Threshold electron impact excitation of the rare gases

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Abstract
Threshold electron impact spectra have been obtained for the rare gases helium, neon, argon, krypton and xenon using a velocity selected electron beam. Zero-energy electrons ejected at excitation thresholds are scavenged by sulphur hexafluoride and the resulting SF- 6 ions are detected using a masses filter. Optically forbidden transitions are observed to make a large contribution to the total threshold excitation. The shape of the helium scavenging curve above the first ionization potential is discussed with regard to threshold laws for ionization by single electron impact. Excitation of outer s and p electrons in neon, argon, krypton and xenon is observed as well as other structure which is attributed to double electron excitation.