The laser-induced fluorescence and resonantly enhanced two-photon ionization spectra for the 1 1Σ u +-X 1Σ g + vibronic transitions of jet-cooled Cl 2 are observed by using a tunable vacuum ultraviolet laser. A rotational analysis of the (ν′, ν″) = (39, 0) transitions for 35Cl 2 and 35Cl 37Cl reveals that the ν′=39 levels of the 1 1Σ u + state of these two isotope species are localized in a Rydberg well of the double-minimum potential. The mixing of the Rydberg and ion-pair potentials is evidenced by the localization in the Rydberg well.