Laser-Induced Fluorescence from N2+ 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 ≈1010 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 108-1010 cm−3 were determined. With moderate increases in laser power and collection efficiency, ion concentrations of less than 106 cm−3 may be measurable, potentially enabling applications in atmospheric standoff detection of ionizing radiation from hazardous radioactive sources.