We present the fifth release of the UMIST Database for Astrochemistry (UDfA). The new reaction network contains 6173 gas-phase reactions, involving 467 species, 47 of which are new to this release. We have updated rate coefficients across all reaction types. We have included 1171 new anion reactions and updated and reviewed all photorates. In addition to the usual reaction network, we also now include, for download, state-specific deuterated rate coefficients, deuterium exchange reactions and a list of surface binding energies for many neutral species. Where possible, we have referenced the original source of all new and existing data. We have tested the main reaction network using a dark cloud model and a carbon-rich circumstellar envelope model. We present and briefly discuss the results of these models.

/pdf/the-umist-database-for-astrochemistry-2012-29l3ep98yt.pdf

The UMIST database for astrochemistry 2012

Inhomogeneous RF Fields: A Versatile Tool for the Study of Processes with Slow Ions

We present a novel chemical database for gas-phase astrochemistry. Named the KInetic Database for Astrochemistry (KIDA), this database consists of gas-phase reactions with rate coefficients and uncertainties that will be vetted to the greatest extent possible. Submissions of measured and calculated rate coefficients are welcome, and will be studied by experts before inclusion into the database. Besides providing kinetic information for the interstellar medium, KIDA is planned to contain such data for planetary atmospheres and for circumstellar envelopes. Each year, a subset of the reactions in the database (kida.uva) will be provided as a network for the simulation of the chemistry of dense interstellar clouds with temperatures between 10 K and 300 K. We also provide a code, named Nahoon, to study the time-dependent gas-phase chemistry of zero-dimensional and one-dimensional interstellar sources.

/pdf/a-kinetic-database-for-astrochemistry-kida-21via2ms84.pdf

A KInetic Database for Astrochemistry (KIDA)

Experiments have lagged theory in exploring chemical interactions at temperatures so low that translational degrees of freedom can no longer be treated classically. The difficulty has been to realize in the laboratory low-enough collisional velocities between neutral reactants to access this regime. We report here the realization of merged neutral supersonic beams and the manifestation of clear nonclassical effects in the resulting reactions. We observed orbiting resonances in the Penning ionization reaction of argon and molecular hydrogen with metastable helium, leading to a sharp absolute ionization rate increase in the energy range corresponding to a few degrees kelvin down to 10 millikelvin. Our method should be widely applicable to many canonical chemical reactions.

https://science.sciencemag.org/content/sci/338/6104/234.full.pdf

Observation of Resonances in Penning Ionization Reactions at Sub-Kelvin Temperatures in Merged Beams

Rate coefficients k1 and k2 have been determined for the association reaction N+2+2 N2→k1N+4+N2 and O+2+2 O2→k2O+4+O2 in the temperature range 20–160 K, using a supersonic jet apparatus (CRESU), which is described in detail. For the highest temperatures where other measurements exist, the measured values of the rate coefficients are in excellent agreement with previous measurements. The temperature dependence of k1 can be represented by a power law of the form k1=6.0×10−29 (300/T)1.85 from 20 K to higher temperatures. For k2, only lower limits for 20 and 45 K are reported but the present data do not confirm the existence of a maximum in k2 as suggested by other authors. These results are examined in the light of recent theoretical arguments.

Study of the reactions N+2+2N2→N+4+N2 and O+2+2O2→O+4+O2 from 20 to 160 K by the CRESU technique

Axisymmetric contoured Laval nozzles have been calculated in order to obtain uniform supersonic flow. For nitrogen and oxygen gases the calculation uses only similarity techniques, but for helium, complete calculations of the isentropic core and the boundary layer were performed for two sets of boundary conditions. Experimental testing has shown that the nitrogen or oxygen flows exhibit a good uniformity but that the helium flow was uniform only for one set of boundary conditions. The purpose of this work is to provide flow reactors for studies of ion‐molecule reactions in the real temperature range of interstellar clouds (T<80 °K).

Design and testing of axisymmetric nozzles for ion‐molecule reaction studies between 20 °K and 160 °K

The reactions of N+(3P) ions with normal, para, and deuterated hydrogens have been examined at temperatures below 163 K by using the CRESU technique with both helium and nitrogen buffer gases. All these reactions have decreasing rate coefficients with decreasing temperature. Comparison of the data for normal and para‐H2 reveals the drastic importance of rotational energy in promoting these processes. Analysis of these results shows a better agreement between the n‐H2, p‐H2, and HD data if the spin–orbit energy of N+(3P) is also considered as efficient as kinetic and rotational energies in driving the reactions. It follows that the endothermicity of the reaction N+(3P0) +H2(X 1Σ+g,J=0) →NH+(X 2Π,J=0) +H(2S) is (18±2) meV. This yields a proton affinity of N equal to (3.524±0.003) eV and a dissociation energy of NH of (3.42±0.01) eV.

Reactions of N+(3P) ions with normal, para, and deuterated hydrogens at low temperatures

Ion—polar-molecule reactions: A CRESU study of He+, C+, N+ + H2O, NH3 at 27, 68 and 163 K

Rate coefficients k have been determined for the reaction O+2+CH4 → CH3O+2+H at several temperatures in the uniquely wide temperature range 20–560 K using a new expanding jet (CRESU) apparatus and a selected ion flow tube (SIFT) apparatus. In the overlapping temperature range of the experiments the rate coefficients are in excellent agreement. A minimum is observed in the rate constant near 300 K but the outstanding feature is the rapid increase at low temperatures from the minimum value k (290 K)=5.4×10−12 cm3 s−1 to k (20 K)=4.7×10−10 cm3 s−1, the latter being about half of the collisional limiting value kc for the reaction (kc=1.16×10−9 cm3 s−1). Indeed the data show that k → kc as T → 0 K and the low temperature values can be fitted to a power law of the form k=1.1×10−7 T−1.8. The results strongly indicate that the CH3O+2 product ion is formed via rearrangement in a long‐lived intermediate (CH4O2)+ complex, the lifetime against unimolecular decomposition of which largely controls the rate of the react...

J. B. Marquette

Papers

Study of the reactions N+2+2N2→N+4+N2 and O+2+2O2→O+4+O2 from 20 to 160 K by the CRESU technique

Design and testing of axisymmetric nozzles for ion‐molecule reaction studies between 20 °K and 160 °K

Reactions of N+(3P) ions with normal, para, and deuterated hydrogens at low temperatures

Ion—polar-molecule reactions: A CRESU study of He+, C+, N+ + H2O, NH3 at 27, 68 and 163 K

The reaction O+2+CH4 → CH3O+2+H studied from 20 to 560 K in a supersonic jet and in a SIFT