TALIF measurements of atomic nitrogen in the afterglow of a nanosecond capillary discharge

The atomic nitrogen (N) density in a nanosecond pulse capillary discharge is measured using two-photon laser induced fluorescence. The capillary discharge is favored for its unique combination of both large reduced fields (E/N) and high specific deposited energies. Under such conditions, we find that a pure nitrogen (N₂) capillary discharge at a pressure of 27 mbar and initial temperature of about 300 K, produces a peak N-atom density of 1.29 × 10¹⁷ cm^-3, corresponding to an extremely high dissociation degree of about 10%. The time evolution of the N-atom density is tracked from a few hundred ns after discharge initiation, up to several ms when the concentration of N-atoms falls below the detection limit. The temporal evolution curve exhibits a trapezoidal-like shape, characterized by an initial rise in the N-atom density up to a few μs, followed by a relatively flat and constant profile until about 1 ms, and finally terminating with a drop to near detection limits at about 10 ms. The high electron densities (≈10¹⁵ cm^-3) and efficient production of electronically excited states associated with this type of discharge is found to have a profound effect on the consequent kinetics. A process of stepwise dissociation through electron impact of the N₂(A³σ⁺ _μ,B³II_g) excited states is examined and proposed as a possible explanation for the unusually high energy efficiency of N-atom production. The present study shows that the capillary discharge is an extremely effective source of N-atoms, and forms the impetus for continued study of discharges with both high levels of specific deposited energies (≥1 eV/molecule) and reduced electric fields (E/N ≥ 150 Td).