387 research outputs found
Aqueous reactions of organic triplet excited states with atmospheric alkenes
Triplet excited states of organic matter are formed when colored organic
matter (i.e., brown carbon) absorbs light. While these âtripletsâ can be
important photooxidants in atmospheric drops and particles (e.g., they
rapidly oxidize phenols), very little is known about their reactivity toward
many classes of organic compounds in the atmosphere. Here we measure the
bimolecular rate constants of the triplet excited state of benzophenone
(3BPâ), a model species, with 17Â water-soluble
C3âC6 alkenes that have either been found in the
atmosphere or are reasonable surrogates for identified species. Measured rate
constants (kALK+3BPâ) vary by a factor of 30 and are in the
range of (0.24â7.5) Ă109 Mâ1 sâ1. Biogenic alkenes
found in the atmosphere â e.g., cis-3-hexen-1-ol, cis-3-hexenyl acetate, and
methyl jasmonate â react rapidly, with rate constants above 1Ă109 Mâ1 sâ1. Rate constants depend on alkene characteristics
such as the location of the double bond, stereochemistry, and alkyl
substitution on the double bond. There is a reasonable correlation between
kALK+3BPâ and the calculated one-electron oxidation potential
(OP) of the alkenes (R2=0.58); in contrast, rate constants are not
correlated with bond dissociation enthalpies, bond dissociation free
energies, or computed energy barriers for hydrogen abstraction. Using the OP
relationship, we estimate aqueous rate constants for a number of unsaturated
isoprene and limonene oxidation products with 3BPâ: values are in
the range of (0.080â1.7) Ă109 Mâ1 sâ1, with
generally faster values for limonene products. Rate constants with less
reactive triplets, which are probably more environmentally relevant, are
likely roughly 25 times slower. Using our predicted rate constants, along
with values for other reactions from the literature, we conclude that
triplets are probably minor oxidants for isoprene- and limonene-related
compounds in cloudy or foggy atmospheres, except in cases in which the triplets
are very reactive.</p
Density Dependent Hadron Field Theory
A fully covariant approach to a density dependent hadron field theory is
presented. The relation between in--medium NN interactions and
field--theoretical meson--nucleon vertices is discussed. The medium dependence
of nuclear interactions is described by a functional dependence of the
meson--nucleon vertices on the baryon field operators. As a consequence, the
Euler--Lagrange equations lead to baryon rearrangement self--energies which are
not obtained when only a parametric dependence of the vertices on the density
is assumed. It is shown that the approach is energy--momentum conserving and
thermodynamically consistent. Solutions of the field equations are studied in
the mean--field approximation. Descriptions of the medium dependence in terms
of the baryon scalar and vector density are investigated. Applications to
infinite nuclear matter and finite nuclei are discussed. Density dependent
coupling constants obtained from Dirac--Brueckner calculations with the Bonn
NN-potentials are used. Results from Hartree calculations for energy spectra,
binding energies and charge density distributions of , and
are presented. Comparisons to data strongly support the importance
of rearrangement in a relativistic density dependent field theory. Most
striking is the simultanuous improvement of charge radii, charge densities and
binding energies. The results indicate the appearance of a new "Coester line"
in the nuclear matter equation of state.Comment: 48 LateX pages, 12 Figures, figures and full paper are available as
postscript files by anonymous ftp at ftp://theorie.physik.uni-giessen.de/dd
Momentum-Dependent Mean Field Based Upon the Dirac-Brueckner Approach for Nuclear Matter
A momentum-dependent mean field potential, suitable for application in the
transport-model description of nucleus-nucleus collisions, is derived in a
microscopic way. The derivation is based upon the Bonn meson-exchange model for
the nucleon-nucleon interaction and the Dirac-Brueckner approach for nuclear
matter. The properties of the microscopic mean field are examined and compared
with phenomenological parametrizations which are commonly used in
transport-model calculations.Comment: 15 pages text (RevTex) and 4 figures (postscript in a separate
uuencoded file), UI-NTH-930
Kinetics of isothermal and non-isothermal precipitation in an Al-6at%Si alloy
A novel theory which describes the progress of a thermally activated reaction under isothermal and linear heating conditions is presented. It incorporates nucleation, growth and impingement and takes account of temperaturedependent solubility. The model generally fits very well to isothermal calorimetry and differential scanning calorimetry data on precipitation in an Al-6 at.% Si alloy. Analysis of the data shows that two processes occur in this precipitation reaction: growth of large Si particles and growth of pre-existing small nuclei. Determination of the sizes of Si precipitates by transmission electron microscopy indicates that interfacial energy contributions are small and have a negligible influence on solubilit
