Population trapping and rotational revival of N2molecules during filamentation of a femtosecond laser pulse in air

We study the fluorescence emitted from filaments in air using a pump–probe scheme with a femtosecond Ti–sapphire laser. The fluorescence intensities from the first negative band (B 2 � + → X 2 � +) and the second positive band (C 3 � u→B 3 � g) show enhancement and change periodically as a function of the pump–probe time delay. We attribute this phenomenon to the universal yet probably forgotten phenomenon of population trapping of nitrogen molecules in highly excited states together with field-induced alignment of nitrogen molecules followed by revivals of the rotational wavepackets. Theoretical calculation of the alignment dynamics of nitrogen molecules is consistent with the experimental data. (Some figures in this article are in colour only in the electronic version)