Experimental cesium line shapes

Abstract

The widths of some of the spectral lines emitted from a dense plasma are sensitive to the ion density but insensitive to the electron temperature. In partially-ionized cesium plasmas with number densities greater than 10 13 ions-cm −3 these plasma-broadened lines are easily measured and can be used as a simple diagnostic method for the determination of ion density, once the width versus ion density relationship is established either theoretically or experimentally. Here we present measured line widths and compare results with theoretical widths. The profiles of various lines of the sharp, diffuse, and fundamental series lines of cesium were measured at known ion densities with a scanning monochromator. The plasma was generated in a hot-cathode parallel-geometry cesium diode. For each case the ion density and the electron temperature were determined by measuring the intensity of the radiative recombination continuum. Line intensities gave independent measurements of temperature that were in agreement with the electron temperatures, showing the existence of local thermodynamic equilibrium and providing a confirmation of ion density through the application of the Saha equation. Detailed results of line width measurements are presented for a range of plasma density from 3·2 × 10 13 to 1·6 × 10 15 ions-cm −3 and for electron temperatures between 2000° and 6000°K. The results, valid in the vicinity of N ϵ = 10 15 ions-cm −3, are summarized in the following table: Series Transition Electron temp. Line width (°K) ω( A ̊ ) Sharp 6P-nS, 9 ≤ n ≤ 11 6000 w = 2 ( n-9) × 0·30 N ϵ × 10 −15 Diffuse 6P-nD, 7 ≤ n ≤ 10 6000 ω = 2 ( n-7) × 0·35 N ϵ × 10 −15 Fundamental 5D-nF, 5 ≤ n ≤ 8 2000 ≤ T ϵ ≤ 10, 000 ω = 2 ( n-5) × 1·5 N ϵ × 10 −15 In the table w is the full width at half maximum and has an accuracy of 20 per cent. The widths of the fundamental series lines are essentially independent of electron temperature, but temperature independence can not be claimed for the sharp and diffuse series lines.