Nonlinear excitation and dissociation kinetics in discharges through mixtures of rare and attaching gases

Using space-time-resolved laser-induced fluorescence and plasma-induced emission spectroscopy, we find metastable and dissociation product densities to vary nonlinearly as Ar is diluted with SF₆, Cl₂, and BCl₃. Because rare gases are relatively inert in the ground state, it is often assumed that rare gases change discharge chemistry through metastable interactions. Surprisingly, the measured changes cannot be accounted for by considering only direct metastable energy transfer. A model is proposed in which Ar metastable states indirectly enhance molecular dissociation by enhancing the ion density via Penning reactions; the increase in ion density in turn causes an increase in the dissociative recombination rate. This rate varies nonlinearly with concentration because both ion-producing metastable states and recombining electrons are quenched by addition of the attaching, molecular gases. The model satisfactorily explains discharge changes when small concentrations of molecular gases are added to an Ar discharge. Less well understood are changes over a broader concentration range, where dissociation and excitation rates decrease with decreasing Ar concentration because of electron attachment and cooling.