Absolute O atom density measurements by actinometry: comparison to cavity ring-down spectroscopy

Optical emission actinometry is widely used to determine the densities of reactive gas-phase atomic species like oxygen atoms, since it is a simple, non-intrusive technique requiring little equipment. The accuracy of absolute measurements however depends on the excitation cross-sections of both the studied species and the actinometer gas, which are typically poorly characterised.

We have evaluated the validity of O actinometry by comparison to cavity ring-down (CRDS) measurements. We measured both in continuous glow discharges in various mixtures of oxygen, CO2, Ar and Kr at pressures between 1 and 5 Torr.

The emission is monitored with an Isoplane SCT320 spectrometer and with an OceanOptics Maya USB spectrometer. The O atom density is determined by CRDS using the O(1D2)↔O(3PJ) transition at 15,867.8cm-1.

The electric field in the tube is measured directly, allowing computation of the EEDF with the LoKI Boltzmann solver. The O atom density was then calculated from the excitation cross-sections and the line intensities.

Large discrepancies are found between CRDS and actinometry using the IST-Lisbon oxygen dataset for the EEDF and common cross-sections for the lines of interest. We also propose a method to estimate the reduced electric field from the ratio of atomic emission lines.