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 0u+ symmetry based on the lowest excited state of the ion core, previously labelled a(4Σu,1/2-), has been identified using optical triple resonance via the B(0u+)andE0g+(3P2) 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 E0g+(3P2) state, whilst the A[2Πu,1/2]c;6s0u+ state is not, are explained in terms of its electronic configuration.