Meson-induced correlations of nucleons in nuclear Compton scattering
The non-resonant (seagull) contribution to the nuclear Compton amplitude at
low energies is strongly influenced by nucleon correlations arising from meson
exchange. We study this problem in a modified Fermi gas model, where nuclear
correlation functions are obtained with the help of perturbation theory. The
dependence of the mesonic seagull amplitude on the nuclear radius is
investigated and the influence of a realistic nuclear density on this amplitude
is dicussed. We found that different form factors appear for the static part
(proportional to the enhancement constant ) of the mesonic seagull
amplitude and for the parts, which contain the contribution from
electromagnetic polarizabilities.Comment: 15 pages, Latex, epsf.sty, 9 eps figures
Hartree Fock Calculations in the Density Matrix Expansion Approach
The density matrix expansion is used to derive a local energy density
functional for finite range interactions with a realistic meson exchange
structure. Exchange contributions are treated in a local momentum
approximation. A generalized Slater approximation is used for the density
matrix where an effective local Fermi momentum is chosen such that the next to
leading order off-diagonal term is canceled. Hartree-Fock equations are derived
incorporating the momentum structure of the underlying finite range
interaction. For applications a density dependent effective interaction is
determined from a G-matrix which is renormalized such that the saturation
properties of symmetric nuclear matter are reproduced. Intending applications
to systems far off stability special attention is paid to the low density
regime and asymmetric nuclear matter. Results are compared to predictions
obtained from Skyrme interactions. The ground state properties of stable nuclei
are well reproduced without further adjustments of parameters. The potential of
the approach is further exemplified in calculations for A=100...140 tin
isotopes. Rather extended neutron skins are found beyond 130Sn corresponding to
solid layers of neutron matter surrounding a core of normal composition.Comment: Revtex, 29 pages including 14 eps figures, using epsfig.st
Renormalization of the P- and T-odd nuclear potentials by the strong interaction and enhancement of P-odd effective field
Approximate analytical formulas for the self-consistent renormalization of
P,T-odd and P-odd weak nuclear potentials by the residual nucleon-nucleon
strong interaction are derived. The contact spin-flip nucleon-nucleon
interaction reduces the constant of the P,T-odd potential 1.5 times for the
proton and 1.8 times for the neutron. Renormalization of the P-odd potential is
caused by the velocity dependent spin-flip component of the strong interaction.
In the standard variant of -exchange, the conventional strength
values lead to anomalous enhancement of the P-odd potential. Moreover, the
-meson exchange contribution seems to be large enough to generate an
instability (pole) in the nuclear response to a weak potential.Comment: 5 pages, Revtex3, no figure
Relativistic Brueckner-Hartree-Fock calculations with explicit intermediate negative energy states
In a relativistic Brueckner-Hartree-Fock calculation we include explicit
negative-energy states in the two-body propagator. This is achieved by using
the Gross spectator-equation, modified by medium effects. Qualitatively our
results compare well with other RBHF calculations. In some details significant
differences occur, e.g, our equation of state is stiffer and the momentum
dependence of the self-energy components is stronger than found in a reference
calculation without intermediate negative energy states.Comment: 13 pages Revtex, 5 figures included seperatel
Highâresolution rock magnetic cyclostratigraphy in an Eocene flysch, Spanish Pyrenees
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95346/1/ggge1746.pd
Relativistic Contributions to Deuteron Photodisintegration in the Bethe-Salpeter Formalism
In plane wave one-body approximation the reaction of deuteron
photodisintegration is considered in the framework of the Bethe-Salpeter
formalism for two-nucleon system. Results are obtained for deuteron vertex
function, which is the solution of the homogeneous Bethe-Salpeter equation with
a multi-rank separable interaction kernel, with a given analytical form. A
comparison is presented with predictions of non-relativistic, quasipotential
approaches and the equal time approximation. It is shown that important
contributions come from the boost in the arguments of the initial state vertex
function and the boost on the relative energy in the one-particle propagator
due to recoil.Comment: 29 pages, 6 figure
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