Negative ion photoelectron spectra of ISO 3 – , IS 2 O 3 – , and IS 2 O 4 – intermediates formed in interfacial reactions of ozone and iodide/sulfite aqueous microdroplets
Three short-lived, anionic intermediates, ISO 3 – , IS 2 O 3 – , and IS 2 O 4 – , are detected during reactions between ozone and aqueous iodine/sulfur oxides microdroplets. These species may play an important role in ozone-driven inorganic aerosol formation; however their chemical properties remain largely unknown. This is the issue addressed in this work using negative ion photoelectron spectroscopy (NIPES) and ab-initio modeling. The NIPE spectra reveal that all of the three anionic species are characterized by high adiabatic detachment energies (ADEs) - 4.62 ± 0.10, 4.52 ± 0.10, and 4.60 ± 0.10 eV for ISO 3 – , IS 2 O 3 – , and IS 2 O 4 – , respectively. Vibrational progressions with frequencies assigned to the S–O symmetric stretching modes are also discernable in the ground state transition features. Density functional theory (DFT) calculations show the presence of several low-lying isomers involving different bonding scenarios. Further analysis based on high level CCSD(T) calculations reveal that the lowest energy structures are characterized by formation of I–S and S–S bonds and can be structurally viewed as SO3 linked with I, IS, and ISO for ISO 3 – , IS 2 O 3 – , and IS 2 O 4 – , respectively. The calculated ADEs and vertical detachment energies (VDEs) are in excellent agreement with the experimental results, further supporting the identified minimum energy structures. In conclusion, the obtained intrinsic molecular properties of these anionic intermediates and neutral radicals should be useful to help understand their photochemical reactions in the atmosphere.