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 $3^{3}S$ and $3^{3}P$ with ground-state atoms are suggested by the results of these calculations. A deexcitation cross section corresponding to $3^{3}S\ensuremath{\rightarrow}2^{3}P$ 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.