Low-temperature chemistry induced by cosmic rays: positive and negative
  ions desorption from nitrile-bearing astrophysical ice analogues
  In cold core of dark molecular clouds, where the UV radiation from external
sources is strongly attenuated, cosmic rays can induce chemical reactions on
the surface of ice-covered grains promoting the ejection of the processed
material to the gas phase. We report the positive and negative secondary ion
emission from pure CH3CN, C2H3CN and i-C3H7CN ices due to the bombardment of
heavy ions (252Cf fission fragments), simulating the incidence of cosmic rays
onto icy surfaces. The secondary ions emitted from each sample were analysed by
time-of-flight mass spectrometry (TOF-MS), using Plasma Desorption Mass
Spectrometry (PDMS) technique. Several ionic species were identified,
indicating strong fragmentation on the frozen surface. Proton-transfer
processes are suggested to play a role for positive ion desorption, as
evidenced by the protonated RCNH+ parent molecules and (RCN)nH+ ionic clusters.
The high electron affinity of the cyano radical seems to contribute to the
strong emission of CN-, as well as anions attributed to the CHmCN- fragment and
(RCN)nCN- cluster series. Sputtering and desorption of ion clusters (positive
and negative) induced by heavy ion bombardment are suggested to constitute a
route by which new neutral or ionised molecular species may be delivered to the
gas phase where thermal desorption is negligible.
