The hydrogen peroxyde radical, HO2 – a potential tracer of interstellar molecular oxygen

Methane/oxygen ice mixtures are irradiated in an ultra-high vacuum chamber at 10 K with 9.0 MeV α-particles to investigate the role of suprathermal oxygen atoms in the formation of oxygen-containing hydrocarbon molecules and to elucidate the formation of the hydrogen peroxyde radical, HO2, as a tracer for hitherto unobserved interstellar molecular oxygen, O2. Computer simulations calculate the elastic as well as inelastic energy loss of the implant to the irradiated targets. Our experimental data and calculation reveal that the kinetic energy of the implant is transferred almost exclusively via inelastic interactions to the ices yielding methyl radicals, CH3, and atomic hydrogen, H. These H atoms are mobile even at temperatures as low as 10 K and recombine barrier-less with matrix isolated molecular oxygen to form HO2 radicals monitored via Fourier transform infrared spectroscopy. In addition, alcohols and carbonic acids with up to 20 carbon atoms are found to be synthesized via O atom insertion, neighboring radical recombination, and/or a multi-center reaction mechanism. Formed on interstellar grains, subsequent grain-grain collisions, cosmic ray particle induced grain mantle evaporation, or explosive grain ejection could release these newly formed molecules into the gas phase of the interstellar medium.