UV photoprocessing of NH3 ice: photon-induced desorption mechanisms

Martín Domenech, R., Cruz Díaz, G. A., Caro, G. M. M. 2018. UV photoprocessing of NH3 ice: photon-induced desorption mechanisms. Monthly Notices of the Royal Astronomical Society 473, 2, 2575-2582, DOI: 10.1093/mnras/stx2510

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 H-2, N-2 and N2H4. In addition, photodesorption to the gas phase of the original ice component, NH3, and two of the three detected photoproducts, H-2 and N-2, 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(-1.0)(+2.1) x 10(-3) molecules/incident photon, which remained constant during the whole experiments, while photodesorption of H-2 and N-2 increased with fluence, pointing towards an indirect photodesorption mechanism involving energy transfer for these species. Photodesorption yield of N-2 molecules after a fluence equivalent to that experienced by ice mantles in space was similar to that of the NH3 molecules (Y-pd (N-2) = 1.7(-0.9)(+1.7) x 10(-3) molecules/incident photon).

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