A Spectroscopic Study of the D(0u+) Ion-Pair State of I₂ by Optical–Optical Double Resonance

The D(0u+) ion-pair state of I₂ has been analyzed by the optical–optical double resonance (OODR) technique. In a stepwise three-photon excitation scheme, D(0u+)–B³Π(0u+)–X¹Σg+, the D(0u+) state appeared in the OODR spectrum as the vibrational progressions consisting of O, Q and S branches in accord with the rotational selection rule of ΔJ = 0 and ±2 for the coherent two-photon transition from the B³Π(0u+) state. The D(0u+)–X¹Σg+ fluorescence was resolved to determine the absolute vibrational numbering of the D(0u+) state. We derived Dunham parameters effective for ν = 0 through ν = 124 which were used to construct a Rydberg–Klein–Rees (RKR) potential curve.