The anomalous sodium doublet D2/D1 spectral line intensity ratio – a manifestation of CCD's presaturation effect

Abstract

In this paper we present the results of intensity measurements of D₁ (589.5224 nm) and D₂ (588.9950 nm) sodium doublet spectral lines emitted from a short-lifetime plasma randomly appearing across the aluminum anode surface during its electrolytic oxidation from the water solution of boric acid with sodium tetraborate. We found that the D₂ to D₁ intensity ratios were not constant, varying from 2 to 1.2. Assuming that the plasma is in local thermal equilibrium it is expected that the intensity ratio of sodium doublet components equals the ratio of the statistical weights of the atomic sublevels corresponding to the electronic transitions (D₁: 3²P_1/2–3²S_1/2, D₂: 3²P_3/2–3²S_1/2) i.e. 2. After detailed analyses of a series of spectra obtained by applying different detection parameters, like exposure time, slit width, detection mode, and number of accumulations, we attributed the anomalous D₂/D₁ intensity ratio to the effect of approaching a saturation condition of the charge coupled device detector and thus to an early indication of saturation occurrence.