Insights into the limitations to vibrational excitation of CO2: validation of a kinetic model with pulsed glow discharge experiments

Vibrational excitation represents an efficient channel to drive the dissociation of CO2 in a non-thermal plasma. Its viability is investigated in low-pressure pulsed discharges, with the intention of selectively exciting the asymmetric stretching mode, leading to stepwise excitation up to the dissociation limit of the molecule. Gas heating is crucial for the attainability of this process, since the efficiency of vibration-translation (V-T) relaxation strongly depends on temperature, creating a feedback mechanism that can ultimately thermalize the discharge. Indeed, recent experiments demonstrated that the timeframe of V-T non-equilibrium is limited to a few milliseconds at ca. 6 mbar, and shrinks to the mu s-scale at 100 mbar. With the aim of backtracking the origin of gas heating in pure CO2 plasma, we perform a kinetic study to describe the energy transfers under typical non-thermal plasma conditions. The validation of our kinetic scheme with pulsed glow discharge experiments enables to depict the gas heating dynamics. In particular, we pinpoint the role of vibration-vibration-translation relaxation in redistributing the energy from asymmetric to symmetric levels of CO2, and the importance of collisional quenching of CO2 electronic states in triggering the heating feedback mechanism in the sub-millisecond scale. This latter finding represents a novelty for the modelling of low-pressure pulsed discharges and we suggest that more attention should be paid to it in future studies. Additionally, O atoms convert vibrational energy into heat, speeding up the feedback loop. The efficiency of these heating pathways, even at relatively low gas temperature and pressure, underpins the lifetime of V-T non-equilibrium and suggests a redefinition of the optimal conditions to exploit the 'ladder-climbing' mechanism in CO2 discharges.

Automated summary

Vibrational excitation is an efficient channel for driving the dissociation of CO2 in a non-thermal plasma. Gas heating is crucial for this process, as the efficiency of vibration-translation relaxation strongly depends on temperature. The redistribution of energy from asymmetric to symmetric levels of CO2 through vibration-vibration-translation relaxation and collisional quenching of CO2 electronic states play important roles in gas heating and feedback mechanism. The efficiency of these heating pathways supports the existence of vibration-translation non-equilibrium and suggests a redefinition of the optimal conditions to exploit the 'ladder-climbing' mechanism in CO2 discharges.