Time-Resolved Optical Emission Spectroscopy Reveals Nonequilibrium Conditions for CO$_{2}$ Splitting in Atmospheric Plasma Sustained with Ultrafast Microwave Pulsation

Among the pool of Power-to-X technologies, plasmas show high potential for the efficient use of intermittent renewable energies. High efficiencies of CO$_{2}$ conversion have been reported while using microwave plasmas at vacuum conditions which are, however, not suitable for CO$_{2}$ mitigation at industrial scales. Here we show that ultrafast pulsation of microwaves allow significant improvements of energy efficiencies during CO$_{2}$ splitting at atmospheric pressure as compared to continuous wave operation of the microwave source. Moreover, by the interrogation of the plasma with time-resolved optical emission spectroscopy we can, for the first time, observe the evolution of the vibrational and rotational temperatures and define a time window where nonequilibrium can be expected at the beginning of the pulse of an atmospheric CO$_{2}$ microwave plasma. In spite of the evidence of nonequilibrium in our system, thermal mechanism appears to dominate the CO$_{2}$ dissociation. It is shown that a fine control of the energy deposition in the plasma is possible with ultrafast pulsation of the microwave energy supply.