[SiPAH]+ pi-Complexes in the Interstellar Medium

Abstract

We investigate the presence of silicon atoms adsorbed on the surface of interstellar polycyclic aromatic hydrocarbons (PAHs) to form SiPAH pi-complexes. We use quantum chemistry calculations to obtain structural, thermodynamic and mid-IR properties of neutral and cationic SiPAH complexes. The binding energy was found to be at least 1.5 eV for [SiPAH]+ complexes whereas it is roughly 0.5 eV for their neutral counterparts. From the spectral analysis of the calculated IR spectra, we found that the coordination of silicon 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. 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 silicon that could account for (i) the blueshifted position of the 6.2 micron AIB and (ii) the presence of satellite bands observed on the blue side of the 6.2 and 11.2 micron AIBs. From such an assignment, we can deduce that typically 1% of the cosmic silicon appears to be attached to PAHs.

Accepted for publication in Astronomy and Astrophysics

References in corpus (5)

• The CH out-of-plane bending modes of PAH molecules in astrophysical environments
• The profiles of the 3 to 12 $\mu$m PAH features
• The infrared spectra of very large, compact, highly symmetric, polycyclic aromatic hydrocarbons (PAHs)
• Depletion patterns and dust evolution in the ISM
• Carriers of the mid-IR emission bands in PNe reanalysed. Evidence of a link between circumstellar and interstellar aromatic dust

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• Coupled Blind Signal Separation and Spectroscopic Database Fitting of the Mid Infrared PAH Features
• The PAH hypothesis after 25 years
• Positron Annihilation on Polycyclic Aromatic Hydrocarbon molecules in the Interstellar Medium
• Interpreting the subtle spectral variations of the 11.2 and 12.7 μm polycyclic aromatic hydrocarbon bands
• The 6 $\mu$m Feature as A Tracer of Aliphatic Components of Interstellar Carbonaceous Grains