The MgO B 1ÎŁ +- a 3Î i (0-0) and (0â1) and D 1Î- a 3Î (0-0) and (1-1) intercombination bands have been observed, rotationally analyzed, and reduced to molecular constants by a nonstandard procedure which made extensive use of an elaborate but highly constrained effective Hamiltonian model. The MgO a 3Î state is important because it is low lying ( T e = 2620.6 cm â1), it correlates to Mg( 1S) + O( 3P) ground state atoms (unlike the X 1ÎŁ + state), it is the lower state of the exceedingly complex near UV triplet-triplet bands, and many Mg + oxidant reactions significantly populate various Ί , e f components of the a 3Î state. The fit model, observed transitions, and computed eigenvalues and eigenvectors for MgO a 3Î v = O and 1 will aid in the analysis of triplet-triplet bands and will be especially valuable in providing transition frequencies and relative Ί , e f , and J-dependent rotational linestrength factors for population monitoring. The present results reaffirm the validity and utility of various semiempirical relationships between fine structure, Î-doubling, and perturbation parameters. Moreover, by showing that the Î-doubling in the a 3Î state is dominated by interactions with the X 1ÎŁ + and b 3ÎŁ + states, an ab initio prediction of the location of the as yet unobserved b 3ÎŁ + state is confirmed. The B 1ÎŁ +- a 3Î transition borrows its oscillator strength from the B 1ÎŁ +- X 1ÎŁ + and B 1ÎŁ +- A 1Î transitions. The pattern of a 3Î , A 1Î , and X 1ÎŁ + vibrational levels is such that, for v ⤠4, the predominant perturber characters admixed into the a 3Î , v a level are v x = v a + 3 and v A = v a . Interference effects between transition amplitudes borrowed from B- X and B- A transitions will cause intensity to be transferred from R Ί (J) to P Ί (J) or vice versa. These e-level interference effects will be strongly dependent on v B , v a , J, and Ί . Although the pattern of interference effects will appear complicated, all such effects are determined, in sign and magnitude, by the following product of eight signed quantities. ăν A = ν Îą|ν Îąă ăν Îą|ν X = ν Îą + 3ă ăν X = ν Îą + 3|ν Bă ăν B|ν A = ν Îąă A ÎąâX A ÎąâA Îź BâA Îź BâX. Note that each vibrational wavefunction and each electronic wavefunction appear twice, once as a bra and once as a ket. The sign of the interference effect can be experimentally determined and cannot depend on arbitrary phase choices.