| AVS 63rd International Symposium & Exhibition | |
| Plasma Science and Technology | Thursday Sessions |
| Session PS-ThP |
| Session: | Plasma Science and Technology Division Poster Session |
| Presenter: | Tam Nguyen, University of Houston |
| Authors: | T. Nguyen, University of Houston E. Hernández, University of Houston D.J. Economou, University of Houston V.M. Donnelly, University of Houston |
| Correspondent: | Click to Email |
Non-thermal atmospheric pressure plasmas are of interest for their potential use in surface treatment and biomedical applications. Even though considerable progress has been made, less is known about the species generated in close proximity to a surface. A novel approach using optical emission spectroscopy (OES) has been developed to probe emissions close to the surface. The plasma jet splays along the flat face of a hemispherical quartz prism. Emission was observed through the prism as a function of the angle of incidence. In this manner, emission integrated from a line-of-sight across the jet was obtained. Emission was also recorded as a function of angle through a MgF2 window coupled to a VUV spectrometer. At normal incidence (0o), light was detected mainly from the discharge within the plasma source, which consisted of a quartz tube surrounded by two electrodes, powered by a 200 kHz AC source. He emission at 706 nm peaked twice per cycle, near the positive and negative voltage maxima. Conversely, VUV-UV-visible emissions from H, O, N, OH, NO and N2+ impurities contained in the He feed gas within the discharge were hardly modulated. The only exception was N2(C-B) emission, which peaked strongly near the maximum positive voltage and weakly near the maximum negative voltage. When the angle of incidence was reduced to just below the critical angle (43.4o) to observe the region within ~1 mm of the surface, all emissions were strongly modulated and peaked near (lead or lag) the maximum positive voltage; no emission was detected at the maximum negative voltage. All of these observations are consistent with excitation of O, N, OH, H, and NO being predominantly due to dissociative excitation of precursors O2, N2, H2O and NO2 resulting from collisions with He metastables (He*). Similarly, N2+ emission was attributed to He* Penning ionization and formation of excited N2+. Only He 706 nm and N2(C-B) emissions were due to electron impact excitation. Inside the discharge tube, He* is long lived because it is quenched very slowly by diffusion to the walls or by collisions with He or electrons, hence He*-induced emission are only weakly modulated. Near the surface, air diffuses into the He and leads to rapid He* quenching and hence a strong modulation of the emissions.