Photochemical Processes in CO 2 /H 2 O Ice Mixtures with Trapped Pyrene, a Model Polycyclic Aromatic Hydrocarbon

Abstract

Vacuum ultraviolet irradiation of a polycyclic aromatic hydrocarbon molecule, pyrene, trapped in carbon dioxide (CO 2 )-enriched astrophysical ice analogues has been studied and presented here. Pyrene is co-deposited with a gaseous mixture of H 2 O and CO 2 (100% CO 2 ; 50:50 CO 2 :H 2 O; and 100% H 2 O) 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 H 2 O content in the ice, indicating that H 2 O ice stabilizes charged ions and electrons better than CO 2 ice by a factor of 10. Caution needs to be exercised not to overinterpret our finding that pyrene is ionized in pure CO 2 ice, because even trace amounts of water in the CO 2 matrix could result in an increased ionization of pyrene. In addition to ionization of pyrene in CO 2 ice, photochemical products of the CO 2 ice itself, namely CO and CO 3 , were found to form efficiently, as detected using Fourier transform infrared spectroscopy, in agreement with earlier studies. UV–vis spectra showed formation of ozone (O 3 ) with prolonged irradiation. Surprisingly, O 3 yields followed CO 2 concentration in the ice mixtures, with the strongest bands in photolyzed CO 2 ice and the weakest in photolyzed H 2 O ice, indicating that CO 2 ice is very protoreactive and produces more O atoms than H 2 O ice. Pyrene-containing photoproducts, incorporating CO 2 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 O 3 .