Anharmonicity is often neglected in the calculation of the near-infrared spectrum of polycyclic aromatic hydrocarbons (PAHs) as these calculations are time consuming. At the University of Amsterdam, we performed mass and conformational-resolved, high-resolution spectroscopy of cold (~10K) linear PAH molecules starting with naphthalene in the 3-µm CH stretching region. The measured infrared spectra show vibrationally resolved bands and, surprisingly, many more strong modes than expected. Harmonic and preliminary anharmonic DFT calculations were performed at SurfSARA supercomputer, for each investigated acene using Gaussian '09 and the B9-71 functional. Both the harmonic and anharmonic calculations are not able to reproduce the observed molecular reality. A measurement of the deuterated counterparts that separates the normal modes from the combination bands, lead to zero signal, demonstrating the anharmonic bands to be Fermi mediated. Preliminary assignments are presented, aided by comparison between the observed rotational contour and the symmetry of candidate bands.

Anharmonic bands in the 3-µm region of acenes: A combined expermental and theoretical study

249th National Meeting (2015)

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