High time-resolved spectroscopy of filament plasma in air

The results of spectral-temporal studies of the filament plasma emission in air with a time resolution of 2 ps were presented. The filament was created by laser radiation with pulse duration of 60 fs and wavelength of 940 nm. It was shown that a spectral composition of the filament plasma depended on the numerical aperture (NA). At NA > 0.035, the atomic and ion lines of oxygen and nitrogen dominate in the spectrum on the background of an intense continuous continuum in the visible region. Molecular lines of the second positive system of nitrogen and the first negative system of molecular ions appear in the spectrum for NA < 0.03. In case of NA < 0.01, the spectrum consists of only the molecular and ion lines of N2 and N2+. The emission intensity maxima of these lines delay relative to the laser pulse. The atomic lines begin to glow per (80-100) ps after the laser pulse. The characteristic time of the luminescent lines decay at 1/e level for ions is 18 ps, for molecules is 27 ps, and for atomic lines is 20 ns. Rapid decrease in the intensity of molecular lines emission is due to the deactivation of the particles upper states by electrons. The formation of N2+(B2Σu+) excited states is most likely through their N2+(X2Σg+) ground state in the process of multiphoton absorption of pumping radiation by nitrogen molecules and subsequent their excitation by electron impact.