Rotation-vibration non-equilibrium measurement using pure rotational fs/ps CARS coherence beating

Rotation-vibration non-equilibrium plays a key role in the chemistry and thermalization in low-temperature plasmas as well as thermal loading of hypersonic vehicles. Here, we report the development of a simple and sensitive two beam hybrid femtosecond/picosecond pure rotational coherent anti-Stokes Raman scattering (fs/ps CARS) approach to simultaneously measure the rotational and vibrational temperature of N2. This approach takes advantage of probe delay specific interferences between ground state and vibrationally excited N2 molecules to intentionally induce so-called “coherence beating,” that leads to apparent non-Boltzmann distributions in the pure rotational fs/ps CARS spectra. These distortions in the spectra enable simultaneous inference of both the rotational and vibrational temperatures. The coherence beating effects were observed in single shot fs/ps CARS measurements of a 75 Torr N2 DC glow discharge and were successfully modeled for rotational and vibrational temperature extraction. Rotational temperatures around 400 K and vibrational temperatures from 1000 K to 4500 K were measured. It is shown that this method can be more sensitive than a pure rotational fs/ps CARS approach using a spectrally narrow probe pulse. This method is general to most diatomic molecules and opens the possibility of high speed 1-D and 2-D simultaneous rotation-vibration non-equilibrium measurements in plasmas and hypersonic flows.