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Abstract.A two-electron model potential method is proposed to compute diabatic electronic excited states for Na2. The configuration space is first divided into two subspaces corresponding to singly and doubly excited configurations respectively. Next this partition is modified to ensure a correct dissociation limit for the ground state. The matrix element of the electronic Hamiltonian between the two subspaces can be extrapolated along a Rydberg series up to the ionization continuum. The first order M.Q.D.T. treatment of Giusti (1980) is then used to estimate the cross-sections for the reaction Na (3 p) + Na (3 p) ~ Na~-+ e-, considering various symmetries of the intermediate Na2 molecule. A marked selectivity in favour of the as,+ s~cmmetry is found and the estimated cross-section o-~ 5 A 2 for a collision energy of 0.05 eV is in satisfactory agreement with the experimental results.