High resolution studies of electron excitation - I. The 4S states of helium and the energy scale

Abstract

We describe an electron monochromator using hemispherical deflecting electrodes and its application to the measurement of electron excitation functions. We present excitation functions of the 4$^{1}$S and 4$^{3}$S states of helium for energies within about 1 V of threshold. We find a prominent peak in each function and determine its energy as 4$^{1}$S 23.937 $\pm $ 0.007 $\pm $ 0.011 eV, 4$^{3}$S 23.939 $\pm $ 0.007 $\pm $ 0.009 eV, where the first limits represent a systematic error and the second are standard deviations of several determinations.