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 41S and 43S states of helium for energies within about 1 V of threshold. We find a prominent peak in each function and determine its energy as 41S 23.937 ± 0.007 ± 0.011 eV, 43S 23.939 ± 0.007 ± 0.009 eV, where the first limits represent a systematic error and the second are standard deviations of several determinations.
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Heddle et al. (1973) studied this question.