Rotational Energy Transfer in Highly Excited States of Lithium Dimer: Experiment and Modeling.

We report level-resolved rate coefficients for collision-induced rotational energy transfer in the Li-Ne system, with Li in the highly electronically excited E(3)Σ( = 4, = 31) and F(4)Σ( = 10, = 31) states. The distributions of rate coefficients are strikingly different from those previously measured for the A(1)Σ( = 2-24, = 30) state of the same molecule, falling off much more rapidly with increasing rotational quantum number change |Δ|. The reason for the difference was explored by means of an inverse Monte Carlo approach employing classical trajectories and a model potential, which was adjusted to give agreement with experiment. The modeling strongly suggests that the E and F state interaction potentials are much more nearly isotropic than that of the A state. The resulting dramatic reduction in rate coefficient, especially for large |Δ|, may be relevant in the relaxation of gases at high temperatures.