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

TL;DR

This study uses quantum scattering methods to calculate electron attachment rates for PAH molecules in the interstellar medium, revealing significant variations based on chemical properties and implications for chemical network models.

Contribution

First-principles quantum calculations of electron attachment rates for PAHs, highlighting the dependence on chemical properties and providing updated rates for astrophysical models.

Findings

Attachment rates vary significantly among different PAHs.

Previous estimates of PAH anion formation rates were higher than current findings.

Differences in rates impact chemical evolution models in the ISM.

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). AIM: 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.