THEORETICAL AND LABORATORY STUDIES ON THE INTERACTION OF COSMIC-RAY PARTICLES WITH INTERSTELLAR ICES .2. FORMATION OF ATOMIC AND MOLECULAR-HYDROGEN IN FROZEN ORGANIC-MOLECULES

Methane ices are irradiated at 4 x 10(-10) mbar at temperatures betwee n 10 and 50 K with 9.0 MeV alpha-particles and 7.3 MeV protons to eluc idate the formation of atomic as well as molecular hydrogen via intera ction of Galactic cosmic-ray particles with extraterrestrial organic i ces. Theoretical calculations focus on computer simulations of ion-ind uced collision cascades in irradiated targets. Our data reveal that mo re than 99% of the energy is transferred via inelastic interactions to the electronic system of the target to form electronically excited CH 4 molecules decomposing to a CH3--H radical pair. Two H atoms recombin e in a diffusion limited step to H-2. Further, secondary dissociation of CH3 to H and CH2 contributes to H production. To a minor amount, im planted ions generate C and H knock-on atoms via elastic encounters wh ich abstract hydrogen atoms or insert into chemical bonds (carbon atom s only). Fourier transform infrared spectroscopy (FTIR) and quadrupole mass spectrometry (QMS) analyses indicate that if these energy-loss p rocesses accumulate up to 6 +/- 3% H atoms in the CH4 target, more tha n 90% of the ice is released in an explosive ejection into the gas pha se. This mechanism represents a powerful pathway to supply newly forme d molecules from interstellar grains back to the gas phase of the inte rstellar medium even at temperatures as low as 10 K.