Abstract
The digital images of plasmas induced by tightly focused Ti:Sapphire laser (1.1mJ, 48fs) 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 1ns) and long-time emission of atomic lines (up to 150ns). Initially, second positive system of N2 and first negative system of N2+ are observed. The atomic lines are formed with delay of 50-80ps 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 50ps time interval.
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