Electronic structure methods and nonperturbative resonance theory are applied to study the radiative and radiationless decay mechanisms of the MgBr (A2ΠΩ) vibrational levels. The X2Σ+ and 1,2 2ΠΩ adiabatic electronic states are characterized using ab initio state-averaged multiconfigurational self-consistent field/second order configuration interaction wave functions. Interstate derivative couplings between the 2Π states have been calculated and used to construct a rigorous diabatic basis. The nonrelativistic potential energy curves are modified in the first order of degenerate perturbation theory to take account of the spin–orbit interactions treated within Breit–Pauli approximation. All vibrational levels in the A2ΠΩ manifold are resonances predissociated by the repulsive 2 2Π state. A recently developed computational approach [S. Han and D. R. Yarkony, Mol. Phys. 88, 53 (1996)] based on a Feshbach formalism is employed to determine energies, linewidths, and radiative and radiationless decay rates in a coupled diabatic states basis within a Hund’s case (a) approximation. Large nonadiabatic interactions cause significant energy shifts in the resonances levels. It is shown that a pronounced Ω-dependence in the radiationless decay rates results from the large fine structure splitting in the 2 2ΠΩ diabatic state which corresponds to Mg(1S)Br(2P). Comparisons with absorption and fluorescence spectra reveal important insights into A2ΠΩ state decay. The spectroscopic constants of the A2ΠΩ, Ω=3/2 and 1/2 states and the A2Π3/2 state predissociation are well described in a Hund’s case (a) approximation. However it is found that the A2Π1/2 state predissociation is significantly underestimated in this limit. Rather the A2Π1/2 state is indirectly predissociated by the 2 2Π3/2 state through rotational coupling to the A2Π3/2 state.

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