Multistep initial ionization behind strong shock waves in argon

A spectroscopic investigation of the initial ionization behind strong shock waves in argon was made to determine the role of atomic levels in addition to the first excited state. For a numerical modeling of the ionization process a six-level argon atom was assumed. A comparison of analytical and numerical results shows that in the initial ionization process, two-step ionization and multistep (``ladder-climbing'') ionization as originally suggested by de Boer, compete with each other. For an initial pressure of 5 torr and a Mach number of 9 the contributions of two-step ionization and multi-step ionization are roughly equal, with increasing pressure and temperature favoring the latter. The appropriate atom-atom excitation and ionization cross sections have been determined.