Electron-impact rotational excitation of linear molecular ions

Fauré, Alexandre; Tennyson, Jonathan

doi:10.1046/j.1365-8711.2001.04480.x

Cited by 79 publications

(58 citation statements)

References 30 publications

(40 reference statements)

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“…Electron excitation will be important for an ionization fraction x(e − ) > ∼ 10 −3 . The critical electron densities are in reasonable agreement (difference is < ∼ 20%) with those published by Faure & Tennyson (2001) for T kin = 500 K.…”

Section: Molecular Datasupporting

confidence: 86%

Excitation and abundance study of CO⁺ in the interstellar medium

Stäuber¹,

Bruderer²

2009

A&A

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Context. Observations of CO+ suggest column densities on the order 10 12 cm −2 that can not be reproduced by many chemical models. CO + is more likely to be destroyed than excited in collisions with hydrogen. An anomalous excitation mechanism may thus have to be considered when interpreting CO + observations. Other uncertainties in models are the chemical network, the gas temperature or the geometry of the emitting source. Similar is true for other reactive ions that will be observed soon with the Herschel Space Observatory. Aims. Chemical constraints are explored for observable CO + abundances. The influence of an anomalous excitation mechanism on CO + line intensities is investigated. Model results are compared to observations. Methods. Chemical models are used to perform a parameter study of CO + abundances. Line fluxes are calculated for N(CO + ) = 10 12 cm −2 and different gas densities and temperatures using a non-LTE escape probability method. The chemical formation and destruction rates are considered explicitly in the detailed balance equations of the radiative transfer. In addition, the rotational levels of CO + are assumed to be excited upon chemical formation according to a formation temperature. Collisional excitation by atomic and molecular hydrogen as well as by electrons is studied for conditions appropriate to dense photon-dominated regions (PDRs) and star-forming environments.Results. Chemical models are generally able to produce high fractional CO + abundances (x(CO + ) ≈ 10 −10 ). In a far-ultraviolet (FUV) dominated environment, however, high abundances of CO + are only produced in regions with a Habing field G 0 > ∼ 100 and T kin > ∼ 600 K, posing a strong constraint on the gas temperature. For gas densities < ∼ 10 6 cm −3 and temperatures > ∼ 600 K, the combination of chemical and radiative transfer analysis shows little effect on intensities of CO + lines with upper levels N up ≤ 3. Significantly different line fluxes are calculated with an anomalous excitation mechanism, however, for transitions with higher upper levels and densities > ∼ 10 6 cm −3 . The Herschel Space Observatory is able to reveal such effects in the terahertz wavelength regime. Ideal objects to observe are protoplanetary disks with densities > ∼ 10 6 cm −3 . It is finally suggested that the CO + chemistry may be well understood and that the abundances observed so far can be explained with a high enough gas temperature and a proper geometry.

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Section: Molecular Datasupporting

confidence: 86%

Excitation and abundance study of CO⁺ in the interstellar medium

Stäuber¹,

Bruderer²

2009

A&A

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“…2 for the transitions 0 → 2 and 1 → 3, in comparison with those obtained by Faure and Tennyson [31] using the ANR approximation, with which they agree quite satisfactorily both in shape and magnitude.…”

Section: Very Low Energy: Ro-vibronic Interactionssupporting

confidence: 79%

Advances in the MQDT approach of electron/molecular cation reactive collisions: High precision extensive calculations for applications

Motapon

Niyonzima

Chakrabarti

et al. 2015

EPJ Web of Conferences

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Abstract. Recent advances in the stepwise multichannel quantum defect theory approach of electron/molecular cation reactive collisions have been applied to perform computations of cross sections and rate coefficients for dissociative recombination and electron-impact rovibrational transitions of H + 2 , BeH + and their deuterated isotopomers. At very low energy, rovibronic interactions play a significant role in the dynamics, whereas at high energy, the dissociative excitation strongly competes with all other reactive processes.a Corresponding author: Ioan.Schneider@univ-lehavre.fr This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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“…The CoulombBorn approximation (Chu & Dalgarno 1974) is expected to be accurate for polar molecules with dipoles in excess of ∼2 D because the dipolar cross sections are entirely dominated by longrange effects and cross sections for transitions with ΔN ≥ 2 are significantly smaller (Faure & Tennyson 2001). In practice, Coulomb-Born cross sections were computed for collision energies below 2 eV and rate coefficients were deduced for temperatures ranging from 10 to 2000 K. The IOS approximation was employed to derive the fine-structure rate coefficients in terms of the rotational rates for excitation out of the lowest rotational level N = 0.…”

Section: Discussionmentioning

confidence: 99%

Spatially extended OH⁺emission from the Orion Bar and Ridge

Tak

Nagy

Ossenkopf

et al. 2013

A&A

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ContextWe analyze these HIFI data with non-local thermodynamic equilibrium radiative transfer and PDR chemical models, using newly calculated inelastic collision data for the e-OH + system. Results. Line emission is detected over ∼1 (0.12 pc), tracing the Bar itself as well as a perpendicular feature identified as the southern tip of the Orion Ridge, which borders the Orion Nebula on its western side. The line width of ≈4 km s −1 suggests an origin of the OH + emission close to the PDR surface, at a depth of A V ∼ 0.3-0.5 into the cloud where most hydrogen is in atomic form. Steadystate collisional and radiative excitation models for OH + require unrealistically high column densities to match the observed line intensity, indicating that the formation of OH + in the Bar is rapid enough to influence its excitation. Our best-fit OH + column density of ∼1.0 × 10 14 cm −2 is similar to that in previous absorption line studies, while our limits on the ratios of OH + /H 2 O + ( > ∼ 40) and OH + /H 3 O + ( > ∼ 15) are somewhat higher than seen before. Conclusions. The column density of OH + is consistent with estimates from a thermo-chemical model for parameters applicable to the Orion Bar, given the current uncertainties in the local gas pressure and the spectral shape of the ionizing radiation field. The unusually high OH + /H 2 O + and OH + /H 3 O + ratios are probably due to the high UV radiation field and electron density in this object. In the Bar, photodissociation and electron recombination are more effective destroyers of OH + than the reaction with H 2 , which limits the production of H 2 O + . The appearance of the OH + lines in emission is the result of the high density of electrons and H atoms in the Orion Bar, since for these species, inelastic collisions with OH + are faster than reactive ones. In addition, chemical pumping, far-infrared pumping by local dust, and near-UV pumping by Trapezium starlight contribute to the OH + excitation. Similar conditions may apply to extragalactic nuclei where H n O + lines are seen in emission.

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Electron-impact rotational excitation of linear molecular ions

Cited by 79 publications

References 30 publications

Excitation and abundance study of CO⁺ in the interstellar medium

Excitation and abundance study of CO⁺ in the interstellar medium

Advances in the MQDT approach of electron/molecular cation reactive collisions: High precision extensive calculations for applications

Spatially extended OH⁺emission from the Orion Bar and Ridge

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Electron-impact rotational excitation of linear molecular ions

Cited by 79 publications

References 30 publications

Excitation and abundance study of CO+ in the interstellar medium

Excitation and abundance study of CO+ in the interstellar medium

Advances in the MQDT approach of electron/molecular cation reactive collisions: High precision extensive calculations for applications

Spatially extended OH+emission from the Orion Bar and Ridge

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Product

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Excitation and abundance study of CO⁺ in the interstellar medium

Excitation and abundance study of CO⁺ in the interstellar medium

Spatially extended OH⁺emission from the Orion Bar and Ridge