Vibrational excitation measurements by CARS in a nanosecond discharge

A broadband, dual-pump CARS (coherent anti-Stokes Raman scattering) setup is realized for simultaneous measurements of nitrogen and carbon dioxide vibrational distribution functions in the electronic ground state. Measurements are performed in a 1 mm discharge gap between two plane-parallel molybdenum electrodes at sub-atmospheric pressure (0.1 to 1 bar). In a pure nitrogen discharge it is found that for vibrational states with v < 8 a two-temperature distribution function is a very good approximation to the vibrational distribution. The excitation conditions for vibrational states are constant during most of the discharge pulse and agree very well with the excitation rates from the literature for the given electric field value, which was measured by E-FISH (electric field induced second harmonic generation). The development of the vibrational states in the afterglow is compared to a state-to-state kinetic model, which is dominated by vibrational-vibrational transfer and transport losses. Here too, good agreement was found for rates available in the literature. It is planned to extend the measurements to nitrogen/carbon dioxide gas mixtures, to investigate the vibrational interaction between both species.