Signature of [SiPAH]⁺ π-complexes in the interstellar medium

Abstract

Aims: We investigate the presence of silicon atoms adsorbed on the surface of interstellar polycyclic aromatic hydrocarbons (PAHs) to form SiPAH π-complexes.
Methods: We used quantum chemistry calculations to obtain the structural, thermodynamic, and mid-IR properties of neutral and cationic SiPAH π-complexes.
Results: The binding energy was found to be at least 1.5 eV for [SiPAH]⁺ complexes, whereas it is ~0.5 eV for their neutral counterparts. From the spectral analysis of the calculated IR spectra, we found that the coordination of Si to PAH⁺ does not strongly affect the intensities of the PAH⁺ spectra, but systematically introduces blueshifts of the C-C in-plane and the C-H out-of-plane bands.
Conclusions: The thermodynamic data calculated for [SiPAH]⁺ complexes show that these species are stable and can be easily formed by radiative association of Si⁺ and PAH species that are known to be abundant in photodissociation regions. Their mid-IR fingerprints show features induced by the coordination of Si that could account for (i) the blueshifted position of the 6.2 μm AIB and (ii) the presence of satellite bands observed on the blue side of the 6.2 and 11.2 μm AIBs. From such an assignment, we can deduce that typically 1% of the cosmic Si appears to be attached to PAHs.

This work is based on observations made with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands, and the UK) and with the participation of ISAS and NASA.