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A new technique has been developed for obtaining high resolution threshold excitation spectra in electron-atom and electron-molecule collisions. A very weak field penetrating into the target chamber produces a saddle point in the potential distribution which has the effect of focusing and enhancing the extraction efficiency of very low energy inelastically scattered electrons. Also the electron energy analyser is used in a special mode which sharpens the pass band for low energy electrons. Examples of threshold excitation spectra of He, Ar and N 2 are given, and the onsets of the 2 3 S and 2 1 S states of helium have been used to determine accurately the energy (19.361+or-0.009 eV) of the 2 2 S resonance in helium.
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Cvejanović et al. (1974) studied this question.
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