Kinetics of Thermal Ionization. II. Xenon and Krypton

The rate and mechanism of the thermal ionization of xenon has been further studied by a microwave and shock‐tube method. The negligible effect of impurities or background on the observed ionization rates was demonstrated. For a variety of added gases, the activation energy for the observed ionization was about 8 eV, not 12.1 eV, the ionization energy. Detailed time studies show an induction period for buildup of free electrons as a result of populating intermediate electronic states of xenon; however, the simple three‐step mechanism proposed previously $(\mathrm{Xe}+\mathrm{M}\leftrightharpoons {\mathrm{Xe}}^{*}+\mathrm{M},{\mathrm{Xe}}^{*}+\mathrm{M}\to {\mathrm{Xe}}^{+}{\mathrm{+e}}^{-}+\mathrm{M})$ was shown to be insufficient to explain certain quantitative features of the data. At these high temperatures a large number of parallel paths are possible, and in general, entropy differences are more important than energy differences. On this basis it is argued that the final ionization process leads to Xe⁺+e^—+M and not MXe⁺+e^—. Limited data are presented on the ionization of krypton, and an activation energy of about 10 eV, not 14 eV (the ionization energy), is indicated. Reaction cross sections are given for xenon and krypton.