223 research outputs found
Multilayer Graphene Synthesized by CVD Using Liquid Hexane as the Carbon Precursor
We produce multilayer graphene by the Chemical Vapor Deposition (CVD) method
at atmospheric pressure and 1000 {\deg}C, using flexible copper substrates as
catalyst and liquid hexane as the source of carbon. We designed an optical
device to measure the transmittance of the carbon films; with this information
we calculate that the approximate number of layers is 11.Comment: 6 pages, 5 figures. Accepted for publication in World Journal of
Condensed Matter Physic
Updated opacities from the opacity project
Using the code autostructure, extensive calculations of inner-shell atomic data have been made for the chemical elements He, C, N, O, Ne, Na, Mg, Al, Si, S, Ar, Ca, Cr, Mn, Fe and Ni. The results are used to obtain updated opacities from the Opacity Project (OP). A number of other improvements on earlier work have also been included. Rosseland-mean opacities from the OP are compared with those from OPAL. Differences of 5-10 per cent occur. The OP gives the 'Z-bump', at log(T) 5.2, to be shifted to slightly higher temperatures. The opacities from the OP, as functions of temperature and density, are smoother than those from OPAL. The accuracy of the integrations used to obtain mean opacities can depend on the frequency mesh used. Tests involving variation of the numbers of frequency points show that for typical chemical mixtures the OP integrations are numerically correct to within 0.1 per cent. The accuracy of the interpolations used to obtain mean opacities for any required values of temperature and density depends on the temperature-density meshes used. Extensive tests show that, for all cases of practical interest, the OP interpolations give results correct to better than 1 per cent. Prior to a number of recent investigations which have indicated a need for downward revisions in the solar abundances of oxygen and other elements, there was good agreement between properties of the Sun deduced from helioseismology and from stellar evolution models calculated using OPAL opacities. The revisions destroy that agreement. In a recent paper, Bahcall et al. argue that the agreement would be restored if opacities for the regions of the Sun with 2 × 106T 5 × 106 K (0.7-0.4 R) were larger than those given by OPAL by about 10 per cent. In the region concerned, the present results from the OP do not differ from those of OPAL by more than 2.5 per cent
Atomic data for the K-vacancy states of Fe XXIV
As part of a project to compute improved atomic data for the spectral
modeling of iron K lines, we report extensive calculations and comparisons of
atomic data for K-vacancy states in Fe XXIV. The data sets include: (i) energy
levels, line wavelengths, radiative and Auger rates; (ii) inner-shell electron
impact excitation rates and (iii) fine structure inner-shell photoionization
cross sections. The calculations of energy levels and radiative and Auger rates
have involved a detailed study of orbital representations, core relaxation,
configuration interaction, relativistic corrections, cancellation effects and
semi-empirical corrections. It is shown that a formal treatment of the Breit
interaction is essential to render the important magnetic correlations that
take part in the decay pathways of this ion. As a result, the accuracy of the
present A-values is firmly ranked at better than 10% while that of the Auger
rates at only 15%. The calculations of collisional excitation and
photoionization cross sections take into account the effects of radiation and
spectator Auger dampings. In the former, these effects cause significant
attenuation of resonances leading to a good agreement with a simpler method
where resonances are excluded. In the latter, resonances converging to the K
threshold display symmetric profiles of constant width that causes edge
smearing.Comment: 18 pages, 8 figures, submitted to Astronomy & Astrophysics 200
Decay Properties of K-Vacancy States in Fe X-Fe XVII
We report extensive calculations of the decay properties of fine-structure
K-vacancy levels in Fe X-Fe XVII. A large set of level energies, wavelengths,
radiative and Auger rates, and fluorescence yields has been computed using
three different standard atomic codes, namely Cowan's HFR, AUTOSTRUCTURE and
the Breit-Pauli R-matrix package. This multi-code approach is used to the study
the effects of core relaxation, configuration interaction and the Breit
interaction, and enables the estimate of statistical accuracy ratings. The
K-alpha and KLL Auger widths have been found to be nearly independent of both
the outer-electron configuration and electron occupancy keeping a constant
ratio of 1.53+/-0.06. By comparing with previous theoretical and measured
wavelengths, the accuracy of the present set is determined to be within 2 mA.
Also, the good agreement found between the different radiative and Auger data
sets that have been computed allow us to propose with confidence an accuracy
rating of 20% for the line fluorescence yields greater than 0.01. Emission and
absorption spectral features are predicted finding good correlation with
measurements in both laboratory and astrophysical plasmas.Comment: 13 pages, 4 figures. Submitted to A&A. Electronic Table 3-4 available
at http://lheawww.gsfc.nasa.gov/users/palmeri/patrick.htm
Photoionization Modeling and the K Lines of Iron
We calculate the efficiency of iron K line emission and iron K absorption in
photoionized models using a new set of atomic data. These data are more
comprehensive than those previously applied to the modeling of iron K lines
from photoionized gases, and allow us to systematically examine the behavior of
the properties of line emission and absorption as a function of the ionization
parameter, density and column density of model constant density clouds. We show
that, for example, the net fluorescence yield for the highly charged ions is
sensitive to the level population distribution produced by photoionization, and
these yields are generally smaller than those predicted assuming the population
is according to statistical weight. We demonstrate that the effects of the many
strongly damped resonances below the K ionization thresholds conspire to smear
the edge, thereby potentially affecting the astrophysical interpretation of
absorption features in the 7-9 keV energy band. We show that the centroid of
the ensemble of K lines, the K energy, and the ratio of the
K to K components are all diagnostics of the ionization
parameter of our model slabsComment: 38 pages, submitted to Ap.J. Sup
Radiative and Auger decay data for modelling nickel K lines
Radiative and Auger decay data have been calculated for modelling the K lines
in ions of the nickel isonuclear sequence, from Ni up to Ni. Level
energies, transition wavelengths, radiative transition probabilities, and
radiative and Auger widths have been determined using Cowan's Hartree--Fock
with Relativistic corrections (HFR) method. Auger widths for the third-row ions
(Ni--Ni) have been computed using single-configuration average
(SCA) compact formulae. Results are compared with data sets computed with the
AUTOSTRUCTURE and MCDF atomic structure codes and with available experimental
and theoretical values, mainly in highly ionized ions and in the solid state.Comment: submitted to ApJS. 42 pages. 12 figure
K-shell photoionization of Nickel ions using R-matrix
We present R-matrix calculations of photoabsorption and photoionization cross
sections across the K edge of the Li-like to Ca-like ions stages of Ni.
Level-resolved, Breit-Pauli calculations were performed for the Li-like to
Na-like stages. Term-resolved calculations, which include the mass-velocity and
Darwin relativistic corrections, were performed for the Mg-like to Ca-like ion
stages. This data set is extended up to Fe-like Ni using the distorted wave
approximation as implemented by AUTOSTRUCTURE. The R-matrix calculations
include the effects of radiative and Auger dampings by means of an optical
potential. The damping processes affect the absorption resonances converging to
the K thresholds causing them to display symmetric profiles of constant width
that smear the otherwise sharp edge at the K-shell photoionization threshold.
These data are important for the modeling of features found in photoionized
plasmas.Comment: 23 pages, 6 figures. Accepted in ApJS
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