The time and spatial dependence of the chemical conversion of CO 2 to CO were studied in a closed glow discharge reactor (p = 50 Pa, I = 2-30 mA) consisting of a small plasma zone and an extended stationary afterglow. Tunable infrared diode laser absorption spectroscopy has been applied to determine the absolute ground state concentrations of CO and CO 2. After a certain discharge time an equilibrium of the concentrations of both species could be observed. The spatial dependence of the equilibrium CO concentration in the afterglow was found to be varying less than 10%. The feed gas was converted to CO more predominantly between 43% and 60% with increasing discharge current, forming so-called quasi-equilibrium states of the stable reaction products. The formation time of the stable gas composition also decreased with the current. For currents higher than 10 mA the conversion rate of CO 2 to CO was estimated to be 1.2 × 10 13 molecules J −1. Based on the experimental results, a plasma chemical modelling has been established.

The spatial distribution of ozone and of oxygen atoms was studied along the active and the passive zone of a dc discharge (positive column, pressure: p = ( 4 . . .lo) . 10' Pa, current: I = 2 . . .50 mA, flow rate: F = 5 . . .lo0 sccm) in flowing oxygen. The composition of the final output 02/03-mixture is controlled by relaxation processes in the passive reactor zone. It is affected sensitively by the total number density and the gas temperature in the afterglow. Steady states meaning reversible chemical qua+-equilibria were observed and analysed extensively. Within a detailed kinetic model the formation of these equilibria can be explained quantitatively. The synthesis to ozone is controlled above all by the metastable O2 (a'd,) species, which modify drastically the results for the basic mechanism, considering the Oatoms in the 3 P and 'D states.

Chemical quasi-equilibria represent the product composition of stable components at the outlet of a nonisothermal plasma chemical reactor under operating conditions of very high or vanishing electric power input. The properties of these quasi-equilibria in relation to the thermodynamic equilibrium are discussed first of all using an instructive kinetic model. The investigations show that for the formation of the quasi-equilibria the existence of two time scales for the chemical conversions in the system is necessary. The quasi-equilibrium states are formed on a short time scale by fast relaxation and deactivation processes of unstable components. After that the system passes into the thermodynamic equilibrium for very long times. The transition from a chemical quasi-equilibrium state into the thermodynamic equilibrium was detected in the H2−I2−HI system experimentally, too. The analysis of a detailed microphysical model for this system shows that the transition into the thermodynamic equilibrium is caused by the increasing thermal production of unstable components (e.g. iodine atoms) at higher gas temperatures. Under these conditions the electronic production of atoms is only a small correction of the thermal degree of dissociation, and the thermodynamic equilibrium is the only stationary state for the system.