Stark polarization spectroscopy of neon spectral lines for estimating cathode sheath parameters in Grimm-type glow discharge sources

Abstract

We present the results of the optical emission spectroscopy (OES) study of the Ne I 503.775 nm and Ne I 508.038 nm atomic spectral lines observed in two DC glow discharge sources (GDS), standard Grimm analytical source and the modified plane cathode Grimm design. The standard Grimm GDS enables OES recording end-on only, i.e. when the radiation is collected from the entire discharge, in the direction perpendicular to the cathode surface. Our modification of Grimm GDS, on the other hand, allows for spectra recording of the complex line shapes from two optical directions, both end-on and side-on, the latter collecting radiation from a narrow slice of the discharge parallel to the cathode surface. Side-on observations enable spatially resolved Stark polarization spectroscopy at different positions along the whole cathode sheath (CS) region of the discharge. This allows the study of the Stark shifts of differently polarized (σ, π, or unpolarized) line components' dependence on the macroscopic CS electric field strength. Stark components of neon lines, encountered in side-on observations in the CS, are preserved in the end-on recordings as characteristic features – wavy wings with intensity maxima corresponding to the distinctly resolved Stark components in the CS. Following OES measurements in a broad range of discharge conditions (pressure, voltage, current), we report on a sustained correlation between the value of the maximum electric field strength Emax in the CS and the end-on recorded wing feature shifts Δλe. This correlation can be extrapolated to estimate the cathode sheath parameters Emax and the CS thickness dc in the standard Grimm-type GDS, with end-on optical access available only.