Recombination and detachment in oxygen discharges: the role of metastable oxygen molecules

A global (volume averaged) model of oxygen discharges is used to study the transition from a recombination dominated discharge to a detachment dominated discharge. The model includes the metastable oxygen molecules O-2 (a (1)Delta(g)) and O-2 (b (1)Sigma(g)(+)) and the three Herzberg states O-2(A (3)Sigma(u)(+), A' (3)Delta(u), c (1)Sigma(u)(-)). Dissociative attachment of the oxygen molecule in the ground state O-2 ((3)Sigma(g)(-)) and the metastable oxygen molecule O-2 (a (1)Delta(g)) are the dominating channels for creation of the negative oxygen ion O-. At high pressures, dissociative attachment of the Herzberg states contributes significantly to the creation of the negative oxygen ion, O-. The detachment by a collision of the metastable oxygen molecule O-2(b (1)Sigma(g)(+)) with the oxygen ion, O-, is a significant loss process for the O- at pressures above 10 mTorr. Its contribution to the loss is more significant at a lower applied power, but at the higher pressures it is always significant. Detachment by collision with O(P-3) is also an important loss mechanism for O-. We find that ion-ion recombination is the dominating loss process for negative ions in oxygen discharges at low pressures and calculate the critical pressure where the contributions of recombination reactions and detachment reactions are equal. This critical pressure depends on the applied power, increases with applied power and is in the range 5-14 mTorr in the pressure and power range investigated.