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Astron. Astrophys. 344, 310-316 (1999)

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Do H atoms stick to PAH cations in the interstellar medium?

E. Herbst 1,2 and V. LePage 3

1 Department of Physics, The Ohio State University, Columbus, OH 43210, USA
2 Department of Astronomy, The Ohio State University, Columbus, OH 43210, USA
3 Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA

Received 8 October 1998 / Accepted 7 December 1998


Recent experimental work has shown that under relatively high density conditions, selected PAH cations associate efficiently with H atoms to form protonated PAH ions. If such association reactions occur efficiently under low density conditions, the protonated PAH ions can be considered as candidate molecular species in diffuse interstellar clouds. The high density and low density processes are not the same, however, since at high density association proceeds by collisional stabilization while at low density it proceeds by radiative stabilization. In this paper, we show using theoretical techniques that for the cation C6H[FORMULA], the laboratory results do not prove that the association with H is efficient at low densities, whereas for C10H[FORMULA] and most probably larger PAH cations, the radiative association with H is very efficient. For these latter cations, the competitive H-atom transfer channels (e.g. [FORMULA]), which can lead to catalytic formation of H2 by regeneration of the PAH cations, are probably not important.

Key words: molecular processes – ISM: molecules – ISM: clouds – ISM: abundances

Send offprint requests to: Eric Herbst (herbst@ohstpy.mps.ohio-state.edu

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© European Southern Observatory (ESO) 1999

Online publication: March 10, 1999