Picosecond CARS measurements of nitrogen vibrational loading and rotational/translational temperature in non-equilibrium discharges

Picosecond coherent anti-Stokes Raman spectroscopy (CARS) is used to study vibrational energy loading and relaxation kinetics in nitrogen and air ns pulsed non-equilibrium plasmas, in both plane-to-plane and pin-to-pin geometries. In 10 kHz repetitively pulsed plane-to-plane plasmas, up to similar to 50% of coupled discharge power is found to load vibrations, in good agreement with a master equation kinetic model. In the pin-to-pin geometry, similar to 40% of total discharge energy in a single pulse in air at 100 Torr is found to couple directly to nitrogen vibrations by electron impact, also in good agreement with model predictions. Post-discharge, the total quanta in vibrational levels v = 0-9 is found to increase by similar to 60% in air and by a factor of similar to 3 in nitrogen, respectively, a result in direct contrast to modelling results which predict the total number of quanta to be essentially constant until ultimately decaying by V-T relaxation and mass diffusion. More detailed comparison between experiment and model show that the vibrational distribution function (VDF) predicted by the model during, and directly after, the discharge pulse is in good agreement with that determined experimentally. However, for time delays exceeding similar to 1 mu s, the experimental VDF shows populations of vibrational levels v >= 2 greatly exceeding modelling results, which predict their predominant decay due to net downward V-V transfer and corresponding increase in v = 1 population. This is at variance with the experimental results, which show a significant monotonic increase in the populations of levels v = 2-9 at t similar to 1-10 mu s after the discharge pulse, both in nitrogen and air, before gradually switching to relaxation at t similar to 10-100 mu s. It is concluded that a collisional process is likely feeding high vibrational levels at a rate which is comparable to the rate at which population of the high levels is lost due to net downward V-V energy transfer. A likely candidate for the source of additional vibrational quanta is quenching of metastable electronic states of nitrogen to highly excited vibrational levels of the ground electronic state. The effect of electronic-vibrational (E-V) coupling on time-resolved N-2 vibrational populations and temperature, estimated using a phenomenological E-V energy transfer model, provides qualitative interpretation of the present experimental results.