Breakdown of the Born-Oppenheimer approach for a diatomic molecule: LiH in the D state

Breakdown of the Born-Oppenheimer adiabatic approach is found for the third electronic excited state of LiH of 1 Σ symmetry. A comparison between adiabatic and vibronic calculations indicates changes in the arrangement of the levels and strong level-shifts, rising up to 90 cm −1 . In the present case, the predictions can be easily confirmed by experiment with the help of the rotational constants. The generality of such breakdown of the adiabatic approach for highly excited states of molecular systems (even diatomic) is further discussed by considering the diabatic picture.