Resonances in the rotational constants of $0_g^{-}(6s_{1/2}+6p_{1/2})$ Cs₂ levels analysed by an improved-B_v-formula associated to a 2-channel model

Spin-orbit coupling in $0_g^{-}(6s_{1/2}+6p_{1/2})$ long-range molecular levels of Cs₂ assigned to resonances in the rotational constant $\left(B_v\right)$ spectrum (Lignier L. et al., Phys. Chem. Chem. Phys., 13 (2011) 18910; Pichler M. et al., J. Chem. Phys., 121 (2004) 1796) has been analyzed with a model which associates an improved-B _v-formula (established here) and a 2-channel model. The approach explains the Lorentzian shape of the observed resonances superimposing a non-horizontal background and the fit of the B _v-spectrum allows us to deduce the spin-orbit coupling parameters. We compare this approach with an analysis of the binding energies using the vibrational quantum defect (VQD). The good agreement between the methods lets one conclude that the improved-B _v-formula associated with a 2-channel model provides a pertinent approach for analyzing coupled molecular series. (letter)