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

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.

Optical emission spectroscopy study in CO2 methanation with plasma

“…OES was performed to estimate the excited species generated by electron impact. [22][23][24][25][26][27][28][29][30][31][32] The light from the whole plasma area is focused using a lens. The lens is placed on the upper side of the reactor, and the focused light was guided to the spectrometer with an optical fiber.…”

“…In addition, OES is a useful method for estimating the vibrational temperature. 25,[30][31][32] The vibrational excitation state is often represented by a parameter referred to as the vibrational temperature, which is calculated based on the assumption that the distribution of the vibration excitation state follows the Boltzmann distribution. Plasma-based CO 2 recycling and decomposition technologies are attracting more attention, and more studies aim to elucidate the behavior of vibrationally excited species by OES.…”

Effect of gas flow rate and discharge volume on CO2 methanation with plasma catalysis

“…OES is an effective method for diagnosing the plasma state. [28][29][30][31] Figure 7 shows the gas flow rate dependence of the Hα and Hβ/Hα emission intensities at a discharge volume of 30.1 cm 3 . Hα is affected by the electron temperature and density, whereas Hβ/Hα is only affected by the electron temperature.…”