Total Cross Sections for Ionization and Attachment in Gases by Electron Impact. I. Positive Ionization

The total ionization cross sections of He, Ne, Ar, Kr, Xe, H2, D2, N2, O2, CO, NO, CO2, N2O, and CH4 have been measured from threshold to 1000 eV in a total ionization tube. More limited measurements were performed in C2H4 and SF6. Great care was taken to assure complete collection of electron and ion currents, and the absence of spurious instrumental errors. A new method was devised for obtaining absolute cross sections of gases relative to H2, and a McLeod gauge was used to obtain the absolute cross section in H2. The cross sections in NO and O2 could not be obtained by this method, and an approximate correction to direct McLeod-gauge readings was used for these gases. It is believed that the results are as accurate as is possible with the present method. It is difficult to explain the differences found between cross sections measured by various investigators. McLeod-gauge errors appear to account for most of the difference in absolute magnitude.