Perturbations in the 3sσ_g ^1,3∏_g Rydberg states of O₂: Bound-bound interactions with the second¹∏_g and¹Δ_g valence states

Existing experimental (2 + 1) REMPI spectra for transitions into rotationally resolved levels of the 3sσ_gd ¹∏_g and 3sσ_g C ³∏_g Rydberg states of O₂ have been rotationally analyzed, resulting in the first characterization of rotational perturbations in d(v = 1 - 3) and C(v = 2). In addition, the results of this analysis have been interpreted with the aid of a coupled-channel Schrödinger-equation (CSE) model of the interacting electronic states. The identification and characterization of perturbations in the d and C states have allowed the nature of the interactions between the 3sσ_g ^1,3∏_g Rydberg states and the ∥ ¹∏_g valence state to be clarified and a realistic empirical potential-energy curve for the ∥ ¹∏_g state to be determined. While it is found that first- and second-order interactions with the ∥ ¹∏_g valence state are responsible for the strongest perturbations observed in d(v = 1-3) and C(v = 2), additional weak perturbations found in d(v = 2 and 3) are shown to result from a second-order interaction with the ∥ ¹Δ_g valence state. These weak perturbations, including an extra level observed for d(v = 3, J = 17), appear to be the first experimental evidence for the ∥ ¹Δ_g state, long predicted theoretically. Finally, detailed comparisons between experimental spectra and d ¹∏_g ←←a ¹Δ_g(1,0), (2,0) and (3,0) (2 + 1) REMPI spectra calculated using the CSE model are presented which support and illustrate these conclusions.