Nonequilibrium vibrational populations and dissociation rates of oxygen in electrical discharges

Abstract

Nonequilibrium vibrational distributions and dissociation rates of molecular oxygen in both electrical and thermal conditions have been calculated by solving a system of master equations including V-V (vibration-vibration), V-T (vibration-translation) and e-V (electron-vibration) energy exchanges. The dissociation constant under thermal conditions (i.e. without electrons) follows an Arrhenius law with an activation energy of 120 kcal/mole, while the corresponding rates under electrical conditions (5000 ⩽ T e ⩽ 15000 K, 300 ⩽ T g ⩽ 1000 K, 10 11 ⩽ n e ⩽ 10 12 cm −3,5 ⩽ p ⩽ 20 torr) increase with decreasing gas ( T g) and electron ( T e) temperatures and pressure ( p) and with increasing electron density ( n e). These results are explained on the basis of the different interplay of V-V and V-T energy exchanges and are rationalized by means of simplified models proposed in the literature. The accuracy of the present results is discussed paying particular attention to the dependence of V-V and V-T rate coefficients on the vibrational quantum number. A comparison of the calculated dissociation rates with the corresponding ones obtained by the direct electron impact mechanism shows that the present mechanism prevails at low electron and gas temperatures. Finally a comparison is shown between theoretical and experimental dissociation rates under electrical and thermal conditions.