[SiPAH]+ pi-Complexes in the Interstellar Medium

TL;DR

This study uses quantum chemistry to analyze silicon-PAH complexes in space, revealing their stability, spectral features, and potential role in explaining observed infrared bands and silicon abundance in the interstellar medium.

Contribution

It provides the first detailed quantum chemical analysis of SiPAH pi-complexes, linking their spectral signatures to astronomical observations.

Findings

SiPAH+ complexes have binding energies of at least 1.5 eV.

Silicon coordination causes blueshifts in IR spectral features.

Approximately 1% of cosmic silicon may be attached to PAHs.

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.