Electron attachment rates for PAH anions in the ISM and dark molecular clouds: dependence on their chemical properties

Department of Chemistry and CNISMThe University of Rome “Sapienza”, P.le Aldo Moro 5, 00185 Rome, Italy
e-mail: fa.gianturco@caspur.it

Received: 11 July 2012
Accepted: 14 November 2012

Abstract

Context. The attachment of free electrons to polycondensed aromatic ring molecules (PAHs) is studied for the variety of these molecules with different numbers of condensed rings and over a broad range of electron temperatures, using a multichannel quantum scattering approach. The calculations of the relevant cross sections are used in turn to model the corresponding attachment rates for each of the systems under study, and these rates are parametrized as a function of temperature using a commonly employed expression for two-body processes in the interstellar medium (ISM).

Aims. The scope of this work is to use first principles to establish the influence of chemical properties on the efficiency of the electron-attachment process for PAHs.

Methods. Quantum multichannel scattering methods are employed to generate the relevant cross sections, hence the attachment rates, using integral elastic cross sections computed over a broad range of relevant energies, from threshold up to 1000 K and linking the attachment to low-energy resonant collisions.

Results. The rates obtained for the present molecules are found to markedly vary within the test ensemble of the present work and to be lower than the earlier values used for the entire class of PAHs anions, when modelling their evolutions in ISM environments. The effects of such differences on the evolutions of chemical networks that include both PAH and PAH-species are analysed in some detail and related to previous calculations.