Population trapping and rotational revival of N2 molecules 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 Sigma(+)(u) -> X-2 Sigma(+)(g)) and the second positive band (C-3 Pi(u) -> B-3 Pi(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.