843 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
Evidence for two-electron processes in the mutual neutralization of O- with O+ and N+ at Subthermal Collision Energies
We have measured total absolute cross sections for the Mutual Neutralization
(MN) of O- with O+/N+. A fine resolution (of about 50 meV) in the kinetic
energy spectra of the product neutral atoms allows unique identification of the
atomic states participating in the mutual neutralization process. Cross
sections and branching ratios have also been calculated down to 1 meV
center-of-mass collision energy for these two systems with a multi-channel
Landau-Zener model and an asymptotic method for the ionic-covalent coupling
matrix elements. The importance of two-electron processes in one-electron
transfer is demonstrated by the dominant contribution of a core-excited
configuration of the nitrogen atom in N+ + O- collisions. This effect is
partially accounted for by introducing configuration mixing in the evaluation
of coupling matrix elements.Comment: 5 pages, 4 figure
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A novel merged beam apparatus to study the cosmic origins of organic chemistry
Reactions of atomic carbon with molecular ions play a critical role for gas phase molecular formation in interstellar clouds. These interactions are the first links in the chain of chemical reactions leading to the synthesis of complex organic species. Much of our knowledge of this process is through spectroscopic observations and theoretical models. However, our understanding of this chemistry is constrained by uncertainties in the underlying reaction rate coefficients. Data from quantum calculations are limited to reactions involving three or fewer atoms. Meanwhile, previous experimental studies have been hampered by the difficulty in generating a sufficiently intense and well characterized neutral carbon beam. To address these issues and to study these reactions experimentally, we are building a novel laboratory device which does not suffer from such limitation
Local perceptions and traditional methods for Carapa procera oil production in Mali
Carapa procera is a wild oil tree species traditionally exploited in Mali for seed oil. Carapa oil is highly sought because of therapeutic, cosmetic, insecticidal and repellent properties. The purpose of this work was to contribute to the understanding of local practices in C. procera oil production and local perceptions related thereto. The method used was based on surveys in four production localities and tests of oil production according to the traditional processes practiced by the processors. Two methods of seed storage and oil extraction processes were identified. Women were the main actors in seed collection and oil processing. The average oil extraction efficiency from the seeds was 23.1% and varied among sites. The wet extraction process was the most efficient. In addition to socio-cultural considerations, difficulties related to technology (lack of adequate extraction equipment) and the decrease of carapa populations limit the full development of processing activities. It is necessary to undertake options to improve traditional oil production processes such as education and introduction of shea presses in order to reduce labour intensity, improve efficiency and ensure quality standards. It also requires action for the sustainable management and conservation of the carapa species.Carapa procera est une espèce ligneuse oléagineuse naturelle traditionnellement exploitée au Mali pour l'huile des graines. L'huile de carapa est très recherchée pour ses propriétés thérapeutiques, cosmétiques, insecticides et répulsives. L'objectif de ce travail était de contribuer à la compréhension des pratiques locales en matière de production d'huile de C. procera et des perceptions locales y afférentes. La méthode utilisée a été basée sur des enquêtes dans quatre localités de production et la réalisation des tests de production d'huile selon les procédés traditionnels pratiqués par les transformatrices. Deux méthodes de conservation des noix de Carapa et d'extraction de l'huile ont été identifiées. Les femmes sont les principales actrices de la collecte des noix et de la production de l'huile. Le rendement moyen d'extraction de l'huile des graines a été de 23,1 % et a varié entre les sites. Le procédé d'extraction de l’huile par voie humide a été le plus efficace. Outre les considérations socioculturelles, les difficultés liées à la technologie (manque d'équipements d'extraction adéquats) et la diminution des populations de Carapa limitent le plein développement des activités de transformation. Il est nécessaire de développer des options pour améliorer les processus traditionnels de production de l'huile comme la formation et l'introduction de presses à karité afin de réduire l'intensité de travail, améliorer l'efficacité et garantir des normes de qualité. Cela implique également une action pour la gestion durable et la conservation de l'espèce Carapa
Vibrational state distribution of 2-Na^+ ions created in ultracold collisions
The vibrational distribution P(v) of 2-Na^+ ions created in
ultracold collisions in a magneto-optical trap has been deter-
mined. Only two vibrational states with v = 2 and 3 are popu-
lated and we find P(2)=0.29±0.02 and P(3)=0.71±0.02. The
results provide conclusive evidence that the ionization mech-
anism is photo-associative autoionization,and not photo-
associative photoionization and will form a fundamental test
for the theoretical description of the process
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Recommended Thermal Rate Coefficients for the C + H3+ Reaction and Some Astrochemical Implications
We incorporate our experimentally derived thermal rate coefficients for C + forming CH+ and CH2 + 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 instead we use a more versatile fitting formula. At a temperature of 10 K and a density of 104 cm−3, 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. In addition, we find that the relatively small error on our thermal rate coefficients, ~15%, 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
Recommended from our members
Recommended Thermal Rate Coefficients for the C + H3+ Reaction and Some Astrochemical Implications
We incorporate our experimentally derived thermal rate coefficients for C + forming CH+ and CH2 + 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 instead we use a more versatile fitting formula. At a temperature of 10 K and a density of 104 cm−3, 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. In addition, we find that the relatively small error on our thermal rate coefficients, ~15%, 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
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