Hybrid fs/ps CARS for Measuring Thermal Non-Equilibrium in a Glow Discharge

This work presents experimental measurements of rotational and vibrational temperature using hybrid fs/ps Coherent Anti-Stokes Raman Scattering (CARS) for the analysis of thermal non-equilibrium. A low-pressure electric glow discharge served as a stable testbed to probe a range of thermal conditions, providing a repeatable plasma where rotational and vibrational temperatures can decouple, making it well suited for evaluating non-equilibrium CARS performance. Experimental CARS spectra were collected from two gas mixtures: 100% N₂ and air where the pressure ranged from 2.5 to 15 Torr. A custom time-resolved, non-equilibrium CARS simulation was used to generate synthetic rotational and vibrational CARS spectra with varying non-resonant background and probe delays. The experimental data were analyzed by using a fitting algorithm to determine both the vibrational and rotational temperature for each condition. A novel approach for quantifying CARS uncertainty in single-shot measurements was discussed using a model that includes experimental noise and fitting error. Significant thermal non-equilibrium was observed using hybrid CARS across all tested pressures and gas mixtures, with vibrational temperatures notably higher than rotational temperatures in the low-pressure glow discharge.