Temperature measurement utilizing the vibrational CARS signal from the ground level X₂Σ_g⁺ of N₂⁺ in high-temperature flames

We analyze the generation of the vibrational coherent anti-Stokes Raman scattering (CARS) signal from the ground level X₂Σ_g⁺ of N₂⁺ and demonstrate the feasibility for gas-phase thermometry utilizing the vibrational CARS signals of N₂⁺ in high-temperature flames. The vibrational CARS signal of N₂⁺ is obtained using the filamentation-based and two-beam hybrid femtosecond/picosecond CARS system and is employed to extract the fitting temperatures in high-temperature flames. The accuracy and precision of the fitting temperatures from the 1000 consecutive single-shot vibrational CARS spectra of N₂⁺ at a target temperature of 1706 K are 1.23% and 5.76%, respectively, which are similar to the fitting results of N₂ in previous filamentation-based CARS thermometry and could also demonstrate that the measured temperature in the filamentation is indeed not affected by the ionization. We further introduce the CARS signal of N₂⁺ in multiple species measurement to obtain more details about the CARS process during filamentation. This work would provide an optional target molecule for CARS thermometry and help researchers further understand the molecular dynamics of N₂⁺ ions during the filamentation.