Spatially-resolved time-evolution of rotation-vibration non-equilibrium and CH$_{\mathrm{4}}$ concentration measured by fs/ps CARS in a nanosecond-pulsed pin-to-pin discharge*

To develop quantitative understanding of non-equilibrium plasmas for methane reforming, temporally and spatially resolved measurements of reactant concentration and rotation-vibration non-equilibrium are necessary. In this study, a recently developed rotational fs/ps CARS method was used to simultaneously measure rotational and vibrational temperatures of a pin-to-pin CH$_{\mathrm{4}}$/N$_{\mathrm{2}}$ nanosecond-pulsed discharge at 60 Torr. The CH$_{\mathrm{4}}$ concentration was measured separately using vibrational CARS in the same experimental setup. These measurements were conducted across a 2 mm length along the electrode axis within 150 $\mu $m of the cathode and from delays of 50 ns from the voltage pulse up to 800 $\mu $s. Significant gradients in N$_{\mathrm{2}}$ rotational and vibrational temperature and CH$_{\mathrm{4}}$ number density were observed across the measurement length. Peak vibrational temperature exceeding 6000K was observed, 0.8 mm from the cathode and 100 $\mu $s after the voltage pulse. Majority of the CH$_{\mathrm{4}}$ consumption occurred during the voltage pulse, but additional decrease was observed within the first 5 $\mu $s of the afterglow.