Probing the Primary Solvation Shell of Hydroxide and Observing Spin-Orbit Excited Dipole-Bound Resonances Using Cryogenic Photoelectron Spectroscopy Xue-Bin Wang*

Size-selective cryogenic photoelectron spectroscopy (cryoPES) coupled with electrospray ionization (ESI) is a powerful experimental technique to investigate electronic structures, energetics, and proton transfers of a wide variety of hydrogen bound clusters. Our latest research includes probing the primary hydration shell of hydroxide and electronic Feshbach resonances of arginine-iodide anion clusters. A key property associated with the behavior of hydroxide, one of the most important ions in aqueous chemistry, is the binding motif in its primary hydration shell. We carried out a joint experimental – theoretical study for hydrated hydroxide clusters OH^–(H₂O)_n (n = 0-6). Our study, based on the first reported photoelectron spectra and high-level ab-initio calculations, determined the primary gas phase hydration shell of hydroxide containing four water molecules, and reconciled a long-standing debate on hydroxide solvation shell between previous IR action spectroscopic data and theory. We demonstrated ‘iodide-tagging’ PES is a sensitive probe for identifying distinct anion binding sites for polar molecules. We spectroscopically distinguished and characterized a plethora of isomers and tautomers of arginine-iodide clusters. We observed the relaxation of spin-orbit excited dipole bound states, known as an electronic Feshbach resonance, for the first time and confirmed the mechanism by which the electrons relax. The Feshbach resonance has a distinct signature directly related to the dipole of the cluster. This has enabled us to differentiate between arginine-iodide cluster isomers that are too large with too many conformers for traditional vibrational spectroscopy approaches.