Production of singlet oxygen in a non-self-sustained discharge

Abstract

The production of O2(a1Δg) singlet oxygen in non-self-sustained discharges in pure oxygen and mixtures of oxygen with noble gases (Ar or He) was studied experimentally. It is shown that the energy efficiency of O2(a1Δg production can be optimized with respect to the reduced electric field E/N. It is shown that the optimal E/N values correspond to electron temperatures of 1.2–1.4 eV. At these E/N values, a decrease in the oxygen percentage in the mixture leads to an increase in the excitation rate of singlet oxygen because of the increase in the specific energy deposition per O2 molecule. The onset of discharge instabilities not only greatly reduces the energy efficiency of singlet oxygen production but also makes it impossible to achieve high energy deposition in a non-self-sustained discharge. A model of a non-self-sustained discharge in pure oxygen is developed. It is shown that good agreement between the experimental and computed results for a discharge in oxygen over a wide range of reduced electric fields can be achieved only by taking into account the ion component of the discharge current. The cross section for the electron-impact excitation of O2(a1Δg and the kinetic scheme of the discharge processes with the participation of singlet oxygen are verified by comparing the experimental and computed data on the energy efficiency of the production of O2(a1Δg and the dynamics of its concentration. It is shown that, in the dynamics of O2(a1Δg molecules in the discharge afterglow, an important role is played by their deexcitation in a three-body reaction with the participation of O(3P) atoms. At high energy depositions in a non-self-sustained discharge, this reaction can reduce the maximal attainable concentration of singlet oxygen. The effect of a hydrogen additive to an Ar: O2 mixture is analyzed based on the results obtained using the model developed. It is shown that, for actual electron beam current densities, a significant energy deposition in a non-self-sustained discharge in the mixtures under study can be achieved due to the high rate of electron detachment from negative ions. In this case, however, significant heating of the mixture can lead to a rapid quenching of O2(a1Δg molecules by atomic hydrogen.