A Spectroscopic Study of the D() Ion‐Pair State of I2 by Optical–Optical Double Resonance

The D(0+u) ion-pair state of 12 has been analyzed by the optical-optical double resonance (OODR) technique. In a stepwise three-photon excitation scheme, D(O+u)-BaH(O+u)-X1E-, the D(0) 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 AJ 0 and _+2 for the coherent two-photon transition from the Bal-I(0+u) state. The D(0+u)-XIE fluorescence was resolved to determine the absolute vibrational numbering of the D(0+u) state. We derived Dunham parameters effective for v 0 through v 124 which were used to construct a Rydberg-Klein-Rees (RKR) potential curve.