Observation of a substantially-bound excited-core Rydberg state in I2 by optical triple resonance

A substantially-bound (De ∼ 7000 cm−1, Re ∼ 3.1 Å, Te ∼ 60 546 cm−1) 6s Rydberg state of I2 with 0 u + symmetry based on the lowest excited state of the ion core, previously labelled a ( 4 Σ u , 1 / 2 - ) , has been identified using optical triple resonance via the B ( 0 u + ) and E 0 g + ( 3 P 2 ) states together with resonance ionization. The interactions with ground-core Rydberg states that the new state undergoes and the reasons why it is observed in the resonance ionization spectrum via the E 0 g + ( 3 P 2 ) state, whilst the A [ 2 Π u , 1 / 2 ] c ; 6 s 0 u + state is not, are explained in terms of its electronic configuration.