Population trapping and rotational revival of N2 molecules during filamentation of a femtosecond laser pulse in air

Abstract

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 (B2Σ+u → X2Σ+g) and the second positive band (C3Πu→B3Πg) 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.