UV photoprocessing of NH3 ice: photon-induced desorption mechanisms

Abstract

Ice mantles detected on the surface of dust grains toward 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 onto a substrate at 8 K. The ice analogs 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 quantifica- tion of the photodesorption yields, leading to Ypd(NH3)=2.1+2.1-1.0 x 10^-3 molecules per incident photon, which remained constant during the whole experiments, while photodesorption of H2 and N2 increased with fluence, pointing toward an indirect photodesorption mecha- nism 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 (Ypd (N2) = 1.7+1.7-0.9 x 10^-3 molecules incident photon ).

References in corpus (18)

• Observations of the Icy Universe
• The Spitzer ice legacy: Ice evolution from cores to protostars
• Formation rates of complex organics in UV irradiated CH3OH-rich ices I: Experiments
• Photochemistry and astrochemistry: photochemical pathways to interstellar complex organic molecules
• Photodesorption of ices I: CO, N2 and CO2
• Photodesorption of water ice: a molecular dynamics study
• CO ice photodesorption: A wavelength-dependent study
• UV photodesorption of methanol in pure and CO-rich ices: desorption rates of the intact molecule and of the photofragments
• Molecular Excitation and Differential Gas-Phase Depletions in the IC 5146 Dark Cloud
• Vacuum-UV spectroscopy of interstellar ice analogs. I. Absorption cross-sections of polar-ice molecules
• CO and N${}_2$ desorption energies from water ice
• The negligible photodesorption of methanol ice and the active photon-induced desorption of its irradiation products
• Indirect ultraviolet photodesorption from CO:N2 binary ices - an efficient grain-gas process
• Wavelength-dependent UV photodesorption of pure N2 and O2 ices
• Vacuum-UV spectroscopy of interstellar ice analogs. II. Absorption cross-sections of nonpolar ice molecules
• Spectrally-resolved UV photodesorption of CH4 in pure and layered ices
• Herschel Observations of Far-Infrared Cooling Lines in intermediate Redshift (Ultra)-luminous Infrared Galaxies
• Interstellar ice analogs: band strengths of H${}_2$O, CO${}_2$, CH${}_3$OH, and NH${}_3$ in the far-infrared region

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