TALIF measurements of atomic nitrogen in the afterglow of a nanosecond capillary discharge
Tat Loon Chng, N D Lepikhin, Inna Orel, Н. А. Попов, Svetlana Starikovskaia
Abstract
Abstract 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 (N 2 ) capillary discharge at a pressure of 27 mbar and initial temperature of about 300 K, produces a peak N-atom density of 1.29 × 10 17 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 μ s, 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 (≈10 15 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">N</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:math> ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">A</mml:mi> </mml:mrow> <mml:mn>3</mml:mn> </mml:msup> <mml:msubsup> <mml:mrow> <mml:mi mathvariant="normal">Σ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">u</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> </mml:mrow> </mml:msubsup> <mml:mo>,</mml:mo> </mml:math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">B</mml:mi> </mml:mrow> <mml:mn>3</mml:mn> </mml:msup> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">Π</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">g</mml:mi> </mml:mrow> </mml:msub> <mml:mo>,</mml:mo> </mml:math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">C</mml:mi> </mml:mrow> <mml:mn>3</mml:mn> </mml:msup> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">Π</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">u</mml:mi> </mml:mrow> </mml:msub> </mml:math> ) 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).