Visible Light Induced Photochemistry of IO₂

IO₂^- and IO₂ have been detected in the atmosphere and suggested as reactive intermediates in the destruction of ozone. Here, photoelectron action spectroscopy provides evidence for a vibrational progression of anion resonances, characterized by a vibrational frequency of ∼0.03 eV, originating from a weakly bound (close to the photodetachment threshold) electronically excited state of IO₂^-. Photoelectron spectroscopy of IO₂^- at visible wavelengths shows indirect electron loss processes, likely autodetachment of the vibrational resonances leading to internally excited IO₂, alongside direct detachment. Given that the excited state has a vertical excitation energy in the visible portion of the solar spectrum, it is likely that photoexcitation of IO₂^- is occurring in the atmosphere, providing a new route for the formation of electrons and internally excited IO₂.