Ionization of electron-excited molecules in a strong electric field

Abstract

In this paper, we prove the direct effect of a strong electric field on the ionization process occurring in dense hightemperature mixtures of molecular gases containing electronexcited particles. Our proof is based on the analysis of systematic divergences of experimental and theoretical data on the value of the electric current onto an electric probe placed into these mixtures. For a wide number of mixtures of var� ious compositions, the electric current calculated on the basis of the conventional theory of electric probes always turns out to be several times lower than the measured current. Introducing into the conventional theory an additional ionization source stipulated by the strong electric field near the probe surface has resulted in satisfactory consistency between the calcu� lated data and the available experimental data. The electricprobe theory for dense chemically reacting plasmas at normal pressure was developed by the authors (together with G.S. Aravin, P.A. Vlasov, and Yu.K. Karasevich, Institute of Chemical Physics, Russian Academy of Sciences) in the 1980s (1-5). A review of these studies was presented in (6). The theory is based on experimental data on the unsteady chemi� calionization process in a shock tube beyond the reflected shock wave in various gas mixtures. In these measurements, a twoconductor microwave interfer� ometer was used, with one of the conductors being a cylindrical probe maintained at a constant negative electric potential. Upon significantly elevating the temperature in the region beyond the reflected shock wave in the gas mix� ture under study, unsteady chemical processes began to develop. Among them, chemicalionization reac� tions including the production of positive ions and electrons were present. In the experiment, the unsteady electric current onto a probe was measured. Then, in accordance with microwave measurements, the electronconcentration profiles were plotted as functions of time. These data have made it possible to find the effective timedependent ionization rate. This function obtained experimentally was used for numer� ically solving the probe problem. The conventional setting of the problem of immobile lowtemperature ( T ~ 2000-3500 K) ionized gas mixtures at pressures on the order of atmospheric pressure can be formu� lated in the following manner. The unsteady set of equations in a cylindrical coor� dinate system can be written out in the form