Formation of unsaturated hydrocarbons in interstellar ice analogs by
  cosmic rays
  The formation of double and triple C-C bonds from the processing of pure
c-C6H12 (cyclohexane) and mixed H2O:NH3:c-C6H12 (1:0.3:0.7) ices by
highly-charged, and energetic ions (219 MeV O^{7+} and 632 MeV Ni^{24+}) is
studied. The experiments simulate the physical chemistry induced by medium-mass
and heavy-ion cosmic rays in interstellar ices analogs. The measurements were
performed inside a high vacuum chamber at the heavy-ion accelerator GANIL
(Grand Accel\'erat\'eur National d'Ions Lourds) in Caen, France. The gas
samples were deposited onto a polished CsI substrate previously cooled to 13 K.
In-situ analysis was performed by a Fourier transform infrared (FTIR)
spectrometry at different ion fluences. Dissociation cross section of
cyclohexane and its half-life in astrophysical environments were determined. A
comparison between spectra of bombarded ices and young stellar sources
indicates that the initial composition of grains in theses environments should
contain a mixture of H2O, NH3, CO (or CO2), simple alkanes, and CH3OH. Several
species containing double or triple bounds were identified in the radiochemical
products, such as hexene, cyclohexene, benzene, OCN-, CO, CO2, as well as
several aliphatic and aromatic alkenes and alkynes. The results suggest an
alternative scenario for the production of unsaturated hydrocarbons and
possibly aromatic rings (via dehydrogenation processes) in interstellar ices
induced by cosmic ray bombardment.
