Recommended Thermal Rate Coefficients for the C + H3+_3^+ Reaction and Some Astrochemical Implications

We have incorporated our experimentally derived thermal rate coefficients for C + H$_3^+$ forming CH$^+$ and CH$_2^+$ 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 use instead a more versatile fitting formula. At a temperature of 10 K and a density of 10$^4$ 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. Additionally, we find that the relatively small error on our thermal rate coefficients, $\sim15\%$, 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.Comment: 19 pages, 5 figures. Accepted for publication in Ap

Merged-beams Reaction Studies of O + H_3^+

We have measured the reaction of O + H3+ forming OH+ and H2O+. This is one of the key gas-phase astrochemical processes initiating the formation of water molecules in dense molecular clouds. For this work, we have used a novel merged fast-beams apparatus which overlaps a beam of H3+ onto a beam of ground-term neutral O. Here, we present cross section data for forming OH+ and H2O+ at relative energies from \approx 3.5 meV to \approx 15.5 and 0.13 eV, respectively. Measurements were performed for statistically populated O(3PJ) in the ground term reacting with hot H3+ (with an internal temperature of \approx 2500-3000 K). From these data, we have derived rate coefficients for translational temperatures from \approx 25 K to \approx 10^5 and 10^3 K, respectively. Using state-of-the-art theoretical methods as a guide, we have converted these results to a thermal rate coefficient for forming either OH+ or H2O+, thereby accounting for the temperature dependence of the O fine-structure levels. Our results are in good agreement with two independent flowing afterglow measurements at a temperature of \approx 300 K, and with a corresponding level of H3+ internal excitation. This good agreement strongly suggests that the internal excitation of the H3+ does not play a significant role in this reaction. The Langevin rate coefficient is in reasonable agreement with the experimental results at 10 K but a factor of \approx 2 larger at 300 K. The two published classical trajectory studies using quantum mechanical potential energy surfaces lie a factor of \approx 1.5 above our experimental results over this 10-300 K range.Comment: 43 pages, 11 figures. Submitted to the Astrophysical Journa

Generation of neutral atomic beams utilizing photodetachment by high power diode laser stacks

We demonstrate the use of high power diode laser stacks to photodetach fast hydrogen and carbon anions and produce ground term neutral atomic beams. We achieve photodetachment efficiencies of $\sim$7.4\% for H$^-$ at a beam energy of 10\,keV and $\sim$3.7\% for C$^-$ at 28\,keV. The diode laser systems used here operate at 975\,nm and 808\,nm, respectively, and provide high continuous power levels of up to 2\,kW, without the need of additional enhancements like optical cavities. The alignment of the beams is straightforward and operation at constant power levels is very stable, while maintenance is minimal. We present a dedicated photodetachment setup that is suitable to efficiently neutralize the majority of stable negative ions in the periodic table

Focused Information Criterion for Locally Misspecified Vector Autoregressive Models

This paper investigates the focused information criterion and plug-in average for vector autoregressive models with local-to-zero misspecication. These methods have the advantage of focusing on a quantity of interest rather than aiming at overall model t. Any (suciently regular) function of the parameters can be used as a quantity of interest. We determine the asymptotic properties and elaborate on the role of the locally misspecied parameters. In particular, we show that the inability to consistently estimate locally misspecied parameters translates into suboptimal selection and averaging. We apply this framework to impulse response analysis. A Monte Carlo simulation study supports our claims

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

Vibrational state distribution of 2-Na^+ ions created in ultracold collisions

The vibrational distribution P(v) of 2-Na^+ ions created in ultracold collisions in a magneto-optical trap has been deter- mined. Only two vibrational states with v = 2 and 3 are popu- lated and we find P(2)=0.29±0.02 and P(3)=0.71±0.02. The results provide conclusive evidence that the ionization mech- anism is photo-associative autoionization,and not photo- associative photoionization and will form a fundamental test for the theoretical description of the process

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