The Aromatic Infrared Bands (AIBs), attributed to Polycyclic Aromatic Hydrocarbon (PAH) species, are observed throughout space. These AIBs designate the vibrations of the CC and CH modes and are general for the entire group of PAHs. Nonetheless, variations of PAH populations are observed across the ISM represented by variations in band positions, intensities, and profiles. Charge induces large variations, leading to a clear distinction between neutral and cationic species. More subtle variations are harder to interpret and additional information on molecular specificity is required in order to differentiate between smaller PAH subgroups. Periphery and anharmonicity may strongly influence the vibrational bands and thus hold a key in the identification of certain subgroups. We performed mass and conformational-resolved, high-resolution spectroscopy of cold (~10K) 2-4 ring linear [1,2] and 3-5 ring isomeric [3,4] PAH molecules in the 3-µm CH stretching region. The measured IR spectra show many surprisingly strong modes, which we demonstrate to be Fermi mediated. All PAH isomers give unique spectra, dominantly affected by the periphery. We show that standard harmonic and anharmonic DFT calculations are not able to reproduce the observed molecular reality. Using our experimental findings and spectra as benchmark, large advances have been made with the programme Spectro and have lead to first band assignments in our high-resolution spectra.

[1] E. Maltseva et. al., Astrophys. J. 2015, 814, 23
[2] C. J. Mackie et. al., J. Chem. Phys. 2015, 143, 224314
[3] E. Maltseva et. al., in preparation
[4] C. J. Mackie et. al., in preparation