Knowledge Bank

Using results from photoassociation experiments involving ultracold $\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}Li$ atoms [1] and dispersed fluorescence data from optical-optical double resonance spectroscopy [2], we have constructed an experimental RKR curve for the $A1$ state extending to 74 \AA. The dissociation energy $De$ with respect to the bottom of the $A1$ state is obtained directly from a fit of the energy levels, $De=9352.03(1)$ cm $\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. The potential energy curve V(R) for the $A1$ state deviates slowly from the usual $V(R)=De-C3/R3+\dots$ 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 $1/Rn$ in a more complex way because of interactions between the two $0u+$ components dissociating to Li(2s) + Li(2p). We present the general analytical form of the long-range part of the $A1$ 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 $0.008cm-1$ for 12$R74$ \AA, with six parameters.