Observation of the Rabi splitting of oxygen atomic levels in filament plasma formed by a femtosecond Ti:Sa laser pulse

Dynamics of the emission spectrum of a highly focused femtosecond laser pulse under filamentation in air have been investigated. A peak at 735 nm and a drop in the laser-beam intensity at the fundamental wavelength are observed in the spectrum of a laser pulse passed through the filamentation zone. It is shown that the peak is likely due to the Rabi splitting of the O I levels and breaking of the $LS$ coupling for the 3p$^{5}$P level in a strong laser field.