Determination of concentrations via the diminution of Doppler shifts by radiation trapping Original Research Article

Under typical discharge conditions, atoms or molecules excited by electrons have a unique velocity component along the plasma electric field direction. This component results in Doppler shifts that can be observed and measured by a high resolution Fourier transform spectrometer. We show theoretically that if the upper state of a given transition suffers radiation trapping, its Doppler shift will be reduced by a factor dependent upon the average number of trapping cycles a photon of that frequency experiences. We further show how the diminution of the Doppler shift can be related to the volume and the density of the trapping species. These results can be used to measure the unknown concentration of a trapping species provided a “standard” is available. A typical application of this technique would be the measurement of the concentration of a metastable discharge species, e.g., He, using the trapping of its known ground state density as a standard.