Energy redistribution under argon dilution in a low-pressure methane RF plasma

In this work, the influences of argon dilution on energy redistribution in a low-pressure RF-ICP methane plasma discharge are studied. A combination of Optical Emission Spectroscopy, Residual Gas Analysis, and x-ray Photoelectron Spectroscopy is employed to correlate plasma energetics with gas-phase chemistry and film composition. Argon addition is shown to increase electron density while lowering electron, excitation, and vibrational temperatures, thereby redistributing the absorbed power and reducing the high-energy tail of the electron energy distribution that drives bond scission. As a result, methane conversion and hydrogen yield decline, which is consistent with a reduction in vibrationally primed targets rather than electron scarcity. Importantly, the methane conversion rate varies nonlinearly with argon concentration: small fractions (∼15\% Ar) can induce disproportionately higher conversion rates compared to simple dilution expectations. Thus, the active role of argon in shaping plasma reactivity through metastable-driven pathways is revealed. On the other hand, at the surface of sample holder inside the RF-ICP reactor, the modest argon additions improve film chemistry by lowering oxygen incorporation and reducing oxygenated functionalities, which is attributable to gentle Ar+/Ar* sputter-cleaning during growth. Taken together, these results define a practical operating window at low-moderate argon fractions (≈25\%–30\%), sufficient to stabilize the discharge and enhance film purity without excessively suppressing vibrational excitation. In addition, this study highlights the broader technological implications of low-pressure Ar/CH4 plasmas discharges, offering practical guidelines for optimizing hydrogen production and advanced carbon materials in industrial plasma processes.