Signature of [SiPAH]$^+$ $\pi$-complexes in the interstellar medium

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  μ m AIB and (ii) the presence of satellite bands observed on the blue side of the 6.2 and 11.2  μ m AIBs. From such an assignment, we can deduce that typically 1% of the cosmic Si appears to be attached to PAHs.