Recent measurements of electron ejection cross sections, differential in electron energy and emission angle, for energetic protons were analyzed to give the total ionization cross section, and the mean energy of the first-generation secondary electron produced in single collisions of protons with hydrogen molecules. These data are used to determine the fraction of the energy lost by the proton in overcoming the electron-binding energy in the target molecules and also the fraction transformed into kinetic energy of first-generation secondary electrons. The fraction of the energy loss which goes to overcome the binding energy of the ejected electron is about 30% for 0.1 MeV protons and decreases monotonically to approximately 20% at 1.5 MeV. The fraction of the energy which is carried away as kinetic energy by the first-generation secondary electrons is about constant at approximately 60% over the same proton energy range. The remaining fraction of the total stopping power is accounted for in excitation and recoil energy of the residual ions and atoms. This information gives a quantitative picture of the energy partition for energetic protons at the beginning of the physical stage of radiation action.

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