Interactions of excited (n=3) and ground-state helium atoms: Potential curves and inelastic processes

Adiabatic potential-energy curves have been calculated for the interaction of several n=3 triplet states of helium with ground-state helium. Mechanisms for excitation-transfer in collisions of 3³S and 3³P with ground-state atoms are suggested by the results of these calculations. A deexcitation cross section corresponding to 3³S-->2³P excitation-transfer has been calculated using the Stueckelberg-Landau-Zener approximation and is consistent with existing measurements. The existence of a small hump in the potential curve causes the cross section to decrease rapidly at low energies. Thus, a strong temperature dependence of effective cross sections is expected. A possible mechanism for associative ionization is also discussed.