Knowledge Bank

While a number of studies of the weakly bound $a3\Sigma u+$ state of $\mathrm{<mrow\; data-mjx-texclass="ORD">6,6</mrow>}$Li$2$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">7,7</mrow>}$Li$2$ have been reported, the only potential functions obtained for these systems were based on point-wise semiclassical RKR curves generated from Dunham or near-dissociation expansions for the vibrational energies and $Bv$ constants.} {\bf 44A}, 1369 (1988); (b) C.\ Linton {\em et al., J.\ Chem.\ Phys.}\ {\bf 91}, 6036 (1989); (c) W.T.\ Zemke and W.C.\ Stwalley, {\em J.\ Phys.\ Chem.}\ {\bf 97}, 2053 (1993); (d) A.J.\ Moerdijk {\em et al., Phys.\ Rev.\ Lett.}\ {\bf 72}, 40 (1994); (e) C.\ Linton {\em et al., J.\ Mol.\ Spectrosc.}\ {\bf 196}, 20 (1999).}~ Moreover, to date the data for $\mathrm{<mrow\; data-mjx-texclass="ORD">6,6</mrow>}$Li$2$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">7,7</mrow>}$Li$2$ have always been treated independently, so the effect of Born-Oppenheimer breakdown in this system is unknown. The present work reports a combined-isotopologue direct-potential-fit analysis of all available fluorescence and PAS data for the $13\Sigma g+-a3\Sigma u+$ and $23\Pi g-a3\Sigma u+$ systems of $\mathrm{<mrow\; data-mjx-texclass="ORD">6,6</mrow>}$Li$2$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">7,7</mrow>}$Li$2$. The analytic potential energy functions used to characterize the $a3\Sigma u+$ and $13\Sigma g+$ states are extended versions of the Morse-Long-Range' (MLR) potential model which explicitly incorporates the theoretically-known inverse-power long-range behaviour within a unified potential function form.}\ {\bf 125}, 164310 (2006);\, R.J.\ Le Roy and R.D.E.\ Henderson, {\em Mol.\ Phys.}\ {\bf 105}, 663 (2007).}~ Adiabatic' Born-Oppenheimer breakdown functions are required to yield a consistent analysis of the data for the two isotopologues, and they yield isotopologue-dependent well depths for these two states. The $-C3/r3$ limiting long-range behaviour of the $13\Sigma g+$-state potential presented a challenge to the use of the MLR potential function form with two or more long-range terms, and this led to better understanding and a significant extension of the MLR model.