The cross section for ionization of the hydrogen atom on electron impact has been measured as a function of electron energy. A modulated beam of atoms and molecules in varying proportions, taken from a furnace, is crossed by a dc electron beam, and the positive ions formed are taken into a mass spectrometer. By using such modulation techniques, the ions formed by ionization of the beam are distinguished from the much larger number of ions formed by collisions of electrons with the residual gas in the vacuum chamber. From the study of the mass-spectrometer peak strengths as a function of temperature with constant gas flow, the ratio of the cross sections for ionization of the atom and the molecule is directly determined. The absolute atomic cross sections, determined by multiplying this ratio by the known molecular cross sections, are cross-checked by taking relative cross sections for the atom. Complete agreement with the first Born approximation is found only above about 250 ev. Deviations of experiment from theory at lower energies are as predicted qualitatively by theory.

• Received 12 May 1958

DOI:https://doi.org/10.1103/PhysRev.112.1141