Excitation processes in low-energy collisions between ground state helium atoms

Apparent threshold energies have been measured for excitation of the He states 2 1 P, 3 1 D, 3 3 P, 3 3 D, 4 1 S, and 4 3 S in collisions between ground state He atoms. The energy dependence of the cross sections for emission of the He lines 2 1 P to 1 1 S, 3 3 P to 2 3 S, 3 1 D to 2 1 P, 3 3 D to 2 3 P, 4 1 S to 2 1 P, and 4 3 S to 2 3 P has been measured from the respective thresholds up to about 600 eV collision energy. Estimates are given for the absolute values of the cross sections in this range. The emission from states He(3D) shows structure which is due to phase-interface effects.