Time-of-flight detection coupled to a flowing afterglow: Improvements and characterization

Dissociative recombination of electrons with HCl, HCl, DCl, and DCl has been measured under thermal conditions at 300, 400, and 500 K using a flowing afterglow-Langmuir probe apparatus. Measurements for HCl and DCl employed the variable electron and neutral density attachment mass spectrometry (VENDAMS) method, while those for HCl and DCl employed both VENDAMS and the more traditional technique of monitoring electron density as a function of reaction time. At 300 K, HCl and HCl recombine with k = 7.7± × 10 cm s and 2.6 ± 0.8 × 10 cm s, respectively, whereas DCl is roughly half as fast as HCl with k = 1.1 ± 0.3 × 10 cm s (2σ confidence intervals). DCl recombines with a rate coefficient below the approximate detection limit of the method (≲5 × 10 cm s) at all temperatures. Relatively slow dissociative recombination rates have been speculated to be responsible for the large HCl and HCl abundances in interstellar clouds compared to current astrochemical models, but our results imply that the discrepancy must originate elsewhere.

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Dissociative recombination of HCl+, H2Cl+, DCl+, and D2Cl+ in a flowing afterglow

Wiens

Miller

Shuman

et al. 2016

The Journal of Chemical Physics

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Contrast between the mechanisms for dissociative electron attachment to CH3SCN and CH3NCS

Miller

Viggiano

Shuman

2018

The Journal of Chemical Physics

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The kinetics of thermal electron attachment to methyl thiocyanate (CHSCN), methyl isothiocyanate (CHNCS), and ethyl thiocyanate (CHSCN) were measured using flowing afterglow-Langmuir probe apparatuses at temperatures between 300 and 1000 K. CHSCN and CHSCN undergo inefficient dissociative attachment to yield primarily SCN at 300 K (k = 2 × 10 cm s), with increasing efficiency as temperature increases. The increase is well described by activation energies of 0.17 eV (CHSCN) and 0.14 eV (CHSCN). CN product is formed at <1% branching at 300 K, increasing to ∼30% branching at 1000 K. Attachment to CHNCS yields exclusively SCN ionic product but at a rate at 300 K that is below our detection threshold (k < 10 cm s). The rate coefficient increases rapidly with increasing temperature (k = 6 × 10 cm s at 600 K), in a manner well described by an activation energy of 0.51 eV. Calculations at the B3LYP/def2-TZVPPD level suggest that attachment to CHSCN proceeds through a dissociative state of CHSCN, while attachment to CHNCS initially forms a weakly bound transient anion CHNCS that isomerizes over an energetic barrier to yield SCN. Kinetic modeling of the two systems is performed in an attempt to identify a kinetic signature differentiating the two mechanisms. The kinetic modeling reproduces the CHNCS data only if dissociation through the transient anion is considered.

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Toward a quantitative analysis of the temperature dependence of electron attachment to SF6

Poutsma

Shuman

Miller

et al. 2020

The Journal of Chemical Physics

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New flowing afterglow/Langmuir probe investigations of electronic attachment to SF6 are described. Thermal attachment rate constants are found to increase from 1.5 × 10−7 cm3 s−1 at 200 K to 2.3 × 10−7 cm3 s−1 at 300 K. Attachment rate constants over the range of 200–700 K (from the present work and the literature), together with earlier measurements of attachment cross sections, are analyzed with respect to electronic and nuclear contributions. The latter suggest that only a small nuclear barrier (of the order of 20 meV) on the way from SF6 to SF6− has to be overcome. The analysis shows that not only s-waves but also higher partial waves have to be taken into account. Likewise, finite-size effects of the neutral target contribute in a non-negligible manner.

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Time-of-flight detection coupled to a flowing afterglow: Improvements and characterization

Cited by 3 publications

References 26 publications

Dissociative recombination of HCl+, H2Cl+, DCl+, and D2Cl+ in a flowing afterglow

Dissociative recombination of HCl+, H2Cl+, DCl+, and D2Cl+ in a flowing afterglow

Contrast between the mechanisms for dissociative electron attachment to CH3SCN and CH3NCS

Toward a quantitative analysis of the temperature dependence of electron attachment to SF6

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