On the Energetics of the HCO+^+ + C →\to CH+^+ + CO Reaction and Some Astrochemical Implications

We explore the energetics of the titular reaction, which current astrochemical databases consider open at typical dense molecular (i.e., dark) cloud conditions. As is common for reactions involving the transfer of light particles, we assume that there are no intersystem crossings of the potential energy surfaces involved. In the absence of any such crossings, we find that this reaction is endoergic and will be suppressed at dark cloud temperatures. Updating accordingly a generic astrochemical model for dark clouds changes the predicted gas-phase abundances of 224 species by greater than a factor of 2. Of these species, 43 have been observed in the interstellar medium. Our findings demonstrate the astrochemical importance of determining the role of intersystem crossings, if any, in the titular reaction.Comment: Accepted for publication in ApJ; 14 pages, 2 figures, and 1 tabl

Absolute energy-resolved measurements of the H-+H→H2+e- associative detachment reaction using a merged-beam apparatus

Using a merged-beam configuration, we have performed absolute measurements for the associative detachment reaction H-+H→H2+e-. Our energy-resolved measurements for this process remove a long-standing discrepancy between theory and experiment for this fundamental reaction. In particular, we find excellent agreement with theoretical results which previously seemed to be ruled out by earlier experiments performed using a flowing afterglow technique

A novel merged beam apparatus to study the cosmic origins of organic chemistry

Reactions of atomic carbon with molecular ions play a critical role for gas phase molecular formation in interstellar clouds. These interactions are the first links in the chain of chemical reactions leading to the synthesis of complex organic species. Much of our knowledge of this process is through spectroscopic observations and theoretical models. However, our understanding of this chemistry is constrained by uncertainties in the underlying reaction rate coefficients. Data from quantum calculations are limited to reactions involving three or fewer atoms. Meanwhile, previous experimental studies have been hampered by the difficulty in generating a sufficiently intense and well characterized neutral carbon beam. To address these issues and to study these reactions experimentally, we are building a novel laboratory device which does not suffer from such limitation

Associative detachment of H− + H → H2 + e−

Using a merged-beams apparatus, we have measured the associative detachment (AD) reaction of H−+H→H2+e− for relative collision energies up to Er≤4.83 eV. These data extend above the 1-eV limit of our earlier results. We have also updated our previous theoretical work to account for AD via the repulsive 2Σg+ H2− potential energy surface and for the effects at Er≥0.76 eV on the experimental results due to the formation of long-lived H2 resonances lying above the H+H separated atoms limit. Merging both experimental data sets, our results are in good agreement with our new theoretical calculations and confirm the prediction that this reaction essentially turns off for Er≳2 eV. Similar behavior has been predicted for the formation of protonium from collisions of antiprotons and hydrogen atoms

Molecular Hydrogen Formation in the Early Universe: New Implications From Laboratory Measurements

We have performed the first energy-resolved measurement of the associative detachment (AD) reaction H- + H → H2 + e-: This reaction is the dominant formation pathway for H2 during the epoch of first star formation in the early universe. Despite being the most fundamental anion-neutral chemical reaction, experiment and theory have failed to converge in both magnitude and energy dependence. The uncertainty in this rate coefficient severely limits our under- standing of the formation of the first stars and protogalaxies

Molecular Hydrogen Formation in the Early Universe: New Implications From Laboratory Measurements

We have performed the first energy-resolved measurement of the associative detachment (AD) reaction H- + H → H2 + e-: This reaction is the dominant formation pathway for H2 during the epoch of first star formation in the early universe. Despite being the most fundamental anion-neutral chemical reaction, experiment and theory have failed to converge in both magnitude and energy dependence. The uncertainty in this rate coefficient severely limits our under- standing of the formation of the first stars and protogalaxies

Recommended Thermal Rate Coefficients for the C + H3+ Reaction and Some Astrochemical Implications

We incorporate our experimentally derived thermal rate coefficients for C + ${{\rm{H}}}_{3}^{+}$ 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 Thermal Rate Coefficients for the C + H3+ Reaction and Some Astrochemical Implications

We incorporate our experimentally derived thermal rate coefficients for C + ${{\rm{H}}}_{3}^{+}$ 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

Isotope effect for associative detachment: H(D)−+H(D)→H2(D2)+e

We report experimental and theoretical results for associative detachment (AD) of D−+D→D2+e−. We compare these data to our previously published results for H−+H→H2+e−. The measurements show no significant isotope effect in the total cross section. This is to be contrasted with previously published experimental and theoretical work which has found a significant isotope effect in diatomic systems for partial AD cross sections, i.e., as a function of the rotational and vibrational levels of the final molecule formed. Our work implies that though the rovibrational distribution of flux is different for AD of H− + H and D− + D, the total flux for these two systems is essentially the same when summed over all possible final channels

Supplemental material: Dynamics of the isotope exchange reaction of D with H3+, H2D+, and D2H+

No abstract has been provided for this article at this time