Formation of Excited Molecular States in the Positive Column of a Helium Discharge

Abstract

Excited molecular states are shown to be populated in the positive column of a glow discharge in helium by electron excitation of metastable molecules. At higher pressures in the range 2–20 torr, metastable molecules are formed primarily by three-body conversion of metastable atoms, Hem+2 He→He2m+He. The rate coefficient is low because of the repulsive potential between Hem and He. At lower pressures a second mechanism is effective, He(2p,3P)+2He→He2(2pπ,3Πg)+He followed by He2(2pπ, 3Πg)→He2(2s,3Σu+)+hν. The metastable molecule is thus formed indirectly by a three-body reaction involving the 2p, 3P state, where the absence of a repulsive potential indicates a large cross section which together with a relatively high population at lower pressures results in a significant contribution to the molecular metastable concentration. The data permit derivation of the three-body coefficient for the conversion of He (2p, 3P) to He2(2pπ, 3Πg).