K. A. Miller

Reaction Studies of Neutral Atomic C with H3+ using a Merged-Beams Apparatus

We have investigated the chemistry of C + H3+ forming CH+, CH2+, and CH3+. These reactions are believed to be some of the key gas-phase astrochemical processes initiating the formation of organic molecules in molecular clouds. For this work, we have constructed a novel merged fast-beams apparatus which overlaps a beam of molecular ions onto a beam of ground-term neutral atoms. Here, we describe the apparatus in detail and present cross section data for forming CH+ and CH2+ at relative energies from ≈9 meV to ≈20 and 3 eV, respectively. Measurements were performed for statistically populated C(3PJ) in the ground term reacting with hot H3+ (at an internal temperature of ~2550 K). Using these data, we have derived rate coefficients for translational temperatures from ≈72 K to 2.3 10^5 and 3.4 10^4 K, respectively. For the formation of CH3+, we are only able to place an upper limit on the rate coefficient. Our results for CH+ and CH2+ are in good agreement with the mass-scaled results from a previous ion trap study of C + D3+, at a translational temperature of ~1000 K. That work also used statistically populated C(3PJ) but internally cold D3+ (~77 K). The good agreement between the two experiments implies that the internal excitation of the H3+ is not significant so long as the reaction proceeds adiabatically. At 300 K, the C fine-structure levels are predicted to be essentially statistically populated, enabling us to compare our translational temperature results to thermal equilibrium calculations. At this temperature, our rate coefficient for forming CH+ lies a factor of ≈2.9 below the Langevin rate coefficient currently given in astrochemical databases, and a factor of ~1.8–3.3 below the published classical trajectory studies using quantum mechanical potential energy surfaces. Our results for CH2+ formation at 300 K are a factor of ≈26.7 above these semi-classical results. Astrochemical databases do not currently include this channel. We also present a method for converting our translational temperature results to thermal rate coefficients for temperatures below ~300 K. The results indicated that CH2+ formation dominates over that of CH+ at temperatures ≲50 K. Key words: astrobiology – astrochemistry – ISM: molecules – methods: laboratory: molecular – molecular data – molecular processes

Merged-beam study of mutual neutralization of H− and H+

Total and partial cross sections have been measured for the mutual neutralization of H− and H+ by means of a merged and inclined beam set-up. The low energy data between 10 meV and 5 eV contradict one previous set of measurements, while above 5 eV the data fall in excellent agreement with previously published results by two other groups.

Measurement of the associative detachment reaction H− + H → H2 + e− using a merged-beams method

Using a merged-beams method, we have performed absolute, energy-resolved measurements for the associative detachment reaction H− + H → H2 + e−. Our results 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 a recent flowing afterglow experiment.

Associative detachment of H{sup -} + H {yields} H{sub 2} + e{sup -}

Using a merged-beams apparatus, we have measured the associative detachment (AD) reaction of H{sup -}+H{yields}H{sub 2}+e{sup -} for relative collision energies up to E{sub r}{ =}0.76 eV on the experimental results due to the formation of long-lived H{sub 2} 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 E{sub r}(greater-or-similar sign)2 eV. Similar behavior has been predicted for the formation of protonium from collisions of antiprotons and hydrogen atoms.

Laboratory Simulations of Molecular Hydrogen Formation in the Early Universe: A Progress Report

During the epoch of protogalaxy and first star formation, H/sub 2/ is the main coolant of primordial gas for temperatures below ~8,000 K. The H/sub 2/ is formed via associative detachment (AD) of H/sup -/ and H. Uncertainties in the rate coefficient for this reaction have limited our understanding of protogalaxy formation during this epoch and of the characteristic masses and cooling times for the first stars. Recently we have carried out a series of laboratory measurements which remove these uncertainties. Here, we present the cosmological motivation for our work, describe the experimental approach, and point the reader to the relevant works where our AD results are reported and their cosmological implications explored.

Laboratory studies of the H2 production mechanism which led to the formation of the first stars

We have developed a novel apparatus to study associative detachment of H- and H forming H2. Beginning with an H- beam, we use laser photodetachment to neutralize a fraction of the beam. This generates self-merged, anion-neutral beams. Laboratory beam energies are in the keV range. Because the beams run co-linear, center-of-mass energies from the meV to keV range are achievable. Our measurements will help to resolve the nearly order of magnitude scatter in previously published results. Resolving this issue will have major implications for understanding the early universe chemistry which led to the formation of the first stars and protogalaxies.