Radical Recombination during the Phase Transition of Interstellar CO Ice

Complex organic molecules (COMs) can be produced efficiently in ice mixtures that simulate the ice mantle on cosmic dust grains, according to prior experimental studies. However, the mechanism that brings the reactive species together in the ice has been debated. Thermal diffusion, which is widely regarded as the main mechanism to bring reactants together, is inefficient at a typical dense cloud temperature of 10 K. A recent experimental study found that the transition of a thin CO ice film from the amorphous to crystalline phase happens at about 10 K. When a small fraction of CO2 was mixed with CO, the CO2 molecules can separate and form clusters during CO phase transition. It was further proposed that the separation of minor species in the CO ice during phase transition may be an important mechanism to form interstellar COMs without the need for thermal diffusion. In this study, we try to verify this new mechanism through laboratory experiments. An ice mixture of CH3OH and CO, which is an analog of the outer layer of the ice mantle on cosmic dust grains, was exposed to UV irradiation to produce radicals such as HCO and CH2OH, whose concentration was monitored during the subsequent warm-up of the ice. We find clear evidence that during the CO phase transition, most of the radicals recombine to form other molecular species, therefore supporting the recently proposed mechanism of COM formation via CO phase transition.