Rotational Non-Equilibrium of CO Excited States in Microwave Discharge CO$_2$ Plasma

Carbon dioxide plasmas are widely studied for practical engineering. For example, they are applied to CO$_2$ laser technologies and considered as the decomposition process of CO$_2$ into CO, etc. However, in these plasmas containing CO$_2$ or CO, the excitation kinetics of electronically excited states of the CO molecule are not fully understood yet. In this study, spectroscopic characteristics of the CO Angstrom band in microwave discharge CO$_2$ plasma is investigated, and a discussion comparison the third positive system (3$^{rd}$ PS) of CO. For this purpose, vibrational temperature $T_v$ and rotational temperature $T_r$ of the CO excited states in low-pressure CO$_2$ plasma are investigated. Angstrom band spectra of CO are calculated theoretically as functions of $T_v$ and $T_r$, after which a fitting is conducted to analyze experimental results. The best theoretical fitting is obtained using two-rotational temperature, a bulk component $T_r$ = 0.04 eV with high energy tail $T_r$ = 0.17 eV, occupying 3/4 and 1/4 fraction of the number density, respectively. For the vibrational temperature $T_v \approx$ 0.4 eV is found as a unique value. From comparing these results with the CO 3$^{rd}$ PS values, it is turned out that there is a difference of 0.11 eV as to the value of $T_r$.