Stark-width measurements of singly ionized calcium resonance lines in a wall-stabilized arc

Stark widths of the Ca II 3933.7- and 3968.5-Å resonance lines are measured around a temperature of 13 000 K and an electron density of ${10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$ in a stationary wall-stabilized arc burning in argon with a small addition of Ca vapor. The electron density is measured using the Stark width of the hydrogen H $\beta$ line. Optical-depth measurements are performed at the peaks of the investigated lines and the calcium concentration is carefully controlled to minimize the reabsorption. A Fabry-Pérot interferometer, whose instrumental profile is measured, yields the required high spectral resolution. The experimental profiles are analyzed using a least-squares fit of the Voigt function to all profile intensity points. The width of the Ca II resonance doublet is found about a factor of 2 lower than both semiclassical and full quantum calculations, in agreement with the earlier experiments of Yamamoto and of Roberts and Eckerle.