Method for the Determination of Atomic-Resonance Line-Oscillator Strengths from Widths of Optically Thick Emission Lines inT-Tube Plasmas

Using the neutral-helium resonance line as an example, a method is described for the measurement of the product of oscillator strength, Stark width, and ground-state density. Directly measured is the width of the line (as emitted by an essentially homogeneous but optically thick layer), which is proportional to the square root of the above product. A T tube is filled with a known helium-hydrogen mixture, and the temperature is calculated from the measured intensity ratios of helium and hydrogen lines. Measured widths of (optically thin) visible lines yield electron densities, which then give the theoretical Stark width of the ultraviolet line. These densities are also used in Saha equations to calculate the ground-state density from the measured temperature and the mixing ratio. This leaves the oscillator strength as the only unknown, whose accuracy is limited in the helium case mostly by the error in the ground-state density to about 25%, when the plasma is sufficiently dense and long-lived for local thermal equilibrium to hold.