Rotation–vibrational states of and the adiabatic approximation

Abstract

We discuss recent progress in the calculation and identification of rotation–vibrational states of at intermediate energies up to 13 000 cm −1 . Our calculations are based on the potential energy surface of Cencek et al . which is of sub-microhartree accuracy. As this surface includes diagonal adiabatic and relativistic corrections to the fixed nuclei electronic energies, the remaining discrepancies between our calculated and experimental data should be due to the neglect of non-adiabatic coupling to excited electronic states in the calculations. To account for this, our calculated energy values were adjusted empirically by a simple correction formula. Based on our understanding of the adiabatic approximation, we suggest two new approaches to account for the off-diagonal adiabatic correction, which should work; however, they have not been tested yet for . Theoretical predictions made for the above-barrier energy region of recent experimental interest are accurate to 0.35 cm −1 or better.