Diabatic-states representation for He* (n > or = 3) + He collisions

A diabatic-states representation for scattering of triplet helium atoms, with n > or = 3, by the ground-state helium atom is presented. Ab initio calculations have been performed on the 3Σ+/subg/,/subu/, 3Pi/subg/,/subu/, and 3Δ/subg/ Rydberg states of He2. Matrix elements for radial and angular coupling between the orthogonalized diabatic states have also been evaluated. The angular couplings are quite weak, with the result that molecular states with different electronic symmetries are effectively uncoupled in low-energy collisions. The crossing by a repulsive diabatic-potential curve through the Rydberg series and into the continuum is shown to be an effective mechanism for associative ionization. Some qualitative observations on associative ionization and dissociative recombination are made