The Resonance Potentials of Helium and Double Impacts by Electrons in Helium

Abstract

First and second critical potentials in helium.—A tube with five platinum electrodes was used to determine the first two critical potentials in carefully purified helium. The inelastic impact method of Franck was used over a pressure range from 0.13 to 13.00 mm and gave values of 19.87±0.06 for the first, and 20.62±0.06 volts for the second critical potential. The method of Lenard for pressures from 0.96 to 2.85 mm gave values of 19.83±0.02 and 20.57±0.06 volts. The spectroscopic value corresponding to the transition $1S\to 2s \mathrm{}\left(A\right)\mathrm{}$ is 19.77 volts and that corresponding to $1S\to 2S \mathrm{}\left(B\right)\mathrm{}$ is 20.55 volts. The observed difference between first and second critical potentials, 0.75 (Franck method), 0.74 volts (Lenard method), is in excellent agreement with the spectroscopic value of the difference $B-A$, 0.78 volts. The average initial velocity correction was determined by two different methods which led to concordant results.