Fluorescence excitation spectra of the D0 u + ion-pair state of Br 2 in a supersonic jet have been recorded using monochromatic, tunable, and coherent vacuum ultraviolet radiation generated by four-wave sum mixing in Mg vapor. Vibrational bands between ν′ = 109 and 182 were assigned and spectroscopic constants were obtained. A quasiperiodic perturbation in the D0 u + state was observed for the first time and is tentatively assigned to interactions with the higher-lying F0 u + ion-pair state.