The Efficiency of Electron Impact Leading to Resonance in Helium

The efficiency of resonance impact between electrons and helium atoms has been studied as a function of the energies of the impinging electrons. The transition studied is $1{}_{}{}^1{}_{}{}^{}S\to 2{}_{}{}^3{}_{}{}^{}S$ at 19.77 volts. The range of energies is 0.8 volt, i.e. from 19.77 to 20.55 volts. The efficiency with which electrons make inelastic impacts rises to a maximum 0.18 volts beyond the resonance potential of 19.77 volts and then decreases. The maximum average efficiency for electrons of 19.95 volts energy is estimated to be 0.002. The measurements were carried out by comparing the velocity distribution of the electrons leaving an equipotential surface with the drop in current as observed in the inelastic impact method of Franck, and similarly by comparing the velocity distribution of the electrons with the distribution of the positive current caused by them in a Lenard experiment. Both types of measurement yield similar results.