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

ThePAHradicalcationquaterrylene + (QTR + ,C40H20)isfoundtobestableinamorphouswatericeupto120K.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 icebegins. AthighertemperaturestheabsorptionduetotheQTRradicalcationdiminishesrapidlybutpersistsuntil 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. Subject headinggs: ISM: clouds — methods: laboratory — molecular processes — radiation mechanisms: nonthermal — ultraviolet: ISM — ultraviolet: solar system