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 p articles with extraterrestrial, organic ices and to elucidate a mechan istic model to synthesize experimentally detected polycyclic aromatic hydrocarbons (PAHs). Theoretical calculations center on computer simul ations of ion-induced collision cascades in irradiated methane targets . MeV ions induce hydrogen and carbon knock-on particles in elastic en counters with the target atoms. Each primary knock-on triggers one col lision 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 o r addition to chemical bonds of a reactant molecule. In the relaxation phase of this energized volume, overlapping reaction zones likely for m observed PAHs napthalene, phenanthrene/azulene, and coronene. This m ulticenter mechanism establishes a versatile route to synthesize compl ex molecules in extraterrestrial ices even at temperatures as low as 1 0 K within cosmic-ray-initiated single collision cascades.