2018MNRAS.473.2575M

Ice mantles detected on the surface of dust grains towards the coldest regions of the interstellar medium can be photoprocessed by the secondary ultraviolet (UV) field present in dense cloud interiors. In this work, we present UV-irradiation experiments under astrophysically relevant conditions of pure NH₃ ice samples in an ultra-high vacuum chamber where solid samples were deposited on to a substrate at 8 K. The ice analogues were subsequently photoprocessed with a microwave-discharged hydrogen-flow lamp. The induced radiation and photochemistry led to the production of H₂, N₂ and N₂H₄. In addition, photodesorption to the gas phase of the original ice component, NH₃, and two of the three detected photoproducts, H₂ and N₂, was observed thanks to a quadrupole mass spectrometer (QMS). Calibration of the QMS allowed quantification of the photodesorption yields, leading to Y_pd (NH₃) = 2.1^{+2.1}_{-1.0} x 10^–3 molecules/{incident photon}, which remained constant during the whole experiments, while photodesorption of H₂ and N₂ increased with fluence, pointing towards an indirect photodesorption mechanism involving energy transfer for these species. Photodesorption yield of N₂ molecules after a fluence equivalent to that experienced by ice mantles in space was similar to that of the NH₃ molecules (Y_pd (N₂) = 1.7^{+1.7}_{-0.9} x 10^–3 molecules/{incident photon}).