Where does the energy go during the interstellar NH3 formation on water ice? a computational study
Ferrero, Stefano (Universitat Autònoma de Barcelona. Departament de Química)
Pantaleone, Stefano (Università degli Studi di Torino. Dipartimento di Chimica)
Ceccarelli, Cecilia (Centre National de la Recherche Scientifique. Institut de Planétologie et d'Astrophysique de Grenoble)
Ugliengo, Piero (Università degli Studi di Torino. Dipartimento di Chimica)
Sodupe Roure, Mariona (Universitat Autònoma de Barcelona. Departament de Química)
Rimola, Albert (Universitat Autònoma de Barcelona. Departament de Química)

Data:	2023
Resum:	In the coldest (10-20 K) regions of the interstellar medium, the icy surfaces of interstellar grains serve as solid-state supports for chemical reactions. Among their plausible roles, that of third body is advocated, in which the reaction energies of surface reactions dissipate throughout the grain, stabilizing the product. This energy dissipation process is poorly understood at the atomic scale, although it can have a high impact on astrochemistry. Here we study, by means of quantum mechanical simulations, the formation of NH3 via successive H-additions to atomic N on water ice surfaces, paying special attention to the third-body role. We first characterize the hydrogenation reactions and the possible competitive processes (i. e. , H-abstractions), in which the H-additions are more favorable than the H-abstractions. Subsequently, we study the fate of the hydrogenation reaction energies by means of ab initio molecular dynamics simulations. Results show that around 58%-90% of the released energy is quickly absorbed by the ice surface, inducing a temporary increase of the ice temperature. Different energy dissipation mechanisms are distinguished. One mechanism, more general, is based on the coupling of the highly excited vibrational modes of the newly formed species and the libration modes of the icy water molecules. A second mechanism, exclusive during the NH3 formation, is based on the formation of a transient H3O+/NH2− ion pair, which significantly accelerates the energy transfer to the surface. Finally, the astrophysical implications of our findings relative to the interstellar synthesis of NH3 and its chemical desorption into the gas are discussed.
Ajuts:	European Commission 741002 European Commission 865657 European Commission 811312
Drets:	Aquest document està subjecte a una llicència d'ús Creative Commons. Es permet la reproducció total o parcial, la distribució, la comunicació pública de l'obra i la creació d'obres derivades, fins i tot amb finalitats comercials, sempre i quan es reconegui l'autoria de l'obra original.
Llengua:	Anglès
Document:	Article ; recerca ; Versió publicada
Publicat a:	The Astrophysical journal, Vol. 944, Issue 2 (February 2023) , art. 142, ISSN 1538-4357

DOI: 10.3847/1538-4357/acae8e

12 p, 4.1 MB

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