Absorption and emission characteristics of femtosecond laser plasma filaments in the air

• Atomic lines are formed with delay of 50-80 ps after laser pulse action.
• Peak intensities of N2 337 and N2+ 391.2 nm lines are separated by 50 ps time interval.
• Molecular emission lasts up to 1 ns while atomic one observed up to 150 ns.
• Absorbed laser energy grows nonlinearly with increase of incident laser energy.
• Lens aberrations lead to complicated fluorescent pattern of laser filamentation.

The digital images of plasmas induced by tightly focused Ti:Sapphire laser (1.1 mJ, 48 fs) in air reveal new fluorescent structures not observed for slightly focused beams earlier. The fluorescent structures arise because of lens aberrations. The energy absorbed by femtosecond laser plasma has nonlinear dependence on incident laser energy. The threshold peak power for filament formation is 5.2 GW. Time-resolved spectroscopy of plasmas shows short-time emission of molecular bands (up to 1 ns) and long-time emission of atomic lines (up to 150 ns). Initially, second positive system of N2 and first negative system of N2+ are observed. The atomic lines are formed with delay of 50-80 ps relative to molecular ones. Lines of N2 and N2+ demonstrate different temporal behavior, while lines of N I and O I have the same intensity time dependence. Peak intensities of second positive system of N2 and first negative system of N2+ are separated by 50 ps time interval.