Parallel Monitoring of the Dynamics of Oxygen and Water Molecule Concentrations under the Action of a Glow Discharge Using Diode Laser Spectroscopy

Abstract

A two-wavelength diode laser spectrometer has been developed and simultaneous changes in the concentrations of water and oxygen molecules in their mixtures with helium were observed. The concentrations behavior has been investigated at different stages: at the gas injection cycle into an evacuated tube, during burning of a DC glow discharge and at the phase after its shutdown. The measurements were carried out by the absorption of light on weak lines of vibrational-rotational intermode transitions of H₂O and spin-forbidden electron transitions of O₂ at reduced pressures of several millibars with concentration sensitivities of 10¹⁴ and 10¹⁵ cm^–3, respectively. Under such conditions, in addition to homogeneous interactions, heterogeneous interactions of particles with each other and with active surface centers are significant in the formation of the chemical composition of gases. It has been established that for different molecules that are stable under normal conditions, the concentrations at the discharge stage and its afterglow may not correspond to each other. This depends on the type of molecule, the composition of the plasma-forming mixture containing these molecules, the conditions of preparation and the operating mode of the reactor; imbalances of elements in the bulk phase are noted. The experimental results were interpreted using a plasma-chemical kinetic model of a direct current discharge in an O₂–He–H₂O gas mixture taking into account the interactions of electrons, ions and neutral particles in the ground and excited states. In addition to solving the kinetic equations for chemical reactions and the Boltzmann equation for electron energy distribution function, the model provides the profiles of gas temperatures and axial electric field strengths, and takes into account the effect of heating of the neutral gas in the non-equilibrium discharge plasma on the measurement results. To explain the imbalance of elements, in addition to the known heterogeneous processes of particle recombination, it is postulated that previously undiscussed chemical reactions of the decay products of the initial molecules in the plasma occur with particles adsorbed on the walls at the stage of filling the reactor with the plasma-forming gas. The model satisfactorily describes the results of measurements of particle concentrations in the discharge and afterglow stages, and the main macroscopic parameters of the discharge.