Probing the Structures, Electronic Properties, and Bonding Patterns of RbOn–/0 and CsOn–/0 ( n = 4–7) by Photoelectron Spectroscopy and Theoretical Calculations

This study combines anion photoelectron spectroscopy with density functional theory calculations to explore the geometric structures, electronic structures, and bonding properties of RbOn–/0 and CsOn–/0 (n = 4–7). The measured spectra exhibit a clear size-dependent evolution: as the number of oxygen atoms increases, all features shift progressively toward higher binding energies. Theoretical analysis reveals that RbOn– and CsOn– have similar structures. They can be described as polyoxide units electrostatically stabilized by alkali metal cations, with structural motifs that follow systematic trends: [O2]−–M+–[O2]− (n = 4), [O3]−–M+–[O2]− (n = 5), [O2]−–M+–[O2]2– (n = 6), and [O3]−–M+–[O2]2– (n = 7). These results confirm that electrostatic interactions dominate the bonding between alkali metal cations and oxygen-rich units, and the polarization effects become more pronounced with the increase of cluster size. The neutral RbOn and CsOn clusters undergo pronounced structural reorganization compared with their corresponding anionic structures yet largely retain the same basic framework as their anionic counterparts.