission products, has good linearity, and adequate energy resolution for these continuous beta energy distributions. Response functions are measured below 1 MeV and trial sets of response functions are tested with known beta spectra extending to 5 MeV.

Kinetic energy study of the ion fragments produced by dissociative photoionization of nitric oxide

Abstract The kinetic energy distribution spectra of the ion fragments N+ and O+ produced by dissociative photoionization of NO and the dissociative photoionization efficiency curve for the ion fragments with specific kinetic energies are observed at photon energies from 19 to 60 eV. Three groups of N+ fragments are observed with their kinetic energies centered at 0, 0.32 and 1.1 eV, respectively. The two groups of energetic N+ fragments correspond to the predissociation of the c 3π and B′ 1∑+ states of NO+, respectively. These two states are found to predissociate primarily into N + ( 3 P) + O( 3 P) . Shape resonances are found for the c 3π and B′ 1∑+ states centered at about 28.7 and 29 eV respectively. The O+ fragments are found to have a continuous kinetic energy distribution concentrated in the region from 0 to 0.6 eV. The dissociations proceeded mainly through the vibrational levels of the ionic states of NO leading to N + ( 3 P) + O( 3 P) . By use of the undispersed synchrotron radiation, the O− fragment is found to have a continuous kinetic energy distribution concentrated from 0 to 2 eV.

XPS and SR-XPS techniques applied to sulphide mineral surfaces

X-ray photoelectron spectroscopy (XPS) is one of the major techniques in basic research of flotation-related surface studies of sulphide minerals. The advantages of the method, in general, are good surface sensitivity, rather straightforward elemental and chemical state analysis and reliable quantification of the data. The most serious disadvantage is the ex-situ nature of the technique. Surface sensitivity of the XPS technique can be considerably improved if an optimal excitation energy is chosen for a characteristic emission of each different element. This can be done by using a synchrotron radiation (SR) source, providing a continuous energy distribution over large energy region, instead of a fixed excitation energy source (either AlKα or MgKα radiation from sealed off X-ray tube) used in conventional XPS. In the case of identification of sulphur species, which often is the main task in surface analysis of sulphide surfaces, this leads to approxinnately an order of magnitude improvement in surface sensitivity and, hence, essentially better possibility to detect surface species of submonolayer coverage. Air oxidation of PbS and FeS2 has been studied with SR excitation. On PbS cleavage surface, oxidized sulphur (sulphate and/or polysulphide type) and lead species were detected after 10 min exposure time in air at room temperature, indicating congruent oxidation of PbS surface. The oxidation reactions proceeded relatively quickly during the first 4 h of contact with air. SR-XPS experiments with cleaved FeS2 also show the formation of sulphate as a primary oxidation product in air. However, the initial layer, formed immediately after cleavage, seemed to passivate the surface against further oxidation.

Short pulse plasma immersion ion implantation of oxygen into silicon: determination of the energy distribution

Plasma immersion ion implantation was used to implant oxygen ions into silicon with applied voltage pulses of 40 kV and 2.5 μs total length. Positive ions from the plasma, O 2 + and O +, with a continuous energy distribution between 0 and 40 keV were implanted with nominal doses between 2 × 10 16 and 2 × 10 17/cm 2. The resulting oxygen depth profiles were measured with elastic recoil detection analysis using 13.4 MeV α particles. The obtained depth profiles were simulated using a linear superposition of single-energy profiles calculated with TRIM, in order to determine relevant parameters of the accelerated ions. The energy distribution of the incident ions is derived from the results obtained and compared with theoretical models. The agreement found is very good. The plasma is found to be composed of 35(8)% O 2 + and 65(8)% O + ions. An Fe contamination in the plasma is observed using Rutherford backscattering.

