ON THE LONG RANGE PART OF THE POTENTIAL CURVE OF THE $A_{1}$ STATE IN $^{6}Li_{2}$

Abstract

Using results from photoassociation experiments involving ultracold $^{6}Li$ atoms [1] and dispersed fluorescence data from optical-optical double resonance spectroscopy [2], we have constructed an experimental RKR curve for the $A_{1}$ state extending to 74 \AA. The dissociation energy $D_{e}$ with respect to the bottom of the $A_{1}$ state is obtained directly from a fit of the energy levels, $D_{e} =9352.03(1)$ cm $^{-1}$. The potential energy curve V(R) for the $A_{1}$ state deviates slowly from the usual $V(R) = D_{e} - C_{3}/R^{3} +\ldots$ expression [3], reflecting a gradual transition from Hund#### case a) to Hund#### case c) coupling in the molecule. The outer limb of the curve varies with $1/R^{n}$ in a more complex way because of interactions between the two $0_{u}^{+}$ components dissociating to Li(2s) + Li(2p). We present the general analytical form of the long-range part of the $A_{1}$ state derived from a treatment which includes all the off-diagonal terms in the interaction matrix. The lithium dimer data have provided a test of the validity of this model; energies are recalculated to within $0.008 cm^{-1}$ for 12$R_{74}$ \AA, with six parameters.