Condensed-Phase Photochemistry in the Absence of Radiation Chemistry

We report postirradiation photochemistry studies of condensed ammonia using photons of energies below condensed ammonia's ionization threshold of similar to 9 eV. Hydrazine (N2H4), diazene (also known as diimide and diimine; N2H2), triazane (N3H5), and one or more isomers of N3H3 are detected as photochemistry products during temperature-programmed desorption. Product yields increase monotonically with (1) photon fluence and (2) film thickness. In the studies reported herein, the energies of photons responsible for product formation are constrained to less than 7.4 eV. Previous post-irradiation photochemistry studies of condensed ammonia employed photons sufficiently energetic to ionize condensed ammonia and initiate radiation chemistry. Such studies typically involve ion-molecule reactions and electron-induced reactions in addition to photochemistry. Although photochemistry is cited as a dominant mechanism for the synthesis of prebiotic molecules in interstellar ices, to the best of our knowledge, ours is one of the first astrochemically relevant studies that has found unambiguous evidence for condensed-phase chemical synthesis induced by photons in the absence of ionization.