Alpha-elastic recoil detection analysis of the energy distribution of oxygen ions implanted into silicon with plasma immersion ion implantation

Plasma immersion ion implantation was used to implant oxygen ions into silicon with applied voltage pulses of −40 kV and 2.5 μs length. Positive ions, O2+ and O+, with a continuous energy distribution between 0 and 40 keV were implanted. Between 3×104 and 3×105 pulses, corresponding to nominal doses from 2×1016 to 2×1017/cm2, were used. The resulting oxygen depth profiles were measured with elastic recoil detection analysis using 13.4 MeV α particles. Rutherford backscattering was used to determine possible co-implanted contaminants. The obtained depth profiles were simulated using a linear superposition of calculated single-energy profiles. The results obtained for the energy distribution of the incident ions are compared with calculations obtained from a theoretical model, and the agreement is very good. The incident flux is found to be composed of 34(5)% O2+ and 66(5)% O+ ions with an Fe contamination of ∼0.5%.

Comparison of computer generated and ERDA depth profiles of oxygen implanted into silicon with plasma immersion ion implantation

Plasma immersion ion implantation was used to implant oxygen into silicon with nominal doses ranging from 2 × 10 16 to 2 × 10 17 /cm 2. Positive ions, O 2 + and O +, with a continuous energy distribution between 0 and 40 keV were implanted. The plasma parameters and the geometrical configuration of the implantation were chosen in order to follow closely the assumptions of a theoretical model, which allowed to calculate the energy distribution of the incident ions. Computer simulations were performed to derive the resulting theoretical oxygen depth distribution. The agreement found with the experimental depth profiles, measured by elastic recoil detection analysis, is good. The small discrepancy observed is discussed in terms of the shortcomings of the model used.

Pure Monte Carlo Simulation of Model Heterogeneous Substrates: From Random Surfaces to Many-Site Correlations

A conceptually simple general method to simulate model heterogeneous surfaces is proposed. It allows one to distribute adsorption energies over a lattice of adsorption sites according to an arbitrary energy distribution which can include spatial site energy correlations. The method provides a powerful tool to simulate either discrete or continuous site energy distributions with any kind of spatial correlations between site energies which makes it useful to test theoretical model of surface processes against MC simulations on the model surface. Effect of correlations on equilibrium and kinetics of gas adsorption, thermal desorption, surface diffusion and reactions can be studied by MC simulation. More sophisticated model heterogeneous surfaces incorporating distributions of saddle point energies (consequently, activation barriers for diffusion) can also be treated. The method is the same as that used for evaluating the equilibrium states of an ensemble of interacting particles through Monte Carlo simulation.

Capacitance and conductance of semiconductor heterojunctions with continuous energy distribution of interface states

Electrical conduction in semiconductor heterojunctions with interface states presenting a continuous density of states distribution inside the gap is studied theoretically. The defect states are given with their density of states function and capture cross sections for transitions to the valence and conduction band. Conduction is considered under steady-state dc applied voltage V and under small-signal ac conditions. The formal developments are based on the resolution of Poisson equation and the continuity equations for electrons, holes, and occupied defect states. The numerical integration yields the position dependent dc and ac components of the concentrations of electrons, holes, occupied defect states, and of the current densities and recombination rates. For any value of x, one obtains the occupation function ft as function of the defect energy Et and the transition rates to the valence and conduction band. This allows a detailed analysis of the response of the interface state system to the external applied voltage. The main electrical characteristics obtained in the final analysis are the dc current–voltage and the ac capacitance–frequency and capacitance–voltage curves. The method is applied to a InGaAs/InP heterojunction with interface DOS functions being either constant inside the gap or having gaussian shapes with a given mean value and standard deviation. Discrete levels are treated as a particular case. The I(V) curve has an ideality factor n different from one, reaching the theoretical value of 2 for discrete midgap states. Comparison of C(f ) curves for different DOS functions allows us to show the appearance of characteristic features depending on the explicit form of the DOS function and different from those of the discrete level case. The C(f ) curves, computed for different values of applied voltage V, show cutoff frequencies whose values increase with V. It is shown that this is due to the fact that different defect states are involved in the dynamical response of the system. The low frequency C−2(V) curves are not linear, as for the ideal junction case, and present structures which are correlated with the slope of the interface charge density are represented as a function of applied voltage V.

