The X 2A 1–A 2B 2 conical intersection in NO 2: determination of the coupling parameter λ from high-resolution experimental data

Abstract

The experimentally known vibronic energy levels of NO 2 [J. Chem. Phys. 108 (1998) 6638] have been reanalyzed to determine the strength of the electronic coupling at the conical intersection between the ground X 2A 1 and excited A 2B 2 electronic states. The full set of vibronic levels located below 11 400 cm −1 were included in our analysis, i.e., 276 vibrational levels with predominant X 2A 1 ground electronic character and seven vibrational levels with predominant A 2B 2 excited electronic character. Each diabatic electronic state was modeled by an effective Dunham-like Hamiltonian, while the standard λq 3 interaction was assumed for the coupling between the two diabatic states ( q 3 is the antisymmetric stretch normal coordinate). The energy shifts induced by the λq 3 interaction were first calculated using second-order perturbation theory. The net effect of λq 3 can be divided into two contributions: (i) a nonresonant energy shift, which affects all of the X 2A 1 vibrational levels and corresponds to very weak mixing coefficients, and (ii) a resonant shift, which is significant for only few X 2A 1 levels below 11 400 cm −1 and induces a significant mixing of these levels with one (or few) neighboring A 2B 2 level(s). The resonant shifts were next recalculated by diagonalizing small sub-matrices limited to resonantly coupled levels. The interaction parameter, λ=600±100 cm −1 , was determined from the analysis of the resonant couplings. This value of λ is in agreement with a recent determination obtained from the comparison of ab initio calculations with the experimental NO 2 − photoelectron spectrum.