657 research outputs found
Recommended Thermal Rate Coefficients for the C + H Reaction and Some Astrochemical Implications
We have incorporated our experimentally derived thermal rate coefficients for
C + H forming CH and CH into a commonly used astrochemical
model. We find that the Arrhenius-Kooij equation typically used in chemical
models does not accurately fit our data and use instead a more versatile
fitting formula. At a temperature of 10 K and a density of 10 cm, we
find no significant differences in the predicted chemical abundances, but at
higher temperatures of 50, 100, and 300 K we find up to factor of 2 changes.
Additionally, we find that the relatively small error on our thermal rate
coefficients, , significantly reduces the uncertainties on the
predicted abundances compared to those obtained using the currently implemented
Langevin rate coefficient with its estimated factor of 2 uncertainty.Comment: 19 pages, 5 figures. Accepted for publication in Ap
Merged-beams Reaction Studies of O + H_3^+
We have measured the reaction of O + H3+ forming OH+ and H2O+. This is one of
the key gas-phase astrochemical processes initiating the formation of water
molecules in dense molecular clouds. For this work, we have used a novel merged
fast-beams apparatus which overlaps a beam of H3+ onto a beam of ground-term
neutral O. Here, we present cross section data for forming OH+ and H2O+ at
relative energies from \approx 3.5 meV to \approx 15.5 and 0.13 eV,
respectively. Measurements were performed for statistically populated O(3PJ) in
the ground term reacting with hot H3+ (with an internal temperature of \approx
2500-3000 K). From these data, we have derived rate coefficients for
translational temperatures from \approx 25 K to \approx 10^5 and 10^3 K,
respectively. Using state-of-the-art theoretical methods as a guide, we have
converted these results to a thermal rate coefficient for forming either OH+ or
H2O+, thereby accounting for the temperature dependence of the O fine-structure
levels. Our results are in good agreement with two independent flowing
afterglow measurements at a temperature of \approx 300 K, and with a
corresponding level of H3+ internal excitation. This good agreement strongly
suggests that the internal excitation of the H3+ does not play a significant
role in this reaction. The Langevin rate coefficient is in reasonable agreement
with the experimental results at 10 K but a factor of \approx 2 larger at 300
K. The two published classical trajectory studies using quantum mechanical
potential energy surfaces lie a factor of \approx 1.5 above our experimental
results over this 10-300 K range.Comment: 43 pages, 11 figures. Submitted to the Astrophysical Journa
Generation of neutral atomic beams utilizing photodetachment by high power diode laser stacks
We demonstrate the use of high power diode laser stacks to photodetach fast
hydrogen and carbon anions and produce ground term neutral atomic beams. We
achieve photodetachment efficiencies of 7.4\% for H at a beam energy
of 10\,keV and 3.7\% for C at 28\,keV. The diode laser systems used
here operate at 975\,nm and 808\,nm, respectively, and provide high continuous
power levels of up to 2\,kW, without the need of additional enhancements like
optical cavities. The alignment of the beams is straightforward and operation
at constant power levels is very stable, while maintenance is minimal. We
present a dedicated photodetachment setup that is suitable to efficiently
neutralize the majority of stable negative ions in the periodic table
Isotope effect for associative detachment: H(D)−+H(D)→H2(D2)+e
We report experimental and theoretical results for associative detachment (AD) of D−+D→D2+e−. We compare these data to our previously published results for H−+H→H2+e−. The measurements show no significant isotope effect in the total cross section. This is to be contrasted with previously published experimental and theoretical work which has found a significant isotope effect in diatomic systems for partial AD cross sections, i.e., as a function of the rotational and vibrational levels of the final molecule formed. Our work implies that though the rovibrational distribution of flux is different for AD of H− + H and D− + D, the total flux for these two systems is essentially the same when summed over all possible final channels
High mortality associated with tapeworm parasitism in geladas (Theropithecus gelada) in the Simien Mountains National Park, Ethiopia
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138266/1/ajp22684.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/138266/2/ajp22684_am.pd
Assignment of resonances in dissociative recombination of HD+ ions: high-resolution measurements compared with accurate computations
The collision-energy resolved rate coefficient for dissociative recombination
of HD+ ions in the vibrational ground state is measured using the photocathode
electron target at the heavy-ion storage ring TSR. Rydberg resonances
associated with ro-vibrational excitation of the HD+ core are scanned as a
function of the electron collision energy with an instrumental broadening below
1 meV in the low-energy limit. The measurement is compared to calculations
using multichannel quantum defect theory, accounting for rotational structure
and interactions and considering the six lowest rotational energy levels as
initial ionic states. Using thermal equilibrium level populations at 300 K to
approximate the experimental conditions, close correspondence between
calculated and measured structures is found up to the first vibrational
excitation threshold of the cations near 0.24 eV. Detailed assignments,
including naturally broadened and overlapping Rydberg resonances, are performed
for all structures up to 0.024 eV. Resonances from purely rotational excitation
of the ion core are found to have similar strengths as those involving
vibrational excitation. A dominant low-energy resonance is assigned to
contributions from excited rotational states only. The results indicate strong
modifications in the energy dependence of the dissociative recombination rate
coefficient through the rotational excitation of the parent ions, and underline
the need for studies with rotationally cold species to obtain results
reflecting low-temperature ionized media.Comment: 15 pages, 10 figures. Paper to appear in Phys. Rev. A (version as
accepted
Static and Dynamic Properties of a Viscous Silica Melt Molecular Dynamics Computer Simulations
We present the results of a large scale molecular dynamics computer
simulation in which we investigated the static and dynamic properties of a
silica melt in the temperature range in which the viscosity of the system
changes from O(10^-2) Poise to O(10^2) Poise. We show that even at temperatures
as high as 4000 K the structure of this system is very similar to the random
tetrahedral network found in silica at lower temperatures. The temperature
dependence of the concentration of the defects in this network shows an
Arrhenius law. From the partial structure factors we calculate the neutron
scattering function and find that it agrees very well with experimental neutron
scattering data. At low temperatures the temperature dependence of the
diffusion constants shows an Arrhenius law with activation energies which
are in very good agreement with the experimental values. With increasing
temperature we find that this dependence shows a cross-over to one which can be
described well by a power-law, D\propto (T-T_c)^gamma. The critical temperature
T_c is 3330 K and the exponent gamma is close to 2.1. Since we find a similar
cross-over in the viscosity we have evidence that the relaxation dynamics of
the system changes from a flow-like motion of the particles, as described by
the ideal version of mode-coupling theory, to a hopping like motion. We show
that such a change of the transport mechanism is also observed in the product
of the diffusion constant and the life time of a Si-O bond, or the space and
time dependence of the van Hove correlation functions.Comment: 30 pages of Latex, 14 figure
Removal of imidacloprid using activated carbon produced from ricinodendron heudelotii shells
In this study, Ricinodendron heudelotii (akpi) shells are used as precursor to prepare activated carbon via chemical activation using phosphoric acid. The characterization of the obtained activated carbon is performed using X-ray diffraction (XRD), Boehm titration method and adsorption of acetic acid. The results show that the prepared activated carbon has a microstructure and a higher specific surface area (1179 m2/g), suggesting that the acid treatment has a significant positive influence on its sorption properties. The maximum adsorption capacity and pollutant elimination efficiency are found to be 43.48 mg/g and 90%, respectively. These results suggest that this low cost agent is an efficient tool to remove organic pollutants especially imidacloprid from wastewater
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