Radiationless decay of the 1,2,3 3Πg states of Al2: A fully first principles treatment using adiabatic and rigorous diabatic states

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Abstract
The decay mechanisms of the metastable 2,3 3Πg states of Al2 are investigated. Both nonadiabatic radiationless decay to the dissociative 1 3Πg state and radiative decay to the ground X 3Πu state are considered. The 1,2,3 3Πg states are described using state averaged multiconfiguration self consistent field/configuration interaction wave functions [ψam(r,Q)]. The derivative couplings famn(Q)≡〈ψam(r, Q)‖(d/dQ)ψan(r,Q)〉r are determined and used to construct a rigorous diabatic basis for this strongly interacting three state problem. The 2 3Πg state and somewhat surprisingly the 3 3Πg state are rapidly predissociated by the dissociative 1 3Πg state. The lifetimes for nonradiative decay of the vibrational levels of the 2 3Πg state are on the order of picoseconds while those of the 3 3Πg state are on the order of nanoseconds being reduced from the direct coupling (3 3Πg∼1 3Πg) rate of milliseconds by indirect coupling through the 2 3Πg state, (3 3Πg∼2 3Πg∼1 3Πg). Radiative decay is found to be on the order of 102 and 30 ns for the 2 3Πg and 3 3Πg states, respectively, so that radiationless decay is principal decay mechanism. Significant variation in the lifetimes of the individual vibrational levels of the 2,3 3Πg states is expected. This is attributed to the mechanism of the predissociation which involves nonadiabatic interactions near the ‘‘inner walls’’ of the 1,2 3Πg states. Although avoided crossings strongly affect the properties of the 1,2,3 3Πg states the adiabatic basis is preferred over the diabatic basis both conceptually and computationally.