The mechanism of nitrogen fluorescence inside a femtosecond laser filament in air

The visible and near-UV fluorescence spectroscopy of air generated inside a femtosecond laser plasma filament was studied. The primary reactions, N2++N2⇒N4+;N4++e⇒N2(C3Πu)+N2, populates the electronic excited state N2(C3Пu) of N2. The N2 fluorescence is not by direct electron-ion recombination of N2++e. Using a pump-probe method, the fluorescence lifetime of C3Пu of N2 at atmospheric pressure was determined to be about 85 ps.