There are several astrophysical, atmospheric and combustion problems for which there exists a need for the electronic and vibrational spectra of aromatic radicals and ions. These comprise the identity of the carriers of the diffuse interstellar bands; the Red Rectangle bands, and the "unidentified infrared bands" (aromatic infrared bands); atmospheric oxidative intermediates; and precursors to soot formation.
Leading candidates for all three astrophysical problems are polycyclic aromatic hydrocarbons (PAHs). Yet, despite decades of spectra being recorded, no definite match between any PAH and an astrophysical spectroscopic feature has been obtained. One explanation is that these bands are carried by larger, more complicated molecules than studied before, perhaps in exotic spin and charge states. There is thus a need to develop techniques to observe spectra of large PAH radicals and ions.
I will present spectra recorded in the Sydney laboratory over the last decade of a range of PAHs, their radicals and lately cations. In the last months, we have developed a three-laser scheme to record the excitation spectrum of a very cold (few K) PAH cation - namely protonated naphthalene, and recently, we have recorded the infrared spectrum of its neutral, the hydronaphthyl radical, using a three laser scheme.