(a–e) Reduced term values, T(J) − B_AJ(J + 1) + D_AJ²(J + 1)², for the A ¹Π, v = 0–4, rovibronic levels of ¹²C¹⁸O and for the hypothetical unperturbed and crossing perturber state levels. B_A and D_A are appropriately chosen values for the rotational constants of A ¹Π. Note that different wavenumber scales are used to show the perturbations in different v‐levels of A ¹Π. Filled and open circles refer to e and f parity levels, respectively. (f–j) Reduced term values for the A ¹Π, v = 5–9, rovibronic levels of ¹²C¹⁸O and for the hypothetical unperturbed and crossing perturber state levels.