Within the framework of the analytical single-channel quantum defect theory, the contributions of discrete and continuous Rydberg states to the adiabatic correction and regular nonadiabatic shifts of vibronic levels of low-lying terms of a diatomic molecule are investigated. It has been established that the partial contribution of the electron continuum can reach several tens of percent even for the first members of the Rydberg series. It has been shown that model estimates of the total contributions to the adiabatic correction of e3 ₔ^ + (3p{ ₔ}) and a3 ₆^ + (2sσg) states of molecular hydrogen are in good agreement with high-precision ab initio data at small and medium internuclear distances and those calculated taking into account the adiabatic correction and nonadiabatic shift of the energy of vibronic levels are in good agreement with experimental electron-vibrational terms of its homonuclear isotopologs.
Лихарев et al. (Wed,) studied this question.