The energy distribution of ions obtained by field ionization of He, Ne, Ar, and H2near the surface of a tungsten emitter is measured with an improved retarding potential tube. Most ions are found to originate immediately beyond the critical distance determined by the condition that the electron tunnels above the Fermi level. The ion energy is determined by the ionization energy and an image force contribution, while there is no indication of a shift of the ground level of the gas atom due to the proximity of the metal surface. In the case of hydrogen the atomic ion is found to have an additional energy of frac12; of the dissociation energy of the molecular ion. The halfhyphen;width of the distribution is between 0.5 and 0.7 eV for all the above gases, indicating an extremely narrow ionization zone with a depth of a few tenths of an angstrom. Calculation of the energy distribution by applying the WKB method to a square well gives the proper general features and the temperature effect but a halfhyphen;width too wide by a factor of 2, thereby exhibiting the limitation of a linear model.
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