ANALYSIS OF LONG-RANGE DISPERSION AND EXCHANGE INTERACTIONS OF 2 LITHIUM ATOMS

This article critically surveys the best available data for the two lowest states (X1SIGMA(g)+ and a3SIGMA(u)+) of Li2. These two states both dissociate to two ground-state Li atoms. Since both states are known precisely up to near dissociation (a rare situation), they can be used to determine the separate Coulombic and exchange contributions to the intermediate and long-range interaction potentials. This article represents the first such experimental determination for any atomic interaction. This work clearly shows that exchange is exponential over 2 orders of magnitude in energy and that accurate theoretical calculations of Coulombic and exchange contributions agree well with experiment for this benchmark system. In addition, improved ground-state dissociation energies D(e)(X1SIGMA(g)+) are determined for Li-6(2) (8517.15 +/- 0.07 cm-1) and (Li2)-Li-7 (8516.61 +/- 0.08 cm-1), showing a significant non-Born-Oppenheimer isotope shift of 0.54 +/- 0.15 cm-1. Improved spectroscopic constants, potential energy curve, and dissociation energy (333.4 +/- 0.3 cm-1) are determined for the a3SIGMA(u)+ state of Li-6(2). It should be clearly understood that precise dissociation energies such as these are a prerequisite for a meaningful long-range exchange/Coulomb analysis.