Perturbation shifts in selected ^aR-branch rotational transitions in the ground-state tunnelling doublet. The plotted quantity is the scaled difference between frequencies from the full spectroscopic model (ν) and perturbation-removed frequencies (ν₀) calculated without the interstate coupling terms. The matching character of the observed resonance behaviour identifies the interacting K_a sequences in the two substates. Circles denote experimental measurements, and the perturbation contribution at the resonance peak at J″ + 1 = 30 is near 2.4 GHz. The 0⁺ sequence hasa smaller resonance at J″ + 1 = 19, which we identify as resulting from a resonance with a different, K_a = 2 sequence in substate 0^-. All measured transitions involved in the resonances are fitted to within their experimental uncertainty.