418 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
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
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
Ro-vibrational analysis of the XUV photodissociation of HeH ions
We investigate the dynamics of the photodissociation of the hydrohelium
cation HeH by XUV radiation with the aim to establish a detailed comparison
with a recent experimental work carried out at the FLASH free electron laser
using both vibrationally hot and cold ions. As shown in previous theoretical
works, the comparison is hindered by the fact that the experimental
ro-vibrational distribution of the ions is unknown. We determine this
distribution using a dissociative charge transfer set-up and the same source
conditions as in the FLASH experiment. Using a non-adiabatic time-dependent
wave packet method, we calculate the partial photodissociation cross sections
for the coupled electronic states of HeH. We find a good agreement
with the experiment for the total cross section into the He + H
dissociative channel. By performing an adiabatic calculation involving the
states, we then show that the experimental observation of the importance
of the electronic states with cannot be well explained theoretically,
especially for cold () ions. We also calculate the relative contributions
to the cross section of the and states. The agreement with the
experiment is excellent for the He + H channel, but only qualitative for
the He + H channel. We discuss the factors that could explain the remaining
discrepancies between theory and experiment.Comment: 10 pages, 8 figure
Spectral matching and outdoor solar to electrical conversion efficiency in thin-film silicon multi-junction solar cells
Semi-empirical computer modelling is used to investigate spectral matching in tandem and triple-junction thin film silicon solar cells. In amorphous/microcrystalline silicon (a-Si:H/µc-Si:H) tandem cells, current mis-match is offset by an increase in fill-factor, resulting in a broad peak in efficiency versus average photon energy. For a-Si:H/a-Si:H tandem cells, photo-generated currents in both sub-cells increase with increasing average photon energy, and efficiency is predicted to increase monotonically over a wide spectral range. a-Si:H/a-Si:H/µc-Si:H triple cells exhibit spectral behaviour similar to a-Si:H/µc-Si:H tandem cells, but with a smaller fill-factor dependence. Variations in spectral quality are predicted to account for only a small reduction in annual electrical energy yield, of some 2 to 4%
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
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
The dependency pair framework: Combining techniques for automated termination proofs
Abstract. The dependency pair approach is one of the most powerful techniques for automated termination proofs of term rewrite systems. Up to now, it was regarded as one of several possible methods to prove termination. In this paper, we show that dependency pairs can instead be used as a general concept to integrate arbitrary techniques for termination analysis. In this way, the benefits of different techniques can be combined and their modularity and power are increased significantly. We refer to this new concept as the “dependency pair framework ” to distinguish it from the old “dependency pair approach”. Moreover, this framework facilitates the development of new methods for termination analysis. To demonstrate this, we present several new techniques within the dependency pair framework which simplify termination problems considerably. We implemented the dependency pair framework in our termination prover AProVE and evaluated it on large collections of examples.
La création de valeur : glas ou Graal ? Revue et modélisation du concept
La littérature en sciences de gestion mobilise le concept de « création de valeur » sans pour autant en discuter le sens et encore moins sa dynamique. Dans les milieux de la pratique stratégique et de la recherche en stratégie d’entreprise, le mot valeur est étroitement associé au concept de modèle d’affaires, depuis une quinzaine d’années, avec une création de valeur présentée comme un « objectif » surplombant voire comme une formule incantatoire permettant aux acteurs de dépasser leurs désaccords. La présente communication est motivée par de nouveaux questionnements face à nos insuffisances pour concevoir des stratégies et des modèles d’affaires qui créent de la valeur pour un ensemble plus étendu de parties prenantes et non plus seulement pour les propriétaires / actionnaires. Notre question de recherche porte sur l’essence de la « création de valeur »: comment les chercheurs qui publient sur le concept de création de valeur définissent-ils ce concept ? Après une présentation du cadre conceptuel mettant en relief les raisons d’une nécessaire synthèse tant pour les besoins du praticien que pour ceux du pédagogue-chercheur, nous présentons la démarche méthodologique. Cette dernière consiste en une revue de littérature utilisant la base de données Scopus et l’exploitation sous Alceste (logiciel de traitement de données textuelles) des définitions de la création de valeur extraites des 50 articles anglophones les plus cités contenant « value » et « creation » dans leur titre. Les classes de discours obtenues de la double classification automatique du corpus textuel agrègent des contenus autour de cinq thèmes : 1) la nature de la valeur créée, 2) l’architecture de la valeur, 3) la valeur perçue par l’usager ou l’acheteur, 4) le partage économique de la valeur entre parties prenantes, et enfin 5) la co-création de la valeur, sur la base d’une interaction entre le client et le fournisseur. Enfin, nous discutons nos résultats, à partir d’un cadre conceptuel inductif et sur la base d’une littérature étendue (anglophone et francophone), à savoir une proposition de modélisation du concept de « création de valeur» autour d’enjeux épistémologiques et même paradigmatiques
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