HOMO is the abbreviation for the highest occupied molecular orbital of a molecule and LUMO is the abbreviation for the lowest unoccupied molecular orbital. The difference in energy between the HOMO and LUMO can be determined by taking the electronic absorption (UV-visible) spectrum of the molecule. The HOMO-LUMO transition corresponds to molecules going from their ground state to their first excited state. Identify the orbital transition that the lowest energy peak of O₂ corresponds to.