Microwave and optical studies of a helium afterglow

Investigations of dissociative recombination between molecular ions He2+ and electrons were carried out as a function of the electron temperature, T/sub e/, up to approximately 1.5 eV in order to provide optimal conditions for its occurrence. Observations of the spectral line profile of the lambda6678 emissions during the afterglow did not reveal the characteristic line broadening of the dissociative process. However, the broadening may have been masked as a result of large excitation transfer cross sections between the excited atoms (31D and 33D) and the ground state (11S) helium atoms. By using proper electron density values, it has been possible to correlate afterglow atomic radiation intensity with the square of the electron density during the early afterglow indicating the occurrence of two-body electron-ion recombination. Afterglow quenching of the type expected if electron-ion recombination is the source of the excited atoms and molecules is observed. The principal afterglow processes found in these studies are strong electron production by metastable-metastable ionizing collisions balanced by enhanced ambipolar diffusion loss. The He(23S) metastable-metastable ionizing coefficient at 3000K is found to be (1.7 +- 0.3) x 10-9 cm3/sec in good agreement with previous results. An electron production term sensitive to the electron mean energy--possibly electron impact ionization of metastable atoms--is deduced from pulsed microwave heating experiments