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 occ urring isotopic variants of lithium hydride have been measured to an u nprecedented accuracy of a few parts in 10(8) with a tunable far-infra red spectrometer. The values of the vibrational and rotational quantum numbers nu and J involved range up to 2 and 12, respectively. The mea surements have been fitted with already existing data on the vibration -rotation energy levels of lithium hydride in its ground (1) Sigma(+) state to a single model based on the Dunham expansion. This model has been modified to take into account the Dunham and nonadiabatic correct ions and their effects on the reduced mass dependence of the energy le vels in the way first described by Watson [J. Mol. Spectrosc. 80, 411 (1980)]. The data are fitted to experimental accuracy to give an impro ved set of reduced Dunham parameters, U-kl, and an improved value for the Born-Oppenheimer equilibrium bond length for LiH, r(e)(BO)=0.15949 0811(16) nm. The fit constitutes a stringent test of this type of mode l for the breakdown of the Born-Oppenheimer approximation.