Theoretical and laboratory studies on the interaction of cosmic-ray particles with interstellar ices .1. Synthesis of polycyclic aromatic hydrocarbons by a cosmic-ray-induced multicenter mechanism

Methane, ethylene, and acetylene ices were irradiated in a ultra-high vacuum vessel between 10 K and 50 K with 7.3 MeV protons as well as 9.0 MeV He2+ nuclei to simulate the interaction of galactic cosmic-ray particles with extraterrestrial, organic ices and to elucidate a mechanistic model to synthesize experimentally detected polycyclic aromatic hydrocarbons (PAHs). Theoretical calculations center on computer simulations of ion-induced collision cascades in irradiated methane targets. MeV ions induce hydrogen and carbon knock-on particles in elastic encounters with the target atoms. Each primary knock-on triggers one collision cascade with up to 70 suprathermal carbon atoms concentrated in one to two subcascades in 0.6-5 x 10(3) Angstrom(3). At the end point of each single trajectory, every suprathermal carbon atom can form an individual reaction center of hydrogen abstraction and insertion in or addition to chemical bonds of a reactant molecule. In the relaxation phase of this energized volume, overlapping reaction zones likely form observed PAHs napthalene, phenanthrene/azulene, and coronene. This multicenter mechanism establishes a versatile route to synthesize complex molecules in extraterrestrial ices even at temperatures as low as 10 K within cosmic-ray-initiated single collision cascades.