Temporal dynamics of air plasma glow under different focusing conditions of a femtosecond radiation pulse

We report the results of the spectral and temporal investigations of air plasma glow with a temporal resolution of ~2 ps. The plasma was produced by 950-nm laser pulses 60 fs in duration. It is shown that the spectrum obtained for a numerical aperture NA > 0.1 is dominated by atomic and ionic lines of oxygen and nitrogen against the background of a high-intensity continuous spectrum in the visible region. For NA < 0.07, the lines of the second positive system of molecular nitrogen N₂ and the first negative system of N₂⁺ molecular ions make their appearance in the spectrum. For NA < 0.03, the spectrum consists only of the molecular lines of N₂ and N₂⁺. The emission intensity peaks of these lines lag behind the laser pulse by 10–12 ps, but the atomic lines begin to shine 80–100 ps after the instant of irradiation. The characteristic time of line glow decay to a 1/e level amounts to 18 ps for N₂⁺, to 27 ps for N₂, and to 20 ns for atomic lines.