Mathematics – Probability
Scientific paper
May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280p.287p&link_type=abstract
The Molecular Universe, Posters from the proceedings of the 280th Symposium of the International Astronomical Union held in Tole
Mathematics
Probability
Scientific paper
As part of the Molecules and Interstellar Ices project, we report the first theoretical results obtained on one of the possible mechanisms for the reconstruction of the ice mantle of interstellar grains after irradiation by the radiation field. In the ISM, the growth of the ice mantle is assumed to proceed more by radical recombination O + H + H than by direct condensation on the grain (Kouchi et al. A&A 1994). Due to the extremely small probability of finding these three species on the same site simultaneously, water formation is thought to be a two-step process:
H + O → OH (1)
OH + H → H2O (2a)
Another mechanism has recently been found to operate in the cryogenic environments of noble gas matrices (Krim et al. in preparation)
OH + OH → H2O + O (2b)
and the O:H2O complex has been characterized by FTIR spectroscopy. Reaction (2b) opens a new scenario that could be effective in the reconstruction of the ice by using the same OH radicals formed by the irradiation. Here we present a theoretical study of reaction (2b) in a model environment - in the presence of an increasing number of water molecules (DFT ; MP2 ; CCSD ; CCSD(T)) - in the presence of an ice surface extracted from the ice crystal (DFT) - in the presence of an ordered and periodic crystalline surface (periodic DFT/VASP) The first study performed in the gas phase shows that the reaction is exothermic (ΔH˜16 kcal/mol) with an activation barrier (ΔE˜2 kcal/mol) in such condition. In a second step, the OH + OH reaction is studied, taking into account the interaction of all species with the ice surface along the whole reaction path; The characteristic points on the potential energy surface, including adsorption sites and transition states, have been determined for each situation. It is found that the activation barrier vanishes as soon as an aggregate of three water molecules or more represents the surface. In addition to the formation of H2O, reaction (2b) provides atomic oxygen ready to take a hydrogen atom to form another OH radical to restart the process. The ice mantle could then be reformed by in situ recombination. Our results suggest also that the OH radical that is a very reactive species in the ice could be involved in a large number of processes implying complex organic molecules (see communication by Pernet et al).
Ellinger Yves
Markovits A.
Pauzat Francoise
Redondo Pablo
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