Polycyclic aromatic hydrocarbon (PAH) molecules are now widely accepted to occur abundantly in interstellar clouds, as evidenced by the comparison of interstellar infrared emission spectra with laboratory recorded spectra of PAH compounds. However, only a relatively small subset of PAH compounds are amenable to experimental spectroscopic investigation using standard IR spectrometers, in particular neutral closed-shell PAHs and – with the use of matrix isolation spectroscopy – some of their radical cations. The vast majority of spectra used for comparison are based on results from quantum-chemical calculations, which obviously pose fewer constraints on the classes of species that can be investigated; species include for instance closed-shell cations, neutral radicals, metal-bound species, anionic PAHs, odd-carbon PAHs, endohedral PANHs, etc.<br/><br/>Using a tandem mass spectrometer, a larger variety of classes of PAHs including some of those mentioned above becomes experimentally accessible. Ion chemistry can be used to produce these species and their state of complete isolation in the vacuum ensures their stability. Recording IR spectra of these mass-selected isolated ionic species has become possible with the use of IR photodissociation spectroscopy applying powerful and tunable IR laser sources, such as free electron lasers and OPO sources. In this contribution, we present IR spectra for various species that are otherwise hard, if not impossible, to access experimentally. Use of the FELIX source moreover allows us to probe the critically important wavelength range between 6 and 16 μm. Species include anionic PAHs (see Figure), even-electron cationic PAHs and metal complexes.

Laboratory infrared spectroscopy of ‘hard-to-get’ ionized polyaromatics

249th National Meeting (2015)

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