Formation of nitrogen-and oxygen-bearing molecules from radiolysis of nitrous oxide ices - implications for Solar system and interstellar ices

The radiolysis of pure N2O ice at 11 and 75 K by 90 (MeVXe23)-Xe-136+ ion irradiation has been studied by Fourier-transformed infrared spectroscopy (FTIR). Six daughter molecular species have been observed: NO2, (NO)(2), N2O3, N2O4, N2O5, and O-3. The chemical evolution of the new molecules formed in the sample was followed by the measurement of the column densities of the precursor and products as a function of the beam fluence. This procedure allows the determination of their formation and dissociation cross-sections. Other processes monitored by FTIR were sublimation (non-existent at 11 K, but present at 75 K) and ice compaction by the ion beam. Comparison between results obtained for the 11 and 75 K ices shows that formation and destruction cross-sections are higher (for light products) or much higher (for heavy products) at 75 K. This enhancement of chemical activity at higher temperature should not be attributed to higher projectile ionization rate but rather to a higher mobility of the radiolysis products in an ice undergoing slow sublimation. Although N2O ice has not yet been observed in space, it is reasonable to expect its occurrence since N and O are very abundant and reactive. Furthermore, if this ice is actually absent, the knowledge of the chemical-physical processes induced by ion irradiation on N2O ice at low temperature is necessary to explain its depletion.