On the electron-induced isotope fractionation in low temperature 32O2/36O2 ices—ozone as a case study

Abstract

The formation of six ozone isotopomers and isotopologues, ¹⁶O¹⁶O¹⁶O, ¹⁸O¹⁸O¹⁸O, ¹⁶O¹⁶O¹⁸O, ¹⁸O¹⁸O¹⁶O, ¹⁶O¹⁸O¹⁶O, and ¹⁸O¹⁶O¹⁸O, has been studied in electron-irradiated solid oxygen ¹⁶O₂ and ¹⁸O₂ (1 ∶ 1) ices at 11 K. Significant isotope effects were found to exist which involved enrichment of ¹⁸O-bearing ozone molecules. The heavy ¹⁸O¹⁸O¹⁸O species is formed with a factor of about six higher than the corresponding ¹⁶O¹⁶O¹⁶O isotopologue. Likewise, the heavy ¹⁸O¹⁸O¹⁶O species is formed with abundances of a factor of three higher than the lighter ¹⁶O¹⁶O¹⁸O counterpart. No isotope effect was observed in the production of ¹⁶O¹⁸O¹⁶Oversus¹⁸O¹⁶O¹⁸O. Such studies on the formation of distinct ozone isotopomers and isotopologues involving non-thermal, non-equilibrium chemistry by irradiation of oxygen ices with high energy electrons, as present in the magnetosphere of the giant planets Jupiter and Saturn, may suggest that similar mechanisms may contribute to the ¹⁸O enrichment on the icy satellites of Jupiter and Saturn such as Ganymede, Rhea, and Dione. In such a Solar System environment, energetic particles from the magnetospheres of the giant planets may induce non-equilibrium reactions of suprathermal and/or electronically excited atoms under conditions, which are quite distinct from isotopic enrichments found in classical, thermal gas phase reactions.