Nonlocal nature of the electron energy spectrum in a glow-discharge in pure O2: II. Actinometry of O(3 P) atoms in a plasma at low gas pressures

Abstract

Results are presented from measurements of the density of oxygen atoms in the positive column of a dc discharge in pure oxygen by the actinometric technique using Ar atoms. Based on the excitation rate constants calculated using two different approaches (namely, the two-term approximation and the Monte Carlo method) to solving the Boltzmann equation for a spatially inhomogeneous electron distribution function, the applicability of the actinometric technique is analyzed. The effects of the discharge kinetics and the nonlocal character of the electron energy spectrum on the accuracy of actinometric measurements are studied. It is shown that the results of measurements depend only slightly on the accuracy with which the electron energy distribution function is described. Over a wide range of the reduced electric field E/N≈40–250 Td, the oxygen atom density calculated using the spatially homogeneous distribution function differs by several percent from that calculated accurately, taking into account the nonlocal character of the electron energy spectrum. It is shown that using the actinometric technique to measure the absolute concentration of oxygen atoms in a plasma requires a detailed description of the discharge plasmochemical kinetics, including a thorough analysis of all possible processes (particularly, surface heterogeneous reactions) that determine the density of active particles at low pressures. At the same time, the use of the actinometric technique for monitoring the behavior of the density of oxygen atoms in a plasma is justified over a wide range of reduced electric fields up to ∼200 Td when the O(3p3P-3p3S) transition (λ=844.6 nm) is used and the degree of dissociation is [O]/[O2]>0.02.