2010ApJ...712...69S

The effects of the π-coordination of an Fe atom on the mid- and far-infrared spectra of a mixture of cationic polycyclic aromatic hydrocarbons (PAHs), e.g., pyrene (C₁₆H₁₀), anthanthrene (C₂₂H₁₂), coronene (C₂₄H₁₂), ovalene (C₃₂H₁₄), circumpyrene (C₄₂H₁₆), and circumcoronene (C₅₄H₁₈), are studied by Density Functional Theory based calculations. In the mid-infrared range (3-20 µm), by comparison with the bare PAH⁺ spectrum, we found (1) an increase of the intensity ratio of the C-H stretching and C-H out-of-plane bending bands with respect to the intense CC stretching band and (2) a shift of the band positions and a characteristic profile with a steep blue rise and an extended red tail for the CC stretching and CH out-of-plane bending bands. None of these features appears inconsistent with the observed aromatic infrared band spectrum. In the far-infrared range (λ > 20 µm), the presence of a π-coordinated Fe atom induces many new bands as (1) some vibrational modes of the PAH are activated due to symmetry reduction and (2) new modes involving the motion of the Fe atom occur. In particular, an accumulation point due to the activation of the Fe-PAH stretching mode is observed at around 40 µm. This range is suggested to contain the spectral fingerprint for the presence of [M-PAH]⁺ (M=Fe, Si, Mg) complexes in the interstellar medium. Additional features in the [60-300] µm range are found for complexes with large PAHs. The obtained results are discussed in the light of past, present, and future astronomical missions, among which are the Herschel Space Observatory and the SPICA telescope for the far-infrared domain.