Initial Ionization Rates and Collision Cross Sections in Shock-Heated Argon

The initial ionizing processes in shock-heated argon are investigated in a low-pressure, 4.5-in.-diam, shock tube using double electrostatic probes to measure the ionization rates. The experimental conditions cover a range of initial pressures of spectroscopically pure argon from 0.2 to 3.0 Torr and temperatures from 7000 to 12 000°K. The maximum impurity level at 1 Torr pressure is less than 7 parts per million. Correction terms are applied to take account of flow nonuniformities in shock tubes operating at low initial pressures. The cross section for excitation, σ*, in the reaction A + A → A* + A is assumed to vary as S1*(E − Ea*) and S1* is measured to be 2.5 × 10×20cm2/eV. The cross section for ionization, σ+, in the reaction A* + A → A+ + A + e is assumed to vary as S+(E − Ea′) and S+ is found to be 5.1 × 10−17cm2/eV. The corresponding rate constants k* and k+ are given for this temperature range.