The Quantum Mechanical Cross Section for Ionization of Helium by Electron Impact

By using Born's collision theory without exchange the effective ionization cross section is calculated for helium when the resulting ion is left in the normal state. A plane wave is used as a mathematically manageable substitute for the hyperbolic functions previously employed to represent the ejected electron. A comparison is made of the calculated cross section with the experimental values obtained by P. T. Smith. The maximum of the calculated curve falls at approximately three times the ionization potential. The calculation predicts that the maximum probability of ionization occurs when the energy transfer is about 3.5 volts greater than the ionization potential.