Ionisation mechanism in the relaxation zone of weakly instationary shock waves in argon and krypton

The primary ionisation relaxation up to electron densities of n e = 4·10 13 cm -3 is investigated by means of 4 mm - microwave - interferometry. The values of the timedependent gas temperature T a , and gas density n a behind the shock front are calculated using a gasdynamic model which strictly takes into account the instationarity of the flow. From the results it is concluded that neither the familiar two step process dominated by atom collisions nor the assumption of additional excitation processes by electron collisions can fully describe the observed ionisation rates. There is evidence that both the ionisation rates and the electron temperature are influenced by transitions between the first four excited states due to superelastic electron collisions. As a result the electron temperature may even exceed the gas temperature.