Visible-light photoionization of aromatic molecules in water-ice: Organic chemistry across the universe with less energy

Ionization of gas-phase organic molecules such as polycyclic aromatic hydrocarbons (PAHs) requires vacuum ultraviolet photons at wavelengths shorter than 200 nm (~6–9 eV). We present here for the first time that visible photons – accessible through sunlight – can cause photoionization of trapped PAHs in cryogenic water–ice, accounting for 4.4 eV less ionization energy than in the gas phase. This finding opens up new reaction pathways involving low-energy ionization in many environments where water and organic matter coexist. These include the interstellar medium, molecular clouds, protoplanetary disks, and planetary surfaces and atmospheres (including Earth).

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