The N₃Radical as a Discriminator between Ion‐irradiated And UV‐photolyzed Astronomical Ices

Infrared spectroscopy has been used to show that irradiation of solid N2 and N2-rich ices with 0.8 MeV protons produces the N3 (azide) radical. In contrast, no N3 was observed after solid N2 and N2-rich ices were photolyzed by far-UV photons. Isotopic substitution experiments support the N3 identification, as does an analysis of the reactions occurring in the ices. This is the first documented difference in reaction products between the radiation chemistry and photochemistry of a nonpolar astronomical ice analog. We suggest that this difference in reaction chemistries could be used to identify ion-irradiated ices on interstellar grains and in the outer solar system. Further, N3 might be used as a tracer of solid-phase interstellar N2, which is expected to exist in dark, molecular clouds but is difficult to observe directly. Although the absolute strength of the N3 band in solid N2 is unknown, we estimate that it is at least 100 times greater than that of the fundamental vibration of N2. Subject headings: ISM: molecules — line: formation — line: identification — molecular processes