Vibrational excitation of methane is believed to promote chemistry and improve product selectivity, compared to thermal conversion methods. We report on unique direct measurements of vibrational–rotational non-equilibrium in methane plasma. The non-equilibrium is sustained for 50 μs, after which the gas temperature equilibrates with the vibrational temperature at around 900 K. The plasma is generated by applying 200 μs, 30 Hz pulses of microwave radiation to methane at 25 mBar. We demonstrate that in microwave discharges, power transfer to vibrational modes of CH4 is the dominant power transfer mechanism, which leads to creation of a vibrational–translational (VT) non-equilibrium. VT relaxation is determined to be the dominant translational heating mechanism in the discharge. However, the high electron temperature at breakdown also leads to strong electronic excitation which may be responsible for some of the heating. Furthermore, we find that the CH4 vibrational levels are in equilibrium with each other due to fast intra-polyad relaxation (VV), and therefore bending vibrational modes population density is greatly in excess of stretching vibrational modes. The window of opportunity to exploit this non-equilibrium is limited by the VT relaxation timescale, which is approximately 50 μs in our experiment.

中文翻译：

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CH4 的等离子体诱导振动激发——其模式选择处理的窗口

与热转化方法相比，甲烷的振动激发被认为可促进化学反应并提高产品选择性。我们报告了对甲烷等离子体中振动-旋转非平衡的独特直接测量。非平衡持续 50 微秒，之后气体温度与振动温度在 900 K 左右平衡。通过在 25 毫巴下向甲烷施加 200 微秒、30 Hz 的微波辐射脉冲来产生等离子体。我们证明，在微波放电中，向 CH4 振动模式的功率传输是主要的功率传输机制，这导致了振动平移 (VT) 非平衡的产生。VT 弛豫被确定为放电中主要的平移加热机制。然而，击穿时的高电子温度也会导致强烈的电子激发，这可能是部分加热的原因。此外，我们发现 CH4 振动能级由于快速的多聚体内弛豫（VV）而彼此平衡，因此弯曲振动模式人口密度大大超过拉伸振动模式。利用这种非平衡的机会窗口受到 VT 弛豫时间尺度的限制，在我们的实验中约为 50 μs。