76 research outputs found
Calibration of oxygen 1s ionization energies. Accurate energies for CO2, H2O, CO, and O2
Access to accurate reference data is a prerequisite in order to translate chemical shifts to an absolute scale for inner-shell ionization energies. Calibration standards for oxygen 1s (O 1s) ionization energies are less well established than, for instance, for carbon 1s. To improve upon this situation, adiabatic and vertical O 1s ionization energies for gaseous carbon dioxide (CO2) are critically reviewed and used to establish the most accurate values currently available: 541.085(17) and 541.253(17) eV, respectively. Combining these with new precise measurements of shifts in O 1s ionization energies for H2O, CO, and O2 allows us to establish equally accurate absolute ionization energies for these molecules as for CO2. The resulting adiabatic and vertical energies are 539.728(17) and 539.827(17) eV for H2O, 542.439(17) and 542.495(17) eV for CO, 543.285(17) and 543.294(17) eV for O2 (4Σ final state), and 544.338(17) and 544.423(17) eV for O2 (2Σ final state). It is proposed that O 1s in CO2 be adopted as a standard of higher precedence, and that H2O, CO, and O2 be used also. The O 1s ionization energies in these molecules occur in the range 540–543 eV. These proposed standards should provide optimal internal calibration for a wide range of oxygen-containing compounds.publishedVersio
Smoothed Particle Hydrodynamics and Magnetohydrodynamics
This paper presents an overview and introduction to Smoothed Particle
Hydrodynamics and Magnetohydrodynamics in theory and in practice. Firstly, we
give a basic grounding in the fundamentals of SPH, showing how the equations of
motion and energy can be self-consistently derived from the density estimate.
We then show how to interpret these equations using the basic SPH interpolation
formulae and highlight the subtle difference in approach between SPH and other
particle methods. In doing so, we also critique several `urban myths' regarding
SPH, in particular the idea that one can simply increase the `neighbour number'
more slowly than the total number of particles in order to obtain convergence.
We also discuss the origin of numerical instabilities such as the pairing and
tensile instabilities. Finally, we give practical advice on how to resolve
three of the main issues with SPMHD: removing the tensile instability,
formulating dissipative terms for MHD shocks and enforcing the divergence
constraint on the particles, and we give the current status of developments in
this area. Accompanying the paper is the first public release of the NDSPMHD
SPH code, a 1, 2 and 3 dimensional code designed as a testbed for SPH/SPMHD
algorithms that can be used to test many of the ideas and used to run all of
the numerical examples contained in the paper.Comment: 44 pages, 14 figures, accepted to special edition of J. Comp. Phys.
on "Computational Plasma Physics". The ndspmhd code is available for download
from http://users.monash.edu.au/~dprice/ndspmhd
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Electronic Properties of Chlorine, Methyl, and Chloromethyl as Substituents to the Ethylene Group-Viewed from the Core of Carbon
“Substituent effects” is an important and useful concept in organic chemistry. Although there are many approaches to parametrizing the electronic and steric effects of substituents, the physical basis for the parameters is often unclear. The purpose of the present work is to explore the properties of chemical shifts in carbon 1s energies as a well-defined basis for characterizing substituents to an ethylene C═C moiety. To this end, high-resolution carbon 1s photoelectron spectra of six chloro-substituted ethenes and seven chloro-substituted propenes have been measured in the gas phase. Site-specific adiabatic ionization energies have been determined from the spectra using theoretical ab initio calculations to predict the vibrational structures. For two molecules, 3-chloropropene and 2,3-dichloropropene, the spectral analyses give quantitative results for the conformer populations. The observed shifts have been analyzed in terms of initial-state (potential) and relaxation effects, and charge relaxation has also been analyzed by means of natural resonance theory. On the basis of core-level spectroscopy and models, chlorine, methyl, and chloromethyl have been characterized in terms of their effect on the carbon to which they are attached (α site) as well as the neighboring sp² carbon (β site). The derived spectroscopic substituent parameters are characterized by both inductive (electronegativity) effects and the ability of each substituent to engage in electron delocalization via the π system. Moreover, the adopted approach is extended to include substituent–substituent interaction parameters
Excitation and relaxation in atom-cluster collisions
Electronic and vibrational degrees of freedom in atom-cluster collisions are
treated simultaneously and self-consistently by combining time-dependent
density functional theory with classical molecular dynamics. The gradual change
of the excitation mechanisms (electronic and vibrational) as well as the
related relaxation phenomena (phase transitions and fragmentation) are studied
in a common framework as a function of the impact energy (eV...MeV). Cluster
"transparency" characterized by practically undisturbed atom-cluster
penetration is predicted to be an important reaction mechanism within a
particular window of impact energies.Comment: RevTeX (4 pages, 4 figures included with epsf
Cluster Magnetic Fields from Galactic Outflows
We performed cosmological, magneto-hydrodynamical simulations to follow the
evolution of magnetic fields in galaxy clusters, exploring the possibility that
the origin of the magnetic seed fields are galactic outflows during the
star-burst phase of galactic evolution. To do this we coupled a semi-analytical
model for magnetized galactic winds as suggested by \citet{2006MNRAS.370..319B}
to our cosmological simulation. We find that the strength and structure of
magnetic fields observed in galaxy clusters are well reproduced for a wide
range of model parameters for the magnetized, galactic winds and do only weakly
depend on the exact magnetic structure within the assumed galactic outflows.
Although the evolution of a primordial magnetic seed field shows no significant
differences to that of galaxy clusters fields from previous studies, we find
that the magnetic field pollution in the diffuse medium within filaments is
below the level predicted by scenarios with pure primordial magnetic seed
field. We therefore conclude that magnetized galactic outflows and their
subsequent evolution within the intra-cluster medium can fully account for the
observed magnetic fields in galaxy clusters. Our findings also suggest that
measuring cosmological magnetic fields in low-density environments such as
filaments is much more useful than observing cluster magnetic fields to infer
their possible origin.Comment: Minor revision for publication in MNRA
Cosmological Simulations using GCMHD+
Radio observations of galaxy clusters show that the intra cluster medium is
permeated by \mu G magnetic fields. The origin and evolution of these
cosmological magnetic fields is currently not well understood and so their
impact on the dynamics of structure formation is not known. Numerical
simulations are required to gain a greater understanding and produce
predictions for the next generation of radio telescopes. We present the
galactic chemodynamics smoothed particle magnetohydrodynamic (SPMHD) code
(GCMHD+), which is an MHD implementation for the cosmological smoothed particle
hydrodynamic code GCD+. The results of 1, 2 and 3 dimensional tests are
presented and the performance of the code is shown relative to the ATHENA grid
code. GCMHD+ shows good agreement with the reference solutions produced by
ATHENA. The code is then used to simulate the formation of a galaxy cluster
with a simple primordial magnetic field embedded in the gas. A homogeneous seed
field of 10^-11 G is amplified by a factor of 10^3 during the formation of the
cluster. The results show good agreement with the profiles found in other
magnetic cluster simulations of similar resolution.Comment: 20 pages, 31 figures, accepted for publication in MNRA
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