Electronic design principle of OCCs in diatomic cations investigated in this study. Electronic configurations of radical-cations ML + mimic the electronic structure of the successfully laser-cooled neutral molecules ML. In these neutral species, an alkali-earth metal forms an ionic bond with a ligand L (which needs one extra electron to form a closed shell), creating a +1 local charge and a single valence electron localized on the metal. This pattern results in electronic transitions localized on the metal . The top row shows the electronic configurations of the elements in each column. In the bottom row the full and empty dots depict valence electrons and valence holes (electrons needed to complete the shell). Color highlights elements studied in this work.