Laboratory Investigation of Simultaneous Ultraviolet Photoprocessing and Temperature-Programmed Desorption of Interstellar Ice Analogs

Ice mantles on dust grains in the interstellar medium and protoplanetary disks are sites that allow for complex chemistry to occur. The formation of interstellar complex organic molecules (iCOMs) in these astronomical environments relies on energetic processes such as photochemistry driven by ultraviolet (UV) photons and thermal processes. Simultaneous versus subsequent UV photoprocessing and temperature-programmed desorption (TPD) on pure methanol and methanol and water mixtures (14-21%) under laboratory conditions were studied to mimic conditions in dense clouds and disks. In experiments where the ice was irradiated and heated simultaneously, results suggest that comparing between different experiments with fluence but not flux or time held constant may be unreliable for mixed ices for some chemical species. This finding indicates that experiments might not be comparable to interstellar conditions if ice mixtures are involved. For experiments where the ice was irradiated and then warmed in sequential steps, the methanol and water ice mixtures show an enhancement of CO, H₂CO, and CH₄ production as compared to experiments with simultaneous irradiation and heating. Additionally, the production of more complex species (i.e., (CH₂OH)₂, HOCH₂CHO, and CH₃OCH₃) is suppressed. This effect is best explained by the enhanced mobility of larger radicals with simultaneous heating and irradiation, yielding more complex molecules. Additional studies with a well characterized UV lamp are needed to explore this trend with other ice mixtures, photon fluxes, and fluences. Nonetheless, these results call into question a common assumption made in the study of ice analogs and may impact the interpretation of experimental and observational results.