Precise experimental test of models for the breakdown of the Born-Oppenheimer separation: The rotational spectra of isotopic variants of lithium hydride

The frequencies of 22 rotational transitions in the four naturally occurring isotopic variants of lithium hydride have been measured to an unprecedented accuracy of a few parts in 10⁸ with a tunable far-infrared spectrometer. The values of the vibrational and rotational quantum numbers v and J involved range up to 2 and 12, respectively. The measurements have been fitted with already existing data on the vibration-rotation energy levels of lithium hydride in its ground ¹Σ⁺ state to a single model based on the Dunham expansion. This model has been modified to take into account the Dunham and nonadiabatic corrections and their effects on the reduced mass dependence of the energy levels in the way first described by Watson [J. Mol. Spectrosc. 80, 411 (1980)]. The data are fitted to experimental accuracy to give an improved set of reduced Dunham parameters, U_kl, and an improved value for the Born-Oppenheimer equilibrium bond length for LiH, r_e^BO=0.159 490 811(16) nm. The fit constitutes a stringent test of this type of model for the breakdown of the Born-Oppenheimer approximation.