Abstract：Atmospheric-pressure plasma has attracted increasing interest in energy conversion and combustion enhancement because of its ability to selectively excite molecular vibrational energy levels. To accurately investigate the excitation and relaxation behavior of nitrogen molecules, this work employs coherent anti-Stokes Raman scattering (CARS) spectroscopy with a BoxCARS phase-matching configuration to achieve in situ high-resolution measurements in both pulsed and direct-current (DC) glow discharges. In the pulsed discharge regime, nitrogen molecules exhibit strong vibrational-rotational non-equilibrium immediately after breakdown (<10 μs). The higher-order vibrational temperature (Tv1v) reaches about 5 500 K, far exceeding the first-order vibrational temperature (Tv01≈1 600 K) and rotational temperature (Tr≈600 K), revealing significant overpopulation of high vibrational states. As the discharge proceeds, rapid vibrational- vibrational (V-V) and vibrational-rotational (V-R) relaxation processes gradually drive the system toward thermal equilibrium. Around 60 μs after breakdown, the vibrational temperature becomes nearly equilibrated, while Tr continues to rise, peaking at approximately 3 600 K near 125 μs. These results indicate that pulsed discharges promote transient non-equilibrium excitation and efficient energy deposition into vibrational modes. Under DC glow discharge conditions, nitrogen molecules remain close to vibrational-rotational equilibrium, with temperature gradually decreasing from the discharge center outward. The central temperature increases with discharge current but decreases with gas flow rate, while a larger electrode gap extends the high-temperature region and broadens the spatial energy distribution. Comparative analysis shows that pulsed discharges are dominated by selective vibrational excitation and fast relaxation kinetics, whereas DC glow discharges exhibit steady collisional processes and uniform thermal behavior. Together, these findings reveal the distinct energy-transfer pathways and equilibrium characteristics of nitrogen molecules across different atmospheric plasma regimes, elucidating the partitioning of electrical energy among vibrational, rotational, and translational degrees of freedom. This study demonstrates the reliability and precision of CARS spectroscopy for diagnosing vibrational excitation dynamics in atmospheric-pressure plasma. The experimental results provide key benchmarks for validating kinetic modeling of plasma-molecule interactions and offer insights for optimizing plasma-assisted combustion and energy conversion.
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