Measurement of Radical Density in an Atmospheric Plasma by Molecular Beam Mass Spectrometry

Radical species produced in an atmospheric discharge have been measured by appearance potential mass spectrometry (APMS) using a Hiden EQP differentially pumped mass/energy analyser. The non-thermal discharge was generated by a radio frequency driven atmospheric plasma source.\footnote{E. Stoffels et\textit{ al}., Plasma Sources. Sci. Technol.11(2002) 383-388} Species were sampled from atmospheric pressure using a triple stage differentially pumped molecular beam inlet system. The discharge was generated mainly in Helium and, for the present investigation, different percentages of Oxygen, Nitrogen, Nitrous oxide, Carbon dioxide and Carbon monoxide gases were added to the plasma discharge. Radical densities produced in the atmospheric discharge were studied as a function of the power and the distance between the plasma source and the entrance to the analyser. Other species observed from the Oxygen/ Helium and Nitrogen/Helium mixtures were NO radicals formed by recombination.