Simple determination of Na

A simple model is developed to determine a scattering length from the values of the energy of the last, least bound, vibrational of the ground state, either ¹\Σ g⁺ or ³\Σ ᵤ⁺, an alkali dimer. It is based on an extrapolation of the positions of the outermost nodes of the bound vibrational wave functions towards the threshold. It uses the asymptotic part of the molecular only. The method is applied to recently measured levels at the (3s+3s) of Na2. We obtain precise values for the two scattering respectively associated to the dissociation limits F₁=F₂=1,F=2(55.1±1.6 a0) and F₁=F₂=1,F=0 (50.0±1.6 a0); the first corresponds to the a1,-1 scattering length usually considered in collisions. The extrapolation procedure is also applied to Li2, existing experimental data.