High resolution photoelectron imaging of cryogenically cooled alkaline-earth metal complexes with the BO₂...

The BO₂ molecule is a superhalogen with a very high electron affinity, resulting in an extremely stable BO₂ ^- anion suitable as a building block to form ionic compounds. Here we report the generation of M(BO₂ ^-) (M = Ca, Sr, Ba) complexes and the investigation of their structures and bonding using high-resolution cryogenic photoelectron imaging. All three M(BO₂ ^-) alkaline-earth complexes are found to have linear M(O-B-O^-) (¹Σ⁺) structures. Photodetachment removes an electron from the alkaline-earth metal atom and produces the neutral M⁺(O-B-O^-) (²Σ⁺) ionically bonded ground state. The change of the charge state on the metal center induces a significant reduction of the M-O bond length in the neutral final state, resulting in an extensive M-O stretching vibrational progression in all the photoelectron spectra. The electron affinities of MBO₂ are measured to be 1.574 eV, 1.487 eV, and 1.291 eV and the M-O stretching frequencies are measured to be 411 cm^-1, 339 cm^-1, and 290 cm^-1 for M = Ca, Sr, and Ba, respectively. The strong electron-withdrawing power of BO₂ leads to the ionically bonded ground state for MBO₂ (²Σ⁺), resulting in a single electron localized on the metal center. The ionic interaction between M⁺ and BO₂ ^- in MBO₂ makes their low-lying electronic excitations resemble atomic transitions, rendering MBO₂-type molecules promising candidates for laser cooling.