Measurements of the vibrationally resolved dynamics of N₂(A) on nanosecond time scales in a pulsed sub-atmospheric pressure discharge

The number densities of various vibrational states of the metastable N₂(A) electronic state are measured in a sub-microsecond-pulsed DC discharge.

The discharge is created by applying high-voltage pulses of approximately 200 ns duration to a 1 mm discharge gap with plane, parallel molybdenum electrodes. Typical discharge parameters are a 1 kV discharge voltage with several Ampere-level currents through a 1 mm x 20 mm discharge cross-section at pressures around 200 mbar. Resulting energy dissipated per discharge pulse is in the order of 1 mJ, or 50 mJ/cm³. Tunable diode laser absorption spectroscopy measurements with a time resolution in the order of 10 ns are achieved by employing a photodiode detector with a bandwidth of 150 MHz. Such time resolution allows observation of the dynamics of the excited states during the discharge pulse. The pressure and number of pulses a given gas volume experiences are varied. Furthermore, the total energy coupled into the plasma is changed by adjusting either the instantaneous discharge power or pulse duration. The peak densities of the individual vibrational states are in the order of 10²⁰ m^-3. The time evolution of the densities strongly depends on the vibrational quantum number, with significant differences in both the excitation rate and the decay times between low and higher vibrational states. The measurement results are discussed in detail by comparison with calculations based on reaction processes found in the literature.