96/06041 Modelling of a vibrating reactor boundary and calculation of the induced neutron noise

Neutron spectrum analyses of spallation neutrons are conducted in the accelerator-driven system (ADS) facility at the Kyoto University Critical Assembly (KUCA). High-energy protons (100 MeV) obtained from the fixed field alternating gradient accelerator are injected onto a tungsten target, whereby the spallation neutrons are generated. For neutronic characteristics of spallation neutrons, the reaction rates and the continuous energy distribution of spallation neutrons are measured by the foil activation method and by an organic liquid scintillator, respectively. Numerical calculations are executed by MCNPX with JENDL/HE-2007 and ENDF/B-VI libraries to evaluate the reaction rates of activation foils (bismuth and indium) set at the target and the continuous energy distribution of spallation neutrons set in front of the target. For the reaction rates by the foil activation method, the C/E values between the experiments and the calculations are found around a relative difference of 10%, except for some reactions. For continuous energy distribution by the organic liquid scintillator, the spallation neutrons are observed up to 45 MeV. From these results, the neutron spectrum information on the spallation neutrons generated at the target are attained successfully in injecting 100 MeV protons onto the tungsten target.

Electronic properties of defects introduced during low-energy He-ion bombardment of epitaxially grown n-GaAs

We have employed deep level transient spectroscopy to investigate the defects introduced by low-energy (5 keV) He-ion bombardment of epitaxially grown n-GaAs. This introduced four electron traps, EHe1–EHe4, with discrete energy levels identical to those of the Eα1–Eα4 defects introduced during high-energy (5.4 MeV) α-particle irradiation of the same n-GaAs, but in different relative concentrations. EHe3 with a level at Ec−0.35 eV exhibits the same metastability as Eα3. In addition, low-energy ion bombardment introduced a band of defects close to the GaAs surface with a continuous energy distribution.

COMPLETE STELLAR MODELS: SPECTRAL AND INTERIOR EVOLUTION OF MASSIVE STARS

ABSTRACT This thesis work presents the first "complete stellar models" for massive stars, which consistently treat the stellar interior, the atmosphere, and the stellar winds. This approach allows to simultaneously predict basic stellar parameters (luminosity, radii, temperatures), nucleosynthesis (abundances), as well as the detailed emergent spectrum through the relevant evolutionary phases (corresponding to OB, LBV and Wolf--Rayet stars). On the other hand, our modelling including the stellar winds also allows to study the influence of the outer layers on the stellar structure and evolution. Conceptually the thesis is divided in two main parts. In the first part we construct the first non-LTE line blanketed hydrodynamic models of spherically expanding atmospheres of hot stars. The entire domain from the optically thick photosphere out to the terminal velocity of the wind is treated. We discuss in detail the effects of line blanketing on the atmospheric structure and on the predicted spectrum. We study the influence of the hydrodynamic structure on the profiles of both photospheric and wind lines. Our results also show that for precise determinations of stellar parameters and abundances of hot luminous stars, the use of plane parallel models may lead to systematic errors. In the second part we develop the "complete stellar models" (CoStar). As a first application we study the main sequence (MS) interior and spectral evolution of massive stars at solar metallicity. The evolutionary tracks and the interior evolution are found to be basically unchanged by the realistic treatment of the outer layers. The main CoStar predictions presented and discussed for the MS are the following: 1. Ejected mass of the most important elements. Deposition of wind momentum and mechanical energy 2. Estimates of mass loss rates due to radiation pressure including multiple scattering and line overlap 3. Continuous spectral energy distribution (EUV to IR) and ionising fluxes 4. UBVRIJHKLMN photometric evolution 5. UV colours 6. Detailed metal line blanketed UV spectra 7. Non-LTE hydrogen and helium line spectra in the visible and IR, including theoretical K band spectra 8. Fit formulae to derive mass loss rates from H-alpha, P-alpha and B-alpha equivalent width measurements As a second application we present Wolf-Rayet (WR) models with spherically outflowing envelopes. We introduce a simple analytical method to describe the structure of a spherically expanding envelope with strong mass outflow. Independently of the wind parameters (wind density, opacity, velocity law) the interior structure and evolution of WR stars is found to be unaffected by the outer layers. On the other hand, the stellar parameters (radii, effective temperatures) depend on the wind structure. We derive subphotospheric radii (taucont ~10-20) for WNE and WC/WO stars with strong mass loss rates. With respect to wind-free stellar models the predicted radius increase may reach up to a factor of ~4 for the most luminous WNE or WC stars, improving the comparisons with observations. The stellar parameters obtained with the new treatment allow a better assignment of theoretical spectra to evolutionary tracks of evolved WR stars (WNE, WC). This provides the base for future studies of the spectral evolution of post main-sequence massive stars and their descendants. We also point out the possible importance of the iron opacity peak for the acceleration of WR winds in the optically thick part, which may be essential for the understanding of the dynamics of WR winds.

