118 research outputs found
On the positive eigenvalues and eigenvectors of a non-negative matrix
The paper develops the general theory for the items in the title, assuming
that the matrix is countable and cofinal.Comment: Version 2 allows the matrix to have zero row(s) and rows with
infinitely many non-zero entries. In addition the introduction has been
rewritte
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
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 ArrheniusâKooij 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 cmâ3, 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 ArrheniusâKooij 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 cmâ3, 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
Decay pathways for protonated and deprotonated adenine molecules
This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Journal of Chemical Physics 151.4 (2019): 044306 and may be found at https://aip.scitation.org/doi/abs/10.1063/1.5109963We have measured fragment mass spectra and total destruction cross sections for protonated and deprotonated adenine following collisions with He at center-of-mass energies in the 20-240 eV range. Classical and ab initio molecular dynamics simulations are used to provide detailed information on the fragmentation pathways and suggest a range of alternative routes compared to those reported in earlier studies. These new pathways involve, for instance, losses of HNC molecules from protonated adenine and losses of NH2 or C3H2N2 from deprotonated adenine. The present results may be important to advance the understanding of how biomolecules may be formed and processed in various astrophysical environmentsThis work was supported by the Swedish Research Council (Constant Nos. 2017-00621, 2015-04990, 2016-04181, and 2018-04092). Furthermore, we acknowledge the European Joint on Theoretical Chemistry and Computational Modelling (INT-EJD-TCCM). We acknowledge the generous allocation of computer time at the Centro de Computacion Cientifica at the Universidad Autonoma de Madrid (CCC-UAM). This work was partially supported by Project No. CTQ2016-76061-P of the Spanish Ministerio de Economia y Competitividad (MINECO
Ultraslow radiative cooling of Cn-(n = 3â5)
Ultraslow radiative cooling lifetimes and adiabatic detachment energies for three astrochemically relevant anions, Cân (n = 3â5), are measured using the Double ElectroStatic Ion Ring ExpEriment (DESIREE) infrastructure at Stockholm University. DESIREE maintains a background pressure of â10â14 mbar and temperature of â13 K, allowing storage of mass-selected ions for hours and providing conditions coined a âmolecular cloud in a box.â Here, we construct two-dimensional (2D) photodetachment spectra for the target anions by recording photodetachment signal as a function of irradiation wavelength and ion storage time (seconds to minute time scale). Ion cooling lifetimes, which are associated with infrared radiative emission, are extracted from the 2D photodetachment spectrum for each ion by tracking the disappearance of vibrational hot-band signal with ion storage time, giving 1e cooling lifetimes of 3.1 ± 0.1 s (Câ3), 6.8 ± 0.5 s (Câ4), and 24 ± 5 s (Câ5). Fits of the photodetachment spectra for cold ions, i.e., those stored for at least 30 s, provide adiabatic detachment energies in good agreement with values from laser photoelectron spectroscopy on jet-cooled anions, confirming that radiative cooling has occurred in DESIREE. Ion cooling lifetimes are simulated using a simple harmonic cascade model, finding good agreement with experiment and providing a mode-by-mode understanding of the radiative cooling properties. The 2D photodetachment strategy and radiative cooling modeling developed in this study could be applied to investigate the ultraslow cooling dynamics of a wide range of molecular anions
Time- and energy-resolved effects in the boron-10 based Multi-Grid and helium-3 based thermal neutron detectors
The boron-10 based Multi-Grid detector is being developed as an alternative
to helium-3 based neutron detectors. At the European Spallation Source, the
detector will be used for time-of-flight neutron spectroscopy at cold to
thermal neutron energies. The objective of this work is to investigate fine
time- and energy-resolved effects of the Multi-Grid detector, down to a few
eV, while comparing it to the performance of a typical helium-3 tube.
Furthermore, it is to characterize differences between the detector
technologies in terms of internal scattering, as well as the time
reconstruction of ~ s short neutron pulses. The data were taken at the
Helmholtz Zentrum Berlin, where the Multi-Grid detector and a helium-3 tube
were installed at the ESS test beamline, V20. Using a Fermi-chopper, the
neutron beam of the reactor was chopped into a few tens of s wide pulses
before reaching the detector, located a few tens of cm downstream. The data of
the measurements show an agreement between the derived and calculated neutron
detection efficiency curve. The data also provide fine details on the effect of
internal scattering, and how it can be reduced. For the first time, the chopper
resolution was comparable to the timing resolution of the Multi-Grid detector.
This allowed a detailed study of time- and energy resolved effects, as well as
a comparison with a typical helium-3 tube.Comment: 37 pages, 21 figure
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