Methane decompostion in short glow-discharge

Glow discharges in Ar–CH₄ mixtures at low pressure offer a unique opportunity to investigate spatially resolved non-equilibrium chemistry in short plasmas without a positive column. In this talk, we present recent measurements from a large-area DC glow discharge (0.4 Torr, 1000 V), where only the cathode glow (CG) and cathode dark space (CDS) are present. Using planar laser-induced fluorescence (LIF) and two-photon TALIF, we obtain absolute 2D density maps of H, CH, and C₂, revealing strong spatial differentiation in plasma chemistry.

Atomic hydrogen reaches peak densities of ~10²¹ m⁻³ near the cathode, pointing to a high degree of methane dissociation—tens of percent under steady-state conditions. The CH and C₂ fragments, in contrast, are confined to the CDS, with C₂ concentrations consistently exceeding that of CH by two orders of magnitude. This spatial partitioning arises from the interplay of electron energy, residence time, and species-specific wall losses across the two regions.

To interpret these findings, we present a simplified global (0D) kinetic model that reproduces the measured species densities and offers insights into the dominant production and loss mechanisms across the discharge. This comparison helps bridge the gap between spatially resolved diagnostics and integrated chemical behavior.

These results illustrate how distinct plasma sub-regions within the cathode structure drive different chemical outcomes, underscoring the importance of coupling diagnostics and modeling for understanding and controlling methane-containing discharges.