Geometric bonding effects in the X A21, A Σ2u+, and B Π2g states of Li2F

Published ab initio and pseudopotential calculations for the dialkali halide systems suggest that the preferred colinear geometry is for the metal to approach the metal end of the alkali halide. Here, ab initio calculations on the Li2F system reveal that the well depth on the halide side in this radical is much deeper and is a local saddle point associated with the ionic nonlinear global minima. Although many features of the pseudopotential surfaces are confirmed, significant differences are apparent including the existence of a linear excited A Σ2u+ state instead of a triangular one, a considerably deeper global minimum some 50% lower in energy and a close approach between the X A21 and the A Σ2u+ states, with the A Σ2u+ minimum 87 kJ mol−1 below the ground state asymptote. All the results can be rationalised as the avoided crossings between a long range, covalent potential dominant within the LiLiF geometry and an ionic state that forms the global minimum. Calculations on the third A2′ potential indicate that even for Li+LiF collisions at ultracold temperatures the collision dynamics could involve as many as three electronic states.