Contribution Contributed Talk
UV-VIS spectroscopy of PAH cations in water ice
Polycyclic aromatic hydrocarbons (PAHs) are volatile species that are expected to freeze out under dark interstellar cloud conditions on icy grains where they are exposed to UV photons. In the laboratory, PAHs embedded in water matrices, as well as their photo-induced derivatives, have been largely studied using infrared spectroscopy (IR) as the principle diagnostic tool. This method benefits from characteristic vibrational modes, but suffers from water absorptions in this range. As a consequence, relatively concentrated ice mixtures are needed. This does not reflect astronomically realistic conditions and moreover, the high concentrations may influence the overall chemistry. Here UV-VIS spectroscopy offers a powerful alternative. Water is practically transparent in this range and oscillator strengths are typically orders of magnitude larger. In this way highly dilute PAH:water mixtures become accessible. Here we focus on the formation of PAH cations in the solid state. Using our setup, OASIS (Optical Absorption Setup for Ice Spectroscopy 1, electronic solid state spectra of PAHs and their cations are recorded in the 250-750 nm range using the dispersed light of a xenon arc lamp that crosses the ice sample. This ice is irradiated with vacuum UV photons generated by a microwave discharge lamp. Besides spectroscopic data this also provides dynamical information as the consumption of precursor material and the formation of new species can be recorded in quasi real time.
Here we present results of UV processed PAHs embedded in water ice with the focus on a number of recently obtained spectra triphenylene and its cation. We also discuss the possibility to use optical solid state spectra of PAHs in ice as a new concept to search for larger aromatic species in the ISM, namely through their electronic ice spectra.
1 Bouwman, J.; Paardekooper, D. M.; Cuppen, H. M.; Linnartz, H.; Allamandola, L. J., The Astrophysical Journal 700 (2009) 56-62.
 Kofman V.; Sarre, P. J.; ten Kate, I.L.; Linnartz H., Astronomy and Astrophysics, submitted