FTIR measurements of ammonia formation by the successive H-atom addition to N-atom in an N_2 matrix at 10K

Details FTIR measurements of ammonia formation by the successive H-atom addition to N-atom in an N_2 matrix at 10K FTIR measurements of ammonia formation by the successive H-atom addition to N-atom in an N_2 matrix at 10K

May 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280p.198h&link_type=abstract

The Molecular Universe, Posters from the proceedings of the 280th Symposium of the International Astronomical Union held in Tole

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Scientific paper

The abundant ammonia (NH_3) has been observed in not only the gas phase but also the solid phase in molecular clouds. The clarification of the formation mechanism of NH_3 is an important theme in the field of astrochemistry, because that formation process is a crucial piece in the story of the chemical evolution in molecular clouds and relates to the synthesis of N-bearing molecules, including amino acids. As the formation process of NH_3 in the gas phase, the combination of successive H-atom abstraction reactions, N^+ + H_2 → NH^+ + H NH^+ + H_2 → NH_2^+ + H NH_2^+ + H_2 → NH_3^+ + H NH_3^+ + H_2 → NH_4^+ + H and subsequent dissociative recombination reaction, NH_4^+ + e^- → NH_3 + H have been proposed. However, this route is considered to be inefficient in cold molecular clouds, because the first abstraction reaction was reported to be slightly endothermic. As the other process, especially for the low temperature region in molecular clouds, the formation by the grain-surface reaction which is the successive H-atom addition to N atom on grain surface, N → NH → NH_2 → NH_3 was predicted theoretically. Since the grain surface can act as the third body for absorbing the excess energy of addition reactions, this process can proceed efficiently without dissociation. In addition, these reactions are expected fast because of the radical-radical reaction. However, the experimental study of this reaction process was little conducted. The NH_3 formation by this process was only in a temperature programmed desorption measurement. We report the FTIR measurements of NH_3 formation by the co-deposition of H- and N-atoms on a cold substrate at temperatures below 20K. NH_3 was detected at 10K but not at 20 K. This indicates that the NH_3 formation proceeds by the Langmuir-Hinshelwood mechanism at 10K, while the H-atoms hardly adsorb and/or those residence time on the surface is very short to react at 20K. Furthermore, even when the sample of N atom in the N_2 matrix was covered with the further deposited pure N_2 solid, NH_3 was produced significantly by the H-atom exposure on the top layers. This implies that NH_3 formation proceeds with the H-atom diffusion in solid N_2 at 10K.

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