Evidence of production/losses of NO on a pyrex surface under and after plasma exposure.

Molecules production on plasma exposed surfaces is of great interest for plasma/catalyst coupling used in air treatment. Interaction of real surfaces with reactive plasmas is barely studied and surface-produced molecules can provide a valuable fingerprint of the underlying processes (adsorption, desorption, recombination, chemical reactions). We use CCP discharge in 60 cm pyrex tube with \textit{in-situ }tunable laser diagnostics to monitor evolution of NO and NO$_{2}$. The tube surface is pre-treated using either Ar, O$_{2}$, N$_{2}$ or air plasma and then two types of experiments are performed i) molecules production in a pure O$_{2}$ plasma reacting with adsorbed species ii) the study of molecules losses/conversion on the surface by introducing a controlled amount of NO/NO$_{2}$ in the reactor. It was found that after N$_{2}$ plasma pre-treatment, pyrex surface is covered with nitrogen species that initiate NO production when exposed to O$_{2}$ plasma. Assuming these species are N atoms, their density was estimated [N$_{ads}$]=3$\cdot $10$^{13}$ cm$^{-2}$, what gives an assessment of the surface active sites density. Similarly, O$_{2}$ plasma leaves adsorbed oxygen species with [O$_{ads}$] $\approx $2$\cdot $10$^{14}$ cm$^{-2}_{. }$These species are capable of NO oxidation to NO$_{2}$ and inhibit NO$_{2}$ adsorption on pyrex (which is pronounced after N$_{2}$ and Ar plasma preparation).