2006ApJ...646..666S

The observed depletion of iron in the interstellar medium has been suggested to result from its efficient complexation with, among others, polycyclic aromatic hydrocarbon molecules (PAHs). We present here the first experimental vibrational spectra of cationic iron-PAH complexes with benzene, naphthalene, and fluorene. The spectra were obtained by infrared multiple-photon dissociation (IRMPD) spectroscopy of the complex ions trapped in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. A continuously tunable free electron laser provided intense radiation in the astrophysically interesting wavelength range of 6-16 µm. Supporting calculations of the geometries, relative stabilities, and harmonic vibrational frequencies of the complexes were carried out using density functional theory with the MPW1PW91/6-31+G(d,p) functional/basis set. In all cases, the experimental spectra indicate that Fe⁺ is bound to the six-membered carbon ring of the aromatic ligand in a high-spin (quartet) electronic ground state for the mono-complexes, Fe(benzene)⁺ and Fe(naphthalene)⁺, and in a low-spin (doublet) electronic ground state for the bis-complexes, Fe(benzene)⁺₂, Fe(naphthalene)⁺₂, and Fe(fluorene)⁺₂ complexes. Comparison of the complex spectra to the bare (neutral and cationic) PAH spectra reveals their spectroscopic fingerprint, e.g., in the splitting of the out-of-plane CH-bending modes, which could aid in their interstellar detection.