Unusual Stability of Polycyclic Aromatic Hydrocarbon Radical Cations in Amorphous Water Ices up to 120 K: Astronomical Implications

Abstract

The PAH radical cation quaterrylene+ (QTR+, C40H20) is found to be stable in amorphous water ice up to 120 K. A careful, 30 day, slow warm-up of water ice containing QTR+from 20 to 190 K revealed that QTR+ does not show any sign of reactivity up to 120 K, the temperature at which the phase transition of amorphous to crystalline water ice begins. At higher temperatures the absorption due to the QTR radical cation diminishes rapidly but persists until the water ice itself sublimes around 170 K. From the absence of evidence for the recovery of neutral QTR upon warm-up or reactions with other trapped reaction intermediates in the ice, we infer that QTR+ reacts with the water ice itself during warm-up above 120 K. Earlier we found that PAH ionization is quantitative in water ice and PAH ionization energy is lowered by up to 2 eV. Some implications of these unusual findings for cosmic ice chemistry and physics are briefly discussed.