2018ApJ...864..151R

Vacuum ultraviolet irradiation of a polycyclic aromatic hydrocarbon molecule, pyrene, trapped in carbon dioxide (CO₂)-enriched astrophysical ice analogues has been studied and presented here. Pyrene is co-deposited with a gaseous mixture of H₂O and CO₂ (100% CO₂; 50:50 CO₂:H₂O; and 100% H₂O) and is subjected to Ly_α photons from a hydrogen-flow microwave discharge lamp. To our surprise, we found ionization of pyrene in all three ices, with ionization yields following the H₂O content in the ice, indicating that H₂O ice stabilizes charged ions and electrons better than CO₂ ice by a factor of 10. Caution needs to be exercised not to overinterpret our finding that pyrene is ionized in pure CO₂ ice, because even trace amounts of water in the CO₂ matrix could result in an increased ionization of pyrene. In addition to ionization of pyrene in CO₂ ice, photochemical products of the CO₂ ice itself, namely CO and CO₃, were found to form efficiently, as detected using Fourier transform infrared spectroscopy, in agreement with earlier studies. UV-vis spectra showed formation of ozone (O₃) with prolonged irradiation. Surprisingly, O₃ yields followed CO₂ concentration in the ice mixtures, with the strongest bands in photolyzed CO₂ ice and the weakest in photolyzed H₂O ice, indicating that CO₂ ice is very protoreactive and produces more O atoms than H₂O ice. Pyrene-containing photoproducts, incorporating CO₂ or CO or O, such as Py-COOH, Py-OH, or Py-CHO, are not explicitly seen in the UV-vis absorption spectra, but we cannot rule out the possibility that their UV absorption may be swamped under the strong absorption of O₃.