Spectroscopic Study on Vibrational Nonequilibrium of a Microwave Discharge Nitrogen Plasma

Abstract

We generate a microwave discharge nitrogen plasma using a rectangular waveguide and a quartz tube (30 mm i.d.) inserted along the vector of the electric field, with an adjustable short-circuited plunger. The typical discharge conditions used are as follows: microwave frequency 2.45 GHz, input power 550 W, nitrogen (99.5%) gas flow rate 0.01–1.6 l/min, and discharge pressure p =0.5–10 Torr. Electron temperature T e , and density n e are measured using a double probe. Vibrational temperature T v is obtained from the population density of the vibrational exited states of N 2 C 3 Π u by spectroscopic examination. Rotational temperature T r is obtained from a comparison of the rovibronic spectra observed experimentally with the calculated ones. It is found that T e ≃0.5–2 eV, T v ≃0.45–1 eV, and T r ≃0.06–0.1 eV under the present pressure conditions, whereas n e ≃10 11 –10 12 cm -3 at a low discharge pressure, and n e ≃10 8 –10 9 cm -3 at a high discharge pressure. We discuss the measured vibrational populations by theoretical calculations in which the variation in their number density is expressed in terms of molecular processes in the nitrogen plasma.