An accurate semi-empirical potential model for the a Σ state of the alkali dimers Na, K, Rb, and Cs which reproduces the scattering length.

A new four parameter Tang-Toennies type potential model is described for the a  Σ triplet state of the alkali dimers Na, K, Rb, and Cs. Compared to an earlier three parameter semi-empirical model based on the experimental well depth, D, well location, R, and the harmonic vibrational frequency, ω [Lau et al., J.