On the use of end-on observation in optical emission spectroscopy measurements of the maximum electric field strength in the cathode sheath of Grimm-type glow discharges in helium

To confirm the fundamental principles that enable using conventional end-on view optical emission spectroscopy (OES) to estimate maximum electric field strength in the cathode sheath of a standard Grimm-type DC glow discharge source (GDS), we have extended our previous investigation (Nedić et al., J. Anal. At. Spectrom., 2022, 37, 1318) in neon and argon to helium. We compared the spectral line profiles recorded from the standard Grimm GDS, with the only available optical path through the anode cylinder from the cathode to the end of the discharge (end-on view) with those recorded from the modified Grimm-type source, which enables not only the end-on view, but also the observations along the optical path parallel to the cathode surface, from the side of the discharge (side-on view). While the end-on OES collects the integral light from the entire discharge, the side-on view enables spatially resolved OES and “sliced” studies of different discharge regions, from the cathode surface towards the negative glow plasma. The forbidden components of the neutral helium spectral lines He I 447.148 nm and He I 492.193 nm were detected in the integral end-on recordings, testifying to the possibility to “look behind” the bright negative glow into the Stark effect influence of the macroscopic electric field in the cathode sheath. The comparison of the end-on recorded forbidden (F) line shift Δλ_(0–F)e from its central wavelength λ₀ with the wavelength separation of the allowed (A) and forbidden component Δλ_(A–F)s in direct side-on observations of the cathode sheath at the position of the maximum electric field F_max yields a stable linear correlation in a broad range of typical discharge conditions (pressure, voltage, current) and cathode materials. This correlation confirms that the effects of the macroscopic electric field can be registered in end-on recordings, which enables the use of conventional OES from the end of the Grimm-type GDS to extract useful information on the maximum electric field strength and thickness of the cathode sheath.