Plasma luminescence from femtosecond filaments in air: Evidence for impact excitation with circularly polarized light pulses

Filaments produced in air by intense femtosecond laser pulses emit UV luminescence from excited N2 and N2+ molecules. We report on a strong dependence at high intensities (I≥1.4×1014W/cm2) of this luminescence with the polarization state of the incident laser pulses. We attribute this effect to the onset of new impact excitation channels from energetic electrons produced with circularly polarized laser pulses above a threshold laser intensity.