Mode resolved heating dynamics in pulsed microwave CO₂ plasma from laser Raman scattering

Efficient CO₂ reduction is predicted for CO₂ microwave plasma by virtue of predominant excitation of the asymmetric stretch vibration. Although interpretation of ongoing research is generally based on this mechanism, direct measurement of the power partitioning to support the assumed preferential vibrational excitation in CO₂ microwave plasma is currently lacking. Here, such measurements are performed on a 100 μs pulsed microwave CO₂ discharge. The <1% duty cycle ensures low gas temperature conditions at the discharge onset. Raman and Rayleigh scattering are employed to reveal vibrational, rotational, and gas temperatures in a spatially and temporally resolved manner. A novelty in the approach is that asymmetric stretch excitation is determined from the bending - symmetric stretch Raman spectrum. During the first 40 μs a significant inter-vibrational non-equilibrium is observed with the symmetric stretch and bending temperature reaching 750 K and the asymmetric stretch temperature reaching 1150 K. A maximum rotational-vibrational non-equilibrium occurs after 60 μs when the rotational temperature is half of the 1150 K vibrational temperature. Rotational and translational modes are measured to be in equilibrium at all times. The power partitioning is analyzed to estimate the power consumed by vibrational excitation, which is used to estimate the reduced electric field in the discharge. This work confirms strong vibrational excitation in CO₂ microwave plasma albeit less predominant than often assumed.