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 ∼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, ∼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 ∼60% in air and by a factor of ∼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 ∼1 µs, 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 ∼ 1-10 µs after the discharge pulse, both in nitrogen and air, before gradually switching to relaxation at t ∼ 10-100 µs. 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.

A suite of laser diagnostics is used to study kinetics of vibrational energy transfer and plasma chemical reactions in a nanosecond pulse, diffuse filament electric discharge and afterglow in N 2 and dry air at 100 Torr. Laser-induced fluorescence of NO and two-photon absorption laser-induced fluorescence of O and N atoms are used to measure absolute, time-resolved number densities of these species after the discharge pulse, and picosecond coherent anti-Stokes Raman spectroscopy is used to measure time-resolved rotational temperature and ground electronic state N 2 (v = 0-4) vibrational level populations. The plasma filament diameter, determined from plasma emission and NO planar laser-induced fluorescence images, remains nearly constant after the discharge pulse, over a few hundred microseconds, and does not exhibit expansion on microsecond time scale. Peak temperature in the discharge and the afterglow is low, T ≈ 370 K, in spite of significant vibrational nonequilibrium, with peak N 2 vibrational temperature of T v ≈ 2000 K. Significant vibrational temperature rise in the afterglow is likely caused by the downward N 2 -N 2 vibration-vibration (V-V) energy transfer. Simple kinetic modeling of time-resolved N, O, and NO number densities in the afterglow, on the time scale longer compared to relaxation and quenching time of excited species generated in the plasma, is in good agreement with the data. In nitrogen, the N atom density after the discharge pulse is controlled by three-body recombination and radial diffusion. In air, N, NO and O concentrations are dominated by the reverse Zel'dovich reaction, N + NO → N 2 + O, and ozone formation reaction, O + O 2 + M → O 3 + M, respectively. The effect of vibrationally excited nitrogen molecules and excited N atoms on NO formation kinetics is estimated to be negligible. The results suggest that NO formation in the nanosecond pulse discharge is dominated by reactions of excited electronic states of nitrogen, occurring on microsecond time scale.

The present kinetic modelling calculation results provide key new insights into the kinetics of vibrational excitation of nitrogen and plasma chemical reactions in nanosecond pulse, 'diffuse filament' discharges in nitrogen and dry air at a moderate energy loading per molecule, ∼0.1 eV per molecule. It is shown that it is very important to take into account Coulomb collisions between electrons because they change the electron energy distribution function and, as a result, strongly affect populations of excited states and radical concentrations in the discharge. The results demonstrate that the apparent transient rise of N 2 'first level' vibrational temperature after the discharge pulse, as detected in the experiments, is due to the net downward V-V energy transfer in N 2-N 2 collisions, which increases the N 2 (X 1 , v = 1) population. Finally, a comparison of the model's predictions with the experimental data shows that NO formation in the afterglow occurs via reactive quenching of multiple excited electronic levels of nitrogen molecule, N * 2 , by O atoms.

Picosecond CARS spectroscopy and phased-locked schlieren imaging are used to measure time-resolved temperature and visualize compression waves in a diffuse filament, nanosecond pulsed discharge sustained between a pair of spherical electrodes in nitrogen and air at P=100 torr. The discharge generates stable plasma with high specific energy loading (up to ~0.5 eV/molecule), and with spatial dimensions sufficiently large to enable laser diagnostic studies. The results demonstrate that significant temperature rise, up to ∆T~200 K, occurs both in nitrogen and in air, on the time scale shorter than the acoustic time scale. The characteristic time for the rapid temperature rise in air, ~100 ns, is significantly shorter compared to that in nitrogen, ~1 µs. In air, a second significant temperature rise, up to ∆T~350 K, occurs on a time scale of ~100-500 µs. This "slow" temperature rise is almost entirely missing in nitrogen. Phase-locked schlieren images demonstrate a near cylindrical shape compression wave formed around the discharge filament, both in nitrogen and in air. An additional, near spherical shape compression wave is formed near the cathode, due to significant energy release in the cathode layer of the discharge. The compression waves, caused by rapid localized heating quantified by the present measurements, are similar to the ones produced by a surface nanosecond pulse discharge in atmospheric air used for high-speed flow control, where comparable temperature rise was detected previously.

Picosecond Coherent Anti-Stokes Raman Spectroscopy is used to study vibrational energy loading and relaxation kinetics in nitrogen and air nsec 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 ~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, ~33% 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 a factor of ~2 in air and by a factor of ~4 in nitrogen, respectively, a result in direct contrast to modeling 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 ~10 μsec, the experimental VDF shows populations of vibrational levels v≥2 greatly exceeding modeling predictions, which predicts their monotonic 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 an increase in the populations of levels v=2 and v=3, reaching a maximum at t~50-100 μsec after the discharge pulse, and relatively steady v=4-9 populations at t~10-100 μsec. It is concluded that a collisional process is 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.