Oxygen atom actinometry reinvestigated: Comparison with absolute measurements by resonance absorption at 130 nm

Resonance absorption at 130 nm was used to measure absolute oxygen atom concentrations in O₂-containing distributed electron cyclotron resonance plasmas. The dissociation fraction [O]/[O₂] in pure O ₂ plasmas (1-6 mTorr) was in the range 0.01-0.06, but was significantly increased by the addition of SF₆, N₂ or Kr. At 2 mTorr total pressure a maximum [O]/[O₂] of 0.3 was observed for 10% SF₆ added. The results were compared to those obtained by optical emission actinometry measurements. The quantity I₀ (844 nm)/I _Ar (750 nm) (with 0.1 mTorr Ar added) was poorly correlated with [O] but well correlated with [O₂]. This suggests that, for dissociation fractions lower than 0.1, dissociative excitation, O₂+e→O* (3p ³P)+O, is the most important mechanism for the production of 844 nm emission.