Study of strong R–P and spin–orbit vibronic coupling effects in linear triatomic molecules

The vibronic coupling between P and R electronic states of a linear molecule is considered with the inclusion of the spin–orbit coupling of the P electronic state, employing the microscopic (Breit–Pauli) spin–orbit coupling operator in the single-electron approximation. The 6 · 6 Hamiltonian matrix in a diabatic spin-electronic basis is derived by an expansion of the molecular Hamiltonian in powers of the bending amplitude up to second-order. The symmetry properties of the Hamiltonian are analyzed. It is pointed out that there exist zeroth-, first-, and second-order R–P vibronic-coupling terms of spin–orbit origin, which are absent when the usual phenomenological form of the spin–orbit coupling operator is used instead of its microscopic form. The influence of the R–P and spin–orbit vibronic-coupling terms on the adiabatic potential energy curves as well as on the vibronic spectra is analyzed for selected models. It is demonstrated that the interplay of strong R–P vibronic-coupling and strong spin–orbit splitting of the P state can result in unexpectedly complex vibronic spectra. 2006 Elsevier B.V. All rights reserved.