Rotational perturbations in the (2 + 1) REMPI spectrum of the Rydberg state [²II_3/2]_c5d; 1g of I₂

Partly rotationally resolved (2 + 1) resonance-enhanced multiphoton ionization contour spectra of the vibrational bands corresponding to the [²II_3/2]_c5d; 1g ← ← X transition in iodine, recorded at room temperature, have been analyzed. Irregular band spectral structures could be simulated by assuming rotational line strengths to be dependent on rotational quantum number in the resonantly excited state at the two-photon level. The analysis allowed improved rotational constants (B_v′ and D_v′) and corresponding internuclear distances (r_v′) for the Rydberg state to be evaluated, as well as the characteristic rotational parameters, B′_e = 0.04047 ± 0.00010 cm^-1, α′_e = (1.3 ± 0.2) × 10^-4 cm^-1 and r′_e = 2.562 ± 0.003 Å. The anomalies in the rotational structure are ascribed to perturbation effects due to heterogeneous interaction between an ion-pair state and the Rydberg state. A candidate for the perturbing state is the D′(2g) state.