Oxygen atom and negative ion densities and kinetics by cavity ringdown spectroscopy*

Accurate absolute density and kinetic measurements of oxygen atoms and negative ions are necessary for rigorous testing of oxygen plasma models. We have developed cavity ringdown spectroscopy of the forbidden $^1$D-$^3$P transition at 630nm using an external cavity tuneable diode laser, and used it to measure oxygen atom densities and translational temperatures (from the Doppler profile) in DC positive column discharges as a function of O$_{2}$ pressure (0.5-7 Torr) and current (10-40 mA). We also developed time-resolved CRDS in (partial and full) modulated discharges, allowing the loss kinetics to be investigated and showing dramatic differences in the loss mechanisms between the discharge and the afterglow. We were also able to observe the weak photodetachment continuum absorption of O$^{-}$ negative ions, The negative ion density is minimal when the O atom mole fraction is highest, confirming that associative detachment is the dominant loss mechanism.