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 molecular ions make their appearance in the spectrum. For NA < 0.03, the spectrum consists only of the molecular lines of N2 and . 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 , to 27 ps for N2, and to 20 ns for atomic lines. The bibliography includes 18 references.