Effect of structural characteristics of water on the infrared absorption and raman spectra

An analysis of band contour in the IR absorption (overtone region) and Raman spectra of water suggests that the high-frequency components of spectral bands correspond to a single continuous distribution in energy of H-bonds. A local increase in the probability density function at high frequencies, which leads to an increase in the population of states with weak hydrogen bonds, is caused by the specific behavior of the angular dependence of the energy of hydrogen bonds between water molecules.

Effect of inversion on the grain boundary potential of a polycrystalline semiconductor

The potential barrier at a semiconductor grain boundary is studied considering an inversion layer at the interface between the two grains. The predicted values of the barrier height are found to be considerably different from those obtained using a conventional model based on depletion approximation. It is found that, once the inversion layer is formed, the potential barrier does not change appreciably with the density of the localized states, A critical value of the density of states above which an inversion layer may be formed at the interface is obtained. Both monoenergetic and continuous energy distribution of localized states are considered for the estimation of the potential barrier. Approximate expressions for grain boundary potential are derived for strongly inverted grains. The barrier height values obtained from approximate relations are found to be very close to those obtained numerically.

Reconstruction of Spectra obtained with PYTHEAS through Photometric Methods

PYTHEAS is an imaging spectrophotometer giving high spectral and spatial resolutions in joining together several optical concepts (Le Coarer et al 1992, and Georgelin et al. in these Proceedings). It will be shown in this paper that, contrary to other spectrographs using somewhat complex methods of data reduction, the PYTHEAS imaging spectrometer requires simple photometric methods in order to reconstruct the spectra of astrophysical objects.The CCD image of a continuous spectral energy distribution for a permanent gap between the layers of the Fabry-Perot consists in a series of channelled spectra: each elementary beam, spectrally filtered by the FP interferometer and sampled by a microlens in the frame, gives light to the whole surface of the grism which separates the various F-P orders by displaying them in a line on the detector. Each line or channelled spectrum consists of a series of Fabry pupils (spectral elements), each of them containing the luminous flux emitted on a certain wavelength by the object under investigation. After scanning the FP interferometer across its free spectral range, we obtain a series of shifted channelled spectra whose set provides us with a chart showing the photometric values of flux according to the wavelength. Consequently, some forms of calibration (continuum lamp, spectral lamp) allow the reconstruction of the spectra of the astrophysical object through simple photometric measures.

Continuum and emission line observations of Wolf-Rayet stars: single and “WR+abs” objects

The UV — IR properties of the continuous energy distribution of single WR stars in the Galaxy and LMC are reviewed. The observed, dereddened distributions may be approximated as a power law Fλ ∼ λ−α with a non-Rayleigh Jeans range of α directly associated with the emission of excess flux over the full wavelength range and across all spectral subtypes. The observational results are compared to published NLTE models, where we find an important discrepancy in the frequency distribution of α for Galactic WN stars. A dependency of emission line equivalent widths on the continuum flux at line center is discussed; the systematic offset of the absorption line (but otherwise presumed single) objects is examined, and the observed behavior is compared to models and explained with simple theory.

On the Sampling Depth of Total Electron Yield (Tey) Measurements

Photoabsorption of x-rays in solids causes primary and secondary photo and Auger electrons. The latter are from self absorption of characteristic radiations which have been produced in the solid by radiative de-excitation of photoionized atoms. Whereas the energy of the electrons at the origin is described by discrete values of a few eV up to nearly the energy of the photoabsorbed photons, we observe after the escape of the electrons from the solid a continuous energy distribution with a superposition of the original line spectrum. This is explained by inelastic collisions of the electrons in the course of their migration through the solid and the subsequent loss of kinetic energy. Since the electron range decreases with decreasing kinetic energy, only a relatively small amount of electrons is able to reach the surface and to escape from there.