Simulated Interstellar Photolysis of N2O Ice: Selectivity in Photoproducts

Abstract

Thermal diffusion and recombination control the kinetics of photochemical reactions of reactive radicals formed by ultraviolet photon irradiation in interstellar ices. Here, we show that upon vacuum ultraviolet photolysis, N₂O ice produces O₃ and several oxides of nitrogen, such as NO, NO₂, N₂O₂, N₂O₃, N₂O₄, and N₂O₅ in interstellar ice mimics. Photoproducts within the bulk and on the surface were analyzed using reflection absorption infrared spectroscopy and Cs⁺ ion-based secondary ion mass spectrometry, while desorbed species were studied using temperature-programmed desorption mass spectrometry. Notably, thermal annealing of the photoirradiated ice to 90 K resulted in a significant increase in NO and N₂O₃. Photoirradiation at 10 K revealed the dominance of three atom photoproducts, such as NO₂ and O₃. In contrast, irradiation at 50 K significantly enhanced the production of four or higher atom photoproducts (N₂O₂, N₂O₃, N₂O₄, and N₂O₅). This behavior is attributed to the restricted diffusion of reactive radicals and unstable oxygen species (O and O₃) at 10 K, which confines radical–radical reactions to three or fewer atom photoproducts, whereas higher temperatures facilitate oxygen and other radical diffusion and recombination, yielding heavier photoproducts. These results throw light on the thermal diffusion effects on the kinetics of photoproducts in interstellar ice mimics.