UV photoprocessing of NH3 ice: photon-induced desorption mechanisms
  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 ).
