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Register a new userExperimental and theoretical studies of D + H$_3^+$ $rightarrow$ H$_2$D$^+$ + H
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Added by
Pierre-Michel Hillenbrand
Publication date
2019
fields
Physics
and research's language is
English
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Deuterated molecules are important chemical tracers of prestellar and protostellar cores. Up to now, the titular reaction has been assumed to contribute to the generation of these deuterated molecules. We have measured the merged-beams rate coefficient for this reaction as function of the relative collision energy in the range of about 10 meV to 10 eV. By varying the internal temperature of the reacting H$_3^+$ molecules, we found indications for the existence of a reaction barrier. We have performed detailed theoretical calculations for the zero-point-corrected energy profile of the reaction and determined a new value for the barrier height of $approx$ 68 meV. Furthermore, we have calculated the tunneling probability through the barrier. Our experimental and theoretical results show that the reaction is essentially closed at astrochemically relevant temperatures. We derive a thermal rate coefficient of $<1times 10^{-12}$ cm$^3$ s$^{-1}$ for temperatures below 75 K with tunneling effects included and below 155 K without tunneling.
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We calculated reaction rate constants including atom tunneling of the reaction of dihydrogen with the hydroxy radical down to a temperature of 50 K. Instanton theory and canonical variational theory with microcanonical optimized multidimensional tunneling (CVT/$mu$OMT) were applied using a fitted potential energy surface [J. Chem. Phys. 138, 154301 (2013)]. All possible protium/deuterium isotopologues were considered. Atom tunneling increases at about 250 K (200 K for deuterium transfer). Even at 50 K the rate constants of all isotopologues remain in the interval $ 4 cdot 10^{-20}$ to $4 cdot 10^{-17}$ cm$^3$ s$^{-1}$ , demonstrating that even deuterat
The non-adiabatic quantum dynamics of the H+H$_2^+$ $rightarrow$ H$_2$+ H$^+$ charge transfer reactions, and some isotopic variants, is studied with an accurate wave packet method. A recently developed $3times$3 diabatic potential model is used, which is based on very accurate {it ab initio} calculations and includes the long-range interactions for ground and excited states. It is found that for initial H$_2^+$(v=0), the quasi-degenerate H$_2$(v=4) non-reactive charge transfer product is enhanced, producing an increase of the reaction probability and cross section. It becomes the dominant channel from collision energies above 0.2 eV, producing a ratio, between v=4 and the rest of vs, that increases up to 1 eV. H+H$_2^+$ $rightarrow$ H$_2^+$+ H exchange reaction channel is nearly negligible, while the reactive and non-reactive charge transfer reaction channels are of the same order, except that corresponding to H$_2$(v=4), and the two charge transfer processes compete below 0.2 eV. This enhancement is expected to play an important vibrational and isotopic effect that need to be evaluated. For the three proton case, the problem of the permutation symmetry is discussed when using reactant Jacobi coordinates.
Advances in merged-beams instruments have allowed experimental studies of the mutual neutralisation (MN) processes in collisions of both Li$^+$ and Na$^+$ ions with D$^-$ at energies below 1 eV. These experimental results place constraints on theoretical predictions of MN processes of Li$^+$ and Na$^+$ with H$^-$, important for non-LTE modelling of Li and Na spectra in late-type stars. We compare experimental results with calculations for methods typically used to calculate MN processes, namely the full quantum (FQ) approach, and asymptotic model approaches based on the linear combination of atomic orbitals (LCAO) and semi-empirical (SE) methods for deriving couplings. It is found that FQ calculations compare best overall with the experiments, followed by the LCAO, and the SE approaches. The experimental results together with the theoretical calculations, allow us to investigate the effects on modelled spectra and derived abundances and their uncertainties arising from uncertainties in the MN rates. Numerical experiments in a large grid of 1D model atmospheres, and a smaller set of 3D models, indicate that neglect of MN can lead to abundance errors of up to 0.1 dex (26%) for Li at low metallicity, and 0.2 dex (58%) for Na at high metallicity, while the uncertainties in the relevant MN rates as constrained by experiments correspond to uncertainties in abundances of much less than 0.01~dex (2%). This agreement for simple atoms gives confidence in the FQ, LCAO and SE model approaches to be able to predict MN with the accuracy required for non-LTE modelling in stellar atmospheres.
Decays of beauty baryons to the $D^0 p h^-$ and $Lambda_c^+ h^-$ final states (where $h$ indicates a pion or a kaon) are studied using a data sample of $pp$ collisions, corresponding to an integrated luminosity of 1.0 fb$^{-1}$, collected by the LHCb detector. The Cabibbo-suppressed decays $Lambda_b^0to D^0 p K^-$ and $Lambda_b^0to Lambda_c^+ K^-$ are observed and their branching fractions are measured with respect to the decays $Lambda_b^0to D^0 p pi^-$ and $Lambda_b^0to Lambda_c^+ pi^-$. In addition, the first observation is reported of the decay of the neutral beauty-strange baryon $Xi_b^0$ to the $D^0 p K^-$ final state, and a measurement of the $Xi_b^0$ mass is performed. Evidence of the $Xi_b^0to Lambda_c^+ K^-$ decay is also reported.
Young massive stars are usually found embedded in dense and massive molecular clumps and are known for being highly obscured and distant. During their formation process, deuteration is regarded as a potentially good indicator of the formation stage. Therefore, proper observations of such deuterated molecules are crucial, but still, hard to perform. In this work, we test the observability of the transition o-H$_2$D$^+(1_{10}$-$1_{11})$, using a synthetic source, to understand how the physical characteristics are reflected in observations through interferometers and single-dish telescopes. In order to perform such tests, we post-processed a magneto-hydrodynamic simulation of a collapsing magnetized core using the radiative transfer code POLARIS. Using the resulting intensity distributions as input, we performed single-dish (APEX) and interferometric (ALMA) synthetic observations at different evolutionary times, always mimicking realistic configurations. Finally, column densities were derived to compare our simulations with real observations previously performed. Our derivations for o-H$_2$D$^+$ are in agreement with values reported in the literature, in the range of 10$^{!10-11}$cm$^{!-2}$ and 10$^{!12-13}$cm$^{!-2}$ for single-dish and interferometric measurements, respectively.
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