A new technique for threshold excitation spectroscopy

Abstract

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 N2 are given, and the onsets of the 23S and 21S states of helium have been used to determine accurately the energy (19.361+or-0.009 eV) of the 22S resonance in helium.