Analysis of Hollow-Cathode-Discharge-Excited Ar i, Ar ii, and Au i Spectral-Line Profiles Measured with a Fabry–Perot Interferometer*

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Abstract
Spectral profiles of Au i, Ar i, and Ar ii lines obtained with a scanning Fabry–Perot spectrometer from a low-current, 3.2-mm-diam Au hollow-cathode discharge in ~6 torr of Ar are fitted with Voigt functions to determine the apparent lorentzian half-widths, ΔLs, and apparent Doppler temperatures, Ts, as functions of the discharge current. Ts values of the Au i 4792-Å line are approximately equal to those of the Ar i lines, whereas for the Au i 3123-Å line, the Ts values are much greater. Arguments are presented to show that these high Ts values result from the high initial ejection energy of the sputtered atoms and the short lifetime of the upper state. Typically, Ts = 1010, 1180, and 1760 K for the Ar i 4511, Au i 4792, and Au i 3123-Å lines, respectively. The Ts values of the Ar ii doublet-state lines equal those of the Ar i lines, whereas for the Ar ii quartet-state lines, the Ts values are significantly lower. The ΔLs values of the Ar i lines, whose transitions terminate on resonance states, are predicted by resonance-broadening theory. The ΔLs values of the Ar ii lines range from 0.010 to 0.031 cm−1 and far exceed the calculated resonance- and Stark-broadening terms. A proposed model of the Ar ii spectral-line profile consists of a superposition of two pure gaussian distributions, which represent one group of high-energy ions accelerated by axial electric fields in an electrical double layer (DL) at the end of the cathode, and a second group in equilibrium with Ar atoms. The sum closely approximates a Voigt function whose analysis yields apparent values, ΔLc and Tc, which equal the corresponding observed quantities. The ΔLs values are found to vary with discharge conditions, presumably depending upon the completion of the DL formation.