Time-resolved kinetics of vibrationally excited methane in nanosecond pulsed discharge plasmas

Non-equilibrium plasmas have been shown to be effective for low temperature reforming of methane (CH₄), leading to improved hydrogen production efficiency and selectivity. Recent studies highlight the critical role of vibration-to-vibration energy transfer involving vibrationally excited methane (CH₄(ν)) in enhancing the efficiency of plasma-driven CH₄ reforming. However, the mechanisms through which CH₄(ν) exchanges energy with other species and the timescales over which this transfer occurs remain poorly understood. To understand the kinetic role of CH₄(ν), we performed time-resolved quantitative measurements of the rotational and vibrational temperatures of the ν₄ mode of CH₄(ν) with 500 nanoseconds (ns) time resolution and examined the rates of key excitation and relaxation reactions. These measurements were conducted in a ns-pulsed plane-to-plane dielectric barrier discharge operated in burst mode, using step-scan infrared laser absorption spectroscopy (IR-LAS) to capture the transient dynamics of pure CH₄ vibrational excitation and relaxation. A steady-state rotational and vibrational temperature difference of approximately 20 K is established in the ν₄ mode within a 3.5 milliseconds. Kinetic modelling is conducted to identify the dominant CH₄(ν)-involved reaction pathways and confirm the reaction rates of e+CH₄→e+CH₄(ν₄) and CH₄(ν₄)+CH₄→CH₄+CH₄. The measured vibrational temperature is used to determine CH₄(ν) concentration and determine the vibration kinetic rates.