A reduced method of coupled vibrational channels: analysis of regular perturbations in the $c^3\Sigma_{\Omega}^{+}$-state of a KRb molecule

To reduce the dimension of a vibrational problem to be solved by the coupled channel method in the case of interacting electronic states of diatomic molecules, we propose to use the contact van Vleck transformations, which make it possible to take into account the nonadiabatic intramolecular interactions with distant states by modifying the initial potential energy matrix. The efficiency of the reduced coupled vibrational channel method (RCVCM) has been demonstrated by an example of analyzing the regular spin–orbital, electron–rotational, and spin–rotational perturbations discovered in the fine structure of vibrational–rotational levels of the $c^3\Sigma_{\Omega}^{+}$-state of a KRb molecule by methods of high-precision laser-emission spectroscopy. The adiabatic potentials and nonadiabatic electron matrix elements, as functions of the internuclear distance necessary for application of the RCVCM, were obtained in the framework of the nonempirical high-level calculation. It has been demonstrated that the RCVCM has broad extrapolation possibilities and makes it possible to describe the position of the regularly perturbed energy levels of $\Omega$-components of the triplet $c^3\Sigma_{\Omega}^{+}$ state at the spectroscopic accuracy level.