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.

Interstellar fullerene compounds and diffuse interstellar bands

“…Although the 5 Si is known to bound on fully hydrogenated PAHs with a modest adsorption energy ∼1.5 eV (e.g. Joalland et al 2009) and with a large rate of accretion for Si + (Dunbar et al 1994). However, such exhohedral Si cannot survive long to interstellar UV photolysis.…”

The Infrared Emission Bands

“…Peeters et al 2002;Hudgins et al 2005;; ii) PAH-metal complexes (e.g. Hudgins et al 2005;Bauschlicher & Ricca 2009;Simon & Joblin 2010;Joalland et al 2009); iii) PAH clusters (e.g. Rapacioli et al 2005;).…”

Astronomical observations of the PAH emission bands

“…ii) PAH-metal complexes (e.g. Hudgins et al 2005;Bauschlicher & Ricca 2009;Simon & Joblin 2010;Joalland et al 2009). These are species in which a metal atom is located either below or above the carbon skeleton.…”