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Theoretical determination of adsorption and ionisation energies of polycyclic aromatic hydrocarbons on water ice

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Authors and affiliations : Eric Michoulier (a ;b) Nadia Ben Amor (a) Mathias Rapacioli (a) Jennifer A. Noble (b) Joëlle Mascetti (c) Céline Toubin (b) and Aude Simon* (a)
Abstract : In dense interstellar environments, Polycyclic Aromatic Hydrocarbons (PAHs) are likely to condense onto or integrate into water ice mantles covering dust grains. Understanding the role of ice in the photo-induced processes involving adsorbed PAHs is therefore a key issue in astrochemistry. This requires -(i)- the knowledge of PAH-ice interactions, i:e : PAH-ice adsorption energies and local structures at the PAH-ice interface, as well as -(ii)- the understanding of the fate of electrons in the PAH-ice system upon excitation. Regarding -(i)-, in this work, we determined the lowest energy structures of PAH-ice systems for a variety of PAHs ranging from naphthalene to ovalene on three types of ice – crystalline (Ih and Ic) and amorphous (low density) – using an explicit description of the electrons and a finite-sized system. The electronic structure was determined using the Self Consistent Charge Density Functional based Tight Binding
(SCC-DFTB) scheme with modified Mulliken charges in order to ensure a good description of the studied systems. Regarding -(ii)-, the influence of the interaction with ice on the Vertical Ionisation Potentials (VIPs) of the series of PAHs was determined using the constrained SCC-DFTB scheme benchmarked against correlated wavefunction results for PAH-(H2O)n (n=1–6,13) clusters. The results show a deviation equal, at most, to ∼ 1.4 eV of the VIPs of PAHs adsorbed on ice with respect to the gas phase values. Our results are discussed in the light of experimental data and previous theoretical studies.
Journal : Journal : Physical Chemistry Chemical Physics PCCP