Nonequilibrium characteristics in the rotational temperature of CO excited states in microwave discharge CO2 plasma

Abstract

Abstract Optical emission spectroscopy (OES) measurements are performed on the third positive system (3PS) and Ångstrom system bands for microwave discharge CO2 plasma with its discharge pressure of 1 Torr. The molecular vibrational temperature T v and rotational temperature T r of CO b 3Σ+ and CO B 1Σ+ states, which are the CO electronically excited upper states of the 3PS band and of the Ångstrom band, respectively, are determined as the best fitting parameters in the theoretical calculation of the optical emission spectra for both emission bands. The general theoretical fitting procedures showed that T v ≃ 0.15–0.24 eV and T r ≃ 0.08–0.15 eV for the CO b 3Σ+ state based on the OES analysis of the 3PS band. Meanwhile, the general fitting of the CO Ångstrom band is difficult with a mono-rotational temperature, whereas two rotational temperatures with bulk component T r bulk and high-energy tail T r tail can fit the experimentally observed spectrum with sufficient agreement. It is found that T v ≃ 0.3–0.4 eV, T r bulk ≃ 0.03 – 0.04 eV , and T r tail ≃ 0.13 – 0.17 eV for the CO B 1Σ+ state in the microwave discharge CO2 plasma. The observed nonequilibrium characteristics are discussed in terms of molecular processes in the CO2 plasma not only for the differences between CO b 3Σ+ and CO B 1Σ+ states, but also for the two-temperature characteristics in the CO B 1Σ+ state.