Exchange reactions in selectively excited Br2(B3Π ou+)

Laser‐controlled bimolecular isotopic exchange reactions in bromine leading to alterations in natural isotopic abundance ratios have been directly observed using laser‐modulated molecular beams and mass analysis. Several instances of selective excitation and subsequent rapid isotope exchange are observed within the 7 GHz gain curve of the exciting 514.5 nm line of an argon‐ion laser. These exchange processes are attributed to the direct photochemical participation of excited molecular states in the B3Πou+ system of Br2.