Dissociation of O₂ in low pressure glow discharges in He- O ₂, Ne- O₂, and Ar- O₂ gas mixtures

Low-pressure glow discharges in He, Ne, or Ar gases containing small amounts of O₂ can produce considerable dissociation of O₂ molecules. High-lying metastable states in these noble gases play a predominant role in this process. Due to the difference in energies of metastable states of the noble gas atoms, there is a significant variation in the product states of O atoms created as a result of dissociation. Apparently, for He and Ne, superexcited O₂ molecules are formed through collisions with high-lying metastable states of the noble gases. The superexcited O₂ molecule could either autoionize to form an O₂⁺ ion that subsequently captures an electron and dissociates or it could directly dissociate through some curve-crossing interactions yielding at least one atom in highly excited state. However, in the case of Ar, the formation of superexcited O₂ may be possible through a prior creation of Ar ²⁺ ion. We report some interesting observations on spectral emissions from O atoms in the three gas mixtures covering transition from 5d, 4d ^23,5,D and 5s ^3,5,S to 3p 3,5 P levels and 4p and 3p P ^3,5 to 3s S ^3,5 levels. The emission from the b 4 g- -a Π4 u system of O₂⁺ ions is also reported. In general, populations of the above-stated O and O₂⁺ levels are highest in the case of He- O₂ mixture and lowest for the Ar- O ₂ mixture. However, the Ne- O₂ mixture appears to strongly populate the 4p P⁵ levels, while the Ar- O₂ mixture strongly populates the 3p ³ levels. The He- O₂ mixture appears to be more efficient among the three gas mixtures considered in dissociating the O₂ molecules.