Inelastic Electron Scattering by Helium Atoms

The angular distribution of electrons scattered by helium atoms when the electrons have lost 21.12 volts energy, thereby exciting the atom to the 2¹P state, has been studied for energies between 27.5 and 400 volts and over an angular range from 6^∘{} to 150^∘{} in most cases. For the higher energies the angular range over which measurements could be made was less, e.g., for 400 volt-electrons measurements could not be made beyond 22^∘{}. The higher the original energy of the electrons, the less the number of electrons scattered with a 21.12 volt energy loss, and the steeper the angular distribution curve. For the lower energies (below about 100 volts) the curves fall gently to about 60^∘{} and then become practically flat up to 150^∘{}. One curve is given for the distribution of 42 volt-electrons which have lost 22.97 volts energy (excitation to the 3¹P state) and another for 400 volt-electrons which have lost 25 volts energy (thereby ionizing the atom). Comparisons are made between the experimental results and theories proposed by Morse and by Massey and Mohr. A very direct comparison is possible with the theory of Massey and Mohr and a very satisfactory agreement is found.