The dipole moment of 7LiH and 7LiD in the excited A 1Σ + state: A test of the born-oppenheimer approximation

Abstract

Continuing our Stark quantum beat experiments on 7LiH at electric field strengths of 100 kV/cm and above, we were le to observe quantum beat signals for the vibrational level v′ = 5 of the A 1 v + state. The evaluation of these signals yields a field reduced splitting Δ W/ E 2 = 1.157(70) kHz/(kV/cm) 2 which would correspond to an “effective” value of 1μ eff( v′ = 5)1 = 0.051 (5) D for the dipole moment, if the ordinarily used formula were applicable. A serious check of all available data, however, shows that neighbouring vibrational levels of the A 1 v + state and also the sum over the levels of the electronic ground state X 1Σ +, though counteracting, have the main effect on the residual splitting thus preventing a straightforward evaluation for the dipole moment. Taking account of these polarizability effects and including the possibility for a slight shift of the theoretical dipole moment function a value of 1μ( v′ = 5)1 = 0.089(14) D is the most probable one. In the case of 7LiD according to the usual formula an “effective” value of 1μ eff( v′ = 6)1 = 0.425(20) D was deduced from a beat signal obtained at an electric field strength of 24 kV/cm. Here the influence of neighbouring vibrational levels and electronic states is only of minor importance leading to a corrected value of 1μ( v′ = 6)1 = 0.43(2) D for the dipole moment. This value differs by 13% from the theoretical one calculated on the basis of the dipole moment function of 7LiH and the adiabatically corrected potential energy curve of 7LiD, thus indicating non-adiabatic effects.