Spectroscopical Observation of a Relaxing High Speed Nitrogen Plasma Jet

A near sonic nitrogen plasma jet operating at pressures between 100 Torr and 1 atm has been investigated spectroscopically. From the absolute emission coefficient of a NI spectral line, local values for the electron temperature T_e have been derived. For pressures above 200 Torr, T_g was found to coincide within the limits of experimental error with the gas temperature T_g. The latter quantity has been determined via the relative emission coefficient of selected rotational line components of the N₂⁺ molecular band at 3914 Å. The results of these measurements together with control data for the electron density derived from the continuous emission coefficient indicate that at pressures above 200 Torr the existence of a thermal equilibrium between the degrees of freedom corresponding to particle translation, electron excitation, and ionization can be accepted, at least for the inner zone of the plasma jet. To the contrary, the data for the absolute emission coefficient of N₂⁺ molecular band lines show that the degree of dissociation in the plasma jet is much in excess of that corresponding to equilibrium. This phenomenon can be explained as a result of the rapid temperature decay in the plasma from initially 13000 K in the arc heating zone to T ≦ 9000 K in the plasma jet zone proceeding in a time interval of 10^?5 s which is much shorter than the time necessary for adjustment of dissociation equilibrium. In the outer cool zone of the plasma jet, an unusual high intensity of the N₂⁺ radiation was found thus indicating the existence of a nonequilibrium excitation mechanism typical for a decaying nitrogen plasma. From the supernormal high degree of dissociation in the high-speed subatmospheric nitrogen plasma jet, conclusions are drawn with respect to its applicability as source of reactive particles in plasma-chemical experiments.