Laser-Induced Fluorescence from N 2 + Ions Generated by a Corona Discharge in Ambient Air

In this work, we present the measurement of laser-induced fluorescence from N₂⁺ ions via the B²Σ⁺ _u − X²Σ⁺ _g band system in the near-ultraviolet. The ions were generated continuously by a plasma glow discharge in low pressure N₂ and by a corona discharge in ambient air. The fluorescence decay time was found to rapidly decrease with increasing pressure leading to an extrapolated decay rate of ≈10 ¹⁰ s⁻¹ at atmospheric pressure. In spite of this quenching, we were able to observe laser induced fluorescence in ambient air by means of a time-gated spectral measurement. In the process of comparing the emission signal with that of N₂ spontaneous Raman scattering, ion concentrations in ambient air of order 10 ⁸ -10 ¹⁰ cm⁻³ were determined. With moderate increases in laser power and collection efficiency, ion concentrations of less than 10 ⁶ cm⁻³ may be measurable, potentially enabling applications in atmospheric standoff detection of ionizing radiation from hazardous radioactive sources.