Optical Emission Reveals Absolute Density of Energetic Metastable Atoms Published June 15, 2009 By Thomas Brown Propulsion Directorate WRIGHT-PATTERSON AIR FORCE BASE, Ohio -- AFRL propulsion researchers optimized a spectral analysis tool that leverages simple optical emission measurements to yield information on absolute argon metastable concentrations in an ionized gas. Metastable atoms are long-lived energetic species of great interest to scientists due to their influence on the energy distribution and energy transfer properties within an excited gas. Typically with optical emission spectroscopy, spectral line wavelengths can reveal the presence of a metastable species, while emission intensity can measure the metastable concentration relatively, but not absolutely. Absolute concentration measurements often involve a more complex absorption experiment, but it is also theoretically possible to determine the absolute species concentration by fitting optical emission spectra using electron-impact cross-section data, if available. In recent years, scientists have compiled electron excitation cross sections for excitation from these argon metastable states. AFRL researchers combined this cross-section data with electric probe measurements within an argon-pulsed direct current discharge in order to accurately simulate experimental optical emission spectra, with the only fitting parameter being the absolute argon metastable concentration. The initial results of this spectral analysis tool are very promising, with the accuracy of metastable concentration measurements currently being verified with atomic absorption measurements.