Mechanistic model of atomic recombination

Recently we have started a new approach of studying atomic recombination at the reactor walls, by rapidly rotating a spinning substrate between the plasma and differentially pumped diagnostic chamber, thereby exposing the surface to the plasma, and then analyzing the reaction products few ms thereafter. We have investigated atom recombination in Cl$_{2}$ and O$_{2}$ plasmas with this technique. In a Cl$_{2}$ plasma, Cl$_{2}$ physisorbs and then desorbs over the time scale comparable to that for Cl recombination, thereby competing with Cl adsorption for active sites. With the plasma off, Cl$_{2 }$desorption flux increases nearly linearly with pressure. Cl recombination probabilities, $\gamma _{Cl}$, ranged from 0.01 to 0.1 and were found to increase with increasing n$_{Cl}$/n$_{Cl2}$ number density ratio (from 0.1 to 0.8) in Cl$_{2}$ plasmas. A multi-site adsorption model has been developed to explain the desorption kinetics of physisorbed Cl$_{2}$. The total surface site density of $\sim $ 10$^{15}$ cm$^{-2}$ was distributed with a Gaussian profile over binding energies (B.E) ranging from 7.8 to 19.8 Kcal/mol. For Cl$_{2}$ physisorption, the sites with B.E $<$ 14Kcal/mol are mostly unsaturated, and give rise to the pressure scaling of Cl$_{2}$ desorption flux. For sites with B.E $>$14Kcal/mol, the fractional surface coverage (\textit{$\theta $}$_{i})$ rises sharply with increasing B.E and pressure. The competitive adsorption of Cl$_{2}$ vs Cl at these high B.E sites is likely responsible for the observed dependence of $\gamma _{Cl}$ on n$_{Cl}$/n$_{Cl2}$.