Orateur
M.
Christian Fernando MEJíA GUAMáN
(PUC Rio)
Description
Methane (CH$_{4}$) and methanol (CH$_{3}$OH) ices are present in various astrophysical environments, from dense molecular clouds to several small objects in the outer solar system, in particular on Saturn satellite Triton and to be a constituent of the icy mantle on interstellar grain [1]. There is a clear lack of information about the phenomena induced by the heavy-ion component of cosmic-rays in the electronic-energy-loss regime.
In this work, the chemical and physical effects induced by fast heavy ions irradiation on frozen pure methane and methanol at 15 K are studied. Measurements were performed at the medium energy
beam-line of the heavy ion accelerator GANIL (Grand Acc\'el\'erateur National d'Ions Lourds), Caen-France [2]. The analysis was done by infrared spectroscopy (FTIR) during irradiation by 220 MeV $^{16}$O$^{7+}$ ion beam. For the case of methane, the principal molecular species identified as a product after irradiation are: CH$_{3}$, C$_{2}$H$_{2}$, C$_{2}$H$_{4}$, C$_{2}$H$_{6}$ and C$_{3}$H$_{8}$. For methanol ices are: H$_{2}$CO, CH$_{2}$OH, CH$_{4}$, CO, CO$_{2}$, HCO and HCOOCH$_{3}$ other products are identified with ambiguity. Their formation and dissociation cross sections are determined. The cross section of CH$_{4}$ and CH$_{3}$OH and its daughters species follows a power law as a function of the electronic stopping power. It is found that, some daughters species cross sections increase with the electronic stopping power roughly as $\sigma\propto$ $S_{e}^{3/2}$. As astrophysical implication, the $S_{e}^{n}$ power law, where $n\approx3/2$ should be very helpful for predicting the CH$_{4}$ and CH$_{3}$OH formation and the dissociation cross sections for other ion beam projectiles and energies in the ISM rich in hydrocarbons that are continuously bombarded by cosmic rays.
