Shock tube study of ionization rates of NaCl-contaminated argon

Abstract

Electron density, electron temperature, and concentration of excited sodium atoms are measured in the weakly ionized regime behind a shock wave in impure argon in a shock tube using microwave techniques and spectrally resolved radiometry. Evidence is presented to show that an apparent increase in the rate of ionization is due to electron detachment of negative chlorine ions produced from sodium chloride vapor contained as an impurity. To be consistent with this chemical model, rate coefficients are found for 5500<T<8600 °K to be as follows: for NaCl+Ar+5.8 eV→Na++Cl−+Ar, k4=0.082exp(−67 315/T)/T2 cm3 sec−1; for Cl−+Ar+3.61 eV→Cl+e+Ar, k5=6×10−12exp(−41 900/T) cm3 sec−1; and for the branching ratio between the reaction NaCl+Ar+5.8 eV→Na++Cl−+Ar(k4) and reaction NaCl+Ar+4.27 eV→Na+Cl+Ar(k3), k4/k3=1.6×104exp(−17 760/T). Electron temperature is lower than heavy particle temperature by roughly 1000 °K. The electron-argon impact ionization rate coefficient is a weak function of electron temperature in contradiction to expectation.