Excitation and dissociation of CO heavily diluted in noble gas atmospheric pressure plasma

The excitation and dissociation of CO admixed to argon and helium atmospheric pressure radio frequency plasmas is analyzed. The absorbed plasma power is determined by voltage and current probe measurements and the excitation and dissociation of CO and CO by transmission mode Fourier-transform infrared spectroscopy (FTIR). It is shown, that the vibrational temperatures of CO and CO are significantly higher in an argon compared to a helium plasma. The rotational temperatures remain in both cases close to room temperature. The conversion efficiency, expressed as a critical plasma power to reach almost complete depletion, is four times higher in the argon case. This is explained by the lower threshold for the generation of energetic particles (electrons or metastables) in argon as the main reactive collision partner, promoting excitation and dissociation of CO, by the less efficient quenching of vibrational excited states of CO and CO by argon compared to helium and by a possible contribution of more energetic electrons in an argon plasma compared to helium.